Microstructure and electrical properties of Lu₂O₃-doped ZnO-Bi₂O₃-based varistor ceramics

XU Dong(徐 东)^{1, 2}, SHI Xiao-feng(史小锋)¹, CHENG Xiao-nong(程晓农)¹,

YANG Juan(杨 娟)¹, FAN Yue-e(樊曰娥)¹, YUAN Hong-ming(袁宏明)³, SHI Li-yi(施利毅)²

1. School of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013, China;

2. Research Center of Nano Science and Technology, Shanghai University, Shanghai 200444, China;

3. State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry,

 Jilin University, Changchun 130012, China

Received 16 October 2010; accepted 25 November 2010

Abstract: Lu₂O₃-doped ZnO-Bi₂O₃-based varistor ceramics samples were prepared by a conventional mixed oxide route and sintered at temperatures in the range of 900-1 000 °C, and the microstructures of the varistor ceramics samples were characterized by X-ray diffractometry (XRD) and scanning electron microscopy (SEM); at the same time, the electrical properties and V—I characteristics of the varistor ceramics samples were investigated by a DC parameter instrument for varistors. The results show that the ZnO-Bi₂O₃-based varistor ceramics with 0.3% Lu₂O₃ (molar fraction) sintered at 950 °C exhibit comparatively ideal comprehensive electrical properties. The XRD analysis of the samples shows the presence of ZnO, Bi-rich, spinel Zn₇Sb₂O₁₂ and Lu₂O₃-based phases.

Key words: ZnO-Bi₂O₃ ceramics; varistor; rare earth; electrical properties

1 Introduction

Zinc oxide varistors are polycrystalline semi- conducting ceramic devices, which are widely used for voltage stabilization and transient surge suppression in electric power systems and electronic circuits[1-3]. The non-linear current—voltage characteristics of ZnO varistor ceramics results from the formation of double Schottky barriers at the grain boundaries, and non-ohmic conduction in ZnO varistors is a grain-boundary phenomenon, which has been explained by thermionic emission enhanced by lowering barrier at low fields with a combination of other mechanisms at high fields[4]. These non-ohmic ZnO-ZnO grain boundaries have a break-down voltage of 3 V, and so the overall break-down voltage of the varistor builds up from the non-ohmic grain boundaries between the electrodes of the varistor and can be controlled either by the varistor thickness or the ZnO grain size. To increase the breakdown voltage, it is necessary to decrease the average size of the ZnO grains[5-7].

Recently, many studies have been made in order to understand the influence of different rare earth oxides on the microstructure and electrical properties of the ZnO varistor ceramics. BERNIK et al[8-9] reported the microstructural and electrical characteristics of ZnO-Bi₂O₃-based varistor ceramics samples doped with Y₂O₃ in the molar fraction range of 0-0.9%. The addition of Y₂O₃ resulted in the formation of a fine-grained Bi-Zn-Sb-Y-O phase along the grain boundaries of the ZnO grains, which inhibits the grain growth. The mean ZnO grain size decreased from 11.3 to 5.4 mm with increasing amount of Y₂O₃. The threshold voltage of the ceramics samples increased from 150 to 274 V/mm, the non-linear coefficient was not influenced and remained at approximately 40, and the leakage current also increased with the amount of Y₂O₃ added. LIU et al[10] investigated ZnO-Bi₂O₃-based varistor ceramics samples doped with 0-3% (molar fraction) Y₂O₃. They discovered that the average grain size of the varistor ceramics samples decreased from about 9.2 μm to 4.5 μm, and the corresponding varistor’s voltage gradient markedly increased from 462 to 2340 V/mm, while the nonlinear coefficient decreased from 22.3 to 11.5. ASHRAF et al[7] reported the microstructure and electrical properties of Ho₂O₃-doped ZnO-Bi₂O₃-based varistor ceramics. The bulk density varied between 5.41 and 5.47 g/cm³ with the maximum value of 5.47 g/cm³ for 0.50% (molar fraction) Ho₂O₃-doped ZnO-Bi₂O₃- based. The average grain sizes for all the samples were calculated from the scanning electron micrographs to be between 5.1 and 7.1 mm. The nonlinear coefficient obtained from electric field—current density plots had a maximum value of 78 for the ceramics with 0.50% Ho₂O₃. The leakage current had a minimum value of 1.30 μA for 0.50% Ho₂O₃-doped ZnO varistor ceramics. The breakdown field was found to increase with the increase of Ho₂O₃ content.

It can be noticed that the rare earth oxides play an important role in controlling operation parameters of these kinds of varistor devices from the review of the research work on ZnO-Bi₂O₃-based varistor ceramics. To address the influence of rare earth oxides on the ZnO varistor ceramics, in the present work, the influence of the amount of added Lu₂O₃ on the microstructure and electrical characteristics of Lu₂O₃-doped ZnO-Bi₂O₃- based varistor ceramics was investigated. The objective of the work is also to understand how the composition controls the microstructure and electrical properties of the ZnO varistors doped with Lu₂O₃. The relation between the electrical characteristics of the Bi₂O₃-based ZnO varistor ceramics with various Lu₂O₃ content was investigated and the results were analyzed.

2 Experimental

ZnO-Bi₂O₃-based varistor ceramics samples with the nominal composition (96.5%–x) (molar fraction) ZnO+0.7%Bi₂O₃+0.8%Co₂O₃+0.5%MnO₂+0.5%Cr₂O₃+ x Lu₂O₃ for x=0, 0.1%, 0.2%, 0.3% and 0.4% (sample labeled N0, N_­1, N2, N3 and N4, respectively) were prepared. Reagent grade oxides were mixed and homogenized in absolute ethanol media at 250 r/min for 5 h by planetary high-energy ball milling in a polyethylene bowl with zirconia balls. The slurry was dried at 70 °C and pressed into discs with 12 mm in diameter and 2 mm in thickness. The pressed discs were sintered at three fixed sintering temperatures of 900, 950 and 1 000 °C for 2 h with heating and cooling rates of  5 °C/min in air ( labeled as A, B, C, respectively). The sintered samples were all lapped and polished to 1.0 mm thick. The final samples with about 10 mm in diameter and 1.0 mm in thickness were obtained. The relative bulk densities (D) of the samples were measured in terms of their mass and volume. For the characterization of DC current—voltage, the silver paste was coated on both faces of samples and the silver electrodes were formed by heating at 600 °C for 10 min in air. The diameters of electrodes were 5 mm. The voltage—current characteristics were measured using a voltage-current source/measure unit (CJP CJ1001). The nominal varistor voltages (V_0.1, V_1.0) at 0.1 and 1.0 mA were measured, and the threshold voltage V_T (V_T = V_0.1/t, where t is the thickness of the sample in mm) and the nonlinear coefficient α (α = 1/lg(V_1.0 /V_0.1)) was determined. The leakage current (I_L) was measured at 0.75V_1.0 [3, 11-15]. The surface microstructure was examined by a scanning electron microscope (SEM, JEOL JSM-7001F). The crystalline phases were identified by an X-ray diffractometer (XRD, Rigaku D/max 2500, Japan) using a Cu K_α radiation.

3 Results and discussion

The relative bulk density values of Lu₂O₃-doped ZnO-Bi₂O₃-based varistor ceramics sintered at sintering temperatures of 900, 950 and 1 000 °C for 2 h are shown in Fig.1. The results represent that the sintered densities of the varistor ceramics samples have a little change at different sintering temperature, and the bulk densities almost increase with increasing the Lu₂O₃ molar fraction increasing and decrease with the sintering temperature of 900, 950 and 1 000 °C, respectively. Lu³⁺ ion has a larger radius (0.085 nm) than Zn²⁺ ion (0.074 nm). Molar mass of Zn (65.39 g) is less than that of Lu (174.97 g). Thus, initial addition of Lu₂O₃ affects the grain distribution and develops different phases in the ceramic matrix, increasing the bulk density initially. Further increase of the Lu₂O₃ content may mainly contribute to the change in grain size and phase distribution. So, bulk density increases with the increase of the Lu₂O₃ content and then decreases at the different temperatures. The decrease in bulk density of the varistor ceramics with higher Lu₂O₃ content may be due to the increase of intragranular porosity[7].

Fig.1 Relative bulk density of Lu₂O₃-doped varistor ceramics samples sintered at different temperatures

Fig.2 shows the leakage current of Lu₂O₃-doped ZnO-Bi₂O₃-based varistor ceramics samples sintered at different sintering temperatures for 2 h as a function of Lu₂O₃ molar fraction. From Fig.2 we can find that the leakage current decreases with the increase of Lu₂O₃ content, and the leakage current is not almost affected by sintering temperature in the range of 950–1 000 °C. The leakage currents of the samples with Lu₂O₃ are all lower than those without Lu₂O₃ sintered at different temperatures, indicating that Lu₂O₃ can significantly decrease the leakage current of varistor.

Fig.2 Leakage current of Lu₂O₃-doped varistor ceramics samples sintered at different temperatures

The variation of the threshold voltage of Lu₂O₃- doped ZnO-Bi₂O₃-based varistor ceramics samples sintered at different temperatures for 2 h is shown in Fig.3. It is observed that the threshold voltage firstly increases and then decreases and finally increases with the increase of Lu₂O₃ content. The changes of the threshold voltage at different sintering temperatures are similar. The threshold voltage increases as the grains size decreases. The higher the sintering temperature is, the lower the threshold voltage is, due to the growth of grain at the elevated temperatures.

As shown in Fig.4, the nonlinear coefficients of Lu₂O₃-doped ZnO-Bi₂O₃-based varistor ceramics samples first decrease, then increase and last decrease at different sintering temperatures, and reach a maximum for 0.3% Lu₂O₃-doped ZnO-Bi₂O₃-based varistor ceramics.

As a result, for the performance index of Lu₂O₃- doped ZnO-Bi₂O₃-based varistor ceramics (high threshold voltage, low leakage current, high nonlinear coefficient and high density), the sample with 0.3% Lu₂O₃ among Lu₂O₃-doped ZnO-Bi₂O₃-based varistor ceramics samples sintered at 950 °C is the best. The threshold voltage is 874 V/mm, the nonlinear coefficient is 20.9, and the leakage current is 0. 61 μA.

Figs.5-7 show the electric field intensity against current density (E—J) characteristics of Lu₂O₃-doped ZnO-Bi₂O₃-based varistor ceramics samples sintered at three fixed sintering temperatures of 900, 950 and 1 000 °C for 2 h, respectively. As we know, the sharper the knee of the curve between the two regions is, the better the nonlinear properties are[3, 14, 16-18]. From Fig.6, we can find that the nonlinear coefficient decreases in the order of N3B→N0B→N4B→N2B→N1B and the threshold voltage decreases in the order of N4B→N1B→NB3→N2B→N0B.

Fig.3 Threshold voltage of Lu₂O₃-doped varistor ceramics samples sintered at different temperatures

Fig.4 Nonlinear coefficient of Lu₂O₃-doped varistor ceramics samples sintered at different temperatures

Fig.5 E—J curves of Lu₂O₃-doped varistor ceramics samples sintered at 900 °C

Fig.6 E—J curves of Lu₂O₃-doped varistor ceramics samples sintered at 950 °C

The SEM images of Lu₂O₃-doped ZnO-Bi₂O₃- based varistor ceramics samples sintered at 950 °C for 2 h are shown in Fig.8. Those varistor ceramics samples are composed of ZnO phase, Bi₂O₃-rich phase, Zn₇Sb₂O₁₂ spinel-type phase and Lu₂O₃ phase, as determined by XRD analysis (Fig.9). From Fig.8, we can find that the grains size is fine when the amount of Lu₂O₃ added is 0.1%, then it increases with increasing the molar fraction of Lu₂O₃ to 0.2%.

Fig.7 E—J curves of Lu₂O₃-doped varistor ceramics samples sintered at 1 000 °C

XRD patterns of Lu₂O₃-doped ZnO-Bi₂O₃-based varistor ceramics samples sintered at 950 °C for 2 h are presented in Fig.9. The samples consist typically of three phases: ZnO phase, Bi-rich phase and spinel phase; ZnO is the predominant phase. However, for the samples doped with Lu₂O₃, additional peaks are evident due to the formation of Lu-based phases in the ceramic and their intensity increases with increasing Lu₂O₃ molar fraction in the starting composition. They may mainly disperse in the grain boundary[18]. At the same time, some of the peaks corresponding to spinel phase of Zn₇Sb₂O₁₂ for varistor ceramics diminish for samples with higher content of Lu₂O₃. So, 0.3% Lu₂O₃-doped ZnO-Bi₂O₃- based varistor ceramics appears to have an optimal composition of spinel phase of Zn₇Sb₂O₁₂ and Lu-based phase, which results in homogeneous grain size of ZnO among those samples as observed by SEM.

Fig.8 SEM images of Lu₂O₃-doped varistor ceramics samples sintered at 950 °C: (a) N0B; (b) N1B; (c) N2B; (d) N3B; (e) N4B

Fig.9 XRD patterns of Lu₂O₃-doped varistor ceramics samples sintered at 950 °C

4 Conclusions

1) Lu₂O₃-doped ZnO-Bi₂O₃-based varistor ceramics samples were prepared by a conventional mixed oxide route and sintered in the temperatures rang of 900-     1 000 °C.

2) The ZnO-Bi₂O₃-based varistor ceramics with 0.3% (molar fraction) Lu₂O₃ sintered at 950 °C exhibits comparatively ideal comprehensive electrical properties with the threshold voltage of 874 V/mm, the nonlinear coefficient of 20.9 and the leakage current of 0.61 μA.

3) The X-ray diffraction analysis of the samples show the presence of ZnO, Bi-rich, spinel Zn₇Sb₂O₁₂ and Lu₂O₃-based phases; at the same time, some of the peaks corresponding to spinel phase of Zn₇Sb₂O₁₂ diminish for samples with higher content of Lu₂O₃.

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(Edited by YANG Hua)

Foundation item: Project(50902061) supported by the National Natural Science Foundation of China; Project(2011-22) supported by the State Key Laboratory of Inorganic Synthesis and Preparative Chemistry of Jilin University, China; Project(20100471380) supported by the China Postdoctoral Science Foundation; Project(J50102) supported by the Leading Academic Discipline Program of Shanghai Municipal Education Commission, China; Project(10KJD430002) supported by the Universities Natural Science Research Program of Jiangsu Province, China; Project(2010002) supported by the Jiangsu University Undergraduate Practice-Innovation Training Program, China

Corresponding author: XU Dong; Tel: +86-511-88783396; E-mail: frank@ujs.edu.cn

DOI: 10.1016/S1003-6326(10)60645-0