When diamond is heated in vacuum to temperatures above about 1200° C, an allotropic transformation to graphite occurs. This change has been studied using two types of sample — fine diamond powder, and large single crystals of diamond. The graphitised diamond powder has been examined by transmission electron microscopy and diffraction; the graphitised single crystals by reflection electron microscopy and diffraction. It was found that the graphite forms as crystallites about 100 A across; that plastic deformation of the diamond can occur at 1800° C under the stresses set up by an expanding nucleus of graphitisation; and that the presence or absence of preferred orientation of the graphite on the diamond depends on the temperature of graphitisation.
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