Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560 8531, Japan.
Nat Commun. 2013;4:2343. doi: 10.1038/ncomms3343.
Diamond is the stiffest known material. Here we report that nanopolycrystal diamond synthesized by direct-conversion method from graphite is stiffer than natural and synthesized monocrystal diamonds. This observation departs from the usual thinking that nanocrystalline materials are softer than their monocrystals because of a large volume fraction of soft grain-boundary region. The direct conversion causes the nondiffusional phase transformation to cubic diamond, producing many twins inside diamond grains. We give an ab initio-calculation twinned model that confirms the stiffening. We find that shorter interplane bonds along [111] are significantly strengthened near the twinned region, from which the superstiff structure originates. Our discovery provides a novel step forward in the search for superstiff materials.
钻石是已知最硬的材料。在这里，我们报告说，通过直接转化法从石墨合成的纳米多晶金刚石比天然金刚石和合成单晶金刚石更硬。这一观察结果与通常的想法相反，即由于软晶界区域的体积分数较大，纳米晶材料比单晶材料更软。直接转化导致非扩散相转变为立方金刚石，在金刚石晶粒内部产生许多孪晶。我们提出了一个基于第一性原理的孪晶模型，证实了这一增强效果。我们发现，在孪晶区附近，[111]方向上的面间键变短，强度显著增强，超硬结构由此产生。我们的发现为寻找超硬材料提供了新的进展。
