Shock synthesis of nanocrystalline Si by thermal spraying

Shock synthesis of nanocrystalline Si was accomplished for the first time u sing 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 strea m 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 pr opagated through the underlying Si layer, which experienced a phase transit ion 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 transmis sion electron microscopy and x-ray diffraction studies. Back-transformed me tastable Si grains, with a size range from 2 to 5 nm, were found to be disp ersed within Si-I (cubic diamond-Si), The metastable phases formed mostly i n deposits on the (100) substrate compared to those of the (111) substrate orientations, This behavior can be correlated with the anisotropic nature o f the pressure-induced transformations of Si-I.