E-Book Overview
The Materials Handbook is an encyclopedic, A-to-Z organization of all types of materials, featuring their key performance properties, principal characteristics and applications in product design. Materials include ferrous and nonferrous metals, plastics, elastomers, ceramics, woods, composites, chemicals, minerals, textiles, fuels, foodstuffs and natural plant and animal substances --more than 13,000 in all. Properties are expressed in both U.S. customary and metric units and a thorough index eases finding details on each and every material. Introduced in 1929 and often known simply as "Brady's," this comprehensive, one-volume, 1244 page encyclopedia of materials is intended for executives, managers, supervisors, engineers, and technicians, in engineering, manufacturing, marketing, purchasing and sales as well as educators and students. Of the dozens of families of materials updated in the 15th Edition, the most extensive additions pertain to adhesives, activated carbon, aluminides, aluminum alloys, catalysts, ceramics, composites, fullerences, heat-transfer fluids, nanophase materials, nickel alloys, olefins, silicon nitride, stainless steels, thermoplastic elastomers, titanium alloys, tungsten alloys, valve alloys and welding and hard-facing alloys. Also widely updated are acrylics, brazing alloys, chelants, biodegradable plastics, molybdenum alloys, plastic alloys, recyclate plastics, superalloys, supercritical fluids and tool steels. New classes of materials added include aliphatic polyketones, carburizing secondary-hardening steels and polyarylene ether benzimidazoles. Carcinogens and materials likely to be cancer-causing in humans are listed for the first time. (20021001)
E-Book Content
Source: Materials Handbook
Part
1 Materials, Their Properties and Uses
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Materials, Their Properties and Uses
2
ABLATIVES
ABLATIVES. Materials used for the outward dissipation of extreme-
ly high heats by mass removal. Their most common use is as an external heat shield to protect supersonic aerospace vehicles from an excessive buildup of heat caused by air friction at the surface. The ablative material must have a low thermal conductivity in order that the heat may remain concentrated in the thin surface layer. As the surface of the ablator melts or sublimes, it is wiped away by the frictional forces that simultaneously heat newly exposed surfaces. The heat is carried off with the material removed. The less material that is lost, the more efficient is the ablative material. The ablative material also should have a high thermal capacity in the solid, liquid, and gaseous states; a high heat of fusion and evaporation; and a high heat of dissociation of its vapors. The ablative agent, or ablator, is usually a carbonaceous organic compound, such as a phenolic plastic. As the dissociation products are lost as liquid or vapor, the char is held in place by the refractory reinforcing fibers, still giving a measure of heat resistance. The effective life of an ablative is short, calculated in seconds per millimeter of thickness for the distance traveled in the atmosphere. Single ablative materials seldom have all the desirable factors, and thus composites are used. Phenolic or epoxy resins are reinforced with asbestos fabric, carbonized cloth, or refractory fibers, such as asbestos, fused silica, and glass. The refractory fibers not only are incorporated for mechanical strength, but also have a function in the ablative process, and surface-active agents may be added to speed the rate of evaporation. Another composite, polyarylacetylene (PAA) reinforced with carbon