Abstract
Nano-polycrystalline diamond (NPD) is a super-hard pure polycrystalline aggregate of nano-diamonds and has a characteristic microtexture composed of a mixture of granular and lamellar crystals. We investigated the origin of the unique microtexture and the influence of the crystallinity of initial graphite sources on the resulting microtexture of NPDs. Polycrystalline graphite rods used for NPD synthesis were found to consist of coke-derived relatively large crystals and pitch-derived nanocrystalline particles. Upon conversion to NPD, the former are converted to cubic and hexagonal diamond mixtures by the martensitic transformation and left a lamellar texture behind, while the latter transform to granular nano-diamonds by diffusion-controlled nucleation and subsequent crystal growth, which initiate preferentially at lattice defects and crystal surfaces. A clear correlation between the crystallite size of the initial graphite and the grain size of the granular nano-diamonds in the NPDs was also found. Our results suggest that the average grain size and the relative abundance of lamellar domains in NPD can potentially be controlled by carefully choosing initial graphite sources based on their crystallinity.
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Acknowledgments
The authors thank T. Shinmei of Ehime Univ. for his technical support in performing the experiments. This study was supported by Grant-in-Aid for Young Scientists (B) (#20740255) from Ministry of Education, Science and Culture, Japan and also supported partly by Grant-in-Aid for Specially Promoted Research (#20001005, Representative: T. Irifune) from Japan Society for the Promotion of Science. The synchrotron-based X-ray diffraction analysis was conducted at BL10XU of Spring-8 with the approval of JASRI (Proposal Nos. 2010A1545 and 2010B1588).
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Ohfuji, H., Okimoto, S., Kunimoto, T. et al. Influence of graphite crystallinity on the microtexture of nano-polycrystalline diamond obtained by direct conversion. Phys Chem Minerals 39, 543–552 (2012). https://doi.org/10.1007/s00269-012-0510-3
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DOI: https://doi.org/10.1007/s00269-012-0510-3