Structure of chemically vapour deposited silicon carbide for coated fuel particles

Abstract

The silicon carbide coating layers prepared under various conditions were examined by density measurement, X-ray diffractometry, and optical and scanning electron microscopies in order to clarify the relation between deposition conditions and structure of the coating layers. It was found that the deposition temperature was the main parameter affecting the content of free silicon, density, crystallite size and lattice distortion, and microstructure. The dependence of these properties on the coating rate and the composition of fluidizing gas was not observed clearly. Free silicon was co-deposited withβ-SiC at temperatures lower than 1400 to 1500° C, and the content of free silicon increased with decreasing deposition temperature. The density of the layers without free silicon was more than 3.210 Mg m⁻³ and the density decreased with increasing content of free silicon. Crystallite size increased with deposition temperature and lattice distortion decreased with increasing deposition temperature. The outer surfaces of the layers without free silicon consisted of large interlocked grains, whereas those of the layers with free silicon showed a cauliflower-like structure of which the apparent grain size was small.