VAPOR-PRESSURES OF URANIUM AND URANIUM NITRIDE OVER UN(S)

The vaporization of uranium mononitride is investigated in the temperature range 1757 to 2400 K by Knudsen effusion cell mass spectrometry. The vaporization occurs incongruently by the preferential loss of nitrogen and the formation of U(1) in equilibrium with UN phase. In addition the vapour phase has U(g) and UN(g). The vapour pressures of U(g) and UN(g) are measured and their dependence with temperature can be represented by: log(p(U)(Pa)) = [(10.59 +/- 0.18)-(26857 +/- 357)/T(K)] (1757 < T < 2396 K), log(P(UN)(Pa)) = [(12.19 +/- 0.57)-(37347 +/- 235)/T(K)] (2190 < T < 2400 K). The enthalpy of vaporization at 298 K (third law) of U(g) is evaluated to be (564.37 +/- 7.37) kJ/mol. The enthalpy of vaporization of UN(g), at an average temperature of 2297 K, is determined to be (715.05 +/- 21.13) kJ/mol. The molar Gibbs free energy of formation of UN(g) is given as: DELTA(f)G-degrees (UN, g, T)(kJ/mol) = 352.75 - 0.0494T(K). The equilibrium constants for the dissociation of UN(s) (K1) and UN(g) (K2) into gaseous elements are given by: log(K1) = (13.03 +/- 0.18)-(42857 +/- 357)/ T(K), log(K2) = (0.84 +/- 0.60) - (5510 +/- 427)/ T(K).