Effect of high-energy mechanical activation on the microstructure and electrical properties of ZnO-based varistors
Introduction
Varistors are electronic materials capable of protecting circuits from overvoltages. They exhibit a nonlinear current–voltage relationship that can be expressed by the empirical equationwhere V is the applied voltage; I the current and C a constant corresponding to the nonlinear resistance and α is a characteristic exponent. ZnO-based ceramic compositions are among the most widely investigated varistors, the applications of which include voltage stabilization, transient surge suppression in electronic circuits, and electronic power systems [1], [2], [3], [4]. The typical composition of a ZnO-based varistor consists of ZnO and a few mole percent of other oxide additives and dopants such as Sb2O3, Bi2O3, Co3O4, MnO and Cr2O3 [5]. Sintered ZnO varistors consist of ZnO grains surrounded and separated by a thin continuous intergranular phase [6]. In addition to forming the intergranular layer, the functions of various additives in ZnO-based varistors have been investigated. For example, the densification and grain growth kinetics of ZnO can be controlled by the occurrence of a Bi2O3-rich eutectic liquid at the grain boundaries and grain junctions [7], [8]. The addition of Co3O4 or MnO can prevent Bi2O3 evaporation at the sintering temperature, and the addition of Cr2O3 can control ZnO grain growth [9]. Co3O4 and MnO also improve the non-ohmic property dramatically, resulting in an α-value as high as 40 compared to below 10 without them. The presence of Sb2O3 impedes grain growth by the formation of crystalline phases, which act as pinning sites for ZnO grain growth [9], and further improves the non-ohmic property [10].
ZnO-based varistors are commonly fabricated via the conventional ceramic processing route, which involves mixing of constituent oxides, with [5], [11], [12] or without [2], [10], [13], [14], [15], [16], [17] phase-forming calcination, followed by pressing and sintering at high temperatures. Various chemistry-based processing routes have also been devised to process ZnO-based ceramic varistors with the aim of producing more homogeneous and finer grained microstructure. Haile and co-workers [18] employed an aqueous precipitation of spherical ZnO particles, while Fan and Sale [19] employed a citrate gel route which led to a high breakdown voltage of 4 kV/cm and an α-value of about 20. Other chemistry-based processing routes include colloidal suspension and centrifugal separation techniques, leading to a breakdown voltage of 30 kV/cm and an α-value of 50 upon sintering [20]. Microemulsion has also been attempted, resulting in varistors of low leakage current and α-values in the range of 76–120 [21], [22]. However, these chemistry-based processing techniques are expensive and of low production yield. In addition to the conventional sintering, microwave sintering was attempted to improve the densification of ZnO-based varistors [23]. Similarly, hot pressing was employed to enhance the sintered density and control the grain growth of ZnO-based varistors [24]. Unfortunately, the hot-pressing technique is constrained by the simple shapes of sintered ceramic body. Other researchers used grain growth inhibitors [25] and introduction of ‘seed’ grains to the varistor composition before sintering [26], in order to control the unwanted abnormal grain growth. In this work, the mixed oxide composition of 97 mol% ZnO, 1 mol% Sb2O3 and 0.5 mol% each of Bi2O3, Co3O4, Cr2O3 and MnO was subjected to a degree of high-energy mechanical activation. The objectives of this work are twofold: (i) to improve the mixing homogeneity of the mixed oxides and therefore improve the sintering behavior of the mixed oxide composition; and (ii) to investigate the effects of high-energy mechanical activation on the electrical properties of ZnO-based composition, such as the breakdown voltage, leakage current and nonlinear coefficient.
Section snippets
Experimental procedure
The composition of 97 mol% ZnO (> 99.0% purity, Fluka), 1 mol% Sb2O3 (> 99.0% purity, Fluka) and 0.5 mol% each of Bi2O3 (> 97% purity, Fluka), Co3O4 (> 97% purity, Fluka), Cr2O3 (> 99% purity, Fluka) and MnO (> 98% purity, Fluka), was chosen for investigation in this work. The commercially available ZnO powder was mixed by conventional ball milling together with the additive oxides in a polyethylene jar using zirconia balls of 5 mm diameter as the milling media in alcohol. The slurry was dried
Results and discussion
Fig. 1 shows the XRD traces of the powders that were subjected to 5 and 10 h of high-energy mechanical activation respectively, together with that of the powder that was not subjected to any high-energy mechanical activation. The spectrum for the powder without subjecting to any high-energy mechanical activation is dominated by the peaks of hexagonal ZnO at 36.25°, 31.75° and 34.41° corresponding to the (101), (100) and (002) planes, respectively. The additive oxides were not observed because
Conclusions
High-energy mechanical activation of the mixed oxide powders of ZnO and oxide dopants significantly refines the crystallite and particle sizes and therefore improves the mixing homogeneity. This leads to a refined microstructure and sintered density upon sintering at 1100°C. As such, the breakdown voltage is increased and the leakage current is lowered, due to the refined grain size and narrowed grain size distribution. The nonlinear coefficients of ZnO-based varistors are also significantly
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