Kinetics and mechanisms of carbon self-diffusion in uranium carbides

Depending upon the temperature, uranium carbide, $\text{UC}{}_{\mathrm{x}}$, has the B1 (NaCl type) crystal structure and is characterized by a wide range of the carbon-to-uranium ratio ($\text{0.95 < x < 2.0}$). In this report, the kinetics of carbon diffusion in uranium carbides are examined and mechanisms are proposed which explain the diffusivities determined by numerous investigators for compositions in the range $\text{0.95 < x < 2.0}$. When the composition is hypostoichiometric, the defect structure in the carbon sub-lattice consists of constitutional vacancies; the concentration of these vacancies is fixed by the carbon-to-uranium ratio and equals ($\text{1?x}$). Carbon diffusion occurs by the random migration of carbon atoms from one octahedral site to an adjacent vacant octahedral site. The defect structure in the carbon sub-lattice of stoichiometric and hyperstoichiometric uranium carbides consists of C₂ groups and single carbon atoms in the octahedral sites; the concentration of unoccupied octahedral sites (vacancies) is negligibly small. Diffusion occurs by the random migration of single carbon atoms from doubly occupied sites to adjacent singly occupied octahedral sites. Quantitative expressions of these mechanisms are developed which accurately describe the diffusion kinetics. The preponderant discordance in the experimental results of other investigators are reviewed and examined; it is concluded that the wide variability in these results was probably caused by microstructural effects,