Effect of Ni2O3 Doping on the Electrical Properties and Intrinsic Defect Concentration of ZnO Varistor Ceramics

With the development of high voltage transmission, there is an urgent need to develop ZnO varistor ceramics with high anti-aging properties. The key is to manipulate the intrinsic defect concentration of ZnO varistor ceramics precisely. In this paper, ZnO varistor ceramics doped with different contents of Ni2O3 are taken to study the relationship between the microstructure and electrical properties, and the effect of ZnO varistor ceramics doped with different contents on intrinsic defect concentration is also considered. The results show that, best electrical performance is shown when the content of Ni2O3 is 1.2mol%, the electrical breakdown field E1mA is 356 V/mm, the nonlinear coefficient reaches 32, and the leakage current IL is 3.4 . While the amount of the doped Ni2O3 is more 0.8mol%, a new phase of Co2Cr5Sb5O4 phase is observed from X-ray diffraction. The SEM micrographs showed that the average grain size decreased monotonously from 14.56 m to 5.73 m with the amount of the doped Ni2O3 increased. According to the results of dielectric spectroscopy, the intrinsic defect concentration increased with the contents of the doped Ni2O3 increased. The increase of Zinc interstitial is much greater than that of Oxygen vacancies, which is harmful to Long-term aging characteristics of ZnO varistor ceramics.


Introduction
ZnO varistor ceramics are widely used in the overvoltage protection of power systems, electronic devices and communication systems because of its excellent I-V characteristics and large energy absorption capacity [1][2]. It is generally believed that the nonlinearity of ZnO varistor ceramics results from Schottky Barrier [3]. The formation and degradation of the double Schottky barrier at the grain boundary of ZnO varistor ceramics and the barrier height, width and other important parameters are determined by the grain boundary depletion layer and the non-uniform distribution of point defects at the interface. In its dynamic process, the types of point defects include intrinsic point defects and extrinsic point defects, which together have a profound impact on the electron transport and relaxation process of the Schottky barrier. In addition， Intrinsic point defect plays a critical role in modern functional material, such as semiconductors, organic electronic devices, high-efficiency batteries and dielectric ceramics [4][5][6][7]. However, the drop in the height of the Schottky barrier at the grain boundary is the direct cause of the deterioration of the electrical properties of ZnO varistor ceramics. The main reason affecting its height is the migration of zinc interstitials and oxygen vacancies. When the zinc interstitials in the depletion layer move towards the grain boundary, the negative charge of the interface state is neutralized, resulting in a decrease in the barrier height. Therefore, reducing the zinc interstitial concentration and increasing the oxygen vacancy concentration become critical. He [8] found that the appropriate concentration of Ni 2 O 3 doping can help reduce the residual voltage ratio of ZnO varistor ceramics, and the nonlinear coefficient can reach up to 47.6. He [9] found that although Zr doping can reversely manipulate the intrinsic defect concentration, that is zinc interstitial concentration increased, the oxygen vacancy concentration reached up to the opposite direction, which resulted in bad aging performance. Moreover, there is a lack of relevant research on the effect of Ni 2 O 3 doping on the intrinsic defect concentration.
In this paper, the solid-phase sintering was used to prepare ZnO-Bi 2 O 3 series multi-element ceramic samples doped with different contents of Ni 2 O 3 , and the effect of Ni 2 O 3 doping on the microstructure, electrical properties and intrinsic defect structure of ZnO varistor ceramics was studied.

Experiments
In this paper, ZnO . First of all, put the configured powder and an appropriate amount of deionized water into a polyamides battle and ball milling for 12 h and add a concentration of 2 wt% polyvinyl alcohol solution (PVA) and drying treatment. The powder was pressed into green discs of 14 mm in diameter and 1.3 mm in thickness. In the last, the discs were placed in the sintering furnace and sintered in the air at 1200 ℃ for 2 h, the heating rate was 150 ℃/h, and the cooling rate is 100 °C/h (≈12 mm, d≈1 mm).
The voltage source used to measure the low current I-V characteristics of the sample is WJ10001D precision linear high-voltage DC power, and the instrument used to measure the current is HP 34401A multi-meter. The electrical breakdown field of the sample is E 1mA =U 1mA /d, where U 1mA is the voltage when the current is 1 mA, d is the thickness of the sample, and the nonlinear coefficient is defined as： log -log log -log In the formula, U 1 and U 2 are the voltage values when the current is at I 1 and I 2 respectively, I 1 =0.1 mA, I 2 =1.0 mA.
In order to analyze the crystalline phase of the sample, X-ray diffraction (XRD: d8 advanced 3.0) was used. The surface microstructure images were conducted by a scanning electron microscopy (SEM: VE-9800S). The broadband dielectric spectrum test system (Novocontrol) produced in Germany was used to measure the dielectric properties of the sample at a temperature of -110 ℃～-60 ℃, an interval of 10 ℃, and a frequency of 10 -1 ～10 7 Hz.

Results and discussion
The electrical properties of the ZnO varistor ceramic samples doped with different contents of Ni 2 O 3 are shown in Figure 1. With the amount of the doped Ni 2 O 3 increased, the electrical breakdown field E 1mA rises from 185 V/mm to 356 V/mm. The reason of E 1mA increased may be related to the spinel phase formed at the grain boundaries, the spinel phase inhibits the growth rate of the crystal grains, led to a decline in the size of the grains. Therefore, the number of grain boundaries per unit thickness raises, which makes the electrical breakdown field increase; the nonlinear coefficient  first decreases and then rises, and reaches up to 32 when the content of      Figure 5 that there are relaxation peaks at low frequency and high frequency respectively, and the peak position moves toward the high temperature and high frequency direction, which is a typical Debye relaxation process, and after calculating the activation energy, that is 0.29 eV and 0.19 eV from low frequency to high frequency respectively, which corresponds to oxygen vacancies and zinc interstitials. And the dielectric loss rises sharply in high frequency. It can be known from the literature that the process may be caused by the Maxwell-Wager effect [10], which is closely related to the resistance of the crystal grains and grain boundaries. As the doping contents of Ni 2 O 3 increases, the zinc interstitial concentration and the oxygen vacancy concentration both augment, and the rising amplitude of the zinc interstitial concentration is greater than the oxygen vacancy concentration. Therefore, K1 /K2 decreases monotonously with the amount of doped Ni 2 O 3 increased.

Conclution
In this paper, a solid-phase sintering was used to prepare ZnO varistors ceramics doped with different contents of Ni 2 O 3 . The effect of Ni 2 O 3 doping on the microstructure, electrical properties and intrinsic defect concentration of ZnO varistor ceramics was studied. Studies have shown that Ni 2 O 3 will inhibit the growth of grains. The most excellent electrical performance is presented when the content of Ni 2 O 3 is 1.2 mol%. Dielectric spectroscopy studies have shown that ZnO samples doped with Ni 2 O 3 will not affect the intrinsic defect energy level, but will increase the zinc interstitial and oxygen vacancy concentration in the same direction, and the amplitude of zinc interstitial concentration is much greater than that of the oxygen vacancy concentration, which is not conducive to the long-term stability of ZnO varistor ceramics.