Strontium Titanate-based Multi-varistor Ceramics And Microstructure

SrTiO₃-series capacitor-varistor ceramics, a kind of important voltage-sensitive ceramic materials with double functions, have low breakdown voltage, excellent nonlinear coefficient, high dielectric constant and low dielectric loss. They belong to multifunctional ceramics (MFC) and have characteristics of high frequency noise absorption, burst pulse absorption, surge absorption and self-recovery. SrTiO₃ varistors have been widely used as transient high-voltage protector and surge arrestor in the field of low voltage, such as restraining the noise of micro direct-current machine or source input, protecting the switch of inductive load or silicon controlled rectifier transwitch, and by-pass capacitors. According to the development and recent status of SrTiO₃ varistors, the present market prospect and the trend of development are overviewed in the paper. The preparation techniques, compositions, electrical properties and microstructure of SrTiO₃ varistors are discussed in detail, the influence of surface layer on electrical properties of SrTiO₃ voltage-sensitive ceramics are also discussed. And some correlated results have been attained.In this experiment, N₂ and graphite are adopted to create reducing atmosphere, and SrTiO₃ ceramics are mainly prepared by single-fired method. Non-atmosphere graphite reducing method is also adopted for the sake of contrast. The experimental results indicate that the properties of samples prepared by non-atmosphere graphite reducing method are fluctuant, but the best properties of sample is better than that of samples prepared by the first method, the average grain resistivity (ρg)is lower than that of the former.Under the condition of (N2+C) reducing atmosphere, when doping La₂O₃ as single donor, the sample with 0.6mol%La₂O₃ addition has the lowest ρg(8Ω.cm), samples with 1.5mol%La₂O₃ addition have relatively good comprehensive electrical properties, V₁mA=6.35V, a=7.8, ε_r=5.57×10⁴, tan5=0.209. Doping Nb₂O₅ and La₂O₃ as double donors will increase their solid solubility at the same time, and samples with the composition of 0.3mol%Nb₂O₅+0.3mol%La₂O₃ have relatively good comprehensive properties, V₁mA=8.64V, a=7.6, ε_r=3.1×10⁴, tan5=0.209.As contrasted with MnCO₃, doping CuO as acceptor can decrease V₁mA and increase ε_r, but a is decreased; Doping CuO and MnCO₃ as acceptors doesn't means good properties, the results of wave spectra may provide a qualitative interpretation. SEM of samples indicates that raisingsintering temperature can promote the growth and homogenization of grain, and form relatively perfect grain boundary layer. These factors will decrease VimA. But higher sintering temperature may easily cause abnormal growth of grain. Wave and Energy spectra show that the sintering aid mainly exists in polygon grain boundaries.The relaxation frequency of sample is lower than lOOHz. sr is still over 104 under the condition of 104Hz.SrTiC>3 voltage-sensitive ceramics have surface layer which thickness is about 60p.m. With the reduction of thickness, VimA and a are greatly reduced, er and tan8 are raised. This phenomenon indicates that the inner of materials is a region which grain boundaries and their potential barrier still not formed very well. Energy Spectra shows that the concentration of oxygen has a gradient from surface to inner. The increment of the temperature or time of oxidizing heat-treatments will increase VimA and a, but er and tan8 are decreased. The chemical adsorptional oxygen greatly increase the density of interfacial state, this is the main factor of the formation of grain boundary potential barrier. Samples doping 0.15mol%Nb2O5+0.75mol%La2O3 as donors and prepared by optimized ingredient and technique have the best properties, VimA=5.81V, a=8.3, er=5.18xl04, tan5=0.06.