SiC, from Amorphous to Nanosized Materials, the Exemple of SiC Fibres Issued of Polymer Precursors

Abstract

Silicon carbide materials are interesting because of their high thermal stability, high thermal conductivity, extreme hardness and good electrical properties. Some of these properties are directly related to the SiC structure that alternates the Si and C atom layers. SiC have applications as single crystal wafer (wide-gap semiconductor for high power device), as air, spacecraft and nuclear plant thermostructural composites because it is possible to prepare the materials in various forms (fibre, matrix and composite) from organic liquid or vapour precursors. Thus it is possible to adjust the crystallinity, from the amorphous to the crystalline state, including intermediate nanocrystalline state. The later state guaranties the optimal mechanical properties. Non-destructive Raman microspectrometry intrinsically probes the matter at the subnanoscale and offers a "bottom-up" approach that is especially efficient for the analysis of ill-crystallised and nanostructured materials. The great advantage of Raman spectroscopy is first to make possible the recording of series of spectrum by automatic mapping. However, because the Raman intensity depends on the polarisability tensor derivative, the scattered intensity – and the sensitivity varies order of magnitude with the nature of the chemical bond. Comparison with transmission electron microscopy (TEM) is in general required to discriminate if the phonon coherence length is determined by the domain or the grain size. Both techniques are efficient to identify the different polytypes as well as the nature and degree of the disorder.