The Growth of Bamboo-Structured Carbon Tubes Using a Copper Catalyst
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The Growth of Bamboo-Structured Carbon Tubes Using a Copper Catalyst B.L. Farmer, D.M. Holmes, L.J. Vandeperre, R.J. Stearn and W.J. Clegg Ceramics Laboratory, Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge, CB2 3QZ. ABSTRACT Catalytic decomposition of methane has been used to grow bamboo-structured carbon tubes at temperatures ranging from 1233 K to 1291 K. No tube growth was observed at temperatures less than 1233 K, whilst above 1291 K pyrocarbon was the dominant product. It is shown that the average size of the copper catalyst particles was influenced by the reaction temperature, with the reciprocal of the maximum size of the copper particle decreasing linearly with temperature. This is consistent with the idea that the melting point can be reduced by surface energy effects. Observations show that under the conditions here the catalyst particle penetrates into the carbon fibre and a mechanism is proposed for development of the bamboo structure based upon the energy changes that take place. INTRODUCTION A wide range of carbon structures can be formed by the catalytic decomposition of hydrocarbons or the disproportionation of CO including various types of tubes and fibres as well as shells and stacks. In this paper hollow carbon fibres grown from copper catalyst particles are described. The most striking feature of these bamboo fibres is the repeating nature of the structure due to the formation of a transverse wall of carbon across the tube at regular intervals. Several authors have suggested that this structure might involve capillary forces causing the liquid or solid metal catalyst to move suddenly up the carbon tube 1-5, although the detailed mechanism is not understood. In this paper we describe some experiments using a copper catalyst and how capillary forces may give rise to such a sudden movement of the liquid catalyst enabling the bamboo structure to form. EXPERIMENTAL DETAILS A copper catalyst was produced by precipitation from a solution of hydrated copper nitrate, Cu(NO3)2)ยท3H2O, and ammonium hydrogen carbonate, NH4HCO3. The precipitate was removed using a centrifuge and dried at 378 K for 12 hours. The resulting particles were then ground with a pestle and mortar before placing a small quantity on an alumina substrate. This was placed in a tube furnace and calcined at 673 K in air for 4 hours before purging with Ar then H2. The furnace was then heated to the desired reaction temperature. At 25 K below the final temperature CH4 was introduced. The flow rates for H2 (99.995% pure) and CH4 (99.999% pure) were 1.4 l min-1 and 0.2 l min-1 respectively through a tube with a 60 mm internal diameter. The tube was held at constant temperature for 1 hour and then cooled under flowing Ar. The carbon grown was examined using a Jeol JSM-6340F field emission scanning electron microscope (SEM) and a Jeol JSM 2000FX transmission electron microscope (TEM). For SEM the samples were examined directly and for TEM the carbon products were scraped off the substr
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