| Summary: | 碩士 === 國立成功大學 === 材料科學及工程學系碩博士班 === 93 === The Positive temperature coefficient of resistivity (PTCR) materials have become very important components, and among these materials barium titanate (BaTiO3) compounds are the most important group. BaTiO3 is a perovskite structure with ferroelectric properties at room temperature. Small amounts of donor doping in BaTiO3 cause semiconductivity and PTCR effect in this material. The PTCR effect of semiconducting BaTiO3 ceramics can be remarkably influenced by both donor dopants and ceramic processing.
The effects of La dopant （0.2∼0.4at%）and different cooling rates on the room resistance and the PTCR effect were investigated in this paper. DC resistivity measurements were carried out as a function of temperature from the room temperature up to 280℃ and data of ac impedance was measured at different temperatures to know the electricity of the individual part of the polycrystalline BaTiO3. Besides, Scanning electron microscope (SEM) was employed to study the microstructures. In this paper, we found that the resistance reached minimum when dopant concentration reached 0.3at%, and the specimen brought about the better PTCR effect, resulting in an increase in the steepness of the resistivity-temperature plots in the region of the transition from low to high resistance. We can separate the grain and the grain boundary resistance by impedance analyses and we found that the origin of the PTCR effect was the grain boundary resistance. It was found that the cooling rates didn’t have significant influence on the microstructure such as grain sizes.
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