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

Abstract

The microstructure and nonohmic properties of SnO 2 – Ta 2 O 5 – ZnO based ceramics sintered at 1450°C for 2 h were investigated in accordance with TiO 2 content (0–8 mol%). Without TiO 2 the prepared sample is nonstoichiometric SnO 2 semiconductor with excessive oxygen; but for the samples doped with TiO 2 , Sn 0.9 Ti 0.1 O 2 phase can be identified, and the incorporation of TiO 2 into the ternary system SnO 2 – Ta 2 O 5 – ZnO ceramics can compensate the defects of Sn 4+ ions loss, promote the sample densification, and facilitate the growth of SnO 2 grains. After 4.0 mol% of TiO 2 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 μA/cm 2 , which are promising in high-voltage applications. The improvement in nonohmic performance of the varistors after the doping of TiO 2 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.