MICROSTRUCTURE AND NONOHMIC PROPERTIES OF SnO2-Ta2O5-ZnO BASED CERAMIC VARISTORS DOPED WITH TiO2

The microstructure and nonohmic properties of SnO2-Ta2O5-ZnO based ceramics sintered at 1450 degrees C for 2 h were investigated in accordance with TiO2 content (0-8 mol%). Without TiO2 the prepared sample is nonstoichiometric SnO2 semiconductor with excessive oxygen; but for the samples doped with TiO2, Sn0.9Ti0.1O2 phase can be identified, and the incorporation of TiO2 into the ternary system SnO2-Ta2O5-ZnO ceramics can compensate the defects of Sn4+ ions loss, promote the sample densification, and facilitate the growth of SnO2 grains. After 4.0 mol% of TiO2 is doped, the samples present no precipitated substances residing in the grain juncture, resulting in varistors with maximum nonlinear exponent of 21, varistor voltage of about 1000 V/mm, and minimum leakage current of 100 mu A/cm(2), which are promising in high-voltage applications. The improvement in nonohmic performance of the varistors after the doping of TiO2 is mainly attributed to the increase in effective barrier height in grain boundary, which can be supported by the decrease in band gap caused by defects and impurities from periodic density function theory calculation.