| With the rapid development of information technology, miniaturization, integration and high reliability have become the main research direction of electronic component nowadays. Accordingly miniaturization, chip type, low room temperature resistance and high performance are the development trend of BaTiO3-based PTCR. The resistance of conventional bulk or single PTCR cannot further be reduced because of the high inherent resistivity of BaTiO3-based semiconducting materials, and a feasible way to reduce the resistance is to fabricate multilayered chip PTCR, which has the characteristics of small shape, high current flux and good current-inrush-proof, therefore become a research direction with academic significance and application perspective. The dissertation relies on the national "863" project of the tenth five-year plan "Investigation of materials system and key technology of multilayer chip PTCR ceramics"(No. 2001AA320504). Based on the study of component and sintering scheme of Ba0.75Sr0.25Ti1.01O3 PTCR material system, ceramic with room resistivity of 8.0?·cm, temperature coefficient of 10.3%℃-1, ratio of maximum to minimum of resistance of 4.3, and resisting electrical strength of 50V/mm were successfully prepared. Polyacrylamide gel process was first used to synthesize BaTiO3-based PTCR ceramic powders. The influence of synthesis conditions on the objective phase was studied, the results showed that high crystallized objective phase could be obtained when pH being between 7-8, polyacrylamide gel being strongly polymerized, and sintering temperature being between 700-900℃. The experiment showed that polyacrylamide gel process could incorporate donor at a wide range and lower the sintering temperature of ceramics to 1280℃ which provided the foundation for the prparation of mutilayer PTCR ceramics by co-firing. Compared with that prepared by solid-state reaction method, PTCR ceramic with more homogeneous structure and better PTC effect could be prepared by polyacrylamide gel process, and the ratio of maximum to minimum of resistance of the ceramic was higher by one magnitude. Gelcasting process of chip PTCR was systemically investigated, including the preparation of aqueous slurry with high solids loading and low viscosity, gelcasting of thick films, drying and sintering of films, and microstructure and properties of the chip PTCR. Chip thermistors with room resistance of 2?, temperature coefficient of 12%℃-1, and ratio of maximum to minimum of resistance of more than 4 were successfully prepared. The size of the chip thermistor was 12.0mm×8.0mm×0.34mm. The thickness-dependence of room temperature resistance of chip PTCR was studied from the microcosmic and macroscopical viewpoints. First the oxygen adsorption and diffusion process was taken into account to explain the relationship, second, the impacts of some micro-structural factors, such as the arrangement of grains between electrodes, and crystal axis orientation on dielectric properties were analyzed theoretically, and the microcosmic mechanism was presented to describe the thickness-dependence of room temperature resistance by considering c-axis orientation toward the direction vertical to the film surfaces as the thickness decreases, which results in the decrease of dielectric constant of the ceramic, therefore decrease of ferro-electric compensation according to Heywang and Jonker model, and increase of room temperature resistance occured. For the present film specimens, the room temperature resistance of chip PTCR increases sharply only when thickness decreased to a certain extent (less than 0.5mm). In order to fabricate chip PTCR with good evenness of the surface, a novel process "gelcasting large bar part-drying-sintering-cutting" was present, and the polyethylene glycol (PEG) solution drying method was first used to dry the gelcast parts of BaTiO3-based ceramics in this dissertation. The impacts of factors, such as loading level of ceramic powders, liquid desiccant concentration, effective thickness and geometry of parts, on the drying rate were studied. Lowering solid content in the gel, increasing liquid desiccant concentration, and lowering effective thickness of parts, the drying rate would increase. Decrease of aspect ratio in cylindrical parts changed the diffusion mechanism from radial to longitudinal direction. Based on the analysis of the defects deriving from drying process in liquid desiccant method, it can be found that loading level of ceramic powders and the liquid desiccant concentration directly connect with the drying defect and the sintering defect of ceramic parts. It is proposed that increasing the loading level of ceramic powders is the approach to obtain high perfomance PTCR ceramics. Flat chip PTCR ceramic was successfully prepared by means of the process "gelcasting large bar part-drying-sintering-cutting" and a five-layer chip PTCR thermistor with flat surface and reliable adherence of internal electrode was fabricated by electrode bonding method. The size of the five-layer chip PTCR thermistor was 10.2mm×5.0mm×1.94mm,with room resistivity of 1.2?, temperature coefficient of 11.0%℃-1,and ratio of maximum to minimum of resistance of 4.1. Reduction-reoxidation firing was investigated to prepare multilayer PTCR thermistor, which provide the foundation for preparing multilayer PTCR thermistor by co-firing.
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