PREPARATION AND CHARACTERIZATION OF CERAMIC OXIDE THIN FILMS FOR ELECTRICAL VARISTORS

Abstract

The composition, microstructure and electrical properties of ZnO based ceramic films (ZnO-V205-MnO) have been analysed. Samples were prepared by novel sol-gel process with ZnO-0.5 mol% V205-x mol% MnO (x = 0.5, 1.0, 1.5) compositions. ZnO based ceramic films, with a thickness of 0.25-3 pm, were deposited on ordinary glass and ITO coated glass substrate by spin coating method. The precursors were prepared initially by dispersing doped-ZnO ceramic nano-powders into the sols, prepared by dissolving zinc acetate into 2-methoxyethanol and stabilized by ethanolamine and acetic acid glacial and doped with amonium metavanadate and manganese acetate. The doped-ZnO ceramic nano-powders were prepared by sol-gel processing through dried and calcined at 150°C and 800°C respectively. The films were prepared by the spin coating method with the substrate spinning rate between 2000 and 3000 rpm. After each deposition, the films were heat treated at 240°C for 10 min. After 10 to 30 layers of the film deposition on ordinary glass and ITO coated glass, the films were annealed for 60 min at 550°C and 700°C respectively. The films have been characterized by X-ray diffraction for phase determination and field emission scanning electron microscopy (FESEM) for grain structure observation. The nonlinear V-I characteristics of the films for use as varistors were studied using KEITHLEY 2400 source meter. All the microstructures consisted of ZnO grains with zinc vanadate and zinc manganate compounds as minority secondary phases. The secondary phases Zn3(VO4)2 and Zn4V2O9 were found in the samples annealed at 700°C. FESEM micrographs show that the film prepared is uniform and the grain sizes are in the range of 25-40 nm and 70-90 nm for doped ZnO films annealed at 550°C and 700°C respectively. All the samples exhibited nonlinear current-voltage behavior, with nonlinear coefficient, a, ranging between 103 and 420. The highest values of a is achieved for the films annealed at 700°C and having composition of ZnO- 0.5 mol% V205-1.5 mol% MnO with a thickness of 3μm.