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

Abstract

Heterogeneous addition of alumina, up to 10 mol%, as a grain boundary modifier in $nBaTi0_3$ PTCR ceramics modifies the p-T characteristics whereby $T_{max}$ is shifted from 455 to 700 K, accompanied by the decrease in $p_{max}$. Whereas $B_2O_3$ shifts the $T_{max}$ to 410 K with little change in $p_{max}$. The grain boundary segregation of alumina leads to the formation of secondary phases, $BaAl_6TiO_{12 }$ and $Ba_3Al_{10}TiO_{20}$. The changing pattern of trap states is evident from the presence of $Ti^{4+}-O^--Al^{3+}$ 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.