ANNEALING EFFECTS OF SEMICONDUCTING BARIUM-TITANATE THERMISTER

Abstract

Two Positive Temperature Coefficient of Resistivity (PTCR) ceramics with composition A, (Ba0.997Sb0.003)Ti1.005O3, and composition B, (Ba0.997Sb0.003)Ti1.005O3+0.1 mole% MnO2 were fabricated and examined herein. The ceramic grain size, oxygen pressure and annealing time need to be simultaneously controlled to obtain an optimum PTCR resistor. Additionally, the stoichiometric proportions of the constituent elements must be weighted carefully in order to produce good quality PTCR devices. This study thoroughly explores all the factors affecting the PTCR characteristics. Experiments were developed to verify that PTCR phenomena are strongly influenced by oxygen absorption. As the samples were annealed in Air atmosphere, the numbers of foreign ions compensation were increased by this oxidation process. The Schottky barrier is formed between grains and grain boundaries, improving the PTCR effect and resulting in very steep gradient Resistivity-Temperature (R-T) plots. Annealing samples in oxygen atmosphere also improves the PTCR effect and reduces the annealing time dramatically, but increases the room-temperature resistivity. By contrast, annealing in reduced air (98% N2+2% H2) atmosphere decreases the resistivity of the ceramics, and also diminishes the PTCR effect of the ceramics. The complex-plane-impedance method was used to analyze the influence of annealing condition (temperature, time and atmosphere) on the PTCR devices. Our study inferred that PTCR mechanism originated from the grain boundaries rather than the bulk. Annealing at 1200℃ was found to control the PTCR characteristics of the ceramics more effectively than the traditional cooling rate control process. The annealing processing technology and theoretical mechanism affecting the PTCR ceramics were designed and discussed in detail. Factors influencing the PTCR phenomena were studied, showing that the design of the PTCR devices is practical.