Refractory Characteristics

A criterion of this book is that only those carbides and nitrides having a melting point above 1800 C and good chemical resistance are considered. Chromium carbide is a marginal case as mentioned in Ch. 3, Sec. 

Its cubic form, Cr23Cg, decomposes on melting at approximately 1500°C and cannot be considered refractory. The hexagonal form, Cr7C3 melts without decomposition at 1755°C but, because of its industrial importance, it is considered here. Only the orthorhombic form, Cr3C2, with a melting point of 1810°C, truly (if barely) meets the refractory criteria. 

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Riverine DOC is expected to behave conservatively and to be distributed throughout the floodplain. In a stable isotope study of autotrophic carbon sources for heterotroph-ic bacteria in Lake Calado, Waichman (1996) found that DOC in all lacustrine habitats was derived predominantly from C-3 plants and was refractory, characteristics similar to those of riverine DOC. 

The ceramic slurry is typically made up of colloidal silica as the liquid medium and Ohe or more ceramic powders selected for their refractory characteristics. Typical ones are Zircon, Silica, Alumina, and Aluminosilicates... 

The GT-MHR has very high proliferation resistance due to low fissile fuel volume fractions and the refractory characteristics of the TRISO fuel particle coating system that forms a containment from which it is difficult to retrieve fissile materials. 

With the exception of glass fiber, asbestos (qv), and the specialty metallic and ceramic fibers, textile fibers are a class of soHd organic polymers distinguishable from other polymers by their physical properties and characteristic geometric dimensions (see Glass Refractory fibers). The physical properties of textile fibers, and indeed of all materials, are a reflection of molecular stmcture and intermolecular organization. The abiUty of certain polymers to form fibers can be traced to several stmctural features at different levels of organization rather than to any one particular molecular property. 

A third group includes silver—nickel, silver—cadmium oxide, and silver—graphite combinations. These materials are characterized by low contact resistance, some resistance to arc erosion, and excellent non sticking characteristics. They can be considered intermediate in overall properties between silver alloys and silver or copper—refractory compositions. Silver—cadmium oxide compositions, the most popular of this class, have wide appHcation in aircraft relays, motor controllers, and line starters and controls. 

Properties. Uranium metal is a dense, bright silvery, ductile, and malleable metal. Uranium is highly electropositive, resembling magnesium, and tarnishes rapidly on exposure to air. Even a poHshed surface becomes coated with a dark-colored oxide layer in a short time upon exposure to air. At elevated temperatures, uranium metal reacts with most common metals and refractories. Finely divided uranium reacts, even at room temperature, with all components of the atmosphere except the noble gases. The silvery luster of freshly cleaned uranium metal is rapidly converted first to a golden yellow, and then to a black oxide—nitride film within three to four days. Powdered uranium is usually pyrophoric, an important safety consideration in the machining of uranium parts. The corrosion characteristics of uranium have been discussed in detail (28). 

A comprehensive analytical program for characterising wastewaters should be based on relevance to unit treatment process operations, the poUutant or pollutants to be removed ia each, and effluent quality constraints. The qualitative and quantitative characteristics of waste streams to be treated not only serve as a basis for sising system processes within the facility, but also iadicate streams having refractory constituents, potential toxicants, or biostats. Such streams are not amenable to effective biological treatment, as iadicated by the characterization results, and requite treatment usiag alternative processes. 

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. 

TABLE 28 33 General Physical and Chemical Characteristics of Refractory Brick ... 

A refractory quarl is usually an integral part of forced-draft burners. Suitable design of burner and quarl can determine the flame characteristics. Long, short, pencil or even flat flames are possible. 

Neuropathic pain is initiated or caused by a primary lesion in the peripheral or central nervous system. The causative agent may be trauma, nerve-invading cancer, herpes zoster, HIV, stroke, diabetes, alcohol or other toxic substances. Neuropathic pain is refractory to most analgesic drugs. Altered sodium channel activity is characteristics of neuropathic pain states. 

Hafnium carbide (HfC) is an interstitial carbide which, with tantalum carbide, is the most refractory compound known. Its characteristics and properties are summarized in Table 9.4. 

Niobium carbide, also known as columbium carbide, is a important refractory material with a high melting point. It is used as a CVD coating mostly on an experimental basis. Niobium carbide has two phases Nb2C and the monocarbide NbC. The latter is the only phase of industrial importance and the only one reviewed here. Its characteristics and properties are summarized in Table 9.5. 

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