Densification behavior and microstructure evolution of yttrium-doped ThO2 ceramics

Sintering of Th1-xYxO2-x/2 ceramics (x = 0.01, 0.08, 0.15 and 0.22), planned to be used as solid electrolytes in oxygen sensors for sodium-cooled fast nuclear reactors, was investigated. High densification state (i.e. up to 98% TD) was reached after 4 h of heat treatment at 1600 °C and beyond. In addition, ESEM observations showed a major effect of yttrium on grain size due to solute drag effects. Sintering maps were plotted for all the samples and evidenced different stages driven by densification and grain growth. Grain growth was found to be strongly slowed down for x > 0.01, resulting in high values of relative density correlated to submicrometric grain size. Also, activation energies related to densification and grain growth were evaluated around 450 and 500-650 kJ mol−1, respectively. These results led to deliver guidelines for the formulation and sintering of Th1-xYxO2-x/2 ceramics in prospect of their use as a solid electrolyte.