Ceramic varistors based on ZnO, Bi2O3, Sb2O3, MnO and Co3O4 were prepared by the mixed oxide route. After milling the powders, disc shaped samples were pressed and sintered for 2 hours at temperatures in the range 950°C to 1250°C. Products were characterised in terms of phase development and microstructure by X-ray diffraction and scanning electron microscopy; electrical characterisation included current-voltage (I-V), capacitance-voltage (C-V) and degradation behaviour. Most varistors contained a three phase microstructure comprising ZnO grains (3–54 μm in size), a Bi-rich grain boundary phase and a spinel phase. Product densities were approximately 5.45 g cm-3, and nonlinear coefficients were typically in the range 5 to 53. By selective removal of either Sb2O3, or MnO or Co3O4, the role of individual components was investigated. The absence of Sb2O3 encouraged grain growth (to 40 μm) but reduced nonlinearity (to 5) and degraded the stability. The absence of Co3O4 improved the stability whilst the absence of MnO reduced the nonlinear coefficient and leakage current. The use of attrition milling yielded powders of small particle size (0.3 μm). The resulting sintered varistors exhibited a slightly smaller grain size (~ 5 μm) lower nonlinear coefficients (~ 30) but higher breakdown fields and leakage currents. Samples sintered at high temperatures (1050°C to 1250°C) degraded under electrical stress more slowly than the samples sintered at low temperature (950°C). Al-doping enhanced the nonlinearity but reduced the stability. In contrast Ag-doping reduced the nonlinearity but improved the ageing behaviour. An appropriate combination of Al and Ag doping led to varistor improvements in both nonlinearity and stability. The optimum aluminium nitrate and silver oxide levels for the varistor composition studied were found to be 500 ppm and 250 ppm, respectively. C-V measurements on this material gave barrier heights of ~ 1.7 V, and donor densities of 1.3x10^18 cm-3. One batch of samples was prepared Ga2O3 additions of 750 – 4000 ppm. Doping with Ga caused a significantly decrease in grain size to ~ 6 μm), and reduced the nonlinear coefficients (to ~25) but increased the stability in terms of breakdown fields and leakage currents.

<p>The work presented in this thesis is concerned with the microstructure of zinc oxide varistor materials used in surge protecting devices. This class of material has been characterised with special emphasis on the functional microstructure and the development of the microstructure during sintering. Several different techniques have been used for the analysis, especially scanning electron microscopy (SEM) in combination with electron beam-induced current (EBIC) analysis and <i>in-situ</i> studies of heat-treatment experiments and transmission electron microscopy (TEM) in combination with energy dispersive X-ray spectrometry (EDS) and electron holography. </p><p>Detailed TEM analyses using primarily centred dark-field imaging of grain boundaries, especially triple and multiple grain junctions, were used to reveal the morphological differences between the various Bi₂O₃ phases. The triple and multiple grain junctions were found to exhibit distinct differences in morphology, which could be attributed the difference in structure of the crystalline Bi₂O₃ polymorphs present in the junctions. </p><p>Electrical measurements were performed on individual ZnO/ZnO grain boundaries using EBIC in the SEM. The EBIC signal was found to depend strongly on the geometric properties of the interface and also on the symmetry of the depletion region at the interface. A symmetric double Schottky barrier was never observed in the experiments, but instead barriers with clear asymmetry in the depletion region. Experimental results together with computer simulations show that reasonably small differences in the deep donor concentrations between grains could be responsible for this effect.</p><p>Electron holography in the TEM was used to image the electrostatic potential variation across individual ZnO/ZnO interfaces. The sign of the interface charge, the barrier height (about 0.8 eV) and the depletion region width (100 to 150 nm) were determined from holography data. Asymmetries of the depletion region were also found with this technique. </p><p>The full sintering process of doped ZnO powder granules was studied <i>in-situ</i> in the environmental SEM. The densification and grain growth processes were studied through the sintering cycle. The formation of a functional microstructure in ZnO varistor materials was found to depend strongly on the total pressure.</p>

The work presented in this thesis is concerned with the microstructure of zinc oxide varistor materials used in surge protecting devices. This class of material has been characterised with special emphasis on the functional microstructure and the development of the microstructure during sintering. Several different techniques have been used for the analysis, especially scanning electron microscopy (SEM) in combination with electron beam-induced current (EBIC) analysis and in-situ studies of heat-treatment experiments and transmission electron microscopy (TEM) in combination with energy dispersive X-ray spectrometry (EDS) and electron holography. Detailed TEM analyses using primarily centred dark-field imaging of grain boundaries, especially triple and multiple grain junctions, were used to reveal the morphological differences between the various Bi2O3 phases. The triple and multiple grain junctions were found to exhibit distinct differences in morphology, which could be attributed the difference in structure of the crystalline Bi2O3 polymorphs present in the junctions. Electrical measurements were performed on individual ZnO/ZnO grain boundaries using EBIC in the SEM. The EBIC signal was found to depend strongly on the geometric properties of the interface and also on the symmetry of the depletion region at the interface. A symmetric double Schottky barrier was never observed in the experiments, but instead barriers with clear asymmetry in the depletion region. Experimental results together with computer simulations show that reasonably small differences in the deep donor concentrations between grains could be responsible for this effect. Electron holography in the TEM was used to image the electrostatic potential variation across individual ZnO/ZnO interfaces. The sign of the interface charge, the barrier height (about 0.8 eV) and the depletion region width (100 to 150 nm) were determined from holography data. Asymmetries of the depletion region were also found with this technique. The full sintering process of doped ZnO powder granules was studied in-situ in the environmental SEM. The densification and grain growth processes were studied through the sintering cycle. The formation of a functional microstructure in ZnO varistor materials was found to depend strongly on the total pressure.

The author's aim is to investigate ZnO varistors with high voltage to 1000V/mm and good resistance to electrical degradation, which could be applied to the UHV transmission systems and reduce ZnO varistor element amounts in surge arresters. The conduction processes in ZnO varistors is clarified. By adjusting additive type and amounts in ZnO the electrical properties of ZnO varistors can be controlled. Based on this recipe, Y, Cr, Ni, B, Si oxides were added in this study. These oxides can be divided into two categories, the Y doping group and the Si doping group. / 博士(工学) / Doctor of Philosophy in Engineering / 同志社大学 / Doshisha University