Overpotential deposition characterization

It is well known [57] that dendrites are not formed at overpotentials of electrodeposition lower than a critical overpotential for dendritic growth initiation, t/j, and that powdered deposits characterized by a very large surface area are formed at overpotentials higher than some critical value, rjc-... 

Anyway, increasing overpotential leads to the formation of a more disperse deposit characterized by the decreased particle size. This can be explained by the fact that increasing overpotential leads to the decrease of the height of protrusion, / o,i, at which dendrites start to... 

As in the cases discussed above, one can define a resistance characterizing the mass-transport limitation and a corresponding concentration overpotential conc The value of this parameter, compared to the Faradaic resistance, determines the brightness and smoothness of the deposit, but has little influence on the uniformity of the thickness, since the characteristic length associated with it (ca. 0.01 cm) is... 

At values of ZoAl ratio lower than 1, the complete diffusion cOTitrol of the electrodeposition process arises at aU overpotentials. The lower limit of the region of the complete diffusion control can be determined as follows it is obvious that the convex shape of the polarization curve characterizes the diffusion control of deposition process and the concave one the activation control of deposition process. The Z/Zl ratio as function of tj is shown in Fig. 1.3 and the Z as a function of in Fig. 1.4. In both cases, the convex shape of curves changes in the concave one at approximately Zo/ l 01> meaning that the diffusion control changes in the activation one at the beginning of the polarization curve at low rj and At larger overpotentials, the diffusion control occurs. Hence, the diffusion ccaitrol at aU overpotentials appears at 0.1 activation control appears at Zq/Zl< 0.1 at low overpotentials. 

The crystallization overpotential strongly decreases with increasing ///q ratio. As a result of this, it can be measured only in the case of a metal deposition which is characterized by very high values of the exchange current density [48]. 

It is obvious that the larger nucleus density, the thinner is the thickness of the metal film required to isolate the substrate from the solution. At the same time, a thinner surface film will be less coarse than a thicker one. This means that a smoother and thiimer surface film will be obtained at larger deposition overpotentials and nucleation rates, i.e., by electrodeposition processes characterized by high cathodic Tafel slopes and low exchange current densities. 

The overpotential-log (current density) plot is given in Fig. 2.24. A well-defined Tafel line characterized by /q 10 A cm and be = 160 mV dec was observed at higher potentials also. This phenomenon is explained by the formation of a film of the organic additive which completely covers the cathode at sufficiently negative potentials [69, 70]. Tafel linearity was also observed over a short overpotential range at low overpotentials. The values of /q 10 A cm and be = 60 mV dec obtained in this case are close to the values expected for deposition from a pure solution [71]. 

Equation (2.121) describes the linear part of the polarization curves for tin [109], silver [1, 6], and lead [34, 35, 110, 111] deposition, and Eq. (2.122) foresees the inflection point in the cases when rj is low and the resistance of electrolyte is large. Finally, Eq. (2.123) describes the part of the polarization curve after the inflection point characterized by the fast increase of current density with increasing the overpotential (see Chap. 1 Sect. 1.3.1.1). 

In systems characterized with low exchange current density values, electrodeposition process enters the full diffusion control at sufficiently large overpotentials [13], On the other hand, if condition 0.1 < 1 is fulfilled, deposition wiU be... 

The first term in Eq. (1.31) corresponds to the activation part of deposition overpotential, and the second one is due to the mass transfer limitations. If one and the same process occurs under two different hydrodynamic conditions, characterized by two different values of the limiting diffusion current densities Il i and Jl,2> the Eq. (1.31) can be rewritten in the forms ... 

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