Modification of PTCR behavior of (Sr0 center dot 2Ba0 center dot 8)TiO3, materials by post-heat treatment after microwave sintering

(Sr(0.2)Bn(0.8))TiO3 materials have been effectively densified by microwave sintering process. The grain size (similar to 6 mu m) and PTCR characteristics (T-c = 50 degrees C, rho(max)/rho(min) = 10(2)) of the as-sintered samples vary insignificantly with sintering temperature (1100-1180 degrees C) and soaking time (10-40 min). However, lowering the cooling rate after sintering substantially increases the resistivity jump (rho(max)/rho(min)) from 10(2) to 10(5.3), without altering the microstructure. Subsequent annealing, on the other hand substantially modifies the micr ostructure and PTCR characteristics. The resistivity jump increases monotonously with heat treatment temperature (T-ht) and reaches (rho(max)/rho(min)) = 10(7) for samples heat-treated at 1300 degrees C for 2 h. The grain size remains nearly unchanged for T-ht less than or equal to 1200 degrees C and grows markedly for samples heat-treated at a higher temperature. The effective tr ap level is estimated to be E(s) = 1.46 eV. The activation energy for the densification rate is Q(MS) = 8.2 kcal/mol for microwave sintering and Q(cs) = 62.5 kcal/mol for conventional sintering process.