Improvement of sintering, nonlinear electrical, and dielectric properties of ZnO-based varistors doped with TiO₂

The effects of TiO₂ on sintering and nonlinear electrical properties of (98.5-x)ZnO-0.5MnO₂-0.5Co₂O₃-0.5Bi₂O₃-xTiO₂ (x = 0.3, 0.5, 0.7, 0.9 mol%) ceramic varistors prepared by the ceramic technique are investigated in this work. The optimum sintering temperature of the prepared samples is deduced by determining the firing shrinkage and water absorption percentages. The optimum sintering temperature is found to be 1200 ℃, at which each of the samples shows a maximum firing shrinkage and minimum water absorption. Also minimum water absorption appears in a sample of x=0.9 mol%. Higher sintering temperature and longer sintering time give rise to a reduction in bulk density due to the increased amount of porosity between the large grains of ZnO resulting from the rapid grain growth induced by the liquid phase sintering. The crystal size of ZnO decreases with increasing TiO₂ doping. The addition of TiO₂ improves the nonlinear coefficient and attains its maximum value at x = 0.7 mol% of TiO₂, further addition negatively affects it. A decrease in capacitance consequently in the dielectric constant is recorded with increasing the frequency in a range of 30 kHz-200 kHz. The temperature and composition dependences of the dielectric constant and AC conductivity are also studied. The increase of temperature raises the dielectric constant because it increases ionic response to the field at any particular frequency.