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Papers from Across the Disciplines: Mechanical and electrical applications of carbon nanotubes

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Papers from Across the Disciplines: Mechanical and electrical applications of carbon nanotubes
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Cruz, Aura G.
Dickrell, Pamela
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Gainesville, Fla.
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University of Florida
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This project investigates the use of carbon nanotube films as potential materials for electrical and mechanical contacts. Carbon nanotubes are closed cylindrical objects that can perform as a passageway for electrical conduction. The mechanical flexibility and electrical contact resistance of vertically aligned carbon nanotube films are experimentally investigated in this study. Results are obtained on a laboratory scale and are discussed based on their potential for tests in real world applications. Two main possible applications considered are low force electrical contacts used in micro-scale electrical devices and brush materials within electrical motors.

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Mechanical and Electrical Applications of Carbon Nanotubes


Aura G. Cruz


College of Engineering, University of Florida


This project investigates the use of carbon nanotube films as potential materials for electrical and mechanical contacts. Carbon
nanotubes are closed cylindrical objects that can perform as a passageway for electrical conduction. The mechanical flexibility and
electrical contact resistance of vertically aligned carbon nanotube films are experimentally investigated in this study. Results are
obtained on a laboratory scale and are discussed based on their potential for tests in real world applications. Two main possible
applications considered are low force electrical contacts used in micro-scale electrical devices and brush materials within electrical
motors.


Introduction

Background. Vertically aligned and multi-walled carbon
nanotubes (MWCNTs) are currently studied for electrical
contact and mechanical applications. This investigation
takes carbon nanotube films and tests their electrical
contact resistance under a static compression load. The
project examines the use of aligned carbon nanotube films
as possible materials for various electrical contact
functions. Multi-walled carbon nanotubes are closed
cylindrical objects consisting of a few graphitic sheets
wrapped around each other with an equal separation
between the layers [1]. The inner diameter of the
MWCNTs ranges from Inm to 3nm and the outer diameter
is approximately 2 nm to about 25 nm [1].

CVD Process. The vertically aligned, multi-walled
nanotube arrays are produced through a chemical vapor
deposition (CVD) procedure. This CVD technique is used
to deposit nanotube films on substrates. As gas
decomposes, it releases carbon atoms, which can
recombine in the form of nanotubes [2]. The original
materials used to make the nanotubes are ferrocene and
xylene [3]. Multi-walled carbon nanotubes are known for
growing on substrates based on a tip growth method. This
process is where the carbon nanotubes adhere to the
surface of the substrate and the particle grows vertically on
the opposite end [4]. The CVD process organizes
nanotubes as semi-aligned films perpendicular to the
substrate surface. Shown in Figure 1, this orientation
allows for more facilitation in their handling and
investigating experimental attributes [5]. The image is
taken as an Scanning Electron Microscopy (SEM) on a
scale of 20 im.

Electrical Properties. Carbon nanotubes can serve as an
alternate pathway for electrical conduction in the
replacement of high-performance electrical brushes [6].
The carbon nanotubes demonstrate flexible properties
similar to the electrical brushes. They can be bent
numerous times through large angles and strains without


disorganizing their arrangement [3]. They are also small,
extremely lightweight, and able conductors of heat and
electricity [7]. In another application, nanotubes have been
examined as brushes used in electrical contacts, where
brush contacts are conducting pads held onto a spinning
metal disc or rod by spring-loaded arms [7]. In this study,
the current is passed from the spinning disc to the nanotube
brush contacts and then to the other parts of the device [7].
This project focuses on the carbon nanotubes showing any
signs of electrical degradation within a certain voltage
range and their mechanical performance to a range of
applied normal loadings.


Figure 1: SEM image of vertically-oriented MWCNTs growing
perpendicularly to the substrate.

Mechanical Properties. Since the discovery of
nanotubes in 1985, properties such as their Young's
modulus, tensile strength, failure processes, and
mechanisms of carbon nanotubes have been the main topic
of research [5]. Previous research states that carbon
nanotubes exhibit large structural flexibility and are less


University of Florida I Journal of Undergraduate Research I Volume 12, Issue 2 I Spring 2011





AURA G. CRUZ


likely to fail when they are placed under large amounts of
bending and strain than other materials useful in
mechanical applications [3]. One important property
previously studied is that a single nanotube contains a
Young's modulus of 1.2 TPa [5]. Once the MWCNTs are
bent, their film-thickness recovery happens faster than the
general recovery rate for most flexible foams. Their
compressive strain is a lot higher than typical low-density
flexible foams that are capable of sustaining large strains
[3]. In a study conducted on their compressible property,
vertically aligned films of carbon nanotubes tend to fold
into waves, or buckled folds [3]. SEM images in that study
show how the waves form during the first cycle of
compression and then work cohesively to remain
unchanged after 200 cycles of compression. Even after
>1000 cycles of compressive forces acting upon that
particular carbon nanotube film, the folds are still visible
[3].

Challenges

One of the main challenges that carbon nanotubes face in
the real world is that many applications require a large
production of nanotubes for the prospective use in devices
[5]. One application includes composites and hydrogen
storage, which requires high quality nanotubes to be
reproduced by the ton or kilogram. The fabrication of the
nanotubes includes self-assembly techniques and plans of
restricted growth [5]. Many studies have proved that the
CVD process will meet expectations to produce tons of
high quality MWCNTs [5].
Other challenges existing that vertically-aligned multi-
walled carbon nanotubes may address is in the field of
MEMs based electrical switches, where the occurrence of
electrical noise/degradation, surface adhesion, and
electrical interface wear are reliability issues. Noise is an
electronic property that can affect the performance of small
electrical switches. Adhesion is another challenge, which is
necessary for conduction across electrical contact surfaces,
but also needs to be controlled so electrical contacts can
separate easily when needed. Too little or lowering of
adhesion can be caused by aging of the contacts due to air
and environmental conditions [8]. On the other hand, too
much adhesion explains the reduced lifetime and
functionality at high bias voltage with actual capacitive
MEMS switches [8].

Experimental Approach

Carbon nanotubes are grown vertically on a substrate
using a CVD process. They range in electrical conductivity
and are not perfectly aligned (Figure 1). The diameters of
the carbon nanotubes in this investigation vary from 10-20
nm. Their closed-form, non-reactive structure and their
supercompressibility makes these carbon nanotube films
potentially useful in high current-density switching


functions [6]. When studying current-carrying applications,
carbon nanotubes distribute themselves across the footprint
of the contact area, and their high elasticity permits films
under compression to provide a multi-faceted interface for
electrical conductivity [6]. Just one carbon nanotube can
carry a current of 25 piA, which pertains to an extreme high
current density of about 109 A/cm2 [4].
To test the electrical resistance of carbon nanotubes, this
project uses a four-point electrical contact resistance
measurement to source current and measure voltage and
relate through Ohm's Law, which is
V
R = - maintaining unitsoffl (Ohms)

Ohm's Law is used in the examination of these carbon
nanotubes films as a group of electrical contact points to
determine the resistance they have to certain levels of
current. The carbon nanotubes are electrically examined in
the open air, each cycle with the same climatic conditions.
Certain motors require high current-density sliding
electrical contacts against a smooth counter surface. To
experimentally simulate such contacts, carbon nanotubes
are brought into contact with a smooth, rounded surface.
The image shown in Figure 2 depicts how the test is set up.
The gold sphere shown is used to make its curved surface
come into contact with the flat nanotube surface. The gold
sphere is placed on a glass cantilever that moves down to
come into contact with the carbon nanotubes. This test uses
the four-wire electrical contact resistance measurements
because it assumes that only the contact resistance
dominates. The test also neglects the resistance through the
films and wires.


conductive
adhesive


glass cantilever
gold
sphere


current
source

copper
wires D


glass plate


MWNT
film


traditional 4 point measurement neglects
resistance through the films and wires
(assumes contact resistance dominates)


Figure 2: The glass cantilever that contains the gold sphere moves
downwards to come into contact with the MWCNT film. The copper wires
attached to the conductive adhesives measure voltage and current
through a source that reads the exact amounts.

Test Matrix

Experiments were structured to investigate a range of
mechanical and electrical loading conditions on the
MWCNT films.
The mechanical and electrical specifications for a normal
test contain all of the specifications shown in Table 1. The


University of Florida I Journal of Undergraduate Research I Volume 12, Issue 2 I Spring 2011





MECHANICAL AND ELECTRICAL APPLICATIONS OF CARBON NANOTUBES


cycles are run a few times to make sure that the results are
repeatable.

Table 1: Initial Conditions for Testing the Electrical and Mechanical
Properties of the MWCNTs
Category Specifications
Cantilever Serial Number GL -062
Cantilever Material GI
Cantilever Range Low Load
Contact Surface Gold-Coated (SiN4i 4 Ball
Pin Radius 7.78 mm
Lateral Stiffness 0.034900 mN/jpm
Normal Stiffness 0.077900 mN/ipm
Time Seconds
Tip Displacement ipm
Fn (normal load) mN
Velocity pm/s

Table 1 also shows the mechanical and electrical
information that is attained through the studies. When the
tests begin, the time (seconds), the tip displacement (jim),
the applied force Fn (mN), and the velocity of the contact
(jim/s) are formulated in a Microsoft Excel spreadsheet
with the other testing conditions. These measurements led
to displacement and resistance results.

Results

The responses attained through this project contain the
tip displacement in the nanotubes through a certain load
applied and the resistance levels that the carbon nanotubes
hold at a certain voltage and current series. Two items are
examined in the experimental results. The first, shown in
Figure 3, is the load-displacement curve for mechanical
loading/broadly indenting the nanotube films. The second,
shown in Figure 4, is the electrical contact resistance seen
across the nanotube-gold sphere interface under four peak
indentation loads.


0.12

- 0.10

-E 0.08


o 0.02
.o 0.06

E 0.04

c 0.02


Displacement y=0.07
was prescribed
up to .10 mN.








1.0 0.2 0.4 0.6 0.8
displacement (gnm)


Figure 3: The normal load force measured in mN
displacement measured in pm.


79x +8E-1 0


IOU
S140
c 120
U
i 100
S80
t 60
c 40 -
0
20

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
normal load (mN)

Figure 4: The points on the graph indicate that as the normal load
increases, the contact resistance measured in Ohms decreases.

Results of Mechanical Properties. An indent was
made to a maximum load of 0.1 mN of the 3 mm gold
sphere into the vertically aligned nanotube film to look for
mechanical response. The best fit line to the load
displacement data returned a slope of 0.0779 mN/jim. This
slope is the same as the normal stiffness of the cantilever
used to apply the normal load, indicating that the MWCNT
film was not indented far enough to gauge a measurable
mechanical response at the 0.1 mN normal load. This
indicated that future testing at normal load values above
the 0.1 mN level will be required to obtain quantifiable
mechanical response of these aligned MWCNT films.
Since carbon nanotubes have supercompressible
capabilities, they are able to resist a large amount of force
from a load and still maintain a positive linear
displacement [3]. After repeated cyclic contacts, the carbon
nanotubes are able to fold into a wavelike form and can
endure thousands of cycles without fracture or shear. A
study done on their compressible characteristics
demonstrates that at a speed of 2000 ipm/s a surface came
into contact with the film, and after over 1000 cycles the
thickness of the nanotubes only decreased from 860 jim to
720 jim [3]. These unique mechanical results demonstrate
that the film of MWCNTs shows a large resistance to many
kinds of deformation [3].


Results of Electrical Properties. Out of the five
,alo cycles, the fourth one was chosen to demonstrate the
resistance levels shown in Figure 4 since the nanotubes
films behaved repetitively. The study kept the voltage at
0.50 V and the current source at 1.00 mA. At a load of 0.1
mN, the resistance remained at over 140 Q. Once the
normal load hit 0.2 nM, the resistance at the contact site
dropped to 75 Q. When the force keeps growing at 0.5 mN,
the resistance is at less than 50 Q. When the normal force
1.0 1.2 is at about 0.75 mN, the resistance is shown at 40 Q. It is
crucial to point out that this trend is nonlinear and extends
from top to bottom as the normal force grows. As the area
with increasing of contact rises and more force is applied on the nanotubes,
the resistance drops. As shown in Figure 4, this nonlinear


University of Florida I Journal of Undergraduate Research I Volume 12, Issue 2 I Spring 2011





AURA G. CRUZ


trend demonstrates that as more force in mN is applied on
the MWCNTs, the resistance levels drop as well.
Studies have been done on a larger scale where
MWCNTs are being researched for their current-carrying
capacities and their display of having no resistance change
under high temperatures and long periods of time. A study
done on the resistance of MWCNTs to certain current
densities showed on a large scale that their electrical
properties are similar to this project's. The nanotubes
displayed resistance on two carbon nanotubes of 2.4 kQ
with a constant voltage of 25 V and a current of 10.4 mA
[9]. The 2.4 kQ resistance level remained constant after
various cycles, which indicates the high stability of carbon
nanotubes to serve in electrical applications.

MEMS Applications. The results from this study provide
a better understanding of MWCNTs and prove that they
can be used as the base of electrochemical brush contacts
and switches. In a large application where nanotubes are
integrated as electrical contacts, it was found that after 0.1
million cycles the nanotubes can reduce contact stresses as
the brushes touch a solid surface during each cycle [10].
The nanotubes are part of an electrically-driven brush,
which is fixated on where the head of where a motor
rotates [10]. The brushes undergo rotational tests in which
they retained their structure and the nanotubes did not shed
[10]. Some of these conductive brushes are used in parts of
commutators where they maintain an electrical bond in the
rotary and sliding contact applications [10].


References


[1] A. Loiseau, P. Launois, P. Petit, S. Roche, J.P. Salvetat, Understanding
Carbon Nanotubes: From Basics to Applications. New York: Springer Berlin
Heidelber, 2006, pp.53, 221.

[2] M. Ali, "A review of current carbon nanomaterials and other nanoparticle
technologies,"unpublished. [Online]. Available http://faculty.kfupm.edu.sa/
CHE/motazali/files/Nanotechnologysynthesisstructures andproperties.pd
f, pp.49.

[3] A.Y Cao, P.L. Dickrell, W.G. Sawyer, M.N Ghasemi-Nejhad, P.M. Ajayan.
(2005). Super-compressible foamlike carbon nanotube films," Science, 310,
pp.1307-1310.

[4] M.J. O'Connell Carbon Nanotubes: Properties andApphlcations, Boca Raton:
Taylor & Francis Group. 2006, p.88.

[5] H Dai, "Carbon nanotubes: opportunities and challenges," Surf Sci., vol. 500,
no. 1-3, pp. 218-241. 2002

[6] G. Toth, J. Miklin, N. Halonen, J. Palosaari, J. Juuti, H. Jantunen, K. Kordas,
W.G. Sawyer, R. Vajtai, P.M. Ajayan, "Carbon-Nanotube-Based Electrical
Brush Contacts," Adv. Mater., vol. 21, pp.2054-2058, Mar. 2009.


Future Work

The applications for which carbon nanotubes are
currently being studied are endless. Carbon nanotubes are
tested for their electrical capability of acting as brushes
within electrical motors and their design for the use of
MEMs switches. Their synthetic technology makes the
nanotubes worth researching because they are low cost;
comprised of certain controlled properties, such as lack of
entanglement; and are easily separable.
Studies of carbon nanotubes prove that they can replace
various materials used in today's technology. The physical
and electrochemical characterization of the surfaces
demonstrated the well alignment of the nanostructures and
highly improved electron transfer properties with respect to
other carbon based electrodes [11]. Testing carbon
nanotubes can prove if they are capable of replacing
indium tin oxide, which is the main material used to make
transparent conductive coatings for touch panels, plasma
displays, or solar cells. The carbon nanotube films have
proved to be conductive and sufficiently transparent in the
visibility range to replace indium tin oxide [12]. The
military would benefit from this breakthrough for their
infrared sensors, cameras, and projectors. Future studies of
carbon nanotubes will expand their use in replacing
popular materials that require high mechanical stiffness
and high electrical contact resistance.


[7] J. Boyd, "Nanotubes find niche in electrical switches," EurekAlert!, (10 Mar.
2009). Available http://www.eurekalert.org/pub releases/2009-03/ru-
nfn031009.php

[8] S.T. Patton, J.S. Zabinski,., 2006, "Failure mechanisms of DC and capacitive
RF MEMS switches," Proc. Spze vol. 6111, issue 1, pp. 61110E. Jan. 2006.

[9] B.Q. Wei, R. Vajtai, P.M. Ajayan, P. M., "Reliability and current carrying
capacity of carbon nanotubes," Appl. 'i .. Lett., vol. 79, no. 8, pp.1172,
2001.

[10] A. Cao, V.P. Veedu, X. Li, Z. Yao, M.N. Ghasemi-Nejhad, P.M. Ajayan,
"Multifunctional brushes made from carbon nanotubes," Nat. Mater., vol. 4,
no. 7, pp.540-545, 2005.
[11] F. Berti, L. Lozzi, I. Palchetti, S. Santucci, G. Marrazza, "Aligned carbon
nanotube thin films for DNA electrochemical sensing," Electrochim. Acta,
vol. 54, no. 22, pp. 5035-5041, 2009.

[12] L. Hu, D.S. Hecht, G. Grfiner, "Infrared transparent carbon nanotube thin
films," Appl. _.'i .. Lett., vol. 94, no. 8, pp.081103, 2009.


University of Florida I Journal of Undergraduate Research I Volume 12, Issue 2 I Spring 2011