Zinc varistors

Grain boundary defects are primarily responsible for the operation of zinc oxide (ZnO) varistors, a shortened form of variable resistor. The varistor behaves like an insulator or poor semiconductor at lower electrical field strengths, but at a critical breakdown voltage the resistance decreases enormously and the material behaves like an electrical conductor (Fig. 3.36). When a varistor is connected in parallel with electrical equipment, negligible power flows through it under normal low... 

Figure 3.36 Variation of current versus electrical field for a typical zinc oxide varistor.

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

Zinc oxide is a semiconducting ceramic used to make varistors (variable resistors). Write a balanced equation for the hydrolysis of zinc ethoxide in the sol-gel method for making ZnO powders. 

Fig. 4.15 (a) Silicon carbide varistors, and (b) zinc oxide varistors. (Courtesy of P.D. Devices Ltd. UK.) (c) PTC thermistors for motor protection and (d) NTC thermistors inset, catheter fitted with biomedical NTC thermistor for measuring blood temperature. (Courtesy of Thermometries.)... 

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. 

Electronic ceramics include barium titanate (BaTiOs), zinc oxide (ZnO), lead zirconate titanate [Pb(ZrJ ii ()03], aluminum nitride (AIN), and HTSCs. They are used in applications as diverse as capacitor dielectrics, varistors. 

ZnO is a semiconductor where the conductivity depends on an excess of zinc atoms its use in varistors... 

The largest consumers of ZnO are the rubber and adhesives industries. Zinc oxide is also found in some latex paints, tiles, glazes, and porcelain enamels, and is the most widely used material in the manufacture of varistors. 

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. 

Figure 9.10. The voltage-dependent conductance of a zinc oxide varistor. Published with permission from the Journal of Materials Education.

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... 

Zinc oxide is liquid-phase sintered with 0.5 mol% of Bi203 (and small additions of other oxides) for the production of varistors. The phase diagram for the Zn0(Bi203) system (Fig. 10.41) shows the formation of a Bi-rich liquid phase above the eutectic temperature of 740°C (63). Samples that have been quenched from the sintering temperature show a liquid film that completely penetrates the... 

A useful avalanche diode that works in either direction is the "varistor," sometimes called the "metal oxide varistor," or "MOV." It is not just a single crystal device as the previously described semiconductors are, but instead it is polycrystalline ceramic material, usually zinc oxide, with many small PN junctions at the grain boundaries. It is quite inexpensive and is very rugged, so it can be found as an important part inside most "surge protector power strips" for connecting multiple computer modems, printers, etc., to protect them against inductive kicks and even most "spikes" ("transients") from lightning. ... 

Zinc oxide, bismuth oxide, titanium oxide Varistors... 

Hingorani, S., Pillai, V., Kumar, R, Multani, M.S., and Shah, D.O. 1993. Microemulsion mediated synthesis of zinc-oxide nanoparticles for varistor studies. Mater. Res. Bull., 28,1303-1310. 

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... 

Similar research has been necessary to optimize formulations for applications on alternative substrate types such as BeO, low-temperature tape, and nonhybrid substrates such as glass substrates for displays, silicon (Si) for photovoltaic apphcations, and various ceramic bodies for component apphcations such as titanates for MLCs, zinc oxide varistors, and zir-conia sensors. In each apphcation, not only wiU the chemistry vary, but the particle size, surface area, and volume percentage of the respective glass compositions wiU also vary. 

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. 

Bwmasconi J, Klein HP, Knecht B, Strassler S. Zinc oxide based varistor a possible mechanism. Solid State Commun. 1976 20 1053-1056. DOI 10.1016/0038-1098(77)90351-9... 

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