Origin of unique microstructures in nano-polycrystalline diamond synthesized by direct conversion of graphite at static high pressure
Origin of unique microstructures in nano-polycrystalline diamond synthesized by direct conversion of graphite at static high pressure
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DOI：
10.2465/jmps.090622i
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发表时间：
2009-10-01
影响因子：
0.7
通讯作者：
Kuroki, Kiyoshi
中科院分区：
文献类型：
--
作者：
Ohfuji, Hiroaki;Kuroki, Kiyoshi
利用激光加热的金刚石压腔和两种具有不同结晶特征的石墨样品，研究了石墨的局部应力状态和结晶度对石墨-金刚石转变过程的影响。用高取向石墨(HOG)回收的样品具有层状结构，由同轴排列的长英石和钻石组成，表明它们是由石墨形成的马氏体。HOG垂直于石墨c轴的压缩导致了原始层状结构的部分碎裂，但即使在碎裂区域也没有发现明显的扩散过程形成金刚石的证据。另一方面，用机械研磨的石墨粉回收的样品由大小为20-50 nm的正面体金刚石纳米晶组成，表明金刚石是通过扩散过程形成的。研究结果表明，石墨-金刚石的转变过程和微观结构主要受石墨的初始结构特征(如结晶度和微晶尺寸)控制，而压缩状态下石墨的局部应力状态对形成过程和微观织构的影响可能不大。
The influence of the local stress state and crystallinity of graphite on the graphite-diamond trans formation process was investigated using a laser-heated diamond anvil cell and two types of graphite samples with contrasting crystalline characteristics. The samples recovered from the experiments using highly oriented graphite (HOG) have layered structures composed of lonsdaleite and diamond arranged in a coaxial relation, indicative of their martensitic formation from graphite. The compression of HOG perpendicular to the graphite c axis results in a partial fragmentation of the original layered structure, but no clear evidence of diamond formation by the diffusion process is found even in the fragmented regions. On the other hand, the sample recovered from the experiment using mechanically milled graphite powder consists of euhedral diamond nanocrystals (size: 20-50 nm), indicating that the diamond is formed by the diffusion process. The results of the present study suggest that the graphite-diamond transformation process and the resulting microstructures are controlled mainly by the initial structure characteristics (such as crystallinity and crystallite size) of the graphite, while the influence of the local stress state in the graphite under compression is likely less significant for the formation process and the microtexture.