GRAIN-BOUNDARY OXIDATION IN PTCR BARIUM-TITANATE THERMISTORS

The phenomenon of grain boundary oxidation in PTCR BaTiO3 thermistors is discussed. In particular, the energy spectra of the surface states were calculated for different samples, and these were related to the n ominal composition, the impurity content of the base BaTiO3 powder use d, and the prevalent atmospheric conditions during cooling and/or anne aling. It is proposed that the interaction of manganese with oxygen cr eates deep-lying traps, and, in general, some proof is offered that th e majority of the surface states are due to different oxidizing chemis orbed gases. It is believed that the ability of a particular sample to adsorb such gases in adequate amounts, and thus exhibit an appreciabl e PTCR effect, is related to the presence of acceptor-type dopants per ferentially segregated onto the grain surfaces. Notably, the role of 3 d transition metal cations in this process is discussed in some detail .