Crystalline alloys

As discussed in several sections of this book, electrical resistivity in ferrites can be 4-10 orders of magnitude larger than in metals this difference explains the dominance of ferrites in most high-frequency applications. In metals, considerable efforts have been made to increase resistivity however, due to the nature of metallic bonding, improvements by a factor of 2-3 are considered as exceptional. 

One of the basic problems in 3d metals and alloys is how to account for the existence of localised magnetic moments in free-electron bands. Electrons in 3d metals do exhibit a localised character, as shown by the accurate fitting of the susceptibility of most ferromagnetic metals (in the paramagnetic state) to the Curie-Weiss law on the other hand. 

This model qualitatively accounts for most of the apparently conflicting properties of metallic ferromagnets. The difference in occupancy between spin-up and spin-down half-bands leads to a spontaneous, localised magnetic moment per atom the filling process usually results in a 

Instead of giving a detailed description of all soft magnetic materials, this overview describes the basic mechanisms that can be used to modify 

Order-disorder transformation Directional order Preferential orientation Grain boundary effects 

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Materials for PC Media. Crystalline alloys of elements from the fifth and sixth main group are preferred (3,103,109—111). As the first PC materials, tellurium suboxides as well as Te/Se or Te films that had been doped with small amounts of other elements like Ge, As, or Sb to shift the crystallization point to >100°C have been described. 

Fig. 3.70 Corrosion rates of amorphous and crystalline alloys measured in 87 wt% H3PO4 at...

Au-substituted /U-Cu-Fe quasi crystalline alloys, a Au and Fe study Replacement of Cu by Au on BCl sites and strong hybridization of Au with A1 neighbors... 

Rosalbino F., Maccio D., Angelini E., Saccone A., Delfino S., Electrocatalytic properties of Fe-R (R = rare earth metal) crystalline alloys as hydrogen electrodes in alkaline water electrolysis, ]. Alloys Compd., 403(1-2), 275-282,2005. 

The second type of impurity, substitution of a lattice atom with an impurity atom, allows us to enter the world of alloys and intermetallics. Let us diverge slightly for a moment to discuss how control of substitutional impurities can lead to some useful materials, and then we will conclude our description of point defects. An alloy, by definition, is a metallic solid or liquid formed from an intimate combination of two or more elements. By intimate combination, we mean either a liquid or solid solution. In the instance where the solid is crystalline, some of the impurity atoms, usually defined as the minority constituent, occupy sites in the lattice that would normally be occupied by the majority constituent. Alloys need not be crystalline, however. If a liquid alloy is quenched rapidly enough, an amorphous metal can result. The solid material is still an alloy, since the elements are in intimate combination, but there is no crystalline order and hence no substitutional impurities. To aid in our description of substitutional impurities, we will limit the current description to crystalline alloys, but keep in mind that amorphous alloys exist as well. 

Homogeneous crystalline alloys of selenium and tellurium, Se Tei-x, of various compositions (x = 0.1-0.9) are prepared by the co-reduction of solutions of Se(IV) and Te(IV) compounds, e.g. dialkylselenites and tetraalk-oxytelluranes (or glycol solutions of Se02 and Te02), with hydrazine. ... 

We note here that gel is a coherent solid because its structure is characterized by a polymer network, and hence, the above theoretical considerations on crystalline alloys should be applicable to gels without essential alteration. It is expected that the curious features of the first-order transition of NIPA gels will be explained within the concept of the coherent phase equilibrium if the proper calculation of the coherent energy and the elastic energy of the gel network is made. This may be one of the most interesting unsolved problems related to the phase transitions of gels. 

Of the various physical properties, it is the mechanical properties that make metallic glasses so unique when compared to their crystalline counterparts. A metallic glass obtains its mechanical strength in quite a different way from crystalline alloys. The disordered atomic structure increases the resistance to flow in metallic glasses so that these materials approach their theoretical strength, An attractive feature is that metallic glasses are equally strong in all directions because ul the random order of their atomic structure... 

TABLE 12. Catalytic activity and selectivity of amorphous and crystalline alloys in the hydrogenation of 1-hexene (0.4 g alloy, 5 ml 1-hexene, 5 ml 1,4-dioxane, 323 K)... 

The combination of the above factors has rendered the nanocrystalline solution competitive, not only with amorphous Co-based alloys, but also with classical crystalline alloys and ferrites. The consequence is a steadily increasing level of applications in magnetic cores for ground fault interrupters, common mode chokes and high frequency transformers. Fig. 14 shows some typical examples. The worldwide production rate meanwhile approaches an estimated 1000 tons/year, and the trend is increasing. The only drawback of the nanocrystalline material appears to be the embrittlement that occurs upon crystallization, which requires final shape annealing and, thus, restricts application mainly to toroidally wound cores. 

Activity. The catalytic activity of the Fe2oNi6oP20 alloys for the Fischer-Tropsch synthesis was analyzed and it was found that the amorphous alloy has the catalytic activity about three hundred times higher than the crystalline alloy ( 5,6). For Fe9o Zrio alloy, the catalytic activity of the amorphous and crystalline phases was analyzed for the same reaction at 248 and 255°C. 

At present, a number of quasi-crystalline alloys with icosahedral, decagonal, and octagonal symmetry are synthesized by different methods. The quasicrystalline form of the solids turned out to be widespread in a great extent. The absence of the translation symmetry and the presence of numerous interstitial sites of the different types in the structure of icosahedral quasicrystals makes some of them interesting objects for hydride chemistry. We cannot wait for any sensational discoveries here, as the general laws of M-H interaction do not depend on matrix structure. However, encouraging results were obtained for icosahedral Ti45Zr38Nii7... 

The various classes of metallic phases that may be encountered in crystalline alloys include substantially pure elements, solid solutions of one element in another and intermetallic compounds. In crystalline form, alloys are subject to the same type of defects as pure metals. Crystalline alloys may consist of a solid solution of one or more elements (solutes) in the major (base) component, or they may contain more than one phase. That is, adjacent grains may have slightly or extremely different compositions and be of identical or disparate crystallographic types. Often, there is one predominant phase, known as the matrix, and other secondary phases, called precipitates. The presence of these kinds of inhomogeneities often results in the alloy having radically different mechanical properties and chemical reactivities from the pure constituent elements. (Noel)5... 

Alloys.—Ruthenium forms a beautiful crystalline alloy with tin,1 which is cubic in structure. It is readily formed by heating to redness in a carbon crucible one part of ruthenium with 10-15 parts of tin. On cooling, the excess of tin is dissolved out by treatment with hydrochloric acid, the residue having the composition represented by the formula RuSn3. 

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