Applications refractory materials

Thermal conductivity and heat capacity In practical applications, refractory materials processing high thermal capacity as well as low thermal conductivity are required, depending upon (of course) the functional requirements. In most situations, a refractory that serves as a furnace wall should have a low thermal conductivity in order to retain as much as heat as possible. However, a refractory used in the construction of the walls of muffles or retorts or coke ovens should have a high thermal conductivity in order to transmit as much heat as possible to the interior. The charge remains separated from flame in these specific examples of installations. 

Silicate ceramics are generally alumino-silicate based materials obtained from natural raw materials. They exhibit a set of fundamental properties, such as chemical inertia, thermal stability and mechanical strength, which explain why they are widely used in construction products (sanitary articles, floor and wall tiles, bricks, tiles) and domestic articles (crockery, decorative objects, pottery). They are often complex materials, whose usage properties depend at least as much on microstructure and aesthetics as on composition. Silicate products with an exclusively technical application (refractory materials, insulators or certain dental implants) will not be explicitly discussed in this chapter. 

Today, the term solid electrolyte or fast ionic conductor or, sometimes, superionic conductor is used to describe solid materials whose conductivity is wholly due to ionic displacement. Mixed conductors exhibit both ionic and electronic conductivity. Solid electrolytes range from hard, refractory materials, such as 8 mol% Y2C>3-stabilized Zr02(YSZ) or sodium fT-AbCb (NaAluOn), to soft proton-exchange polymeric membranes such as Du Pont s Nafion and include compounds that are stoichiometric (Agl), non-stoichiometric (sodium J3"-A12C>3) or doped (YSZ). The preparation, properties, and some applications of solid electrolytes have been discussed in a number of books2 5 and reviews.6,7 The main commercial application of solid electrolytes is in gas sensors.8,9 Another emerging application is in solid oxide fuel cells.4,5,1, n... 

Flammable atmospheres can be assessed using portable gas chromatographs or, for selected compounds, by colour indicator tubes. More commonly, use is made of explos-imeters fitted with Pellistors (e.g. platinum wire encased in beads of refractory material). The beads are arranged in a Wheatstone bridge circuit. The flammable gas is oxidized on the heated catalytic element, causing the electrical resistance to alter relative to the reference. Instruments are calibrated for specific compounds in terms of 0—100% of their lower flammable limit. Recalibration or application of correction factors is required for different gases. Points to consider are listed in Table 9.10. 

Refractory Rrick Nonmetallic refractory materials are widely used in high-temperature applications in which the service permits the appropriate type of construction. The more important classes are described in the following paragraphs. 

Refractories are materials that resist the action of hot environments by containing heat energy and hot or molten materials (1). There is no well-established line of demarcation between those materials that are and those that are not refractory. The ability to withstand temperatures above 1100°C without softening has, however, been cited as a practical requirement of industrial refractory materials (see CERAMICS). The type of refractories used in any particular application depends on the critical requirements of the process. For example, processes that demand resistance to gaseous or liquid corrosion require low permeability, high physical strength, and abrasion resistance. Conditions that demand low thermal conductivity may require entirely different refractories. Combinations of several refractories are generally employed. 

Magnesium occurs in many igneous rocks and in dolomite. It is usually obtained from seawater (1300 mg kg-1) or from the minerals magnesite (MgCC>3) or carnallite (KCl-MgCl2-6H20). The metal is used in lightweight alloys, MgO is employed as a refractory material and as an adsorbent for water treatment, and other Mg compounds find applications in the pharmaceutical and chemical process industries. 

Various forms of carbon, semigraphite, and graphite materials have found wide application in the metals industry, particulady in connection with the production of iron, aluminum, and ferroalloys. Carbon has been used as a refractory material since 1850, though full commercial acceptance and subsequent rapid increase in use has occurred only since 1945. 

The successful application of the sol-gel route has been realized in the International technological practice of many branches of the industries—electronics, optics, construction of engines, nuclear energetics, chemical, and food industry equipment. Among these materials are ferro-, piezo-, and dielectrics, solid electrolytes, refractory materials, membranes, protective and decorative coatings, and also films with special optical and electrophysical properties, like hightemperature superconductors. 

According to the data in Table 25.5 and to Eq. (25.6) the compressive strength of filaments of refractory materials such as carbon and silicon carbide have compressive strengths about 10 times as large as those of organic fibres. This would seem to be a serious restriction to the use of organic polymers such as aramids in their application in composites. For most of the applications this restriction is of minor importance, however, since long before ac max is reached, instability in the construction will occur. The resistance of a column or a panel under pressure is proportional to the product of a load coefficient and a material efficiency criterion ... 

Following these two surveys, we conducted a literature review based upon Ceramic Abstracts for the years 1988-2002. The results are summarized in Table 2.1. The results presented in Table 2.1 indicate that there has been a significant increase in the literature on CBPCs in recent years. The major thrust of the research has been in biomaterials and dental cements. Though small in number, there have been several articles in structural materials applications, which also include oil well cements. Interest in conventional refractory materials has continued, and as expected, all the applications have been supported by research in materials structure and properties of the CBPCs. 

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