Preparation And Electrical Properties Of The Thick Film NTC Thermal Paste

With the expansion of thermistor applications, temperature measurement, sensing and control accuracy requirements are increasing. In this paper, tracking the thermistor material development at home and abroad, based on lead-free thick-film NTC thermistor for the study of the development objectives, dedicated to developing low resistance lead-free thick-film NTC thermistor paste. Main research contents and results are as follows:The BaBiO₃ thick film NTC resistor pastes were prepared, the effects of various factors on the properties of thick-film paste were investigated. A suitable organic carrier formula for thick film resistors was, and the firing process of BaBiO₃ thick film was developed. The effects of Ti⁴⁺-doped amount on the microstructure and electrical properties were investigated. With an increase in BaTiO₃ content the grain size decreased, the room temperature square resistivity for thick film decreased to a minimal value and then began to increase. When BaTiO₃ amount x=1, the thick film sample with good NTC characteristics was obtained and its resistivity RS was about 2.4×10⁵Ω·sq ⁽-10 , B_25⁸5 value was about 3100 K andα₂₅ was about -0.0348℃^-1.The BaFe_1-xSn_xO₃ ceramic were prepared by solid-state reaction method. The effects of x value on the microstructure and electrical properties of thick-film were investigated. When x(Sn) valued between zero and 0.4, the structure of ceramic was solid solution, with a increase in Sn content the grain size decreased to a minimal value and then began to increase. When x(Sn) valued between 0.05 and 0.15, BaFe_1-xSn_xO₃ ceramic obtained a good density after sintering at 1300℃, with a increase with x value the grain size and resistivity increased, B_25⁸5 andα₂₅ value changed as"U"type. The composition with the best comprehensive performance was obtained as x=0.1.The BaFe_1-xSn_xO₃ thick films were prepared and investigated. The best preparing of BaFe_1-xSn_xO₃ powder is 1250℃for 6h, and the best preparing for thick films as follow: heated to 900℃for 1h with the heating rate of 2℃/min, then heated to 1300℃for 2h with the heating rate of 5℃/min, and then cooled with furnace. The functional phase with a formula BaFe_0.9Sn_0.1O₃ obtained the best property and its resistivity RS was about 2.5×10⁷Ω·sq^-1 , B_25⁸5 value was about 3542K.The BFS paste was prepared for the first time, and the effects of BaBiO₃ content on the electrical properties of thick-film paste were investigated. The BFS films had a Good NTC characteristics of low resistance and high B_25⁸5 value. BaBiO₃, played a role as a binger, could reduce the sintering temperature of BaFe_0.9Sn_0.1O₃ film. With the temperature increased the square resistance varied as "U"-type while B_25⁸5 value increased when the BaFe_0.9Sn_0.1O₃ and BaBiO₃ mixed as a certain percentage. With BaBiO₃ amount increased the square resistance varied as "U"-type while B_25⁸5 value decreased when sintering temperature had certain. The best composition of thermal phase and glass phase is half and half, while the best preparing is 950℃for 4h, and its resistivity RS was about 4.2×105Ω·sq^-1 , B_25⁸5 value was 3973K. The sintering process of BFS thick-film resistors were investigated according to the kinetic equation of the initial stages of sintering. The conduction mechanism of BFS films was also analyzed by tunnel barrier model.The effects of iron-doped amount on the microstructure and electrical properties were investigated with the theory of semiconductor doping. Take CuO-doped as a stand for the acceptor doping, Cu²⁺ substitute for Bi⁵⁺ ions and Sn₄₊ ions for the first when they doped into the lattice of conductive particles, then followed by the substitute for Fe3+ ions. With an increase in CuO content the square resistivity for thick film decreased slowly and then down quickly, the B value changed in"Z"-type. When wt(CuO) =14% the thick film sample with good NTC characteristics was obtained and its resistivity RS was about 2.8×10⁵Ω·sq^-1 , B_25⁸5 value was 3285 K andα₂₅ was -0.0369℃^-1. Sb³⁺ was oxidation to Sb⁵⁺ ions at a high temperature then to donor doping. With the doping amount gradually increases, the donor concentration in excess of acceptor concentration, load concentration decreased after the first increase, the square resistivity and B value for thick film changed in"Z"-type together. When wt(Sb₂O₃) =1% the thick film sample with good NTC characteristics was obtained and its resistivity RS was about 1.7×10⁶Ω·sq^-1 , B_25⁸5 value was 3512 K andα₂₅ was -0.0395℃^-1.