Varistor properties, ceramic materials

L. M. Levinson and H. R. Philipp. Application and Characterization of ZnO Varistors. In Ceramic Materials for Electronics—Processing, Properties, and Applications. (R. C. Buchanan, Ed.) Marcel Dekker, New York, 1986, pp.375-402. 

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. 

Thin film ceramic materials with important magnetic, optical, electronic, and mechanical properties are often highly anisotropic. Thus, the ability to control orientation is critically important in thin film applications. For many of the oxide materials, as well as Ae ionic materials, aqueous solution or sol-gel routes are the most convenient or the only method of preparation. Examples of these include barium titanate (BaTiOs) used in multilayer capacitors, lead-zirconate-titanate (Pb(Zr,Ti)03, "PZT") used as a piezoelectric material, and zinc oxide (ZnO) used in varistors. Thus, the use of substrates to control orientation can eliminate major problems in deposition of thin films. In some cases, e.g., the many magnetic and non-magnetic phases of iron oxide, the ability to control the phase formed is critical to production of the desired properties. While this can be controlled by solution conditions, the proper surface can add an additional and very effective mechanism of control. 

Historically ceramics were exploited for their electric insulation properties, which together with their chemical and thermal stability rendered them ideal insulating materials in applications ranging from power lines to cores bearing wire-wound resistors. Today their use is much more ubiquitous — in addition to their traditional role as insulators, they are used as electrodes, catalysts, fuel cells, photoelectrodes, varistors, sensors, and substrates, among many other applications. 

The mechanical properties of polycrystalline ZnO ceramics are of special importance for their applications as varistors (see Section 1.4.4.1). When a varistor experiences a high-current pulse, the electrical energy is quickly converted to heat. The inertia of the material, which resists its thermal expansion, and the resonances of the resultant elastic waves in the block, may lead to microcracks and finally to mechanical failure [107]. Characteristic values of mechanical properties of ZnO ceramics are 1.2-1.4 MPa m for fracture toughness, and 100-125 MPa for flexural strength. 

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... 

Interfaces have a substantial impact on the properties (in particular, functional properties) of ceramics, such as varistors, dielectrics, sensor-type materials, catalysts, and superconductors. In many cases the properties of these materials are determined by the chemical composition and stracture of the interfaces. 

Zinc oxide (ZnO) is a cheap but important material with many unique properties and potential applications [123-125]. Firstly, it is a gas/pressure-sensing ceramic and can be used as non-linear varistors and sensors [126]. Secondly, piezoelectric ZnO can find applications in surface and bulk acoustic wave devices [127-129]. Thirdly, it is a semiconducting oxide with a wide band gap of 3.37 eV at room temperature and a large exciton energy of 60 meV,... 

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