Equilibrium codeposition

Using specific metal combinations, electrodeposited alloys can be made to exhibit hardening as a result of heat treatment subsequent to deposition. This, it should be noted, causes solid precipitation. When alloys such as Cu-Ag, Cu-Pb, and Cu-Ni are coelectrodeposited within the limits of diffusion currents, equilibrium solutions or supersaturated solid solutions are in evidence, as observed by x-rays. The actual type of deposit can, for instance, be determined by the work value of nucleus formation under the overpotential conditions of the more electronegative metal. When the metals are codeposited at low polarization values, formation of solid solutions or of supersaturated solid solutions results. This is so even when the metals are not mutually soluble in the solid state according to the phase diagram. Codeposition at high polarization values, on the other hand, results, as a rule, in two-phase alloys even with systems capable of forming a continuous series of solid solutions. 

Recently, two types of surfaces with coadsorbed cesium and oxygen have shown promise for low work function operation in thermionic converters. The first type of surface is made by codeposition of a "thick layer" ( 30 A) of cesium. As shown in Figure 13 ij), if the proportions of cesium and oxygen are properly controlled, a work function as low as 1.0 ev can be obtained. The substrate material does not affect the low obtained with a thick Cs - 0 layer. This type of surface could potentially be maintained in a thermionic converter by an equilibrium mixture of cesium, oxygen, and cesium oxide. Experiments to demonstrate this in operating diodes are underway ( ). 

It is possible to extend the feasibility of codeposition of two metals to cases where this does not look possible owing to a very large difference in the equilibrium potentials of individual metals, A ( eq,l Ee q,2) (Fig. 27). 

The required reduction potential of Ga is higher than the oxidation potential of Fe electrode. The deposition potential is composed of the equilibrium reduction potentials (Ecq), the overpotential, and the ohmic potential drop (iRs) in the solution. The rest potential of the deposition bath solution is about 0.3 V. The applied potential (E) during codeposition of Cu-In-Ga-Se using Fe electrode is (0.447 + rest potential), which probably make the Ga deposition possible. 

Classification of different types of alloy electrodeposition was made by Brenner [3] in 1962, by defining five groups equilibrium, irregular, regular, anomalous, and induced codeposition. More detailed explanations including samples for each type were given in Ref. [5]. 

The assumption of exclusive deposition on to the surface of the like metal applies, of course, only as a limiting case. In practice, some codeposition at the molecular level is expected. Indeed, in some systems which show immisci-bility in the solid phase under equilibrium conditions (e.g., Cu-Pb and Cu-Tl), there is evidence that, in an electrodeposited layer, each phase contains some concentration of the other component. Thus, reduction of the less noble component occurs at more positive potentials than those estimated on the basis of standard potentials of the pure metal. The potential shifts can be of the order of several tens of millivolts, and can be explained by the crystallites of the eutectic being supersaturated solid solutions of one component in the... 

Though alloy deposition is subject to the same scientific principles as individual metal plating, thermodynamics and kinetics of codeposition processes are more complicated than that of the deposition of a single metal. Electrochemical deposition of more than one metal often results in formation of different microstructures and phases. Moreover, electrodeposited binary alloys may or may not be the same in phase structure as those formed by simple melting. Besides, electrodeposited phases are not always in the thermodynamic equilibrium. 

Favorable conditions for codeposition oftwo metals, Mj and M2 (Mj is assumed to be more noble), are usually achieved when their equilibrium potentials are close. Both and M2 IM couples acquire the same equilibrium potential... 

Most of regularities that are typical of partial processes persist in the case of codeposition of Cu and Sn. Typical cathodic voltammograms are shown in Figure 9.35. Several regions corresponding to different electrochemical processes may be distinguished. Cu(II) reduction starts at its equilibrium potential equal to 0.24 V. It follows from Nernst equation that similar characteristic of Sn Sn + electrode is -0.24 V therefore, the phase consisting of free tin should be stable at more... 

Panicker, et al. (53), investigated the cathodic codeposition of cadmium and tellurium to form CdTe layers. This paper analyzed the deposition reactions in light of the existence range of CdTe and the equilibrium potentials developed in this range. Under all conditions cadmium is the potential-determining species and the deposition potential is given by ... 

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