Electrical Property Improvements of Multiple Dopant ZnO/Bi2O3 Varistor By Liquid Phase Method

Although ZnO Varistor is widely used in surge protection due to its unique I-V characteristics, its key influencing factors of grain size and uniformity are difficult to be guaranteed by traditional solid phase method. In this paper, the ZnO/Bi2O3 varistor was prepared by liquid phase co-deposition method and the sintering temperature also has been optimized. For samples prepared by the liquid-phase method, as the sintering temperature increases, the grain size gradually increases, the grain distribution becomes initially more uniform and later more disordered, and the crystallinity gradually increases. In comparison to solid-phase method, samples prepared by liquid-phase method exhibit smaller grain sizes and more uniform grain size distribution. Based on this, samples sintered at 1100 °C by the liquid-phase method demonstrate optimal electrical performance: the voltage gradient is 520 V/mm, the nonlinear coefficient is 44, and the leakage current is 5.8 μA/cm2. This paper has theoretical support for the study of preparation of varistor for achieving excellent electrical performance for being used in the field of surge protection.


Introduction
ZnO varistor has been widely used in the field of lightning arrester and surge absorption with its outstanding characteristics of excellent nonlinear current-voltage and strong absorption capacity [1][2][3].Especially, ZnO/Bi 2 O 3 based varistors have attracted many researchers to investigate their microstructure and electrical characteristics since they were first developed by Panasonic in Japan in 1968, due to its low raw material price, sintering process without higher sintering temperature and fewer pores products [4,5].However, there is an increasing demand for the electrical performance of ZnO varistors, such as nonlinear coefficient, leakage current, flow capacity, and so on, as the development of transmission systems towards ultra-high voltage and circuits towards integration [6].
It is widely recognized that grain size and uniformity play crucial roles in influencing the performance of ZnO varistors [7,8].Meanwhile, optimizing the powder formula has been proven to be an effective way for reducing the size and improving the distribution of the grains.But, for now, ZnO/Bi 2 O 3 varistors are mainly prepared by solid phase method [9,10], which results large particle size and low grain size uniformity.Luckily, some researchers have brought up the liquid phase method to synthesize the powder, which can make the components evenly mixed during the preparation process and produce highly activity, smaller grain size powder.Scholars Haile S M et al. [11] have showed that water deposition method could prepare spherical powders with more uniform particle size and good dispersion compared with traditional methods.What's more, the sintering temperature also is a great influence on the micromorphology and electrical properties of varistors, because higher sintering temperatures can lead to larger grain sizes and increased density, which can improve the electrical properties of the varistor, however, excessively high sintering temperatures can also lead to grain growth and the formation of voids or cracks, which can degrade the electrical performance of the varistor.Scholars, including Roy et al. [12], have conducted research on the performance of varistors at varying sintering temperatures.They found that the sintering temperature has a significant impact on the overall performance of the varistors, and determined that the most suitable temperature for optimal performance is 875 ℃.
In terms of liquid-phase preparation methods, the co-deposition method offers advantages such as shorter processing time, simplicity of operation, and high film density compared with sol-gel method [13,14].Therefore, in this study, the composite powder required for varistor preparation was synthesized using the co-deposition method, and the sintering temperatures were also studied to obtain the best electrical properties.

Figure 1. Experimental process
The process of preparing varistors via the liquid-phase method can be divided into three main steps: precursor powder preparation, ball milling, and sintering.Firstly, The precursor powder consists of three parts: precursor A (Bi 2 O 3 , Al 2 O 3 , and Cr 2 O 3 ), precursor B (Co 2 O 3 and MnO 2 ), and precursor C (ZnO, Er(NO 3 ) 3 , Y(NO 3 ) 3 , and other conventional dopants).The preparation process of precursor A involves preparing a metal nitrate solution with the corresponding proportions according to the formula.Ammonium hydroxide and ammonium salt are used as precipitants.Under heating conditions at 50 °C in a water bath, the metal nitrate solution is titrated together with the precipitants into a buffer solution prepared with ammonium hydroxide with the solution pH controlled at 7.5.After the reaction is complete, it is aged for an additional 30 minutes.Then, it is filtered and washed three times.The washed precipitate is dried at 120 °C for 3 hours and then removed for calcination at 600 °C for 2.5 hours.The preparation process for precursor B is the same as that for precursor A, except that the pH value of the reaction solution is controlled at 9.5.Precursor C uses purchased raw materials.Next, three parts of precursor powders are weighed according to the specified mass ratios of the formulation.These powders are mixed with zirconia beads and deionized water at a 1:2:1 mass ratio and placed in a ball milling jar.The mixture is ball-milled at a speed of 480 r/min for 10 hours with alternating directions.After ball milling, the slurry is dried at 120 °C for 10 hours and then subjected to grinding through an 80-mesh sieve.The ground powder is weighed and mixed with 5 wt% polyvinyl alcohol in a 9:1 mass ratio, followed by grinding.After grinding, pass the resulting powder through a 100-mesh sieve, and then use a pellet press to prepare circular embryo with a specification of Φ = 13×1 mm.At last, the prepared samples are sintered at temperatures of 1050 °C, 1100 °C, and 1150 °C respectively, to investigate the optimal sintering temperature.Meanwhile, samples are prepared by the solid-phase method with the same formulation at the determined optimal sintering temperature.These samples are then compared to the ones prepared through solid-phase method to assess the performance and evaluate the quality of the precursor powders.Both methods follow the same process, except that the solid-phase method skips the precursor powder preparation step, instead, it directly uses purchased raw materials.

Studies of ZnO varistor grains
The cross section of the prepared samples was taken and their microscopic morphology was observed using the SU8010 high resolution field emission scanning electron microscope (SEM) produced in Japan, as shown in Figure .2. The microscopic particle size was measured by Nano measurer and recorded in Table 1.displays the varistors, referred to as sample Z 0 , prepared by the traditional solid phase method at 1100 ℃.Table 1 clearly illustrates that the grain size of the varistor, prepared using liquid-phase method, gradually increases as the sintering temperature rises.Among these, the grain size of Z 1 is the smallest at approximately 4.37 μm, while that of Z 2 is nearly 4.69 μm.However, the grain size distribution of Z 2 is significantly more uniform compared to that of Z 1 .Numerous small grain aggregations can be observed in Z 1 , which can be seen from Figure .2. Therefore, 1100 ℃ is the optimal sintering temperature, which is consistent with the later electrical property tests.The average grain size of the sample Z 0 is 5.22 μm, which is larger than the grain size of Z 2 .Detailed data can be found in Table 1.This aligns with theoretical situations, because the precursor powder produced through chemical reactions in the liquid-phase method enables uniform mixing at the molecular or even atomic level, the resulting samples demonstrate smaller grain sizes and more uniform microstructure.Figure .3 displays the X-ray diffraction (XRD) patterns of varistors prepared using the solid phase method and liquid phase method, respectively.The XRD analysis conducted using the Empyrean polycrystalline XRD analyzer manufactured in the Netherlands.All the diffraction peaks remain consistent, because of the formulation is the same, but the intensity of the diffraction peaks enhanced as the sintering temperature increasing.And the patterns obviously show that the tree mainly characteristic peaks for 2θ values are 32.183°,34.862°, and 36.674°,corresponding to the (1, 0, 0), (0, 0, 2), (1, 0, 1) plane of hexagonal lead-zinc ore structure of ZnO.In addition to the diffraction peaks of ZnO, several small diffraction peaks corresponding to Zn 7 Sb 2 O 12 spinel phases, Bi-rich phases, and Zn 2 SiO 4 willemite phases.Among the Bi-rich phases, mainly β-Bi 2 O 3 and γ-Bi 2 O 3 are observed.Elements such as Co and Mn were not detected in the samples, which is supposed to be due to these doped metal ions entering the grain boundaries or forming solid solutions with Zn 7 Sb 2 O 12 and Bi 2 O 3 , resulting in the absence of diffraction peaks.Furthermore, a composite phase of Bi, Er, and O was detected with 2θ = 28.187° with weak intensity, which is likely attributed to the low Er doping concentration.The diffraction peaks of sample Z 3 exhibited an overall leftward shift, which is presumed to be caused by an increase in grain size due to excessively high sintering temperatures, ultimately leading to the observed peak shift.V-I) curve in the pre-breakdown region to the area where the leakage current reaches 10 -3 A/cm 2 .The electric properties can be calculated from the E-J curve, as shown in Table 1.The formula for calculating the nonlinearity coefficient (α) is given by α = (lgI 2 -lgI 1 ) / (lgE 2 -lgE 1 ), where I 2 and I 1 represent the product of the sample's area with 1 mA and 0.1 mA, respectively.E 2 and E 1 represent the electric fileds at the current densities of 1 mA/cm 2 and 0.1 mA/cm 2 , respectively.The leakage current is the current when a voltage of 0.75 V 2 is applied across its terminals.The formula for voltage gradient (E) is E = V 2 / d, where d is the thickness of the varistor.

Electrical performance of ZnO varistor
Figure .5(b) illustrates that the voltage gradient of the varistors prepared through the liquid-phase method decreases as the sintering temperature increases.Among these varistors, the voltage gradient of Z 1 is the highest of 658 V/mm, while Z 2 has a voltage gradient of 519 V/mm.However, Z 2 exhibits the lowest leakage current at 5.8 μA/cm 2 , and possesses the highest nonlinear coefficient of 44.These electric properties remain superior across all sintering temperatures, aligning with the SEM morphology discussed earlier.Therefore, varistors prepared through the liquid-phase method exhibit the best electric properties when sintered at 1100 °C, which is consistent with the SEM result in the previous study.Although the voltage gradient of the varistor Z 1 prepared by the liquid-phase method is just only 519 V/mm, which is lower than 535 V/mm achieved by the traditional solid-phase method (Z 0 ) varistor.However, Z 1 exhibits a higher nonlinearity coefficient of 44 and lower leakage current 5.8 μA/cm 2 compared to Z 0 .Therefore, varistors prepared through the liquid-phase method demonstrate superior electrical properties due to the smaller particle size and more uniform distribution of powder particles prepared by the co-deposition method.This is beneficial for the solid solution of Co, Mn, and rare earth elements within the ZnO grains and the Bi-rich phase, leading to a more even distribution of on the grain and grain boundary surface.This promotes the uniform growth of grains, resulting varistors with better electrical properties when prepared through the co-deposition method.

Conclusion
The varistors were prepared using the liquid phase method and sintered at temperature of 1050 ℃, 1100 ℃ and 1150 ℃, respectively.Their properties were then tested and compared with varistors prepared using solid-phase method.The XRD results indicate the spinel and willemite structure of ZnO/Bi 2 O 3 varistors.The SEM results show that the varistors prepared with liquid phase method sintered at 1100 ℃ exhibits smaller grain sizes, most uniform grain size distribution, fewer agglomeration phenomena.And the electrical performance also demonstrates that the varistors prepared with liquid phase method sintered at 1100 ℃ Z 1 performs a high voltage gradient of 519 V/mm, the highest nonlinearity coefficient of 44, and the lowest leakage current of 5.8 μA/cm 2 .All these results indicate that liquid-phase method is good for improving the electrical performance of varistors compared with the solid-phase method and the sintering temperature could be optimized to enhance the electrical performance of varistors prepared by liquid-phase method furtherly.

Acknowledgement
This work is supported by the National Nature Science Foundation of China (62204080), Xiangyang Industrial Research Institute of Hubei University of Technology (XYYJ2022C16, XYYJ2022C09)

Figure 2 (
Figure 2 (a)、(b) and (c) depict the varistors, referred to as samples Z 1 , Z 2 , and Z 3 , respectively, prepared by co-deposition at temperatures of 1050 ℃, 1100 ℃, and 1150 ℃.Additionally, Figure.2(d)displays the varistors, referred to as sample Z 0 , prepared by the traditional solid phase method at 1100 ℃.Table1clearly illustrates that the grain size of the varistor, prepared using liquid-phase

Figure. 5
(a) and 5(b) show the E-J curve and the 3y axis diagram of the electric properties of the varistor, respectively (The test equipment is the Keithley 2410 source table produced in the United States).The enlarged portion of Figure.5(a) represents the voltage-current (

Figure 4 .
Figure 4. Pressure sensitive performance test diagram (a: E-J curve; b: Pressure sensitive performance 3y axis diagram)

Table 1 .
Internal particle size of ZnO varistors

Table 2 .
Pressure sensitive performance