Transmission electron microscopy microstructure of (K _0.5Na _0.5)NbO ₃ ceramics with CuO addition

The microstructures in Na _0:5K _0:5NbO ₃ (NKN) ceramics sintered at 960 °C with CuO additives were investigated with transmission electron microscopy (TEM). As a new microstructural constituent, CuO pockets have been observed at the grain boundaries of NKN and also inside the NKN matrix. The melting of CuO is caused by the element Na from the matrix, which forms Na-Cu-O eutectoid compounds whose melting point is lower than the sintering temperature. The kinetics of melting reaction largely depends on the size of the pocket. The interaction starts at the interfaces between the pocket and the matrix and advances into the interior of the pocket. The smaller pocket would melt earlier during the sintering process, flow into triple junctions between matrix grains, and participate in sintering via liquid phase sintering. In the larger pockets, the melting starts at the interfaces. Thus the outer areas are melted, but the CuO particles in the center remained unmelted. The NKN matrix then grows further into the pocket through liquid phase sintering, leaving low-angle grain boundaries behind that interface with the remaining CuO particles. The unmelted CuO particles in the pocket remain as the second-phase particles.