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.
Nano‐sized diamond powder, shock‐compacted in the present study, showed neither grain growth nor graphitization, but the compact was fragmented and not strongly consolidated. In contrast, a mixture of the nano‐sized powder and submicrometer‐sized diamond powder consolidated into a disk with 93 % of theoretical density and a Vickers micro‐hardness of 25 GPa also exhibited no grain growth and showed interparticle bonds only among large particles. Although the samples obtained were neither hard nor strong, microstructural analyses and considerations based on the shock‐compaction theories and models clarified issues surrounding the shock‐compaction method of producing nanocrystalline materials. Both the propriety and the limitation of these theories and models are discussed in this paper.
The effect of microtexture on diamond transition was examined for graphite starting materials under shock compressions of 50 to 60 GPa and 80 to 90 GPa. Each of the starting materials used in the present study possessed a fully homogeneous microtexture. To distinguish the effect of microtexture from that of other experimental parameters, the shock conditions were standardized for all specimens tested. Three graphite materials-a glassy carbon, a carbon black, and a natural graphite-were selected and shock compressed using a quenching technique to generate conditions common to all samples. Detailed characterization by transmission electron microscopy and electron energy-loss spectroscopy revealed a clear tendency: The lower the crystallinity and crystallite size of the starting graphite, the more easily the graphite transformed to diamond when the transition mechanism was reconstructive.
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