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Microstructure and nonohmic properties of Sn[O.sub.2]-[Ta.sub.2][O.sub.5]-ZnO system doped with Zr[O.sub.2].

1. Introduction

Sn[O.sub.2] varistors are semiconducting ceramic devices, which possess nonlinear voltage-current characteristics due to their grain boundary effects formed commonly by sintering Sn[O.sub.2] powder with minor additives (impurity). Due to their excellent energy handling capabilities, they can be applied extensively to protect electronic circuits, various semiconductor devices, and electric power systems from dangerous abnormal transient overload [1, 2].

The first impurity-doped Sn[O.sub.2] varistor was reported by Glot and Zlobin [3], and Pianaro et al. also made great contributions to the knowledge of varistor behavior of impuritydoped Sn[O.sub.2] ceramics [4]. Through a series of studies on Sn[O.sub.2]-based varistors for decades, it is well known that an excellent Sn[O.sub.2] varistor system consists of three kinds of dopants: resistance reducers (varistor forming oxide, VFO), densifiers, and modifiers, respectively [5]. To date, the commonly applied VFOs are [Nb.sup.5+] [6-8] or [Ta.sup.5+] [9-11], which possesses high chemical valence and is soluble in Sn[O.sub.2] grains to decrease the grain resistivity; the densifier is insoluble ion of low chemical valence that will segregate at Sn[O.sub.2] grain boundary regions to promote the densification by producing oxygen vacancies, for example, [Co.sup.2+] [4, 6, 7, 9, 11], [Mn.sup.2+] [12, 13], and [Zn.sup.2+] [8, 14], and the modifiers can effectively improve the electrical properties of the varistors, such as [Cr.sup.3+], [Fe.sup.3+], [Cu.sup.2+], and rare earth elements [6-9, 15, 16].

Moreover, during modern ceramics processing, high energy attrition milling and Zr[O.sub.2] grinding media were often applied. As a result, [Zr.sup.4+] contamination in ceramic samples is a common phenomenon. However, up to now, no literature about the role of [Zr.sup.4+] ion (Zr[O.sub.2]) in Sn[O.sub.2]-based varistors has been reported.

Recently, we optimized a Sn[O.sub.2]-[Ta.sub.2][O.sub.5]-ZnO varistor system, which presents varistors of good nonlinear properties but very low varistor voltage [17]. Based on it, in the present study, Sn[O.sub.2]-[Ta.sub.2][O.sub.5]-ZnO-based varistor system was doped with Zr[O.sub.2] (0-2.0 mol%), and the effect of Zr[O.sub.2] doping on the microstructure and nonohmic properties of Sn[O.sub.2]-[Ta.sub.2][O.sub.5] based varistors was investigated. To our surprise, varistors with fully dense structure and high breakdown voltage could be obtained.

2. Experimental Procedure

2.1. Sample Preparation. The samples were prepared using a conventional ceramic processing method with a nominal composition of (99.45-%) mol% Sn[O.sub.2] + 0.05 mol% [Ta.sub.2][O.sub.5] + 0.5mol% ZnO + % mol% Zr[O.sub.2] (x = 0, 0.25, 0.5, 1.0, 2.0). All the oxides were raw powders of analytical grade. At beginning, the raw powders were mixed in deionized water and ball-milled in polyethylene bottle for 24 h with 0.5 wt% of PVA as binder and highly wear-resistant Zr[O.sub.2] balls as grinding media. Subsequently, the obtained slurries were dried at 110[degrees]C in an open oven. After drying, the powder chunks were crushed into fine powders, sieved, and pressed into pellets of 6 mm in diameter and 1.5 mm in thickness under a pressure of 40 MPa. Then, the pressed pellets were sintered at 1400[degrees]C for 2h in a Muffle oven by heating at a rate of 300[degrees]C/h and cooling naturally. To measure the electrical properties, silver electrodes were prepared on both surfaces of the sintered disks by heat treatment at 500[degrees]C for half an hour.

2.2. Materials Characterization. The density of the samples was measured by Archimedes method according to international standard (ISO18754). Their crystalline phases were identified by X-ray diffractometer (XRD, D/ max2550HB+/PC, Cu K[alpha], and [lambda] = 1.5418 [Angstrom]) through a continuous scan mode with speed of 8[degrees]/min. The microstructure was examined on the fresh fracture surfaces of the samples via a scanning electron microscope (SEM, Tescan XM5136). And the average size of Sn[O.sub.2] grains in the samples was determined using linear intercept method from the SEM images.

A high-voltage source measurement unit (Model: CJ1001) was used to record the characteristics of the applied electrical field versus current density (E-J) of the samples. The varistor voltage ([V.sub.B]) was determined at 1 mA/[cm.sup.2] and the leakage current ([I.sub.L]) was the current density at 0.75 [V.sub.B]. Then, the nonlinear coefficient ([alpha]) was obtained by the following equation:

[alpha] = log ([J.sub.2]/[J.sub.1])/log([E.sub.2]/[E.sub.1]) = 1/log([E.sub.2]/[E.sub.1]), (1)

where [E.sub.1] and [E.sub.2] are the electric fields corresponding to [J.sub.1] = 1 mA/[cm.sup.2] and [J.sub.2] = 10 mA/[cm.sup.2], respectively.

3. Results and Discussion

3.1. Composition and Microstructure. Figure 1 illustrates the XRD patterns of the as-prepared Sn[O.sub.2]-[Ta.sub.2][O.sub.5]-ZnO-based varistor ceramics doped with different amounts of Zr[O.sub.2]. All the sharp diffraction peaks were assigned, corresponding to the (110), (101), (200), (111), (211), (220), (002), (310), (112), (301), (202), and (321) reflections of Sn[O.sub.2] cassiterite phase (JCPDS card no. 77-0451). No extra phases were identified, possibly because the doping levels of the additives were too low to be detected in XRD limits. And, because of the same ionic valence and almost no radius difference between [Sn.sup.4+] (0.071 nm) and [Zr.sup.4+] (0.072 nm) ions, the doped Zr[O.sub.2] is fully soluble in Sn[O.sub.2] lattice, which can be seen from almost the same positions of XRD diffraction peaks of the prepared samples as shown in Figure 1(b) in a close view to the patterns in 2[theta] from 50 to 55[degrees]. As for the splitting of the XRD peaks in the figure, it might be due to the peak doublet of K-alpha 1 and K-alpha 2.

SEM images of the as-prepared Sn[O.sub.2]-[Ta.sub.2][O.sub.5]-ZnO based varistor ceramics also confirmed the solubility of Zr[O.sub.2] into Sn[O.sub.2] lattice (please see Figure 2). The images reveal that, although doped with different amounts of Zr[O.sub.2], the typical microstructure of the samples almost has no change: almost fully dense structure of Sn[O.sub.2] grains without any obvious second phases. The detailed microstructural parameters are also summarized in Table 1. With increasing doping amount of Zr[O.sub.2], the density of samples decreases in a very narrow range from 6.93 to 6.80 g/[cm.sup.3] partly because the density of Zr[O.sub.2] (5.89 g/[cm.sup.3]) is lower than that of the matrix Sn[O.sub.2] (6.95 g/[cm.sup.3]), but the relative density of the samples also decreases although also in a very narrow range from 99.8% to 98.2%, which indicates a decreased sample densification and could be attributed to the lower diffusion ability of solid Zr[O.sub.2] particles in Sn[O.sub.2] matrix at the designed sintering temperature because the melting point of Zr[O.sub.2] (2680[degrees]C) is much higher than that of Sn[O.sub.2] (1630[degrees]C). Moreover, from these SEM images, it can be clearly seen that, with increasing Zr[O.sub.2] contents in the ceramics, the average size of Sn[O.sub.2] grains decreases, which might be owing to the inhibited transportation of Sn[O.sub.2] during sintering by the doped Zr[O.sub.2] with lower diffusion ability.

3.2. Electrical Properties. The E-J characteristics of the as-prepared Sn[O.sub.2]-[Ta.sub.2][O.sub.5]-ZnO-based ceramic varistors doped with different contents of Zr[O.sub.2] are illustrated in Figure 3, and their corresponding detailed electrical parameters calculated from the E-J curves are listed in Table 1.

The results indicate that, with increasing doping content of Zr[O.sub.2] up to 1.0 mol%, the nonlinear coefficient of the samples increased up to 15.9, possibly owing to the increased carrier concentration in the varistors, decreased electrical resistivity of Sn[O.sub.2] grains and thus enhanced barrier height by doping, and higher number of voltage barriers due to the decrease in grain size, but it would drop down with more Zr[O.sub.2] doped, due to the corresponding less dense sample structure, degraded effective grain boundary, destroyed depletion layer structure, and thus decreased barrier height. The leakage current of the samples presented an opposite trend to that of nonlinear coefficient with Zr[O.sub.2] doping, and the varistors with 1 mol% Zr[O.sub.2] presented the lowest leakage current, 110 [micro]A/[cm.sup.2], which is completely consistent with classic theory on their relationship [18]. Thus, it can be concluded that the optimum doping amount of Zr[O.sub.2] in the proposed Sn[O.sub.2]-[Ta.sub.2][O.sub.5]-ZnO-based ceramic varistor system was 1 mol%. The varistor voltage of the samples increased monotonously with the doping amount of Zr[O.sub.2], which could be mainly attributed to the decreased Sn[O.sub.2] grain size, thus increasing the number of grain boundary in unit thickness after doping.

4. Conclusions

Sn[O.sub.2]-[Ta.sub.2][O.sub.5]-ZnO varistors doped with different amounts of Zr[O.sub.2] (0-2.0 mol%) were prepared by sintering at 1400[degrees]C for 2h with conventional ceramic processing method. The doping of Zr[O.sub.2] would degrade the densification of the varistor ceramics, but inhibit the growth of Sn[O.sub.2] grains. In the designed range, varistors with 1.0 mol% Zr[O.sub.2] presented the maximum nonlinear exponent of 15.9 and lowest leakage current of 110 [micro]A/[cm.sup.2]; but the varistor voltage increased monotonously with the doping amount of Zr[O.sub.2].

http://dx.doi.org/10.1155/2014/754890

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

The authors would like to thank the financial support for this work from the National Natural Science Foundation of China (grant nos. 61274015, 11274052 and 51172030), and the Transfer and Industrialization Project of Sci-Tech Achievement (Cooperation Project between University and Factory) from Beijing Municipal Commission of Education.

References

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Xiuli Fu, (1) Feng Jiang, (2) Ruichao Gao, (1,2) and Zhijian Peng (2)

(1) School of Science, Beijing University of Posts and Telecommunications, Beijing 100876, China

(2) School of Engineering and Technology, China University of Geosciences, Beijing 100083, China

Correspondence should be addressed to Xiuli Fu; xiulifu@bupt.edu.cn and Zhijian Peng; pengzhijian@cugb.edu.cn

Received 7 August 2013; Accepted 26 November 2013; Published 20 January 2014

Academic Editors: M. J. Hua, R. Parra, and K. Prabhakaran

TABLE 1: Basic physical parameters of Sn[O.sub.2]-[Ta.sub.2]
[O.sub.5]-ZnO varistor ceramics doped with different contents
of Zr[O.sub.2].

Doping amount of    Apparent density    Relative     Sn[O.sub.2]
Zr[O.sub.2] (mol)    (g/[cm.sup.3])    density (%)   grain size
                                                     ([micro]m)

0                         6.93            99.8          7.87
0.25                      6.89            99.2          6.67
0.5                       6.88            99.1          5.45
1.0                       6.84            98.6          4.55
2.0                       6.80            98.2          3.03

Doping amount of    [alpha]   [V.sub.B]    [I.sub.L]
Zr[O.sub.2] (mol)              (V/mm)     ([micro]A/
                                          [cm.sup.2])

0                     4.6        81           660
0.25                  6.0        103          560
0.5                   8.2        250          220
1.0                  15.9        500          110
2.0                  11.6        720          170
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Title Annotation:Research Article
Author:Fu, Xiuli; Jiang, Feng; Gao, Ruichao; Peng, Zhijian
Publication:The Scientific World Journal
Article Type:Report
Date:Jan 1, 2014
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