Metal Oxide Varistors

No short-term or long-term damage should ever occur. Note that these fine surges are of frequent occurrence in these areas (rather than a rare one in a blue-moon type of thing), so we just cannot rely on MOV s (metal oxide varistors) which have inherent lifetime/wearout issues. 

Metal oxide varistors are ceramic semiconductive devices having highly nonlinear current-voltage characteristics, as shown in Figure 2.1.9, expressed as... 

Figure 10.19 Current-voltage characteristics of a metal oxide varistor at 77 K and for a small range of temperatures near 300...

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. ... 

Solid-state reduced-voltage starters must also be protected against voltage transients. This is usually accomplished by installing metal oxide varistors (MOVs) hne-to-line on line and load, or by connecting them from line-to-load across the SCRs. If MOVs are not provided, the SCRs will likely short-circuit due to voltage transients. 

Several kinds of conduction mechanisms are operative in ceramic thermistors, resistors, varistors, and chemical sensors. Negative temperature coefficient (NTC) thermistors make use of the semiconducting properties of heavily doped transition metal oxides such as n-ty e Ti O andp-ty e... 

Vapor pressure - The pressure of a gas in equilibrium with a liquid (or, in some usage, a solid) at a specified temperature. Varistor - A device that utilizes the properties of certain metal oxides with small amounts of impurities, which show abrupt nonlinearities at specific voltages where the material changes from a semiconductor to an insulator. 

Horio N, Hiramatsu M, Nawata M, hnaeda K, Torri T. Preparation of zinc oxide/ metal oxide multilayered thin films for low-voltage varistors. Vacuum. 1998 51 719-722. DOI 10.1016/S0042-207X(98)00293-0. 

For a large number of applications involving ceramic materials, electrical conduction behavior is dorninant. In certain oxides, borides (see Boron compounds), nitrides (qv), and carbides (qv), metallic or fast ionic conduction may occur, making these materials useful in thick-film pastes, in fuel cell apphcations (see Fuel cells), or as electrodes for use over a wide temperature range. Superconductivity is also found in special ceramic oxides, and these materials are undergoing intensive research. Other classes of ceramic materials may behave as semiconductors (qv). These materials are used in many specialized apphcations including resistance heating elements and in devices such as rectifiers, photocells, varistors, and thermistors. 

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... 

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