URANIA-YTTRIA SOLID-SOLUTION ELECTRODES FOR HIGH-TEMPERATURE ELECTROCHEMICAL APPLICATIONS

Measurements of total electrical conductivity on fluorite-type U3O8-Y2O3 (Sc2O3) solid solutions have been made as a function of temperature and U/Y(Sc) ratio. The following compositions were studied: (U0.7Y0.3)O2+x, (U0.6Y0.4)O2+x, (U0.5Y0.5)O2+x, (U0.45Y0.55)O2+x, (U0.4Y0.6)O2+x, (U0.35Y0.65)O2+x, (U0.3Y0.7)O2-x, (U0.5Sc0.5)O2+x and (U0.38Sc0.62)O2+x . Preliminary measurements on the latter two compositions were carried out for comparison purposes. The maximum conductivity value occurred for the U3O8-Sc2O3 solid solutions, and for (U0.7Y0.3)O2+x in the U3O8-Y2O3 system. The conductivity in these fluorite-type solid solutions is mainly electronic, the conduction mechanism being hopping-type. The energy of activation lay between 25 and 40 kJ mol-1. The (U0.3Y0.7)O2-x composition appeared to be an ionic conductor with an activation energy of ∼110 kJ mol-1 below 800 to 850° C. The diffusion of cations of U3O8-Y2O3 into ZrO2-Y2O3 was studied during passage of current: no observable diffusion occurred over the period of current passage (384 h). Attempts were made to determine the anionic contribution to the total conductivity in U3O8-Y2O3 solid solutions using the blocking electrode technique. Results indicated that complete isolation of the specimen-blocking electrode (YSZ) interface from the ambient gases is necessary if such measurements are to be reliable. The diffusion coefficients calculated from the conductivity data using the Nernst-Einstein relation were two orders of magnitude higher than those obtained by a direct method. © 1979 Chapman and Hall Ltd.