General characteristics of metals

Metals are generally crystalline materials. Under very rapid cooling rates, say, greater than 10 Ks , one can also produce amorphous metals. Crystalline metals have the following three common crystal structures  

In general, metals can be worked extensively, either at room temperature or at high temperatures. This is so mainly because of the availability of a large of number slip systems for plastic deformation. This allows us to use metal drawing techniques to obtain filamentary metals. Metallic fibers are, generally, not spun from a molten state, although this can be done in some cases (see Section 5.2). When metals are cold worked (i.e. below the recrystallization temperature), they 

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Describe the general characteristics of metals, nonmetals, and metalloids. 

Interpreting Data Metals are usually malleable and good conductors of electricity. They are generally lustrous and silver or white in color. Many react with acids. Write the word metal beneath the Classification heading in the data table for those element samples that display the general characteristics of metals. 

In the example of Table 4.4, the surface energy of solid W in equilibrium with a saturated vapour of Cu is lower than °sv due to adsorption of Cu atoms on the W surface. This is generally characteristic of metallic A-B pairs having a low mutual miscibility (Eustathopoulos and Joud 1980). For this reason, results of sessile drop experiments for such systems cannot be interpreted by taking for the surface energy of the solid metal the value of equilibrium with its own vapour (see Sections 1.4.2 and 5.2). 

The general characteristics of metal colloidal dispersions are weU understood even though the mechanisms by which such metallic particles are formed in the colloidal state aic not This is easily understood if one recognises the complexity of the processes [1] involved in nucleation, growth and coagulation. In order to understand fully and control the preparation, these three steps of course have to be studied individually. Water purity, trace impurities, concentration, cleanliness of the reaction vessel and other factors all have a significant effect on the size and morphology of the particles [1,2] and therefore need to be controlled. 

The general characteristics of all these elements generally preclude their extraction by any method involving aqueous solution. For the lighter, less volatile metals (Li, Na, Be, Mg, Ca) electrolysis of a fused salt (usually the chloride), or of a mixture of salts, is used. The heavier, more volatile metals in each group can all be similarly obtained by electrolysis, but it is usually more convenient to take advantage of their volatility and obtain them from their oxides or chlorides by displacement, i.e. by general reactions such as... 

Thermal expansion mismatch between the reinforcement and the matrix is an important consideration. Thermal mismatch is something that is difficult to avoid ia any composite, however, the overall thermal expansion characteristics of a composite can be controlled by controlling the proportion of reinforcement and matrix and the distribution of the reinforcement ia the matrix. Many models have been proposed to predict the coefficients of thermal expansion of composites, determine these coefficients experimentally, and analy2e the general thermal expansion characteristics of metal-matrix composites (29-33). 

Space does not permit a survey of all the various weldable metals and their associated problems, although some suggestions are made in Table 9.9. It is sufficient to state that with a knowledge of the general characteristics of the welding process and its effects on a metal (e.g. type of thermal cycle imposed, residual stress production of crevices, likely weldability problems) and of the corrosion behaviour of a material in the environment under consideration, a reliable joint for a particular problem will normally be the rule and not the exception. 

The high values of E generally characteristic of the decomposition reactions of metal oxyhalides are widely interpreted as evidence that the initial step in anion breakdown is the rupture of the X—O bond and that the energy barrier to this reaction is not very sensitive to the properties of the cation present. Information of use in the formulation of reaction mechanisms has been obtained from radiolytic studies of oxyhalogen salts [887-889],... 

C). However, the characteristics of metals that physically distinguish them are their metallic luster, electrical conductivity, malleability, and ductility. Elements generally classified as metals exhibit wide variation in these properties. Chemically, metals are also reducing agents as a result of their having comparatively low ionization potentials. Another characteristic that differs enormously is their cost. Some of the base metals sell for a few cents per pound, whereas some of the exotic metals sell for a few thousand dollars per gram. 

Although the subject of stability of complexes will be discussed in greater detail in Chapter 19 it is appropriate to note here some of the general characteristics of the metal-ligand bond. One of the most relevant principles in this consideration is the hard-soft interaction principle. Metal-ligand bonds are acid-base interactions in the Lewis sense, so the principles discussed in Sections 9.6 and 9.8 apply to these interactions. Soft electron donors in which the donor atom is sulfur or phosphorus form more stable complexes with soft metal ions such as Pt2+ or Ag+, or with metal atoms. Hard electron donors such as H20, NH3( or F generally form stable complexes with hard metal ions like Cr3+ or Co3+. 

In this section the salts based on metallocenium cations and metal bisdichalcogenate anions will be reviewed according to the previously referred structural classification. After referring to the general characteristics of the crystal structures the supramolec-ular features will be correlated with the magnetic properties. 

General characteristics of alloys such as those presented in Fig. 3.3 have been discussed by Fassler and Hoffmann (1999) in a paper dedicated to valence compounds at the border of intermetallics (alkali and alkaline earth metal stannides and plumbides) . Examples showing gradual transition from valence compounds to intermetallic phases and new possibilities for structural mechanisms and bonding for Sn and Pb have been discussed. Structural relationships with Zintl phases (see Chapter 4) containing discrete and linked polyhedra have been considered. See 3.12 for a few remarks on the relationships between liquid and amorphous glassy alloys. 

Within the lanthanides the first ones from La to Eu are the so-called light lanthanides, the other are the heavy ones. Together with the heavy lanthanides it may be useful to consider also yttrium the atomic dimensions of this element and some general characteristics of its alloying behaviour are indeed very similar to those of typical heavy lanthanides, such as Dy or Ho. An important subdivision within the lanthanides, or more generally within the rare earth metals, is that between the divalent ones (europium and ytterbium which have been described together with other divalent metals in 5.4) and the trivalent ones (all the others, scandium and yttrium included). 

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