Breakdown Characteristics of Varistor Ceramics

Abstract

Breakdown characteristics are of great importance for varistor ceramics, which largely depend on Schottky barriers at grain boundaries. In order to enhance breakdown performance for meeting the requirement of device miniaturization, different doping methods are introduced to not only restrict grain size from additional phase but also manipulate defect structure of Schottky barrier at grain boundaries from substitution. Distribution of barriers is another key point affecting breakdown characteristics in varistor ceramics. Dimensional effect, which is detected in not only ZnO ceramics but also CaCu 3 Ti 4 O 12 ceramics, is practically and theoretically found to be closely correlated with uniformity of grains. As a result, breakdown characteristics of varistors are dominated by combination effect of single barrier performance and spatial barrier distribution. In this chapter, enhanced breakdown field in Ca x Sr 1Àx Cu 3 Ti 4 O 12 ceramics, in situ synthesized CaCu 3 Ti 4 O 12-CuAl 2 O 4 ceramics, and CaCu 3 Ti 4 O 12-Y 2/3 Cu 3 Ti 4 O 12 composite ceramics are investigated from the aspect of Schottky barriers at grain boundaries. In addition, dimensional effect is found in both ZnO and CaCu 3 Ti 4 O 12 ceramics, which are investigated from grain size distribution through theoretical and experimental analysis.

Effects of Dopants and Processing Parameters on the Properties of ZnO-V2O5-Based Varistors Prepared by Powder Metallurgy: A Review

“…The size of grains and the number of grain boundaries (GBs) in the microstructures of MOVs, along with the intergranular effects, play a crucial role in altering the varistor properties, according to several reviewed literature reports [ 63 , 80 , 199 ]. However, controlling the grain size and grain size distribution in multicomponent ZnO-based varistors is a challenging task due to the many interdependent MO dopants needed to achieve the desired varistor characteristics [ 59 ].…”

“…The ZnO grain size determines the electrical breakdown (E B ) of MOVs for a specific formulation and thickness of the MOV disc [ 80 ]. The number of GBs across the thickness of an MOV disc increases with the reduction in grain size during sintering, which improves the E B .…”

“…An efficient strategy for improving the non-ohmic behavior of multiternary ZnO-based systems consists of developing novel compositions with appropriate types and content of VFO and MO dopants, using adequate PM processing parameters. A better microstructure can be achieved by reducing the ZnO grain size, obtaining a narrow ZnO grain size distribution, and improving the uniformity and long-term stability of electrical potential barriers (DSBs) across the GBs [ 33 , 80 , 83 , 200 ]. The use of IB-forming dopants can tailor the microstructure and the electronic and optical characteristics of ZnO-based systems, benefiting from enhanced functional properties at reduced costs [ 59 ].…”

“…Most commercial ZnO-based varistors are composed of ZnO-Bi 2 O 3 systems with up to 8–12 MO dopants and typically yield ZnO grain sizes (d ZnO ) of 10–100 μm and a depletion length of the doped GBs of 50–100 nm, a breakdown field (E B ) of 2–3 kV/cm, and a nonlinear coefficient of about 40–50 [ 52 , 56 , 59 , 80 , 81 , 82 ]. MOVs with coarse-grained microstructures (mean d ZnO > 30 µm) and lower E B values are used for low-voltage (LV) applications, while MOVs with fine-grained microstructures (mean d ZnO < 10 µm) are required for high-voltage (HV) and medium-voltage (MV) applications [ 59 , 83 ].…”