Homogeneously alloyed

Homogeneous alloys have a single glass transition temperature which is determined by the ratio of the components. The physical properties of these alloys are averages based on the composition of the alloy. 

Alloys are metallic materials prepared by mixing two or more molten metals. They are used for many purposes, such as construction, and are central to the transportation and electronics industries. Some common alloys are listed in Table 5.5. In homogeneous alloys, atoms of the different elements are distributed uniformly. Examples include brass, bronze, and the coinage alloys. Heterogeneous alloys, such as tin-lead solder and the mercury amalgam sometimes used to fill teeth, consist of a mixture of crystalline phases with different compositions. 

Homogeneous alloys of metals with atoms of similar radius are substitutional alloys. For example, in brass, zinc atoms readily replace copper atoms in the crystalline lattice, because they are nearly the same size (Fig. 16.41). However, the presence of the substituted atoms changes the lattice parameters and distorts the local electronic structure. This distortion lowers the electrical and thermal conductivity of the host metal, but it also increases hardness and strength. Coinage alloys are usually substitutional alloys. They are selected for durability—a coin must last for at least 3 years—and electrical resistance so that genuine coins can be identified by vending machines. 

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N1 surface concentrations determined from ESCA are plotted as a function of bulk N1 content in Figures 1 and 2. In the case of homogeneous alloys the points should fall on the 45 diagonal line. It can be seen that In both (N1 SI ) and (N1 Th ) series the surfaces of the alloys are nickel-poor, Ss compared to tHe bulk. Similar observations have been made In the case of N1 A1 (11,12) and Co Th (13) alloys. Surface enrichment In Si or tS i2 to be expected be cause of the higher heats of formation of S10 and ThO, compared to NiO (-210, -292, and -58.4 kcal/mol, respectively). This would lead to a higher chemical affinity of SI and Th toward the ambient gas and consequently an Increased driving force of SI and Th for segregation. 

Protons are not the sole species that can be incorporated into the lattices of different host materials. At the beginning of the 1960s, Boris N. Kabanov showed that during cathodic polarization of different metals in alkaline solutions, intercalation of atoms of the corresponding alkali metal is possible. As a result of such an electrochemical intercalation, either homogeneous alloys are formed (solid solutions) or heterogeneous polyphase systems, or even intermetallic compounds, are formed. 

The overall compositions of films 2 and 4 was such that homogeneous alloys were not expected, and the X-ray diffraction peaks showed evidence of phase-separation with two maxima corresponding to the compositions recorded in column 8, Table II. When palladium was deposited on top (and the film was rather light) then the apparent surface composition after annealing was 10% Rh and X-ray diffraction indicated a phase also containing 10% Rh (film 2). This convenient result was not observed when the order of deposition was reversed (film 4). These differences between apparent surface composition and the overall composition of the homogeneous alloy (or one of the phases in the miscibility gap) are discussed in Moss and Gibbens (34), with further examples. The main point to be made here is the rather variable nature of the surface composition compared with that expected, due to the operation of a number of factors. 

Composition range 80-100% Rh. At this end of the composition range, reasonably homogeneous alloys were again formed, and for present purposes it is unnecessary to reiterate the comments made on the activity of these alloys and about pure rhodium (73). 

Fig. 28. Results in Fig. 27 replotted using apparent surface compositions derived from X-ray data where phase separation was detected (A) or assumed for pure metals and homogeneous alloys, ( ) (73). 

Fig. 29. CO oxidation at 240°C over Pd-Rh alloy films as a function of apparent surface composition homogeneous alloys and pure metals ( ) alloys showing phase separation (3) (34).

Ruthenium and copper are not miscible hence, homogeneous alloy particles will not be formed in supported Ru-Cu catalysts. As copper has a smaller surface free energy than ruthenium, we expect that if the two metals are present in one particle, copper will be at the surface and ruthenium in the interior (see also Appendix 1). This is indeed what chemisorption experiments and catalytic tests suggest [40], EXAFS, being a probe for local structure, is of particular interest here because it investigates the environment of both Ru and Cu in the catalysts. 

Let us first consider what EXAFS might tell us in the case of bimetallic particles that are not too small - say a few nanometer in diameter. For a truly homogeneous alloy with a 50 50 composition, EXAFS should see a coordination shell of nearest neighbors with 50% Cu and 50% Ru around both ruthenium and copper atoms. If, on the other hand, the particle consists of an Ru core surrounded by a Cu shell of monatomic thickness, we expect that the Ru EXAFS shows Ru as the dominant neighbor, because only Ru atoms in the layer directly below the surface are in contact with Cu. The Cu EXAFS should see both Cu neighbors in the surface and Ru neighbors from the layer underneath, with a total coordination number smaller than that of the Ru atoms. The latter situation is indeed observed in Ru-Cu/Si02 catalysts, as we shall see below. 

Using this method, homogeneous alloys, segregated alloys, layered bi-metallics, and decorated particles are all readily accessible. An obvious advantage of the precursor concept over the conventional salt-impregnation method is that both the size and the composition of the colloidal metal precursors may be tailored independent of the support. Further, the metal particle surface may be modified by lipophilic or hydrophilic protective shells and coated by intermediate layers, e.g., of oxide. The modification of the precursor by dopants is also possible. 

Alloys were prepared from metals of 99.9% purity by arc melting on a water-cooled copper hearth under an argon atmosphere. The alloys were homogenized at 800° C. Diffraction patterns of cast material were equally as sharp as those of homogenized alloys. X-ray diffraction patterns were taken with filtered FeKa radiation. Computer programs verified x-ray pattern indexes. 

In this particular case, there is no transport of component B towards the surface. BO is homogeneously precipitated in the region < F, and the BO fraction corresponds to the concentration of B in the initially homogeneous alloy. Although the BO fraction is spatially constant in this case, the size of the BO particles is not. The increase in supersaturation becomes slower as the reaction front F advances. Thus, the number of precipitating particles becomes smaller with increasing time and, consequently, their volumes become larger since the local product of number times volume remains constant. 

Pure iron is relatively flexible and malleable, but the carbon atoms make cast iron very hard and brittle. It is used for objects that experience little mechanical and thermal shock, such as ornamental railings, engine blocks, brake drums, and transmission housings. Steel is a homogeneous alloy, a solid solution of 2% or less carbon in iron. 

Range 1 x2 < x < 1. Equilibrium is established after all the nickel has been dissolved. A homogeneous alloy, rich in copper, is formed and the concentration of either metal at the surface is equal to the surface enrichment proper to the concentration of the metal in the segment (see Section II, B). 

Range 3 0 < x < xv A homogeneous alloy is formed, containing more than 95% nickel. 

Figure 6 shows, however, that alloys of CdSex Te x can have optical bandgaps comparable to that of CdTe, even if x>0.5 (10). CdSe and CdTe form homogeneous alloys over the whole composition... 

An important consideration for the electronics of semiconductor/metal supported catalysts is that the work function of metals as a rule is smaller than that of semiconductors. As a consequence, before contact the Fermi level in the metal is higher than that in the semiconductor. After contact electrons pass from the metal to the semiconductor, and the semiconductor s bands are bent downward in a thin boundary layer, the space charge region. In this region the conduction band approaches the Fermi level this situation tends to favor acceptor reactions and slow down donor reactions. This concept can be tested by two methods. One is the variation of the thickness of a catalyst layer. Since the bands are bent only within a boundary layer of perhaps 10-5 to 10 6 cm in width, a variation of the catalyst layer thickness or particle size should result in variations of the activation energy and the rate of the catalyzed reaction. A second test consists in a variation of the work function of the metallic support, which is easily possible by preparing homogeneous alloys with additive metals that are either electron-rich or electron-poor relative to the main support metal. 

Homogeneous alloys have a single glass transition temperature which is determined by the ratio of the components. The physical properties of these alloys are averages based on the composition of the alloy. Heterogeneous alloys can be formed when graft or block copolymers are combined with a compatible polymer. Alloys of incompatible polymers can be formed if an interfacial agent can be found. 

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