Microcontact measurement is employed to locally investigate the electric and dielectric properties of individual grains and grain boundaries in CaCu3Ti4O12 ceramic. The measurements give more detail of the impedance spectroscopy, capacitance, and I-V characteristics of the microstructure, and will help with further understanding of the mechanism of the electric and dielectric properties of CaCu3Ti4O12 ceramics.
In this paper discussions on ZnO based varistor ceramics doped with different ratios of Y2O3 are presented.Analysis on the phase and microstructures of the samples indicates that an additional phase is detected in the samples doped with Y2O3,and the average grain size of the specimens decreases from about 9.2μm to 4.5μm,with an increase in the addition of Y2O3 from 0 mol%to 3 mol%.The corresponding varistor’s voltage gradient markedly increases from 462 V/mm to 2340 V/mm,while the nonlinear coefficient decreases from 22.3 to 11.5,respectively.Furthermore,the characteristics of deep trap levels in these ZnO samples are investigated by measuring their dielectric spectroscopies.The trap energy level and capture cross section evaluated by relaxation peak of the Cole-Cole plot vary slightly as the addition of Y2O3 increases.These traps may be ascribed to the intrinsic defects of ZnO lattice.
LIU Jun 1 ,HU Jun 1 ,HE JinLiang 1 ,LIN YuanHua 2 &LONG WangCheng 1 1State Key Laboratory of Power Systems,Department of Electrical Engineering,Tsinghua University,Beijing 100084,China
In order to investigate the influence of sintering temperature on the Schottky barrier and bulk electron traps of ZnO varis- tors, ZnO-Bi2O3 based varistor ceramic samples were sintered at 1000, 1100, 1200 and 1300℃, respectively. The measured results indicate that the sample sintered at 1300℃ possesses the lowest voltage gradient and nonlinear coefficient, compared with other samples. The barrier height of the samples decreased as the sintering temperature increased, which resulted in the deterioration of nonlinearity. Furthermore, two bulk electron traps determined by admittance spectroscopy were generally independent of sintering temperature, which indicated that these two traps might originate from the intrinsic defects in ZnO lattice.
The surge arrester of 1000 kV gas-insulated substation (GIS) needs ZnO varistor with high voltage gradient to effectively improve the potential distribution along ZnO varistor column inside the metal-oxide surge arresters. In this paper, the elec-trical and structural parameters of ZnO varistors are changed by doping with some rare-earth oxides, and the mechanism which leads these changes is discussed. When rare-earth oxide additives are added into ZnO varistors, the growing speed is slowed down due to the stabilization of the new spinel phases formed in the grain-boundary by rare-earth oxide additives, then the size of ZnO grains is smaller, and the voltage gradient of varistor increases obviously. By adding suitable amount of oxides of metal Co and Mn, the leakage current can be effectively decreased and the nonlinearity coefficient increased. The novel ZnO varistor samples sintered with the optimal additives have a voltage gradient of 492 V/mm, and the nonlinearity coefficient of 76, but their leakage currents are only 1 μA.
The surge arrester with excellent protection characteristics would decrease the overvoltage level ap- plied on the power apparatus to reduce their insulation levels and manufacturing bottleneck. The arrester for the 1000-kV ultra-high voltage ac power transmission system is designed as tank-type structure. The field-circuit combination numerical method combining the three-dimensional finite element method with circuit is proposed to analyze the potential distribution of GIS arrester. By comparing several design schemes, the most effective method to improve the potential distribution along the varistor column is to increase the voltage gradient of the ZnO varistor. Synthesizing several influential factors, the suitable voltage gradient of ZnO varistor should be controlled to 435 V/mm, and the resulted nonuniform degree of the potential distribution along the varistor column inside the GIS arrester would be controlled smaller than 10%. The result in this paper provides the fundamental technical index for the study of the high voltage gradient ZnO varistors.
HE JinLiangHU JunMENG BoWenZHANG BoZHU BinCHEN ShuiMingZENG Rong
By sintering CaCu3Ti4O12ceramics at different temperatures and testing the samples＇ properties, the segregation behavior of Cu and its effects were carefully studied. As the temperature rises, CuO segregates to the grain boundaries and gradually volatilize. Complete decomposition and volatilization occur at 1150℃. The behavior of CuO is found to be closely related to the formation of the grain boundary barrier, as well as the acquirement and evolution of the electrical properties, which are revealed in the research.
