Thermodynamic study of UO2+x by solid state EMF technique

A complete set of thermodynamic parameters of UO2+x - the relative partial molar thermodynamic quantities of oxygen anti g(O2), anti h(O2) as a function of nonstoichiometry x and temperature T - have been determined with sufficient accuracy by the precise emf measurements of the solid state galvanic cell of the type Ni, NiO/Stabilized ZrO2/UO2+x, at 0.003≤x≤0.23 between 500 and 11000C. Nonstoichiometry x was controlled and determined by the coulometric titration of oxide ions at 10000C by using NiO in the Ni/NiO reference mixture as a source of oxygen. UO2+x samples of two different preparation procedures give almost identical results and show that anti g(O2) versus T plots at various compositions x are not a linear function of temperature, but curve downwards with temperature in the composition and temperature ranges studied. This tendency becomes more pronounced with decreasing nonstoichiometry x and indicates that both anti h(O2) and anti s(O2) of UO2+x increase with temperature, their temperature dependence becoming stronger with decreasing nonstoichiometry x. (orig./TW)