Realization of the extraordinary properties presented by carbon nanotubes holds much promise of revolutionize several fields and we are tantalizingly close to achieving these remarkable changes. Many of these applications will require mass production of aligned carbon nanotubes (NTs) in high purity and at low cost if they are to achieve industrial viability. In this regard, chemical vapor deposition (CVD) is the most promising synthesis route for producing large quantities of carbon nanotubes at a low cost. We have developed a low temperature CVD process for the production of aligned, high purity multiwall carbon nanotubes. The process has production rates up to 1.5 g/m2/min with carbon yields approaching 70% conversion of all carbon fed to MWNT product. Product yield is found to increase with reaction temperature and carbon partial pressure, while reaction time passes through a maximum after which amorphous carbon is primarily formed in favor of MWNT. While the process produces nanotubes of high purity, 5% iron catalyst remains in the tubes. A simple post-production graphitization step is an effective way to remove both the catalyst and anneal out structural defects in the nanotubes.
Harnessing the unique physical properties of multiwalled carbon nanotubes (MWNTs) in materials applications has yet to be fully realized. One practical approach to producing MWNT composites is by shear mixing of MWNT into polymer matrices followed by extrusion or injection molding. Sufficient dispersion has been found to be the key in realizing the potential of these unique nano-reinforcements. The dispersion of low concentrations (<0.5vol%) of MWNTs in a polymer matrix resulted in substantial decreases in the electrical surface resistivity of the derived composite material. In polypropylene the inclusion of only 0.05vol% MWNTs produced a reduction in resistivity from >1012 Ohm/square to a value of ~105 Ohm/square.
MWNTs were derivatized by the addition of aryl- or diarylcarbenes. Addition of (4-bromophenyl)phenylcarbene followed by analysis of bromine content showed the addition of approximately one carbene added per 6,000 carbon atoms. To determine the effect of such addition on the properties of composites, a series of carbenes containing alkyl groups were added to the nanotubes. The first derivative was prepared using 4-octadecylphenylcarbene, and led to composites with improved properties. In order to determine whether the nature or quantity of alkyl substituent had any bearing on the observed improvements, we next added 4,4'di(octadecyl)diphenylcarbene (a carbene with two long-chain alkyl groups) and 4,4'di(t-butyl)diphenylcarbene (a carbene with two small, very compact alkyl groups) to MWNTs.
Polystyrene-nanotube composites were prepared using these chemically modified multiwalled carbon nanotubes and unmodified MWNT for comparison. After shear blending, thin composite films were pressed and the mechanical properties of the films measured. The composites prepared from modified nanotubes exhibited improved tensile and flexural properties over native polystyrene films, and were significantly higher than unmodified MWNT composite films. This suggests that the interaction between the nanotube and the polymer matrix has been significantly enhanced by the chemical treatment, and presents a path to super-strong composite materials based on carbon nanotubes.
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