Rate of Transformation of Rhombohedral Graphite at High Temperatures

Abstract

THE hexagonal modification of graphite (common graphite) is composed of identical layers of stacking sequence ABAB …, while in the rhombohedral modification the sequence is ABCABC …, the latter modification being confirmed by Lipson and Stokes1. The occurrence of this modification and the enhancement and weakening of the X-ray diffraction lines belonging to it by chemical, mechanical and thermal treatments have been described by Hofmann2, who states that it is transformed to the hexagonal modification in a graphitizing oven in an atmosphere of carbon monoxide at 3,000° C. In the course of experiments on the thermal expansion of graphite by X-ray diffraction3, I observed that the corresponding lines were markedly weaker at much lower temperatures in vacuo. The rates of transformation were measured using the following improvements in technique. (1) Plate specimens were used in place of rods; to achieve a definite degree of orientation of crystals, 21 mgm. of powder of Ceylon graphite of average diameter 0.05 mm., purified with hydrochloric and hydrofluoric acids, was compressed to a plate 0.5 mm. thick and 6 mm. in diameter with a pressure of 40 kgm. without adding any adhesive substances. (2) The plate was mounted on a Debye–Scherrer camera 20 cm. in diameter so as to bisect the angle between the incident and diffracted rays and oscillated about this angle with a half-amplitude of 30°. (3) Since, with cobalt K-radiation, the curvatures of the diffraction lines were not large, a stepped absorber made by piling aluminium foils was placed in front of the photographic film so that the blackening of each line was graded by known ratios, permitting more accurate determination of the intensities than by the use of intensity marks photographed on one side of a film.