JIANG Zongheng ¹, WANG Huanping ¹, LIN Xiaojie ², MA Hongping ³, GAO Zhao ¹, XU Shiqing ¹

(1. School of Optics and Electronic Technology, China Jiliang University, Hangzhou 310018, Zhejiang, China; 2. Haining Yongli Electronic Ceramics Co., Ltd., Haining 314415, Zhejiang, China; 3. School of Mechanical and Energy Engineering, Zhejiang University of Science and Technology, Hangzhou 310023, Zhejiang, China)

Extended Abstract:[Background and purpose] Barium titanate (BT) based positive temperature coefficient (PTC) ceramics can be used as heating components in electric vehicles. In order to ensure heating efficiency and safety of the heating components, the PTC ceramics should have high positive temperature coefficient and high interior temperature. Pure BT is an insulator with a Curie temperature of about 120 ℃. Generally, donor elements, acceptor elements and Curie temperature shift agents need to be doped to achieve high PTC effect and high Curie temperature. In recent years, the electric vehicle industry has developed rapidly, where the voltage that the PTC ceramic components need to withstand is increasing very rapidly. Excessive voltage can lead to electrical breakdown. Therefore, it is necessary to further improve the breakdown strength of the PTC ceramics. The common method is doping modification. Sn doping can effectively improve the breakdown strength of BT ceramics, but its effect on PTC effect has not been reported. Therefore, in this article, Sn was introduced into BT ceramics, to achieve high Curie point and high PTC effect.[Methods] Ba_0.7Pb_0.3Ti_0.9952−xSn_xNb_0.004Mn_0.0008O₃ ceramics were prepared by using the traditional solid-phase method. The raw materials with the designed compositions were mixed through ball milling with zirconia balls as media in anhydrous ethanol. After ball milling, the mixtures were dried. to the dried mixtures were calcined at 1050 ℃ for 2 h. is the calcined powders were ground, with the addition of 5 wt.% PVA solution. and the powders were granulated at a pressure of 10 MPa to produce circular embryo pieces. The embryos were heated at 800 ℃ for 90 min to remove the binder and then sintered at 1310 ℃ and for 1 h. The sintered ceramics were polished for XRD and SEM analysis. Silver paste was applied on both sides of the ceramics and baked at 550 ℃ for 30 min for measurement of resistance temperature, complex impedance and breakdown strength.[Results] As evidenced by the XRD results, the Sn doped BT ceramics is still phase pure perovskite. With increasing content of Sn, the diffraction peaks at 42°–45° gradually merge into one diffraction peak. The calculated lattice constant shows that the c/a ratio decreased with increasing concentration of Sn. From SEM images, it is found that the grain size of the ceramics gradually decreased, with increasing level of Sn. Room temperature resistivity and breakdown strength gradually increased, with the rise to resistance ratio showing a trend of first increasing and then decreasing. The maximum value was reached in the sample with x=0.08, while the Curie temperature gradually decreased. With increasing content of Sn, there was no significant change in grain resistance, but the grain boundary resistance gradually increased. The density of acceptor states gradually decreased, while the thickness of the loss layer first increased and then decreased.[Conclusions] Sn doping reduced the Curie temperature. Sn doping inhibited grain growth, reduced grain size, increased grain boundary resistance and grain boundary voltage capacity, thereby increasing room temperature resistivity and breakdown strength. Moderate Sn doping could increase the thickness of the loss layer, thereby improving the rise to resistance ratio and enhancing the PTC effect. The sample with x=0.08 displayed a maximum rise to resistance ratio of 1.39×10⁴, a loss layer thickness of 66.18 nm, a room temperature resistivity of 292.4 Ω·cm, a Curie temperature of 185.5 ℃ and a breakdown strength of 88.96 V·mm⁻¹.

Key words: barium titanate; PTC ceramics; low resistivity; high curie point; breakdown strength