Predominant parameters in the shock-induced transition from graphite to diamond

Abstract

Predominant parameters in the shock-induced transition from graphite materials to diamond were examined in the present study by using quenching and powder methods under pressures of 50-60 GPa and 80-90 GPa, respectively, in the temperature range from 750 to 3500 K. Effects of the material parameters of the starting graphite - i.e., crystallite size and crystallinity - were distinguished from effects of the experimental parameters by standardizing the shock conditions for the materials examined. In addition, only a few graphite materials possessing wholly identical parameters throughout were selected as starting materials. Detailed characterization by transmission electron microscopy and electron energy-loss spectroscopy revealed that the transition ratios of diamond and the degrees of graphitization varied with those parameters. The various changes observed were plotted in terms of pressure, temperature, and material-parameter axes to create a tentative pressure-temperature-material diagram representing the behavior of the graphite materials under shock compression. The material parameters of the initial graphite structure primarily affected the diamond transition: The lower the crystallinity and crystallite size, the more easily the diamond transition occurred in the case of a reconstructive mechanism. Smaller crystallite size and lower crystallinity elevated the initial energy states of the graphite materials because of surface energy and strain energy, making it relatively easier to transcend the activation-energy barrier to diamond transition. Temperature was fairly effective and pressure ineffective in regard to the diamond transition, a result consistent with the belief that the transition is a diffusion-controlled process. Moreover, differentiation of the transition pathways, the diamond transition, and graphitization fit a concept of alternative metastable behavior; graphitization was more favored kinetically than diamond transition under the shock conditions examined. © 1995 American Institute of Physics.