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在高温高压条件下以洋葱碳为原料合成的纳米孪晶金刚石具有前所未有的硬度和稳定性,且二者随纳米孪晶厚度的减小而提高.目前为止,在金刚石纳米晶中纳米孪晶的形成机制尚不明确本研究通过分析在不同条件下合成的纳米孪晶金刚石块材中的孪晶厚度,发现在合成压力约为20 GPa时孪晶厚度达到一个极小值(~5 nm).TEM结果表明在合成压力低于20 GPa时,纳米孪晶金刚石中同时存在因马氏体相变而形成的相变孪晶和塑性形变所导致的形变孪晶.针对马氏体相变后形成的相变孪晶内部塑性变形,基于密度泛函理论的紧束缚方法和位错运动学理论的分析表明:纳米孪晶金刚石的塑性变形存在一个依赖于压力的机制转变,而机制转变的临界压力能够解释孪晶厚度随压力变化时出现的极小值.本研究对通过优化合成条件制备出更高硬度和稳定性的纳米孪晶金刚石具有指导意义
The nano-twinned diamond synthesized from onion carbon under high temperature and high pressure has unprecedented hardness and stability, and both increase with the decrease of the thickness of nano-twins. So far, nano-twins in diamond nanocrystals The mechanism of formation is not clear. In this study, by analyzing the twin thickness in the nanocomposites synthesized under different conditions, it was found that the twin thickness reaches a minimum (~ 5 nm) at a synthetic pressure of about 20 GPa. TEM results show that the twinning deformation caused by the martensite transformation and the twinning deformation caused by the plastic deformation exist in the nano-twinned diamond at the synthesis pressure lower than 20 GPa.For the formation of martensite transformation The analysis of the tight binding method based on density functional theory and the theory of dislocation kinematics shows that there is a mechanism dependent on the pressure in the plastic deformation of nanocrystalline twins and the critical pressure of mechanism transformation Which can explain the minimum value of twins’ thickness with the change of pressure.This study is instructive to prepare nano-twinned diamond with higher hardness and stability by optimizing the synthesis conditions