Popov K.I., Djokic S.S., Grgur B.N. Fundamental Aspects of Electrometallurgy

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Chapter 2

It is obvious from Figs. 2.7c and 2.7a that after the formation of a low index

plane, two-dimensional nucleation is necessary for the growth to be continued.

In the case of a reentrant grove, Fig. 2.8, the growth of new layers can be

started by one-dimensional nucleation. In the case of a screw dislocation, the

step provokes the growth by retaking itself with one end fixed at the point

where the screw dislocation emerges.

If a crystal plane lacks steps and kinks, i.e., points of growth, or if growth

at these sites is sufficiently inhibited so that a large concentration of ad-ions

builds up compared to the equilibrium concentration, the probability

increases that new growing centers will form as two-dimensional nuclei. A

very convincing illustration of such a situation was made by Budevski et

al.

after a remarkable achievement of preparing metal surfaces free of any

dislocations. At constant current, polarizations much larger than those upon

2. Definitions, Principles and Concepts

ordinary planes of such metals were obtained

. Moreover, they were distin-

guished by periodic oscillations. These phenomena are ascribed to fluctua-

tions in the formation of two-dimensional nuclei. Under potentiostatic condi-

tions, a current can be observed on a dislocation-free surface only at over-

potentials exceeding 8 to 12 mV, whereas the cell is electrically cut off at

lower overpotentials. When a short voltage pulse in excess of this value is

applied to a cell in the cut off condition, supersaturation by ad-ions is

achieved and a nucleus of a new lattice net is formed. The propagation of the

produced step is accompanied by a certain current flow. When the new layer

has spread out over the whole surface the current again drops to zero, since

the steady-state potential is insufficient to form a new nucleus

The current-time curves for a series of successive voltage pulses vary in

form, but the integral of the current over time has the same value in all cases.

The amount of electricity given by this integral corresponds exactly to the

amount required for the completion of a monoatomic layer over the cubic

plane

11,13

The growth rates in the cases of one-dimensional and two-dimensional

nucleation as rate determining steps can be compared to each other by consi-

dering the growth of two dimensional flat cadmium dendrites from Fig 2.9.

The tip of the twined cadmium dendrite precursor from Fig. 2.9a

represents the physical equivalent of the scheme of the growth site from Fig.

2.8. As shown in Fig. 2.8 an layer of atoms advance in the direction deter-

mined by twining laws, an edge is constantly renewed, in which the new

Chapter 2

layers can be started by one-dimensional nucleation. Further growth and

branching of precursor like that from Fig. 2.9a produces the dendrites shown

in Figs. 2.9b and 2.9c. The deposition on the lateral flat dendrite surfaces

takes place by repeated two-dimensional nucleation, as in earlier described

deposition on dislocation free surface. This makes the deposition rate in the

direction of tip motion many times larger, which results in dendrite shape

like that from Fig. 2.9c.

Let it be supposed now, that an advancing microstep suddenly stops

advancing. The movement may cease, e.g., owing to the adsorption of

impurities from the solution at the step. On a solid surface with its hierarchy

of sites, there will be a hierarchy of free energies of adsorption and it may

occur that impurities seek adsorption at steps in preference to adsorption on

flat planes.

So think of a microstep which, for a reason such that given above, has

stopped advancing somewhere within the boundaries of the crystal (Fig.

2.10). Now imagine that a layer B of atoms is growing on top of the layer A.

The step B will keep advancing until it comes to the point where the advance

of step A was blocked. The layer B will then act as though it has reached the

edge of the crystal. If the same process is repeated with another layer C on

top of layer B, and then another layer on top of layer C, and so on, then there

is a pile-up of layer upon layer. Microsteps bunch into macrosteps and

sometimes the pile-up reaches such proportions that it can be seen under a

microscope as a macrostep.

2. Definitions, Principles and Concepts

The macrosteps can be clearly seen on the lateral surfaces of flat

cadmium dendrites as shown in Fig. 2.11.

The flat dendrite lateral surfaces behaves as monocrystals ones and the

questions arises: How valid is the picture of deposition, developed above,

when the electrodes are polycrystalline metals? The answer is simple. One

Chapter 2

can consider the surface exposed to the solution by each grain as a single-

crystal macrosubstrates. That is, one would have charge transfer followed by

surface diffusion, transfer to steps, then to kinks. etc., and one would also

have rotating addition, however steps, resulting from screw dislocation,

growth spirals, faceting, etc. In addition, however, at the grain boundaries

where the single-crystal microsubstrates meet and the periodic atomic

arrangement of each grain is interrupted, the deposition and growth

processes will be abnormal. But the actual area of an electrode surface

occupied by the grain boundaries is so negligible that the abnormal processes

occurring there can be largely ignored. In conclusion, therefore, the basic

picture of deposition and growth developed for single crystals is valid as a

basis for understanding the electrogrowth of polycrystals

2.5 CONCLUSIONS

In this chapter some basic definitions, principles and concepts necessary

for understanding of the following chapters have been treated. Assuming that

riders have some electrochemical knowledge, for all necessary questions we

strongly recommends further reading of excellent books of J. O’M Bockris

and A. K.N. Reddy, Modern Electrochemistry.

2.6 FURTHER READINGS

Bockris, O’M John; Reddy, K.N. Amulya, Modern Electrochemistry, Vol 1, New York:

Plenum Press, 1970.

Bockris, O’M John; Reddy, K.N. Amulya, Modern Electrochemistry, Vol 1, second

edition, New York: Plenum Press, 1998.

Bockris, O’M John; Reddy, K.N. Amulya, Modern Electrochemistry, Vol 2, New York:

Plenum Press, 1970.

Bockris, O’M John; “Electrode kinetics”. In Modern Aspects of electrochemistry, Vol 1,

John O’M Bockris, Brian E. Conway, eds. London, Butterworths, 1954.

Diggle J.W., A.R., Bockris J.O’M. The mechanism of the dendritic

electrocrystallization of zinc. J. Electrochem. Soc. 1969; 116:1503-14

Barton J.L., Bockris J.O’M. The electrolytic growth of dendrites from ionic solutions.

Proc. Roy. Soc. London 1962; A268:485-505

Aleksandar; Popov, Konstantin I., “Transport Controlled Deposition and

Dissolution of Metals.” In Modern Aspects of Electrochemistry, Vol. 7, Brian E.

Conway, John O’M. Bockris, eds. New York, NY: Plenum Press, 1972.

Popov, Konstantin I., Miomir, “Electrodeposition of Metal Powder with

Controlled Grain Size and Morphology”. In Modem Aspects of Electrochemistry, Vol.

24, Ralph E. White, John O'M. Bockris, Brian E. Conway, eds., New York, NY, Plenum

Press, 1993.

Popov, K.I., E.R., V., M.G., Morphology of lead

dendrites electrodeposited by square-wave pulsating overpotential, Powder Technology,

1997; 93:55-67.

2. Definitions, Principles and Concepts

10.

11.

12.

13.

14.

Nickols R.J., Kolb D.M., Belum R.J. STM observations of the initial stages of copper

deposition on gold single crystal electrodes. J. Electroanal. Chem. 1991; 313:109-19;

Nickols, J. Richard,. “Imagining Metal electrocrystallization at high resolution” In

Imagining of Surfaces and Interfaces, Series: Frontiers in Electrochemistry, Vol IV,

Philip N. Ross; Jacek Lipkovski, eds, New York: VCH Publ. Inc., 1997.

Bockris, O’M John, Razumney, G. A, Fundamental Aspects of Electrocrystalization,

New York: Plenum Press, 1967.

Budevski, Evgeni; Georgi, Stoikov; Lorenz, Wolfgang, Electrochemical Phase

Formation and Growth, Weinheim: VCH, 1996.

Budevski E., Bostanov W., Vitanov T., Stojnov Z., Kotzeva A., Kaishev R., Phys.

Status. Sol. 1966; 13:577-88

Popov K.I., M.I. Dendritic electrocrystallization of cadmium from acid

sulphate solution II: The effect of the geometry on dendrite precursors on the shape of

dendrites. Surf. Coat. Technol. 1989; 37:435-40.

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Chapter 3

SURFACE MORPHOLOGY OF METAL

ELECTRODEPOSIT

Morphology is probably the most important property of electrodeposited

metals. It depends mainly on the kinetic parameters of the deposition process

and on the deposition overpotential or current density. The morphology of an

electrodeposited metal depends also on the deposition time until the deposit

has attained its final form. When the electrodeposition is from pure simple or

complex salt solutions, the form of the metal electrodeposit can be:

compact,

dendritic,

spongy, granular (grains, boulders), etc.

There are three main cases which are divided according to the exchange

current density of deposition processes.

Deposition processes which are characterized by very large exchange

current densities. Boulders are formed at lower overpotentials and dendrites at

higher ones. In the limiting case, dendrites grow at practically all overpoten-

tials in the electrodeposition of metals with low melting points (e.g. Sn, Pb),

deposition processes which are characterized by large exchange current

densities. Spongy-deposits appear at lower and dendrites at larger

overpotentials,

deposition processes which are characterized by medium and low

exchange current densities. Compact deposits are obtained at lower and

dendritic and spongy-dendritic deposits at larger deposition overpotentials.

On the other hand, it is known that the morphology of electrodeposits can

be substantially changed if the electrodeposition is carried out in the

presence of organic or inorganic additives. For example, a smooth deposit

can be obtained in the presence of additives instead of rough one in the

absence of additives.

Chapter 3

The processes of metal electrodeposition can be categorized into three

main groups:

electroplating and electroforming

electrorefining and electrowinning, and

metal powder production

each of which have different requirements with respect to the physical state

of the cathodic product.

In electroplating the crystal layer is required to be fine grained, smooth,

strongly adhesive, glassy, i.e. to be easily polished or bright. In refining and

electrowinning relatively coarse grained, rough, but adhesive deposits are

required. They have to be of high purity and firm enough to endure handling

before melting and casting into shapes suitable for further processing. In

metal powder production by electrodeposition a controlled product particle

size is necessary and it is preferable if the product is only weakly adhesive.

The first kind of deposit can be obtained from solutions characterised by

low exchange current densities at overpotentials close to the end of the Tafel

linearity in the presence of different organic additives, with different effects

on the cathode deposit.

The deposits in electrorefining and electrowinning are obtained from

electrolytes characterised by medium exchange current density at

overpotentials close to the end of Tafel linearity. Powdered electrodeposits

are obtained from the same solutions, but at current densities considerably

larger than the limiting diffusion current density for the electrodeposition

process.

Spongy and granular electrodeposits appear during deposition by

processes characterised by large exchange current densities at low overpo-

tentials. They are not often met in electrodeposition practice. Spongy zinc

can be formed during the charging cycle of same alkaline storage batteries,

and can be easily removed by electrodeposition at a periodically charging

rate, as can the whiskers which can be formed in some plating baths in the

presence of some organic additives.

The above facts are well known, but there is no complete analysis of

them so far. The main aim of this chapter is to remedy this. This analysis is

performed for electrodeposition of metals from pure simple and complex salt

solutions at a constant rate in the presence or in the absence of additives.

3.1

THIN COMPACT SURFACE METAL FILMS

The first stage of metal electrodeposition on an inert substrate is the

formation of a thin surface film of deposited metal. It isolates the initial

substrate from the solution, and the aim of this section is to show how this

can be achieved using a minimum quantity of electrodeposit.

3. Surface Morphology of Metal Electrodeposits

3.1.1

Crystallization overpotential

The formation of the first crystals during galvanostatic metal electro-

deposition on an inert substrate is sometimes accompanied by a pronounced

increase in the overpotential

1,2

. The dependence of the overpotential on time

in such situations is shown in Fig. 3.1.

The overpotential changes with deposition time from point b to point c

according to the equation

where is the concentration of adatoms at time t and is the

concentration of adatoms at t = 0.

On the other hand, the surface concentration of adatoms changes

according to: