ZnO-Based Varistors

The current-voltage curve of a varistor consists of three distinct regions [113]  

An inhomogeneous microstmcture results in the formation of preferred current paths in a varistor, its local overheating, and mechanical failure. Microstructural 

---

ZnO-based varistor have become popular because of the high non-linearity on voltage-current characteristics. 

It was recalled that the models which were used to describe ZnO-based varistors relied upon diffusion and defect data which had been obtained by using techniques that had been superseded by methods such as secondary ion mass spectrometry. Values were reported here for O diffusivities in undoped monocrystalline ZnO as a function of temperature and orientation. Evaporation was taken into consideration when analyzing the experimental results. It was found that, to within experimental error, the energetics of diffusion were isotropic, but were slightly faster in the c-direction. The present results could be described by ... 

Two types of ceramic varistors are manufactured. Zinc oxide based ceramic varistors were developed in 1970. They exhibit a high non-linearity on voltage-current characteristics. Their a value is in the range of 40-50, and the adjustable to values in the range from 50 to 250 V/mm. Strontium titanate based varistors were developed in 1980. The feature of these varistors is their larger electrostatic capacitance compared with ZnO varistors. The SrTi03 ceramics are... 

The Sn02-based varistors were introduced by Pianaro et al. [7] as an alternative to the ZnO varistors commercial, presenting nonlinear electrical characteristics similar to ZnO varistors. The Sn02-based system shows more advantages, for example, their simpler microstructure and no formation of secondary phases require a lower concentrahon of agents modifiers to promote the varistor characteristics and densification and higher chemical and thermal resistances. The use of IVF ion as dopant improved significantly the densificahon of the varistor, the addihon of ion promoted electrical conductivity, and the IVP ion influenced on nonlinearity coefficient. 

Varistor compositions based on ZnO contain a number of dopants at a level of approximately 1 mol.% a typical formulation is (in mol.%) 96.5ZnO-0.5Bi2O3-... 

Figure 2. Potential barrier model to ZnO varistors proposed by Gutpa et al. [22] and Leite et al. [23] and to SnOj vans tors base proposed by Bueno et al. [14] and Pianaro et al. [1].

---