Superior performing nano‐enabled metal oxide varistors

Abstract

Commercial metal oxide varistor devices are subject to nonuniformity, defects and coarse grain size, low working voltage, high leakage current, and mechanical cracking issues. This work tempted to make use of nanopowders and alternative sintering processes to achieve uniform microstructure and superior varistor performance. These nano‐enabled compositions exhibit lower sintering temperatures (650‐1050°C), smaller grain sizes (0.5‐2 μm), enhanced nonlinear coefficient (30‐130) in DC I‐V characteristics, higher varistor voltages (240‐2500 V/mm), and low leakage current (<1 μA/cm2). Conventional sintering offers highest nonlinearity. Spark plasma sintering and microwave sintering techniques show advantages in reducing grain sizes and sintering temperatures. Spark plasma sintering is particularly effective in achieving better varistor performance for the nano‐enabled MOV compositions after a post‐SPS sintering for sufficient oxygen diffusion. Microwave sintering needs higher sintering temperatures and additional research for the nano‐enabled MOV compositions to offer equivalent electrical properties.

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

“…Metal oxide varistors (MOV) are voltage-dependent resistors (VDR) which have nonlinear, non-ohmic current-voltage characteristics to provide protection against the voltage transients and high current surges [22]. 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].…”

“…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.…”

Comparative Characterization of Different Cutting Strategies for the Sintered ZnO Electroceramics

“…high surge current catastrophically flowing through the edge of the varistors as the easier (less resistance) path rather than through the ZnO grains [22,23].…”

“…Consequently, this study investigates the abrasive waterjet machining performance of sintered ZnO varistors with respect to conventional milling to compare and obtain the parametric range (cutting mechanism), causing edge chipping or transgranular/sudden failure in hard varistor ceramics. Predominantly the edge chipping occurs due to Bi-rich spacer phase coerced out, which in turn causes high surge current catastrophically flowing through the edge of the varistors as the easier (less resistance) path rather than through the ZnO grains [22,23].…”