Ceramic varistors

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... 

In denominated "smart ceramics," the ceramic varistor acts as variable resistors, with resistive behavior at low voltages and conductive behavior starting from a specific voltage value, known as the breakdown voltage (V ) or breakdown electric field ( p,) [9,10],... 

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. 

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... 

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. 

MiscelEneous. Small quantities of cobalt compounds are used in the production of electronic devices such as thermistors, varistors, piezoelectrics (qv), and solar collectors. Cobalt salts are useful indicators for humidity. The blue anhydrous form becomes pink (hydrated) on exposure to high humidity. Cobalt pyridine thiocyanate is a useful temperature indicating salt. A conductive paste for painting on ceramics and glass is composed of cobalt oxide (62). 

A book edited by Levinson (1981) treated grain-boundary phenomena in electroceramics in depth, including the band theory required to explain the effects. It includes a splendid overview of such phenomena in general by W.D. Kingery, whom we have already met in Chapter I, as well as an overview of varistor developments by the originator, Matsuoka. The book marks a major shift in concern by the community of ceramic researchers, away from topics like porcelain (which is discussed in Chapter 9) Kingery played a major role in bringing this about. 

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. 

It is worth noting that ZnO occupies already an enviable place in the industrial market. Tens of thousands tons of ZnO powder are industrially produced each year which are used in the rubber industry as vulcanisation activator ( 36 %), in the industry of ceramics as a flux ( 26 %), in the chemical industry (desulphuration of gases, fabrication of stearates, phosphates, etc) ( 20 %), as trace elements in the animal food ( 12 %), in the paints ( 3 % 50 % in 1961 ). The last 3 % are used for different applications, in electronics (ferrites, varistors), ends of matches, pharmaceutic industry (fungicidal properties of ZnO for skin-problems, trace elements. ..etc.). 

On the other hand, the uses of nonorganic compounds of bismuth in alloys, metallurgical additives and chemicals are still increasing, especially in pearlescent pigments for cosmetics and plastics, and in varistors and ceramic capacitors. 

Finally, it is worth noting that charged interfaces are created not only at free surfaces, but whenever two dissimilar phases come into contact. Electrified interfaces are at the heart of much of today s technology from drug manufacturing to integrated circuits. Life itself would be impossible without them. More specifically in ceramics, the electric double layer is responsible for such diverse phenomena as varistor behavior, chemical sensing, and catalysis, to name but a few. 

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. 

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. 

Magnetic ceramics represent an important fraction of the magnetic industry in the US, an estimated 40% of the total hard magnetic materials market value is dominated by ferrites, and in spite of the continuous development of new materials, ferrite consumption is still growing. In soft material applications, ferrites participate with an estimated 20% of the market value. In 1990, the estimated world production was 159 500 metric tons of soft ferrites, and 431 100 metric tons of hard ferrites (Ruthner, 1989). In addition to the versatility of ferrites, there are two essential factors which explain this success the low electrical conductivity, and the low production cost. The market value of ferrites ( 3/kg) is very low compared with other electroceramics 33/kg for varistors, 330/kg for thermistors and 1100/kg for ceramic capacitors (Cantagrel, 1986). 

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 particularly important ceramic semiconductor its conductivity decreases with increasing p02- A major application for ZnO that makes use of its electrical properties is the varistor (variable resistor). 

Example 3 GBs in ZnO are processed intentionally to include a glass film. This film allows the ceramic to be used as a varistor (voltage-dependent resistor), a device that protects circuits from high-voltage spikes (Chapter 30). Figure 14.38 illustrates an IGF of varying thickness in ZnO and how this film controls the resistance. 

Figure 30.16 Data from Barsoum, M.W. (1996) Fundamentals of Ceramics, Institute of Physics, Bristol, p. 208. Figure 30.17 Data from Gupta, T.K. (1991) Varistors , in Ceramics and Glasses, Engineered Materials Handbook Vol. 4, ASM International, p. 1151.

Of the 700 000 tons SiC produced per year, about 33% is used in metallurgy as a deoxidizing plus alloying agent, and about 50% in the abrasive industry [257]. The remainder is used in the refractory and structural ceramics industries and to a small extent also in electric and electronic industries as heating elements, thermistors, varistors, light-emitting diodes, and attenuator material for microwave devices. 

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