GRAPHITE ROD PRECURSORS FOR ISOTOPICALLY PURE FULLERENES AND DIAMOND

Graphite tubes of C-12 or C-13 carbon are made by cracking isotopicall y pure (CH4)-C-12 or (CH4)-C-13, respectively, at subatmospheric press ures on a Ta tube that is Joule heated to 2100 degrees C and under a c ritical, predetermined mechanical tension. Conversion efficiencies of methane to graphite of up to 80% are routinely obtained. The density o f the graphite tubes varies from 96 to 99% of the theoretical density of graphite. Following conversion of Ta to TaC, pyrolitic graphite dep osits at a rate of up to 700 microns/hour with its basal plane paralle l to the TaC substrate. When the graphite deposit reaches the required thickness, the assembly is cooled to room temperature. The anomalous negative thermal expansion of graphite in its basal plane allows the m echanical removal of the TaC inner tube from the graphite tube to leav e an isotopically pure graphite tube. In addition to isotopically pure graphite, graphite of other intermediate isotopic compositions are ma de by using suitable mixtures of C-12 and C-13 methane gases. Copyrigh t (C) 1996 Elsevier Science Ltd.