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Copyright 2018. Used by CAMX – The Composites and Advanced Materials Expo. CAMX Conference Proceedings. Dallas, TX,
October 15-18, 2018. CAMX – The Composites and Advanced Materials Expo
POST-GROWING CNTS ON CNT WIRES TO STUDY THE
PHYSICAL PROPERTY CHANGES
Md. Nizam Uddin, Maninder Dhillon, Heath E. Misak and *Ramazan Asmatulu
Department of Mechanical Engineering
Wichita State University, 1845 Fairmount, Wichita, KS, 67260
*Email: ramazan.asmatulu@wichita.edu
ABSTRACT
Carbon nanotubes (CNTs) possess the huge potential in becoming a viable replacement for
materials in a wide variety of mechanical, electrical and thermal applications. CNT wires (yarns)
made of individual CNTs are able to carry power over vast distances, and the amount of power
lost from heat or resistivity could be significantly reduced along with the amount of materials
needed. CNT wires can be used in satellites, space stations, and other low earth orbit
applications. Studying the viability of such applications would require some additional methods
to reduce the overall size and weight of the objects being sent into the space. The aim of this
study was to investigate the mechanical and electrical properties of CNT wires after post
growing of CNTs on 60 plies CNT yarns/wires. The CNTs were grown on the CNT yarn by
chemical vapor deposition (CVD) process and it was observed that “fuzzy” CNTs were produced
vertically on the CNT wires. The CNTs growth process was simulated and optimized for each
test condition. The experimental result showed that some variations of the ultimate tensile
strength (UTS) and the tenacity were observed by growing CNTs on the untreated yarns. The
CNT yarns have potential in composite materials as the yarn can produce a good interface
between the matrix and the yarns.
1. INTRODUCTION
Carbon nanotubes (CNTs) have been a topic of hot debate and research over the past two
decades when they were first observed by Sumio Iijima from the NEC Cooperation in 1991 [1].
Due to the CNTs’ structures, they possessed the potential in becoming a viable replacement for
various materials in a wide variety of mechanical, electrical and thermal applications. The carbon
atoms in CNTs are constructed with sp2 bonding which allows for a variety of amazing structures
[2]. Depending on the structure of the CNTs, they can have a very high tensile strength that is
between 13 and 53 GPa with a modulus of 1 TPa [3]. Along with being stronger than steel, CNTs
can have electrical conductivity and current density that can be better than copper while having
thermal conductivity similar to that of a diamond or higher. With researchers trying to create
CNTs that excel at varies properties, CNTs have been created to have electrical resistivity down
to 10-7 Ω.m while being able to carry current between 104 and 105 A/cm2 [4]. Along with having
their electrical conductivity between 150 to 8300 S/cm and thermal conductivity of over 2000
W/mK have been observed [5-10].
One of the many ways to produce CNTs is to use a chemical vapor deposition (CVD) process.
The CVD process is one of the oldest methods used in producing CNTs. Originally, this method
was employed to apply solid thin-film coatings on to surfaces to produce very high-quality bulk
material and powders efficiently and cheaply. Due to its ability to produce very controlled and
high-quality materials, it was the perfect choice for producing CNTs. These CNTs are produced
by having a catalyst be placed within the tube or is mixed into the carbon solution that is pumped
into the chamber to fuel the reaction with the catalysts. There are many different ways of
conducting the CVD processes, but all rely on having a chamber for containing samples or
substrates while heating elements are placed around it with an inlet and outlet for gases to flow
through [9].
With the CVD method, there are three main factors that control the growth rate and morphology
of CNTs no matter what the mixture or catalyst is being used. Those three factors are reaction
temperature, flow rate and the concentration of the metal catalyst [10]. To examine these three
factors further, a recent study should be reviewed [10]. The authors have used a plain carbon
source while using ferrocene as a catalyst to produce single wall carbon nanotubes (SWCNTs).
First, the temperature effects were observed using a steady flow of 1000 sccm (standard cubic
centimeters per minute) and a ferrocene concentration of 10% while adjusting the temperature
setting between 900ºC and 1000ºC. It was noted that the deposition rate of carbon increased as
the temperature amplified. This leads to the catalyst having an abundance of carbon. Since the
extra carbon is left in the system, the surplus of carbon was able to create amorphous impurities
on the CNT surface. As these impurities stacked on to the surface of the CNT, the diameter
increased from 1.2 to 1.5nm [10]. Next, the flow rate was observed by fixing the temperature and
ferrocene content while running the experiments at 400, 800, 1000 and 1200 sccm. It was noted
that as the flow rate was increased, the amount of defects on the surface of the decreased. As the
flow rate of the gas was increased, there was less time for the carbon molecules to condense onto
the metal catalyst [10]. Finally, the effects of adjusting the ferrocene concentration in ethanol
while the flow rate and the temperature remained constant. As the concentration was changed, so
did the diameter of the CNTs. As more catalyst was introduced into the mixture, the diameter
also increased. The reason behind this was that as more ferrocene was present, larger catalyst
particles can be formed during the condensing process which in return allows for larger carbon
nanotubes (e.g., multiwall carbon nanotubes - MWCNTs) to be formed. By analyzing and
optimizing these factors, a consistent and defect free CNTs can be produced.
The CNT yarn is manufactured by using a wet spin method where CNTs are dispersed in a
solution that spun to strains, and then farther spun into yarns. The idea behind this approach was
to develop an easy way to utilize the extremely high tensile strength and Young’s modulus of
CNT yarns. Nevertheless, the major drawback of this method was the difficulties in the
dispersion process, low use of CNTs yarns and the reduction in the electrical and thermal
conductivity from the CNTs being covered with surfactant or polymer molecules [11-18]. To
combat the disadvantages of wet spinning, methods for dry spinning were improved. Dry
spinning is prepared during the manufacturing process of the CNTs and allows for super-aligned
CNTs [6]. CNT yarns can be used in satellites, space stations and other low earth orbit
applications [7]. CNTs can take this trend for light and stronger composite to a whole new level
by being woven into a fabric that can be used in composite layups and has a density of 0.2-0.4
g/cm3 that is much lesser than carbon fiber with a density of 1.8 g/cm3 [8].
In this study, a quartz tube furnace was used for the CVD process. With this tube furnace,
premade CNT wire (yarn) was placed inside the tube while a carrying gas was pumped into the
system along with an ethanol-based solution. The purpose of this was to use the preexisting
catalyst left over on the CNT samples to grow more CNTs on in the hopes of improving the
mechanical and electrical properties.
2. EXPERIMENT
2.1 Materials
The CNT yarns used in this study was untreated 60-yarn, which was manufactured by Nanocomp
Technologies, Inc. Since the material was received as untreated, no additional process was
conducted on the CNT wires after the spinning process. Due to this, the individual strains of the
yarn still had the original catalysts used for the production of the CNTs. As the catalyst is still
present on the yarn, no extra step would be needed to introduce catalysts on the CNT yarn. This
was an important aspect in regards to the success of this experiment, cost and the scalability. For
the growth of CNT laterally/vertically, a carbon source was needed to fuel the reactions with the
catalyst during the CVD process. To ensure ease and availability, 200 proof,
HPLC/spectrophotometric grade ethyl alcohol was used. This mixture has less than 0.2%
impurities and less than 0.001% evaporation residue which allows for a very clean consumption
as the ethyl alcohol flows through the chamber. Ethyl alcohol was mixed with distilled water as
an additional hydrogen source for all of the experiments. The purpose of having the distilled
water mixed with the ethyl alcohol was to provide additional hydrogens during the reaction to
improve the chances of pyrolysis of ethyl alcohol into pure carbon. For the CVD process to
work, an inert gas was needed to flow through the system and act as a carrier for the mixture
from the inlet to the outlet. The gas chosen to perform this roll was argon due to its inertness,
availability and cost. Figure 1 shows the scanning electron microscopy (SEM) images of 60-yarn
CNT wires subjected for the lateral CNT growth [19].
Figure 1. SEM images showing the 60-yarn CNT wires (as-received) subjected to the
lateral/vertical CNT growth.
2.2 Methods
2.2.1 Experimental Setup
The equipment used in this study is the MTI Corporation OTF-1200X quartz tube furnace. This
furnace was designed to allow for heating temperatures up to 1200ºC with a pressure gauge on
the inlet flange. The original setup came with a flange on each side for a 50mm OD quartz tube
with 1/4” bard fittings. These fittings on the inlet side were modified to allow up to three
continues sources of flow. The first two bards on the new inlet side were connected to flow
regulators for the argon and hydrogen tank while the third bard was connected to a syringe
attached to a syringe pump to allow for a continuous and even flow of the ethyl alcohol mixture.
Between the bard and the syringe, a shutoff valve was placed to prevent the liquid from being
pulled into the tube during the vacuuming process. On the exit side of the tube furnace, the
originally bard was used to run a pipe from it to a three-way valve that connected to the vacuum
pump and a bubbler system. The valve setup can vacuum while not reducing the vacuum in the
tube. The sealant was wrapped around the connections to prevent any leaks. Between the three-
way valve and the bubbler system, a two-way valve was placed to prevent the mineral oil from
being sucked into the quartz tube after the vacuum valve was closed. In order to prevent the flow
of the mineral oil through the piping, argon was slowly pumped back into the system until
atmospheric pressure was achieved. Finally, the exhaust from the tube furnace is led into the
bubbler system to prevent any diffusion of oxygen back into the system. The bubbler system is
composed of a simple jar that had sealant placed around the outside and inside of the lid. The
pipes were put into place by drilling holes on the top of the lid and sealant placed on both side of
the lid around the inlet and outlet. The schematic diagram of the experimental setup is presented
in Figure 2 [19].
Figure 2. Schematic diagram of the experimental setup for vertical CNT growth on CNT wires.
2.2.2 Laterally Growing of CNTs on CNT Wires
For the lateral growing of CNTs on the CNT wires, the first step is to create the ethyl alcohol
mixture containing 300 parts ethyl alcohol and 1 part distilled water under a fume hood and then
it was placed in a plastic syringe. Next, the syringe was taken to the syringe pump and connected
to the piping system. When the pipe was connected to the syringe, the sealant was utilized to
prevent any leaks between the connections. After the syringe had been attached, some ceramic
tiles were placed within the quartz tube to allow for a greater heat absorption in the glass tube.
These tiles absorbs a bulk amount of the radiation from the heating elements to help pyro
carbonization. Then the sample (CNT yarn) was placed in the center of the quartz tube and then
side flanges were attached. It is important to note that the side flanges can only be attached after
the furnace has been closed and locked. Now all the valves are turned closed with the
expectation of the three-way valve connecting the vacuum pump to the system. This will allow
the pump to apply a vacuum to the tube without having liquid being sucked up into the quartz
tube either from the syringe or the oil from the bubbler system. By reading the pressure dial on
the inlet flange and once the pressure gauge has stabilized at a negative pressure reading, the
vacuum pump is turned off and the three-way valve is turned on to allow flow between the tube
furnace and the two-way valve. Then, argon gas is slowly allowed into the system. As the
pressure starts to approach zero, the two-way valve is opened. When the furnace has reached the
required temperature, the syringe pump is turned on to allow liquid to flow into the quartz tube.
Table 1 summarizes the fabrication parameters for the lateral growing of CNTs on CNT yarns
[19].
Table 1. Fabrication parameters for the lateral growing of CNTs on CNT yarns.
2.2.3 Mechanical and Electrical Testing
Tensile tests were conducted on both as-received CNT wires and surface treated samples using a
MTS Tytron 250 bench type unit (dynamic) with a 45 N load cell and hand-made tensile testing
Number
Temperature
(°C)
Duration
(H)
Liquid Flow Rate
(mL/H)
Gas Flow Rate
(cm
3
/min)
Ceramic
Blocks
Water
1
650
1
8
4700
No
Yes
2
800
1
8
4700
No
Yes
3
800
3
8
4700
Yes
Yes
4
800
3
12
5300
Yes
Yes
5
800
3
12
4000
Yes
Yes
6
800
3
12
3000
Yes
Yes
7
800
0.5
12
4000
Yes
Yes
8
800
1
12
4000
Yes
Yes
9
800
2
12
4000
Yes
Yes
10
800
1
12
4000
Yes
No
11
800
1
12
4000
Yes
No
12
625
1
12
4000
Yes
Yes
unit (static). The electrical conductivity of the CNT wires was measured via Keithley resistivity
measurement device. Figure 3 shows the MTS Tytron 250 test, and four-point conductivity unit
used for the mechanical and electrical characterizations of CNT nanowires. The CNT wires with
a length of approximately 50 to 100 mm were placed on the grips of the MTS unit and loaded at
different conditions while measuring the mechanical strength and electrical resistivity of the
CNT wires before and after lateral CNT growths.
a)
b)
Figure 3. Photographs showing a) the MTS Tytron 250 test, and b) four-point conductivity
(Keithley) units used for the mechanical and electrical characterizations of CNT nanowires.
3. RESULTS AND DISCUSSION
3.1 Mechanical and Electrical Properties of As-received CNT Yarns
The as-received CNT yarn (without any vertical CNT growths) was tensile tested and
representative stress-strain curve is plotted in Figure 4 for tensile stress and tenacity calculations.
The untreated CNT yarn has an ultimate tensile strength (UTS) of 221.27 MPa and a maximum
tenacity of 38.55 N/tex. Stress and tenacity values provided almost the same trends. The
electrical resistivity of the as-received CNT yarn was found to be around 0.278 Ω-m. These
experiments were conducted using a gauge length of 43 mm (CNT wire length) [19].
Figure 4. Representative (a) stress-strain and (b) tenacity-strain curves of as received CNT yarn
3.2 Analyzing the Vertical CNT Growths on CNT Yarns
In order to achieve the optimum fabrication parameters, several trial tests were performed with
different catalyst, operating temperature, gas flow rate and other necessary setup of the furnace.
Also, to simplify the CNT production process on the sample, the ferrocene would be removed
from the mixture since it would rely on the preexisting catalyst on the CNT yarns. Besides from
removing the ferrocene, hydrogen would be eliminated as well by adding distilled water to ethyl
alcohol mixture. This process likely allowed to increase the safety of the experiment by
removing only chemically combustible component. Finally, the CNTs were laterally grown on
CNT yarn at 800°C furnace temperature based on the process described above. In this process,
“fuzzy” CNT yarn was produced and the SEM images of a dry-spun CNT yarn and fuzzy CNT
yarn are presented in Figure 5.
Figure 5. SEM images of a) as-received 60-plies CNT yarn, and b) the 60-plies CNT yarn called
“fuzzy”.
The CNT production for the CVD is greatly influenced by the flow rate and increasing the
temperature beyond the threshold would only damage the CNT yarns to a greater extent. The
amount of catalyst being used could not be considered since the preexisting catalysts in the
system was used during the process. The effect of the flow rate on the CNT production was
investigated and SEM images of the CNT yarn are illustrated in Figure 6.
Figure 6. SEM images of the “fuzzy” CNT yarns obtained at different liquid flow rates: (a) 3000
cm3/min; (b) 4000 cm3/min; and (c) 5300 cm3/min.
From Figure 6, we can also see that flow rate significantly affects the growth of CNT on CNT
yarns, and higher flow rate leads to more CNTs growth on CNT yarn among the three different
flow rates investigated (Figure 6c). A similar effect was observed by changing the different
runtime as shown in Figure 7. Longer run time produces more CNTs on CNT yarn, which may
be beneficial for physical property improvements of the new CNT-based nanomaterials. Based
on the flow rate and test durations, the length of the CNTs on CNT wires were substantially
changed.
Figure 7. SEM images of the “fuzzy” CNT yarns obtained at different runtimes: (a) 0.5hr; (b) 1.0
hr; and (c) 2 hr.
3.3 Mechanical and Electrical Properties of CNT Yarns after Laterally CNT Growths
Figure 8 shows the stress-strain and tenacity-strain curves of laterally grown CNTs on 60 yarn
CNT wires. The fuzzy CNT yarns exhibited the UTS of about 145.7 MPa and tenacity of 37.1
N/tex. While the tenacity value remains very similar to the untreated yarn, the UTS vale of the
treated CNT yarn was drastically reduced from 221 MPa to 145 MPa. This may be because of
the thermal decomposition effects (based on trapped free oxygen in the system), untangling of
the CNT yarns and flimsiness during the vertical CNT growths.
Figure 8. Images showing a) stress-strain and b) tenacity-strain curves of laterally grown CNTs
on 60 yarn CNT wires.
In addition to these studies, the electrical resistivity of the fuzzy CNT yarns was also measured
before and after CNT growths. The electrical resistivity of the as-received CNT yarn was about
0.278 Ω-m, whereas for the fuzzy CNT yarns, it was about 1.66 Ω-m, which indicates some
alterations on the electrical resistivity of the CNT wires. More studies will be conducted in these
fields to determine the electrical and thermal behaviors of CNT yarns before and after post-CNT
growths.
4. CONCLUSIONS
CNTs have been the subject of hot research over the past two decades. The reason behind this is
their potential to not only replace the currently used materials, but easily outperform the current
competitions. Along with being superior to their current counterparts, CNTs can be easily
modified to provide new or improved physical and chemical properties. CNT wires can carry
power over vast distances, and the amount of power lost from heat or resistivity would be
significantly reduced along with the amount of materials need. Another application of CNT wires
would be their use in satellites, space stations, and other low earth orbit applications. In this
study, the CNTs are successfully grown on the 60-yarn CNT wires using CVP process. The
mechanical and electrical properties of the produced CNTs on CNT yarns were measured before
and after CNT growths. The produced CNTs on CNT yarn (fuzzy CNT yarn) exhibit almost the
same tenacity value, but provide the lower mechanical and electrical properties when compared
to as-received CNT yarns. This may be because of the higher operating temperatures of the CNT
growth process and decompositions. More work is necessary to optimize the production process
to produce the higher quality CNTs on CNT yarns for improved the physical properties of new
materials and devices.
ACKNOWLEDGEMENTS
The authors gratefully acknowledge AFIT/ AFRL and Wichita State University for technical and
financial support of the present research studies.
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