High-temperature thermodynamic properties of hypo- and hyper-stoichiometric uranium carbides

Measurements of carbon activity over univariant and bivariant UC_x compositions, including the difficult region near stoichiometry, have been made in the temperature range 2155–2455°K. A flowing-gas technique was used with H₂-CH₄ mixtures in order to fix the carbon potential in the gas stream. The shape of the isotherms showed a sharp decrease in carbon activity near the stoichiometric composition as the composition of the condensed phase was reduced toward the lower phase boundary in the hypostoichiometric region. With argon as the carrier gas, a congruent vaporization composition of C/U ≈ 1.05 was obtained. Values for the free energy and heats of formation, relative partial molar enthalpies and entropies of solution, enthalpy of vaporization and uranium activity are presented.