Abstract

X-ray and neutron diffraction experiments were performed on nanocrystalline SiC with the average grain diameters of 7, 12, and 16 nm subjected to isostatic pressure. Under pressure the interatomic distances undergo a complex, non-monotonic changes which were correlated with the compression of the SiC lattice determined from the Bragg scattering. The results indicate the presence of non-uniform distribution of stresses between the interior and surface shell of the nanograins. The present results lend a strong support to the concept of a core-shell model of nanocrystals.

Author Keywords: Nanocrystals; Silicon carbide; Diffraction; High pressure

Article Outline

1. Introduction

2. Experimental

3. Results

3.1. Analysis of Bragg scattering from X-ray diffraction: development of macro- and micro-strains under pressure

3.2. PDF analysis: changes of interatomic distances

4. Summary

Acknowledgements

References

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Fig. 1. Tentative models of nanocrystals subjected to an external stress: (a) two-phase (core-shell) powder grains characterized by two different bulk modulus B_o^core and B_o^shell; (b) grains with a uniform initial structure (B₀ = const).

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Fig. 2. Typical experimental results: (a) diffraction pattern of 7 nm SiC powder in high pressure cell measured at HIPD instrument; (b) intensity profile in the real space.

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Fig. 3. Dependence of strains on pressure in SiC. (a) macro-strain; (b) micro-strain obtained from X-ray diffraction at HASYLAB. The boxes mark the range of pressures in the experiment performed with HIPD diffractometer at LANSCE.

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Fig. 4. Dependence of interatomic distances on pressure in SiC nanocrystals. The curves in the figures are somewhat arbitrary due to large pressure intervals between 1 and 7 GPa.

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Fig. 5. Tentative model showing changes of interatomic distances within SiC₄ tetrahedron with pressure: (a) stereoview, interatomic distance r₂ between like atoms; (b) a view from the [1 1 0] direction, interatomic distances r₁ between unlike atoms.

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