Vapor Pressure of Uranium

Abstract

The vapor pressures and the heat of vaporization of liquid uranium have been measured by collecting a known fraction of the vapor effusing from a Knudsen cell. The deposits were ``weighed'' by comparing the number of fissions upon thermal neutron irradiation with the fissions from a standard. The measured rate of vaporization is extremely sensitive to the residual pressure of oxygen in the vacuum at background pressures of the order of 10^—7 mm Hg. With the aid of a theoretically derived expression the effusion rate can be extrapolated to zero oxygen pressure to yield for the vapor pressure the result that $$\log \mathrm{p(}\mathrm{mm}\mathrm{)=-}\frac{\text{(23330±21)}}{T}\text{+(8.583±0.011),}$$ , over the temperature range 1630 to 1970°K. This expression, together with heat capacity data for solid uranium, yields a value of 116.6 kcal/mole for the heat of sublimation at 0°K and predict a value of about 4.7 kcal/mole for the heat of fusion.