| This research evaluated the chemical vapor deposition (CVD) of silicon carbide from methylsilane (CH(,3)SiH(,3)) and coating of nuclear waste ceramics. The phase equilibria of the SiC coatings were investigated and compared with thermodynamic equilibrium calculations. The major variables studied were coating temperature, hydrogen content of the diluent gaseous system (Ar - H(,2)) or (N(,2) - H(,2)), and addition of C(,2)H(,2) to the system. Pyrocarbon coated alumina, and nuclear waste ceramic substrate particle were coated with silicon carbide in a fluidized-bed coater.; The thermodynamic calculation results on the CH(,3)SiH(,3) - Ar system indicate that SiC + Si are codeposited up to (DBLTURN)1450 K. Single phase SiC is computed to form above that temperature. The silicon content of the condensed phase increases as the temperature decreases or the hydrogen content in the diluent gas increases.; Coatings deposited at 1073 K were amorphous and those deposited at 1180 K were crystalline. The major phase observed was (beta)-SiC, however a small amount of (alpha)-SiC was also present. Coatings deposited in an argon diluent were stoichiometric and those deposited in diluents of high hydrogen content had a silicon content greater than that of stoichiometric SiC by (DBLTURN)2 wt %. The phases and their relative concentration present in the coatings were modified by adding C(,2)H(,2) to the system. The experimental results were in agreement with thermodynamic calculations.; The coated particle concept for solidification of high-radiation-level nuclear waste was also evaluated. Sol-gel derived substrates of high-waste loading, and Cs-loaded zeolite were coated with SiC and pyrolytic carbon in a fluidized-bed coater. Leach testing by standard techniques showed them to be highly leach resistant and possess leach rates lower than other nuclear waste forms. |
