Effect of segregative additives on the positive temperature coefficient in resistance characteristics of n-BaTiO3 ceramics

The influence of B2O3, and Al2O3 as segregative additives in modifying the rho-T characteristics has been studied in BaTiO3 ceramics with positive temperature coefficient of resistance (PTCR). Reaction of Al2O3 at the grain boundary regions of BaTiO3 ceramics leads to the segregation of the secondary phase, BaAl6TiO12 resulting in broad PTCR jump, whereas B2O3 addition gives rise to steeper resistivity jump. Microstructure studies by SEM reveal the formation of coherent second phase layer of barium aluminotitanate surrounding the BaTiO3 grains. The EDX results shows varying Al to Ti ratio in the early stage of phase formation in BaAl6TiO12 resulting in electrically active layer around the BaTiO3 grains. The TiO2-excess melt formation results in lower resistivity for 2-4% Al2O3 containing n-BaTiO3 ceramics whereas at higher alumina contents, BaAl6TiO12 phase becomes dominant leading to higher resistivity in the sample. Complex impedance analyses support the three-layer regions, corresponding to the contributions from grain interior resistance (R-g), grain boundary resistance (R-gb), and that from secondary phase (R-sec). Electron paramagnetic resonance spectroscopy (EPR) indicated barium vacancies, V-Ba(/) as the major electron trap centers which are activated across the tetragonal-to-cubic phase transition. A charge trapping mechanism is proposed wherein the segregation of secondary phases bring carrier redistribution among the various acceptor states thereby affecting the electrical conductivity of n-BaTiO3 ceramics. The presence of Al3+-O--Al3+ or Ti4+-O--Al3+ type hole centers at the grain boundary layer (GBL) regions results in charge redistribution across the modified phase transition temperature due to symmetry-related vibronic interactions resulting in broad PTCR characteristics extending to higher temperatures.