Fluidized bed deposition and evaluation of silicon carbide coatings on microspheres
- 1 January 1977
- report
- Published by Office of Scientific and Technical Information (OSTI)
Abstract
The fuel element for the HTGR is an array of closely packed fuel microspheres in a carbonaceous matrix. A coating of dense silicon carbide (SiC), along with pyrocarbon layers, is deposited on the fueled microspheres to serve as a barrier against diffusion of fission products. The microspheres are coated with silicon carbide in a fluidized bed by reaction of methyltrichlorosilane (CH/sub 3/SiCl/sub 3/ or MTS) and hydrogen at elevated temperatures. The principal variables of coating temperature and reactant gas composition (H/sub 2//MTS ratio) have been correlated with coating rate, morphology, stoichiometry, microstructure, and density. The optimum temperature for depositing highly dense coatings is in the range 1475 to 1675/sup 0/C. Lower temperatures result in silicon-rich deposits, while higher temperatures may cause unacceptable porosity. The optimum H/sub 2//MTS ratio for highly dense coatings is 20 or more (approximately 5% MTS or less). The amount of grown-in porosity increases as the H/sub 2//MTS ratio decreases below 20. The requirement that the H/sub 2//MTS ratio be about 20 or more imposes a practical restraint on coating rate, since increasing the total flow rate would eventually expel microspheres from the coating tube. Evaluation of stoichiometry, morphology, and microstructure support the above mentioned optimum conditionsmore » of temperature and reactant gas composition. 18 figures, 3 tables. « lessKeywords
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