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 N₂ have been obtained by silver calibration over the two‐phase system UN_0.4–UN_0.9 in the temperature range 1910–2230°K. The enthalpies ${\mathrm{\Delta H°}}_{298}$ for the simplified reaction (1) UN(s)=U(g)+0.5 N₂ (g) and its partial reactions (2) UN(s)=U(l)+0.5 N₂(g) and (3) U(l)=U(g) have been obtained from second‐ and third‐law treatments. The experimental third‐law 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 ${\mathrm{\Delta H°}}_{298}\text{\hspace{0.17em}=\hspace{0.17em}200.3\hspace{0.17em}±\hspace{0.17em}4.0}{\mathrm{kcal\; mole}}^{\text{−1}}$ yields with appropriate literature data a heat of atomization ${\mathrm{\Delta H°}}_{298}\text{\hspace{0.17em}=\hspace{0.17em}313.3\hspace{0.17em}±\hspace{0.17em}4.5}{\mathrm{kcal\; mole}}^{\text{−1}}$ for solid UN, and a heat of vaporization ${\mathrm{\Delta H°}}_{298}\text{\hspace{0.17em}=\hspace{0.17em}127.9\hspace{0.17em}±\hspace{0.17em}4.9}{\mathrm{kcal\; mole}}^{\text{−1}}$ 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.