Electrodeposition mass transport

Moffat [80] reported the electrodeposition of Ni-Al alloy from solutions of Ni(II) in the 66.7 m/o AlCl3-NaCl melt at 150 °C. The results obtained in this melt system are very similar to those found in the AlCh-EtMcImCI melt. For example, Ni deposits at the mass-transport-limited rate during the co-deposition of Al, and the co-deposition of Al commences several hundred millivolts positive of the thermodynamic potential for the A1(III)/A1 couple. A significant difference between the voltammetric-derived compositions from the AlCl3-NaCl melt and AlCl3-EtMeImCl melt is that alloy composition is independent of Ni(II) concentration at the elevated temperature. Similar to what has been observed for room-temperature Cu-Al, the rate of the aluminum partial reaction is first order in the Ni(II) concentration. Moffat s... 

In this chapter we derive the Butler-Vohner equation for the current-potential relationship, describe techniques for the study of electrode processes, discuss the influence of mass transport on electrode kinetics, and present atomistic aspects of electrodeposition of metals. 

Dendritic. In electrodeposited films, dendritic grains result from mass-transport-controlled growth, and the individual crystals may vary in shape. 

Tellurium is a constituent common to several definite compounds having semiconducting properties which can be obtained by electrolytic deposition (e.g. CdTe, ZnTe,...). The low solubility of tellurium oxide in acidic aqueous solutions explains why its kinetics of electrodeposition, in the binary of tertiary alloys involved, is mainly controlled by mass transport. 

They proposed hence to apply this technique for improving the electrodeposition of composition modulated alloys (CMA) [101]. They showed that combining potential and flow modulations it is possible to control the symmetry of concentration distribution of the element in the alloy which is electrodeposited under mass transport conditions [102],... 

The deposition of Cu, Sn, and Cd occurs at a potential of 0.5, 0.3 and 0.1 V, respectively, more positive than the deposition of Zn. In these studies samples of the alloys were prepared on Ni substrates by constant potential electrolysis and examined with EDX, SEM, and XRD. It was found that the Zn content in the electrodeposits increased as the deposition potential became more negative but decreased with increasing concentrations of Cu(II), Sn(II), and Cd(II) in the solution. Increasing the deposition temperature increases the mass-transport rates... 

As in any electrode process, the potential applied to the electrode determines the reaction rate. In electrodeposition, we expect that it affects the rate of deposition and thence the structure of the deposit a low overpotential signifies more time available to form an electrodeposit of perfectly crystalline structure. This can be observed experimentally (Fig. 15.7). Another factor arises from differences in current density between different parts of the electrode owing to electrode shape, which affects mass transport and thus accessibility to the cations to be deposited. Generally, it is best to apply a potential corresponding to the formation of poly crystalline deposits. A more perfect crystalline structure would be desirable, but the low rate of electrodeposition does not compensate for using such low overpotentials. 

O. Devos, C. Gabrielli, and B. Tribollet, "Nucleation-Growth Process of Scale Electrodeposition Influence of the Mass Transport," Electrochimica Acta, 52 (2006)285-291. 

Dendritic deposits grow under mass transport-controlled electrodeposition conditions. These conditions involve low concentration of electrolyte and high current density. A dendrite is a skeleton of a monocrystal consisting of stem and branches. The shapes of the dendrites are mainly determined by the directions of preferred growth in the lattice. The simplest dendrites consist of the stem and primary branches. The primary branches may develop secondary and tertiary branches. The angles between the stem and the branches, or between different branches, assume certain definite values in accordance with the space lattice. Thus, dendrites can be two dimensional (2D) or three dimensional (3D). 

At overpotentials larger than 175 my the current density is considerably larger than the one expected from the linear dependence of current on overpotential. The formation of dendritic deposits (Fig. 16d-f) confirms that the deposition was dominantly under activation control. Thus, the elimination of mass transport limitations in the Ohmic-controlled electrodeposition of metals is due to the initiation of dendritic growth at overpotentials close to that at which complete diffusion control of the process on the flat part of the electrode surface occurs. 

Convective mass transport from small cavities is relevant to through-mask electrodeposition and to localized corrosion and has hence received much attention. In localized corrosion, mass transport is important for determination of the local environment inside an active pit. For through-mask deposition, an understanding of mass transport may be important for design of mixing methods or for analysis of measured deposit profiles. The electrodeposition in circular or rectangular cavities formed by a photoresist has been studied by Kondo etal. it was found that the shapes of deposited bumps can be explained by calculations of vortex evolution and penetration flow. 

The key factors that control the rate of electrodeposition and the structure, physical properties, uniformity, and composition of electrodeposited metals and alloys are (1) thermodynamics (where the electric potential is based on the standard electromotive series) (2) electrode kinetics (which may vary with the structure of the electrodeposit) and (3) mass transport (which is important at high current densities, where the delivery of reactant to the cathode surface affects the local deposition rate and the structure of the deposit). 

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