Capacitor-varistor materials were studied by using various electron microscopic techniques. They were found to contain 30 grains with a perovskite structure, a Na+-diffused layer at the grain boundaries, and multiple-grain junctions where TinO2n-1 Magneli phases coexisted with an amorphous intergranular phase. Also observed were wide (10 to 30) grain boundaries, thin (about 1) grain boundaries, and boundaries which were free of intergranular phases. It was concluded that the diffusion and distribution of Na+ depended upon the grain boundary structure. High-angle and/or thin boundaries corresponded to infinite-source conditions, while low-angle boundaries corresponded to finite-source conditions.

Q.Zou, Z.Meng: Journal of the American Ceramic Society, 1995, 78[1], 58-64