Varistor microstructure

The microstructure of commercial varistors is extremely complex, and commercial preparations also contain other dopants, mainly oxides of cobalt, manganese, chromium, and antimony, that are used to fine tune the varistor characteristics. The transition-metal dopants are chemically similar to Zn2+ and mainly form substitutional defects within the ZnO grains, such as CoZn, that modify the n-type behavior of the grain interior. (See also Chapter 8 for further discussion of the electronic... 

Figure 3.37 Microstructure of a zinc oxide varistor (schematic).

Yoshimura HN, Molisani AL, Narita NE, Manholetti JLA, Cavenaghi JM. Mechanical properties and microstructure of zinc oxide varistor ceramics. Materials Science Forum 2006 530-531, 408-413. 

The microstructure of a ZnO varistor is the key to its operation. Grains of about 15-20 pm in diameter are separated by a Bi-rich intergranular film (IGF) that varies in thickness from 1 nm to 1 pm, as illustrated in Figure 14.38. Varistor action is a result of a depletion region formed on either side of the IGF. To explain varistor behavior we use an approach very similar to that used to describe Schottky barriers in metal-semiconductor junctions. 

In general, compared with solid state sintering, liquid phase sintering allows easy control of microstructure and reduction in processing cost, but degrades some important properties, for example, mechanical properties. In contrast, many specific products utilize properties of the grain boundary phase and, hence, need to be sintered in the presence of a liquid phase. Zinc oxide varistors and SrTiOs based boundary layer capacitors are two examples. In these cases, the composition and amount of liquid phase are of prime importance in controUing the sintered microstructure and properties. 

Both, spinel and pyrochlore accommodate excess dopants, the concentrations of which exceed their solubility limits in ZnO, and therefore they concentrate at grain boundaries [113]. The microstructure of a ZnO varistor will then comprise ZnO grains, a bismuth-rich phase, and spinel grains, which can be located either inter- or intragranularly (Figure 1.5). 

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

Zinc oxide (ZnO) powders are technologically attractive materials due to their inherent characteristics such as dielectric, piezoelectric, pyroelectric and semiconducting properties. To improve the varistor effect and achieve high breakdown fields from ZnO ceramics it is required to have a microstructure with uniformly distributed smaller grains. The grain size for conventional ZnO varistors is larger than 2.0 /rm (Chu, 2000). Note, however, that... 

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. 

The Vr value provides the varistor voltage application, and it is a function of a grain size of sintered material. If the composition is fixed, the microstructure becomes strongly dependent on the processing conditions [12,15],... 

There are many papers available in the literature [56-58,61-63,66-70], which studied the influence of doping agent into the tin oxide matrix ceramic. The possible microstructural, morphological, and varistor property changes that may occur with the addition of certain elements are searched. 

Aguilar-Martinez et al. [69] investigated the effect of calcium (sample named SCa), barium (sample named SBa), and strontium (sample named Sr) additions on the microstructure and electrical properties of Sn02-Co304-Sb205 ceramic varistors. 

Pianaro SA, Bueno PR, Longo E, Varela JA. Microstructure and electric properties of a SnOj based varistor. Ceram Int. 1999 25 1-6. DOI 10.1016/S0272-8842(97)00076-X... 

Wang CM, Wang JF, Zhao Y, Su WB. Effects of copper oxide on the microstructural morphology and electrical properties of tin oxide-based varistor ceramics. J Phys D Appl Phys. 2006 39 1684-1689. DOI 10.1088/0022-3727/39/8/031... 

Aguilar-Martinez JA, Pech-Canul MI, Hernandez MB, Glot AB, Rodrigez E, Gracia-Ortiz L. Effect of Cr203 on the microstructure and non-ohmic properties of (Co, Sb)-doped Sn02 varistor. Rev Mex Fis. 2013 59 6-9. 

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