Uranium self-diffusion in stoichiometric uranium monocarbide

Self‐diffusion measurements, with ²³³U as the tracer, have been obtained for stoichiometric arc‐cast UC at temperatures between 1480°C and the melting point and for zone‐refined UC single crystals at temperatures between 1500 and 2125°C. α spectroscopy was used to measure the α‐energy degradation from which the diffusion profiles were determined. Conventional sectioning techniques were employed to determine the profiles at high temperatures. The results obtained from arc‐cast UC yielded a curved Arrhenius plot. Computer programs were developed to fit the sum of two exponentials to the experimental data. This was accomplished by changing one or two of the parameters, D_0,1, D_0,2, ΔH₁, and ΔH₂, in steps and adjusting the remaining parameters to obtain the minimum standard deviation σ. The resultant equation was $D_{\mathrm{UC}}^{\mathrm{U}}\text{=6.9}\exp {\text{(−141\hspace{0.17em}000/RT)+3.6×10}}^{\text{−5}}\exp \text{(−84\hspace{0.17em}500/RT)\hspace{0.17em}}{\mathrm{cm}}^2{\sec }^{\text{−1}}$ . The data obtained from the single crystals showed a linear Arrhenius plot described by $D_{\mathrm{UC}}^{\mathrm{U}}\text{=11.7}\exp \text{(−141\hspace{0.17em}900/RT)\hspace{0.17em}}{\mathrm{cm}}^2{\sec }^{\text{−1}}$ . The influence of impurities at low temperatures $\mathrm{(<}{\displaystyle {lim}_{\sim }}\text{2100°}\mathrm{C})$ is the most probable cause of the curvature in the Arrhenius plot for the arc‐cast UC.