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.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:603152 |
| Date | January 2014 |
| Creators | Poosimma, Poonsuk |
| Contributors | Freer, Robert |
| Publisher | University of Manchester |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | https://www.research.manchester.ac.uk/portal/en/theses/stability-of-zinc-oxide-varistors(1986b8ad-9f20-4317-9dd0-2b459bbd6c89).html |
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