Current density codeposition

In addition, EC-ALE offers a way of better understanding compound electrodeposition, a way of breaking it down into its component pieces. It allows compound electrodeposition to be deconvolved into a series of individually controllable steps, resulting in an opportunity to learn more about the mechanisms, and gain a series of new control points for electrodeposition. The main problem with codeposition is that the only control points are the solution composition and the deposition potential, or current density, in most cases. In an EC-ALE process, each reactant has its own solution and deposition potential, and there are generally rinse solutions as well. Each solution can be separately optimized, so that the pH, electrolyte, and additives or complexing agents are tailored to fit the precursor. On the other hand, the solution used in codeposition is a compromise, required to be compatible with all reactants. 

Bath Composition Particle Size and Crystal Phase Particle Loading (g/1) Current Density (mA/cm2) Analytical Method Codeposition Results ... 

It was not until 1987, before a second model on electrocodeposition was published by Buelens [37, 58], From experimental observations on the codeposition of particles on a rotating disk electrode (RDE) as a function of current density, rotation speed and bath composition, that could not be explained by Guglielmi, she suggested that a particle will only be incorporated into the deposit if a certain amount of the adsorbed ions on the particle surface is reduced. This is one possible way to account for the field-assisted adsorption, held responsible for the transition between loosely and strongly adsorbed particles in the model of Guglielmi. This proposition yields the probability P(k/K,i) for the incorporation of a particle based on the reduction of k out of K ions, bound to its surface, at current density i... 

Another critique on Valdes thesis is the authors claim to predict a codeposition maxima. Closer inspection reveals that the predicted codeposition maxima lie close to the limiting current density, which contradicts most experimental evidence. 

Cu, In, Ga, and Se are codeposited from the solution at room temperature in a three-electrode cell configuration, where the reference electrode is a platinum pseudo-reference, the counter electrode is platinum gauze, and the working electrode is the substrate. The substrates typically used are glass, DC-sputtered with about 1 pm of Mo. In all experiments, the applied potential is -1.0V versus the Pt pseudo-reference electrode. The corresponding current density range for the deposition is 5 to 7 mA/cm2. 

The methodology most practiced is referred to here as codeposition, where a single solution contains precursors for all the elements being deposited and is reduced at a fixed potential or current density. The earliest report appears to be that by Gobrecht et al., which was published in 1963 [45]. Two anodes were used in the study, one of Se and one of Cd (or Ag), to form selenite and cadmium ions, respectively. CdSe was then formed by co-reduction of both species at the cathode. Reports of the formation of GaP in 1968 [46] and ZnSe in 1975 [47] via codeposition were subsequently published, and both involved molten salt electrolysis. 

To express the preceding in a different, more specific way, we state that codeposition of two or more metals is possible under suitable conditions of potential and polarization. The necessary condition for simultaneous deposition of two or more metals is that the cathode potential-current density curves (polarization curves) be similar and close together. 

The individual polarization curves for the metals are often modified as a result of interactions resulting from codeposition. If the alloy deposition occurs at low polarization, the nobler metal will be deposited preferentially (Cu in Example 11.1). All factors, however, that increase polarization during electrodeposition, such as high current density, low temperature, and quiescent solution—factors that increase concentration polarization—will favor the deposition of the less noble metal (Zn in Example 11.1). 

An increase in current density tends to increase the proportion of the less noble metal in the alloy deposit. The extent of such change may be expected to be greater in the case of simple primary salt solutions than in complex primary salt solutions, and still more so when the codepositing metals are present in complex ions with a common anion than when the anions of the complex ions are different. In cases where the metals are associated with different complexing ions, a significant change in current density can be accommodated with relatively little change in plate composition. 

It appears in this discussion that electrochemical parameters and not substrate properties are the main deciding factors in determining the texture of deposits. This is indeed so when a deposit s thickness is 1 pum or more. In case of thinner deposits, the substrate plays an important role as well (see the text above). Another nonelectrochem-ical factor may be the codeposition of particulate matter with some metal deposits. To summarize, we note that texture is influenced mostly by deposition current density, as it is itself a function of bath pH, potential, and other parameters. Not surprising, then, is the fact that in the case of physical vapor deposition (PVD), the deposition rate is the determining factor in setting the texture of the coating. 

The electrodeposition of an alloy requires, by definition, the codeposition of two or more metals. In other words, their ions must be present in an electrolyte that provides a cathode film where the individual deposition potentials can be made to be close or even the same. Figure 11.1 depicts typical polarization curves, that is, deposition potentials as a function of current density for two metals (A and B corresponding to curves A and B in Fig. 11.1) separately. From such curves it is possible... 

The deposition variables are the process parameters most suited to regulate the particle composite content within the limits set by the particle properties and plating bath composition. Particle bath concentration is the most obvious process variable to control particle codeposition. Within the limits set by the metal plating process and the practical feasibility also current density, bath agitation and temperature can be used to obtain a particular composite. Consequently the deposition process variables are the most extensively investigated parameters in composite plating. The models and mechanisms discussed in Section IV almost exclusively try to explain and model the relation between these process parameters and the particle codeposition rate. 

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