Solvothermal Formation of SP2- and SP3- Carbon from Carbides

Carbon formation on SiC in the supercritical range of water (T > 374 °C, p > 222 MPa) was first observed in the course of corrosion studies of amorphous Si-Ti-C-0 (Tyranno) fibers. Extending the investigations to a great variety of different forms of SiC showed in ail cases carbon formation which crucially depended on the H₂O:SIC molar ratio. Depending on the reaction conditions and the SiC substrate nanosized graphitic carbon, amorphous carbon and carbons with significant sp'- content were detected by different methods of analysis with emphasis on Raman- spectroscopy. In the case of ß~SiC small particles of about 50 nm with morphology very similar to that of gas phase diamond powders formed in the hydrothermal process. XRD, TEM, and SAD analysis suggested the formation of cubic diamond as well as of lonsdaleite or other diamond polytypes beside graphite. In order to achieve tailored and homogeneous carbon coatings and larger diamond crystals the investigations were extended to SiC single crystals and modified fluids.