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|Title: ||Conductive behaviour of carbon nanotube based composites|
|Authors: ||Sun, Xinxin|
|Keywords: ||Carbon nanotube|
|Issue Date: ||2009|
|Publisher: ||© Xinxin Sun|
|Abstract: ||This project was basically exploratory in the electrical properties of carbon nanotube
(CNT) based materials. The direct current (DC) conductivity of CNT/polymer
composites was computed by using equivalent circuit method and a three dimensional
(3-D) numerical continuum model with the consideration of tunneling conduction.
The effects of the potential barrier of polymer and the tortousity of CNTs on the
conductivity were analyzed. It was found that both of percolation threshold and DC
conductivity can be strongly affected by the potential barrier and the tortousity. The
influence of contact resistance on DC conductivity was also computed, and the results
revealed that contact resistance and tunneling resistance had significant influences on
the conductivity, but did not affect the percolation threshold. The
microstructure-dependent alternating current (AC) properties of CNT/polymer
composites were investigated using the 3-D numerical continuum model. It was found
that AC conductivity and critical frequency of CNT/polymer composites can be
enhanced by increasing the curl ratio of CNTs. In the mid-range CNT mass fraction,
with increasing curl ratio of CNTs, AC conductivity, interestingly, became
frequency-dependent in low frequency range, which cannot be explained by reference
to the percolation theory. A proper interpretation was given based on the linear circuit
theory. It was also found that the critical frequency can also be affected by the size of
CNT cluster. Series numerical formulas were derived by using a numerical
capacitively and resistively junction model. In particular, this work introduced an
equivalent resistor-capacitor (RC) circuit with simple definitions of the values of
contact resistance and average mutual capacitance for CNT/polymer nanocomposites.
Theoretical results were in good agreement with experimental data, and successfully
predicted the effect of morphology on the AC properties of CNT/polymer composites.
DC and AC conductivities of multi-walled carbon nanotube (MWCNT)/graphene oxide (GO) hybrid films were measured for selected MWCNT mass fractions of 10%,
33.3%, 50%, 66.7%, and 83.3% using four-probe method. The experimental results
were fitted using scaling law, and relatively high percolation threshold was found.
This high percolation threshold was understood in terms of the potential energy and
intrinsic ripples and warping in the freestanding graphene sheets. The capacitance of
these hybrid films were measured using the voltmeter-ammeter-wattmeter test circuit
with different voltages and heat treatments. The MWCNT/GO film showed relatively
high specific capacitance (0.192F/cm3 for the mass fraction of 83.3%) and power
factor compared to conventional dielectric capacitors. Both of measured capacitance
and power factor can be enhanced by increasing testing voltages. The capacitance of
MWCNT/GO films rapidly decreased after heat treatments above 160 ℃. This
decrease was caused by redox reaction in the GO sheets. The capacitive behaviour of
MWCNT/GO hybrid films was also interpreted by using the equivalent circuit model.
Single-walled carbon nanotube (SWCNT) and SWCNT/Poly(vinyl alcohol) (PVA)
films were used to form a piezoresistive strain sensor. Both of static and dynamic
strain sensing behaviours of SWCNT and SWCNT/PVA films were measured. It was
found that the sensitivities of these films decreased with increasing their thicknesses.
The SWCNT film with a thickness of 1900 nm and SWCNT/PVA film exhibited
viscoelastic sensing behaviour, because van der Waals attraction force allowed axial
slippages of the smooth surface of nanotubes. A numerical model was derived based
on the dynamic strain sensing behaviour. This model could be useful for designing
CNT strain sensors.
Finally, thermoelectric power (TEP) of deformed SWCNT films with various
thicknesses was measured. It was observed that positive TEP of SWCNT films
increased with increasing stain above the critical point. The experimental results were
fitted by using a numerical model in terms of a variation of Nordheim-Gorter relation
and fluctuation induced tunneling (FIT) model. From the numerical model, it was
found that the increase of TEP above the critical strain resulted from the positive term of the contribution from the barrier region, and the effect of barrier regions decreases
with increasing the thickness of the film.|
|Description: ||A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.|
|Appears in Collections:||PhD Theses (Materials)|
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