Nonoxide materials

The nonmetallic, mechanically resistant materials that are constituents of the ceramic materials can be subdivided into oxidic and nonoxidic materials. High melting points and hardnesses are the outstanding properties, as shown in Table 20.2. 

SbSI (LB Number 20A-7). SbSI is ferroelectric below 20 °C. The phase transition is of the displacive type, a relatively rare characteristic in nonoxide materials. The crystal is photoconductive (Figs. 4.5-47 and 4.5-48). 

Restricted to nonoxidizing materials Nonflammable material is available but more expensive... 

Microporous inorganic materials dominated historically by the 2eohtes and alumosilicates, and the great variety of more recent nonoxide and coordination framework materials should also be mentioned here (171—174) but not discussed in detail. This type of molecular recognition is usually known as molecular sieving. 

The largest-volume phosphoms compounds are the phosphoric acids and phosphates (qv), ie, the oxide derivatives of phosphoms ia the + 5 oxidation state. With the exception of the phosphoric acid anhydride, P O q, and the phosphate esters, these materials are discussed elsewhere (see Phosphoric acids and phosphates). An overview of phosphoms compounds other than the phosphoric acids and phosphates is given herein. These compounds constitute a large variety of phosphoms compounds that are either nonoxide derivatives or derivatives of phosphoms ia oxidation states lower than + 5. These phosphoms compounds are manufactured only from elemental phosphoms (qv) obtained by reduction of naturally occurring phosphate rock (calcium phosphate). 

Carbon and Graphite. Carbon (qv) and graphite [7782 2-5] have been used alone to make refractory products for the lower blast furnace linings, and electrodes for steel and aluminum production. They are also commonly used in conjunction with other refractory raw materials. These materials are highly refractory nonwettable materials and are useful refractories in nonoxidizing environments. Carbon blacks are commercially manufactured, whereas graphite for refractory use has to be mined. 

Fibrous materials may be naturally occurring or synthetically manufactured by thermal or chemical processes (Fig. 1) (see Fibers, survey). Refractory fibers are generally used in industrial appHcations at temperatures between 1000°C and 2800°C. These fibers may be oxides or nonoxides, vitreous or polycrystalline, and may be produced as whiskers, continuous filaments, or loose wool products. 

Fiber chemistry determines whether the material is an oxide or nonoxide and can also influence its vitreous or polycrystalline physical form. Refractory fibers generally have diameters ranging from submicrometer to 10 )J.m, and lengths, as manufactured, may range from millimeters to continuous filaments. 

Nonoxide fibers, such as carbides, nitrides, and carbons, are produced by high temperature chemical processes that often result in fiber lengths shorter than those of oxide fibers. Mechanical properties such as high elastic modulus and tensile strength of these materials make them excellent as reinforcements for plastics, glass, metals, and ceramics. Because these products oxidize at high temperatures, they are primarily suited for use in vacuum or inert atmospheres, but may also be used for relatively short exposures in oxidizing atmospheres above 1000°C. 

Oxide and nonoxide refractory fibers have become essential materials for use in modem high temperature industrial processes and advanced commercial appHcations. Future process improvements, cost reductions, and performance enhancements are expected to expand the uses and markets for these specialized fibrous materials. 

Fusion Reactors. The development of fusion reactors requires a material exhibiting high temperature mechanical strength, resistance to radiation-induced swelling and embrittlement, and compatibUity with hydrogen, lithium and various coolants. One aUoy system that shows promise in this appHcation, as weU as for steam-turbine blades and other appHcations in nonoxidizing atmospheres, is based on the composition (Fe,Co,Ni)2V (30). 

Preceramic polymer precursors (45,68) can be used to make ceramic composites from polymer ceramic mixtures that transform to the desired material when heated. Preceramic polymers have been used to produce oxide ceramics and are of considerable interest in nonoxide ceramic powder processing. Low ceramic yields and incomplete burnout currently limit the use of preceramic polymers in ceramics processing. 

Nonoxide NLO ceramics include Si and compound semiconductors (qv) having the silicon stmcture, eg, GaAs, InP, and InSb, as weU as ferroelectrics such as SbSI. These materials tend to be more highly nonlinear than oxide ceramics, although lack of transparency at visible and uv wavelengths prevents them from competing with the oxides for the same appHcations. 

Pigment Systems. Most of the crystals used for ceramic pigments are complex oxides, owing to the great stability of oxides in molten silicate glasses. Table 3 fists these materials. The one significant exception to the use of oxides is the family of cadmium sulfoselenide red pigments. This family is used because the colors obtained caimot be obtained in oxide systems thus it is necessary to sustain the difficulties of a nonoxide system. 

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