Classes of Solids

The most convenient classification scheme of solids is based on the physical character of the interatomic binding forces in various classes of crystalline materials. According to this classificahon, all solids fall into one of five general categories metallic, covalent, ionic, molecular, and hydrogen-bonded crystals. Some materials may belong to more than one category, thus, the distinction is in many cases not a sharp one. 

Metallic bond In metals the collectivized electrons move in the lattice potential of the posihve ions and are shared by all atoms in the crystal. The ions provide the rigid framework, that is, the crystal lattice, through which the valence electrons move about freely like the particles of a gas. The metallic bond is not a directional one. 

Metallic systems are conductors because there is no energy gap for electronic excitation. The energy bands of metals are partially filled. Then the bands can easily accept different numbers of electrons, leading to the ability of metals to form alloys with different valency, and to the tendency for metals to adopt close-packed structures, such as fee, hep, and bcc. 

Due to presence of free electrons the metals and the metallic solid solutions are excellent conductors of electricity and heat The electrons easily absorb energy of incident radiation or the lattice vibration. 

We have seen in previous chapters that the good examples of metallic crystals are the alkali metals, which can be correctly described by the near-free electron model. The valence electrons in these metals are completely separated from their ion cores and form a nearly uniform gas. 

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There are two classes of solids that are not crystalline, that is, p(r) is not periodic. The more familiar one is a glass, for which there are again two models, which may be called the random network and tlie random packing of hard spheres. An example of the first is silica glass or fiised quartz. It consists of tetrahedral SiO groups that are linked at their vertices by Si-O-Si bonds, but, unlike the various crystalline phases of Si02, there is no systematic relation between... 

A comparison of these predicted values of E with the measured values plotted in the bar-chart of Fig. 3.5 shows that, for metals and ceramics, the values of E we calculate are about right the bond-stretching idea explains the stiffness of these solids. We can be happy that we can explain the moduli of these classes of solid. But a paradox remains there exists a whole range of polymers and rubbers which have moduli which are lower - by up to a factor of 100- than the lowest we have calculated. Why is this What determines the moduli of these floppy polymers if it is not the springs between the atoms We shall explain this under our next heading. 

F. Hide, M.A. Dtaz-Gatcia, B.J. Schwarts, M.R. Andersson, P Qibing, A.J. Hecger, Semiconducting polymers a new class of solid-state laser materials. Science 1996, 273, 1833. 

Probably the most common compound of +3 chromium is potassium chrome alum, KCr(SOi)r 12H20. We know that the twelve water molecules are distributed equally, six around Cr+a and six around K+. Potassium chrome alum is just one example of the general class of solids called alums which have a 4-1 ion, a +3 ion, two sulfates, and twelve molecules of water. In the dyeing industry chrome alum is used for fixing dyes to fabrics. 

Another class of solid supports is based on rigid structures containing large pores and is often referred to as macroporous nonswelling beads. 

The most common classes of solid adsorbents are Zeolites and Silicagels. The main difference between the two is the way they are built. Zeolites have a crystalline structure and therefore a certain pore size. Silicagel have a pore... 

Silicon-containing ceramics include the oxide materials, silica and the silicates the binary compounds of silicon with non-metals, principally silicon carbide and silicon nitride silicon oxynitride and the sialons main group and transition metal silicides, and, finally, elemental silicon itself. There is a vigorous research activity throughout the world on the preparation of all of these classes of solid silicon compounds by the newer preparative techniques. In this report, we will focus on silicon carbide and silicon nitride. 

The silicates are a large class of solids of great importance in industry as well as science, particularly geology. The prototype silicate is quartz consisting of Si04 tetrahedra which share their comers and edges and are arrayed in various three-dimensional patterns depending on the temperature. In other crystalline minerals the tetrahedra are linked in one-dimensional chains, or two-dimensional sheets. The arrays in these latter cases are combined with various metal ions. 

There are three classes of solid state scintillation phosphors organic crystals, inorganic crystals, and plastic phosphors. 

Composites are an important class of solid materials, whose history goes back to ancient times. For example, bricks made only of clay were not as strong as those in which straw was mixed with the clay. Now we use clay as the filler in new polymeric composites to enhance their mechanical properties. The ionic con-... 

For a broad class of solids, clusters, and molecules, the Standard Model describes ground-state and excited state properties such as ... 

The first half of this chapter concentrates on the mechanisms of ion conduction. A basic model of ion transport is presented which contains the essential features necessary to describe conduction in the different classes of solid electrolyte. The model is based on the isolated hopping of the mobile ions in addition, brief mention is made of the influence of ion interactions between both the mobile ions and the immobile ions of the solid lattice (ion hopping) and between different mobile ions. The latter leads to either ion ordering or the formation of a more dynamic structure, the ion atmosphere. It is likely that in solid electrolytes, such ion interactions and cooperative ion movements are important and must be taken into account if a quantitative description of ionic conductivity is to be attempted. In this chapter, the emphasis is on presenting the basic elements of ion transport and comparing ionic conductivity in different classes of solid electrolyte which possess different gross structural features. Refinements of the basic model presented here are then described in Chapter 3. 

Either MA or MM processes drop under more general class of solid-state amorphization reactions, SSAR. Amorphization by irradiation of solids was observed yet in the era of study of materials for nuclear reactors. In 1962, Bloch [88] amorphized UgFe by exposing it to fluxes of nuclear fission fragments. Others obsawed amorphization... 

Several classes of solids have commonly been used for surface organic chemistry including aluminas, silica gels and clays. Zinc oxide (ZnO) is certainly one of the most interesting of these solids because it has surface properties that suggest that a very rich organic chemistry may occur there. 

The previous examples were all for drops, bubbles, or particles, all objects small in three directions. The other examples in this chapter are for solids that are small only in one direction. These are solid films, a class of solids with many important applications. 

The second class of solid reactions involves situations where the solid does not disappear or appear but rather transforms from one solid phase into another as the reaction proceeds, as shown in Figure 9-6. For transformations of solids there are several models that may be appropriate, depending on the microstmcture of the reacting solid. Limiting cases of concentration profiles within the solid are (1) uniform reaction and (2) film formation. Concentration profiles within the solid for these situations are shown in Figure 9-7. 

The oxides represent one of the most important and widely employed classes of solid catalysts, either as active phases or as supports. They are used for both their acid-base and ReDox properties and constitute the largest family of catalysts in heterogeneous catalysis [76]. 

This acid-catalyzed cleavage of the glycosidic bonds is rather complex and often suffers from a lack of selectivity mainly due to side dehydration or recombination reactions of monosaccharides. In the existing literature, four different classes of solid catalysts are reported (1) cation-exchange resins, (2) siliceous-based materials, (3) metal oxides, and (4) sulfonated amorphous carbons. 

One Interesting and industrially important process developed recently,"the cyclar process" is the catalytic aromatization of light (C3-C5) hydrocarbons over pentasil based catalysts. These new classes of solids have been widely studied (1-9). These Investigations led to the conclusion that the catalysts consisting of gallium, zinc, Pt, and modified H-ZSM-5 were more active and more selective towards aromatics than the parent H-ZSM-5 zeolite. The formation of aromatics from light alkanes comprised several main hydrocarbon reactions alkane... 

A third class of solids, which were tested as catalysts for carbohydrate dehydrations are inorganic phosphates. In a comparative study including vanadyl phosphate (VOPO4 2H2O) and partially metal-substituted vanadyl phosphates xPC>4 2H2O, M being Cr, Mn, Fe, A1 and Ga), it was... 

Supported vanadium oxides represent one of the technologically most important class of solid catalysts. These catalysts are useful for partial oxidation of various hydrocarbons 0), ammoxidation of alkyl substituted N-heteroaromatic compounds (2) and most recently for NO reduction (3) For a catalyst to be a successful one in industry, it should exhibit high activity with maximum selectivity, thermal and mechanical stability and long life etc. For getting some of these functionalities, the active component has to be dispersed uniformly on a support material. 

Although most solids do not have a dipole moment in the absence of an electric field, the classes of solids that do are commercially important, and so form the subject matter of the rest of this chapter. 

The remaining chapters each deal with a property or a special class of solid. Chapter 4 covers low-dimensional solids, the properties of which are not isotropic. Chapter 5 deals with zeolites, an interesting class of compounds used extensively in industry (as catalysts, for example), the properties of which strongly reflect their stracture. Chapter 6 deals with optical properties and Chapter 7 with magnetic properties of solids. Finally, Chapter 8 explores the exciting field of superconductors, particularly the relatively recently discovered high temperature superconductors. 

Five aspects of the preparation of solids can be distinguished (i) preparation of a series of compounds in order to investigate a specific property, as exemplified by a series of perovskite oxides to examine their electrical properties or by a series of spinel ferrites to screen their magnetic properties (ii) preparation of unknown members of a structurally related class of solids to extend (or extrapolate) structure-property relations, as exemplified by the synthesis of layered chalcogenides and their intercalates or derivatives of TTF-TCNQ to study their superconductivity (iii) synthesis of a new class of compounds (e.g. sialons, (Si, Al)3(0, N)4, or doped polyacetylenes), with novel structural properties (iv) preparation of known solids of prescribed specifications (crystallinity, shape, purity, etc.) as in the case of crystals of Si, III-V compounds and... 

Relations between the structure and properties have been investigated in a variety of solids such as metal oxides, chalcogenides, pnictides and halides. In addition to studying model systems for testing theoretical predictions, solid state chemists have been preparing new classes of solids as well as novel members of known types of solids. In this section, we have chosen three classes of solids, viz. metal oxides, metal sulphides and metal fluorides, to discuss structure-property relations we shall concentrate especially on their electrical and magnetic properties. 

Different classes of solid propellants DB, CMDB, and fuel rich (FR) have been developed in order to meet the requirements of various missions in terms of specific impulse (Lsp) and wide range of burn rates with low pressure index (n). High density, low temperature sensitivity and good mechanical properties constitute other essential requirements of these propellants. The salient features of such performance parameters are ... 

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