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The breakdown voltage (Vb) was studied for (Bi0.5Na0.5)TiO3 (BNT)-added positive temperature coefficient of resistance (PTCR) ceramic samples based on (Ba0.659Pb0.341)TiO3, which were prepared by a traditional solid-reaction method and sintered in an air atmosphere. By measuring the electrical properties and microstructures of the materials, the breakdown voltage was found to depend on neither the room-temperature resistivity nor the maximum resistivity, but on the grain size (D) of the samples to some extent. With a decrease in the grain size, the Vb value increases rapidly, especially for samples with fine grains. With the help of Heywang-Jonker theory, the depletion width (LD) was calculated to determine the dominant factor affecting the Vb value using the approximate equation Vb= 600 × LD/D + 100; i.e., the breakdown voltage is approximate directly proportional to the ratio of LD/D. However, the room-temperature resistivity increased slightly with LD/D, and the PTC performance remained at a high level (α > 34%, Rmax/Rmin > 1 × 103). It is more advantageous to increase the breakdown voltage by adjusting LD/D than by reducing grain size alone. The variations in the grain size and the depletion layer width of different samples are mainly due to the generation of VNa′ and VBiʺ′ inside of the lattice. The ratio of LD/D can also be used to explain the variation in room-temperature resistivity. [ABSTRACT FROM AUTHOR]

The phase transition performance and positive temperature coefficient of resistivity (PTCR) effect in Mn-doped (BaK)NbO-based ceramic material were investigated. The results showed that an excellent PTCR effect with a resistance anomaly ( Rmax/ Rmin) up to 5 × 10 and a switch temperature ( Ts) of approximately 60 °C was obtained in the ceramic sample. In contrast to the phase transition of orthorhombic-to-tetrahedral and tetrahedral-to-cubic with an exothermic effect, endothermic effects were observed near the temperature of the rhombohedral-to-orthorhombic ( R- O) phase transition. The XRD results at elevated temperature showed significant cell shrinkage when the lattice structure transferred from the rhombohedral state to an orthorhombic state, accompanied by remarkable internal stress inside the ceramic sample. The XRD results also showed that both the rhombohedral and orthorhombic phase co-existed near the temperature of the R- O phase transition. At this temperature, we observed a considerable dielectric anomaly from the measurement of the temperature dependency of the dielectric constant, which was believed to be from the unique R- O phase transition. The PTCR effect observed in this material was mainly contributed by the anomalous dielectric constant. [ABSTRACT FROM AUTHOR]