Formation of Graphite Single Crystal from the Melt under High Temperature and High Pressure

Abstract

Graphite single crystals were grown by the solidification of the melt under high temperature and high pressure of 2 kb and 10 kb for the durations of about 1 sec and 0.5 sec, using a piston cylinder type high pressure apparatus. Spectroscopic graphite electrode rod (0.3 c X 17 mm, 1.8 qx 3 mm for the central part) was used as a starting material and a heater. In contains only a negligible amount of calcium.
Temperature calibration under high pressure was carried out by the use of a platinum platinum 13% rhodium thermocouple and based on melting points of platinum, molybdenum and tungsten wires. It was found that temperature was almost proportional to electric power input below 2000° C. Since it was uncertain whether this linear relation was able to be extra polated above 2000° C, the temperature was expressed in terms of the eleetric power-input (watt).
Most of large graphite single crystals with metallic luster were grown at the surface of graphite heater at 2 kb, 2500 watt and 10 kb, 3190 watt, with c-plane parallel to the direction of applied pressure. Crystals produced were platelets with maximum dimension of 2.1 mm in width and 0.25 mm thick. On the other hand, graphite single crystals smaller than 0.4 mm in width were mostly produced in the central part of the heater, and their basal planes were generally orientated in a direction normal to the applied pressure. It was found that the content of large graphite single crystals larger than O.2 mm in width incresed with the increase in temperature and time of treatment. The dimensions of graphite single crystals produced by the solidification from temperatures within a vapor phase region of carbon, were comparatively smaller than those of crystals from temperatures within a liquid phase region.
Smaller globules of graphites with maximum diameter up to 6 gm were formed at the surface of the basal planes of grown graphite single crystals above 2600 watt-2 kb or 3300 watt-10 kb. Results from X-ray diffraction of grown graphite single crystals indicated that crystals produced had lattice spacings close to the values of natural graphite given in the literature.