Study On The Preparation Of BaTiO₃-based Lead-free High Curie-point Positive Temperature Coefficient Ceramics

The experimental results on the composition, preparation procedure and performance of lead-free high Curie point positive temperature coefficient resistivity (PTCR) of Bi_0.5Na_0.5TiO₃ (BNT)-modified BaTiO₃ (BT) ceramics were studied. The whole article was divided into three parts: the study on Bi₂O₃ doping method and synthesizing BNT procedures, BNT pre-calcining methods and introducing sequence study, the preparation and mechanism of high Curie point PTCR ceramics.Firstly, Bi₂O₃-doping methods and synthesizing BNT procedures were studied in this article. From the two aspects of commonly used introducing procedures, i.e. pre-calcined Bi₂O₃, Na2CO3 and TiO2 to BNT then doping or directly doping these oxide state materials into BT-based ceramics, taking into account of the different level of Bi₂O₃-excess contents, the effect of pre-calcining temperature, holding time and sintering temperature on PTC property was studied, when BNT was doped at 0.1mol%. The PTCR samples with the highest Curie temperature about 142℃and 4.5 orders of magnitude of resistivity jump could be obtained when using the following two technical processes respectively: pre-calcining BNT at 780℃firstly with Bi₂O₃-excess 0.5mol% then doping into BT materials and doping with oxide state materials while Bi₂O₃-excess 1.0mol%, with holding 30min at 850℃for calcining BNT in the process of synthesizing BT-BNT solid solution. In addition, the influence of Bi₂O₃ doping methods and BNT doping contents on the property of samples when BNT-contents bigger than 1mol% was studied, while all the samples were sintered at air atmosphere, the same condition with previous experiments. The best BNT introducing method was obtained at last, that is, doping with 780℃pre-calcined BNT and Bi₂O₃ excess 0.5mol%.Secondly, a new pre-calcining BNT method was also studied in this article. As the semiconducting difficulties could be increased evidently when doping more BNT contents, we adopted donor complements when pre-calcining BNT to help their semiconductivity, in the molar ratio of Bi₂O₃:Na2CO3:TiO2:Nb2O5:Sb2O3=1¹.005:1: 4:0.00046:0.0010. The ceramics doped with 0.5mol% BNT and possessed PTC property were prepared in this experiment, when introducing 780℃pre-calcined 0.5mol%-Bi₂O₃ excessed above BNT in the starting materials. The correspondingly curie temperature of obtained samples was 149.7℃and resistivity jump 3.5 orders of magnitude.At last, the preparation technology and performance of high Curie temperature PTCR ceramics were studied, according to the above Bi₂O₃ doping methods.â‘ the effect of sintering procedure on the PTC property of materials was discussed. When BNT doping contents (x) smaller than 4mol%, the samples could be sintered at reducing atmosphere directly and the highest Curie temperature obtained in this experiments was about 192℃, with about 3 orders of magnitude of resistivity jump. The samples with bigger x values (<20mol%) could be semiconducting when using the sintering procedure of firstly air atmosphere sintering then reducing atmosphere heat-treatment and firstly reducing atmosphere sintering then another reducing atmosphere heat-treatment, respectively. But the correspondingly obtained samples possessed weak NTC effect, rather than PTC effect.â‘¡using the above new BNT pre-calcining method to synthesis PTCR ceramics with BNT doping contents, x, of 6mol%, 8mol%, 10mol% and 15mol% at reducing atmosphere sintering was studied. The Curie temperatures were 210℃, 226℃, 237℃and 243℃, respectively, but their resistivity jumps were decreased with x increasing, and when x=15mol%, it was only about an order of magnitude.â‘¢the preparation procedure of doping additional contents of MnO₂-acceptor on the basic materials was also studied. The suitable sintering temperature and atmosphere and heat-treatment at certain temperature could be obtained to improve the resistivity jump of PTCR materials.