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Influence of Cr2O3 on ZnO–Bi2O3–MnO2-based varistor ceramics
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CERAMICS INTERNATIONAL
Ceramics International 40 (2014) 10149–10152 www.elsevier.com/locate/ceramint
Short communication
Influence of Cr2O3 on ZnO–Bi2O3–MnO2-based varistor ceramics Shuai Maa, Zhijun Xua,n, Ruiqing Chua, Jigong Haoa, Meijun Liua, Lihong Chengb, Guorong Lib a
College of Materials Science and Engineering, Liaocheng University, Liaocheng 252059, People's Republic of China b Shanghai Institute of Ceramics, Chinese Academy of Science, Shanghai 200050, People's Republic of China Received 27 December 2013; received in revised form 9 February 2014; accepted 9 February 2014 Available online 26 February 2014
Abstract The microstructure and electrical properties of ZnO–Bi2O3–MnO2-based varistor ceramics doped with different Cr2O3 content were investigated. The addition of Cr2O3 caused a transformation of minor secondary phases from Bi7.72Mn0.28O12.14 to Bi2O3 dissolving different amount of Cr. The average grain size of sample with 0.2 mol% Cr2O3 increased remarkably from a range of 6.38–6.62 μm to 9.79 μm, and the barrier voltage decreased to 1.31 V. With the increase of Cr2O3, the nonlinear coefficients of sintered samples decreased to 25.0 at 0.2 mol%, and then it further increased up to 40.0, a comparable value with the one of the undoped sample, at 0.8 mol%. The results showed small amounts of Cr2O3 promote the grain-growth at a relatively low sintering temperature, and thus result in the shift of breakdown voltages to a lower value. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: C. Electrical properties; D. ZnO; E. Varistor; Microstructure
1. Introduction ZnO varistors exhibit highly non-ohmic behavior in current– voltage characteristics and show high-energy absorption capacity, low residual voltage, small leakage and fast response to voltage transients [1,2]. For these properties, they are widely used as a voltage regulator and surge protector [2–4]. In classical ZnO–based varistor ceramics, Bi2O3 is used as the varistor-former and thus is essential for inducing the nonlinearity of ZnO ceramics [5,6]. The another kind of additives in ZnO powders are oxide cations that can dissolve into the ZnO grains to affect the grain resistivity, such as Co2O3 and MnO2 [7]. Transition metal oxide such as MnO2 can generally improve the coefficient of nonlinearity at lower current density region because of the increase in barrier height by trapping of electrons [8]. Additives such as Sb2O3, TiO2 and Cr2O3, are used to improve the reliability of ZnO varistors [9–11]. Some studies have been focused on the role of Cr2O3 as an additive in relation to nonlinear properties and microstructure. It should be noted that these studies have often been n
Corresponding author. Tel./fax: þ 86 6358230923. E-mail address: [email protected] (Z. Xu).
http://dx.doi.org/10.1016/j.ceramint.2014.02.035 0272-8842 & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
considered for different multicomponent systems. Maksuoka [1] reported that Cr2O3 dissolves uniformly in the ZnO grain and segregates at grain boundaries. Cho [12] reported that the addition of Cr changes the nature of liquid phase during sintering, resulting in reduction of grain size in ZnO–Bi2O3– Sb2O3-based varistors. However, little attention has been given to the influence of Cr2O3 on the ZnO–Bi2O3 based varistors at a relatively low sintering temperature. The melting point of Bi2O3 is 825 1C, thus a Bi2O3-rich liquid phase can be formed above this temperature, which will enhance the grain growth of ZnO during sintering [13]. Accordingly, the varistor properties can be detected in ZnO–Bi2O3 based ceramics at a sintering temperature of 950 1C with a well crystallized ZnO phase [14]. In this paper we described the effects of Cr2O3 content on the microstructure and the electrical characteristics in ZnO–Bi2O3– MnO2 ternary systems sintered at 950 1C. 2. Experimental procedure The conventional oxide mixing process was used to fabricate the ceramics. The samples were prepared with the following nominal compositions: (98 x) mol% ZnO þ 1.0 mol% Bi2O3 þ 1.0 mol% MnO2 þ x mol% Cr2O3 (x¼ 0, 0.2,
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0.5, 0.8). The above chemicals were mixed by ball-milling in deionized water for 8 h. The dried powers were pressed into discs with 12 mm in diameter and 1.5 mm in thickness under 200 MPa pressure. Then these disc samples were sintered at 950 1C for 4 h with a heating rate of 3 1C/min and cooled to room temperature in the furnace. During sintering, specimens were muffled with the same compositions powders to restrain elements evaporation. Silver pastes were painted on both surfaces of the specimens and heated at 500 1C for 20 min to serve as electrodes. The bulk density was calculated using the dry weight, diameter and thickness of the sample. The phase composition of samples was analyzed by an X-ray diffraction meter using a Cu K. radiation (λ¼ 1.54178 Å) (D8 Advance, Bruker Inc., Germany). The microstructure was examined using a scanning electron microscope (JSM-6380, Japan). The average grain size (d) was determined from the equation, d¼ 1.56L/MN [15], where L is the random line length on the micrograph, M is the magnification of the micrograph, and N is the number of the grain boundaries intercepted by lines. The nominal breakdown voltages V1 mA at 1 mA, V0.1 mA at 0.1 mA, and the leakage current IL at 75% V1 mA were measured by using a varistor tester (MY-4C, Compute Technology Institute of Hunan, Changsha, China), The breakdown field EB (V/mm) and the nonlinear coefficient α are calculated from Et ¼ V1 mA/h and α ¼ 1/log(V1 mA/V0 1 mA), respectively, where h is the thickness of the sample. 3. Results and discussion Fig. 1 shows the XRD patterns of the samples with various Cr2O3 contents. They revealed that different minor secondary phases appeared in addition to a major phase of hexagonal ZnO. In the sample without Cr2O3, the minor phase was Bi7.72Mn0.28O12.14 (PDF no. 43-0208), which was replaced by the crystalline phase Bi7.38Cr0.62O12 þ x (PDF no. 50-0373), Bi2O3 (PDF no. 65-1209) and CrBi18O30 (PDF no. 24-0303) separately in the samples doped with 0.2, 0.5 and 0.8 mol% Cr2O3. In the present study, we could not find the diffraction peaks corresponding to the phase ZnCr2O4 [12]. Therefore, it
Fig. 1. XRD patterns of the varistor ceramics with 0, 0.2, 0.5, and 0.8 mol% Cr2O3 sintered at 950 1C for 4 h.
can be concluded that Cr2O3 can not react with ZnO at such a low temperature of 950 1C. As Cr could diffuse into the crystalline phase ZnO and dissolve in the Bi2O3 phase during sintering process, it played an important role in the electrical behavior. Fig. 2 shows the microstructure of the samples with different Cr2O3 contents. A significant increase in the average size of grains was obtained only for samples with 0.2 mol% Cr2O3, but the uniformity of grains decreased intuitively with some abnormal grain growth. It seemed that when Mn was partly or completely replaced by Cr in the phase Bi2O3, the average grain size increased significantly. With further increase of Cr2O3, the average grain size changed a little. The behavior for the change in the grain size was not in good agreement with the report by Cho [12]. Therefore, it can be concluded that Cr2O3 does not inhibit the grain-growth at relatively low sintering temperatures. Fig. 3(a) shows the average grain size d and barrier voltage Vgb as a function of Cr2O3 content. The average grain size increased obviously for samples with 0.2 mol% Cr2O3, reaching up to 9.79 μm, while the average grain sizes of the other samples were in the range of 6.38–6.62 μm. The barrier voltages of this system were estimated to be in the range of 1.31–4.25 V, which was not always a constant of 3 V for ZnO-based varistors [4]. Fig. 3(b) shows the sintered densities of the samples as a function of Cr2O3 content. As shown in Fig. 3(b), the densities of ceramics were in the range of 5.03–5.34 g/cm3, corresponding to the relative densities 89.5–94.9% of the theoretical value (5.65 g/cm3 for ZnO). In addition, it can be noted that doping 0.2 and 0.5 mol% Cr2O3 induced a decrease of the density. It seemed that small amounts of Cr2O3 inhibited the densification process of ZnO–Bi2O3–MnO2-based varistor ceramics. However, for samples with 0.8 mol% Cr2O3, the density recovered to the initial value of unmodified ZnO– Bi2O3–MnO2- based varistor ceramics. The breakdown voltage E1 mA and nonlinear coefficient as a function of Cr2O3 content are indicated graphically in Fig. 3(c). The E1 mA decreased noticeably from 305.7 V/mm to 133.8 V/mm as 0.2 mol% Cr2O3 added. However, further increase in Cr2O3 content caused the breakdown voltage increasing to 524.7 v/mm at 0.5 mol%, and 641.7 V/mm at 0.8 mol%. E1 mA can also been written as the following expression, E1 mA ¼ Vgb/d, where Vgb is the voltage per barrier, and d is the average grain size. Therefore, the decrease of E1 mA with 0.2 mol% Cr2O3 was attributed to the increase of the average grain size and the decrease of voltage per barrier. However, as 0.2 mol% Cr2O3 added, the nonlinear coefficient α decreased from 39.9 to 25.0. It has been proposed that the behavior of α is strongly related to the potential barrier, which depends on the electronic states related to the zinc vacancies, interstitial zinc, ionized donor-like, oxygen, etc. at the grain boundaries[16]. It has been reported that Cr2O3 does not form an extensive solid solution with the ZnO [17]. On the other hand, Cr2O3 is thermally stable in the range of the studied temperature [18] and does not suffer decomposition at high temperatures. Thus, we assumed that the excess Cr2O3 was
S. Ma et al. / Ceramics International 40 (2014) 10149–10152
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Fig. 2. Surface SEM micrograph of ceramics with different Cr2O3 contents sintered at 950 1C for 4 h (a) x ¼0 mol%; (b) x¼ 0.2 mol%; (c) x ¼0.5 mol%; (d) x¼ 0.8 mol%.
Fig. 3. Microstructural and nonlinear electrical parameters of the varistor ceramics with different Cr2O3 contents sintered at 950 1C for 4 h. (a) average grain size and voltage per barrier, (b) sintered density, (c) breakdown voltage and nonlinear coefficient, (d) leakage current.
segregated at the grain boundary, or small amounts of Cr3 þ ions segregated at the grain boundaries substituting the Zn2 þ ions and increased the electronic density at the grain interface.
Therefore, the increase or decrease of α with an increase in Cr2O3 content was attributed to the increase or decrease of potential barrier height at the grain boundaries.
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The behavior of leakage current as a function of Cr2O3 content is shown in Fig. 3(d). It can be seen that the leakage current decreased with the increase of Cr2O3 content. On the whole, this system exhibited a relatively low leakage current in a range of 0.3–1.3 μA. 4. Conclusion In this study, the effect of Cr2O3 additive on the microstructure and electrical properties of ZnO–Bi2O3–MnO2-based varistor was examined. The addition of Cr2O3 caused a transformation of minor secondary phases from Bi7.72Mn0.28O12.14 to Bi7.38Cr0.62O12 þ x, Bi2O3 and CrBi18O30 in accordance with 0, 0.2, 0.5, 0.8 mol% of Cr2O3. As 0.2 mol% Cr2O3 added, the average grain size increased and the barrier voltage decreased, resulting in the breakdown voltage decreasing to 133.8 V/mm. However, the sintered density and nonlinear coefficient of the samples doped with 0.2 mol% Cr2O3 were also reduced. We observed that further increase of Cr2O3 increased the nonlinear coefficients as well as the breakdown voltages. It was found that Cr2O3 did not react with ZnO at such a low sintering temperature of 950 1C. Conclusively, the present study showed that ZnO–Bi2O3–MnO2-based varistor ceramics could be desirably designed by controlling the amount of Cr2O3 in order to obtain low breakdown voltages with low leakage current. Acknowledgments This work was supported by the National High Technology Research and Development Program of China (No. 2013AA030801), the National Natural Science Foundation of China (No. 51372110), the Natural Science Foundation of Shandong Province of China (No. ZR2012EMM004), the Ph. D. Programs Foundations of Shandong Province of China (No. BS2010CL010). References [1] M. Matsuoka, Nonohmic properties of zinc oxide ceramics, Jpn. J. Appl. Phys. 10 (1971) 736–746. [2] S.A. Pinaro, E.C. Pereira, L.O.S. Bulhoes, E. Longo, J.A. Varela, Effect of Cr2O3 on the electrical properties of multicomponent ZnO varistor at the pre-breakdown region, J. Mater. Sci. 30 (1995) 133–141.
[3] L.M. Levinson, H.R. Philipp, Zinc oxide varistors—a review, Am. Ceram. Soc. Bull. 65 (1986) 639–646. [4] T.K. Gupta, Application of zinc oxide varistors, J. Am. Cream. Soc. 73 (1990) 1817–1840. [5] M. Elfwing, R. Osterlund, E. Olsson, Differences in wetting characteristics of Bi2O3 polymorphs in ZnO varistor materials, J. Am. Cream. Soc. 83 (2000) 2311–2314. [6] S. Wan, W.Z. Lu, X.C. Wang, Low-temperature sintering and electrical properties of ZnO–Bi2O3–TiO2–Co2O3–MnCO3–based varistor with bi2o3–b2o3 frit for multilayer chip varistor applications, J. Am. Cream. Soc. 93 (2010) 3319–3323. [7] K. Hembram, D. Sivaprahasam, T.N. Rao, Combustion synthesis of doped nanocrystalline ZnO powders for varistors application, J. Eur. Cream. Soc. 31 (2011) 1905–1913. [8] E.D. Kim, C.H. Kim, M.H. Oh, Role and effect of Co2O3 additive on the upturn characteristics of ZnO varistor, J. Appl. Phys. 8 (1985) 3231–3235. [9] D. Nina, R. Aleksander, B. Slavko, Grain growth control in Sb2O3– Doped zinc oxide, J. Am. Ceram. Soc. 86 (2003) 1379–1384. [10] C.C. Zhang, Y.X. Hu, W.Z. Lu, M.H. Cao, D.X. Zhou, Influence of TiO2/ Sb2O3 ratio on ZnO varistor ceramics, J. Eur. Ceram. Soc. 22 (2002) 61–65. [11] Y.H. Kim, H. Kawamura, M. Nawata, The effect of Cr2O3 additive on the electrical properties of ZnO varistor, J. Mater. Sci. 32 (1997) 1665–1670. [12] S.G. Cho, H. Lee, Y.S. Kim, Effect of chromium on the phase evolution and microstructure of ZnO doped with bismuth and antimony, J. Mater. Sci. 32 (1997) 4283–4287. [13] J.C. Wurst, J.A. Nelson, Lineal intercept technique for measuring grain size in two-phase polycrystalline ceramics, J. Am. Ceram. Soc. 55 (1972) 109–111. [14] J. Kim, T. Kimura, T. Yamaguchi, Effect of bismuth oxide content on the sintering of zinc oxide, J. Am. Ceram. Soc. 72 (1989) 1541–1544. [15] A. Peigney, H. Andrianjatovo, R. Legros, A. Rousset, Influence of chemical composition on sintering of bismuth-titanium-doped zinc oxide, J. Mater. Sci. 27 (1992) 2397–2405. [16] W. Choon Nahm, Low-temperature sintering effect on varistor properties of ZnO–V2O5–MnO2–Nb2O5–Bi2O3 ceramics, Ceram. Int. 39 (2013) 2117–2121. [17] M. Inada, Formation mechanism of nonohmic zinc oxide ceramics, Jpn. J. Appl. Phys. 19 (1980) 409–419. [18] G.V. Samsonnov, The Oxide Handbook, IFI/Plenum, New York, 1973.
CERAMICS INTERNATIONAL
Ceramics International 40 (2014) 10149–10152 www.elsevier.com/locate/ceramint
Short communication
Influence of Cr2O3 on ZnO–Bi2O3–MnO2-based varistor ceramics Shuai Maa, Zhijun Xua,n, Ruiqing Chua, Jigong Haoa, Meijun Liua, Lihong Chengb, Guorong Lib a
College of Materials Science and Engineering, Liaocheng University, Liaocheng 252059, People's Republic of China b Shanghai Institute of Ceramics, Chinese Academy of Science, Shanghai 200050, People's Republic of China Received 27 December 2013; received in revised form 9 February 2014; accepted 9 February 2014 Available online 26 February 2014
Abstract The microstructure and electrical properties of ZnO–Bi2O3–MnO2-based varistor ceramics doped with different Cr2O3 content were investigated. The addition of Cr2O3 caused a transformation of minor secondary phases from Bi7.72Mn0.28O12.14 to Bi2O3 dissolving different amount of Cr. The average grain size of sample with 0.2 mol% Cr2O3 increased remarkably from a range of 6.38–6.62 μm to 9.79 μm, and the barrier voltage decreased to 1.31 V. With the increase of Cr2O3, the nonlinear coefficients of sintered samples decreased to 25.0 at 0.2 mol%, and then it further increased up to 40.0, a comparable value with the one of the undoped sample, at 0.8 mol%. The results showed small amounts of Cr2O3 promote the grain-growth at a relatively low sintering temperature, and thus result in the shift of breakdown voltages to a lower value. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: C. Electrical properties; D. ZnO; E. Varistor; Microstructure
1. Introduction ZnO varistors exhibit highly non-ohmic behavior in current– voltage characteristics and show high-energy absorption capacity, low residual voltage, small leakage and fast response to voltage transients [1,2]. For these properties, they are widely used as a voltage regulator and surge protector [2–4]. In classical ZnO–based varistor ceramics, Bi2O3 is used as the varistor-former and thus is essential for inducing the nonlinearity of ZnO ceramics [5,6]. The another kind of additives in ZnO powders are oxide cations that can dissolve into the ZnO grains to affect the grain resistivity, such as Co2O3 and MnO2 [7]. Transition metal oxide such as MnO2 can generally improve the coefficient of nonlinearity at lower current density region because of the increase in barrier height by trapping of electrons [8]. Additives such as Sb2O3, TiO2 and Cr2O3, are used to improve the reliability of ZnO varistors [9–11]. Some studies have been focused on the role of Cr2O3 as an additive in relation to nonlinear properties and microstructure. It should be noted that these studies have often been n
Corresponding author. Tel./fax: þ 86 6358230923. E-mail address: [email protected] (Z. Xu).
http://dx.doi.org/10.1016/j.ceramint.2014.02.035 0272-8842 & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
considered for different multicomponent systems. Maksuoka [1] reported that Cr2O3 dissolves uniformly in the ZnO grain and segregates at grain boundaries. Cho [12] reported that the addition of Cr changes the nature of liquid phase during sintering, resulting in reduction of grain size in ZnO–Bi2O3– Sb2O3-based varistors. However, little attention has been given to the influence of Cr2O3 on the ZnO–Bi2O3 based varistors at a relatively low sintering temperature. The melting point of Bi2O3 is 825 1C, thus a Bi2O3-rich liquid phase can be formed above this temperature, which will enhance the grain growth of ZnO during sintering [13]. Accordingly, the varistor properties can be detected in ZnO–Bi2O3 based ceramics at a sintering temperature of 950 1C with a well crystallized ZnO phase [14]. In this paper we described the effects of Cr2O3 content on the microstructure and the electrical characteristics in ZnO–Bi2O3– MnO2 ternary systems sintered at 950 1C. 2. Experimental procedure The conventional oxide mixing process was used to fabricate the ceramics. The samples were prepared with the following nominal compositions: (98 x) mol% ZnO þ 1.0 mol% Bi2O3 þ 1.0 mol% MnO2 þ x mol% Cr2O3 (x¼ 0, 0.2,
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0.5, 0.8). The above chemicals were mixed by ball-milling in deionized water for 8 h. The dried powers were pressed into discs with 12 mm in diameter and 1.5 mm in thickness under 200 MPa pressure. Then these disc samples were sintered at 950 1C for 4 h with a heating rate of 3 1C/min and cooled to room temperature in the furnace. During sintering, specimens were muffled with the same compositions powders to restrain elements evaporation. Silver pastes were painted on both surfaces of the specimens and heated at 500 1C for 20 min to serve as electrodes. The bulk density was calculated using the dry weight, diameter and thickness of the sample. The phase composition of samples was analyzed by an X-ray diffraction meter using a Cu K. radiation (λ¼ 1.54178 Å) (D8 Advance, Bruker Inc., Germany). The microstructure was examined using a scanning electron microscope (JSM-6380, Japan). The average grain size (d) was determined from the equation, d¼ 1.56L/MN [15], where L is the random line length on the micrograph, M is the magnification of the micrograph, and N is the number of the grain boundaries intercepted by lines. The nominal breakdown voltages V1 mA at 1 mA, V0.1 mA at 0.1 mA, and the leakage current IL at 75% V1 mA were measured by using a varistor tester (MY-4C, Compute Technology Institute of Hunan, Changsha, China), The breakdown field EB (V/mm) and the nonlinear coefficient α are calculated from Et ¼ V1 mA/h and α ¼ 1/log(V1 mA/V0 1 mA), respectively, where h is the thickness of the sample. 3. Results and discussion Fig. 1 shows the XRD patterns of the samples with various Cr2O3 contents. They revealed that different minor secondary phases appeared in addition to a major phase of hexagonal ZnO. In the sample without Cr2O3, the minor phase was Bi7.72Mn0.28O12.14 (PDF no. 43-0208), which was replaced by the crystalline phase Bi7.38Cr0.62O12 þ x (PDF no. 50-0373), Bi2O3 (PDF no. 65-1209) and CrBi18O30 (PDF no. 24-0303) separately in the samples doped with 0.2, 0.5 and 0.8 mol% Cr2O3. In the present study, we could not find the diffraction peaks corresponding to the phase ZnCr2O4 [12]. Therefore, it
Fig. 1. XRD patterns of the varistor ceramics with 0, 0.2, 0.5, and 0.8 mol% Cr2O3 sintered at 950 1C for 4 h.
can be concluded that Cr2O3 can not react with ZnO at such a low temperature of 950 1C. As Cr could diffuse into the crystalline phase ZnO and dissolve in the Bi2O3 phase during sintering process, it played an important role in the electrical behavior. Fig. 2 shows the microstructure of the samples with different Cr2O3 contents. A significant increase in the average size of grains was obtained only for samples with 0.2 mol% Cr2O3, but the uniformity of grains decreased intuitively with some abnormal grain growth. It seemed that when Mn was partly or completely replaced by Cr in the phase Bi2O3, the average grain size increased significantly. With further increase of Cr2O3, the average grain size changed a little. The behavior for the change in the grain size was not in good agreement with the report by Cho [12]. Therefore, it can be concluded that Cr2O3 does not inhibit the grain-growth at relatively low sintering temperatures. Fig. 3(a) shows the average grain size d and barrier voltage Vgb as a function of Cr2O3 content. The average grain size increased obviously for samples with 0.2 mol% Cr2O3, reaching up to 9.79 μm, while the average grain sizes of the other samples were in the range of 6.38–6.62 μm. The barrier voltages of this system were estimated to be in the range of 1.31–4.25 V, which was not always a constant of 3 V for ZnO-based varistors [4]. Fig. 3(b) shows the sintered densities of the samples as a function of Cr2O3 content. As shown in Fig. 3(b), the densities of ceramics were in the range of 5.03–5.34 g/cm3, corresponding to the relative densities 89.5–94.9% of the theoretical value (5.65 g/cm3 for ZnO). In addition, it can be noted that doping 0.2 and 0.5 mol% Cr2O3 induced a decrease of the density. It seemed that small amounts of Cr2O3 inhibited the densification process of ZnO–Bi2O3–MnO2-based varistor ceramics. However, for samples with 0.8 mol% Cr2O3, the density recovered to the initial value of unmodified ZnO– Bi2O3–MnO2- based varistor ceramics. The breakdown voltage E1 mA and nonlinear coefficient as a function of Cr2O3 content are indicated graphically in Fig. 3(c). The E1 mA decreased noticeably from 305.7 V/mm to 133.8 V/mm as 0.2 mol% Cr2O3 added. However, further increase in Cr2O3 content caused the breakdown voltage increasing to 524.7 v/mm at 0.5 mol%, and 641.7 V/mm at 0.8 mol%. E1 mA can also been written as the following expression, E1 mA ¼ Vgb/d, where Vgb is the voltage per barrier, and d is the average grain size. Therefore, the decrease of E1 mA with 0.2 mol% Cr2O3 was attributed to the increase of the average grain size and the decrease of voltage per barrier. However, as 0.2 mol% Cr2O3 added, the nonlinear coefficient α decreased from 39.9 to 25.0. It has been proposed that the behavior of α is strongly related to the potential barrier, which depends on the electronic states related to the zinc vacancies, interstitial zinc, ionized donor-like, oxygen, etc. at the grain boundaries[16]. It has been reported that Cr2O3 does not form an extensive solid solution with the ZnO [17]. On the other hand, Cr2O3 is thermally stable in the range of the studied temperature [18] and does not suffer decomposition at high temperatures. Thus, we assumed that the excess Cr2O3 was
S. Ma et al. / Ceramics International 40 (2014) 10149–10152
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Fig. 2. Surface SEM micrograph of ceramics with different Cr2O3 contents sintered at 950 1C for 4 h (a) x ¼0 mol%; (b) x¼ 0.2 mol%; (c) x ¼0.5 mol%; (d) x¼ 0.8 mol%.
Fig. 3. Microstructural and nonlinear electrical parameters of the varistor ceramics with different Cr2O3 contents sintered at 950 1C for 4 h. (a) average grain size and voltage per barrier, (b) sintered density, (c) breakdown voltage and nonlinear coefficient, (d) leakage current.
segregated at the grain boundary, or small amounts of Cr3 þ ions segregated at the grain boundaries substituting the Zn2 þ ions and increased the electronic density at the grain interface.
Therefore, the increase or decrease of α with an increase in Cr2O3 content was attributed to the increase or decrease of potential barrier height at the grain boundaries.
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The behavior of leakage current as a function of Cr2O3 content is shown in Fig. 3(d). It can be seen that the leakage current decreased with the increase of Cr2O3 content. On the whole, this system exhibited a relatively low leakage current in a range of 0.3–1.3 μA. 4. Conclusion In this study, the effect of Cr2O3 additive on the microstructure and electrical properties of ZnO–Bi2O3–MnO2-based varistor was examined. The addition of Cr2O3 caused a transformation of minor secondary phases from Bi7.72Mn0.28O12.14 to Bi7.38Cr0.62O12 þ x, Bi2O3 and CrBi18O30 in accordance with 0, 0.2, 0.5, 0.8 mol% of Cr2O3. As 0.2 mol% Cr2O3 added, the average grain size increased and the barrier voltage decreased, resulting in the breakdown voltage decreasing to 133.8 V/mm. However, the sintered density and nonlinear coefficient of the samples doped with 0.2 mol% Cr2O3 were also reduced. We observed that further increase of Cr2O3 increased the nonlinear coefficients as well as the breakdown voltages. It was found that Cr2O3 did not react with ZnO at such a low sintering temperature of 950 1C. Conclusively, the present study showed that ZnO–Bi2O3–MnO2-based varistor ceramics could be desirably designed by controlling the amount of Cr2O3 in order to obtain low breakdown voltages with low leakage current. Acknowledgments This work was supported by the National High Technology Research and Development Program of China (No. 2013AA030801), the National Natural Science Foundation of China (No. 51372110), the Natural Science Foundation of Shandong Province of China (No. ZR2012EMM004), the Ph. D. Programs Foundations of Shandong Province of China (No. BS2010CL010). References [1] M. Matsuoka, Nonohmic properties of zinc oxide ceramics, Jpn. J. Appl. Phys. 10 (1971) 736–746. [2] S.A. Pinaro, E.C. Pereira, L.O.S. Bulhoes, E. Longo, J.A. Varela, Effect of Cr2O3 on the electrical properties of multicomponent ZnO varistor at the pre-breakdown region, J. Mater. Sci. 30 (1995) 133–141.
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