The effect of aluminium oxide on the residual voltage of ZnO varistors

The role of aluminium oxide is investigated in an attempt to obtain low residual voltage ZnO varistors fabricated in the traditional way. The samples were sintered at three different temperatures: 1150, 1200, and 1250 °C for three different times: 1, 2, and 3 h. The effect of aluminium additions, in the range of 500-10,000 ppm, on the characteristics of electric field-current density was investigated. Addition of Al2O3 shifts the upturn region to higher current density due to the increase of the conductivity of the ZnO grains up to an optimal value. After 1000 ppm, the beneficial effect on the onset of upturn voltage is no more efficient. This work highlighted the ambivalent role of aluminium (donor or acceptor) and the need for tuning both an optimal concentration and sintering time to obtain the greatest conductivity. Our results suggest that Al3+ substitutes first to Zn2+ sites in the ZnO wurtzite structure and then, when thermodynamic saturation is reached, interstitial sites.