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Influence of SnO2 on ZnO–Bi2O3–Co2O3 based varistor ceramics
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Influence of SnO2 on ZnO–Bi2O3–Co2O3 based varistor ceramics Shuai Maa, Zhijun Xua,n, Ruiqing Chua, Jigong Haoa, Wei Lia, 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 18 April 2015; received in revised form 22 May 2015; accepted 1 June 2015
Abstract ZnO–Bi2O3–Co2O3 based varistors doped with SnO2, from 0 to 0.3 mol% were prepared by conventional ceramic processing. The effect of SnO2 on the microstructure and the electrical characteristics of varistor samples were investigated. Results showed that the addition of small amounts of SnO2 inhibited the grain growth and induced a change of grain morphology. With increasing SnO2 doping level up to 0.3 mol%, Bi2O3 reacted with SnO2 to form a cubic crystalline phase Bi2(Sn2O7) at 950 1C and a pyrochlore phase Bi2Sn2O7 at 1100 1C. Moreover, small amounts of SnO2 produced higher nonlinear coefficients and reduced leakage currents of ZnO–Bi2O3–Co2O3 based varistors. Meanwhile, the breakdown fields increased noticeably with the addition of SnO2, which was due to the decrease of average grain sizes. It was found that the addition of SnO2 as well as the sintering temperature had an obvious effect on the insulating level at grain boundary region, which was indicated from the changes of relative dielectric constants. & 2015 Published by Elsevier Ltd and Techna Group S.r.l.
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 [3,4].The varistor behavior of ZnO ceramics is directly related to the composition and to the microstructure development [5,6]. Therefore, it is important to understand the effect of the individual additives both on the electrical properties and on the microstructural development of ZnO grains. The chemical compositions of traditional varistors consist of minor additions of several metal oxides to the ZnO powder. Among these additives, Bi2O3 is used as the varistor property imposer; thus it is essential for inducing the nonlinearity of the ZnO ceramics [7,8]. Transition metal oxide such as Co2O3 can generally improve the nonlinear coefficient by providing charge carriers and increasing the barrier height [9,10]. n
Corresponding author. Tel./fax: þ 86 6358230923. E-mail address: [email protected] (Z. Xu).
However, it should be noted that very little is known about the effect of small amounts of SnO2 on the electrical properties in ZnO–Bi2O3 based varistors. Elfwing et al. [11] reported that the addition of SnO2 influences ZnO grain growth due to the formation of a Zn2SnO4-type spinel phase and a Bi2Sn2O7type pyrochlore phase. Reports in the field of SnO2-doped ZnO varistors point out that small amounts of tin substituting zinc atoms in ZnO lattice appear to cause a strong donor effect [12]. Furthermore, it is reported that in Bi2O3–MnO2–Co3O4 added ZnO varistors. Spinel particles began to appear by the addition of SnO2 over approximately 0.5 mol% and varistor voltage increased with increasing the amount of SnO2 [13]. In the present work, the effect of SnO2 (the amount of SnO2 is not more than 0.3 mol%) on the microstructure and the electrical characteristics of ZnO–Bi2O3–Co2O3 based varistors have been studied. 2. Experimental procedure Samples with nominal compositions (98.5 x) mol% ZnOþ 0.7 mol% Bi2O3 þ 0.8 mol% Co2O3 þ x mol% SnO2 (x¼ 0, 0.1,
http://dx.doi.org/10.1016/j.ceramint.2015.06.004 0272-8842/& 2015 Published by Elsevier Ltd and Techna Group S.r.l.
Please cite this article as: S. Ma, et al., Influence of SnO2 on ZnO–Bi2O3–Co2O3 based varistor ceramics, Ceramics International (2015), http://dx.doi.org/ 10.1016/j.ceramint.2015.06.004
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S. Ma et al. / Ceramics International ] (]]]]) ]]]–]]]
0.2, 0.3) were fabricated by a conventional oxide mixing process. Raw materials of ZnO (99.5%), Bi2O3 (99.64%), Co2O3 (99.0%) and SnO2 (99.5%) were used. The powers were mixed and homogenized in a polyethylene battle with ZrO2 balls in deionized water for 8 h by planetary high-energy ball milling. The slurry was dried at 110 1C for 10 h; then pulverized by an agate mortar pestle. After adding 3 wt% polyvinyl alcohol (PVA) binder, the powders were granulated by sieving through a 50mesh screen to produce starting powers. The starting powers were pressed into discs with 15 mm in diameter and 1.5 mm in thickness under 200 MPa pressure. The pressed disks were heated in air at 650 1C for 1 h to release the binder. Then these disc samples were sintered between 950 1C and 1100 1C in air for 3 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 electrodes were painted on both surfaces of the specimens followed by thermal treatment at 500 1C for 20 min. Some unpainted electrodes ceramic samples were cleaned with ultrasonic washer for 5 min for microstructure testing. The natural surface microstructures of the ceramics samples were examined using a scanning electron microscope (JSM6380, Japan). The average grain size (d) was determined from the equation, d ¼ 1.56L/MN [14], 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 Eb ¼ V1 mA/h and α ¼ 1/log(V1 mA/ V0.1 mA), respectively, where h is the thickness of the sample. The phase composition of samples was analyzed by an X-ray diffraction meter using a Cu Kα radiation (λ ¼ 1.54178 Å) (D8 Advance, Bruker Inc., Germany). The relative dielectric constant, εr, was measured using a precision analyzer (Agilent 4294A, Agilent Inc., American) in a range of 102–107 Hz.
3. Results and discussion Fig. 1 shows the XRD patterns of the varistor ceramics with various SnO2 contents sintered at 950 1C (Z1) and 1100 1C (Z2) for 3 h. The patterns reveal that no secondary phases were detected in Z1 varistor samples containing lower than 0.3 mol % SnO2. However, small diffraction peaks at 27.51 and 47.81 associated with the cubic crystalline phase Bi2(Sn2O7) (PDF no. 88-0496) appeared in Z1 varistor samples with 0.3 mol% SnO2. For Z2 varistor samples without SnO2, Bi2O3 (PDF no. 51-1161) phase was detected as minor secondary phase according to the weak peaks at 28.71 and 46.81. Whereas with the addition of SnO2, Bi2O3 phase disappeared. Instead, the Bi2Sn2O7 (PDF no. 34-1203) phase appeared in Z2 varistor samples as 0.3 mol% SnO2 doped, indicating that Bi2O3 reacted with ZnO and formed a pyrochlore phase [11]. Comparing with Z1 and Z2 varistor samples, the intensity of ZnO phase reflections in Z1 varistor samples was weaker than that in Z2 varistor samples, which can be explained in terms of the morphological difference in crystallites. The absence of Bi2O3 phase in Z1 varistor samples without SnO2 was probably due to the diffusion of bismuth into the ZnO lattice. It was found that the cubic crystalline phase Bi2(Sn2O7) would transform to pyrochlore phase Bi2Sn2O7 at 1100 1C. Table 1 shows the lattice constant of ZnO phase in Z1 and Z2 varistor samples. In comparison to the samples without SnO2, lattice constant of ZnO phase slightly increased in all the ceramics containing SnO2. This means that some Sn4 þ ions diffused into ZnO phase. The reaction of Bi2O3 and SnO2 occurred only in the case of composition with 0.3 mol% SnO2 both for Z1 and Z2 varistor samples, so the range of solid solution of tin into ZnO lattice should be under this level of doping. Fig. 2 shows the SEM images of Z1 and Z2 varistor samples with various SnO2 contents. Due to the higher sintering temperatures, the grain sizes of Z2 varistor samples were much larger than that of Z1 varistor samples with the same composition. As SnO2 was added to the base composition, the average grain sizes decreased obviously for both Z1 and Z2
Fig. 1. XRD patterns of Z1 and Z2 varistor samples with 0, 0.1, 0.2 and 0.3 mol% SnO2, Z1: sintered at 950 1C; Z2: sintered at 1100 1C.
Please cite this article as: S. Ma, et al., Influence of SnO2 on ZnO–Bi2O3–Co2O3 based varistor ceramics, Ceramics International (2015), http://dx.doi.org/ 10.1016/j.ceramint.2015.06.004
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varistor samples. This behavior shed a serious doubt with the explanation of Recnik [15], who reported that the addition of SnO2 to basic ZnO–Bi2O3 varistor composition resulted in an increase in ZnO grain size. The decreased grain sizes in the present work may be ascribed to the addition of Co2O3, which generally acts grain growth inhibitor in ZnO-based varistor [16]. For Z1 varistor samples without SnO2, the microstructure revealed mainly equiaxed polygon-shaped grains with a small amount of pores situated at triple boundaries, grain boundaries were flat and often 1201 apart from each other. However, for Z1 and Z2 varistor samples containing SnO2, it shows irregular bar-shaped grains. It was observed that continuous crystalline intergranular films were situated between ZnO grains in Z2 varistor samples without SnO2. For further study of these intergranular films, the elements present in the grain and intergranular films regions for Z2 samples without SnO2 were identified using EDS spot analysis. To ensure the accuracy of the results, the cross sections were used for the EDS spot analysis. As shown in Fig. 3, the bismuth cations were completely detected in intergranular films regions, while the cobalt element was detected both in grain regions and intergranular films regions. Combined with the XRD analysis, the intergranular phases were Bi-rich phases, mainly containing ZnO phase and small amounts of Bi2O3 phase. Therefore,
3
it was concluded that small amounts of SnO2 inhibited the grain growth of ZnO and caused an alteration of the grain morphology. Fig. 4 shows the electric field–current density (E–J) characteristics of Z1 and Z2 samples and characteristic parameters which define the electrical properties of the varistors are shown in Table 2. It can be observed from Fig. 4 that the conduction characteristics are divided into two regions: pre-breakdown region at low electric field and breakdown region at high electric field. The shaper is the knee of curves between the two regions; the lager is the nonlinearity coefficient. It shows Z2 samples exhibited a slightly sharper knee in comparison with that of Z1 samples. However, all of Z1 samples exhibited better nonlinear electrical characteristics at breakdown region, because the current densities increased more abruptly with the increase of electric fields in this region.
Table 1 Characteristic parameters for Z1 and Z2 varistor samples. SnO2 content (mol%)
0 0.1 0.2 0.3
Z1
Z2
da (μm)
αb ILc (μA)
E1 mAd (V/ d α mm) (μm)
4.99 2.65 2.78 2.74
4.8 6.7 7.3 5.7
411 683 575 592
191.1 142.4 123.3 177.6
E1 mA (V/ IL (μA) mm)
13.96 3.7 405 97 7.98 8.1 98.9 244 8.42 10.7 50.3 286 8.48 8.5 90.5 255
a
Average grain size. Nonlinear coefficient. c Leakage current. d Breakdown field. b
Fig. 3. EDS spectra of spot analysis of Z2 varistor samples without SnO2 in grain regions and intergranular films regions: (A) intergranular films regions and (B) grain regions.
Fig. 2. Surface SEM micrographs of Z1 and Z2 varistor samples with different SnO2 contents: (a) Z1-0 mol%, (b) Z1-0.1 mol%, (c) Z1-0.2 mol%, (d) Z1-0.3 mol %, (e) Z2-0 mol%, (f) Z2-0.1 mol%, (g) Z2-0.2 mol%, and (h) Z2-0.3 mol%.
Please cite this article as: S. Ma, et al., Influence of SnO2 on ZnO–Bi2O3–Co2O3 based varistor ceramics, Ceramics International (2015), http://dx.doi.org/ 10.1016/j.ceramint.2015.06.004
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Fig. 4. E–J characteristics curves of Z1 and Z2 varistor samples with 0, 0.1, 0.2 and 0.3 mol% SnO2.
Table 2 Lattice constants of ZnO phase in Z1 and Z2 varistor samples. SnO2 content (mol%)
Z1
Z2
a (Å) 0 0.1 0.2 0.3
3.248 3.250 3.253 3.250
c (Å) (8) (1) (9) (1)
5.205 5.207 5.209 5.207
a (Å) (4) (1) (8) (1)
3.24 3.250 3.25 3.242
c (Å) (9) (1) (3) (7)
5.205 5.207 5.21 5.194
(2) (1) (3) (8)
Table 2 shows that between 0.1 and 0.3 mol% of SnO2 addition, the average grain sizes changed very slightly for both Z1 and Z2 varistor samples. The values of nonlinear coefficients were in the range of 4.8–7.3 and the leakage currents were in range of 123.3–191.1 μA for Z1 varistor samples. However, the breakdown filed increased noticeably from 411 V/mm to 683 V/mm for Z1 with 0.1 mol% SnO2, which was mainly attributed to the decrease of the average grain sizes [17]. For Z2 varistor samples doped with SnO2, the nonlinear coefficients were in the range of 8.1–10.7 with the leakage currents below 98.9 μA. For Z2 without SnO2, the leakage current was 405 μA and the nonlinear coefficient was only 3.7. However, unlike those of Z1 samples, the breakdown fields were not increasing in an inverse ratio with the average grain sizes in Z2 samples. This unusual behavior was probably due to the increase of potential barrier height at grain boundaries, which can be indicated from the increase of nonlinear coefficients. Combined with the XRD analysis, the presence crystalline phase Bi2(Sn2O7) and Bi2Sn2O7 in the samples with 0.3 mol% SnO2 had a negative effect on the nonlinear characteristics. We concluded that small amounts of SnO2 improved the nonlinear electrical properties of varistors. In addition, it should be noted that the influence of SnO2 on the samples sintered at 1100 1C was more obvious than those sintered at 950 1C. Fig. 5 shows the relative dielectric constants (εr) of Z1 and Z2 samples at various frequencies in the range 102–107 Hz. It shows that with the increase of sintering temperature, the values of εr increased obviously. In the low frequency range, the effective capacitance is originated from grain boundaries
[18]. The larger dielectric constants of the varistor ceramics indicated the formation of microstructure with more insulating grain boundary regions [19]. With the addition of SnO2, the values of εr decreased for both Z1 and Z2 samples. It should be noted that εr did not decrease monotonously with the increasing of SnO2 content, and the samples with 0.2 mol% had the lowest relative dielectric constants. Therefore, SnO2 had an obvious influence on the grain boundaries of ZnO varistor ceramics by reducing the permittivity values, resulting in a less insulating grain boundary region. 4. Conclusion In summary, small amounts of SnO2 had a great influence on the microstructure and electrical properties of ZnO–Bi2O3– Co2O3 based varistors. With the addition of very small amounts SnO2, the average grain sizes decreased obviously, and the crystal morphology revealed an alteration from equiaxed polygon-shaped grains to irregular bar-shaped grains. The reaction of SnO2 and Bi2O3 occurred when the doping level reaches up to 0.3 mol%. The Bi was combined with Sn as cubic crystalline phase Bi2(Sn2O7) at 950 1C and as pyrochlore phase Bi2Sn2O7 at 1100 1C. For samples sintered at 1100 1C, Bi2O3 phase was only observed in continuous crystalline intergranular films situated between ZnO grains in the samples without SnO2. It was found that small amounts of SnO2 increased the nonlinear coefficients and reduced the leakage currents. The dielectric dispersion studies showed that the addition of SnO2 reduced the dielectric constants, making the grain boundaries less insulating. 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, 51402144, 51302124), the Natural Science Foundation of Shandong Province of China (No. ZR2014JL030), The Project of Shandong Province Higher Educational Science and Technology Program (No. J14LA11,
Please cite this article as: S. Ma, et al., Influence of SnO2 on ZnO–Bi2O3–Co2O3 based varistor ceramics, Ceramics International (2015), http://dx.doi.org/ 10.1016/j.ceramint.2015.06.004
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Fig. 5. εr–f characteristics curves of Z1 and Z2 varistor samples with 0, 0.1, 0.2 and 0.3 mol% SnO2.
No.J14LA10), and the Research Foundation of Liaocheng University (Nos. 318011301 and 318011306).
References [1] M. Matsuoka, Nonohmic properties of zinc oxide ceramics, Jpn. J. Appl. Phys. 10 (1971) 736–746. [2] C. Leach, Grain boundary structures in zinc oxide varistors, Acta Mater. 53 (2005) 237–245. [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] D.R. Clarke, Varistor ceramics, J. Am. Ceram. Soc. 82 (1999) 485–502. [6] P.Q. Mantas, J.L. Baptista, The barrier height formation in ZnO varistor, J. Eur. Ceram. Soc. 15 (1995) 605–615. [7] S.A. Pinaro, E.C. Pereira, L.O.S. Bulhoes, et al., Effect of Cr2O3 on the electrical properties of multicomponent ZnO varistor at the prebreakdown region, J. Mater. Sci. 30 (1995) 133–141. [8] 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. [9] 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.
[10] 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. [11] S. Bernik, N. Daneu, Characteristics of SnO2-doped ZnO-based varistor ceramics, J. Eur. Cream. Soc. 21 (2001) 1879–1882. [12] A. Anastasiou, M.H.J. Lee, C. Leach, R. Freer, Ceramic varistors based on ZnO–SnO2, J. Eur. Ceram. Soc. 24 (2004) 1171–1175. [13] M. Takada, S. Yosikado, Effect of SnO2 addition on electrical degradation characteristics of ZnO varistors, IEEJ Trans. Fundam. Mater. 127 (2007) 621–629. [14] 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. [15] A. Recnil, S. Bernik, N. Daneu, Microstructural engineering of ZnObased varistor ceramics, J. Mater. Sci. 47 (2012) 1655–1668. [16] B. Straumal, A.A. Mazilkin, S.G. Protasova, et al., Increase of Co solubility with decreasing grain size in ZnO, Acta Mater. 56 (2008) 6246–6256. [17] D.C. Halls, C. Leach, Spectroscopic cathodoluminescence studies of additive free zinc oxide and varistor ceramics, Acta Mater. 46 (1998) 6237–6243. [18] L. Ke, D.M. Jiang, X.M. Ma, Low-temperature sintering of high voltage gradient ZnO-based thick film varistors, Ceram.-Silik. 53 (2009) 102–107. [19] T.R.N. Kutty, S. Ezhilvalavan, Zinc oxide ceramic varistors formulated with barium orthosilicate for operation in the 3–15 V battery range, Mater. Lett. 27 (1996) 355–361.
Please cite this article as: S. Ma, et al., Influence of SnO2 on ZnO–Bi2O3–Co2O3 based varistor ceramics, Ceramics International (2015), http://dx.doi.org/ 10.1016/j.ceramint.2015.06.004
CERAMICS INTERNATIONAL
Ceramics International ] (]]]]) ]]]–]]] www.elsevier.com/locate/ceramint
Short communication
Influence of SnO2 on ZnO–Bi2O3–Co2O3 based varistor ceramics Shuai Maa, Zhijun Xua,n, Ruiqing Chua, Jigong Haoa, Wei Lia, 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 18 April 2015; received in revised form 22 May 2015; accepted 1 June 2015
Abstract ZnO–Bi2O3–Co2O3 based varistors doped with SnO2, from 0 to 0.3 mol% were prepared by conventional ceramic processing. The effect of SnO2 on the microstructure and the electrical characteristics of varistor samples were investigated. Results showed that the addition of small amounts of SnO2 inhibited the grain growth and induced a change of grain morphology. With increasing SnO2 doping level up to 0.3 mol%, Bi2O3 reacted with SnO2 to form a cubic crystalline phase Bi2(Sn2O7) at 950 1C and a pyrochlore phase Bi2Sn2O7 at 1100 1C. Moreover, small amounts of SnO2 produced higher nonlinear coefficients and reduced leakage currents of ZnO–Bi2O3–Co2O3 based varistors. Meanwhile, the breakdown fields increased noticeably with the addition of SnO2, which was due to the decrease of average grain sizes. It was found that the addition of SnO2 as well as the sintering temperature had an obvious effect on the insulating level at grain boundary region, which was indicated from the changes of relative dielectric constants. & 2015 Published by Elsevier Ltd and Techna Group S.r.l.
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 [3,4].The varistor behavior of ZnO ceramics is directly related to the composition and to the microstructure development [5,6]. Therefore, it is important to understand the effect of the individual additives both on the electrical properties and on the microstructural development of ZnO grains. The chemical compositions of traditional varistors consist of minor additions of several metal oxides to the ZnO powder. Among these additives, Bi2O3 is used as the varistor property imposer; thus it is essential for inducing the nonlinearity of the ZnO ceramics [7,8]. Transition metal oxide such as Co2O3 can generally improve the nonlinear coefficient by providing charge carriers and increasing the barrier height [9,10]. n
Corresponding author. Tel./fax: þ 86 6358230923. E-mail address: [email protected] (Z. Xu).
However, it should be noted that very little is known about the effect of small amounts of SnO2 on the electrical properties in ZnO–Bi2O3 based varistors. Elfwing et al. [11] reported that the addition of SnO2 influences ZnO grain growth due to the formation of a Zn2SnO4-type spinel phase and a Bi2Sn2O7type pyrochlore phase. Reports in the field of SnO2-doped ZnO varistors point out that small amounts of tin substituting zinc atoms in ZnO lattice appear to cause a strong donor effect [12]. Furthermore, it is reported that in Bi2O3–MnO2–Co3O4 added ZnO varistors. Spinel particles began to appear by the addition of SnO2 over approximately 0.5 mol% and varistor voltage increased with increasing the amount of SnO2 [13]. In the present work, the effect of SnO2 (the amount of SnO2 is not more than 0.3 mol%) on the microstructure and the electrical characteristics of ZnO–Bi2O3–Co2O3 based varistors have been studied. 2. Experimental procedure Samples with nominal compositions (98.5 x) mol% ZnOþ 0.7 mol% Bi2O3 þ 0.8 mol% Co2O3 þ x mol% SnO2 (x¼ 0, 0.1,
http://dx.doi.org/10.1016/j.ceramint.2015.06.004 0272-8842/& 2015 Published by Elsevier Ltd and Techna Group S.r.l.
Please cite this article as: S. Ma, et al., Influence of SnO2 on ZnO–Bi2O3–Co2O3 based varistor ceramics, Ceramics International (2015), http://dx.doi.org/ 10.1016/j.ceramint.2015.06.004
2
S. Ma et al. / Ceramics International ] (]]]]) ]]]–]]]
0.2, 0.3) were fabricated by a conventional oxide mixing process. Raw materials of ZnO (99.5%), Bi2O3 (99.64%), Co2O3 (99.0%) and SnO2 (99.5%) were used. The powers were mixed and homogenized in a polyethylene battle with ZrO2 balls in deionized water for 8 h by planetary high-energy ball milling. The slurry was dried at 110 1C for 10 h; then pulverized by an agate mortar pestle. After adding 3 wt% polyvinyl alcohol (PVA) binder, the powders were granulated by sieving through a 50mesh screen to produce starting powers. The starting powers were pressed into discs with 15 mm in diameter and 1.5 mm in thickness under 200 MPa pressure. The pressed disks were heated in air at 650 1C for 1 h to release the binder. Then these disc samples were sintered between 950 1C and 1100 1C in air for 3 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 electrodes were painted on both surfaces of the specimens followed by thermal treatment at 500 1C for 20 min. Some unpainted electrodes ceramic samples were cleaned with ultrasonic washer for 5 min for microstructure testing. The natural surface microstructures of the ceramics samples were examined using a scanning electron microscope (JSM6380, Japan). The average grain size (d) was determined from the equation, d ¼ 1.56L/MN [14], 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 Eb ¼ V1 mA/h and α ¼ 1/log(V1 mA/ V0.1 mA), respectively, where h is the thickness of the sample. The phase composition of samples was analyzed by an X-ray diffraction meter using a Cu Kα radiation (λ ¼ 1.54178 Å) (D8 Advance, Bruker Inc., Germany). The relative dielectric constant, εr, was measured using a precision analyzer (Agilent 4294A, Agilent Inc., American) in a range of 102–107 Hz.
3. Results and discussion Fig. 1 shows the XRD patterns of the varistor ceramics with various SnO2 contents sintered at 950 1C (Z1) and 1100 1C (Z2) for 3 h. The patterns reveal that no secondary phases were detected in Z1 varistor samples containing lower than 0.3 mol % SnO2. However, small diffraction peaks at 27.51 and 47.81 associated with the cubic crystalline phase Bi2(Sn2O7) (PDF no. 88-0496) appeared in Z1 varistor samples with 0.3 mol% SnO2. For Z2 varistor samples without SnO2, Bi2O3 (PDF no. 51-1161) phase was detected as minor secondary phase according to the weak peaks at 28.71 and 46.81. Whereas with the addition of SnO2, Bi2O3 phase disappeared. Instead, the Bi2Sn2O7 (PDF no. 34-1203) phase appeared in Z2 varistor samples as 0.3 mol% SnO2 doped, indicating that Bi2O3 reacted with ZnO and formed a pyrochlore phase [11]. Comparing with Z1 and Z2 varistor samples, the intensity of ZnO phase reflections in Z1 varistor samples was weaker than that in Z2 varistor samples, which can be explained in terms of the morphological difference in crystallites. The absence of Bi2O3 phase in Z1 varistor samples without SnO2 was probably due to the diffusion of bismuth into the ZnO lattice. It was found that the cubic crystalline phase Bi2(Sn2O7) would transform to pyrochlore phase Bi2Sn2O7 at 1100 1C. Table 1 shows the lattice constant of ZnO phase in Z1 and Z2 varistor samples. In comparison to the samples without SnO2, lattice constant of ZnO phase slightly increased in all the ceramics containing SnO2. This means that some Sn4 þ ions diffused into ZnO phase. The reaction of Bi2O3 and SnO2 occurred only in the case of composition with 0.3 mol% SnO2 both for Z1 and Z2 varistor samples, so the range of solid solution of tin into ZnO lattice should be under this level of doping. Fig. 2 shows the SEM images of Z1 and Z2 varistor samples with various SnO2 contents. Due to the higher sintering temperatures, the grain sizes of Z2 varistor samples were much larger than that of Z1 varistor samples with the same composition. As SnO2 was added to the base composition, the average grain sizes decreased obviously for both Z1 and Z2
Fig. 1. XRD patterns of Z1 and Z2 varistor samples with 0, 0.1, 0.2 and 0.3 mol% SnO2, Z1: sintered at 950 1C; Z2: sintered at 1100 1C.
Please cite this article as: S. Ma, et al., Influence of SnO2 on ZnO–Bi2O3–Co2O3 based varistor ceramics, Ceramics International (2015), http://dx.doi.org/ 10.1016/j.ceramint.2015.06.004
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varistor samples. This behavior shed a serious doubt with the explanation of Recnik [15], who reported that the addition of SnO2 to basic ZnO–Bi2O3 varistor composition resulted in an increase in ZnO grain size. The decreased grain sizes in the present work may be ascribed to the addition of Co2O3, which generally acts grain growth inhibitor in ZnO-based varistor [16]. For Z1 varistor samples without SnO2, the microstructure revealed mainly equiaxed polygon-shaped grains with a small amount of pores situated at triple boundaries, grain boundaries were flat and often 1201 apart from each other. However, for Z1 and Z2 varistor samples containing SnO2, it shows irregular bar-shaped grains. It was observed that continuous crystalline intergranular films were situated between ZnO grains in Z2 varistor samples without SnO2. For further study of these intergranular films, the elements present in the grain and intergranular films regions for Z2 samples without SnO2 were identified using EDS spot analysis. To ensure the accuracy of the results, the cross sections were used for the EDS spot analysis. As shown in Fig. 3, the bismuth cations were completely detected in intergranular films regions, while the cobalt element was detected both in grain regions and intergranular films regions. Combined with the XRD analysis, the intergranular phases were Bi-rich phases, mainly containing ZnO phase and small amounts of Bi2O3 phase. Therefore,
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it was concluded that small amounts of SnO2 inhibited the grain growth of ZnO and caused an alteration of the grain morphology. Fig. 4 shows the electric field–current density (E–J) characteristics of Z1 and Z2 samples and characteristic parameters which define the electrical properties of the varistors are shown in Table 2. It can be observed from Fig. 4 that the conduction characteristics are divided into two regions: pre-breakdown region at low electric field and breakdown region at high electric field. The shaper is the knee of curves between the two regions; the lager is the nonlinearity coefficient. It shows Z2 samples exhibited a slightly sharper knee in comparison with that of Z1 samples. However, all of Z1 samples exhibited better nonlinear electrical characteristics at breakdown region, because the current densities increased more abruptly with the increase of electric fields in this region.
Table 1 Characteristic parameters for Z1 and Z2 varistor samples. SnO2 content (mol%)
0 0.1 0.2 0.3
Z1
Z2
da (μm)
αb ILc (μA)
E1 mAd (V/ d α mm) (μm)
4.99 2.65 2.78 2.74
4.8 6.7 7.3 5.7
411 683 575 592
191.1 142.4 123.3 177.6
E1 mA (V/ IL (μA) mm)
13.96 3.7 405 97 7.98 8.1 98.9 244 8.42 10.7 50.3 286 8.48 8.5 90.5 255
a
Average grain size. Nonlinear coefficient. c Leakage current. d Breakdown field. b
Fig. 3. EDS spectra of spot analysis of Z2 varistor samples without SnO2 in grain regions and intergranular films regions: (A) intergranular films regions and (B) grain regions.
Fig. 2. Surface SEM micrographs of Z1 and Z2 varistor samples with different SnO2 contents: (a) Z1-0 mol%, (b) Z1-0.1 mol%, (c) Z1-0.2 mol%, (d) Z1-0.3 mol %, (e) Z2-0 mol%, (f) Z2-0.1 mol%, (g) Z2-0.2 mol%, and (h) Z2-0.3 mol%.
Please cite this article as: S. Ma, et al., Influence of SnO2 on ZnO–Bi2O3–Co2O3 based varistor ceramics, Ceramics International (2015), http://dx.doi.org/ 10.1016/j.ceramint.2015.06.004
S. Ma et al. / Ceramics International ] (]]]]) ]]]–]]]
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Fig. 4. E–J characteristics curves of Z1 and Z2 varistor samples with 0, 0.1, 0.2 and 0.3 mol% SnO2.
Table 2 Lattice constants of ZnO phase in Z1 and Z2 varistor samples. SnO2 content (mol%)
Z1
Z2
a (Å) 0 0.1 0.2 0.3
3.248 3.250 3.253 3.250
c (Å) (8) (1) (9) (1)
5.205 5.207 5.209 5.207
a (Å) (4) (1) (8) (1)
3.24 3.250 3.25 3.242
c (Å) (9) (1) (3) (7)
5.205 5.207 5.21 5.194
(2) (1) (3) (8)
Table 2 shows that between 0.1 and 0.3 mol% of SnO2 addition, the average grain sizes changed very slightly for both Z1 and Z2 varistor samples. The values of nonlinear coefficients were in the range of 4.8–7.3 and the leakage currents were in range of 123.3–191.1 μA for Z1 varistor samples. However, the breakdown filed increased noticeably from 411 V/mm to 683 V/mm for Z1 with 0.1 mol% SnO2, which was mainly attributed to the decrease of the average grain sizes [17]. For Z2 varistor samples doped with SnO2, the nonlinear coefficients were in the range of 8.1–10.7 with the leakage currents below 98.9 μA. For Z2 without SnO2, the leakage current was 405 μA and the nonlinear coefficient was only 3.7. However, unlike those of Z1 samples, the breakdown fields were not increasing in an inverse ratio with the average grain sizes in Z2 samples. This unusual behavior was probably due to the increase of potential barrier height at grain boundaries, which can be indicated from the increase of nonlinear coefficients. Combined with the XRD analysis, the presence crystalline phase Bi2(Sn2O7) and Bi2Sn2O7 in the samples with 0.3 mol% SnO2 had a negative effect on the nonlinear characteristics. We concluded that small amounts of SnO2 improved the nonlinear electrical properties of varistors. In addition, it should be noted that the influence of SnO2 on the samples sintered at 1100 1C was more obvious than those sintered at 950 1C. Fig. 5 shows the relative dielectric constants (εr) of Z1 and Z2 samples at various frequencies in the range 102–107 Hz. It shows that with the increase of sintering temperature, the values of εr increased obviously. In the low frequency range, the effective capacitance is originated from grain boundaries
[18]. The larger dielectric constants of the varistor ceramics indicated the formation of microstructure with more insulating grain boundary regions [19]. With the addition of SnO2, the values of εr decreased for both Z1 and Z2 samples. It should be noted that εr did not decrease monotonously with the increasing of SnO2 content, and the samples with 0.2 mol% had the lowest relative dielectric constants. Therefore, SnO2 had an obvious influence on the grain boundaries of ZnO varistor ceramics by reducing the permittivity values, resulting in a less insulating grain boundary region. 4. Conclusion In summary, small amounts of SnO2 had a great influence on the microstructure and electrical properties of ZnO–Bi2O3– Co2O3 based varistors. With the addition of very small amounts SnO2, the average grain sizes decreased obviously, and the crystal morphology revealed an alteration from equiaxed polygon-shaped grains to irregular bar-shaped grains. The reaction of SnO2 and Bi2O3 occurred when the doping level reaches up to 0.3 mol%. The Bi was combined with Sn as cubic crystalline phase Bi2(Sn2O7) at 950 1C and as pyrochlore phase Bi2Sn2O7 at 1100 1C. For samples sintered at 1100 1C, Bi2O3 phase was only observed in continuous crystalline intergranular films situated between ZnO grains in the samples without SnO2. It was found that small amounts of SnO2 increased the nonlinear coefficients and reduced the leakage currents. The dielectric dispersion studies showed that the addition of SnO2 reduced the dielectric constants, making the grain boundaries less insulating. 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, 51402144, 51302124), the Natural Science Foundation of Shandong Province of China (No. ZR2014JL030), The Project of Shandong Province Higher Educational Science and Technology Program (No. J14LA11,
Please cite this article as: S. Ma, et al., Influence of SnO2 on ZnO–Bi2O3–Co2O3 based varistor ceramics, Ceramics International (2015), http://dx.doi.org/ 10.1016/j.ceramint.2015.06.004
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Fig. 5. εr–f characteristics curves of Z1 and Z2 varistor samples with 0, 0.1, 0.2 and 0.3 mol% SnO2.
No.J14LA10), and the Research Foundation of Liaocheng University (Nos. 318011301 and 318011306).
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Please cite this article as: S. Ma, et al., Influence of SnO2 on ZnO–Bi2O3–Co2O3 based varistor ceramics, Ceramics International (2015), http://dx.doi.org/ 10.1016/j.ceramint.2015.06.004