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Investigation of Structure-Property Relations in Nanocomposites for Energy Storage

Permanent Link: http://ufdc.ufl.edu/UFE0045233/00001

Material Information

Title: Investigation of Structure-Property Relations in Nanocomposites for Energy Storage
Physical Description: 1 online resource (154 p.)
Language: english
Creator: Tang, Haixiong
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2013

Subjects

Subjects / Keywords: capacitor -- energy -- film -- nanocomposite -- nanowire -- pvdf
Materials Science and Engineering -- Dissertations, Academic -- UF
Genre: Materials Science and Engineering thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: High energy density capacitors arecritically important in advanced electronic devices and electric power systemsthat rely on pulsed-power, such as defibrillators, and high powermicrowaves.  Nanocomposites have greatpotential as high energy capacitors, since they combine the high breakdownstrength of polymers with the high dielectric permittivity of ceramics, to produce energy density greater than either in its pure form.  Most of the currentresearch has focused on improving the energy denisty of nanocomposites by choosinga high dielectric permittivity filler and high breakdown strength matrix.  However, the improvement of dielectricpermittivity comes at the expense of the breakdown strength thus limiting theultimate performance of the capacitors. This dissertation hasinvestigated the relationship between filler’s structure (aspectratio and orientation) and energy storage performance of nanocomposites.  Initially, the effect of the filler’s aspect ratio on the nanocomposite’s energy density was studied.  It isdemonstrated that the nanocomposites with PZT nanowires(NWs) show 77.8% increasein energy density compared to samples with PZT nanorods at 50 % volume fraction.  Second, this work investigates the role of NWorientation towards the improvement in the energy density of nanocomposites.  It is demonstrated that the energy storagecapacity of the nanocomposite can be enhanced by 51.6% through the alignment of PZT nanowires in the direction ofthe applied electric field as compared to the sample with randomly alignednanowires at 20% volume fraction.  Furtherresearch is performed to quantify the dielectric constant of nanocomposites asa function of both aspect ratio and orientation factorof the fillers.  Based on these findings, two different types of nanocomposites withhigh energy density are fabricated.  Thenanocomposites with 7.5 vol.% Ba0.2Sr0.8TiO3NWs in PVDF are shown to have an ultra-high energy density of 14.86 J/cc at 450 MV/mwith microsecond discharge time speed, which exceeds those reported in theliterature for ceramic/polymer composites, and is 1138% greater than the reported commercialcapacitor biaxial oriented polypropylene (1.2 J/cc at 640 MV/m).  This dissertation will serve to disseminate a state-of-the-artmethod of preparing nanocomposites with high energy density and fast dischargefor development of future pulsed-power capacitors.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Haixiong Tang.
Thesis: Thesis (Ph.D.)--University of Florida, 2013.
Local: Adviser: Sodano, Henry.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2015-05-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2013
System ID: UFE0045233:00001

Permanent Link: http://ufdc.ufl.edu/UFE0045233/00001

Material Information

Title: Investigation of Structure-Property Relations in Nanocomposites for Energy Storage
Physical Description: 1 online resource (154 p.)
Language: english
Creator: Tang, Haixiong
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2013

Subjects

Subjects / Keywords: capacitor -- energy -- film -- nanocomposite -- nanowire -- pvdf
Materials Science and Engineering -- Dissertations, Academic -- UF
Genre: Materials Science and Engineering thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: High energy density capacitors arecritically important in advanced electronic devices and electric power systemsthat rely on pulsed-power, such as defibrillators, and high powermicrowaves.  Nanocomposites have greatpotential as high energy capacitors, since they combine the high breakdownstrength of polymers with the high dielectric permittivity of ceramics, to produce energy density greater than either in its pure form.  Most of the currentresearch has focused on improving the energy denisty of nanocomposites by choosinga high dielectric permittivity filler and high breakdown strength matrix.  However, the improvement of dielectricpermittivity comes at the expense of the breakdown strength thus limiting theultimate performance of the capacitors. This dissertation hasinvestigated the relationship between filler’s structure (aspectratio and orientation) and energy storage performance of nanocomposites.  Initially, the effect of the filler’s aspect ratio on the nanocomposite’s energy density was studied.  It isdemonstrated that the nanocomposites with PZT nanowires(NWs) show 77.8% increasein energy density compared to samples with PZT nanorods at 50 % volume fraction.  Second, this work investigates the role of NWorientation towards the improvement in the energy density of nanocomposites.  It is demonstrated that the energy storagecapacity of the nanocomposite can be enhanced by 51.6% through the alignment of PZT nanowires in the direction ofthe applied electric field as compared to the sample with randomly alignednanowires at 20% volume fraction.  Furtherresearch is performed to quantify the dielectric constant of nanocomposites asa function of both aspect ratio and orientation factorof the fillers.  Based on these findings, two different types of nanocomposites withhigh energy density are fabricated.  Thenanocomposites with 7.5 vol.% Ba0.2Sr0.8TiO3NWs in PVDF are shown to have an ultra-high energy density of 14.86 J/cc at 450 MV/mwith microsecond discharge time speed, which exceeds those reported in theliterature for ceramic/polymer composites, and is 1138% greater than the reported commercialcapacitor biaxial oriented polypropylene (1.2 J/cc at 640 MV/m).  This dissertation will serve to disseminate a state-of-the-artmethod of preparing nanocomposites with high energy density and fast dischargefor development of future pulsed-power capacitors.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Haixiong Tang.
Thesis: Thesis (Ph.D.)--University of Florida, 2013.
Local: Adviser: Sodano, Henry.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2015-05-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2013
System ID: UFE0045233:00001


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1 INVESTIGATION OF STRUCTURE PROPERTY R E LATIONS IN NANOCOMPOSITES FOR ENERGY STORAG E By HAIXIONG TANG A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2013

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2 2013 Haixiong Tang

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3 To my parents and wife

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4 ACKNOWLEDGMENTS I would like to thank my advisor Dr. Henry Sodano for providing me with all the opportunities for completing this dissertation and also for his exceptional guidance and assistance throughout my PhD program. Dr. Sodano has not only taught me how to become a good and a productiv e researcher but has also show n me how to balance work and family life. I feel lucky to have had this opportunity to work with him and learn from him. I am pos i tive that my learning experience from him during my PhD program will ben efit my entire life. My three and half years study at the Arizona State University and University of Florida has been intense and rewarding. I also wish to acknowledge the intellectual input from my committee members: Dr. Jacob L. Jo n es, Dr. Wolfgang M. Sigmund, Dr. Jennifer S. Andrew, and Dr. Curtis R. Taylor, who will be the final judges of my worthiness for this degree. I also would like to thank Dr. Yirong Lin who gave me a lot of valuable suggestions and help in the last three years. Dr. Zoubeida Ou naies must be acknowledged for giving me a lot of advice both for research and career development. I also really appreciate Dr. Yu Zhou, Dr. Dechang Jia and Dr. Yujing Wang for their encouragement and advice in the last three years. I want to thank all t he present and My labmates gave me numerous opportunities for laugher and maintained a cordial environment in the lab to make my research work more memorable. I wish I could mention every one, but the list would be too long and I would feel awful for leaving anyone out. I also would like to express my sincere gratitude to Mohammad Malakooti, for their inputs and suggestions in the research; Teng Ma shared a lot of happy momen ts in the last three years.

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5 My parents and brother deserve the special attention for all their complete support to this point in my life. They gave me tremendous encouragement to let me choose the options in life that I desired. Nothing was easy for our life and we faced challenges and choice around every corner during growing up. Fortunately, I learned how to catch and create every opportunity in my life and I became stronger. laughter and optimistic attitude to life make my life full of sunsh ine. I also want to thank study. Last but not the least, I want to thank my wife who is the love of my life.

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6 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF ABBREVIATIONS ................................ ................................ ........................... 13 ABSTRACT ................................ ................................ ................................ ................... 16 CHAPTER 1 INTRODUCTION ................................ ................................ ................................ .... 18 M otivation ................................ ................................ ................................ ............... 18 Fundamental s of Capacitor ................................ ................................ ..................... 19 Capacitance, Dielectric Constant and Loss Tangent ................................ ........ 20 Polarization Mechanisms ................................ ................................ .................. 22 Ener gy Density ................................ ................................ ................................ 25 Capacitor Technolog y ................................ ................................ ............................. 28 Ceramic ................................ ................................ ................................ ............ 28 Polymer ................................ ................................ ................................ ............ 29 Nanocomposite ................................ ................................ ................................ 31 Dielectric Permittivity of Composite ................................ ............................ 32 Breakdown Strength of C omposite ................................ ............................ 35 Nanocomposites with Ferroelectric Filler ................................ ................... 40 Nanocomposites with Non ferroelectric Filler ................................ ............. 44 Dissertation Overview ................................ ................................ ............................. 50 Contributions ................................ ................................ ................................ .... 50 Chapter Su mmary ................................ ................................ ............................ 52 2 THE EFFECT OF FILLER ASPECT RATIO ON THE ENERGY DENISTY OF NANOCOMPOSITES ................................ ................................ .............................. 56 Chapter Introduction ................................ ................................ ............................... 56 Investig ation of the Effects of Filler Aspect Ratio on the Energy Storage of Nanocomposites ................................ ................................ ................................ .. 57 Preparation of Different Aspect Ratio Fillers: PZT NWs and PZT NRs ............ 57 Fabrication of Nanocomposites with Different Aspect Ratio Fillers .................. 62 The Effect of Filler Aspect Ratio on the Dielectric Constant ............................. 64 The Effect of Filler Aspect Ratio on the Energy Density ................................ ... 67 Quantification of the Relationship between the Filler Aspect Ratio and Dielectric Constant of the Nanocomposite s ................................ ................................ ......... 72 Chapter Summary ................................ ................................ ................................ ... 81 3 THE EFFECT OF FILLER ORIENTATION ON THE ENERGY DENSITY OF NANOCOMPOSITES ................................ ................................ .............................. 83

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7 Chapter Introduction ................................ ................................ ............................... 83 Alignment Methods and T heir Effect s ................................ ................................ ..... 83 Preparation of Nanocomposites with Aligned PZT NWs ................................ .. 85 The Effect of Filler Orientation on the Dielectric Constant ................................ 88 The Effect of Filler Orientation on the Energy Density ................................ ...... 89 Quantification of the Relationship between the Filler Orientation Factor and the Dielectric Constant of Nanocomposites ................................ ............................... 93 Chapter Summary ................................ ................................ ................................ 102 4 HIGH ENER GY DENSITY NANOCOMPOSITES ................................ ................. 105 Chapter Introduction ................................ ................................ ............................. 105 High Energy Density Nanocomposite Capacitors Using BaTiO 3 NWs and P(VDF TrFE CFE) ................................ ................................ ............................. 106 Dielectric Property of the Nanocomposites ................................ ..................... 108 Energy Storage Performance of the Nanocomposite Capa citor s ................... 110 Ultra High Energy Density Nanocomposite Capacitors Using Ba 0.2 Sr 0.8 TiO 3 NWs ................................ ................................ ................................ ................... 115 Synthesis of Ba 0.2 Sr 0.8 TiO 3 NWs and Preparation of Nanocomposites .......... 116 Dielectric Property of the Nanocomposites ................................ ..................... 121 Energy Storage Performance of the Nanocomposite Capacitors ................... 124 C hapter Summary ................................ ................................ ................................ 129 5 CONCLUSION S ................................ ................................ ................................ ... 131 Brief Summary of Dissertation and Results ................................ .......................... 132 Contributions ................................ ................................ ................................ ......... 135 Recommendations for Future Work ................................ ................................ ...... 138 LIST OF REFERENCES ................................ ................................ ............................. 140 BIOGRAPHICAL SKETCH ................................ ................................ .......................... 154

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8 LIST OF TABLES Table page 1 1 The dielectric constant of typically used ceramics as capacitors. ....................... 29 1 2 Dielectric properties of the most commonly used polymers ................................ 30

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9 LIST OF FIGURES Figure page 1 1 Energy and power density of several energy storage technologies .................... 19 1 2 Typical mode of the capacitor. ................................ ................................ ............ 20 1 3 Four different polarization mechanisms ................................ ............................. 22 1 4 The frequency dependence of the polarization in the presence of different polarization mechanism ................................ ................................ ...................... 25 1 5 Typical D E loop used to calculate energy density of non linear capacitor ........ 26 1 6 Four typical D E loop s ................................ ................................ ......................... 27 1 7 Ceramic capacitors with different shapes ................................ ............................ 29 1 8 The polarization response under high field for PVDF based copolymers and terpolymers ................................ ................................ ................................ ......... 31 1 9 The effective dielectric permittivity of composite is predicted by the models ....... 33 1 10 P redicted dielectric permittivity of the nanocomposite s with different aspect ratio ................................ ................................ ................................ .................... 35 1 1 1 Muti core model of interface in the nanocomposites. ................................ .......... 38 1 12 ABO 3 perovskite type unit cell ................................ ................................ ............. 40 1 13 The cubic structure of ABO 3 above the Curie temperature. ................................ 41 1 14 The dependence of energy density of the nanocomposites on the volume fraction of BaTiO 3 in the P(VDF TrFE CTFE) and P(VDF CTFE) ....................... 42 1 15 Dielectric property of nanocomp osites with BaTiO 3 in the P (VDF HFP ) ............ 43 1 16 The breakdown strength and maximum energy density of nanocomposites with BaTiO 3 in the P (VDF HFP ) ................................ ................................ .......... 43 1 17 D E hysteresis of loops P(VDF CTFE) /BaTiO 3 nanocomposite films .................. 44 1 18 The unite cell of CCTO crystal ................................ ................................ ............ 45 1 19 Nanocomposites with TiO 2 nanorods for energy storage ................................ .... 47 1 20 Preparation and energy density of nanocomposites with ZrO 2 ........................... 48

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10 1 21 D E loops and breakdown strength of the nanocompo sites with kaolinite fillers 49 2 1 SEM images of PZT particles ................................ ................................ .............. 59 2 2 PZT particle size distribution ................................ ................................ ............... 59 2 3 XRD patterns of PZT NWs and PZT NRs ................................ ........................... 61 2 4 Gauss fit of XRD ................................ ................................ ................................ 61 2 5 SEM images of top view of PZT PVDF nanocomposite ................................ ...... 63 2 6 FTIR spectra of nanocomposites. ................................ ................................ ....... 64 2 7 Comparison of measured dielectric constant (at 1 KHz) of nanocomposites ...... 65 2 8 Dielectric constants of nanocomposites ................................ ............................. 66 2 9 Dielectric loss tangent of different PZT volume fraction in nanocomposites ....... 67 2 10 The Sawyer Tower circuit and mechanism ................................ ......................... 68 2 11 Typical hysteresis loop and energy density calculation of nanocomposites ........ 68 2 1 2 Electri c displacement field (D E) loop of the nanocomposite s ............................ 69 2 1 3 The dependence of energy densit y on the PZT volume fraction ......................... 70 2 1 4 SEM images of hydrogen titanate NWs prepared at different temperatures ....... 74 2 1 5 SEM images of BaTiO 3 NWs prepared at different temperature ........................ 75 2 1 6 BaTiO 3 particle size distribution dependent on the hydrothermal temperature ... 75 2 1 7 Verification of BaTiO 3 NWs ................................ ................................ ................ 77 2 1 8 The relationship between the aspect ratio of BaTiO 3 NWs and hydrothermal reaction temperature ................................ ................................ ......................... 78 2 1 9 BaTiO 3 NWs functionalization and nanocomposites ................................ .......... 79 2 20 Dielectric constant of the nanocomposites as a function of aspect ratio and volume fraction of BaTiO 3 NWs. ................................ ................................ ......... 80 3 1 Alignment of PZT NWs in nanocomposit es ................................ ......................... 86 3 2 SEM images and XRD patterns of PZT NWs nanocomposites ........................... 87 3 3 Comparison of measured dielectric permittivity (at 1 k Hz) of nanocomposites .... 88

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11 3 4 Di electric constants of different PZT volume fraction in nanocomposites from 1 KHz to 1 MHz ................................ ................................ ................................ .. 89 3 5 D E loop measured under different applied fields at room temperature and 100 Hz fo r the polymer and nanocomposite ................................ ....................... 90 3 6 The dependence of energy density on the PZT orientation and volume fraction in the PVDF under 15 kV/mm ................................ ................................ 91 3 7 FTIR patterns of 10 % PZT NWs PVDF nanocomposites ................................ ... 92 3 8 Comparison of measured dielectric constant of nanocomposites with different structures as a function of volume fraction of PZT particle ................................ 93 3 9 Various draw ratios of the n anocomposite to control the orientation fa ctor ......... 95 3 10 SEM images and XRD patterns of PZT NWs and nanocomposites ........................ 96 3 11 The Determination of HOF. ................................ ................................ ................. 98 3 12 SEM images of cross section of the 20. vol% PZT NWs nanocomposites along with their FFT under various draw ratio ................................ .................. 100 3 13 HOF as a function of the draw ratio for the nanocomposites ............................ 101 3 14 Dielectric constant of the nanocomposites as a function of HOF ...................... 102 4 1 Morphology, structure and functionalization of BaTiO 3 NWs ............................. 107 4 2 SEM image of 10 vol.% BaTiO 3 P(VDF TrFE CFE) nanocomposites ............... 108 4 3 Dielectric properties and loss tangent of nanocomposites ................................ 109 4 4 Dielectric permittivity constants of different BaTiO 3 NWs volume fractions ....... 110 4 5 Unipolar electric displacement electric field (D E) loops for nanocomposites ... 111 4 6 Energy density of the nanocomposite with different volume fractions as a function of electric field calculated from D E loops ................................ ........... 112 4 7 Discharge circuit of energy density and discharge speed characterization ....... 113 4 8 An equivalent circuit of a dielectric sample ................................ ....................... 113 4 9 Typical discharged energy density and power density profiles for nanocomposites with 17.5% BTO NWs ................................ ............................ 114 4 10 SEM images and XRD of nanowires and nanocomposites .............................. 118

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12 4 11 TEM analysis of Ba 0.2 Sr 0.8 TiO 3 NWs ................................ ................................ 119 4 12 BST NWs functionalization and nanocomposites ................................ .............. 120 4 13 FTIR spectra and breakdown strength of quenched PVDF ............................... 121 4 14 Dielectric properties of th e nanocomposites ................................ ..................... 122 4 15 Diagram of breakdown strength test using an electrostatic pull down method .. 123 4 16 Weibull distribution and observed dielectric breakdown strength of nanocomposites with di fferent volume fractions of BST ................................ ... 124 4 17 Unipolar electric displacement electric field (D E) loops for nanocomposites ... 125 4 18 Energy density and efficiency of nanocomposites with Ba 0.2 Sr 0.8 TiO 3 NWs ...... 127 4 19 Discharged energy density and power density of nanocomposites with Ba 0.2 Sr 0.8 TiO 3 NWs. ................................ ................................ ......................... 128

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13 LIST OF ABBREVIATION S Dielectric permittivity E b Breakdown strength of the materials PZT L ead zirconate titanate PVDF P olyvinylidene fluoride BST B arium strontium titanate NWs N anowires NRs N anoparticles SEM Scanning Electron Microscopy FTIR Fourier transform infrared XRD X ray diffraction LCR I nductance capacitance resistance BET Brunauer, Emmet, Teller V Voltage d Distance Q Charge A Area 0 Dielectric permittivity of the free space r Dielectric constant of the dielectric materials Complex permittivity P Polarization charge s Charge density D Electric displacement E Electric field U Energy density

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14 tan Loss tangent DF Dissipation factor W Energy loss f Frequency e Electronic polarization i Ionic p olarization d Dipolar polarization s Space charge polarization Volume fraction BOPP Biaxial oriented polypropylene PP P olypropylene PET P olyster PC P olycarbonate PEN P olyethylene naphate PPS P olyphenylene sulfide P(VDF TrFE) P oly(vinylidene fluoride trifluoroethylene) P(VDF HFP) P oly(vinylidene fluoride hexafluoropropylene) P(VDF CTFE) P oly(vinylidene fluoride chlorotrifluoroethylene) P(VDF TrFE CFE) P oly(vinylidene fluoride trifluoroethylene chlorofluoroethylene) 1 Dielectric permittivity of filler 2 Dielectric permittivity of matrix 1 Dielectric permittivity of filler 2 Dielectric permittivity of matrix CCTO Calcium copper titanate ZTO Zr x Ti 1 x O 2 nH 2 O

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15 DMF D imethylformamide D E E lectric displacement electric field DUT D evice under test HOF o rientation f actor FFT Fast f ourier transform MMT O rganomontmorillonite d 33 P iezoelectric strain coefficient

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16 Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy INVESTIGATION OF STRUCTURE PROPERTY RELATIONS IN NANOCOMPOSITES FOR ENERGY STORAGE By Haixiong Tang May 2013 Chair: Henry A. Sodano Major: Materials Science and Engineering High energy density capacitors are critically important in advanced electronic devices and electric power systems that rely on pulsed power, such as defibrillators, and high power microwaves. Nanocomposites have great potential as high energy capacitors, since they combine the high breakdown strength of polymers with the high dielectric permittivity of ceramics, to produce energy density greater than either in its pure form. Most of the current research has focused on improving the energy denisty of n anoc omposites by choosing a high dielectric permittivity filler and high breakdown strength matrix. However, the improvement of dielectric permittivity comes at the expense of the breakdown strength thus limiting the ultimate performance of the capacitors. Th is dissertation has investigated the relationship between filler s structure (aspect ratio and orientation) and energy storage performance of nanocomposites Initially, the effect of the filler aspect ratio on the energy density was studied It is demonstrated that the nanocomposite s with PZT nanowires (NWs) show 77.8% increase in energy density compared to samples with PZT nanorods at 50 % volume fraction. Second, this work investigates the role of NW orientation towards the

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17 improv ement in the energy density of nanocomposites. It is demonstrated that the energy storage capacity of the nanocomposite can be enhanced by 51.6% through the alignment of PZT nanowires in the direction of the applied electric field as compared to the sampl e with randomly aligned nanowires at 20% volume fraction Further research is performed to quantify the dielectric constant of nanocomposites as a function of both aspect ratio and orientation factor of the fillers. Based on these finding s two different types of nanocomposites with high energy density are fabricated. The nanocomposites with 7.5 vol. % Ba 0.2 Sr 0.8 TiO 3 NWs in PVDF are shown to have an ultra high energy density of 14.86 J/cc at 450 MV/m with microsecond discharge time speed, which exceeds th ose reported in the literature for ceramic/polymer composites, and is 1 1 38% greater than the reported commercial capacitor biaxial oriented polypropylene (1.2 J/cc at 640 MV/m). This dissertation will serve to disseminate a state of the art method of prep aring nanocomposites with high energy density and fast discharge for development of future pulsed power capacitors.

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18 CHAPTER 1 INTRODUCTION M otivation The motivation for this research lies in the development of high energy density nanocomposite capacitors with fast discharge speed. Over the past few decades, high energy density c apacitors have gained significant attention for future advanced pulsed power electronic devices and electric power systems, such as lasers, radar, pace makers, electromagnetic armor s defibrillators and high power microwaves [1 8] By releasing large amounts of electrical energy in a short duration, typically less than a microsecond, high energy density capacitors possess large power densities to meet the requirements of pulsed power devices [1,6 8] Theoretically, the energy density is linearly proportional to the dielectric constant and quadratically related to the breakdown strength of the capacitor [6,9,10] Therefore, many efforts have been devoted to energy density. Currently, commercial monolithic materials are re aching a plateau in terms of energy density due to the trade off between the dielectric permittivity and breakdown strength of the materials as shown in Figure 1 1 [11] For example, ceramics have a high dielectric perm ittivity but low breakdown strength, while polymers have a high breakdown strength but low dielectric permittivity [2,5,12] In addition, other energy storage techn ologies, for example, batteries and fuel cells, have greater energy density but have a very long discharge time compared to polymers and ceramics [11] which result in a low power density that is not suitable for pulsed power applications. Therefore, it is important to develop a novel energy storage material having high energy density with fast discharge speed.

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19 Nanocomposites combining a high breakdown strength polymer and high dielectric permittivity ceramic filler offe r significant promise for future high energy density capacitors [9,13 16,16 18] However, while current nanocomposites improve the dielectric permittivity of the capacitor, the gains come at the expense of the breakdown strength, which limits the ultimate performance of the capacitor [14 16,19 21] This dissertation will present a new fabrication route by using high aspect ratio nanowires rather than equiaxial particles T he relationship between the structure (aspect ratio and orientation) and energy storage will be investigated. Based on the findings a novel nanocomposite with higher energy density and higher power density than monolithic material will be fabricate d to fill the green gap as shown i n Figure 1 1. Furthermore, the advances made through the development of high energy density nanocomposite capacitors such as those here will pave the way for the development of future pulsed power devices. Figure 1 1. Energy and power density of several energy storage technologies (Reprinted with permission from K tz et al. [11] ) Fundamental s of Capacitor A capacitor, also known as a condenser, is a simple electronic device that typically consists of two electr odes that are separated by a dielectric material as shown

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20 in Figure 1 2 [22] When there is a voltage across two electrodes, an electric field is formed across the dielectric material. Positive cha rge accumulates on one electrode until the induced electric field across the gap balances the applied field, while the negative charge is on the other electrode [22] By this process, the energy is stored by an electrostatic field in the dielectric material. T he performance of a capacitor can be characterized by the following parameters: capacitance, dielectric loss, breakdown strength, polarization electric field response, energy density and power density, which will be discussed in detail below. Figure 1 2. Typical mode of the capacitor. Capacitance, Dielectric Constant and Loss Tangent As shown in Figure 1 2, it is assumed that there is a voltage V (volt, v) across a distance d (meter, m) in a dielectric material (first assume vacuum). The positive charge +Q (coulomb, c) is accumulated on the bottom electrode with an area A (m 2 ), while the negative charge Q is on the other side. The capacitance C (Farad, F) can be defined as the ratio of the charge to voltage between two electrodes [23] : w here 0 is the dielectric permittivity of vacuum with a value of 8.854*10 12 F/m.

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21 If a dielectric material is inserted between two electrodes, t he charg e is redistributed on the dielectric material and the electrodes in response to the applied field, resulting in inducing a polarization charge P at the surface of the material. The resulting charge density ( s ) of the capacitor consists of two parts: one part is from the vacuum charge, and the other part is P ( pol ) from the charge compensation on the surface of the dielectric in contact with the electrodes as shown in Figure 1 2 [23,24] Ther efore, the total charge density can be expressed by the following equation: where D is the electric displacement (C/m 2 ), E is the electric field (V/m), r is the dielectric constant of the material with respect to the 0 of vacuum r is typically larger than one ; therefore, by inserting a dielectric material, the energy storage capacity of the capacitor will be improved as compared to vacuum The energy density U (J /m 3 ) of the linear capacitor can be expressed [25] : ( 1 1 ) In reality, energy loss is always associated with dielectric materials. In order to understand the energy loss of a material, the complex permittivity ( ) is introduced: where is the dielectric permittivity and r is the re al permittivity part of the material. T he relative magnitude of the losses can be estimated by the loss tangent ( ), defined as the below equation [24] :

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22 The loss tangent is a measurement of e nergy loss in the dielectric material during the AC operation, which is independent of the geometry of the capacitor. The energy loss, W is linearly proportional to the loss tangent and can be determined by: where is the freque ncy of the electric field. Therefore, a dielectric material with low loss tangent can reduce the energy loss of the capacitor and improve the efficiency of energy storage. Polarization Mechanisms M aterial exhibit s a unique dielectric property due to polar ization in which the dipole is switched according to the application of an external electric field. Here, four different mechanisms of polarization are introduced as shown in Figure 1 3: electronic polarization, ionic polarization, orientation polarizatio n and space charge polarization [24,26] Figure 1 3 Four diffe rent polarization mechanisms: A) electronic polariz ation, B) ionic polarization, C ) orientation polarization, D ) space charge polarization (A) (B) (C) (D)

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23 Electronic polarization exists in all dielectrics because every material is composed of atoms [24,26,27] The electronic polarization is formed by shifting the electronic cloud within each atom under an electric field as shown in Figure 1 3a. When an electric field is applied across the dielectric material, the positive charge distribution is shifted with respect to the negative charge distribution. As a result a polarization vector develops in the dielectric material. T he electronic polarization can respond to most frequencies up to 10 17 Hz due to the small mass of atoms. Also, this mechanism is independent o f temperature since the electronic structure is stable over a wide temperature range [24,26,27] However, the electronic polarization is smaller compared to other polar ization mechanisms due to the short arms of the dipoles [24,26,27] Ionic polarization typically occurs in an ionic crystal, which is similar to electronic polarization [24,26,28] As shown in Figure 1 3b, each pair of oppositely charged neighboring ions has a dipole moment in the ionic crystal. In the absence of an electric field, there is no net polarization since the dipole moments of equal magnitude are lined up head to head and tail to tail resulting in a zero net dipole moment. Under an electric field, both the anions and cations are shifted relative to their normal positions leading to the formation of net dipole moment across the dielectric material. Like electronic polarization, ionic polarization is independent of temperature, and respond s to a high frequency of up to 10 13 Hz. As shown in Figure 1 3c, orientation polarizat ion occurs in molecules with a permanent dipole moment such as ferroelectric materials [24,26,29] In the absence of an electric field, there is no net polarization in t he dielectric material because the thermal agitation causes a totally random molecule distribution resulting in a zero net

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24 dipole moment. Under an applied electric field, the dipoles rotate and shift to align along the field l eading to a net average dipol e moment in the material. Orientation polarization is larger compared to other mechanisms and can response to frequency less than 10 13 Hz and dependent on temperature. For example, the dielectric permittivity of the ferroelectric material changes sharply at Curie temperature [24,26,27] Space charge polarization occurs whenever there is an accumulation of charge at an interface between two materials or between two regi ons within a material like grain boundaries and defects in the material [24,26,30] As shown in Figure 1 3d, the absence of a field creates a zero net dipole in the material since there is no separation between all the positive charge carriers and all the negative ones. Under an electric field, the negative charge carriers migrate toward the positive electrode and accumulate there, while the positive ones move to th e negative side. As a result, interfacial polarization arises within the dielectric material because of this separation. This mechanism commonly exists in grain or phase boundaries and free surfaces. Space charge is highly related to temperature, and it can only respond to low frequencies (<10 3 Hz). T he average induced dipole moment per molecule ( ) in the electrical material can be measured by the sum of all contributions: electronic polarization ( e ), ionic polarization ( i ), dipolar polarization ( d ) and space charge polarization ( s ) The frequency dependence of polarization in different mechanisms can be summarized in Figure 1 4. E lectronic polarization and ionic polarization can respond at high frequency, while dipolar polariz ation and space charge polarization can only

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25 respond at low frequenc y A relaxation frequency is the reciprocal of the minimum reorientation time. A dipole will not make a contribution to the polarization when the frequency of the applied electric field is higher than the relaxation frequency and the dipole cannot shift orientation direction to keep up with the field [31] Therefore, the polarizability decreases with an increase in fr equency as shown in Figure1 4. For ferroelectric materials the space charge polarization reduces first with increas ing frequency, and then the orientation polarization dominate s the overall polarization. Figure 1 4 The frequency dependence of the polarization in the presence of different polarization mechanism (Reprinted with permission from Kingery et al. [26] ) Energy Density T he energy density of a linear dielectric capacitor can be directly determined by the dielectric permittivity and the breakdown strength using Equation 1 1 However, for ferroelectric materials, the electric displacement of the material is not linearly dependent on the applied electric field. Even in the commercial linear capacitor b iaxially o riented p olypropylene ( BOPP ) the electric displacement is not linear to the electric field because of electrical conduction at high electric field. Therefore, the energy density is not only dependent on the dielectric permittivity and breakdo wn strength but also related

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26 to the polarization and applied electric field. Therefore, in order to calculate the energy density of the capacitor, it is important to utilize t he Sawyer Tower circuit to determine the electric displacement electric field (D E) relationship, especially at high electric field. The true energy density of the capacitor is shown as the shaded area in Figure 1 5 [32,33] which is computed from the integration of the discharge curve in the D E loop : Figure 1 5 Typical D E loop used to calculate energy density of non linear capacitor T here are four different types of D E loops for a polarization electric field response : linear, relaxor ferroelectric, ferroelectric and anti ferroelectric as shown in Figure 1 6 [32] The linear dielectric has a linear relationship between the polarization and electric field as shown in Figure 1 6a suc h as p olypropylene. The linear dielectric has low loss; however, it does not have high dielectric permittivity and limits the energy density [34] It should be mentioned that even the best linear dielectric can become non linear above a certain electric field due to the increased conduction loss such as BOPP The relaxor ferroelectric and ferroelectric materials have a higher polarization compared to linear capacitor because of the heavy contribution from the orientation

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27 polarization mechanism. However, the ferroelectric properties of these materials have polarization saturation and a hysteresis loop leading to a higher loss compared to linear capacitors as shown in Figure 1 6b and c Another type of D E loop is from the anti ferroelectric materials ( Figure 1 6d ) Anti ferroelectric material has low remnant polarization and relatively narrow D E hysteresis loop, which implies higher energy storage ability [32,35,36] With increasing the electric field, the antiferroelectric phase of the dielectric material is changed into ferroelectric phase resulting in a limited energy storage performance [37,38] Currently, many researchers are trying to maintain the antiferroelectric phase at high electric field to yield higher energy density [39,40] Figure 1 6 Four typical D E loop s : A) linear, B) relaxor ferroelectric, C ) ferroelect ric, D ) antiferroelectric ( Reprinted with permission from Burn et al. [32] ) (A) (B) (C) (D)

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28 Capacitor Technolog y To date, several types of dielectri c materials have been used to manufacture capacitors, such as ceramics, polymers, and glass ceramics so on Here, a review of typical capacitors will be presented with a special focus on the recent progress of the nanocomposite capacitors for energy storage Ceramic Ceramic capacitor is one of the most commercial and popular capacitors characterized by moderate energy density (~2 J/cc) high power density and small to midsize capacitance (pF to 100 uF) [12,41] The ceramic capacitors can be classified into two different categories: one is low dielectric permittivity ceramics (15 500) and the other is high dielectric permittivity ceramics (2000 20000), as shown in Table 1 1 [42] C eramic capacitors are composed of two electrodes attached directly to the ceramic dielectrics as shown in Figure 1 7a [43] In order to improve the capacitance density of the capacitors, two kinds of constructions are developed and employed: multilayer ceramic capacitor and hollow tubular ceramic capacitor as shown in Figure 1 7b and c, respectively [43] T he multilayer ceramic cap acitors are widely produced in industry as they can include hundreds of dielectric layers to achieve the desired capacitance. The obvious disadvantage of the ceramic capacitors is the low breakdown strength (around 10 MV/m) leading to low energy density. For example, BaTiO 3 has a dielectric constant between 1800 and 4500 and a breakdown strength of approximately 6 MV/m, and the energy density can reach 2 J/cc [41,43] Additionally ceramics are very fragile, and hard to process to intricate configurations for electronic and electric devices [44] In summary, ceramic capacitors are more suitable for low and medium voltage application due to their limited b reakdown strength.

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29 Table 1 1 The dielectric constant of typically used ceramics as capacitors. PMN PT(65/35) is the abbreviation for 65% lad magnesium niobate and 35% lead titanate ( Reprinted with permission from Nalwa et al. [42] ) High permittivity dielectric Low permittivity dielectric Composition Dielectric constant Composition Dielectric constant (Ba, Sr)TiO 3 250 2,000 TiO 2 80 Pb( Zr, Ti)O 3 400 3,000 SiO 2 3.9 PMN PT (65/35) 3,640 Al 2 O 3 9 Pb( Mg, Nb)O 3 >3,000 Ta 2 O 5 22 CaCu 3 Ti 4 O 12 ~60,000 ZrO 2 25 La 1.8 Sr 0.2 NiO 4 ~100,000 SiN 7 9 Figure 1 7 Ceramic capacitors with different shapes: A) disk capacitor, B) tube capacitor, C ) multilayer capacitor Polymer Compared to ceramic capacitor s polymer capacitors feature a high breakdown strength (> 3 00 MV/m), moderate energy density (~2 J/cc), light weight, longer operation life and grace failure because of its self healing capability [45 47] The common polymer based capacitors includ e polypropylene (PP) [48,49] polyster (PET) [50] polycarbonate (PC) [51] polyethylene naphate (PEN) [52] polyphenylene sulfide (PPS) (A) (B) (C)

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30 [53] and polyvinylidene fluoride (PVDF) [54 56] The dielectric property and performance of various polymer capacitor s are detailed in Table 1 2 [48] Among them, BOPP is the most widely used commercial capacitor because of its high breakdown strength, low loss, low cost and the ease at which it can be manufactured. However, BOPP offers an energy density of only 1.2 J/cc at 640 MV/m due to its low dielectric permittivit y (2.2) [48] In order to improve the overall capacitance value, polymer capacitors are typically manufactured into a multilayer structure for use as commercial capacitors Additionally, the frequency response of polymer capacitors can extend to the gigahertz range with low loss [57] Table 1 2 Dielectric properties of the most commonly used polymers ( Reprinted with permission from Rabuffi et al. [48] ) Polymer F ilm Dielectric C onstant Maximum T emperature ( o C) Breakdown S trength (MV/m) Loss T angent (% at 1 k Hz) Energy D ensity (J/cc) P P 2.2 105 640 <0.02 1 1.2 P ET 3.2 125 570 <0.5 1 1.5 P C 2.8 125 528 <0.15 0.5 1 PEN 3.2 125 550 <0.15 1 1.5 PPS 3.0 200 550 <0.03 1 1.5 PVDF 12 125 >600 <1.8 2.4 In order to improve the energy density of polymers many efforts have been devoted to improve the ir dielectric permittivity without sacrificing the breakdown strength. Recently, ferroelectric polymers PVDF [56] and copolymers such as p oly(vinylidene fluoride trifluoroethylene) (P(VDF TrFE)) [16] p oly(vinylidene fluoride hexafluoropropylene) (P(VDF HFP)) [18] and p oly(vinylidene fluoride chlorotrifluoroethylene) (P(VDF CTFE)) [16] as well as terpolymer s such as

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31 poly(vinylidene fluoride trifluoroethylene chlorofluoroethylene) (P(VDF TrFE CFE)) simultaneously exhibit hig h breakdown strength ( >300 MV/m ) and relatively high dielectric permittivity (approximately 50) [16,17] Figure 1 8 shows the polarization response under a high field for PVDF copolymer and terpolymer [6] It can be seen that PVDF terpolymers have higher energy density and storage efficiency tha n PVDF copolymers. This results due to the large chlorine atom in CFE and CTFE which acts as a defect in crystal lattice, and then break up the ferroelectric domains into local nano polar regions [58,5 9] It should be mentioned that the polarization property of P(VDF TrFE CFE) is dependent on their composition [58,59] The disadvantage of the PVDF based polymers for energy storage comes from their ferroelectric properties, such as high loss and polarization saturation that limits the energy density. Figure 1 8 The polarization response under high field for PVDF based copolymers and terpolymers. The shaded region indicate s the energy density of the material. (Reprinted with permission from Chu et al. [6]) Nanocomposite Nanocomposite s represent a class of materials with multiple solid phases where at least one of the phases has nanoscale dimension range from one nanometer to hundreds o f nanometers [2,14,16,17,60 63] In this dissertation, the inorganic/polym er

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32 nanocomposites will be discussed. As mentioned, a tradeoff between the dielectric permittivity and breakdown strength exists in nanocomposites and limits the design of high energy density capacitors [14,63 67] Therefore, much research has been permi ttivity and breakdown strength in order to improve the energy storage properties. Dielectric Permittivity of C omposite There are many theoretical models developed to predict the effective dielectric permittivity of a nanocomposite. The simplest model is t he volume faction average to estimate the effective dielectric constant of the composite. w here is the effective dielectric permittivity of the composite 1 and 2 are the dielectric permittivity of the ceramic filler and polymer matrix, respectively, 1 and 2 are the volume fraction of the ceramic and polymer, respectively. This simplest model cannot predict correctly the effective dielectric permittivity of the c omposites, since it predicts very high dielectric constant even at low volume fraction of the filler, which far diverges from the value obtained from experimental research [68,69] Another more reasonable simple model is the Lichtenecker logarithmic rule [70] which is also based on the volume fraction average. Li et al. demonstrated that the experimental data agrees reasonably well with this model, except at volume fraction greater than 20% [16] Lichtenecker logarithmic law over predicts the effective dielectric permittivity of the nanocomposite as compared to experimenta l research.

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33 In order to accurately predict the effective dielectric permittivity of the binary composites, the Bruggeman model is developed. It is based on the mean field theory and treats spherical inclusion in the center of the matrix [70,71] The effective dielectric permittivity of the binary composites can be obtained with the following equation: The Bruggeman model can predict the effective dielectric permittivity of the composites at low volume fraction, but the prediction increases sharply at filler volume fractions larger than 20% and it also over estimates the dielectric permittivity of the binary nanocomposite at high volume fraction. Another mo del based on the mean field theory is Maxwell equation [71] : However, the Maxwell equation is only reasonable for prediction at low volume fraction The abov e mentioned models are summarized in Figure 1 9, which assumes 1 1 =2.3). Figure 1 9 The effective dielectric permittivity of composite is predicted by various model assumed the 1 2 =2.3)

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34 Many experiments have demonstrated that the dielectric constant of a composite is improved by using high dielectric permittivity fillers [14 16,19 21] However, the experimental value is always lower than that predicted by models. In addition, the aforementioned theoretical models all treat the filler as sp herical shape and assume they are homogenously dispersed in the polymer matrix. Therefore, the high aspect ratio filler are motivated to improve the dielectric permittivity of the composites [55,56,67,72 74] The aspect ratio effect can be described by the Maxwell Garnet model where N i is the depolarization factor of ellipsoids in the x y z direction. For needle shaped fillers, where the radii a x > a y = a z a simple expression of N i can be expressed as: The Maxwell Garnet model demonstrates that the dielectric permittivity of composite is improved by high aspect ratio filler. Andrews et a l. also developed micromechanics and finite element models to study the effect of aspect ratio, and show that the higher aspect ratio of the filler play s critical role in achieving high dielectric permittivity as shown in Figure 1 10 [67] Guo et al. experimentally demonstrated that rod shaped T iO 2 /polypropylene has higher dielectric constant than the sample with sphere shaped TiO 2 at the same volume fraction of filler. However, the authors did not test the D E loop to indicate the effect of the filler aspect ratio on the energy density of

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35 the n anocomposites. They also did not show higher dielectric constant nanocomposites with high aspect ratio nanowires, since traditional inorganic materials are not available in high aspect ratio until recently. Additionally, the orientation of the filler play s another important role in the dielectric constant and energy density of the composites. The detailed effect will be discussed in Chapter 3. Figure 1 10 P redicted dielectric permittivity of the nanocomposite s with different aspect ratio PZT 7A filler at different volume fraction ; is the aspect ratio of the filler. ( Reprinted with permission from Andrew et al. [67] ) Breakdown Strength of C omposite Dielectric materials are widely used as insulating materials between two electrodes to avoid flashover or short circuit between two electrodes [24,75] However, the applied electric fiel d in the dielectric material cannot be increased without limit. Eventually, the threshold voltage is reached resulting in mechanical damage and electric conduction, which is called dielectric breakdown. The maximum electric field that the dielectric mate rial can withstand is called breakdown strength. T he breakdown strength of the capacitor is highly dependent on the defect density, where the samples are always broken at the weakest place. There are many mechanisms developed to

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36 understand the dielectric breakdown of material. The most common type of breakdown mechanisms is electron avalanche breakdown [76 78] With increasing electric field, a free electron acquires enough energy and then collides with a host atom of the material to knock out the other electrons. Both the primary and the released electrons can further cause ionization by colliding with other host atoms and thereby generate an electron avalanche, leading to conductivity and breakdown of the material. Electron avalanche breakdown is highly depended on the temperature. With increasing temperature, the possibility of the avalanche breakdown becomes much higher, since it is much easier for the electrons to migr ate between two electrodes. Another common mode of the breakdown of dielectric materials is thought to be caused by intrinsic, thermal and ionization mechanisms [24,75] As the electric f ield in the dielectric material increases, the electric conduction and dielectric losses generate more heat leading to an increase in temperature. Consequently, certain parts of the material become hot spots inducing local melting and triggers physical an d chemical erosion, resulting in breakdown. Therefore, thermal breakdown represents failure due to localization and non uniform fields arising from conduction and dielectric losses. Electromechanical breakdown and electro fracture are another type of the [79,80] By increasing the electric field, two electrodes are pulled to each other because of the electrostatic field accumulated on the surfaces. The gap between two electrodes decreases and induces a mechanical stress in the sandwiched dielectric material. Eventually, the capacitor breaks down when the material cannot withstand this mechanical stress. Another cause for breakdown is the

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37 initiation and growth of the internal cracks in the material because of the mechanical stress, which is called electro fracture [79,80] In conventional composites, the introduction of micro or even larger size fillers into the polymer usually reduces the breakdown strength of the composites [2,81 84] The decreased breakdown strength is due to the aggregation of the fillers, which introduces defect center that distort and enhance the local electric field, leading to reduce the breakdown strength of the material [2,81 84] The distortion of the electric field is primarily caused by the different dielectric permittivity between the filler and mat rix. Additionally, the larger particle size, the higher probability of local field enhancement and the lower breakdown strength. Compared to conventional composites, nanocomposites with much smaller fillers improve the breakdown strength. In addition, t he large interfacial areas of nanocomposites reduce the charge accumulation in the composite system, resulting in the improvement of breakdown strength [85] Therefore, the breakdown strength in the nanocomposite s is much highe r than conventional composite. Roy et al. demonstrated that the nanocomposites with nanofillers have higher breakdown strength compared to the samples with microfillers [86] They believed that the nan oparticles can disrupt the continuity of the path provided to the charge carriers resulting in higher breakdown strength [81] Tanaka et al. proposed a famous model with multi core construction to understand and describe the interfacial structure and charge behavior of spherical filler embedded in a polymer matrix [87] As shown in Figure 1 1 1 the interface is composed of three layers. The first layer is a bonded layer whic h is equivalent to a transition layer tightly bonded to the polymer and particle filler by coupling agents such as silane. The

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38 second layer is an interfacial region including polymer chains interacting with the first layer. The thickness is around 2 9 nm depending on the strength of the polymer particle interaction. The third layer is the loose layer containing free volume and the crystalline region of the polymer matrix. Due to the three different layers, a gradient of charge distribution is establishe d between the particle and matrix. The charge distribution at the interfacial layers has a significant effect on the dielectric properties of the nanocomposite. Suitable alteration at the interface results in a change of charge carriers mobility, free v olume and trap sites. Better charge distribution at the interface can improve the breakdown strength. This can explain the phenomena of higher breakdown strength in the nanocomposites. For example, Ma et al. observed a decrease in the mobility of the ch arge carriers in the nanocomposite by incorporation of modified titanium dioxide with a polar silane coupling in the matrix [81] The low concentration of the filler in the nanocomposite s decease the possibility of overlapping of the local conductive regions, leading to an improvement in the breakdown strength. Figure 1 1 1. Muti core model of interface in the nanocomposites ( Reprinted with permission from Tanaka et al. [87] )

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39 surface functionalization to increase compatibility between filler and matrix and then help the filler in the matrix. Ma et al. reported an improvement in t he breakdown strength by introduction of the polar silane modified TiO 2 nanoparticles in to a polyethylene matrix [81] The author found that the functionalization technique improved the electron scattering by th e polar interfacial groups to decrease the made by Dou et al., who demonstrated that BaTiO 3 /PVDF nanocomposites have increas ed breakdown strength after modifying the filler with iso propyl alcohol [88] Large enhancement of breakdown strength wa s observed at 7 vol.% of BaTiO 3 in PVDF Without surface functionalization, the breakdown strength decrease d with increasing filler concentration. In cont rast, Perry et al. embedded phosphonic acid surfaced modified BaTiO 3 into P(VDF HFP) to fabricate high energy density nanocomposites (7 8 J/cc) [14] Clearly, the surface functionalization can improve the dielectric prop erty of nanocomposite s ; however, there is little agreement on how interfacial modification affects breakdown strength. Based on the discussion about the mechanisms of dielectric permittivity and breakdown strength, nanocomposites offer significant promise for fabricat ing high energy density capacitor. The following sections will review the recent progress about nanocomposites for energy storage. The y are classified into two different categories: nanocomposites with ferroelectric filler s and nanocomposites with non ferroelectric fillers.

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40 Nanocomposites with Ferroelectric Filler Most of the current research has focused on nanocomposites with ferroelectric fillers for energy storage, s ince the y typically have high dielectric constants such as BaTiO 3 PZT, and PbTiO 3 so on [14,55,89 91] Most ferroelectric ceramics have perovskite structure with the general composition of ABO 3 as shown in Figure 1 12. Figure 1 12. ABO 3 perovskite type unit cell, with A 2+ ions shown in green and BO 6 corner sharing octahedron. Perovskites have high dielectric permittivity due to the spontaneous polarization, which is the net dipole moment within the unit cell as shown in Figure 1 13. The dipole moment results from the relative displacement of the O 2 A 2+ and B 4+ ions from their symmetrical positions within the unit cell. The O 2 ions are located near but slightly below the centers of each six faces, whereas the B 4+ ion is displaced upwards from the unit cell center [92] Thus, a permanent dipole moment is associated with each unit cell as shown in Figure 1 13. This asymmetric structure gives rise to f erroelectric behavior with high dielectric permittivity ranging from 1000 6000. Therefore, researchers have tried to develop composites that utilize high dielectric permittivity from the perovskite fillers to produce high energy density.

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41 Figure 1 13. T he cubic structure of ABO 3 above the Curie temperature (left); the tetragonal structure of ABO 3 below the Curie temperature shows the permanent dipole moment arising from misalignment as shown by arrows in the unit cell (right). BaTiO 3 is perhaps the most widely investigated ferroelectric filler for preparing high dielectric constant or high energy density nanocomposites. Wang et al. demonstrated the improvement of dielectric constant by dispersing surface functionalized BaTiO 3 nanopar ticles into two different ferroelectric polymer matrices : P(VDF TrEE CTFE) and P(VDF CTFE) [16] The author s showed that the surface functionaliz ation of BaTiO 3 with ethylenediamine wa s dispe rsion in the matrix and the nanocomposites based on P(VDF Tr F E CTFE) had higher energy density due to its higher permittivity than that with P(VDF CTFE). The incorporation of BaTiO 3 nanoparticles improve d the polarization of the nanocomposites as shown in Figure 1 1 4 However, both nanocomposites showed the breakdown strength saturate d around 150 MV/m. At 150 MV/m, the energy density of the nanocomposites with 30 vol% BaTiO 3 wa s 7.0 J/cc, which show ed 120% improvement compared to neat P(VDF TrFE CTFE). This research generated a lot interest in the research community, since it was perhaps the first report of high energy density nanocomposites. This high energy density is based on the improved dielectric permittivity. Therefore, the incorporation of

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42 high dielectric permittivity filler is an effective way to improve the dielectric permittivity of the overall composites. However, the reduced breakdown strength of the composite limits the improvement of the energy density at higher electric field. Figure 1 14. The dependence of energy density of the nanocomposites on the volume fraction of BaTiO 3 in the P (VDF TrFE CTFE) and P(VDF CTFE). A) energy density of nanocomposites with different concentration under ele ctric field 100 and 150 MV/m. ( B ) D E loops measured under different electric field for nanocomposites. (Reprinted with permission from Li et al. [16] .) Recently, Perry et al. embedded phosphonic acid surface modified BaTiO 3 into P(VDF HFP) with foc us on fabricating high energy density capacitors with relatively high breakdown strength (>200 MV/m) as shown in Figure 1 1 5 [14,15] The authors performed a very detailed study to demonstrate that the surface modification helped particles form uniform and high quality thin films. The research experimentally demonstrated that the nanocomposites with very high concentration fillers produced low energy density. Both dielectric constant and breakdown strength decreased at high volume fraction fillers (>50%) leading to a decrease of energy density as shown in Figures 1 15 and 1 16. The decreases were due to more defects and voids produced in the nanocomposites. The measured energy density was around 3 .2 J/ c c with 60 vol.% BaTiO 3 while the calculated maximum energy density was predicted as high as 7 8 J/c c (A) (B)

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43 as shown in Figure 1 16 The difference of energy density based two methods is because the calculated maximum energy density treated the nanocomposites as linear capacitors, which did not account the energy loss. This suggests that it is important to obtain actual energy densit y by calculation from D E loop at high energy field. Figure 1 15. Dielectric property of nanocomposites with BaTiO 3 in the P (VDF HFP ) A) Dielectric constant, B ) loss tangent dependence on frequency. (Reprinted with permission from Philseok et al. [15] ). Figure 1 16. The breakdown strength and maximum energy density of nanocomposites with BaTiO 3 in the P (VDF HFP ) A breakdown strength, B) maximum energy density of the nanocomposites with different concentration of filler in the P(VDF HFP). (Reprinted with permission from Philseok et al. [15] ) One of the disadvantages of the nanocomposites with ferroe lectric fillers is polarization saturation and high remnant polarization. Zhang et al. fabricated (A) (B) (A) (B)

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44 nanocomposite s by incorporation of functionalized BaTiO 3 particles into the ferroelectric polymer matrix P(VDF CTFE) [19] The incorporation of BaTiO 3 highly improved the dielectric permittivity and polarization of the nanocomposite. However, the D E loop became large and polarization reached saturation at high electric fields as shown in Figure 1 1 7 which resulted in very low energy density The large D E loop came from the ferroelectric property of filler and matrix. Therefore, in order to fabricate high energy density capacitors, it is important to avoid ferroelectric properties from filler and matrix Figure 1 1 7. D E hysteresis of loops P(VDF CTFE) 91/9 mol%/BaTiO 3 nanocomposite films with different concentration BaTiO 3 under unipolar electric field ( Reprinted with permission from Xia et al. [19] ) Nanocomposites with Non ferroelectric Filler In order to capture high dielectric permittivity and simultaneously avoid ferroelectricity from fillers, a perovskite like (ABO 3 ) body centered cubic oxide such as calcium copper titanate (CCTO) and Ba x Sr 1 x TiO 3 (x<0.7), exhibits another promising solution for producing high energy density nanocomposites [93 97] The crystal structure of CCTO is shown in Figure 1 18 This type of material exhibits giant dielectric constant and no curie temperature to affect the dielectric property because of the cubic

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45 structure [98] For example, CCTO exhibits gi ant dielectric constant ranging from approximately 10,000 for polycrystalline to 100,000 for single crystal along with no polarization saturation [99,100] Figure 1 18. The unite cell o f CCTO crystal (green sphere represents Ca 2+ blue sphere represents Cu 2+ and octahedron represents TiO 6 ). Dang et al. incorporated CCTO particles in a polyimide matrix to prepare high dielectric permittivity composite s [101] The dielectric constant of the nanocomposite s with 40 vol % of CCTO (49) was 14 times larger than pure polyimide. In contrast, Prakas et al. created three phase composites with 25 vol % of metallic aluminum powder into the CCTO/epoxy mixture [96] The composite s reached a dielectric constant as high as 700 compared to neat epoxy (4.81) [96] However, most research did not report the breakdown strength to indicate the energy density of th ese composites. Tuncer et al. reported a similar composite with nano and sub micron CCTO fillers in the epoxy matrix [102] Interestingly, they showed that breakdown strength of the composites became more reliable after incorporation of fillers but still lower than 120 MV/m The low breakdown strength may come from the lack of surface functionalization of CCTO. Another disadvantage of CCTO is a high energy loss

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46 associated with the composite s because of the higher dielectric loss of CCTO [94,103] Current researchers are trying to increase breakdown strength and decrease the energy loss of the CCTO nanocomposites, but this type of material is not the subject of this work. Another ABO 3 body centered cubic oxide is Ba x Sr 1 x TiO 3 [20,104] which is a solid solution of BaTiO 3 and SrTiO 3 that transfers from a cubic phase to a tetragonal phase when the Sr fraction decreases. In the Ba x Sr 1 x TiO 3 solid solution, transformation occurs from the ferroelectric phase to the paraelectric phase when the Ba molar fraction decreases below 0.7 (x<0.7) and begins to show low remnant polarization around room temperature [105,106] The use of high dielectric filler such as a Ba x Sr 1 x TiO 3 can provide high dielectric constant while eliminating the remnant polarization to improve efficiency of the capacitor. A novel high energy density nanocomp osite capacitor with Ba 0.2 Sr 0.8 TiO 3 NWs with fast discharge speed will be prepare d in Chapter 4 Beyond high dielectric permittivity fillers, relatively low dielectric constant fillers are also studied to prepare nanocomposite capacitors for energy storag e, such as TiO 2 [17] ZrO 2 [13] and kaolinite [18] Li et al. reported high energy density nanocomposites based on surface functionalized TiO 2 nanorods into P(VDF TrFE CTFE) [17] as shown in Figure 1 19. It is interesting that the incorporation of TiO 2 did not improve the dielectric permittivity but increas ed the electric displacement of the nanocomposites. For the composites with 10 vol.% TiO 2 the energy density was 6.9 J/cc at 200 MV/m, which was 45% higher compared to polymer matrix (4.7 J/cc at 200 MV/m). They argued that the improved energy density w as due to the coalescing of the interface region and a reduction of the interface effect.

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47 Figure 1 19. Nanocomposites with TiO 2 nanorods for energy storage: A ) SEM image of nanocomposite with 30 vol.% TiO 2 B ) the stored energy of nanocomposites with di fferent applied electric field. (Reprinted with permission from Li et al. [17] ) Compared to most composites base on functionalized fillers, a new route toward high energy density nanocomposite s was reported based on chain end functionalized ferroelectric polymers [13] As shown in Figure 1 2 0 a, ferroelectric polymers were first modified with phosphonic acid end couplin g, and then allowed to react with terminal groups of Zr O 2 With the formation of covalent coupling between the fillers and matrix, the fillers had great stability and uniform dispersion in the nanocomposites. As a result of the intimate coupling, the int erfacial interaction regions between the nanoparticles and polymer matrix produced high energy density. The improvement of the energy density was mainly attributed from the polymer microstructures and the rise of the electric displacement was induced by t he nanoparticles. As shown in Figure 1 20b, for the composites with 9.1 wt.% ZrO 2 the energy density was 11.2 J/cc at 270 MV/m, which was a 60% increase compared to the neat polymer matrix. It is noted that the energy density began to decrease when the filler concentration was higher than 9.1 wt.%, since the breakdown strength was highly decreased. (A) (B)

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48 Figure 1 20 Preparation and energy density of nanocomposites with ZrO 2 A ) Synthesis of covalent bonded ferroelectric polymer nanocomposites, B ) e nergy density of the polymer and the nanocomposites with ZrO 2 (Reprinted with permission from Li et al. [13] ) For practical applications, it is desirable to n ot only have a high energy density, but also maintain a high efficiency. In order to improve the efficiency and enhance the energy density of nanocomposites, Tomer et al. explored a promising route to improve the energy storage performance of PVDF, throug h a synergy of hexafluoropropylene (HFP) co monomers and kaolinite clay nanofillers [18] It should be noted that kaolinite fillers have lower dielectric constant (~3) compared to the P(VDF HFP) matrix (~10), and have a layered structure enabling their high breakdown strength. The addition of these kaolinite fillers was expected to decrease the electric displacement of the nanocomposites, but markedly enhanced the breakdown strength as shown in Figure 1 21. The D E lo op was closer to a linear capacitor at a high volume fraction of kaolinite. Specifically, these composite films exhibited reduced high field losses, ma rkedly (A) (B)

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49 increased breakdown strength (780 MV/m), thus, ultra high energy density value of 19 J/cc was ach ieved. Figure 1 21. D E loops and breakdown strength of P(VDF HFP) and its nanocompo sites with kaolinite fillers. A) D E loops, B ) Weibull distribution and observed dielectric breakdown strength of the stretched P(VDF HFP) and its nanocomposite films with 5 wt.% kaolinite. (Reprinted with permission from Tomer et al. [18] ) The papers detailed in this section have shown some of the important research for fabrication nanocomposite capacitors for e nergy storage. Most of the papers detailed have improved dielectric constant of the nanocomposites by incorporation of high dielectric permittivity ceramic fillers. However, the decreased breakdown strength because of the increased defect limits the fina l energy density. Therefore, the integration and geometry of the fillers must be optimized along to reach the maximum possible energy density of the nanocomposites. This dissertation will investigate the fabrication of nanocomposites based on high aspect ratio nanowires rather than the equiaxial particles as discussed by other researchers. The relationship between the will be investigated. Finally, the dissertatio n will fabricate ultra high energy density nanocomposites base on the findings. (A) (B)

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50 Dissertation Overview The following two sections will present the contribution of this research to high energy density nanocomposite capacitors and provide a detailed descripti on of the research performed in each c hapter. Contributions High power capacitors form a critical technology for numerous electronic applications; however, the current state of the art technologies suffer from low energy density making them bulky and costly [2,6,7,107] With the requirement of high energy density, fast discharge time and low loss in pulsed power capacitors, much of the attention in high power capacitors has been applied to this class of materials [2,6,7,107] One method to increase the energy density of these systems is through the use of polymer nanocomposites as an alternati ve to polymeric and ceramic dielectrics, which are commonly used for electrostatic energy storage [13,15 18,55] Nanocomposites derive their high energy density from the use of a high dielectric filler and a high breakdown strength polymer. Additionally, the use of nanocomposites adds a degree of tunability to the capacitor allo wing intricate configurations for electronic and electric devices in addition to the film thickness [13,15 18,55] Therefore, nanocomposites are promising candidates for the next generation of high power capacitors that reach unprecedented energy density. This dissertation investigates the relationship between the structure (asp ect ratio and orientation) and energy density of nanocomposites The research performed here serves as a novel approach for the development of high energy density nanocomposite capacitors with high aspect ratio nanowires.

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51 The first contribution lies in th e development of a novel technique that allows for large scale production of wide ranging compositions of perovskite NWs, many of which have never been demonstrated in this form, such as PZT, BaTiO 3 and Ba x Sr 1 x TiO 3 Besides these synthesis methods, a process to control the aspect ratio of the BaTiO 3 NWs is demonstrated for the first time by tuning the hydrothermal reaction temperature. Additionally, a facile approach to the growth of high aspect ratio Ba x Sr 1 x TiO 3 NWs with high yield and control over the stoichiometry of the solid solution i s successfully developed. These three methods of large scale production of the pervoskite NWs are significant contribution to the ferroelectric and nanomaterials communities. Following the synthesis of high aspect ratio PZT NWs, the effect of the aspect ratio on the energy storage of the nanocomposites is investigated. It is demonstrated that the dielectric constant and energy density of the nanocomposites are both improved by PZT NWs compared to PZT NRs. The impr ovement of the dielectric constant is also found by using high aspect ratio BaTiO 3 and Ba 0.2 Sr 0.8 TiO 3 The relationship between the dielectric constant of the nanocomposites and the aspect ratio of the fillers is also quantified for the first time in this dissertation. These synthesis methods and results have generated high quality papers published in Nanotechnology, Nano Letter and a best paper award at Material Systems. Furthermore, this dissertation has also inve stigated the effect of the orientation on the energy density of the nanocomposites. Experimental results show that the dielectric constant and energy density of the nanocomposites are improved with 3 direction alignment. Additionally, a novel strategy to tune the orientation factor of NW s

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52 by controlling the draw ratio during uniaxial force assembly can be used in any form of thermoplastic matrix. In order to characterize the NWs orientation factor, a method is proposed by d on the f ast Fourier transform analysis of SEM images This represents the first time the relationship between the orientation factor and the dielectric constant of the nanocomposites has been quantified. All these results present a new solutio n to improve the energy density and dielectric constant of the nanocomposites. Finally, ultra high energy density nanocomposites with fast discharge speed are developed based on high aspect ratio Ba 0.2 Sr 0.8 TiO 3 NWs. The energy density reach es to 14.86 J/cc at 450 MV/m, which h as more than an order of magnitude higher energy density than commercial BOPP capacitors (1.2 J/cc at 640 MV/m) with faster discharge time to peak power and vastly improved power density In summary, this dissertation h as made contributions in synthesis of perovskite NWs, and the fabrication, testing and evaluation of high energy density nanocomposites with fast discharge speed. The findings of this dissertation could lead to enhanced interest in nanowire based nanocompo sites due to their potential application in achieving next generation energy storage devices and future pulsed power capacitors. Chapter Summary Chapter 1 provides an introduction to previous work in the areas related to the research in this dissertation. It starts with the motivation of this dissertation. It is detailed that there has been an increasing demanding for the development of high energy capacitor to meet the requirement of the pulsed power device. Nanocomposites are promising candidate for fa bricating high energy density capacitors since they can combine the high dielectric permittivity from filler and high breakdown strength from the

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53 polymer. However, the current nanocomposites still do not reach the desirable energy density. Therefore, the motivation of this research is to develop high energy density nanocomposite capacitors through high aspect ratio and aligned NWs. The C hapter 1 then moves on to detail fundamentals of capacitors. Following, a literature review of the previously research related to nanocomposites for energy st orage has been performed. The C hapter 1 finishes with an outline of the work that has performed using NWs synthesis, fabrication and characterization of high energy density nanocomposites. Chapter 2 investigates the effect of the aspect ratio on the dielectric constant and energy density of the nanocomposites. It begins with a detailed synthesis and characterization of two different aspect ratio particles: PZT NWs and PZT NRs. Nanocomposites are then prepared using the two different aspect ratio fillers Experiment results are demonstrated that the dielectric constant and energy density of the nanocomposites are improved by high aspect ratio. Many researchers are following this route to make high dielectric permit tivity nanocomposites, however, the relationship between the aspect ratio of filler and dielectric constant of nanocomposites is still not quantified in the literature. In order to quantify this relationship, a method to control the aspect ratio of the Ba TiO 3 NWs is proposed. The results demonstrated that the dielectric constant of the nanocomposites increases with increasing the aspect ratio of the fillers. Th ese finding s open a new route to prepare high dielectric constant and high energy density compo sites In order to further enhance the energy density of the nanocomposites, Chapter 3 investigates the effect of the filler orientation on the energy storage of the nanocomposites. It begins with a literature review of the effect of alignment on the

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54 die lectric constant of the nanocomposites along with various alignment methods. In this regard, a novel strategy to align nanowires in a thermop lastic matrix by uniaxial force assembly is developed. The results demonstrate that t he increased dielectric cons tant of the aligned filler in nanocomposites can provide a platform to obtain materials with greater energy density. However, no experimental research is conducted to investigate the relationship between the orientation factor and the dielectric constant of the nanocomposites. Here, the orientation degree of the nanowires in the nanocomposites is controlled by the draw ratio Orientation Factor based on the f ast Fourier transform analysis of cr oss sectional SEM images of the nanocomposites. This is the first time to quantify the relationship between the and the dielectric constant of the nanocomposites. Through alignment control, the effective dielectric permittivit y of the nanocomposite can be tuned without requiring additional fillers This finding provides another way to improve the effective dielectric constant and the energy storage performance of the nanocomposites. Chapter 4 develops t wo techniques for developing high energy density nanocomposites based on high aspect ratio NWs The first technique is based on nanocomposites composed of high aspect ratio BaTiO 3 NWs in P(VDF TrFE CFE). The results show that the ene rgy density reaches to 10.48 J/cc at 300 MV/m. However, this type of nanocomposites exhibits polarization saturation and high ferroelectric loss from ferroelectric filler and matrix which limits the improvement of energy density. In order to overcome th ese limitations, another technique is developed. A new synthesis process is developed to provide a

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55 facile approach to the growth of high aspect ratio nanowires with high yield and control over the stoichiometry of Ba x Sr 1 x TiO 3 NWs T he paraelectric phase Ba 0.2 Sr 0.8 TiO 3 is chosen for the NWs combined with quenched PVDF to fabricate high energy density nanocomposite capacitors Compared to BaTiO 3 / P(VDF TrFE CFE) nanocomposites, t his type of nanocomposite can maintain high polarization with no saturation, high breakdown strength and also reduced ferroelectric loss Therefore, the results demonstrate that high aspect ratio nanowires can be used to produce nanocomposite capacitors with greater performance than the neat polymers thus providing a novel process for the development of the next generation energy storage devices. The final chapter of this dissertation is Chapter 5, which starts with a brief overview of the results found throughout this dissertation. After the overview, a discussion of the contribu tion in this dissertation is made and how will they affect the future research in nanocomposites are presented. The final section of this c hapter and this thesis describes the future work that could be conducted in further the research.

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56 CHAPTER 2 THE EFFECT OF FILLER ASPECT RATIO ON THE ENERGY DENISTY OF NANOCOMPOSITES Chapter Introduction T his c hapter focuses the effects of the filler aspect ratio on the dielectric constant and energy storage performances of the nanocomposites The hypothesis of this c hapter is that the aspect ratio of the filler will impact the dielectric permittivity of the nanocomposites, which in turn influence s their energy density. In order to investigate nt of the nanocomposites, this c hapter will first develop a hydrothermal procedure to produce a high yield of hig h aspect ratio PZT nanowires (NWs) while the low aspect ratio PZT nanorods (NRs) are obtained by decreasing the PZT NWs by mortal pestle. Then, the ef aspect ratio on the dielectric constant and the energy storage performance of nanocomposites will be investigated. It will demonstrate that the dielectric constant and energy density of the nanocomposites can be improved by high aspec t ratio fillers. This method provides a solution to fabricate high dielectric constant nanocomposites. However, there is no research to quantify the relationship between the filler aspect ratio and the dielectric constant of nanocomposites, especially at high aspect ratio. In order to quantify this relationship, a novel method is developed to synthesize and control the aspect ratio of BaTiO 3 NWs based on a two step hydrothermal reaction by adjusting reaction temperature Using the different aspect ratio BaTiO 3 NWs, the relationship be quantified. Finally, the c hapter will conclude with identifying relationship between the aspect ratio of NWs and the energy storage o f nanocomposites.

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57 Investig ation of the Effects of Filler Aspect Ratio on the Energy Storage of Nanocomposites Many theoretical models have demonstrated that the aspect ratio of the fillers play an important role in the dielectric permittivity of the composites [123,124 73] however, there is few experimental research to demonstrate this point. Most of current composites are based on equiaxial partic les since the high aspect ratio NWs are unavailable until recently. This section will focus on the fabrication nanocomposites with nanowires to improve the energy density A method allowing for large scale production of d ifferent aspect ratio fillers (P ZT NWs and PZT NRs) with will be presented T he effect of filler aspect ratio on the dielectric constant and energy density of the nanocomposite will be experimentally investigated to provide a new route for fabricate high energy density capacitor in the future Preparation of Different Aspect Ratio Fillers: PZT NWs and PZT NRs PZT NWs were synthesized through the following hydrothermal process [55,5 6,60,108] Briefly, a mixture of 0.08 M (C 4 H 9 O) 4 Ti (Alfa Aesar, 98%) ethanol solution and 0.10 M ZrOCl 2 (Alfa Aester, 98%) water solution were co precipitated by the dropwise addition of a 0.15 M ammonia solution (Ricca Chemical Company, 28.0 30.0%). The precipitated Zr x Ti 1 x O 2 nH 2 O (ZTO) gel was washed with deionized water through centrifugation (Eppendor f, centrifuge 5810) and vortex (Fisher Scientific) mixing three times, and then dispersed into deionized water under stirring. Then, 0.11 M of Pb(NO 3 ) 2 (Alfa Aesar, 99%), 0.5M of KOH (Mallinckrodt Chemicals, ACS, 88%), and 0.4 g/L of poly(vinyl alcohol) ( PVA) water solution were dissolved in the ZTO solution. The final mixture was transferred into a 150ml Teflon lined stainless steel autoclave with a fill factor of 80%. The reaction vessel was kept at 200 C for 4 hours under vigorous

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58 stirring. After th e hydrothermal process was complete, the precipitate was removed from the reactor and washed with water for four times through centrifugation and vortex mixing, and subsequently dried on a hotplate at 80 C overnight. The resulting powder was a PX phase o f PZT nanowires (PX is used to note the non perovskite phase because of similarity to phase of PbTiO 3 ). The powder was then heated at 600 C for 2 hours to transform PX phase to perovskite PbZr 0.2 Ti 0.8 O 3 phase [108] I n order to effectively evaluate the effect of the filler aspect ratio on the dielectric constant and energy storage performance of nanocomposites, the PZT NWs were ground with a sapphire mortal pestle for 30 minutes in ethanol to form low aspect ratio PZT NRs, then dried overnight at 80 C The morphology of the separated PZT NWs and NRs are shown in Figure 2 1 (a) and (b), respectively. It should be mentioned that there has been a great deal of research devoted towards the synthesis of PZT NWs due to the ir excellent ferroelectric and piezoelectric properties. However, control over the morphology and yield of as grown PZT NWs still poses a challenge in the field of nanotechnology. Here a hydrothermal reaction wa s developed to procedure high yield and hi gh quality PZT NWs. The aspect ratio of the acicular PZT particle was analyzed using SEM images through ImageJ software as shown in Figure 2 2. The PZT NWs are measured to have an aspect ratio of approximately 14 with a mean diameter of 160 nm and a mean nanoparticles with different aspect ratios allows for the characterization of the effect of the filler a spect ratio on the dielectric constant and energy storage performance of the nanocomposites.

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59 Figure 2 1. SEM images of PZT particles A) PZT NWs, B ) PZT NRs Figure 2 2. PZ T particle size distribution: A) length of PZT NWs, B) diameter of PZT NWs, C) length of PZT NRs, D ) diameter of PZT NRs In order to demonstrate that the phase of PZT has not changed during grinding, both the PZT NWs and PZT NRs are characterized using XRD as shown in Figure 2 3. (A) (B) (A) (B) (C) (D)

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60 From the XRD trace it can be determined that the crystalline structure of the PZT before (NWs) and after (NRs) grinding is unchanged and closely match the structure of PbZr 0.2 Ti 0.8 O 3 (JCPDS No. 01070 4261). The Zr/(Zr+Ti) ratio can be verifie d using the XRD characterization along the Gaussian fitting to determine the structure of PZT. The (002) and (200) planes are used to calculate the lattice parameters ( a, c) as well as the c/a ratio. The Gaussian function is used to fit the peaks that account for the 002 and 200 diffraction peaks as shown in Figure 2 4. The lattice parameters can be calculated where d is the lattice plane distance, is the scattering angle, n is an integer determined by the order given, and is the wavelength of the X ray source 0.1541 n m ). It shows that the calculated lattice parameters of PZT are: a =3.935 c =4.135 and c/a =1.051. Funakubo et al [109] performed a detailed study regarding the relationship between Zr/(Zr+Ti) ratio and c/a ratio as shown in Figure 2 4c, which indicates that the c/a ratio is approximately 1.051 when the Zr/(Zr+Ti) is about 0.2. Additiona lly, these lattice parameters ( a =3.935 c=4.135 ) closely match the data found in the literature for PbZr 0.2 Ti 0.8 O 3 [110,111] ; therefore, the composition of the PZT NWs synthesized here is PbZr 0.2 Ti 0.8 O 3 From the SEM and XRD, it can be seen that the only difference between the PZT NWs and PZT NRs is the aspect ratio of the particles, while the crystal structures of PZT NWs and NRs are the same. The next step is to verify the particle dispersion in the matrix by SEM observation and also the crystal phase of the PVDF matrix by FTIR. Following these analys e s, the effect of the filler

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61 aspect ratio on the dielectric properties and energy density of the nanocomposites can be studied more thoroughly. Figure 2 3 XRD patterns of PZT NWs and PZT NRs Figure 2 4 Gauss fit of XRD. A ) (002) p lane XRD data with gauss fit, B ) (200) p lane XRD data with gauss fit, C ) Zr/(Zr+Ti) ratio dependency of c/a ratio, report data from PZT powd er by Funakubo et al [109] (A) (B) (C)

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62 Fabrication of Nanocomposites with Different Aspect Ratio Fillers The nanocomposites were prepared by dispersing the PZT particles into a PVDF/DMF solution by sonicating for 30 minutes, t hen solution cast onto a glass plate and dried at 60 C under vacuum overnight. In order to decrease the thickness and obtain smooth void free films, the resulting films were further hot pressed at 170 C for one hour allowing a final film thickness of 0. 1 0.2 mm. Finally, silver paint was applied to the top and bottom surfaces of the samples to act as the corresponding electrodes for electrical measurement. The microscopic homogeneity of the nanocomposite is observed by imaging their top surfaces as show n in Figure 2 5. It is clearly shown that both of the PZT NWs and PZT NRs show homogeneous dispersion in the PVDF matrix. Each volume fraction is characterized using the same procedure and all demonstrated a homogeneous dispersion. As the previous study indicated, the dispersion of inorganic fillers in these fluorinated polymers is problematical due to the low surface energy of the polymers [14 16,19 21] A gglomeration of the ceramic in the composites will significantly lower the energy density due to a reduction of the breakdown strength. A good dispersion of PZT NWs and NRs in the polymer is required to ensure high energy density of nanocomposite as well as the generation of reproducible measurements.

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63 Figure 2 5. SEM images of top vi ew of PZT PVDF nanocomposite: A ) 20 % PZT NWs/PVDF nanocomposites, B ) 20% PZT NRs/PVDF nanocomposites In order to demonstrate that fillers with different aspect ratios do not influence the phase of the polymer, Fourier transform infrared (FTIR) spectroscopy was performed with a Nicolet 10 FTIR with a Smart Orbit ATR accessory to characterize the phase of P VDF in nanocomposite with different aspect ratio fillers, as shown in Figure 2 6. The FTIR spectrum of the nanocomposites with PZT NWs and NRs are very similar and clearly shows that the phase of PVDF does not change in either nanocomposite while both PV [112,113] The nanocomposites differ only in the aspect ratio of the fillers, since the crystal structure of the two PZT particles and the PVDF do not change during the preparation and both fillers have good dispersion in matrixes. Therefore, the samples with PZT NWs and NRs in PVDF can be used to investigate the effect of aspect ratio on the dielectric property and energy storage performance. (A) (B)

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64 Fig ure 2 6. FTIR spectra of nanocomposites with 10% PZT NWs and 10% PZT NRs, respectively. The Effect of Filler Aspect Ratio on the Dielectric Constant The dielectric constants of nanocomposites with various volume fractions of PZT NWs and NRs are compared with values predicted by the Maxwell model, as shown in Figure 2 7. It indicate s that the dielectric constant of the composites increase with an incre asing volume fraction of either NWs or NRs. This increase is attributed to the higher dielectric constant of PZT fillers compared to the PVDF matrix. More importantly, the nanocomposites with high aspect ratio PZT NWs have higher dielectric constant than the samples with low aspect ratio PZT NRs. For example, the dielectric constant of the nanocomposite containing PZT NWs is 21% larger than that of a nanocomposite containing PZT NRs at 50 vol.%. Many theoretical models have been developed to predict the effective dielectric constant of a 0 3 composite system. For nanocomposites, a third phase in the composite, such as grain boundaries and interfaces, are substantial and cannot be neglected. The Maxwell Model is formulated to include such broad assumptio ns and is based on the mean field theory of a single spherical inclusion surrounded by a

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65 continuous polymer matrix [71] 10 50%, which is the range of interes t for this study. In a 0 3 composite system, the effective dielectric constant ( eff ) can be expressed as where 1 and 2 represent the dielectric permittivity of the polymer matrix and the ceramic filler, respectively, and is the volume fraction of t he filler. In this model, 1 and 2 are chosen as typical values of 10 and 1600, respectively. From Figure 2 7 it can be seen that better agreement is exhibited for the composites with NRs compared to NWs. Since the Maxwell model considers the filler a s a single spherical inclusion, it makes sense that dielectric constant is much closer to the model when the aspect ratio decreases from PZT NWs to PZT NRs. With increasing aspect ratio of the fillers, the dielectric constant of the nanocomposites diverge s from the Maxwell model, as shown in Figure 3 7 [55,56,67,72 74] Figure 2 7. Comparison of measured dielectric constant (at 1 KHz) of nanocomposites as a function of PZT NWs and NRs volume fractions with predicted values from Maxwell model

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66 Figure 2 8 shows the dielectric constant of the nanocomposites with various PZT volume fractions over a frequency range of 1 kHz to 1 MHz. It shows that the dielectric constant of the composites increases with an increase in the concentration of filler. Also, the dielectric constant decreases with increasing the frequency. At low frequenc y, the dipoles can move sufficiently fast to follow the electric field. However, as the frequency increases the dipole cannot shift orientation sufficiently fast as the applied electric field exceeds its relaxation frequency, resulting in a decrease in th e dielectric constant at high frequency [31] Additionally, the dielectric constant of nanocomposites with a high volume fraction of fillers decreases much faster than those with a low er volume fraction of filler. This occurs due to the high dielectric permittivity of PZT fillers which results due to orientation polarization and space charge polarization, which decrease faster at high frequency compared to PVDF matrix [31] Figure 2 8. Dielectric constants of nanocomposites with different PZT volume fracti on depended on the frequency: A) PZT NWs/PVDF nanocomposites, B ) PZT NRs/PVDF nanocomposites Th e loss tangent of nanocomposites with varying volume fraction of PZT particles is shown in Figure 2 9. It is demonstrated that the loss tangent increases with frequency and decreases with increasing volume fraction of the PZT fillers. The (A) (B)

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67 dielectric loss is mainly due to a resonance of the matrix; therefore, the high volume fraction of PZT particles in the polymer matrix leads to a low loss tangent [15] At high frequency, the loss tangent of the nanocomposite with NWs is lower than that with NRs, especially at high volume fractions such as 50% PZT NWs. According to Figure 2 9a, the loss tangent of the 50% PZT NW sample is 0.031 at 1 kHz, which is smaller than other nanocomposites currently reported in the literatur e [16,17] Figure 2 9. Dielectric loss tangent of different PZT volume fraction in PVDF from 1 kHz to 1 MHz: A) PZT NWs/PVDF nanocomposites, B ) PZT NRs/PVDF nanocomposites The Effect of Filler Aspect Ratio on the Energy Density In order to characterize the energy storage capability of the nanocomposites, t he electric displacement electric field (D E) loops of the nanocomposites were measured using a Sawyer Tower circuit, wh ich allowed for the direct computation of the energy density [33,114] As shown in Figure 2 10 the sample capacitor and linear sensing capacitor are in series in the circuit, and connected to a h igh voltage source and oscilloscope. The voltage on the sensing capacitor is recorded on an oscilloscope. In the series circuit, the charge is the same across the sample and capacitor since the capacitors are in series as shown in Figure 2 10 Therefor e, the charge on the tested (A) (B)

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68 sample can be obtained from the sensing capacitor. The electric displacement can be calculated by dividing the charge to the area of the sample. Figure 2 10 The Sawyer Tower circuit and mechanism Based on the measurement of the Sawyer Tower circuit, a typical hysteresis curve of the nanocomposite sample can be obtained as shown in Figure 2 11 The energy density of the nanocomposites can be calculated from the D E loops based on the integration of the discharge curve of the D E loop as the shaded area in Figure 2 11 Figure 2 11 Typical hysteresis loop and energy density calculation of nanocomposites Typical D E loops measured at different electric field and various volume fractions are presented in Figure 2 1 2 The PZT particle concentration notably raises the electric displacement, which is attributed to the high polarization of the PZT fillers

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69 compared to PVDF matrix. Also, the electric displacements of the nanocomposites increase with the applied electric field, which indicates that the energy density can be achieved at higher electric field. It should be noted that the PZT NW nanocomposites have a larger electric displacement than the PZT NR samples. For example, at an electric field of 15kV/mm and a 50% volume fraction, the NWs achieve an electric 2 while NRs only have an electrical displacement of 0.471 2 This difference shows an increase of 67.1% for nanocomposites with high aspect ratio fillers compared to the lower aspect ratio materials. Figure 2 1 2 Electric displacement field (D E) loop measured under different applied fields at room temperature and 100 Hz for t he polymer and nanocomposite: A ) PZT NWs / PVDF nanocomposites, B ) PZT NRs / PVDF nanocompo sites Figure 2 1 3 presents the stored electrical energy density of the PZT nanocomposites calculated from the D E loops of Figure 2 1 2 It indicates that that the energy density of the nanocomposites is strongly dependent on the concentration of PZT part icles and clearly shows that the energy density increases with the electric field. Most importantly, high aspect ratio NW improve s the energy density more than lower aspect ratio NR. For example, the energy storage of a 40% PZT NW nanocomposite is nearly equal to that of a 50% PZT NR nanocomposite. The measured energy density of (A) (B)

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70 a nanocomposite with 50% NWs at an electric field 15 kV/mm is 0.0528 J/cm 3 which is 77.8% larger tha n that of the 50% NR sa m ple (0.0297 J/cm 3 ). Figure 2 1 3 The dependence of energy density on the PZT volume fraction in a PVDF matrix under 15 kV/mm: PZT NWs/PVDF nanocomposite (blue) and PZT NRs/PVDF nanocomposite (red) Results of this study indicate that the aspect ratio of the filler significantly affects the dielectric constant and energy density of the nanocomposites. The nanocomposites with high aspect ratio PZT NWs exhibits improved energy densi ty, which is attributed to the increased dielectric constant with NWs compared to NRs. It is well known that a material with higher dielectric permittivity will have higher energy density at the same electric field, since the energy density is linearly re lated to the dielectric constant. In this work, the dielectric constant of nanocomposites with NWs is shown to be larger than nanocomposites consisting of lower aspect ratio NRs. The phase of the matrix and PZT inclusions are the same from the FTIR and XR D characterization, respectively. Therefore, it is implied that the aspect ratio of the PZT NWs is responsible for the change in the dielectric constant of nanocomposites. Additionally, the Brunauer Emmet Teller (BET) surface analysis can conclude that t he aspect ratio plays a critical

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71 role in defining the dielectric constant and energy density of the nanocomposite. BET surface area of PZT NWs and NRs are 3.46 m 2 /g and 5.07 m 2 /g, respectively, which means that nanocomposites with NRs have higher interfac ial area than the samples with NWs. Some researchers have proposed that the dielectric constant of the composite increases with an increase in the interfacial area between the filler and matrix [115 117] The PZT NRs have higher surface area compared to NWs and therefore, the interfacial area i s not responsible for the increased dielectric permittivity and energy storage in this case. Consequently, the aspect ratio plays important role in enhancing the dielectric permittivity in this work. A study of the electrical conductivity of composites consisting of insulating polymer and conductive filler, results concludes that it does not increase monotonically with increasing filler concen tration. A critical volume concentration exists for these composites and is also called the percolation threshold [118 120] When the concentration reaches the perc olation threshold, the properties are drastically changed due to the formation of a connective passage that resembles a continuous filler network. For example, the insulating state could be changed to a semiconducting or a conducting state at percolation threshold [118 120] Previous research has also shown that the percolation threshold is strongly related to the aspect ratio of filler with a larger aspect ratio leading to a smaller percolation threshold [118 120] Therefore, the high aspect ratio filler (nanowire) reach the percolation threshold at a l ower volume fraction than a low aspect ratio filler (nanorod or nanoparticle). The dielectric properties of composites can be enhanced by several orders of magnitude depending on the manner in which connections are made [121] when the

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72 fillers reach the percolation threshold. Therefore, the dielectric constant of the nanocomposites with PZT nanowires is much higher than that with PZT nanorods since the NW is much easy to reach the percolation threshold in the pol ymer compared to NR Additionally, the high aspect ratio nanowires can improve the dielectric constant of the composites due to the large dipole moment [122] P revious BET results have demonstrated that high aspect ra tio fillers had a lower surface area than low aspect ratio fillers, which helps to reduce the surface energy and thus prevent agglomeration in the nanocomposites. Considering all these reasons, the NWs with large aspect ratio are more effective enhancemen ts of the dielectric constant of the nanocomposites. Thus, the nanocomposites with PZT NWs have higher energy density than that with PZT NRs because of the improvement in the effective dielectric permittivity. The results presented here clearly indicate that the use of high aspect ratio fillers can lead to significant improvement in energy storage density, which will be employed to fabricate high energy density nanocomposite capacitors. Quantification of the Relationship between the Filler Aspect Ratio an d Dielectric Constant of the Nanocomposite s The previous research has demonstrated that high aspect ratio (AR) fillers can improve the dielectric constant of the nanocomposites more efficiently compared to the spherical fillers. Also, many theoretical mod els have shown that high aspect ratio fillers can improve the dielectric constant of the nanocomposites [123,124 73] This discovery has led many resear ches to follow this route to prepare composites with improved dielectric permittivity [21,125 127] However, high AR fillers still have not been investig ated as extensively as spherical fillers because of challenges in manufacturing NWs, especially for AR>15. Here, the relationship between the AR of the filler and the

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73 dielectric constant of the nanocomposites is quantified in this work. A novel and simpl e method has been developed to tailor the AR of BaTiO 3 NWs with an AR as high as 45.8. The dielectric constant of the nanocomposites with 30 vol.% and an AR of 45.8 NWs can be as high as 44.3, which is 3.5 times larger than the neat PVDF polymer. Therefo re, the research performed here can be applied to manufacture high dielectric constant capacitors and provide a solution for the improvement of energy storage performance of future nanocomposites. The synthesis of BaTiO 3 NWs was accomplished through a two step hydrothermal reaction [128,129] First, high aspect ratio sodium titanate nanowires were synthesized. Typically, 1.88g of anatase titanium dioxide powder (Sigma Aldrich, ACS, 99%) was mixed with 91ml of a 10M sodium hydroxide (Fisher, ACS, 99%) aqueous solution. Then, the mixed solution was transferred into a 130ml Teflon lined autoclave. The solution was sealed in a stainless steel autoclave and stirred at certain temperature for 24 hours. After the autoclave was cooled to room temperature, the obtained sodium titanate NWs were washed with water then soaked in diluted 0.2M hydrochloric acid (Fisher, 37%) for 4 hours to yield hydrogen titanate NWs. The resulting powder was then w ashed with water four times through centrifugation and vortex mixing with subsequent drying on a hotplate at 60 C overnight. In order to adjust the aspect ratio of the hydrogen titanate NWs, the hydrothermal temperature is changed from 150 C to 240 C. Figure 2 14 shows the SEM images of the hydrogen titanate NWs synthesized at different temperatures. It is shown that the precursor sodium titanate is free standing NWs. With an increase in temperature, the length of the NWs is increased much greater co mpared to the diameter, resulting in a higher AR.

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74 Figure 2 1 4 SEM images of hydrogen titanate NWs prepared at different temperatures: A) 150 C, B) 180 C, C) 200 C D ) 240 C The BaTiO 3 NWs were synthesized from the hydrogen titanate NWs by a second hydrothermal reaction in an aqueous with barium ions source After the hydrothermal process was completed, the precipitate was collected and washed with 0.2M diluted HCl aqueous solution, wat er and ethanol to yield BaTiO 3 NWs. The second hydrothermal process was designed to specifically maintain the morphology of the nanowires since the morphology is highly dependent on the hydrothermal reaction parameters, such as the temperature, duration a nd nature of the precursors and as can be seen by Scanning Electron Microscopy (FE SEM; 6335F, JEOL) Figure 2 1 5 shows the free standing BaTiO 3 NWs after second hydrothermal transformation from the precursor hydrogen (A) (B) (C) (D)

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75 titanate. The surfaces of BaTiO 3 NWs are much rougher compared to precursor hydrogen titanate, which is because the Ba ions diffuse into the precursors and forms particles on the surface. It should be mentioned that the morphology of the BaTiO 3 is highly dependent on conditions of the second hydrothermal process, such as the temperature, duration and nature of the precursors. The BaTiO 3 can be varied from cubic, starfish like, and snow flake like during the second hydrothermal transformation. Figure 3 13 shows that the morphology of BaTiO 3 was preserved from the hydrogen titanate with free standing NWs. Additionally, the aspect ratio of BaTiO 3 NWs increases with increasing reaction temperature of the hydrogen titanate (Figure 2 16) Figure 2 1 5 SEM images of BaTiO 3 NWs prepared at different temperature A) 150 C, B) 180 C, C) 200 C D ) 240 C (A) (B) (C) (D)

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76 Figure 2 1 6 BaTiO 3 particle size distribution dependent on the hydrothermal temperature: A) 150 C, B) 180 C, C) 200 C D ) 240 C (C) (D) (A) (B) 150 C C 180 C C 200 C C 240 C C

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77 Chemical composition of nanowires was studied by an energy dispersive X ray spectroscopy (EDX, GENESIS), as shown in Figure 2 1 7 a The succ essful transformation of BaTiO 3 nanowires after diffusion of the Ba ions into the precursor NWs during the hydrothermal reaction is confirmed due to the presence of Ba, Ti and O. It should be mentioned that the it is hard to clearly observe the of the sep arate peaks of Ba and Ti, since the main peaks of Ba (L edge) and Ti (K edge) overlap in the energy range of 4.5 5 keV [130] XRD equipped with a curved position sensitive detector s used to determine the structure of the BaTiO 3 NWs and is shown in Figure 2 1 7 b All the diffraction peaks can be assigned to the BaTiO 3 crystal structure (JCPDS, 81 2203) without any indication of crystalline by products such as BaCO 3 or TiO 2 Therefor e, the BaTiO 3 NWs were successfully transformed by the second hydrothermal reaction Figure 2 1 7 Verification of BaTiO 3 NWs A ) typical EDS spectra (B ) XRD patterns The BaTiO 3 NWs are transformed directly from the precursor of hydrogen titanate, therefore, the aspect ratio distributions of BaTiO 3 NWs are dependent on the hydrothermal temperature of the precursor. The aspect ratio of BaTiO 3 are analyzed (A) (B)

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78 from SEM pictures by using ImageJ software and summarized in the Figure 2 1 8 It sho ws that the aspect ratio of the BaTiO 3 NWs is dependent on the hydrothermal temperature used to synthesize the precursor NWs. From the Figure 2 1 8 it is clearly shown that the length of NWs increases at a faster rate than the diameter of the NWs. T he aspect ratio is increased from 9.5 to 45.8 with a corresponding increase in temperature from 150 C to 240 C, respectively, as shown in Figure 2 1 8 Figure 2 1 8 The relationship between the aspect ratio of BaTiO 3 NWs and hydroth ermal reaction temperature In order to increase compatibility and improve dispersion of the fillers in the matrix, the BaTiO 3 NWs were surface functionalized with ethylenediamine. The powder was mixed with ethylenediamine by vortex mixing, sonicated for 1 hour, and heated in a 90 C water bath for another hour The NWs were separated by centrifugation and dried at 70 C on a hotplate overnight. FTIR spectroscopy was performed with a Nicolet 10 FTIR with a Smart Orbit ATR accessory to confirm the functio nalization of the nanowires. Figure 2 1 9 a shows the FTIR spectra of the BaTiO 3 NWs before and after the surface modification with ethylenediamine. Evidence of ethylenediamine binding Temperature ( o C)

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79 can be confirmed by an amine group (N H) adsorption around 1450 cm 1 [131] which is known to act as a bridge to bind BaTiO 3 with a PVDF matrix as shown in the Figure 2 1 9 c The nanocomposites were prepared by first dispersing the BaTiO 3 NWs into a 7 wt.% PVDF in dimethylformamide (DMF) solution through 1 hour of sonication. Then the solution was cast onto a glass plate to obtain thin films. The films were then dried at 80 C under vacuum overnight. The obtained films were heated at 200 C for 10 min, and then dried at room temp erature for 24 hours and peeled from the glass plate. Figure 2 1 8b shows the top surface of a nanocomposite with 10 vol.% BaTiO 3 NWs. It indicates that the functionalized fillers have been homogenously dispersed in the PVDF matrix without voids in the fi lm. Because of the surface modification of BaTiO 3 NWs using amine groups, there is an increase in the compatibility between filler and matrix, and a homogenous dispersion in the PVDF matrix is formed as shown in Figure 2 1 9 b Figure 2 1 9 BaTiO 3 NWs functionalization and nanocomposites. A ) FTIR spectra of BaTiO 3 NWs and modified BaTiO 3 NWs by ethylenediamine B ) SEM topography of the nanocomposites with 10% BaTiO 3 NWs C ) schematic image of the functionalized BaTiO 3 by ethylenediamine reacting with PVDF (A) (B) (C) Reactio n Hydrogen bonding

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80 The dielectric constants of the nanocomposites were measured with an Agilent 4980A LCR meter at a frequency of 1 kHz with a parallel equivalent circuit at room temperature Gold electrodes with a thickness of approximately 10 nm were sputtered onto both surfaces of the film and served as contact points for the dielectric constant measurements. fraction an d aspect ratio of the fillers is shown in Figure 2 20 The dielectric constant of the nanocomposites increases with increasing volume fraction of filler due to the higher dielectric permittivity of BaTiO 3 as compared to PVDF polymer. Figure 2 20 clearly demonstrates that the dielectric constant of the nanocomposite can be significantly increased through an increased aspect ratio of BaTiO 3 NWs. It should be noted that the dielectric constant of the nanocomposite with 30 vol.% BaTiO 3 NWs (aspect r atio 45.8) can reach a dielectric constant as high as 44.3, which is 30.7% higher than samples with a low aspect ratio (9.3) and 352% larger than the polymer matrix. This technique efficiently improves the dielectric property of the nanocomposites without the need for additional fillers. Figure 2 20 Dielectric constant of the nanocomposites as a function of aspect ratio and volume fraction of BaTiO 3 NWs.

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81 This section has determined the relationship between the aspect ratio of fillers and the dielectric constant of the nanocomposites. The result clearly shows that the dielectric constant of the nanocomposites can be improved through increasing higher aspect ratio of the filler. Various theoretical models have correlated improv ed dielectric constant with high aspect ratio fillers [73,74,123] The benefits of using high aspect ratio fillers can be also explained through the following aspects. Firs t, the high aspect ratio fillers reach the percolation threshold at lower volume fraction than low aspect ratio nanowires, which allows connectivity in the system and improves the dielectric properties of the nanocomposites [118,119] Additionally, the high aspect ratio nanowires can improve the dielectric constant of the composites because of their large dipole moment [122] Finally, Brunauer Emmer T eller (BET) surface area analysis, detailed in this c hapter, has also demonstrated that high aspect ratio fillers have lower surface area than low aspect ratio fillers, which helps reduce the surface energy and thus prevents agglomeration in the nanocompos ites. All these aspects work to make high aspect ratio nanowires more effective in increasing the dielectric constant of the nanocomposites. Chapter Summary This c hapter has demonstrated that the energy density of nanocomposites can be enhanced through hi gh aspect ratio NWs. Specifically, nanocomposites are prepared by using two different aspect ratio fillers in a PVDF matrix: PZT NWs and PZT NRs. It was demonstrated that the high aspect ratio PZT NWs can improve the dielectric constant of the nanocompos ites resulting higher energy density. The high aspect ratio PZT NWs show a 77.8% increase in energy density over the lower aspect ratio PZT NRs under an electric field of 15 kV/mm and at a 50 vol.%.

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82 This c hapter present ed a highly efficient method for the large scale synthesis of high quality PZT NWs. In addition, a novel synthesis method based on a two step hydrothermal reaction is presented to achieve a high yield of BaTiO 3 NWs. This work is the first time to report the control of aspect ratio of the B aTiO 3 NWs by changing the hydrothermal reaction temperature from 150 C to 200 C, which corresponding to the AR from 9.3 to 45.8, respectively. Based on these different aspect ratio BaTiO 3 NWs, the relationship between the dielectric constant of nanocomposites and the aspect ratio of the fillers is quantified. The dielectric constant of the nanocomposites is significantly enhanced by using higher aspect ratio NWs. Nanocomposites with a 30 vol.% of BaTiO 3 NWs (aspect ratio 45.8) can reach a dielectric constant as high as 44.3, which is 30.7% higher than samples with an aspect ratio of 9.3 and 352% larger than the PVDF matrix. The results presented in this c hapter demonstrate that the use of high aspect ratio NWs is an effective way to tune and improve the dielectric and energy storage performance of nanocomposite capacitors. Future work related to C hapter 2 includes the synthesis of NWs and development of the high energy density nanocomposi te capacitors based on the fundamental finding in this c hapter. The C hapter 3 will investigate the effects of the orientation of the fillers in the matrix and how it influences the dielectric constant and energy storage performance of the nanocomposites.

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83 CHAPTER 3 THE EFFECT OF FILLER ORIENTATION ON THE ENERGY DENSITY OF NANOCOMPOSITES Chapter Introduction Chapter 2 has demonstrated that high aspect ratio fillers can improve the energy density of the nanocomposites. T his c hapter focuses the effects of the filler orientation on dielectric constant and energy storage performances of the nanocomposites. It begins with a review of the effects of filler orientation on the dielectric property of the composites Then, a novel method is proposed to align the NWs in a thermoplastic matrix by uniaxial force assembly By using the NWs aligned in the polymer, the effects and energy density of the nanocomposites are studied through three different directions: random, 1 direction alignment and 3 direction alignment. It will be shown that the orientation plays an important role in the dielectric property and energy density of the nanocomposites. Additionally, a new characterization method is proposed to quantify the NW Orientation Factor based on the fast Fourier transform analysis of cross sectional SEM images of the nanocomposites. The orientation factor of nanowires is controlled by the draw ratio of the nanocomposites. Finally, the relationship between the orientation factor and the dielectric constant of the nanocomposites is quantified for the first time. The key contribution of C hapter 3 is to provide another solution to improve the energy density of nanocomposites by controlling the orientation of NWs. Alignment Methods and T heir Effect s Past research has been demonstrated composites with aligned filler can influence the dielectric properties of a material, specifica lly the dielectric constant and breakdown strength [72,132,133] Andrew et al. [74] and Patil et al. [13 4] have

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84 developed finite element models and the equivalent capacitance model, respectively, to show that the fillers aligned in the electric field direction can improve the dielectric constant of composites. Bowen et al. created anisotropic fillers for composites by using a dielectrophoretical assembly process [72] T he samples with aligned fillers had a dielectric constant approximately three times higher than that with randomly distributed fillers. However, th e improved dielectric permittivity came at the expense of the breakdown strength [9] This drop in breakdown strength has also been reported in the case of composites with randomly dispersed ceramic filler. Tomer et al. used the dielectrophoretical assembly method to obtain aligned BaTiO 3 in a silicone elastomer thermoset polymer and found the dielectric constant in the applied electric field direction was much larger than random sample [132] However, the breakdown strength decreased when compared to a nanocomposite with random orientation. Later, Tomer et al. aligned high aspect ratio organomontmorillonite (MMT) in a polyethylene (PE) matrix and demonstrated the improvement in both the dielectric constant and breakdown strength, leading to higher energy density [133] T hese results indicate that breakdow n strength to achieve materials with high energy density. Alignment of NWs has been achieved by many methods, such as dielectrophoresis [72,132,133,135] ex trusion methods [136,137] Langmuir Blodgett troughs [138,139] microfluidics [ 140,141] blown bubble films [142,143] strain release assembly [144,145] and contact printing [146,147] However, most of these methods only align the NWs on the surface or in the plane of the substrate, making them unsuitable for preparing bulk nanocomposites. Furthermore, while the use of

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85 dielectrophoresis can obtain bulk nanocomposites with aligned filler, only samples with low volume fraction of filler can be aligned [132,148,149] It is especially diff icult to align NWs in the composites since NWs occupy more space compared to equiaxial particles at the same weight. Here, a simple and useful strategy for aligning NWs in a thermoplastic matrix is proposed. The nanocomposites with aligned PZT NWs are pre pared by uniaxial force assembly. This c hapter mainly covers the effects of orientation of PZT NWs on the dielectric constant and energy density of the nanocomposite. The alignment of the NWs in electric field direction leads to energy densit y up to 51.6 % greater than nanocomposites with random alignment at 20 vol.%. Finally, the relationship between the orientation factor and dielectric constant of the nanocomposite is quantified. The study here indicates that aligned NWs in the electric field directio n can be used to improve the energy density of the nanocomposites and have promising potential to produce high energy density nanocomposite capacitors in the future. Preparation of Nanocomposites with Aligned PZT NWs PZT NWs were synthesized through a hydr othermal process as described in Chapter 2 The nanocomposites were prepared by dispersing the PZT NWs into a PVDF (Kynar 301 F)/dimethylformamide (DMF) solution through bath sonication for at least one hour. The solution was cast o nto a glass pate and d ried under vacuum at 60 C overnight. The resulting films were cut into small pieces and transferred to a stainless steel mold to be laminated in a hot press at 185 C for one hour. This process was used to obtain void free nanocomposites. The dimension of the final sample was 42 mm 6.4 mm 3.2 mm. Nanocomposites with aligned PZT NWs were prepared through uniaxial force assembly as shown in Figure 3 1(a). The samples with randomly

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86 oriented PZT NWs were first loaded into the wedge grips of Instron 59 69 electromechanical load frame as shown in Figure 3 1(b). Then, the samples were heated to 150 C using a heat gun and stretched to 20% draw ratio. Finally, the stretched samples were cooled down to room temperature. The samples were released and final samples showed a dog bone shape with a necked section in the center of the sample (Figure 4 1(b)). Due to the brittle nature of the nanocomposites with 40 vol.% filler, the random films were first stretched to obtain films with aligned PZT NWs, and then were placed into a mold and hot pressed together at 180 C for one hour. Figure 3 1. Alignment of PZT NWs in nanocomposites. A ) Illustration of the alignme nt of PZT NWs in PVDF matrix, B ) Set up of alignment for PZT NWs in the composite under uniaxial force Figure 3 2(a) shows the free standing high aspect ratio PZT NWs and t he crystalline structure of the synthesized PZT NWs is PbZr 0.2 Ti 0.8 O 3 (JCPDS No. 01070 4261) as shown in Figure 3 2(b). The microscopic homogeneity of the nanocomposite is investigated by imaging its top surface as shown in Figure 3 2 (c), which indicates (A) (B)

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87 that the sample with 10 vol.% PZT NWs has a homogeneous dispersion in the PVDF matrix. Well dispersed PZT NWs in the PVDF matrix is required to ensure a high energy density as well as reproducible measurements. In order to observe the aligned PZT NWs in the nanocomposite, the uniaxial stretched samples were soaked in liquid nitrogen then snapped to break the sample s and image the cross section of samples. After uniaxial force assemble the PZT NWs are aligned in the PVDF matrix alon g the axis of the applied force as shown in Figure 3 2 (d). Each volume fraction is characterized using the same procedure and all are demonstrated homogeneous dispersion and uniform alignment of NWs in the nanocomposites. Figure 3 2. SEM images and XRD patterns of PZT NWs and nanocomposites. A ) SEM images B ) XRD patterns of PZT NWs C ) SEM images of nanocomposites with 10% random PZT NWs D ) 10% aligned PZT NWs under uniaxial force (A) (B) ( C ) (D)

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88 The Effect of Filler Orientation on the Dielectric Constant The dielectric constant of the nanocomposites under different orientations (random, 1 direction alignment and 3 direction alignment) with various volume fractions of PZT NWs are shown in Figure 3 3. The 3 direction alignment is along the electric field direction, while the 1 direction is perpendicular to the electric field as shown in Figure 3 3. It shows that th e effective dielectric constant values increase with increasing volume fraction of NWs. This is attributed to the higher dielectric permittivity of the PZT fillers compared to the PVDF matrix. More importantly, the dielectric constants of 3 direction ali gned PZT NW samples are much larger than that of random and 1 direction aligned PZT NW samples at the same volume fraction. The dielectric constants of 3 direction aligned PZT NWs samples are 35.7% and 15.0% larger than the samples with random PZT NWs at 30 vol. % and 40 vol.%, respectively. The drop in dielectric constant of the nanocomposites with aligned NWs perpendicular to the electric field is expected and observed in prior studies [67,74,150,151] as well as through theoretical modeling [67] Figure 3 3. Comparison of measured dielectric permittivity (at 1 k Hz) of nanocomposites under different orientation (random, 1 direction alignment and 3 direction alignment) as a function of PZT NWs volume fractions

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89 Figure 3 4 shows the dielectric constant of the nan ocomposites under different orientation directions with various volume fractions of PZT NWs over the frequency range of 1 k Hz to 1 MHz. It is indicated that the dielectric constant of the nanocomposites decrease with the frequency due to the dipole mobility, which is not sufficiently mobile to displace as the frequency of the applied electric field exceeds the relaxation freq uency [31] Figure 3 4. Dielectric constants of different PZT volume fraction in PVDF from 1 KHz to 1 MHz A ) 3 direction aligned PZT NWs/PVDF nan ocomposites, B ) rando m PZT NWs/PVDF nanocomposites, C ) 1 direction aligned PZT NWs/PVDF nanocomposites The Effect of Filler Orientation on the Energy Density The effects of the filler orientation on the energy storage performance of the nanocomposites can be investigat ed by D E loop characterized by a Sawyer Tower circuit. Figure 3 5 presents the typical D E loops measured at various volume fractions (A) (B) (C)

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90 under different orientations (random, 1 direction alignment and 3 direction alignment). The concentration of PZT filler s notably raises the electric displacement, which is attributed to the higher dielectric permittivity of PZT compared to the PVDF matrix. It is also shown that the 3 direction aligned PZT NW nanocomposites have larger electric displacement than the random and 1 direction aligned PZT NW nanocomposites as a result of the higher dielectric permittivity of the samples. For example, with a field of 15 kV/mm and 40 vol.%, the 3 direction aligned nanocomposites achieve an electric 2 wh ile the random and 1 direction aligned PZT NW 2 2 respectively. Figure 3 5. D E loop measured under different applied fields at room temperature and 100 Hz for the p olymer and nanocomposite A ) 3 direction align ed PZT NWs PVDF nanocomposites, B ) rando m PZT NWs PVDF nanocomposites, C ) 1 direction aligned PZT NWs PVDF nanocomposites (A) (B) (C)

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91 Figure 3 6 presents the stored electrical energy density of 3 direction align ed and random PZT NWs in PVDF calculated from each D E loop. It demonstrates that the energy density of the nanocomposite s is dependent on the orientation of the PZT NWs. Figure 3 6 indicates that the energy density increases with higher electric field a nd PZT concentration. The energy density of 3 direction aligned PZT NW composites is higher than that of random PZT NW composites throughout the entire volume fraction range considered. For example, the nanocomposite with 40 vol.% PZT NWs aligned in the field direction has an energy density of 0.0431 J/cm 3 at an electric field of 15 kV/mm, which is 36.0% larger than that of the 40% random PZT NWs composite (0.0317 J/cm 3 ). Figure 3 6. The dependence of energy density on the PZT orientation and volume f raction in the PVDF under 15 kV/mm: 3 d irection a lignment PZT NWs / PVDF (red) and r andom PZT NWs / PVDF nanocomposites (blue) In this work, the dielectric constant of nanocomposites with NWs aligned in the electric field direction is shown to be larger than that of nanocomposites with random orientation. In order to eliminate the influence of PVDF orientation on the dielectric property of nanocomposites, the phase of the PVDF is characterized. Figure 3 7 shows that the FTIR patterns of PVDF in the nanocompo sites before and after stretching are

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92 similar, which means that the uniaxial force does not change the structure of the PVDF. The filler orientation is the only parameter changed in the experiment; therefore, the change of the dielectric constant. By aligning PZT NWs in single direction as shown in Figure 3 2, the poling direction and dipole can be more easily oriented in the electric field direction. Also, some researchers have proposed models that consider cubi c particles in aligned chains as capacitors in series and showed that nanocomposites with aligned particles have higher dielectric constant than random ones [21,67,72] Bowen propose s that aligned fillers in the matrix experience stronger interactions than randomly dispersed fillers leading to higher dielectric constant [72,151] Considering all these points nanocomposites with aligned PZT NWs will have higher dielectric constant than the samples with random inclusions. The improvement of dielectric constant yield higher energy density at the same electric field, since the energy density is linearly related to the dielectric constant Figure 3 7. FTIR patterns of 10% PZT NWs PVDF nanocomposites before and after stretching Based on the findings in the Chapter 2 and 3, the current results demonstrate that higher aspect ratio fil lers and 3 direction alignment shows the enhancement of

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93 dielectric constant with potential to improve the energy density of nanocomposites. The relationship between the structure of the filler and dielectric constant of the nanocomposites are summarized i n Figure 3 8, which represents the first report in the literature and has generated significant interest in this community. The ability of the dielectric constant of nanocomposite is listed as following: 3 direction alignment PZT NWS> random PZT NWs> rand om PZT NRs>1 direction alignment PZT NRs Figure 3 8. Comparison of measured dielectric constant of nanocomposites with different structures (aspect ratio and orientation) as a function of volume fraction of PZT particle Quantification of the Relationship between the Filler Orientation Factor and the Dielectric Constant of Nanocomposites Prior research has demonstrated that filler alignment in the electric field direction can improve the dielectric constant more efficientl y than randomly oriented fillers or filler s oriented perpendicular to the electric field direction. There are many theoretical models that have shown an improved dielectric constant by aligning the fillers in the electric field direction of the sample [72,74,123,152] Currently, many researchers are using the alignment method to fabricate high dielectric constant composites

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94 [72,132,133] However, the quantification of the orientation factor of filler in nanocomposites is still a challenge using experimental analysis Here, the orientation factor of nanowires is successfu lly controlled by draw ratio of the nanocomposites. Additionally, a method is described to characterize the orientation factor of the fillers. Finall y, the relationship between the dielectric constant and orientation factor is experimentally quantified. It has been demonstrated that NWs can be aligned via shear forces on the NWs when the specimen is subjected to uniaxial force This approach is employed here to draw ratio of the samples, as shown in Figure 3 9. It i s predicted that the orientation factor which represents the alignment of nanowires will be increased by increasing the draw ratio of the nanocomposites. However, the orientation factor will reach a saturation point when most of the NWs have be en aligned and further extension will not change the NW orientation. Here, the dimensional (2 D) fast Fourier transform (FFT) analysis of a cross sectional SEM image of the nanocomposites. Through orientation factor control, it is expected that the effective dielectric constant of the nanocomposite can be tuned without requiring additional fillers, which add defects into the nanocomposite This method provides another solution to improve the energy density of the capacitors in addition to quantifying the relationship between the filler orientation and dielectric constant of the nanocomposites for the first time.

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95 Figure 3 9. Various draw ratios of the nanocomposite to factor The orientation factor of embedded NWs in the polymer matrix can be obtained by performing FFT analysis on the cross sectional SEM image of the nanocomposites (Figure 3 10). In this case, the Fourier transform maps the original grayscale levels of the SEM image from real space into frequency space (Figure 3 10c). The pattern of FFT intensity is identical to the degree of alignment distribution of the components in the original SEM image [153 155] As shown in Figure 3 10b, the FFT intensity profile can us by extracting the intensity profile of each half circle projection. The peak and its sharpness in the intensity profile illustrate the orientation factor of the majority of NWs and the quality of the distribution of the aligned NWs, respectively. It i s observed that perfectly aligned vertical NWs show a sharp peak at 90 while randomly distributed NWs do not illustrate a distinctive peak in the FFT profiles. It is worth mentioning that the edge effect in the SEM image adds fictitious information to th e FFT intensity plot and consequently to the orientation factors [154] The error of the edge effect appears

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96 as crossed lines in the FFT as shown in Figure 3 10d. In order to avoid this type of error, the edges of the SEM images are blurred by using a Gaussian low pass filter as shown in Figure 3 10a. Filtering the edge effects in the SEM image leads to less noise in the FFT output and consequently the intensity profiles (Figure 3 10c). Thus, the blurred images ar e utilized to determine the orientation factor of NWs based on the HOF with the procedure that will be described in detail. Figure 3 10. FFT of the SEM images of the nanocomposites. A ) Edge blurred SEM image of 20 vol % PZT NWs nanocomposites under 25% draw ratio B ) radial projection representing normalized intensity profile. C ) FFT of the edge blurred SEM image D ) corresponding FFT representing the edge effect. (A) (B) (C) (D)

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97 Since the FFT intensity profile does not quantify the distribu orientation factor, HOF is used to quantify the orientation factor of NWs in the nanocomposites based on the FFT intensity profiles. HOF as an orientation factor is a mathematical construction that describes the line structures relative t o another direction of interest. Based on th e definition, HOF can vary between 0.5 and 1. By defining a reference direction, 0.5 and 1 represent perfect alignment in perpendicular and parallel directions, respectively, while 0 represents random orienta tion of components. After performing a FFT on the SEM image, an azimuthal average of the output FFT image is performed to obtain the intensity azimuthal angle curve, which is used to calculate the HOF. The details are as follows: The HOF is defined as th e standard formulation commonly used in polymer science to quantify orientation of semi crystalline polymers: [156,157] where the angular brackets indicate a spatial av erage to account for the spatial distribution of the orientation degree and can be expressed as: where azimuth angle is the angle between the orientation reference axis and component 3 11a), and is the intensity profile of anisotropy as a function of intensity profiles as a function of azimuth angle and integrating numerically from zero to 90 to determine the spatial average. As shown in Figure 3 11, the profile intensities vary with respect to the distance from center; however, they show the same trends

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98 (profiles 1 to 4). At a distance far from center, the projected profile does not con tain useful information due to the resolution limitations (profile 6). Here, the HOF is estimated by averaging the effective profile intensities based on cross sectional SEM images of the nanocomposites. Only minor variations in the determined HOF are observed by altering the range and increments of the averaged profiles. For aligned PZT NWs in the nanocomposite, the components are embedded PZT NWs. Ideally, all the individual PZT NWs should be exposed on the surface of the composite when the sample is prepared. Then, theoretically, the HOF determination should be performed for each individual NW; however, this is nearly impossible by SEM obser vation of the nanocomposites in this study. Figure 3 11. The Determination of HOF A ) FFT image with defined axes azimuth angle. B ) Extracted intensity profiles at different frequencies. Different draw ratios are applied on the samples i n order to control the orientation factor of the N Ws Here, the nanocomposites with 20 vol.% PZT NWs are used to investigate the effects of the filler orientation factor on the dielectric constant of the nanocomposites. Four different samples are cut to the same size and loaded into the wedge grips of an Instron 5969 electromechanical load frame as shown in Figure 3 1(b). (A) (B)

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99 Then, the samples are heated to 150 C using a heat gun and stretched to the desired draw ratio. Figure 3 12 shows the SEM images of the nanocomposites with different draw ratios under uniaxial force accompanied by their respective FFT analyses. Before stretching, randomly dispersed PZT NWs are observed in the PVDF matrix (Figure 3 12a). The corresponding FFT image shows the pixel dis tributed in a symmetrical and circular shape, which indicates all the specific pixel intensities are theoretically identical in any direction and the fillers are randomly distributed in the polymer matrix. After stretching, the PZT NWs beg i n to align in t he draw direction. The FFT analysis of the aligned NWs results in an image distributed as an elliptical shape, which indicates that the pixel intensities are preferentially distributed with a specific orientation thus the NWs are aligned in the composites (Figure 3 12b). With increasing draw ratio, both the SEM images and their corresponding FFT intensity outputs indicate that the NW orientation factor increases (Figure 3 12).

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100 Figure 3 12. SEM images of cross section of the 20. vol% PZT NWs nanocomposites along with their FFT under various draw ratio. A ) 0%, B) 5%, C) 10%, D) 20%, E ) 25% (A) (B) (C) (D) (E)

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101 As shown in Figure 3 13, the HOF of the nanocomposites increases with increasing draw ratio. The HOF is 0.044 for the unstretched samples, which implies the NWs are completely randomly distributed. After stretching about 5 %, the orientation factor increases sharply to 0.22, which means the NWs begin to align in the draw direction. With increasing draw ratio, the HOF of the NWs con tinues to increase however at a decreasing rate. Finally, the HOF of NWs reaches saturation because most of the NWs are aligned. It is noted that the HOF of the embedded NWs in the polymer matrix reaches 0.481 at 25% draw ratio. Previous research has sh own that the HOF values of 0.3 to 0.6 are reached for a high volume fraction of aligned carbon nanotubes in composites or aligned carbon nanotube forest, which are shown to have good alignment in the SEM observation [158 160] Therefore, the HOF (0.481) indicates that most of PZT NWs are aligned in the PVDF matrix. Figure 3 13. HOF as a function of the draw ratio for the nanocomposites with 20 v ol % PZT NWs The dielectr ic constant of the nanocomposites is measured at a frequency range of 1 kHz to 1 MHz by an Agilent 4980A LCR meter. The frequency dependence of the

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102 dielectric constants with various HOFs is shown in Figure 3 14. It is clearly shown that the dielectric constant increases with increasing orientation factor of PZT NWs in the nanocomposites. The dielectric constant of the nanocomposite with 20 vol.% PZT NWs reached as high as 28.7 at a HOF of 0.481, which is 1. 48 times that of the randomly oriented PZT NWs with a dielectric constant of 19.4. Thus, it is demonstrated that the filler orientation factor plays a significant role in controlling the dielectric constant of the nanocomposites, and the proposed techniqu e is able to efficiently improve the dielectric constant of the nanocomposites without the need for additional fillers. Figure 3 14. Dielectric constant of the nanocomposites as a function of HOF at the 20 vol.% PZT NWs Chapter Summary In this c hapter, the relationship between orientation factor of the filler and energy storage performance of the nanocomposites was investigated. It showed that the dielectric constant and the energy density of nanocomposites were enhanced through alignment of the NWs in the applied electric field direction. Nanocomposites with aligned PZT NWs were prepared through uniaxial force and compared to samples with

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103 randomly distributed NWs. The results demonstrated that nanocomposites with NWs aligned in the electric fi eld direction have higher dielectric constants than samples with randomly dispersed NWs. The increased dielectric constant of the aligned nanocomposites provide d a platform to obtain materials with greater energy density. The alignment of the NWs led to e nergy densities up to 51.6% greater than nanocomposites with random alignment at 20 vol.%. These results showed that controlling the orientation of the filler could be used to improve the energy density of the nanocomposites. This c hapter also included de tails about a simple and useful strategy for aligning NWs in a thermoplastic matrix. The degree of NW orientation was tuned by varying the draw ratio of the samples through uniaxial force The HOF was utilized to quantify the orientation factor of NWs ba sed on 2 D FFT analysis of cross sectional SEM images of the nanocomposites. It was observed that HOF initially increased sharply after stretching and saturated when most of the NWs were aligned. The HOF varied from 0.044 to 0.481, corresponding to the d raw ratios of 0 to 25%. It was demonstrated that the dielectric constant of the nanocomposites increased with increasing degree of NW orientation. The dielectric constant of the nanocomposite with 20 vol.% PZT NWs and a HOF of 0.481 reached as high as 28 .7, which is 1.48 times larger than samples with randomly oriented PZT NWs (19.4). The relationship between the orientation factor of the fillers and dielectric constant of the nanocomposites was quantified for the first time. The results presented in thi s c hapter combined with my prior demonstration that high aspect ratios produce significant performance gains over spherical fillers provides a methodology to achieve vastly improved capacitive materials. The Chapter 4 will

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104 introduce the fabrication of hig h energy density nanocomposite capacitors based on high aspect ratio NWs.

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105 CHAPTER 4 HIGH ENER GY DENSITY NANOCOMPO SITES Chapter Introduction Based on the finding of Chapter 2 and 3, t his c hapter will develop and characterize nanocomposites with high er ene rgy density and fast er discharge rate than any appearing prior to the work of this dissertation Two particular cases will be considered, one utilizing a high dielectric PVDF terpolymer matrix and BaTiO 3 NWs and a second using conventional PVDF and Ba 0.2 Sr 0.8 TiO 3 NWs The first type of high energy density nanocomposite capacitor is fabricated with high aspect ratio BaTiO 3 NWs in poly(vinylidene fluoride trifluoroethylene chlorofluoroethylene) (P(VDF TrFE CFE)). It is shown that these nanocomposite capac itors have a high energy density up to 10.3 J/cc. However, this form of nanocomposite capacitor shows polarization saturation and high ferroelectric loss, which is attributed to the ferroelectric properties of BaTiO 3 fillers and P(VDF TrFE CFE) matrix. I n order to achieve higher energy density, it is necessary to overcome the ferroelectric properties of the nanocomposite while maintaining high dielectric permittivity and high breakdown strength. Therefore, it is hypothesized that the fabrication of a nan ocomposite without ferroelectric behavior will yield higher energy density. In order to achieve this goal, a new method is first developed to synthesize high aspect ratio Ba x Sr 1 x TiO 3 NWs with different molar ratios of barium and strontium ions. In the B a x Sr 1 x TiO 3 solid solution, transfor mation occurs from the ferroelectric phase to the paraelectric phase when the Ba mole fraction decreases below 0.7 (x<0.7) and shows little hysteresis behavior at room temperature and above Here, the paraelectric phase Ba 0.2 Sr 0.8 TiO 3 is chosen for the NWs combined with quenched PVDF to fabricate high energy density nanocomposite capacitors, which can reach an energy density as high as 14.86 J/cc This rivals or

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106 exceeds those reports for ceramic/polymer composites [15 17,19,20] and is 1138% greater than the energy density of 1.2 J/cc at 640MV/m for commercial biaxially oriented polypropylene (BOPP). Hi gh Energy Density Nanocomposite Capacitors Using BaTiO 3 NWs and P(VDF TrFE CFE) High aspect ratio BaTiO 3 NWs are used in this case as filler for the fabrication of nanocomposites. Figure 4 1a shows the free standing BaTiO 3 NWs The size of BaTiO 3 particles are analyzed from SEM pictures by using ImageJ and shown that they ha ve a mean length of 14.7 m and a mean diameter of 470 nm, with an aspect ratio of approximately 31. A representative XRD pattern of the BaTiO 3 NWs is shown in Figure 4 1b. All diffraction peaks can be assigned to the BaTiO 3 crystal structure (JCPDS, 81 2203) without any indication of crystalline byproducts such as BaCO 3 or TiO 2 When working with nanocomposites, the high surface area to volume ratio leads to large interfacia l area that can produce defects in the polymer creating a situation where the overall dielectric strength is significantly compromised. In order to improve the compatibility between the filler and matrix and thus increase the breakdown strength, surface m odification of the fillers is often used to improve the performance of the nanocomposite system [9,15 17] Figure 4 1d shows the FTIR spectra of BaTiO 3 NWs befo re and after surface functionalization with ethylenediamine. The binding of ethylenediamine is evidenced by the adsorption around 1450 cm 1 which corresponds to amine groups (N H) [131] T he coordination of the amine group on the functionalized BaTiO 3 with the P(VDF TrFE CFE) matrix is schematically shown in Figure 4 1 d. The presence of amine groups has been shown to increase the compatibility between the

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107 filler and matrix and allow the formation of a homogenou s dispersion in the P(VDF TrFE CFE) matrix more readily [161] Figure 4 1. Morphology, structure and functionalization of BaTiO 3 NWs. A ) SEM image, B) XRD patterns, C ) FTIR spectra of BaTiO 3 NWs and modified BaTiO 3 NWs by ethylenediamine, D ) schematic image of the functionalized TiO 2 by ethylenediamine reacting with P(VDF TrFE CFE) N anocomposites were prepared by dispersing the BaTiO 3 NWs into a 10 wt.% of P(VDF TrFE CFE) (63/29/8% mole ratio, Piezotech S.A.S, France) in dimethylformamide (DMF) solution through sonication for 1 hour then solution casting onto a glass plate to obtain thin films approximately 10m thick. The films were then dried at 60 C under vacuum overnight. The films were peeled from the glass substrates and further dried in a vacuum oven at 120 C for 24 hours to make sure the solvent was completely removed. Finally, gold electrodes approximately 10 nm thick (A) (C) (D) (B) Hydrogen bonding Reaction

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108 were sputtered onto both surfaces of the film for low field measurements and high electric field D E loop measurements. The microscopic homogeneity of the nanocomposite is investigated by imaging its top surface as shown in Figure 4 2. It indicates that the BaTiO 3 NWs have a homogenous dispersion in the PVDF terpolymer matrix, and no void exists in the film, which is required to ensure a high energy density as well as reproducible measurements. The good dispersion of BaTiO 3 NWs is attributed to the compatibi lity between the PVDF terpolymer and functionalized BaTiO 3 NWs with ethylenediamine Figure 4 2 SEM image of 10 vol.% BaTiO 3 P(VDF TrFE CFE) nanocomposites Dielectric Property of the Nanocomposites The increased dielectric constant resulting from NWs over low aspect ratio particles can be seen in Figure 4 3a. It clearly demonstrates that the use of high aspect ratio filler more efficiently imparts its high dielectric permittivity to the bulk nanocomposite than the low aspect ratio particles used by Wa ng et al [16] The dielectric constant can reach as high as 69.5 at 17.5 vol.% of BaTiO 3 NWs, while the dielectric constant is only around 52 at a high concentration of 30 vol.% of nanoparticles in a similar matrix. C onsidering the energy storage scope of this dissertation, high

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109 energy density capacitors need to simultaneously have high breakdown strength as well as high dielectric permittivity. It is well known that increasing the volume fraction of the filler decrea ses the breakdown strength of the capacitor [15,55] In addition, the remnant polarization of the nanocomposite increases with the volume fraction of the ferroelectric fillers, which decrease s the energy density of the material [16] Therefore, this dissertation is focused on using a low concentration of fillers to maintain improved dielectric permittivity and high breakdown strength to create high energy density capacitors. It is also noted that the dielectric loss of the nanocomposite is around 0.09 and is almost independent of the low volume fraction of the BaTiO 3 NWs (Figure 4 3b). In addition, Figure 4 4 indicates that the effective dielectric permit tivity of the nanocomposites decreases with increasing frequency. At low frequency, the dipoles can move sufficiently fast to follow the electric field. However, as the frequency increases the dipole cannot shift orientation sufficiently fast as the appl ied electric field exceeds its relaxation frequency, resulting in a decrease of dielectric constant at high frequency [31] Figure 4 3. Dielectric properties of nanocom posites: A ) comparison of measured dielectric permittivity (at 1kHz) of BaTiO 3 NWs nanocomposites as a function of BaTiO 3 NWs and nanoparticle volume fractions [16] B ) dielectric permittivity and loss tangent (at 1kHz ) of BaTiO 3 NWs nanocomposites (A) (B)

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110 Figure 4 4. Dielectric permittivity constants of different BaTiO 3 NWs volume fractions in P(VDF TrFE CFE) from 1kHz to 1 MHz Energy Storage Performance of the Nanocomposite Capacitor s The D E loops measured with a Sawyer Tower circuit under a unipolar 100 Hz field with varying peak electric field are shown in Figure 4 5. It shows that the electric displacement of the nanocomposite increases with the applied electric field, which indicates that it is reasonable to o btain a larger electric displacement at a higher electric field. Also, the electric displacement of the nanocomposite increases with the volume fraction of BaTiO 3 NWs. This is attributed to the fact that BaTiO 3 has a higher dielectric permittivity than P VDF terpolymer. It can also be observed that the remnant polarization of the D E loops increase with the concentration of the fillers since the ferroelectric ceramic BaTiO 3 NWs has a much higher remnant polarization. For energy storage the high remnant polarization will decrease the discharge energy of the material since the integrated area of the D E loops will decrease. This is one of the reasons that the nanocomposites used here have been prepared with low volume fractions of BaTiO 3 NWs in the nanoco mposite s

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111 Figure 4 5. Unipolar electric displacement electric field (D E) loops for nanocomposites with different BaTiO 3 NWs volume fractions in P(VDF TrFE CFE) matrix The energy densities are summarized in Figure 4 6. It shows the energy densities of nanocomposites calculated from the D E loops as a function of applied field at different volume fractions. It indicates that the energy density of the nanocomposites increases with increasing electric field and volume fraction of the BaTiO 3 NWs. The inc orporation of BaTiO 3 NWs into the terpolymer greatly increases the energy density of the nanocomposites. Figure 4 6 clearly shows that BaTiO 3 NWs can improve the energy density of the nanocomposite compared to a neat polymer. The 17.5 vol.% nanocomposite show a increase in energy density of more than 45.3% over that of the P(VDF TrFE CFE) polymer (10.48 J/cc compared to 7.21 J/cc). This value is significant because it exceeds those reported for the conventional polymer ceramic composites [9,15 17,61] and is also more than seven times larger than high performance commercial polypropylene capacitors (1.2 J/cc).

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112 Figure 4 6. Energy density of the nanocomposite with different volume fractions as a function of electric field calculated from D E loops For pulsed power applications, the capacitor needs to not only have high energy density but also high power density. Therefore, it is important to characterize the discharge speed of the capacitor to calculate discharged power density. The discharge speed and discharge power are measured by using a self designed, high speed capacitor discharge circuit similar to that reported in the literature [162 164] and shown in Figure 4 7. First, the nanocomposite sample is charged with a given voltage. After that, by closing a high speed (<500 ns) high voltage switch (Behlke HTS81), the stored energy in the nanocomposite capacitor is discharged across a load resistor R L The dielectric materials can be modeled as an equivalent series resistor (ESR) as shown in Figure 4 8 [6,162,163] It will be shown that most of the stored energy in the capacitor will be delivered to the load when the R L is much larger than the ESR. Therefore, the measurement of the energy density from the load resistor can be close to the stored energy of the tested sample The voltage across R T is monitored with an oscilloscope

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113 through a high voltage probe as shown in Figure 4 7. The discharged energy can be calculated from the voltage on R T as a function of time, as shown in the follow ing equation: where U is the energy density of the sample, R L is the resistor, V(t) is the voltage from the oscilloscope, t is the time and V is the volume of the measured capacitors. Finally, the power density of the nanocomposite c an be calculated by dividing the discharge energy by the discharge time. Figure 4 7. Discharge circuit of energy density and discharge speed characterization Figure 4 8. An equivalent circuit of a dielectric sample

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114 It is well known that the commercial polypropylene capacitor has fast discharge speed. In order to compare the discharge speed of the prepared nanocomposites with existing commercial materials, a nanocomposite with 17.5 vol.% B a T i O 3 NWs and a commercial biaxially oriente d polypropylene having the same capacitance (73 pF) are 4 9a. The experimental discharge time 0.9 is defined as the time for the discharged energy under load to reach 90% of the final value from the d ischarge profiles. Figure 4 9a shows the discharging time is around 7.08 s, which is comparable to the discharge speed of 5.68 s for commercial polypropylene. More notably, the discharge energy density of the nanocomposites is 4.8 J/cc at an electric f ield of 175 MV/m, which is close to the result measured by the D E loop and 17.5 times larger than a commercial polypropylene capacitor with 0.26 J/cc at 175 MV/m. In addition, the power density of the nanocomposites can reach as high as 1.2 MW/cc at 1.52 s, which is 14 times larger than commercial polypropylene (0.08 MW/cc at 0.6 s). Therefore, the high energy density nanocomposite capacitors with fast discharge speed can satisfy the application requirements for pulsed power devices Figure 4 9. Discharge energy density and power density of nanocomposites: A ) discharged energy density, B ) power density profiles for nanocomposites with 17.5% BTO NWs and commercial polypropylene, respectively. The load resistor R L rical field is 175 MV/m. (A) (B)

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115 In this section, high energy density nanocomposite capacitors were successfully fabricated by using surface functionalized high aspect ratio BaTiO 3 NWs and P(VDF TrFE CFE) polymer. The nanocomposites exhibited increased dielectric permittivity at low volume fractions of the fillers and withstood high electric field for energy storage The 17.5 vol.% nanocomposites showed a significant increase in energy density, more than 45.3% higher than that of the neat P(VDF TrFE CFE) polymer (10.48 J/cc compared to 7.21 J/cc) at electric field 300 MV/m. This value is significant and exceeds those reported for the conventional polymer ceramic composite and is more than seven times larger than a high performance commercial polypropylene capacit or. In addition, the nanocomposites have rapid discharge speed reached a power density of 1.2 MW/cc at 175 MV/m, which is 14 times larger than commercial polypropylene. This simple approach to obtain high energy density capacitors with fast discharge spe ed can be applicable to other ferroelectric nanowires such as PZT and lead titanate. Ultra High Energy Density Nanocomposite Capacitors Using Ba 0.2 Sr 0.8 TiO 3 NWs The previous section has shown that nanocomposites prepared with BaTiO 3 NWs and P( VDF TrFE CFE) could achieve high energy density and fast discharge speed. However, it is shown that this type of nanocomposite capacitor has polarization saturation and high ferroelectric loss because of the ferroelectric properties of the BaTiO 3 and P(VD F TrFE CFE) (Figure 4 5) limiting the max energy recovered. Therefore, it is critical to avoid saturation of the polarization below the breakdown field [165] PVDF phase can be ob tained by quenching the film in ice water and showed it exhibited higher energy density than PVDF due to improved breakdown strength and the absence of early polarization saturation

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116 [166] F erroelectric fillers, such as PZT and BaTiO 3 are defined by their high remnant polarization and polarization saturation, both of which reduce the maximum energy density [19,20,165,166] Other ceramics can be designed such that they have a non ferroelectric structure but high dielectric pr operties, such as Ba x Sr 1 x TiO 3 (BST), which can be tuned to obtain a desired hysteresis behavior by varying the stoichiometry [104,106] In the Ba x Sr 1 x TiO 3 solid solution, transformation from the ferroelectric phase to the paraelectric phase occurs when the molar fraction of Ba decreases below 0.7 (x<0.7) and shows little hysteresis behavior at room temperature and above [104 106] The use of high dielectric fillers such as BST can provide a high dielectric permittivity while eliminating the remnant polarization, ultimately increasing the energy storage efficiency of c apacitor. A second technique to prepare high energy density nanocomposite capacitors with fast discharge speed is proposed. The nanocomposites are prepared with high aspect ratio Ba 0.2 Sr 0.8 TiO 3 NWs in PVDF and are quenched in ice water to achieve high br eakdown strength and energy storage properties. To the best of my knowledge, these nanocomposites have the largest breakdown strength (>450 MV/m) and highest energy density of any nanocomposite incorporating a high dielectric filler reported in the curren t literature while also having low hysteresis and sub microsecond discharge [15 17,19,20] The maximum energy density calculated from t he D E loop is 14.86 J/cc, exceeding the state of the art commercially available capacitors (1.2 J/cc at 640 MV/m) by more than an order of magnitude [48] Synthesis of Ba 0.2 Sr 0.8 TiO 3 NWs and Preparation of Nanocompos ites The synthesis of Ba 0.2 Sr 0.8 TiO 3 NWs was approached by a two step hydrothermal reaction [128] First, the precursors of sodium titanate NWs were

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117 synthesized by hydrothermal reaction. Typically, aqueous sodium hydroxide (Fisher, ACS, 99%) was added to a Teflon lined autoclave followed by the addition of titanium dioxide powder (anatase, Sigma Aldrich, ACS, 99%) and then bath sonicated for 20 min. The stainless steel autoclave was sealed and stirred at 20 0 C for 24 hours. After the autoclave was cooled to room temperature, the obtained powder was sequentially washed with water and then soaked with 0.2M hydrochloric acid (Fisher, 37%) aqueous solution for 3 hours. Then, the powders were washed with water four times through centrifugation and vortex mixing, and subsequently dried on a hotplate at 60 C overnight. Subsequently, the hydrogen titanate nanowires were converted to Ba x Sr 1 x TiO 3 NWs by a second hydrotherm al reaction with an aqueous solution containing barium and strontium ion sources The precipitate was collected, washed with 0.2M HCl aqueous solution, water and ethanol. The precursor to the Ba 0.2 Sr 0.8 TiO 3 phase is sodium titanate synthesized by hydrothe rmal reaction as shown in Figure 4 10a. The precursor nanowires are free standing and have high aspect ratio. The second hydrothermal process is designed to specifically maintain the morphology of the nanowires and as can be seen by Figure 4 10b that the morphology of Ba 0.2 Sr 0.8 TiO 3 is preserved. Chemical composition of nanowires was studied by an energy dispersive X ray spectroscopy (EDX, GENESIS), as shown in Figure 4 10c. The successful transformation of Ba x Sr 1 x TiO 3 nanowires after diffusion of the Ba and Sr ions into the precursor NWs during the hydrothermal reaction is further confirmed due to the presence of only Ba, Sr, Ti and O. It should be mentioned that the it is hard to clearly observe the of the separate peaks of Ba and Ti, since the main peaks of Ba (L edge) and Ti (K edge) overlap in the energy range of 4.5

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118 5 keV. [130] The crystal structure verification of Ba 0.2 Sr 0.8 TiO 3 is performed by XRD. As shown in the XRD patterns of Ba x Sr 1 x TiO 3 NWs (Figure 4 1 0d), it clearly demonstrates the crystal structure of Ba x Sr 1 x TiO 3 NWs after diffusion of the Ba and Sr ions into the precursor nanowires. The (110) diffraction peak of Ba x Sr 1 x TiO 3 gradually shifts from 31.48 to 32.24 degree as the molar fraction of bari um initially in solution decreases from 1 to 0.20, since the radius of a Ba 2+ ion (1.61 ) is larger compared to radius of a Sr 2+ 0.2 Sr 0.8 TiO 3 NWs is calculated as 3.922 which closely matches reported data (3.920 ) [ [167,168] ]. Figure 4 10. SEM images and XRD of nanowires and nanocomposites: A) sodium titanate NWs, B ) Ba 0.2 Sr 0.8 TiO 3 NWs, C ) typical ED S spectra of Ba 0.2 Sr 0.8 TiO 3 nanowire, D ) XRD patterns of Ba x Sr 1 x TiO 3 NWs with different Ba molar ratio Transmission electron microscopy operated at 200 kV (TEM, JEOL TEM 1011) was utilized to further analyze the microstructure and crystal structure of the (A) (C) (B) (D)

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119 Ba 0.2 Sr 0.8 TiO 3 nanowires. Figure 4 11a shows the individual nanowire has a straight cylindrical shape and single crystalline structure with growth along the [100] axis, as evidenced by clear lattice fringes in the high resolution TEM (HRTEM) image of Figure 4 11b. The interlayer distances between adjacent lattice fringes in the HRTEM image of Ba 0.2 Sr 0.8 Ti O 3 nanowire is measured to be 3.915 0.005, which is consistent with XRD data and corresponds closely to the reported distance between two adjacent [100] Ba 0.2 Sr 0.8 TiO 3 crystal planes (3.920) [ [1 67,168] ]. The Ba 0.2 Sr 0.8 TiO 3 has paraelectric phase and shows little hysteresis behavior at room temperature and above, [104,105] which directly decreases the ferroelectric loss and should increase the energy density of the nanocomposites. This result demonstrates a unique process for the growth of high aspect ratio nanowires of any Ba x Sr 1 x TiO 3 stoichiometry. Figure 4 11 TEM analysis of Ba 0.2 Sr 0.8 TiO 3 NWs A) TEM image, B ) representative HRTEM image showing clear crystal lattice fringes The surface of the Ba 0.2 Sr 0.8 TiO 3 NWs was functionalized with ethylenediamine to improve the dispersion and interaction with the PVDF matrix. The functionalization was carried ou t by mixing the BST powder with ethylenediamine followed by vortex (A) ( B )

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120 mixing for 5 minutes then sonicating for 1 hour followed by heated the solution to 90 C in a water bath for one hour The precipitate was separated by centrifugation and dried at 70 C un der vacuum overnight. Figure 4 12 a shows the FTIR spectrum of Ba 0.2 Sr 0.8 TiO 3 NWs before and after treatment with ethylenediamine. The presence of the amine groups on the surface of the Ba 0.2 Sr 0.8 TiO 3 NWs following functionalization is evidenced through t he appearance of the transmission at 1450 cm 1 [131] which acts as the bridge between the Ba 0.2 Sr 0.8 TiO 3 NWs and PVDF matrix as shown in Figure 4 12 c After functionalizing the BST NWs using ehtehylenediamnine, they can be homogenously dispersed in the PVDF polymer matrix, as shown in Figure 4 10 b which also shows the absence of voids in the film following annealing and quenching. Figure 4 12. BST NWs functionalization and nanocomposites. A ) FTIR spectra of BST NWs and functionalized BST NWs by ethylenediamine, B ) top surface of 5% Ba 0.2 Sr 0.8 TiO 3 /PVDF nanocomposites C ) schematic image of the functionalized BST by ethylenediamine reacting with P(VDF TrFE CFE) The quenching process is used to modify the crystallization of the PVDF in the nanocomposites to improve the breakdown strength and decrease the ferroelectric loss. Therefore, FTIR has been collected for PVDF before and after quenching as shown in (A) (B) Hydr ogen bonding Reaction

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121 Figure 4 (840, 812, 775 cm 1 resu lt is as expected and desirable [166] PVDF has reduced ferroelectric loss and higher breakdown strength [166] Figure 4 13b summarizes the characteristic breakdown strength of the quenched PVDF, untreated PVDF and commercial biaxial oriented polypropylene (BOPP). Following the quenching of the PVDF film in ice water, the dielectric strength is improved to 536.1 MV/m, while the untreated PVDF is 443.8 MV/m. It should be noted that the breakdown strength of the quenched PVDF is close to commercial BOPP from Milwek Company with 646.8 MV/m. PVDF reduced ferroelectric loss to yield high energy storage efficiency and high energy density of the nanocomposites, which will be discussed in the energy storage part. Figure 4 13. FTIR spectra and breakdown strength of quenched PVDF. A ) FTIR spectra of untreated PVDF, quenched PVDF B ) Weibull distribution analysis of breakdown strength of untreated PVDF, quenched PVDF and BOPP Dielectric Property of the Nanocomposites T he breakdow n strength of the nanocomposites decreases with increasing concentration of the filler, especially at high volume fraction [15,55] In order to improve the dielectric property of the nanocomp osite while maintaining high breakdown strength, (A) (B)

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122 the nanocomposites are fabricated at a low volume fraction of Ba 0.2 Sr 0.8 TiO 3 NWs ranging from 2.5% 7.5%. In addition, the high aspect ratio Ba 0.2 Sr 0.8 TiO 3 NWs are ground to form low aspect ratio Ba 0.2 Sr 0.8 T iO 3 nanorods (NRs) to effectively demonstrate 4 14a shows that the dielectric constant of the samples increases at low electric field with increasing volume fracti on of Ba 0.2 Sr 0.8 TiO 3 NWs in the nanocomposites, since the Ba 0.2 Sr 0.8 TiO 3 fillers have a higher dielectric constant than the PVDF matrix. Also, it is clearly demonstrated that the nanocomposites with BST NWs have a higher dielectric constant than the sampl es with BST NRs, which has also been demonstrated in the prior research [19,55] Building off these findings, the high energy density capacitors developed here will use high aspect ratio Ba 0.2 Sr 0.8 TiO 3 NWs. Figure 4 14b shows the relationship between the dielectric constant of the nanocomposite and the frequency. The dielectric constant decreases with increasing frequency due to the fact that the dipole cannot shift orientation direction as t he frequency of the applied electric field exceeds the relaxation frequency [31,31] Figure 4 14. Dielectric prop erties of the nanocomposites: A ) comparison of measured dielectric constant (at 1 kHz) of nanocomposites with different aspect ratios of BST NWs and BST NRs, B ) dielectric constant of different BST NWs volume fractions in PVDF from 1 kHz to 1 MHz (A) ( B )

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123 The brea kdown strength wa s measured using an electrostatic pull down method [165,169,170] A diagram of the setup is shown in Figure 4 15. Pull down between the conductive substrate a nd a brass dome typically occur s at an electrical field of 10 MV/m and i s maintained until breakdown occurred over the test area. The pull down method was chosen over a point contact method to avoid any mechanical force that might cause premature breakdow n at the contact point. Breakdown testing is performed in silicon oil to avoid electric arcing and was performed using an Acopian high voltage supply (PO30HP2M) by sweeping the applied voltage from 60 V DC at approximately 500 V/sec until sample failure, as evidenced by spurious current changes. Figure 4 15. Diagram of breakdown strength test using an electrostatic pull down method The dielectric breakdown strength is analyzed using a two parameter Weibull cumulative probability function : P(E)=1 exp[1 (E/E BD ) ] where P(E) is the cumulative probability of failure occurring at the electric field lower or equal to E [18,75] The E BD is the scale parameter for experimental breakdown strength with a 63.2% probability for failure, is the shape parameter associated with the linear regressive fit of the data distribution. The dielectric breakdown strength is then extracted from a fit using Weibull failure statistics across at least 15 tests per sample. Figure 4 16 summarizes the characteristic breakdown strength of the nanocomposites with different volume fractions

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124 of Ba 0.2 Sr 0.8 TiO 3 NWs PVDF, quenched PVDF and BOPP. Following quenching of the PVDF film in ice water, the dielectric strength is improved to 536.1 MV/m, which is close to commercial BOPP from Milwek Company with 646.8 MV/m. As the volume fraction of the BST NWs increases from 2.5% to 7.5%, the dielectric breakdown strength decreases from 505.1 to 450.1 MV/m, since the introduction of the fillers into the polymer results in defects that initiate failure and decrease the breakdown strength. It should be noted that while the breakdown decreases, compared to other composites [15 17,19,20] all nanocomposites maintain relatively high breakdown strength at a low volume fraction of the filler, which provides the opportunity for nanocomposites with very high energy density. Figure 4 16 Weibull distribution and observed dielectric breakdow n strength of nanocomposites with different volume fractions of BST, quenched PVDF and commercial polypropylene films Energy Storage Performance of the Nanocomposite Capacitors The polarization loop was measured by the Sawyer Tower circuit under a unipola r 100 Hz electric field with increasing peak electric field as shown in Figure 4 17 for the neat PVDF and each volume fraction of NWs. The addition of the Ba 0.2 Sr 0.8 TiO 3

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125 NWs into the PVDF polymers greatly increases the maximum polarization of the nanocomp osites. Most notably, it increases sharply at a high volume fraction of Ba 0.2 Sr 0.8 TiO 3 NWs compared to pure PVDF. It should also be noted that the hysteresis increases slowly with increasing concentration of the filler; however, the hysteresis is conside rably smaller than the results presented in prior research efforts utilizing nanocomposites with ferroelectric fillers [19,20] Hysteresis is important since it leads to reduced efficiency, inter nal heating and ultimately limits the maximum energy density and operational frequency of the capacitor. Figure 4 17 Unipolar electric displacement electric field (D E) loops for nanocomposites with different BST NWs volume fractions Figure 4 18a summarizes the energy density of BST nanocomposites calculated from the D E loop. It shows the use of the BST nanowires leads to improved energy densities compared with neat PVDF polymer at high electric field. For the composites with 7.5% Ba 0.2 Sr 0. 8 TiO 3 NWs, the energy density is 14.86 J/cc at 450 MV/m, which represents a 42.9% increase in comparison to the PVDF with an energy density of 10.4 J/cc at the same electric field. This energy density rivals or exceeds those reports for

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126 ceramic/polymer co mposites [15 17,19,20] and is 1138% greater than the 1.2 J/cc energy density at 640MV/m for commercial BOPP [48,171] The measured breakdown strength over 450 MV/m is the highest breakdown strength reported for nanocomposites incorporating a high dielectric filler ceramic/polymer capacitor, all of which are well below 250 MV/m [15 17,19,20] For practical applications, it is desirable to not only have a high energy density, ). It is well known that the energy losses in the capacitor leads to heating, and consequently, to detrimental effects on the performance and reliability of the capacitor. Figure 4 18b shows the efficiency (discharge energy/charge energy) of the nanocomposites with different NW concentration at high electric field, as calculated from the D E loops in Figure 4 17. It is clearly shown that the efficiency decreases with the applied el ectric filed, particularly above 100 MV/m, which is highly related to conduction loss [172] As the concentration of the filler increases, the efficiency of the capacitor decreases due to the larger hysteresis in the p olarization. However at fields below 100 MV/m, the efficiency is greater than 90% and greater than 60% at an electric field of 450 MV/m, which are higher than the nanocomposite capacitors reported in the literature [19,20] The improved efficiency is attributed to the non ferroelectric structure of Ba 0.2 Sr 0.8 TiO 3 NWs and the quenching PVDF phase [104,166] The efficiency results demonstrate that the nanocomposites developed here can capitalize upon the combination of inorganic materials of large permittivity with polymers of high breakdown strength to achieve high energy density and high efficiency.

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127 Figure 4 18 Energy density and efficiency of nanocomposites with Ba 0.2 Sr 0.8 TiO 3 NWs. A ) Energy density of the nanocomposite with different volume fractions as a function of electric fie l d calculated from D E loops; B ) Efficiencies of the nanocomposites with different volume fraction as function of electric field The discharge speed and discharged energy were measured using a specially designed, high speed capacitor discharge circuit as shown in Figure 4 7 In order to compare with the discharge speed of commercial capacitors, the nanocomposites and commercial polypropylene films are designed to have the same capacitance 26 pF at 1 kHz. Both samples are charged to 200MV/m followed by di load, as shown in the Figure 4 19a. I t is demonstrated that the nanocomposites have a discharge speed of approximately 2.3 s, which is faster than commercial polypropylene (2.8 s) that is noted for its fast discharge. Additionall y, nanocomposites can discharge more energy at the same electric field compared to polypropylene. The electric field used here was limited for safety thus the delivered energy should not be the full energy density of the material. The rate of discharge i s also primarily driven by the RC time constant thus the discharge rate is highly dependent on the load resistance. However, the nanocomposites have faster discharge than commercial BOPP when the RC time constant is identical. The discharge speed demonst rates that nanocomposites can be applied to the design of pulsed power capacitors with ultra high energy density and fast (A) (B)

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128 discharge time. It can be seen that the energy density measured from the discharge measurement (Figure 4 19a) and the D E loop measur ement (Figure 4 17) has consistent results (around 3.5 J/cc at electrical field 200 MV/m). The power density calculated from the discharge curve is shown in Figure 4 1 9 b and demonstrates that the nanocomposites can reach 2.38 MW/cc at 0.6 s while the BOP P can only deliver 0.164 MW/cc at 0.92 s. Figure 4 19. Discharged energy density and power density of nanocomposites with Ba 0.2 Sr 0.8 TiO 3 NWs. A ) discharged energy density, B ) power density profiles for nanocomposites with 7.5% BST NWs and commercial BOPP, respectively. The load resistor R L In this section, a novel method for the preparation of high energy density nanocomposite capacitors with fast discharge speed has been developed. It wa s shown that the quenched PVDF and Ba 0.2 Sr 0.8 TiO 3 NWs can improve the breakdown strength of the nanocomposites with the absence of polarization saturation. The energy density with 7.5% Ba 0.2 Sr 0.8 TiO 3 NWs reached 14.86 J/cc at 450 MV/m, which represented a 42.9% increase in comparison to the PVDF ( 10.4 J/cc at 450 MV/m) and 1138% greater than commercial capacitor BOPP ( 1.2 J/cc at 640MV/m ) To the best of my knowledge, this energy density is the highe st reported for any nanocomposite incorporating a high dielectric filler ceramic/polymer capacitor in the literature [9,15 (A) (B)

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129 17,61] Furthermo re, the discharge speed of the nanocomposites is only 2.3 s across BOPP at the same RC time constant. Furthermore, the efficiency of the nanocomposite is high d ue to the use of a paraelectric phase of bar ium strontium titanate and quenched PVDF C hapter Summary This c hapter has studied two techniques to prepare high energy density nanocomposites. The first technique is based on high aspect ratio BaTiO 3 NWs and the ferroelectric polymer P(VDF TrFE CFE). A 17.5 vol.% of BaTiO 3 nanocomposite showed a significant increase in the energy density, more than 45.3% higher than that of the neat P(VDF TrFE CFE) polymer (10.48 J/cc compared to 7.21 J/cc) at an electric field of 300 MV/m. However, this type of nanoco mposite capacitor exhibits polarization saturation and high ferroelectric loss, which limits the maximum energy density of the nanocomposite. In order to overcome these limitations and obtain high energy density nanocomposite capacitors, a second techniqu e is developed focusing on the elimination of the ferroelectric properties. Specificall y, the second technique is based on choosing paraelectric phase Ba 0.2 Sr 0.8 TiO 3 nanowires as fillers along with a quenched PVDF matrix Compared to BaTiO 3 / P(VDF TrFE CFE) nanocomposites, t his type of nanocomposite capacitor can maintain high polarization with no saturation, high breakdown strength and also reduced ferroelectric loss. The energy density of the nanocomposite with 7.5% Ba 0.2 Sr 0.8 TiO 3 NWs reaches to 14.86 J/cc at 450 MV/m, which represents a 42.9% increase in comparison to the PVDF with an energy density of 10.4 J/cc at the same electric field. The capacitors have more than an order of magnitude higher energy density than commercial B OPP capacitors (1.2 J/cc at 640 MV/m) and exhibit faster time to peak power and vastly improved power density. The

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130 techniques presented in this c hapter demonstrate that the use of high aspect ratio nanowires can be used to produce nanocomposite capacitors with greater performance than the neat polymers thus providing a novel process for the development of future pulsed power capacitors.

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131 CHAPTER 5 CONCLUSION S The demand for materials with high energy density and fast discharge speed is rapidly rising due to the requirement of pulsed power devices An example of a research area within the scope of this topic is high energy density and pulsed power capacitors. The energy density of a capacitor is linearly related to its dielectric pe rmittivity and quadraticly to the breakdown strength. However, m onolithic materials are not meeting the increasing demand for flexible, lightweight and compact high energy density capacitors. Th e limitation in energy density is due to the trade off betwe en the dielectric permittivity and breakdown strength. Nanocomposites containing high dielectric permittivity ceramics embedded in high breakdown strength polymers are currently of considerable interest as a solution for the development of high energy den sity capacitors Compared to conventional ceramic or polymer capacitors, nanocomposite capacitors can not only provide lightweight, compact and cost effective devices with higher energy density capabilities, but can also be inexpensively fabricated as lar ge, mechanically flexible and intricate configuration devices using polymer processing techniques Most current nanocomposites are based on a high dielectric permittivity filler in high breakdown strength polymer matrix to improve the energy density. However, the improve ment of the dielectric permittivity comes at the expense of the breakdown strength, which in turn limits the energy density of the capacitor. Therefore, the integration and geometry of the fillers must be optimized together to reach th e highest possible energy density of the nanocomposites. T his dissertation has explored the relationship between structure (aspect ratio and orientation) of the filler and energy

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132 density of the nanocomposites. High energy density nanocomposite capacitors were fabricated by using high aspect ratio nanowires r ather than equiaxial particles as discussed by other researchers. The highest energy density of the nanocomposites fabricated in this work reached as high as 14.86 J/cc, which was an order of magnitud e higher energy density than commercial BOPP capacitors (1.2 J/cc at 640 MV/m) and exhibit ed faster discharged time resulting in higher power density. Th is dissertation provides a novel process for the development of future pulsed power nanocomposite capa citors by using high aspect ratio nanowires with greater performance than the neat polymers Brief Summary of Dissertation and Results This dissertation has investigated the development of high energy density nanocomposite capacitors with fast discharge sp ratio on the energy density of nanocomposites was investigated. High aspect ratio PZT NWs were synthesized by hydrothermal reaction, while low aspect ratio PZT NRs were obtained by decreasing PZT NWs using mo rtal pestle The experimental results demonstrated that both dielectric constant and energy density of the nanocomposites were improved by the hig h aspect ratio fillers. The PZT NWs showed a 77.8% increase in energy density over PZT NRs, under an electri c field of 15 kV/mm and 50 vol. %. This finding generated a lot of interest in this community and many researchers followed this method to prepare high dielectric constant nanocomposites, however, there was no research to quantify the relationship between constant of the nanocomposites. In order to quantify this relationship, a method to control the aspect ratio of BaTiO 3 NWs was the first time to be proposed by adjusting the hydrothermal reaction temperature. The aspect ratio of BaTiO 3 NWs was varied

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133 from 9.3 to 45.3 with respect to the hydrothermal reaction temperature from 150 C to 240 C The results exhibited that the dielectric constant of the nanocomposites increased with increasing aspect ratio of the filler. Nanocomposites with 30 vol.% BaTiO 3 NWs (aspect ratio 45.8) reached a dielectric constant as high as 44.3, which wa s 30.7% higher than samples with low aspect ratio (9.3) and 352% larger than the PVDF matrix. Therefore, using high aspect ratio NWs wa s an effective way to control and improve the dielectric constant and energy density of the nanocomposites. In order to continue to improve the energy density, the effect of orientation on the dielectric constant and energy storage performance wa s studied. T he NWs were aligned in the PVDF matrix by using uniaxial force assembly. The results showed that the dielectric constant and energy density of the nanocomposites were improved by the aligned fillers in the electr ic field direction. The increased dielectric constant with aligned NWs led to an energy density up to 51.6% greater than nanocomposites with ran domly aligned filler at 20 vol.% successfully controlled by the draw ratio of the nanocomposites. Orientation factor was characterized by the f ast Fourier transform analysis of the cross sectional SEM images of the nanocomposites. Following th ese tests the relationship to th nanocomposites was identified for the first time The experimental results demonstrated that the dielectric constant of the nanocomposites increase d with an increas e in the filler orientation factor in the matrix. These results provided another solution to improve the dielectric constant and energy density of nanocomposites by controlling of the orientation

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134 Based on the prior findings two technologies for fabricating high energy de nsity nanocomposite capacitors were developed based on high aspect ratio NWs. The first nanocomposite capacitors wer e prepared by dispers ing BaTiO 3 NWs into a P(VDF TrFE CFE) matrix. The nanocomposites with 17.5 vol.% BaTiO 3 NWs reached an energy density of 10.48 J/cc at an electric field 300 MV/m, which wa s 45.3% higher than that of the neat P(VDF TrFE CFE) polymer (7.21 J/cc at 300 MV/m). However, the high ferroelectric loss and polarization saturation from the filler and matrix limit ed the improvement of energy density of the nanocomposites In order to overcome these limitations a second technique wa s developed. Control over the stoichiometry of Ba x Sr 1 x TiO 3 NWs was exhibited based on a two step hydrothermal reaction. T he paraele ctric phase of Ba 0.2 Sr 0.8 TiO 3 was chosen for NWs combined with quenched PVDF to fabricate high energy density nanocomposite capacitors Compared to BaTiO 3 / P(VDF TrFE CFE) nanocomposites, t his type of nanocomposite maintained high polarization with no satu ration, high breakdown strength and also reduced ferroelectric loss The nanocomposites with 7.5% Ba 0.2 Sr 0.8 TiO 3 NWs were shown to have an ultra high energy density of 14.86 J/cc at 450 MV/m, and provide d microsecond discharge time quicker than commercial BOPP capacitors. The energy density of the nanocomposites exceeds those reported in the literature for ceramic/polymer composites, and wa s 1138% greater than the reported commercial capacitor with an energy density of 1.2 J/cc at 640 MV/m for the current state of the art BOPP. These results demonstrate d that the high aspect ratio NWs could be used to produce nanocomposite capacitors with greater performance than the neat polymers thus providing a novel process for the development of future pulsed power c apacitors.

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135 Contributions Nanocomposites have generated substantial attention for high energy density capacitors; however the improved dielectric permittivity comes at the expense of the breakdown strength thus limiting the final energy density. This dissertation has investigated the structure property relations in the nanocomposites for energy storage and has fabricated high energy density nanocomposite capacitors by using high aspect ratio nanowires. This work has made numerous contributions detaile d in the following paragraph. One dimensional nanostructure of ferroelectric perovskite has been studied with increasing interest due to the potential application in microelectromechanical systems, nonlinear optics and sensors. The application of the nano wires is currently still in the initial phase, and one of the important steps is to produce high quality nanowires. This dissertation reported a series of novel methods allowing for large scale production of wide ranging compositions of perovskite NWs, su ch as PZT, BaTiO 3 and Ba x Sr 1 x TiO 3 Based on these methods, this dissertation is the first report of a simple route to control the aspect ratio of BaTiO 3 NWs by adjusting the hydrothermal reaction temperature. Additionally, the stoichiometry and structur e of Ba x Sr 1 x TiO 3 NWs were successfully controlled in achieving the desired properties for applications, such as the paraelectric phase Ba 0.2 Sr 0.8 TiO 3 These methods for large scale and high quality production of ferroelectric perovskite NWs were expected to provide new alternatives for fabricating new electric devices in the future. Unlike previous theoretical models regarding the effect of aspect ratio, this energy de nsity of nanocomposites. The experimental results showed that the aspect

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136 ratio played an important role in the energy storage of nanocomposites. Both energy density and dielectric constant were improved by high aspect ratio PZT NWs. This finding was als o verified by using high aspect ratio BaTiO 3 NWs and Ba 0.2 Sr 0.8 TiO 3 NWs in this dissertation. This fundamental discovery has led many researchers to follow this route to prepare composites with improved dielectric permittivity However, high aspect ratio fillers still have not been investigated as extensively as spherical fillers because of challenges in manufacturing NWs, especially for aspect ratio larger than 15 Based on the different aspect ratio BaTiO 3 ect ratio and dielectric constant of the nanocomposites was experimentally quantified for the first time. The results clearly showed that the dielectric constant of the nanocomposites was improved with increasing aspect ratio of the NWs. It paved a new w ay to fabricate high dielectric constant and energy density nanocomposites based on high aspect ratio NWs. the energy density of nanocomposites. Compared to conventional alignment of NWs on the substrate, a novel method was proposed to align any NWs in any thermoplastic matrix by uniaxial force assembly The orientation factor was easily controlled by adjusting the draw ratio of the samples The experimental results showed that the dielectric constant and energy density of the nanocomposites were improved by the aligned NWs in the electric field direction. Additionally, this dissertation detailed an image processing method to characterize Orientation Factor based on f ast Fourier transform analysis of SEM images. Unlike other methods requiring complex sample preparation, this method can easily d SEM images.

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137 constant of the nanocomposites was experimentally quantified, which is useful for future nanocomposite design and evaluation of theoretical models. The se results provided another solution to improve the dielectric constant and energy density of the nanocomposites. Following the prior findings, this dissertation has developed two techniques to fabricate high energy density nanocomposite capacitors based o n high aspect ratio NWs, which have never been demonstrated in this form The first technique was prepared by dispersing high aspect ratio BaTiO 3 NWs in ferroelectric polymer P(VDF TrFE CFE), which produced the energy density 1 0.48 J/cc, 45.3% higher than that of the neat P(VDF TrFE CFE) (7.21 J/cc) at an electric field of 300 MV/m However, this type of nanocomposite exhibited polarization saturation and high dielectric loss that limited the maximum energy density. In order to alleviate these restraints a second technique was developed focusing on the elimination of the ferroelectric properties The nanocomposites were based on paraelectric phase Ba 0.2 Sr 0.8 TiO 3 nanowires along with a quenched PVDF matrix The energy density reach ed 14.86 J/cc at 450 M V/m with microsecond discharge speed. These values were significant, since it ha d more than an order of magnitude higher energy density than commercial BOPP capacitors (1.2 J/cc at 640 MV/m) with faster discharge time to peak power and vastly improved po wer density These methods will open a new door to fabricate high energy density nanocomposite capacitors in the future. In closing, this dissertation has made numerous contributions in the fabrication, and characterization of high energy density nanocomp osite capacitors. The synthesis

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138 of ferroelectric nanowires provided a facile method for producing large scale and high quality nanowires to meet the desired properties of NW applications in the future. The investigation of the relationship between struct ure (aspect ratio and orientation) and energy density provided a baseline for fabricating nanocomposites by using high aspect ratio NWs. The use of high aspect ratio NWs and their alignment in the electric field direction provided a new route to fabricate high dielectric constant and high energy density of nanocomposite capacitors. Furthermore, the Ba 0.2 Sr 0.8 TiO 3 NWs allowed the fabrication of nanocomposites capacitors with an energy density as high as 14.86 J/cc at 450 MV/m with microsecond discharge spe ed. This ultra high energy density demonstrated a new way to produce high energy density nanocomposite capacitors based on high aspect ratio nanowires. Recommendations for Future Work This dissertation has performed fundamental research for the developmen t of high energy density nanocomposite capacitors. T he highest energy density of the nanocomposite capacitors reached 14.8 J/cc at 450 MV/mm with an efficiency of 62%. However, t he existing energy loss in the capacitor leads to heating, and consequently degrades the commercial potential, performance and reliability of a capacitor. Therefore, the first recommendation for future work is to develop high energy density nanocomposite capacitors with low loss The suggested nanocomposites can be fabricated by dispersion non ferroelectric filler into polypropylene matrix which is a commercial linear capacitor and has very low loss. It is expected that th is nanocomposite capacitor will have high energy density, low loss and fast discharge speed.

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139 In the case of fabrication high energy density nanocomposites, only the ferroelectric nanowires were chosen as fillers in this dissertation. As mentioned in Chapter 1, there are four different types of D E loops: linear, relaxor ferroelectric, ferroelectric and anti fe rroelectric. Anti ferroelectric material has low remnant polarization and relatively narrow D E hysteresis loop, which implies higher energy storage ability with high efficiency Therefore, it is necessary to fabricate the nanocomposites with high aspect ratio anti ferroelectric nanowires. Hopefully, this can provide another route to fabricate high energy density nanocomposite capacitors with low loss. Lastly, other properties of the nanocomposites could be investigated due to the active fillers PZT. Th e piezoelectric strain coefficient ( d 33 ) and sensitivity of the nanocomposites could be studied in the future work. The goal of this research has been to develop a composite that can perform a wide range of functions including high energy storage and elec tromechanical coupling. This will allow the material system with application s tructural h ealth m onitoring, power harvesting and sensor.

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140 LIST OF REFERENCES [1] Akiyama H, Sakugawa T, Namihira T, Takaki K, Minamitani Y, Shimomura N. Industrial appl ications of pulsed power technology. IEEE Trans Dielectr Electr Insul 2007;14:1051 64. [2] Barber P, Balasubramanian S, Anguchamy Y, Gong S, Wibowo A, Gao H et al. Polymer composite and nanocomposite dielectric materials for pulse power energy stora ge. Materials 2009;2:1697 733. [3] Karden E, Ploumen S, Fricke B, Miller T, Snyder K. Energy storage devices for future hybrid electric vehicles. J Power Sources 2007;168:2 11. [4] Zhang S, Zou C, Zhou X, Anderson D, Zellers B, Zhang Q. Polymer film capacitors with high dielectric constant, high capacitance density, and high energy density. Power Modulator and High Voltage Conference (IPMHVC), 2010 IEEE International 2010:221 4. [5] Zhang S, Zellers B, Henrish J, Rockey S, Anderson D, Zou C et al. High energy density film capacitors. Pulsed Power Conference, 2009 PPC '09 IEEE 2009:779 83. [6] Chu B, Zhou X, Ren K, Neese B, Lin M, Wang Q et al. A dielectric polymer with high electric energy density and fast discharge speed. Science 2006;313:334 6. [7] MacDougall FW, Ennis JB, Cooper RA, Bates J, Seal K. High energy density pulsed power capacitors. Pulsed Power Conference, 2003 Digest of Technical Papers PPC 2003 14th I EEE International 2003;1:513 7. [8] Zheng J, Jow T High energy and high power density electrochemical capacitors. J Power Sources 1996;62:155 9. [9] Li J Zhang L, Ducharme S. Electric energy density of dielectric nanocomposites. Appl Phys L ett 2007;90:132901. [10] Zhou X, Ch u B, Neese B, Lin M, Zhang Q Electrical energy density and discharge characteristics of a poly(vinylidene fluoride chlorotrifluoroethylene) copolymer. IEEE Trans Dielectr Electr Insul 2007;14:1133 8. [11] Ktz R, Carlen M. Principles and applications of electrochemical capacitors. Electrochim Acta 2000;45:2483 98. [12] Chazono H, Kishi H. DC electrical degradation of the BT based material for multilayer ceramic capacitor with Ni internal electrode: impedanc e a nalysis and microstructure. Jpn J Appl Phys 2001;40:5624 9.

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154 BIOGRAPHICAL SKETCH Haixion g Tang was born in Hunan, China He completed his high school at Shaodong No.1 High School in Hunan, China. In the year of 2003, Haixiong entered Harbin Institute of Technology (HIT) to start his fir st year of college, majored in m aterials s cience and e ngineering. Following four years of undergraduate study he received his Bachelors of Science degree in m aterials s cience in July of 2007. After graduation, he was recommended to without the entrance exam in HIT. He spent two happy year s in Institute of Advanced Ceramic under the supervision of Dr. Yu Zhou and Dr. Dechang Jia. He worked as both graduate assista nt and teaching assistant, and focused on research of biomaterials. Following his in China, he came to USA and joined in degree in the mechanical e ngineering department at Arizona State Universit y. In December 2010, he transferred to University of Florida with Dr. Henry Sodano to continue his research towards PhD degree in materials science and e ngineering. The topic of h is PhD research is about investigation of structure property relations for energy storage of nanocomposites. This research is trying to open a new door for high energy density capacitor with fast discharge speed used for future pulsed power application. After 3 years 6 months, Haixiong defended his Doctor of P hilosophy in the s pring of 2013 and looks forward to a career pursuing new t echnologies for energy materials, nanotechnology field.