50
2 Electrochemistry and Electrodeposition
alloy is formed, and the initial part of the composition diagram looks like for irregular codeposition. However, as the concentration of the induced metal increases in
the deposit, the current efficiency decreases, and a limiting deposit composition can
be seen. Finally, the current efficiency drops so much that no deposit is produced,
and the composition function line ends in the middle of the composition diagram.
Interestingly, these systems are the most prone to develop amorphous deposits as the
concentration of the oxoanion-forming element in the deposit increases.
2.12.3 Structural Consequences of Alloy Formation
Even if a metal pair exhibits an equilibrium alloy, there is no guarantee that a particular
alloy can be synthesized by electrochemical codeposition. The general trend is that
the closer the deposition potentials of the constituents are, the easier to deposit a real
alloy with no segregation but with miscibility at the atomic scale. The miscibility
at the atomic scale is often promoted by the application of a complexing agent that
makes a more stable complex with the ions of the more noble element to be deposited.
Hence, the difference in the deposition potentials is diminished, which is a favourable
factor in the tendency of codeposition.
Beside the deposition potential, the ordering and the unit cell size are also important factors in the electrolytic alloy formation. The larger the unit cell size and the
more ordered an alloy must be, the less likely that it can be synthesized by direct
electroplating without an annealing step afterwards. Obviously, alloys with a random
atomic arrangement tend to form the most easily, in accord with the non-equilibrium
nature of the electrodeposition process. Another general trend is that the increase in
the number of components results in a decrease in grain size of the deposit.
Finally, it must be mentioned that phase mixtures are often deposited when the
composition of the deposit is close to an equilibrium phase transition composition of
the system even if the equilibrium phase transition takes place at a definite composition without a miscibility gap. In such cases, electrodeposition leads to a phase
mixture in a 10–15 atomic percent wide composition range. This is related to the
non-equilibrium nature of the electroplating method. The general experience is that
the formation range of a phase is extended with respect to its equilibrium stability
range. It is a direct consequence of the deposition of a phase mixture that the grain
size exhibits a minimum in the mixed-phase deposition regime, simply because the
competitive growth of the various crystals hinders the growth of each other and
minor changes in the local deposition conditions often lead to the nucleation of new
crystals. The nanocrystalline nature of the deposit in the phase mixture composition
range also contributes to the widening of the formation zone of a crystalline form as
compared to the equilibrium phase diagram.
2 Electrochemistry and Electrodeposition
alloy is formed, and the initial part of the composition diagram looks like for irregular codeposition. However, as the concentration of the induced metal increases in
the deposit, the current efficiency decreases, and a limiting deposit composition can
be seen. Finally, the current efficiency drops so much that no deposit is produced,
and the composition function line ends in the middle of the composition diagram.
Interestingly, these systems are the most prone to develop amorphous deposits as the
concentration of the oxoanion-forming element in the deposit increases.
2.12.3 Structural Consequences of Alloy Formation
Even if a metal pair exhibits an equilibrium alloy, there is no guarantee that a particular
alloy can be synthesized by electrochemical codeposition. The general trend is that
the closer the deposition potentials of the constituents are, the easier to deposit a real
alloy with no segregation but with miscibility at the atomic scale. The miscibility
at the atomic scale is often promoted by the application of a complexing agent that
makes a more stable complex with the ions of the more noble element to be deposited.
Hence, the difference in the deposition potentials is diminished, which is a favourable
factor in the tendency of codeposition.
Beside the deposition potential, the ordering and the unit cell size are also important factors in the electrolytic alloy formation. The larger the unit cell size and the
more ordered an alloy must be, the less likely that it can be synthesized by direct
electroplating without an annealing step afterwards. Obviously, alloys with a random
atomic arrangement tend to form the most easily, in accord with the non-equilibrium
nature of the electrodeposition process. Another general trend is that the increase in
the number of components results in a decrease in grain size of the deposit.
Finally, it must be mentioned that phase mixtures are often deposited when the
composition of the deposit is close to an equilibrium phase transition composition of
the system even if the equilibrium phase transition takes place at a definite composition without a miscibility gap. In such cases, electrodeposition leads to a phase
mixture in a 10–15 atomic percent wide composition range. This is related to the
non-equilibrium nature of the electroplating method. The general experience is that
the formation range of a phase is extended with respect to its equilibrium stability
range. It is a direct consequence of the deposition of a phase mixture that the grain
size exhibits a minimum in the mixed-phase deposition regime, simply because the
competitive growth of the various crystals hinders the growth of each other and
minor changes in the local deposition conditions often lead to the nucleation of new
crystals. The nanocrystalline nature of the deposit in the phase mixture composition
range also contributes to the widening of the formation zone of a crystalline form as
compared to the equilibrium phase diagram.