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Mass production of carbon nanotubes (CNTs) by a cost effective process is still a challenge for further research and application of CNTs. Our group has focussed on the deposition of CNTs on a continuously-fed carbon substrate via arc discharge at atmospheric pressure. This process produces MWNTs using carbon from the substrate. The method differs in other respects from the conventional batch arc discharge method by using lower currents (< 20 A) and larger inter-electrode gaps. To help define the local conditions of nanotube growth, the substrate surface temperature (T s ) was measured by optical pyrometry. Here, we report the influence of inter-electrode gap, substrate velocity and arc current on this temperature. It is found that carbon nanotube growth is favourable over a certain temperature range and retention time in the arc. To further understand the effect of arc parameters, we used a computer simulation to model the arc plasma. Computational fluid dynamic (CFD) software, Comsol Multiphysics, was used to simulate the temperature distribution and flow properties of the arc plasma. It was found necessary to include dusty plasma conductivity near to the electrodes to adequately represent observed arc behaviour.

Carbon nanotubes have been extensively studied since their discovery [1] due to their valuable structural and electronic properties. Direct growth of nanotubes on substrates is essential for their integration into various applications, as individual manipulation into position is both difficult and expensive due to their size. A unique single-step method has been developed of continuously depositing nanotubes on a carbon substrate using an arc discharge at atmosphere pressure [8]. This method differs from the conventional arc discharge method in that the nanotubes are grown at low currents on a moving substrate surface which acts as one of the electrodes.The effects of inter-electrode gap, buffer gas flow through the porous substrate and substrate speed on the yield and morphology of carbon nanotubes are investigated. It was found that an inter-electrode gap range of 2.5 -6.0mm is optimal for nanotube occurrence on the carbon substrate. Providing a flushing gas flow into the arc through the substrate reduced the mean diameter but not the number of nanotubes and markedly reduced the number of attached nanoparticles. The residence time of the substrate in the arc was varied by changing substrate velocity, and this was found to be critical for nanotube formation. From the parameters explored, it appears that the substrate temperature alone governs nanotube formation and this occurs at temperatures lower than reasonable sublimation temperatures. This latter fact together with the lack of influence of gas flush, indicate that carbon vapour is unlikely to dominate the formation of nanotubes in arcs.

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