Preparation and properties of ZnO-based ceramic films for low-voltage varistors by novel sol-gel process

ZnO-based ceramic films, with thickness close to 2-4 mum, were deposited on Au/Si substrates by novel sol-gel process. The precursors were fabricated by dispersing doped-ZnO ceramic nano-powders into the sols, which were prepared by dissolving zinc acetate dihydrate into 2-methoxyethanol and stabilized by diethanolamine and glacial acetic acid and doped with a concentrated solution of bismuth nitrate, phenylstibonic acid, cobalt nitrate, manganese acetate and chromium nitrate. The doped-ZnO ceramic nano-powders were prepared by sol-gel processing through dried and calcined in air at 150 and 800 degreesC, respectively. The films were prepared by the spin coating method with the substrate spinning rates of 2000-3000 rpm. After each deposition, the films were heat-treated in air-at 350-400 degreesC for 10 min. After ten to 20 layers, the films were annealed in air at 650-800 degreesC for 2 h. The films have been characterized by X-ray diffraction and scanning electron microscopy. The nonlinear V-I characteristics of the films for use as varistors were studied by transistor characteristics testing instrument. The nonlinear voltage was 2-10 V, the nonlinear coefficient (x) was 3-22. The highest nonlinearity coefficient with nonlinear voltage of 4 V could be achieved at 750 degreesC by the films with a thickness of 2 mum. (C) 2001 Elsevier Science B.V. All rights reserved.