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Nat Commun. 2013;4:2343. doi: 10.1038/ncomms3343.

Observation of higher stiffness in nanopolycrystal diamond than monocrystal diamond.

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Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560 8531, Japan.


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.


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