VAPORIZATION OF URANIUM MONONITRIDE AND HEAT OF SUBLIMATION OF URANIUM

The vaporization of uranium mononitride UN has been investigated by the Knudsen effusion technique in combination with a mass spectrometer. The vaporization occurs incongruently by preferential loss of nitrogen forming the two-phase system nitrogen-saturated liquid uranium-uranium-saturated nonstoichiometric uranium mononitride. The vapor pressures of U and N 2 have been obtained by silver calibration over the two-phase system UN0.4-UN0.9 in the temperature range 1910-2230°K. The enthalpies ΔH°298 for the simplified reaction (1) UN(s) =U(g)+O.S N2 (g) and its partial reactions (2) UN(s) =U(1)+0.5 N 2(g) and (3) U(l)=U(g) have been obtained from second- and third-law treatments. The experimental thirdlaw value of Partial Reaction (3) of 130.7±2.2 kcal mole-1 may be considered an upper value for the heat of vaporization of uranium metal. The third-law enthalpy of Partial Reaction (2) of 68.9±0.4 kcal mole-1 is in general agreement with literature data for this reaction. The selected value for the enthalpy of Reaction (1) of ΔH°298 = 200.3±4.0 kcal mole -1 yields with appropriate literature data a heat of atomization ΔH°298 = 313.3±4.5 kcal mole-1 for solid UN, and a heat of vaporization ΔH°298= 127.9±4.9 kcal mole-1 for uranium. The difference of 2.8 kcal mole-1 between this value and the directly measured value for Partial Reaction (3) is attributed to an activity decrease of uranium in nitrogen-saturated liquid uranium that is in equilibrium with the UN phase.