VOLATILIZATION CHARACTERISTICS OF URANIUM MONONITRIDE

Experiments have been performed by transpiration, vacuum microbalance, effusate collection, and mass spectrometry to determine mass transport in uranium mononitride. It is shown that under equilibrium conditions uranium mononitride will preferentially lose nitrogen, and molten uranium will be a product at all temperatures above the melting point of uranium. Under dynamic vacuum conditions, however, nitrogen vaporization is markedly curtailed probably as a result of the slow rate of diffusion of nitrogen through uranium mononitride. Under dynamic conditions, it is possible to vaporize UN without the accumulation of molten uranium on the surface. The transpiration determinations of vaporization of UN have been utilized in a thermodynamic cycle to establish a value of 126 kcal/mole as the standard heat of vaporization of uranium. This value was further substantiated by effusate collection which yielded a value of 126.5 kcal/mole for the heat of vaporization of uranium. It is indicated that the UN homogeneity range extends to UN0.97 at 1600°C and approaches UN0.96 at 1700°C. Across the portion of the homogeneity range of UN that was examined the activity of uranium changes by a factor of about a thousand at these temperatures. © 1969.