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/cm2, 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.

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The microstructure and nonohmic properties of SnO 2– Ta 2 O 5– TiO 2- CuO varistor system were investigated. The proposed samples were doped with different contents of CuO (0–6 mol%) and sintered at 1400°C for 2 h with conventional ceramic processing method. In all the samples, the commonly identified phase was SnO 2 (rutile); however, with increasing doping amount of CuO , the peaks of CuO phase emerged in the X-ray diffraction (XRD) patterns. Scanning electron microscopy (SEM) examination on the fractured surfaces of the samples revealed that a minor amount of CuO dopant can facilitate the sintering of the varistor ceramics, but excessive CuO would mainly segregate at grain-boundaries. The doped CuO may also act as a modifier in the SnO 2 based varistors. The measured electric-field versus current-density characteristics of the samples indicated that both nonlinear exponent and varistor voltage increased with increasing doping amount of CuO up to 3 mol% and then decreased with excessive CuO .