Phase transition and oxygen deficiency in electrochemically reduced Y-, Ca-PSZ crystals

Phase transition and oxygen deficiency in electrochemically reduced Y-, Ca-PSZ crystals were investigated using XRD and thermogravimetric techniques. PSZ samples were reduced in air at 1650 °C with DC current for 0 to 2 h and were oxidized at 1650 °C, or were quenched to room temperature keeping their reduction states. Reduced Y-, Ca-PSZ's were divided into two regions; severely reduced cathode and weakly reduced anode ones. A metastable tetragonal (t′) phase appeared in the cathode region of Y-PSZ samples reduced and oxidized or of those reduced and quenched. Reduced t′-phase had a small c/a ratio and its lattice volume was slightly contracted compared to the oxidized t′-phase. Phases of cathode region for Ca-PSZ samples reduced and oxidized or of those reduced and quenched were cubic (c). The lattice parameter of the reduced c-phase was slightly smaller than the oxidized c-phase. In thermogravimetric analyses in oxygen from room temperature to 1000 °C for reduced Y-PSZ and Ca-PSZ, two steps of weight gain were observed. The first step seems to be due to oxidation in weakly reduced anode region and the second to that in severely reduced cathode region. The oxygen deficiency in weakly reduced region was independent of the electrochemical reduction time.