Shock synthesis of high-pressure phases of Si, Ge and diamond was accomplished using thermal spray by scientists at the Center for Thermal Spray Research (NSF MRSEC) in collaboration with the Center for High Pressure Research at the State University of New York at Stony Brook.

They report, in the September 1999 and January 2000 issues of the Journal of Materials Research, that thermal spray can be treated as a "dynamic-pressure anvil" allowing synthesis of high-pressure metastable forms of nanocrystalline Si and diamond, where shock pressures of 0 to 30 GPa are achieved. The researchers said that there is exciting scientific and applied interest in forming deposits of novel phases, such as nano-diamond and nano-silicon, by continuously generating high-velocity particles and impacting these on a substrate. Powders are injected into a high energy flame, where they melt and accelerate, impacting on a substrate, creating a shock wave which propagates through the substrate and prior deposited layers, yielding phase transitions to high pressure forms.

During thermal spraying (which yields impact pressures of 10 to12 GPa), cubic Si (Si-I) transforms to Si-II, with a beta-Sn type structure. A number of metastable phases were observed during subsequent spontaneous depressurization. The metastable-high pressure form of Si transforms to nanocrystalline (2 to 5 nm) Si-IX, Si-IV (hexagonal diamond-Si), R-8 and BC-8 phases. In the case of Ge, a metastable phase, ST-12, was observed. The St-12 is a decompression product of Ge-II. with a b-Sn type of structure. The transformation pressure of Ge-II is slightly lower than that of Si-II.

Nanocrystalline-diamonds were deposited (>20 m on steel) by thermal spray of Ni-clad graphite powder. The high-velocity impact generates a shock wave which propagates through the particle and the underlying deposits. TEM and Raman spectroscopy reveal that this deposit contains cubic diamond nanocrystals (5 to10 nm) in the graphite matrix. It was also observed that a portion of the deposit contains "closed-curved graphite." which is tubular form of graphite.

The broader implications of this work include the production of other pressure phases whose bulk synthesis is precluded by economic considerations. The low cost of thermal spray will greatly enhance our capacity to test and utilize high pressure phases, which remain largely a curiosity because of their cost of production. Additionally, preliminary results indicate that these shock-pressure induced structures may have interesting electrical (e.g. Si and Ge) and mechanical (e.g. diamond hardness on functional surfaces) properties.

The shock pressure is obtained by Rankine-Hugoniot equation P = d v u, where d is the density, v is the shock velocity and u is the velocity of the compressed material.