Abstract: Carbon nanotube (CNT) yarns were drawn from the web found on the CVD furnace wall. The yarns were twisted and their tensile behavior was investigated through in-situ SEM and regular tensile tests. The mechanisms involved in the CNT yarn deformation were identified. The twisted CNT yarns were found to have a good ability to stretch and deform and featured a diameter-dependent strength.
Abstract: The rheological and electrical properties of suspensions of carbon nanotubes in an uncured epoxy resin were investigated by means of shear rheology and impedance spectroscopy. It was found that above an onset CNT weight fraction (0.1 wt%), the steady viscosity increased with CNT loading and presented a shear thinning behaviour. The concentration dependence of viscosity changed from a power law to an exponential with increasing shear rate, indicating a loss of interaction between aggregates and CNT network breakage. The fluid-to-solidlike and insulator-to-conductor transitions occurred in the same CNT weight fraction range between 0.5 and 0.6 wt %. The correspondence of these transitions was explained by the reduction of contact resistance between CNT by stiffening of the CNT network leading to improved electronic transport.
Abstract: The nonlinear bulk compressibility of entangled multiwalled carbon nanotubes is studied. The analogy with textile fibre assemblies is explored by means of the well established van Wyk model. In view of the small diameter of the nanotubes, the possible effect of adhesive van der Waals interactions at tube-tube contacts is analysed. It is found, however, that the contribution of adhesive contacts to the bulk stress should be negligible. Compression experiments are performed on multi-walled carbon nanotubes and show that van Wyk's model is able to describe the response, although the values of the dimensionless parameter k of van Wyk's model were lower than expected. There is indeed no indication that van der Waals interactions play any significant role.
Abstract: We proposed to evaluate the contact resistance between carbon nanotubes from bulk measurement on their tangle. Compression tests with simultaneous measurement of the ohmic resistance were performed on carbon nanotube tangles. The experimental data were analyzed using a contact network model to get an estimate of the contact resistance between carbon nanotubes. It was found that in some cases the contact resistance decreased by up to one order of magnitude and reached values in the order of 100 kOhms.
Abstract: Motivated by the widespread and contradictory results regarding the glass transition temperature of carbon nanotube
(CNT)/epoxy composites, we reviewed and analyzed the literature results dealing with the effect of unmodified multiwall
carbon nanotubes (MWNT) on the cure behaviour of an epoxy resin (as a possible source of this discrepancy). The
aim of this work was to clarify the effective role of unmodified multiwall carbon nanotubes on the cure kinetics and glass
transition temperature (Tg) of their epoxy composites. It was found that various authors reported an acceleration effect of
CNT. The cure reaction was promoted in its early stage which may be due to the catalyst particles present in the CNT raw
material. While SWNT may lead to a decrease of Tg due to their bundling tendency, results reported for MWNT suggested
an increased or unchanged Tg of the composites. The present status of the literature does not allow to isolate the effect of
MWNT on the Tg due to the lack of a study providing essential information such as CNT purity, glass transition temperature
along with the corresponding cure degree.
Abstract: The reinforcement efficiency of carbon nanotubes (CNTs) in epoxy matrix was investigated in the elastic regime. Cyclic uniaxial tensile tests were performed at constant strain amplitude and increasing maximum strain. Post-curing of the epoxy and its composite at a temperature close to the glass transition temperature allowed us to explore the effect of aging on the reinforcement efficiency of CNT. It is found that the reinforcement efficiency is compatible with a mean field mixture rule of stress reinforcement by random inclusions. It also diminishes when the maximum strain increased and this effect is amplified by aging. The decrease of elastic modulus with increasing cyclic maximum strain is quite similar to the one observed for filled elastomers with increasing strain amplitude, a phenomenon often referred as the Payne effect.
Abstract: Carbon nanofibers (CNF) were dispersed into an epoxy resin using a combination of ultrasonication and mechanical mixing. The electronic transport properties of the resulting composites were investigated by means of impedance spectroscopy. It was found that a very low critical weight fraction (pc = 0.064 wt%) which may be taken to correspond to the formation of a tunneling conductive network inside the matrix. The insulator-to-conductor transition region spanned about one order of magnitude from 0.1 to 1 wt%. Far from the transition, the conductivity increased by two orders of magnitude. This increase and the low value of the conductivity were explained in terms of the presence of an epoxy film at the contact between CNF. A simple model based on the CNF–CNF contact network inside the matrix was proposed in order to evaluate the thickness of that film.
Abstract: The dielectric properties of a MWNT/epoxy composite have been studied by impedance spectroscopy. The results were very attractive and showed that the addition of a small amount of MWNTs allows increasing the permittivity of the resin by many orders of magnitude. The insulator-to-conductor transition, which occurred when a few wt% of MWNTs was added to the matrix, is related to a percolation phenomenon with non-universal critical exponent. Two types of MWNTs treatment have been compared and the results demonstrate that MWNTs should be used in as-prepared state. The treatment alters their morphology, the aspect ratio can be reduced by a factor of 10, leading to a modification of the shape of the insulator-to-conductor transition curve and a decrease of the global improvement.
Abstract: An experimental investigation of the effect of nanotube length and aggregate size on the mechanical and electrical properties of the composites was reported. Three treatments, that affect mainly the length and aggregate size, were applied on the CVD MWNTs before they were added into a resin matrix. They had a very clear impact on the dielectric properties of the composites and on the improvement efficiency. There was an insulator-to-conductor transition with the as-prepared MWNTs at 0.5wt%. Regarding the mechanical properties, the addition of MWNTs increased the Young's modulus and reduced the fracture strain. In that case, the pre-treatment on MWNTs, however, had a much more moderate effect on the improvement efficiency.
Abstract: Multi-walled carbon nanotube/epoxy resin composites have been fabricated. By choosing an over-aged hardener, relatively soft and ductile matrix, a rubbery epoxy resin, has been obtained. This made possible to evaluate the effect of nanotube addition on the whole stress-strain curve up to high strain level. The mechanical and electrical properties of the composite with different weight percentages of nanotubes have been investigated. The Young's modulus and the yield strength have been doubled and quadrupled for composites with respectively 1 and 4 wt.% nanotubes, compared to the pure resin matrix samples. Conductivity measurements on the composite samples showed that the insulator-to-conductor transition took place for nanotube concentration between 0.5% and 1 wt.%.
Abstract: Carbon nanotubes (CNT) are promising nanofillers for the development of polymer composites with multiple enhanced properties. It has been long recognized that their potential could not be fully exploited without achieving their good dispersion in the matrix. In the literature, different dispersion instruments were used at the laboratory scale (ultrasonic processor, high shear mixer …) to disperse CNT in liquid matrices. The use of a dispersing agent can facilitate the dispersion. The most commonly used are surfactants (SDS or NaDBBS) and polymers like PmPV and PVP. This work will show that the combination of the right dispersion instrument with a suitable dispersing agent can improve the dispersion of multiwall carbon nanotubes in epoxy resin. Due to van der Waals interactions, CNT are in aggregate form with size up to hundreds of 2m. In this study, light transmission optical microscopy was used to evaluate the size and distribution of these aggregates. Dielectric spectroscopy and shear rheology were performed in order to detect and evaluate any enhancement in dispersion. In fact, when the aggregates are broken during dispersion, CNT spread into the matrix to form a complex interconnected network. It is thus expected that the electrical conductivity (when the dispersion quality overcomes the reduction in length of the CNT) and the viscosity increase.
