Preparation Of Varistors Research Articles

High performance ZnO varistors prepared from nanocrystalline precursors for miniaturised electronic devices

An industrially viable solution-based processing route using minimal amounts of solvent has been used to prepare bulk quantity nanopowders (average particle size 15 ± 3 nm) for the fabrication of ZnO varistors. The xerogels, calcined powders and sintered materials were fully characterised. The preparation of varistors from nanopowders has been optimised by studying the effect of temperature on grain growth, densification and breakdown voltage. The varistors are prepared by sintering at 1050 °C for 2 hours, a temperature that is significantly lower than that used in the current industrial process. Highly dense varistor discs prepared from the sintered material produce devices, with a breakdown voltage 85% higher than that of commercial varistors, making them suitable for use in miniaturised electronic circuitry. Improved performance of these materials has been attributed to the small grain size and better dispersion of additives on ZnO grains.

Influence of Spinel Phase Composition on ZnO Varistors Properties

The method of directed synthesis of the constituent phases (DSCP) was applied to investigate the influence of spinel phase composition on characteristics of ZnO varistors. Six varistor mixtures differing only in chemical composition of the spinel phase were prepared. The phase ratio as well as the compositions of ZnO and intergranular γ-Bi 2 O 3 phases were the same in all samples. Characterization of prepared varistors was performed by X-ray diffraction, SEM and electrical measurements. It was shown that it is possible, in certain cases, to estimate additive redistribution taking into account changes in lattice constants of ZnO, spinel phases and varistor composition. Comparison of six different raristor mixtures shows that it is possible to reduce the number of components of the system if all main constituents of the varistor microstructure are still present. Results confirmed advantages of the DSCP method in preparation of varistors of required phase composition and properties.

TiO2 based varistors derived from powders prepared by a sol-gel process

Abstract Nb and Ba doped TiO 2 oxide powders were prepared by a sol-gel process. XRD was used to identify the phase composition of the gels calcined at different temperatures. Green disks of Ti-Nb-Ba were made from the starting powders and sintered at 1250–1300 °C for 2 h. The microstructures of the sintered compacts were examined by SEM. The dielectric and nonlinear electric properties of the samples were investigated and the results show the Ti-Nb-Ba system is suitable for preparation of varistors.

Aqueous Precipitation of Spherical Zinc Oxide Powders for Varistor Applications

Aqueous precipitation has been used to prepare spherical ZnO particles. These powders have been shown to be suitable for use in the preparation of varistors. The uniform size distribution and uniform coating of these powders with the appropriate dopants allowed fabrication of varistors with improved properties over those fabricated from conventionally ball‐milled and calcined powders. The improved properties were a high coefficient of nonlinearity (∼44) in the nonohmic region, a sharp change in electrical behavior from ohmic to nonohmic, and a high resistivity (5×1012 SOM°m) at low voltages.