Thesis (Ph.D.) - Cairo University - National Institute of Laser and Enhanced Sciences - Department of Laser of Applications in Engineering

Fabrication of ZnO varistors follows standard ceramic techniques. The components are mixed, for example by milling in a ballmill and spray drying afterwards. The mixed powder is dried and pressed to the desired shape. The resulting pellets are sintered at high temperature, typically 1000-1400 {u00B0}C. Additives are usually added to improve the electric properties of varistors; increase the non linearity behavior or to improve the breakdown voltage of ZnO. X- ray diffraction analysis of sintered samples reveals no formation of new phases, but lattice constants of the phases were changed after sintering. Lattice constants of each sample were changed in different extents depending on the phases affinity to a particular ion and on overall varistor composition. the structure has a single phase with shift in the position of peaks (change in d-spacing). The present study reveals that the addition of CuO leads to a two phase microstructure; well crystalline ZnO grains in preferred orientation, ZnO crystallize in hexagonal system. It is characterized by perfect cleavage plane along 001 which was distinct in SEM of ZnO. CuO added was partly distributed in ZnO grains themselves, and SEM image of mix (M1) which containing ZnO + 0.3 mol% CuO, show that ZnO agglomerates of various size and shapes, some of these agglomerates are dissected and fine grains are in corporate within these lines of dissection. Also, there are a mass of solid solution of ZnO and CuO in some places are rich in Zn others in Cu showing a kind stereopticons (dark and light lines)

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