Microstructure control to reduce leakage current of medium and high voltage ceramic varistors based on doped ZnO

Abstract

International audienceThe leakage current and the non-linearity coefficient (α) are crucial parameters in varistors. This work, deals with the optimization of the electrical characteristics of medium and high voltage varistors based on the ZnO–Bi2O3–Sb2O3 system. First, the aim was to allow the formation and the stabilization of the spinel phase by reducing the heating rate. To do so, the right sintering temperature and dwell time have to be chosen to obtain a breakdown field suitable for industrial requirements. During cooling, the spinel phase can return to a pyrochlore phase, which contributes to the increase of leakage current. In a second part, this reaction was prevented by faster cooling in an appropriate temperature range. The fine-tuning of both the heating and cooling phases leads to a significant decrease of the leakage current. Moreover, the value of the non-linearity coefficient was increased by 80%, due to better and more homogeneous wetting of the ZnO grains by the Bi-rich phase

Evaluation of Chip Formation Mechanisms in the Turning of Sintered ZnO Electro-Ceramics

“…In order to enhance the threshold voltage and the surge current withstanding capability, the number of intergranular layers in series and parallel, respectively, have to be enhanced. Therefore, for high surge protection applications, a finer ZnO grain size and a high grain boundary surface area are prerequisites [24][25][26][27][28][29] This difference in the acceleration and the frequency FFT signals for the top and bottom parts further required microscopical characterization to determine the plausible changes in the machined surfaces as a result of the variation in feed rates, which is discussed in the ensuing section.…”

“…The non-linear characteristics are strongly dependent on the microstructure, which in-turn is related to the chemical composition and the applied processing methods [23]. The microstructural features in a typical ZnO varistor include approximately 90% of a dark grey matrix composed of ZnO grains in the narrow range of few tens of microns [24]. Usually, for this nonlinear, non-ohmic performance, the compositions are doped with a varistor forming oxide (VFO) and primarily involve a Bi 2 O 3 phase.…”

“…In order to enhance the threshold voltage and the surge current withstanding capability, the number of intergranular layers in series and parallel, respectively, have to be enhanced. Therefore, for high surge protection applications, a finer ZnO grain size and a high grain boundary surface area are prerequisites [24][25][26][27][28][29] [22,[26][27][28][29], which in a pragmatic sense defines the high voltage transient protection of MOVs [25].…”

“…Typically, without Sb 2 O 3 , the Schottky barrier height drops which increases the donor density of the charges, leading to an increase in the leakage current and a reduction in the non-linearity rated protection. These dopants instinctively reduce the mobility of ZnO-Bi 2 O 3 GBs due to the pinning effect of the spinel grains and hence enhance the surge protection capabilities [22,24,26,28,29]. With careful EDXS analysis, the microstructure in Figure 9a for the top side and Figure 9c of the cross-section indicated the loss of Bi-rich oxides and a higher Sb/Bi > 1 ratio.…”

Metal salts as dopants for ZnO ceramics-thermogravimetry coupled with mass spectrometry studies

“…The properties of materials can be also modified during forming process. The example of such approach can be the production process of varistors based on zinc oxide [6,14], where dopants in the form of oxides (Bi 2 O 3 , Cr 2 O 3 , Sb 2 O 3 and others) are mixed with ZnO and homogenized during milling followed by granulation and pressing. The advantages of such approach are simplicity of the process and possibility to use powders which are available on the market.…”