Ceramic compounds

Nuclear Applications. Powder metallurgy is used in the fabrication of fuel elements as well as control, shielding, moderator, and other components of nuclear-power reactors (63) (see Nuclearreactors). The materials for fuel, moderator, and control parts of a reactor are thermodynamically unstable if heated to melting temperatures. These same materials are stable under P/M process conditions. It is possible, for example, to incorporate uranium or ceramic compounds in a metallic matrix, or to produce parts that are similar in the size and shape desired without effecting drastic changes in either the stmcture or surface conditions. OnlyHttle post-sintering treatment is necessary. 

Uses. The unalloyed metal cannot be directly used owing to its bad mechanical properties and its high oxidability. Several thallium alloys are used as semiconductors or ceramic compounds it may be used as additive to gold, silver or copper contacts in the electronic industries. Thallium is dangerously toxic. 

Equation (S.21) is normally used in metallic systems for substitutional phases such as liquid, b.c.c., f.c.c., etc. It can also be used to a limited extent for ceramic systems and useful predictions can be found in the case of quasi-binary and quasi-temary oxide systems (Kaufman and Nesor 1978). However, for phases such as interstitial solutions, ordered intermetallics, ceramic compounds, slags, ionic liquids and aqueous solutions, simple substitutional models are generally not adequate and more appropriate models will be discussed in Sections 5.4 and 5.5. 

Solid oxide fuel cells (SOFC) use a hard, non-porous ceramic compound as the electrolyte. Since the electrolyte is a solid, the cells do not have to be constructed in the plate-like configuration typical of other fuel cell types. SOFCs are expected to be around 50-60 percent efficient at converting fuel to electricity, however, calculations show that over 70 percent may be achievable. In applications designed to capture and utilize the system s waste heat (co-generation), overall fiiel use efficiencies could top 80-85 percent. 

The silicates, made up of base units of silicon and oxygen, are an important class of ceramic compounds that can take on many structures, including some of those we have already described. They are complex structures that can contain several additional atoms such as Mg, Na, K. What makes the silicates so important is that they can be either crystalline or amorphous (glassy) and provide an excellent opportunity to compare these two disparate types of structure. Let us first examine the crystalline state, which will lead us into the amorphous state. 

Ceramic compound Starting materials Type of plasma used... 

The previous section showed that the physical properties of the carbides and nitrides differed substantially from those of the parent metals, and resembled those of ceramic compounds. The electronic and magnetic properties of the carbides and nitrides provide an interesting contrast, because here there are similarities with the metals. 

In some ceramic compounds, the different kinds of ions possess different and strong magnetic dipoles. In addition, both kinds are partially oppositely aligned in an external field. Since the sizes of the magnetization are unequal, the result is a net magnetization which can considerably reinforce the external field. This is called ferrimagnetism. 

In the immediate future, the main objective in ceramic superplasticity will be the search of the right conditions to achieve high strain rate superplasticity (HSRS) ((e > 1CT2 s 1). Although this phenomenon has been found in several ceramic compounds and several inputs have been outlined to achieve it, we are still far from knowing what to do to obtain this effect systematically. This HSRS will enlarge the applications for ceramics. 

There are many factors which contribute to dielectric loss and in the case of the complex ceramic compounds discussed above, to achieve a satisfying understanding of the relative magnitudes of the various loss mechanisms is challenging. There will be contributions to loss intrinsic to the idealized structural chemistry of the material and it is now clear that this is complicated by a domain structure. There will also be contributions of an extrinsic nature, particularly those associated with departures from the ideal structure, point defects and... 

When ceramic compounds are used as reactants in solid-state synthesis, the reaction conditions are often extreme because of the refractory nature of ceramics. 

Corrosion of refractories and ceramics involves consideration of (i) acid-base effects involved in the corrosion of refractory (ii) verification of the thermal stability of each constituent (iii) calculation of the free enthalpy of all possible reactions that might occur in the corrosion of the refractory or ceramic compound. Kinetic data are also useful in understanding the corrosion and selection of the refractory compound for a specific application. 

Ceramics are primarily compounds. Ceramics other than glasses generally have a crystalline structure, while silica-based glasses, a subclass of ceramic materials, are noncrystalline. In crystalline ceramic compounds, stoichiometry dictates the ratio of one element to another. Nonstoichiometric ceramic compounds, however, occur frequently. Some important ceramic materials are listed in Table... 

Fascinating it was, but few people paid attention—except for the Japanese. They had read Powell and Danby s paper with the same keen interest with which years later, they would read the obscure journal detailing the discovery by IBM scientists of a ceramic compound superconducting at a record-high Kelvin. In 1970, they exhibited a model of the train at the Osaka World s Fair. By 1979, they tested another at speeds that hit a top of 321 miles per hour, a world record. In 1985, another maglev carried more than a half-million passengers on short runs at the science fair at Tsukuba. 

IBM researchers Alex Muller and Georg Bednorz make a ceramic compound of lanthanum, barium, copper, and oxygen that superconducts at 35° K. 

The sialons are a group of ceramic compounds of increasing technological importance for their thermal, chemical and mechanical properties. These are similar to Si3N4 but can be more readily tailored to specific requirements because of the wide range of possible sialon compositions. 

The crystalline form of interest in Zr-based ceramic compounds is the cubic fluorite structure based on the mineral CaF2. In this structure, consisting of interpenetrating face-centered-cubic and simple cublic arrays of cations (Zr ) and anions (O ), respectively, oxygen ion conductivity is enhanced by replacing zirconium (Zr ) ions on the cation lattice with soluble dopant cations having a valence less than 4, typically divalent (Mg, Ca ) and trivalent (Y, Yb , Sc ) cations. These dopants, which are in solid solution, are incorporated into the zirconia structure by the following types of defect reaction ... 

The coating formulation includes other ingredients, such as a pigment for color and opacity, modifiers and additives for performance enhancement, etc. Because the coating will be baked at elevated temperatures, the pigment must also be resistant to heat and chemicals. Nonorganic color pigments, such as ceramic compounds, are preferred. 

Mechanical strength and thermal stability of catalyst particles are always of concern to process designers. In some cases it may be the most critical feature. This was emphasized, for e.xample, in steam reforming. Strong pellets with good thermal resistance are required. Catalyst designers use mixed oxides fired at high temperatures to form ceramic compounds, Particles must be preformed and active components added later. 

Silica (Si02) is the most important and versatile ceramic compound of MX2 stoichiometry. As noted above, it is widely available in raw materials in the earth s surface, and silica is a fundamental constituent of a wide range of ceramic products and glasses ... 

Abstract Refractory oxides encompass a broad range of unary, binary, and ternary ceramic compounds that can be used in structural, insulating, and other applications. The chemical bonds that provide cohesive energy to the crystalline solids also influence properties such as thermal expansion coefficient, thermal conductivity, elastic modulus, and heat capacity. This chapter provides a historical perspective on the use of refractory oxide materials, reviews applications for refractory oxides, overviews fundamental structure-property relations, describes typical processing routes, and summarizes the properties of these materials. 

Lead and lead compounds have been used in a multitude of products for centuries. Lead (metal) is occasionally used as a pure material, but this is relatively rare when compared with the extent of its use in alloys and in ceramic compounds and glasses. 

Structure Some 14 different ceramic compounds are reported to... 

Solid oxide fuel cells (SOFCs) use a nonporous ceramic compound as the electrolyte and operate at very high temperatures (1,800°F). Heat can be recaptured for co-generation, making these fuel cells highly efficient (80-85%). Because of size, heat output, and a long start-up time, these fuel cells are more suitable for stationary applications. 

Besides combinations with polymers as mentioned above, there are also some composites of carbon nanotubes with other, and especially with inorganic materials. These are, in most cases, metals or ceramic compounds like aluminum oxide. 

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