Shock synthesis of nanocrystalline Si by thermal spraying
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RAPID COMMUNICATIONS The purpose of this Rapid Communications section is to provide accelerated publication of important new results in the fields regularly covered by Journal of Materials Research. Rapid Communications cannot exceed four printed pages in length, including space allowed for title, figures, tables, references, and an abstract limited to about 100 words.
Shock synthesis of nanocrystalline Si by thermal spraying R. Goswami, S. Sampath, and H. Herman Center for Thermal Spray Research, Department of Materials Science, State University of New York at Stony Brook, Stony Brook, NY 11794-2274
J.B. Parisea) Department of Geosciences and Center for High Pressure Research, State University of New York at Stony Brook, Stony Brook, NY 11794-2100 (Received 3 November 1998; accepted 28 June 1999)
Shock synthesis of nanocrystalline Si was accomplished for the first time using thermal spray in which Si powder is injected into a high-energy flame where the particles melt and accelerate to impact on the substrate. A stream of molten Si particles impacted onto Si wafers of two orientations (100) and (111). The shock wave generated by the sudden impact of the droplets propagated through the underlying Si layer, which experienced a phase transition to a high-pressure form of Si due to propagation of the shock wave. The metastable high-pressure form of Si then transformed to metastable Si-IX, Si-IV (hexagonal diamond-Si), R-8, and BC-8 phases as evidenced by transmission electron microscopy and x-ray diffraction studies. Back-transformed metastable Si grains, with a size range from 2 to 5 nm, were found to be dispersed within Si-I (cubic diamond-Si). The metastable phases formed mostly in deposits on the (100) substrate compared to those of the (111) substrate orientations. This behavior can be correlated with the anisotropic nature of the pressure-induced transformations of Si-I.
Si exhibits a number of high-pressure polymorphs. In the pressure range of 8.8 to 12 GPa,1 cubic diamond Si (Si-I) transforms to Si-II, which has a –Sn type of crystal structure. Although most pressure-induced transformations have been reported in polycrystalline Si, an anisotropic shock-induced transformation has been observed here. A number of metastable phases have been observed during depressurization. On slow pressure release, Si-II transforms to the BC-8 phase,2 which is stable up to 400 K and transforms to Si-IV (hexagonal diamond-Si) above 400 K.2,3 It has been observed recently that the Si-II phase does not transform directly to the BC-8 phase, but to an intermediate phase, R-8,4 which is a rhombohedral distortion of BC-8. On fast pressure release, two forms of Si,5 Si-VIII and Si-IX, have been observed. This work reports for the first time the shock synthesis of nanocrystalline Si-IV, Si-IX, R-8, and BC-8 phases by thermal spray and demonstrates
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Address all correspondence to this author. J. Mater. Res., Vol. 14, No. 9, Sep 1999
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