Showing papers by "Yury Gogotsi published in 1998"

Pressure-induced phase transformations in diamond

Abstract: The stability of diamond under pressure and the structure of hypothetical high-pressure phases have been a controversial issue for a long time. “Will diamond transform under megabar pressures?” asked Yin and Cohen in the title of their paper [Phys. Rev. Lett. 50, 2006 (1983)] which attempted to predict an answer to this question 15 years ago. Before and after that, many other scientists tried to find the answer doing both modeling and experiments. However, the cubic structure of diamond seems to be experimentally stable up to the highest static pressures that the modern high-pressure technology can achieve. We addressed the problem by decreasing the contact area of pressurization instead of increasing the total load. Experimentally this can be easily done in indentation tests using a sharp diamond indenter. In addition to hydrostatic stresses, such a test creates shear stresses as well. Here deformations may be realized, which are either impossible or would require much higher pressures when utilizing only hydrostatic stresses. By coupling the indentation loading with micro-Raman spectroscopy, we were able to drive and monitor phase transformations in diamond. A very similar phenomenon can be observed by scratching a diamond with another diamond. Thus, phase transformations in diamond may in fact be a very common feature of wear.

Hydrothermal degradation of chemical vapour deposited SiC fibres

Abstract: The interaction of chemical vapour-deposited (CVD) SiC fibres with H2O at 200 MPa and 400–700°C (673–973K), was investigated. With increasing temperature and time, the smooth surface of the amorphous SiC fibre becomes rough and sponge-like. This modification can be controlled by adjusting temperature and duration of the hydrothermal treatment. CVD SiC dissolves in supercritical water in a first order reaction with Ea=150 kJ mol-1. According to thermodynamic calculations, the main products are SiO2, CH4 and H2. The formation of carbon is also predicted. Films of amorphous and graphitic carbon have been observed, but only small areas of the fibres were coated with carbon. Amorphous silica, quartz, cristobalite and keatite were deposited on the surface of fibres in larger quantities © 1998 Kluwer Academic Publishers

Phase transformations in semiconductors under contact loading

Abstract: Phase transformations occurring in materials under contact loading are important for a wide range of problems in materials science and engineering. We studied solid-state phase transformation, including metallization and amorphization of semiconductors under high non-hydrostatic pressures using a combination of hardness indentation tests, scratch tests, Raman spectroscopy, FTIR and various microscopic techniques. Our experiments demonstrated metallization due to closing of the band gap and consequent formation of metastable phases upon decompression in silicon, germanium and several other semiconductors. For the first time high-pressure, these phases were unambiguously observed in hardness impressions, scratches and machining debris, and for some of these phases Raman spectra have not been published before.

Formation of Carbon Films on Ceramic Carbides by High Temperature Chlorination

Abstract: Carbon films can be formed on SiC and other metal carbides by selective chlorination at high temperature and atmospheric pressure because of the volatility of compounds formed between chlorine and silicon or other metals, and because of the low thermodynamic stability of the corresponding carbon-chlorine compounds. An apparatus has been constructed in which carbon films are formed on metal carbides by reaction with flowing argon-chlorine-hydrogen gas mixtures in a fused silica furnace tube. The structure of the carbon films can be manipulated by controlling the temperature and composition of the reactive gas mixtures. Electron microscopy and Raman spectroscopy have been used to characterize the carbon films formed on β-SiC in Ar-Cl2-H2 gas mixtures at temperatures between 600 and 1000°C.