COMPLEX IMPEDANCE ANALYSES OF N-BATIO3 CERAMICS SHOWING POSITIVE TEMPERATURE-COEFFICIENT OF RESISTANCE

The influence was studied of grain boundary layer modifiers on the imp edance spectra of semiconducting BaTiO3 ceramics processed in various atmospheres. Ceramic samples of undoped BaTiO3 annealed in an N-2 + H- 2 atmosphere showed a single semicircle in the Cole-Cole plot when ohm ic Ni electrodes were used. Impedance spectra of donor-doped specimens sintered in air showed overlapping semicircles which may indicate rel axational processes within the grains, e.g. electron trapping and detr apping at the deep traps. This is strongly supported by the minima sho wn by the grain resistance (R(g)) around the Curie point (T-c), wherea s the behaviour of the grain boundary resistance (R(gb)) with temperat ure is similar to the d.c. resistance, i.e. a positive temperature coe fficient of resistance (PTCR). Electron paramagnetic resonance (EPR) r esults indicate that the charge trapping is caused by the redistributi on of electrons at native and extrinsic acceptor states during phase t ransitions. Thus, the neutral barium vacancies (V-Ba(X)) present in th e tetragonal and orthorhombic phases change into singly ionized barium vacancies (V-Ba') in the cubic and rhombohedral phases. From the corr elation between the resistivity changes and the charge redistribution at the trap states, it is envisaged that the PTCR effect is related to charge trapping at the acceptor states having higher concentration in the regions of the grain boundary layer. The impedance spectral data support this conclusion.