Effect of charge redistribution through secondary phase formation on PTCR characteristics of n-BaTiO3 ceramics

Heterogeneous addition of alumina, up to 10 mol%, as a grain boundary modifier in nBaTiO(3) PTCR ceramics modifies the rho-T characteristics whereby T-max is shifted from 455 to 700 K, accompanied by the decrease in rho(max). Whereas B2O3 shifts the T-max to 410 K with little change in rho(max). The grain boundary segregation of alumina leads to the formation of secondary phases, BaAl6TiO12 and Ba3Al10TiO20. The changing pattern of trap states is evident from the presence of Ti4+-O--Al3+ type hole centers at the grain boundary layer regions, as identified by the EPR spectra. The charge redistribution among the inter-band gap defect states across the Curie temperature is substantiated by the EPR results. (C) 1998 Elsevier Science B.V.