Shock Synthesis of Nanocrystalline High -Pressure Phases in Semiconductors by High-Velocity Thermal Spray
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Shock Synthesis of Nanocrystalline High -Pressure Phases in Semiconductors by High-Velocity Thermal Spray 1,3
R. Goswami , 2 J.Parise, 1 H.Herman, 1 S.Sampath, 1 R. Gambino, 3 Y. Zhu and 3 D.Welch
1. Center for Thermal Spray Research, Dept. of Materials Sci. & Engg., SUNY at Stony Brook 2. Center for High Pressure Research, Dept. of Geosciences, SUNY at Stony Brook 3. Energy Sciences and Technology Department, Brookhaven National Laboratory, Upton, NY Abstract Shock synthesis of nanocrystalline Si, Ge and CdTe was accomplished using high- velocity thermal spray. Si or Ge powders were injected into a high energy flame, created by a thermal spray gun, where the particles melt and accelerate to impact on a substrate. The shock wave generated by the sudden impact of the droplets propagated through the underlying deposits, which induces a phase transition to a high pressure form. The decompression of the high-pressure phase results in the formation of several metastable phases, as evidenced by transmission electron microscopy and x-ray diffraction studies. The peak pressure is estimated to be ≈23GPa with a pulse duration of 1-5 ns. Transmission electron microscopy revealed that the metastable phases of Si with a size range of 2 to 5 nm were dispersed within Si-I. In Ge, a metastable phase, ST-12, was observed. This is a decompression product of Ge-II which possesses the β-Sn type of structure. In the case of CdTe, a fine dispersion of hexagonal CdTe particles, embedded in cubic-CdTe with an average size of 2 nm was obtained. 1. Introduction: Thermal spray involves the rapid deposition of molten, partially molten or solid particles upon a substrate to form a deposit of one micrometer or more thickness1,2. The deposit is built up by the successive deposition of such droplets. Generally particle speeds range from sub-sonic to supersonic velocities, depending on the specific thermal spray technique employed. We have demonstrated shockinduced transformations of both nanocrystalline Si1 and diamond 2 using the high- velocity thermal spray, a methodology widely used for producing protective coatings of metals, alloys and ceramics.
Fig. 1. X-ray diffraction pattern showing a broad hump around (111) Si-I. The broad hump is due to the hexagonal-Si formed in the shock compressed deposits.
A number of phases of Si have been reported during compression and pressure release. In the hydrostatic pressure range of 11 to12 Gpa3 , Si-I (cubic-diamond ) transforms to Si-II, which has a β-Sn type of crystal structure. Four other transformed phases are reported in Si as pressure increases from 12 to 75 GPa. A number of metastable phases have been observed during depressurization. On slow pressure F15.1.1 1
release, Si-II transforms to the BC-8 phase4 , which is stable up to 400K and transforms to Si-IV (hexagonal diamond-Si) above 400K 5 . 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-86 , which is a rhombohedral distortion of BC-8. On fast pressure rel
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