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.

Carbon isotope fractionation between graphite and diamond during shock experiments

“…This observation suggests that the diffusive reconstruction process is the dominant mechanism for the transformation from graphite into the diamond that was analyzed in this study. This conclusion is consistent with those obtained in similar experiments using porous initial graphite (e.g., Yoshida and Thadhani 1992;Hirai et al 1995). Therefore, the isotopic fractionation discussed above should be associated with the diffusive reconstruction process.…”

Influence of graphite crystallinity on the microtexture of nano-polycrystalline diamond obtained by direct conversion

“…It has been demonstrated through detailed TEM observations that the diamond nucleation initiates preferentially at structural defects and crystal surfaces (terminations), where sp3-hybridized dangling bonds are dominated (Guillou et al 2007). This is because the activation energy required for diamond nucleation can be significantly lowered by the presence of such reactive bonds (Hirai et al 1995). Since the proportion of dangling bonds increases with increase in the surface area (to volume ratio) and lattice defects of graphite particles, the smaller the crystallite size of graphite, the more the nucleation sites for diamond are provided.…”

Natural occurrence of pure nano-polycrystalline diamond from impact crater

“…On the other hand, diamond formation by the latter mechanism proceeds through diffusion-controlled nucleation and subsequent crystal growth, which initiate preferentially at lattice defects and crystal surfaces where sp 3 -hybridized dangling bounds are dominated 21 27 . Experimental studies suggest that the most important and essential factor determining which transformation mechanism is preferable is the crystallinity of initial graphite sources 21 27 28 29 . The martensitic transformation is favored when using well-crystalline graphite as a source material, while the nucleation and growth mechanism becomes dominant when using poor-crystalline, disordered graphite.…”