Thermal evolution of BaO-CuO flux as sintering aid for proton conducting ceramic fuel cells

Abstract

The eutectic melt of BaO-CuO flux is known to be a potential sintering aid for Ba(Zr,Y)O3 (BZY) electrolyte for proton-conducting ceramic fuel cells (PCFCs). A density of BZY higher than 97% of theoretical density can be achieved via sintering at 1300°C for 2 h using a flux composed of 28 mol% BaO and 72 mol% CuO. In the present study, chemical and structural evolution of BaO-CuO flux throughout the sintering process was investigated. An intermediate holding step at 1100°C leads to formation of various impurity compounds such as BaCuO1.977, Ba0.92Cu1.06O2.28 and Cu16O14.15, which exhibit significantly larger unit cell volumes than the matrix. The presence of such secondary compounds with large lattice mismatch can potentially lead to mechanical failure. On the other hand, direct heating to the final sintering temperature produced CuO and Cu2O as secondary phases, whose unit cell volumes are close to that of the matrix. Therefore, the final composition of the flux is strongly affected by the thermal history, and a proper sintering schedule should be used to obtain the desired properties of the final product.