Reorientation effects in vitreous carbon and pyrolytic graphite

Abstract

Tubular specimens of vitreous carbon and of pyrolytic graphite, heated in nitrogen or helium for several minutes at 2850 to 3100° K by RF induction, developed a “skin” of crystalline carbon, nodular in appearance and apparently growing into the carbon from the surface. The vitreous carbon was isotropic, with no preferred orientation of “crystallites”; the pyrolytic graphite had a preferred orientation of basal planes at right angles to the axis of the specimen, so that curved surfaces were composed predominantly of edge atoms. X-ray diffraction studies of the skin indicated that it was partially graphitised and strongly orientated with basal planes parallel to the surface. The mechanism of the reorientation process is as yet not clear, but the driving force may be a change in surface free energy.