Chemical vapor deposition of silicon carbide from methylsilane and coating of nuclear waste ceramics

Author

Peter Angelini

Date of Award

6-1985

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Metallurgical Engineering

Major Professor

E. E. Stansbury

Committee Members

C. R. Brooks, B. F. Oliver, J. E. Spruiell, P. G. Huray, J. Bentley, W. J. Lackey

Abstract

This research evaluated the chemical vapor deposition (CVD) of silicon carbide from methylsilane (CH₃SiH₃) 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 — H₂) or (N₂ — H₂), and addition of C₂H₂to the system. Pyrocarbon coated alumina, and nuclear waste ceramic substrate particle were coated with silicon carbide in a fluidized-bed coater.

Most of the thermodynamic calculations were performed with a mole ratio of CH₃SiH₃/diluent = 5.8 x 10^-3 and a total system pressure of 1.01 x 10⁵ Pa. The results for the CH₃SiH₃ — Ar system indicate that SiC + Si are codeposited up to a temperature of ≈1450 K. Single phase SiC is computed to form above that temperature. The results for the CH₃SiH₃ — H₂ system are similar to those mentioned above except SiC + Si is computed to form over the entire 973 to 1600 K temperature range. The silicon content in the condensed phase increases as the temperature decreases or the hydrogen content in the diluent gas increases. Additions of various amounts of C₂H₂ to the CH₃SiH₃ — (Ar — H₂) system can alter the types and concentration of phases present in the condensed phase (SiC + Si, SiC + C, or SiC). Results for the condensed phases are presented as CVD phase diagrams and those of the gaseous phase as partial pressures of various species.

The coatings were evaluated by x-ray diffraction, Raman spectroscopy, quantitative electron microprobe, and other techniques. Coatings deposited at 1073 K were amorphous and those deposited at 1180 K were crystalline. Annealing of the amorphous SiC coating to 1173 K crystallized the material. The major phase observed is β-SiC, however a small amount of α-SiC is also present and the coatings are isotropic. The SiC crystallite size decreased as the hydrogen content in the diluent gas increased.

Coatings deposited in diluents of high hydrogen content had a silicon content greater than that of stoichiometric SiC by ≈2 wt %. The phases and their relative concentration present in the coatings were modified by adding C₂H₂to 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.

Recommended Citation

Angelini, Peter, "Chemical vapor deposition of silicon carbide from methylsilane and coating of nuclear waste ceramics. " PhD diss., University of Tennessee, 1985.
https://trace.tennessee.edu/utk_graddiss/12509