Cu-Ag alloys

Bimetallic nanoparticles, either as alloys or as core-shell structures, exhibit unique electronic, optical and catalytic properties compared to pure metallic nanopartides [24]. Cu-Ag alloy nanoparticles were obtained through the simultaneous reduction of copper and silver ions again in aqueous starch matrix. The optical properties of these alloy nanopartides vary with their composition, which is seen from the digital photographs in Fig. 8. The formation of alloy was confirmed by single SP maxima which varied depending on the composition of the alloy. 

As alluded to before, the adsorption of atoms and molecules may also induce segregation in alloys. Upon revisiting the thermodynamic behavior of the improved Cu-Ag alloy catalysts for ethylene epoxidation synthesized by Linic et al, (section 2.1) Piccinin et al. calculated that, while in the absence of oxygen Cu prefers to stay in the subsurface layers, oxygen adsorption causes it to segregate to the surface which then phase-separates into clean Ag(lll) and various Cu surface oxides under typical industrial conditions (Fig. 7). This casts doubt on the active state of the previous Cu-Ag catalysts being a well-mixed surface Ag-Cu alloy. 

Temperatures in the 270-290°C range, a pressure of about ambient, and a 1-second contact time give conversions to ethylene oxide of about 60%. The ethylene oxide is absorbed in water from the reactor exit gases, and the aqueous solution is then fractionated for ethylene oxide (b.p. 13.5°C) recovery. The nonabsorbed ethylene and the absorption water recovered from the bottom of the ethylene oxide fractionator are both recycled. Recently use of a Cu/Ag alloy catalyst has shown greater selectivity for ethylene oxide in this process [18]. 

The studies on the oxametallacycle intermediate have led to theoretical work with DFT that predicts that Cu-Ag alloys will have enhanced activity [342] (see Chapter 8), and this has been experimentally verified [323,343], validating the initial reports by Lefort [320,321]. 

Fig. 17. Dehydrogenation of formic acid on Cu-Ag alloys, according to Schwab and Schwab-Agallidis (20), and according to Rienacker. —Rienacker, static decomposition O—Schwab, static decomposition A—Schwab, dynamic decomposition.

Au-Cu-Ag alloys based on the inter-metallic phases CuAu and CujAu have found applications in dentistry because of their extremely high corrosion resistance, their advantageous mechanical properties such as high strength and ductility, and their decorative gold color (Yasuda, 1991). These alloys age-harden as a result of complex ordering and decomposition reactions by which the phases CujAu I, CuAu I, CuAu II, and an Ag-rich tXj phase are formed, depending on the composition. 

Some metals are soluble as atomic species in molten silicates, the most quantitative studies having been made with Ca0-Si02-Al203(37, 26, 27 mole per cent respectively). The results at 1800 K gave solubilities of 0.055, 0.16, 0.001 and 0.101 for the pure metals Cu, Ag, Au and Pb. When these metal solubilities were compared for metal alloys which produced 1 mm Hg pressure of each of these elements at this temperature, it was found drat the solubility decreases as the atomic radius increases, i.e. when die difference in vapour pressure of die pure metals is removed by alloy formation. If the solution was subjected to a temperature cycle of about 20 K around the control temperamre, the copper solution precipitated copper particles which grew with time. Thus the liquid metal drops, once precipitated, remained stable thereafter. 

Niemi, L., Minni, E. and Ivaska, A. An Electrochemical and Multispectroscopic Study of Corrosion of Ag-Pd-Cu-Au Alloys , Journal of Dental Research, 65, 888-891 (1986)... 

Hultquist, G. and Her0, H. Surface Enoblement by Dissolution of Cu, Ag and Zn from Single Phase Gold Alloys , Corrosion Science, 24, 789-805 (1984)... 

When a pure metal A is alloyed with a small amount of element B, the result is ideally a homogeneous random mixture of the two atomic species A and B, which is known as a solid solution of in 4. The solute B atoms may take up either interstitial or substitutional positions with respect to the solvent atoms A, as illustrated in Figs. 20.37a and b, respectively. Interstitial solid solutions are only formed with solute atoms that are much smaller than the solvent atoms, as is obvious from Fig. 20.37a for the purpose of this section only three interstitial solid solutions are of importance, i.e. Fc-C, Fe-N and Fe-H. On the other hand, the solid solutions formed between two metals, as for example in Cu-Ag and Cu-Ni alloys, are always substitutional (Fig. 20.376). Occasionally, substitutional solid solutions are formed in which the... 

Lithium has been alloyed with gaUium and small amounts of valence-electron poorer elements Cu, Ag, Zn and Cd. like the early p-block elements (especially group 13), these elements are icosogen, a term which was coined by King for elements that can form icosahedron-based clusters [24]. In these combinations, the valence electron concentrations are reduced to such a degree that low-coordinated Ga atoms are no longer present, and icosahedral clustering prevails [25]. Periodic 3-D networks are formed from an icosahedron kernel and the icosahedral symmetry is extended within the boundary of a few shells. 

Figure 5. Schematic structure (left) of a ternary alloy on Ru(0 0 1) surface, MPt2/Ru, with M selected from the set Fe, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au, Snj.The two bottom layers represent Ru atoms while Pt atoms form the top surface atoms. The predicted surface activities of various ternary alloys are shown in the plot (right). (Reprinted from Ref [102], 2003, with permission from American Chemical Society.)...

Dilute gold alloys with Cu, Ag, Ni, Pd, and Pt as absorbers Correlation of isomer shift with residual electrical resistivity, wave function at Fermi level, s--band population of gold... 

Au-Cu and Au-Ag alloys Isomer shift and electrical resistivity as function of alloy composition and, in CU3AU, of pressure model to describe 5 in terms of average atomic volume, of short-range parameter and alloy composition average charge density on Au... 

Au-Cu and Au-Ag alloys Correlation of isomer shift with Au 5d population... 

According to the data in Table III, the value of the ratio P)Mm is approximately the same for the metals Au, Fe, Co, Ni, and Pd. Binary alloys formed from any pair of these metals can therefore be expected to evaporate without substantial fractionation. On the other hand, films evaporated from Ag-Pd and Cu-Ni alloys can be expected to be enriched in Ag and Cu, respectively. These predictions are largely confirmed by experiment. For example, the composition of Pd-Au films was found to be the same as the wires which were evaporated (46), but in the case of Pd-Ag, evaporation of a 30% Ag-Pd alloy ware yielded a 50% Ag-Pd alloy film (47)- Alexander and Russell evaporated a number of alloys from pellets in the reaction vessel as shown in Fig. 5 (48) The alloy pellet was placed in a small quartz cup with its surface equidistant from the hemispherical top of the reaction vessel. The pellet was evaporated by... 

The structure of a vapor-quenched alloy may be either crystalline, in which the periodicity of the unit cell is repeated within the crystallites, or amorphous, in which there is no translational periodicity even over a distance of several lattice spacings. Mader (64) has given the following criteria for the formation of an amorphous structure the equilibrium diagram must show limited terminal solubilities of the two components, and a size difference of greater than 10% should exist between the component atoms. A ball model simulation experiment has been used to illustrate the effects of size difference and rate of deposition on the structure of quench-cooled alloy films (68). Concentrated alloys of Cu-Ag (35-65%... 

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