Conductors high temperature

Optical characteristics include transmitting portions of the infrared. Boron is a poor conductor of electricity at room temperature but a good conductor at high temperature. 

Carbon, Carbides, and Nitrides. Carbon (graphite) is a good thermal and electrical conductor. It is not easily wetted by chemical action, which is an important consideration for corrosion resistance. As an important stmctural material at high temperature, pyrolytic graphite has shown a strength of 280 MPa (40,600 psi). It tends to oxidize at high temperatures, but can be used up to 2760°C for short periods in neutral or reducing conditions. The use of new composite materials made of carbon fibers is expected, especially in the field of aerospace stmcture. When heated under... 

Sihcon nitride has good strength retention at high temperature and is the most oxidation resistant nitride. Boron nitride [10043-11 -5] has excellent thermal shock resistance and is in many ways similar to graphite, except that it is not an electrical conductor. 

Soldering materials are alloys that are composed primarily of tin and lead (qv), and have low melting temperatures relative to the conductor metals which are being soldered (see Lead alloys Tin and tin alloys). Welding requires sufficientiy high temperatures for the fusion of metals. 

The contact ends of printed circuit boards are copper. Alloys of nickel and iron are used as substrates in hermetic connectors in which glass (qv) is the dielectric material. Terminals are fabricated from brass or copper from nickel, for high temperature appHcations from aluminum, when aluminum conductors are used and from steel when high strength is required. Because steel has poor corrosion resistance, it is always plated using a protective metal, such as tin (see Tin and tin alloys). Other substrates can be unplated when high contact normal forces, usually more than 5 N, are available to mechanically dismpt insulating oxide films on the surfaces and thereby assure metaUic contact (see Corrosion and corrosion control). 

The elements are all lustrous, silvery metals with high mps and they have typically metallic hep structures which transform to bcc at high temperatures (882°, 870° and 1760°C for Ti, Zr and Hf). They are better conductors of... 

Metals and semiconductors are electronic conductors in which an electric current is carried by delocalized electrons. A metallic conductor is an electronic conductor in which the electrical conductivity decreases as the temperature is raised. A semiconductor is an electronic conductor in which the electrical conductivity increases as the temperature is raised. In most cases, a metallic conductor has a much higher electrical conductivity than a semiconductor, but it is the temperature dependence of the conductivity that distinguishes the two types of conductors. An insulator does not conduct electricity. A superconductor is a solid that has zero resistance to an electric current. Some metals become superconductors at very low temperatures, at about 20 K or less, and some compounds also show superconductivity (see Box 5.2). High-temperature superconductors have enormous technological potential because they offer the prospect of more efficient power transmission and the generation of high magnetic fields for use in transport systems (Fig. 3.42). 

Fe(CN)6]3-(aq) + 6 H20(1). substrate The chemical species on which an enzyme acts, superconductor An electronic conductor that conducts electricity with zero resistance. See also high-temperature superconductor. supercooled Refers to a liquid cooled to below its freezing point but not yet frozen, supercritical fluid A fluid phase of a substance above its critical temperature and critical pressure. supercritical Having a mass greater than the critical mass. 

Electrical conductivity The electrical conductivity of refractories is important when they are used in electric furnaces. Except for graphite and metals, all other refractories are poor conductors of electricity. Graphite is a highly refractory material, and is used for electrodes and furnace linings in all high-temperature electric furnaces. Metals are not important as refractories in electric furnaces, but copper wires or busbars, for example, are utilized to carry current to the graphite electrodes. 

Inorganic polymers based on alternating main group element-nitrogen skeletons (e.g. I - IV) are of interest for their potential as elastomers, high-temperature oils, electrical conductors, biological molecule carriers, and precursors to ceramic materials (J - 6). 

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