V-SHAPED POSITIVE TEMPERATURE-COEFFICIENT OF RESISTIVITY (PTCR) CHARACTERISTICS OF MICROWAVE-SINTERED (SR0.4PB0.6)TIO3

The resistivity-temperature (p-T) characteristics of the (Sr0.4Pb0.6)TiO3 materials prepared by the microwave sintering process were examined. The resistivities varied insignificantly with sintering temperature (1000-1100 degrees C) and soaking time (10-30 min), but increased markedly as the cooling rate reduced or as the heat-treatment temperature increased. The Curie temperature was not altered by the cooling rate control process but was shifted from T-c = 302 to 210 degrees C when the as-sintered samples were post-heat-treated at 1100 degrees C for 1 h. The high Curie temperature of the as-sintered samples was proposed to result from the core-shell structure of the grains. The 'shell', which contained Pb-rich perovskite of composition (Sr0.29Pb0.71)TiO3, was homogenized with the 'core' during heat treatment such that the Curie temperature approached T-c = 210 degrees C, as the composition of the 'shell' reduced to the designed value, (Sr0.4Pb0.6)TiO3.