Conventionally batched varistors in the ZnO–Co3O4–Pr6O11 system that were sintered at or above 1280 °C exhibited a sharp increase in the average breakdown voltage per grain boundary. In these samples, barrier heights and donor concentration values, derived from capacitance-voltage analysis, exhibited sharp changes at the same soaking temperature. At or above 1280 °C, grain growth in this system proceeds with the assistance of an eutectic liquid. The improvement of the distribution of Pr6O11, by precipitation of Pr(OH)3 during batching, resulted in varistors markedly more insensitive to firing temperature than conventionally batched ones. The presence of a liquid phase, at least during grain growth, results in an efficient distribution of grain boundary materials and dopants in general. Suitable chemisorbed gases along the grain boundaries are believed to be responsible for the formation of interface states. Segregated materials appeared to provide for enhanced transport of gaseous species along the grain boundaries. The need for the addition of oxides with large ionic-radius cations, i.e., varistor-forming oxides such as bismuth, barium, praseodymium, or lanthanum oxide, to varistor compositions is explained within this context.

The phenomenon of grain boundary oxidation in PTCR BaTiO, thermistors is discussed. In particular, the energy spectra of the surface states were calculated for different samples, and these were related to the nominal composition, the impurity content of the base BaTiO, powder used, and the prevalent atmospheric conditions during cooling and/or annealing. It is proposed that the interaction of manganese with oxygen creates deep-lying traps, and, in general, some proof is offered that the majority of the surface states are due to different oxidizing chemisorbed gases. It is believed that the ability of a particular sample to adsorb such gases in adequate amounts, and thus exhibit an appreciable PTCR effect, is related to the presence of acceptor-type dopants perferentially segregated onto the grain surfaces. Notably, the role of 3d transition metal cations in this process is discussed in some detail.