Electrical resistivity of ceramics

It was also found that the electrical resistivity of ceramics based on silicon carbide, and, more recently, zinc oxide could be made sensitive to the applied field strength. This has allowed the development of components that absorb transient surges in power lines and suppress sparking between relay contacts. The non-linearity in resistivity is now known to arise because of potential barriers between the crystals in the ceramic. 

Thermistors, which detect temperature variability according to the changes of the electrical resistance of ceramics or polymers. 

Titanium siUcides are used in the preparation of abrasion- and heat-resistant refractories. Compositions based on mixtures of Ti Si, TiC, and diamond have been claimed to make wear-resistant cutting-tool tips (157). Titanium siUcide can be used as an electric—resistant material, in electrically conducting ceramics (158), and in pressure-sensitive elastic resistors, the electric resistance of which varies with pressure (159). 

The electrical characteristics of ceramic materials vary gteady, since the atomic processes ate different for the various conduction modes. The transport of current may be because of the motion of electrons, electron holes, or ions. Electrical ceramics ate commonly used in special situations where reftactoriness or chemical resistance ate needed, or where other environmental effects ate severe (see Refractories). Thus it is also important to understand the effects of temperature, chemical additives, gas-phase equilibration, and interfacial reactions. 

Materials can be classified as conductors, semiconductors or insulators. Conductors typically have resistivity in the range 10 2-103 xQ cm, semiconductors approximately 106-10n iQ cm, and insulators about 1013-1018 (xQ cm. Table 1.5 compares the electrical resistivity of the elements and compounds at room temperature. Although the carbides and nitrides have somewhat higher resistivity than do the pure metals, they still have resistivity in the regime of metallic conductors. In comparison the ceramic materials have much higher values, and are typically insulators. 

Figure 8. Temperature dependence of electrical resistivity of VO2 ceramics, (a) VOi.99, (b) VOi 93.

In contrast to the physical properties, transition metal carbides and nitrides possess electric and magnetic properties that are often similar to metals. For example, electrical resistivities of Ti or W are 39 and 5.39 pft cm at room temperamre, while their respective carbides have only slightiy higher resistivities of 68 and 22 p,fl cm. For comparison the electrical resistivity of the hard SiC ceramics is significantly higher (1000 pfi cm). 

The resistance of ceramically bonded and recrystallized SiC to thermal shock, oxidation and corrosion is utilized in its use as a refractory construction material, for example, in the linings and skid rails for furnaces and hot cyclones, and as a kiln furniture, especially in saggars [164,552-556]. The good electrical conductivity of the material at high temperatures, coupled with its outstanding oxidation resistance, led to its early use in the electric heating industry [557-559], which markets its products... 

Electrical Conductivity of Metals Electrical Resistivity of Metals Electrical Resistivity of Alloy Cast Irons Resistivity of Ceramics Volume Resistivity of Glass Volume Resistivity of Polymers... 

Since exhaust gas temperature changes rapidly during engine operation, ceramic honeycomb substrates must have thermal shock resistance. Thermal shock resistance of ceramic honeycombs is determined by electric furnace or gas burner testing. Thermal shock resistance of ceramic material is generally represented by the following equation [8]. As the coefficient of thermal expansion of extruded cordierite is extremely low, high thermal shock resistance is expected. 

Frandsen, M. V. and Williams, W. S., Thermal Conductivity and Electrical Resistivity of Cemented Transition-Metal Carbides at Low Temperatures, J. Am. Ceramic Soc., (74)6 1411-1416 (1991)... 

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