Peculiar effects of microwave sintering on ZnO based varistors properties

https://doi.org/10.1016/j.jallcom.2011.03.048Get rights and content

Abstract

Non-ohmic properties of doped zinc oxide are widely used in varistors applications. It is well established that final properties of the component are strongly correlated with reactivity of the added phases during sintering process and with final microstructure. In this paper, the specific effects of the hybrid single-mode microwave sintering process on the microstructure and electrical properties of a ZnO-based composition are investigated. Nano-sized ZnO-based powder with a proper amount of Bi2O3, Sb2O3, CoO and MnO is synthesized by a liquid route and is sintered within a short time (less than 10 min) in a conventional (CV) or by an hybrid single-mode microwave (MW) furnaces. Distinct differences can be seen in the density, reaction kinetics and dopant diffusivity: higher kinetics of MW leads to denser pellet, faster reaction among dopants and faster diffusion of cobalt and manganese into ZnO grains although grain sizes are almost identical between CV and MW. These differences in terms of chemistry and microstructure lead to sharp contrasts in electrical properties.

Highlights

► Standard composition for ZnO-based varistors discovered 40 years ago was chosen. ► Liquid route synthesis was implemented to get nanoparticles. ► Samples were sintered during similar short times by microwaves and conventionally. ► Microwaves sintering increases sintered samples’ densities and improve varsitors properties. ► Peculiar effects of microwaves: enhancement of the reactions kinetics and dopants diffusion.

Introduction

Zinc oxide is a well known functional material widely spread in various application fields, like for instance in electronic, optic and spintronic. Among them, the main application is ZnO-based varistors which are used as surge protectors in electrical circuits in either low or high voltage. ZnO is a direct band gap semiconductor [1] which exhibits a non linear current–voltage response, depending on both the processing conditions and the added dopants [2]. This typical electrical response is known to be provided by the addition of dopants like Bi2O3, Sb2O3, CoO or MnO [3]. Some of these dopants, like Bi2O3 and Sb2O3, react with each other resulting in inter-granular secondary phases whereas others, like Co and Mn, coming from the added oxides, go into solution in ZnO grains [4]. Consequently, a p-type semiconducting area is formed in the vicinity of the grain boundaries while the core of the ZnO grain is n-type. Thus a double Schottky barrier is formed between two ZnO grains, which is responsible for the varistor non-ohmic response [5], [6], [7]. It is well understood that microstructure, including doping element distribution and grains size, has a strong influence on the ZnO electrical properties. After the finding of the non-linear properties in this system by Matsuoka in 1969, many investigations have been carried out to unveil the relationships among the starting composition and/or the processing conditions and the electrical properties. One can mention the influence of vibratory milling [8], [9], sintering and annealing conditions [10], [11]. Different synthesis methods have also been investigated such as precipitation [12], [13], solution-coating [14], sol–gel [15], combustion [16] and self-propagating high-temperature [17] methods. Contrariwise, only few works report the effect of a fast sintering process, like microwave sintering, on the microstructure and properties of ZnO based varistors. Leach et al. have studied the local microstructure and the functional property of ZnO sintered by a microwave-assisted process [18], using a conventional heating furnace combined with microwaves radiation. With similar heating program, the microwave assisted heating process leads to similar grain size but slightly higher density over conventional process. Although electrical responses measured through the bulk are similar, local electrical properties showed that spatial variations in non-linear coefficient and leakage current density were significantly reduced in the microwave assisted sintered samples. It is thus suggested that microwaves promote diffusion leading to more homogeneous Bi distribution than in conventionally sintered specimens. Subasri et al. have also investigated the indirect microwave sintering on suitably doped ZnO nano-powder [19] using multimode microwave cavity equipped with SiC rods as susceptors. In their study, they showed that microwave sintered samples exhibit finer grain size and higher density than those obtained on conventionally sintered samples, with similar temperature profiles. Another study [20] reported that microwave sintering in single-mode cavity enhances densification of ZnO based varistors as well as grain growth, leading to degraded properties for long time sintered samples. The enhancement of densification is reported by many works however some contradictory results on grain growth are observed. In fact, the comparison of the results of the literature leads to a misunderstanding due to the different microwave furnaces used (choice of susceptor and microwave mode). The goal of this work is to clarify this issue: the effects of microwave irradiation on the microstructure (grain growth), dopants diffusion and resultant electrical properties of nano-sized based ZnO varistors. To get a temperature distribution as homogenous as possible, hybrid microwave sintering in single mode cavity was designed using a rectangular 2.45 GHz cavity and a cylindrical ZnO susceptor. As comparison, the conventional sintering was also performed with similar short sintering time (<10 min). A conventional sintering experiment with usual temperature–time profiles was also carried out as a reference. Macrostructure observations and chemical analysis were performed and discussed according to the electrical response and Shottky barriers properties in order to understand the peculiar effects of microwave processing.

Section snippets

Synthesis, powder characterization and shaping

The liquid route synthesis developed for the preparation of the varistors is described by Fig. 1. Zinc acetate (Chempur, 99.5%) was first dissolved in absolute ethanol and the dopants were added to the solution from metal oxides powders in proportion of 98 mol% ZnO, 0.5 mol% Bi2O3, 0.5 mol% Sb2O3, 0.5 mol% CoO, 0.5 mol% MnO. The resulting solution was stirred during 1 h at 70 °C to achieve a good dissolution of the precursors. An oxalic acid solution was then poured leading to the precipitation of the

Zinc oxide powder synthesis

Zinc oxide based powder was successfully synthesized as shown by the XRD pattern (Fig. 4). It is also shown that the Bi2O3 phase is detected, but among all the added phases (Bi2O3, Sb2O3, CoO, MnO), it is the only one that can be identified. Using TEM (Fig. 5a), the ZnO particles appear to be nano-crystalline with a nearly spherical shape and a very narrow particle size distribution. The BET specific surface area of the powder is 53 m2/g. An estimated value of the grain diameter (d) of 19 nm is

Conclusion

Starting from nano-sized ZnO powders, doped with usual additives for varistors, hybrid microwave sintering was performed and compared with fast conventional sintering. Sintered samples were thoroughly characterized in terms of structure, at different scales, and electrical properties, including the barrier characterization. Owing to a volumic heating process, it was clearly shown that microwave heating enhances the overall density. The grain growth follows the same trend with conventional way.

Acknowledgements

Etienne Savary thanks the French Ministry of Research for the financial support. Yoshiaki Kinemuchi thanks JSPS for his financial support. The authors would like to thank Mrs François-Xavier Lefèvre, Jérôme Lecourt, Jacques Quillard, Stéphane Lejuez, Etienne Compagnon, Ms Alexandra Kennard and Isabelle Velluet for the technical support.

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