The as-sintered pre-conversion pellets (Ga " O ' + YSZ) were loosely buried in the vapor phase packing powder (Na_β'' -Al " O ' ) in an alumina crucible covered with an alumina lid. The encapsulated as-sintered samples were placed in a muffle furnace (Across International) for 5 to 30 hours with 5-hour intervals at 1250 °C.