Study on the main factors affecting the breakdown voltage of (Bi0.5Na0.5) TiO3-added (Ba0.659Pb0.341)TiO3 PTCR ceramic materials

Abstract

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.