Abstract
This paper investigated the effects of sintering temperature on the microstructure and electrical properties of ZrO 2 -doped zinc oxide (ZnO) varistor ceramics. The results show that, the additive ZrO 2 exists as independent second phase between ZnO grains, which can limit the growth of ZnO grains and improve the voltage gradient. With the increasing of ZrO 2 , the content of extrinsic elements (Mn, Sb, Co, Cr) in the grain boundary layers tends to increase first and then decease. When ZrO 2 content is more than 1.0mol%, the electrical performance of ZnO varistors decreases sharply. With the increasing of sintering temperature, the ZnO grain size increases and the voltage gradient decreases. When the sintering temperature is larger than 1200°C, more monoclinic ZrO 2 phase transformed into cubic phase, and more micropores are generated, causing the non-linear coefficient to decrease and the residual voltage ratio and leakage current to increase. With a sintering temperature of 1150°C and a ZrO 2 content of 1.0mol%, the ZnO varistors can reach the overall optimum electric performance, exhibiting a breakdown voltage of E 1mA = 420V/mm, a nonlinear coefficient of α = 58, a residual voltage ratio of CR = 1.87, and a leakage current of I L = 4μA. The studies in this paper can give reference for the development of high quality ZnO arresters.
Highlights
Because of the excellent nonlinear V-I characteristics and energy absorption capability, metal oxide arresters (MOAs) are widely used in power systems to protect electric equipments from transient over-voltages [1]–[5]
In view of the above, in order to improve the electric characteristics of zinc oxide (ZnO) varistors, this paper studied the combined effects of ZrO2 dopants and sintering temperature on the microstructure and electrical properties of ZnO varistors
As residual voltage ratio and bulk density are regarded as important parameters of surge handling ability, it can be confirmed that ZnO varistors with a sintering temperature of 1150◦C and a ZrO2 content of 1.0mol% can reach the optimum electric characteristics
Summary
Because of the excellent nonlinear V-I characteristics and energy absorption capability, metal oxide arresters (MOAs) are widely used in power systems to protect electric equipments from transient over-voltages [1]–[5]. It has been proved that the electric characteristics of ZnO varistors are determined by their microstructures, which can be effectively improved by the introduction of metal oxides [28], [29]. Hu: Influence of Sintering Temperature and ZrO2 Dopants effectively in improving the electric characteristics of ZnO varistors [33]–[37]. In view of the above, in order to improve the electric characteristics of ZnO varistors, this paper studied the combined effects of ZrO2 dopants and sintering temperature on the microstructure and electrical properties of ZnO varistors. Bulk density of the samples was measured, the V-I characteristics, breakdown voltage, residual voltage ratio as well as leakage current of the manufactured ZnO varistors were obtained. According to the electric model of ZnO varistors in [2], [4], [10], [15]–[18], the resistance of ZnO grain is much smaller than grain boundary, and the grain boundary resistance can be measured when the applied frequency is 0, and the ZnO grain resistance was measured when the applied frequency is 13MHz [9]–[13]
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