Metal deposition current density

Because the plating process produces a clean, bondable surface, freshly plated surfaces often do not require additional preparation. Keep in mind, however, that plating changes surface properties such as adhesion, porosity, and surface stress of the metal deposit. Current density, composition of the plating bath (including brightener content), and bath temperature affect the bondabiUty of the plated surface. 

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. 

Calculate the current density at which an alloy with 25 mol.% Cu and 75 mol.% Cd should be deposited, assuming that the deposition current density of the alloy is the sum of the deposition current density of individual metals. What is the electrode potential value at that current density [Take Z)(Cu2+) = D(Cd2+) = 5.00 x 10-10 m2 s1, u= lx 10-6 kg m-1 s-1, T = 298 K. The current... 

For growth under an excess of oxygen, an increase of the reactive gas partial pressure leads to a reduction in the deposition rate, which is independent of the substrate temperature. This results from the oxidization of the target, since the metallic particle current density j(Zn) is reduced by the low sputtering yield of the oxidized target. 

The initiation of dendritic growth is followed by an increase of the deposition current density, and the overall current density will be larger than the limiting diffusion current on a flat active electrode. Based on the above discussion, the polarization curve equation in the Ohmic-controlled electrodeposition of metals can be determined now by 9... 

Thus, instead of a limiting diffusion current density plateau, a curve inflection point or a short inclined plateau can be expected on the polarization curve in Ohmic-controlled electrodeposition of metals, as observed in the case of silver electrodeposition from nitrate solutions. The exchange current density of the silver reaction in nitrate electrolytes is sufficiently large to permit Ohmic-controlled deposition as well as dendritic growth at low overpotentials.27 After a linear increase of the deposition current density with increasing overpotential, an exponential increase after the inflection point appears, meaning the elimination of mass-transfer limitations due to the initiation of dendritic growth. 

The rate of the crystallization process of a pure metal differs for different crystallographic faces. For comparison, the net deposition current density can be split into partial current densities L... 

The growth follows the same principles as were discussed for pure metals. The rate of deposition of each component can be described by a partial deposition current density i. The total deposition rate is then the sum of all partial current densities (Wagner and Traud ), for a binary alloy... 

In this equation Cy is the volume percent of particles in the electrolyte, K a Langmuir-type adsorption constant, py, the density of the deposited metal (g cm ), Vq a system constant (cm s ), zF the Coulomb charge of the deposited metal ions (in C moL ), M the molar mass of the metal atoms (g mol ), i the deposition current density (A cm ), if) the exchange current density of metal deposition, and P another system constant. This equation starts with an adsorption isotherm and then compares an empirical weight... 

In this equation M is the molar mass of the metal, i the deposition current density of the metal, and zF the equivalent charge of the deposited metal ions. 

Hence, an increase in the surface coarseness can be expected with increasing quantity of deposited metal for the same deposition current density, as well as with increasing current density for the same quantity of electrodeposited metal. 

The deposition current density of metal ions must be close to the end of the Tafel linearity, i.e., it can be treated as the current density in activation-controlled deposition, being independent on the geometry of the system. Hence, the current density at the tip of a protrusion will be equal to the current density on the flat surface in the absence of additive. It should be noted that under such condition geometric leveling occurs, but true leveling requires the presence of an additive. If the additive is consumed at the electrode by the reaction... 

Figure 16.1.7 Scanning electron micrographs of metal particles prepared using the slow growth method. The composition of the plating solutions employed for the electrodeposition of these metal particles is listed in Table 16.1.1. The deposition current density observed in each experiment was as follows (MoOj) 180-140 pA cm , (Cd) 40-60 pA cm , (Cu) 40-60 pA cm , (Ni) 240-260 pAcm , (Au) 30-40 pAcm, and (Pt) 5-100 pAcm". Reprinted with permission of the American Chemical Society and Elsevier.

During metal electtodeposition, for a given value of overpotential, the corresponding cathodic current density [A m ] is observed which can be correlated to the deposition flux [mol m s ] using the expression, flux = j/nF. The relation between the deposition current density j) and electrode surface overpotential JjsfU] is described by Butler V /rwer equation defined below ... 

Much attention, particularly among the Russian workers, has been given to the applicability of the Heyrovsky-Ilkovic versus the Kolthoff-Lingane equation for electrode reactions involving metal deposition. In practice, one plots the electrode potential versus log[(/i — /)//] or log(/ — i) and examines the linearity of the plot and also the value of the slope (theoretically 2.3RTjnF). It is expected that the Heyrovsky-Ilkovic equation should be applicable when alloy formation with the electrode material takes place and the metal formed diffuses away from the surface so that its surface activity is a function of the current density. Alloying and diffusion in the electrode will be functions of the metal deposited, electrode material, temperature, and the rate of deposition (current density) therefore, comparison is difficult or meaningless if several of these variables are varied simultaneously. 

Electrolytic plating rates ate controUed by the current density at the metal—solution interface. The current distribution on a complex part is never uniform, and this can lead to large differences in plating rate and deposit thickness over the part surface. Uniform plating of blind holes, re-entrant cavities, and long projections is especiaUy difficult. 

Electroless plating rates ate affected by the rate of reduction of the dissolved reducing agent and the dissolved metal ion which diffuse to the catalytic surface of the object being plated. When an initial continuous metal film is deposited, the whole surface is at one potential determined by the mixed potential of the system (17). The current density is the same everywhere on the surface as long as flow and diffusion are unrestricted so the metal... 

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