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 μm, 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 °C, 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 °C for 10 min. After ten to 20 layers, the films were annealed in air at 650–800 °C 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 (α) was 3–22. The highest nonlinearity coefficient with nonlinear voltage of 4 V could be achieved at 750 °C by the films with a thickness of 2 μm.