Fabrication of a Shear Stress Sensor Using Vertically Aligned Barium Titanate Nanowire Arrays

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Fabrication of a Shear Stress Sensor Using Vertically Aligned Barium Titanate Nanowire Arrays
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Annual UF Undergraduate Research Symposium
Cheng, Peishi
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Conference Poster


Shear stress sensors are highly relevant to fluid dynamics, especially in cardiology where the wall shear stresses measured by these devices have been implicated in the causes of certain cardiovascular diseases like atherosclerosis. Piezoelectric barium titanate nanowire arrays offer the potential for the fabrication of sensitive, robust, and direct measurement shear stress sensors by virtue of their high aspect ratio and high coupling coefficient. However, the application of a conformal top electrode to these nanowire arrays poses a challenge. In this work, a conductive polymer, PEDOT:PSS, was used to create a conformal top electrode for a prototype sensor. PMMA was spin coated into the nanowire arrays to form a barrier layer to prevent device shorting and enhance the mechanical stability of the device. This nanowire-polymer device was characterized under dynamic vibration testing for both axial and lateral (shear) modes. The spectral characteristics and sensitivity of the device were measured and it was shown that the device had less than unity coherence but a peak sensitivity of 40.86 mV g-1. Still, the device showed significant departure from frequency dynamic sensor ideals of a large flat band region. Further work must be completed to determine whether the stiffness of the PMMA matrix inhibits full functionality of the device.
Collected for University of Florida's Institutional Repository by the UFIR Self-Submittal tool. Submitted by Peishi Cheng.
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Center for Undergraduate Research
Poster presented at the 2015 UF Undergraduate Research Symposium

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Piezoelectric materials, Greek for and generate a measureable voltage upon application of a stress Barium titanate is a common piezoelectric material which can be synthesized in the form of a nanowire array This geometry is highly anisotropic and offers the potential to be integrated into a highly sensitive shear stress sensor Fabrication of a Shear Stress Sensor Using Vertically Aligned Barium Titanate Nanowire Arrays Peishi Cheng and Henry Sodano* Materials Science and Engineering, University of Florida, Gainesville, Florida, 32611, United States Interdisciplinary Microsystems Group, University of Florida, Gainesville, Florida, 32611, United States Introduction and Background 1. http :// media/File :PmdsStructure.png 2. By DrTorstenHenning (Own work, drawn with bkchem ) [Public domain], via Wikimedia Commons 3. http :// / media/File:Polythiophenes_Pedotpss.png 4. Plass 2009, 21 (3) p325 328. DOI: 10.1002/adma.200802006 References Conclusions This work was supported by the University Scholars Program and previously by the HHMI Science for Life Program We would also like to thank the expertise of the Interdisciplinary Microsystems Group members, in particular of Dr Mark group Approach Frequency Response Applying Polymer Electrodes Testing Setup Device Testing 300 nm 300 nm Challenges are posed in the fabrication of such a sensor Finding suitable substrate to bond nanowire array Creating conformal top electrode Promising techniques: Sputtering thin layer of metal on NW array Conductive polymer viscous and can be cured. PEDOT:PSS or polyaniline Using buffer layer of polymer spun in between nanowires like PMMA or PDMS Frequency response plots device characteristics in the frequency domain: Magnitude response : ratio of voltage produced to input excitation Phase response: the phase angle between the input signal and output Coherence: attributable to excitation. Measure of linearity. Left: BaTiO 3 nanowire array Top: accelerometer configuration PEDOT:PSS is a mixture of two ionomers poly(3,4 ethylenedioxythiophene) polystyrene sulfonate was used as a conformal conductive top electrode The insulating barrier layers tested were: PDMS and PMMA poly(methyl methacrylate) and polydimethylsiloxane Polymer Electrodes Above: frequency response of industrial PVDF film sensor Right PEDOT:PSS structure [3] Spin Coating PDMS Barrier Layer 1. Synthesize polymer mixing resin and hardener 2. Dilute with toluene to reduce viscosity [4] 3. Spin coat with static dispense 4. Sit overnight in vacuum dessicator 5. Cure in convection oven at 80 o C OR Spin Coating PMMA Barrier Layer 1. Same process but PMMA dissolved in toluene from polymer pellets Etch the barrier layer with O 2 plasma (PDMS is difficult to etch) Spin coat PEDOT:PSS Pole NW devices Above Left: PMMA structure [2], Right: PDMS structure [1] Image Captions 1. NWs coated in PDMS, unetched 2. NWs in PDMS, etched and covered with PEDOT:PSS 3. NWs in PMMA, etched and covered with PEDOT:PSS 20 m NWs in PDMS 1 20 m NWs in PDMS PEDOT:PSS 2 20 m NWs in PMMA PEDOT:PSS 3 Successful device of PMMA layer Coherence is only near unity or unity at high frequencies Poor response at low frequencies No resonant peak in magnitude response (FRF) No good sensor region Sensitivity: Maximum of 32.04 dB = 40.86 mV g 1 dB = 20 log (ref) where ref = 1V/g for 0 dB Stiffness of PMMA is likely inhibiting efficient transduction of force to the nanowires, preventing magnitude lower than PMMA [4][5]. Future work will involve verifying etch processes for PDMS and fabricating NW in PDMS devices. 5. http : //www mit edu/~ 6 777 /matprops/pmma htm 6. http : //www mit edu/~ 6 777 /matprops/pdms htm Acknowledgements