An Experimental Contribution to the Problem of Diamond Synthesis

Abstract

Pressure up to 30,000 kg/cm², and in one case 45,000 kg/cm², has been applied to graphite alone, or to graphite seeded with diamonds, at temperatures above 2000°C. In order to maintain temperatures for times of the order of seconds it is necessary to make the apparatus large, a commercial 1000‐ton press being used to generate the pressure. Two methods were used, external heating with rapid transfer of the heated specimen to the pressure vessel and application of pressure, and internal heating by a modified thermite reaction set off by the pressure itself. No detectable transformation of graphite to diamond was found in any of the experiments. In the experiments with internal heating it was established that the rate of inversion of diamond into graphite at a temperature above the melting point of molybdenum is a function of the pressure. The individual points are considerably scattered, but the mean points lie roughly on a straight line running from 100 percent graphitization at 15,000 kg/cm² to 0 percent at 30,000 kg/cm². Linear extrapolation would, therefore, indicate reversible transition from graphite to diamond at higher pressures. It has not been possible to check this by direct experiment. The graphite formed from diamond under these conditions has the same x‐ray structure as ordinary graphite, indicating that there are no previously unknown modifications of graphite in the present range to complicate the problem. In an appendix the theoretical evidence is considered and the conclusion drawn that linear extrapolation of the equilibrium curve between graphite and diamond from data up to 1000°C gives too high values of pressure in a range above 2000° where the velocities become appreciable, and that the equilibrium curve more probably curves upward.