146
5 Compositionally Modulated and Multilayered Deposits
a potentiostat/galvanostat device that was suitable for switching the operation mode
within a usual instrumental transient time (at most a few tens of microseconds) [45].
Concerning the other deposition modes (i.e., anomalous or induced), the polarization curves usually show no characteristic features (i.e., plateau) which could
be indicative of the deposition rate of either of the components. Since the alloying
elements in these composition modes are codeposited in the entire range of current
density in which a coating can be produced at all, the measurement of the composition–current density relationship has to be determined experimentally. It has to be
emphasized that, as it will be described in Sect. 5.4.1, a transient may also take place
when the current density (or electrode potential) is modified, which is expected give
rise to a difference between the composition of a nanolayer and that measured at
otherwise identical deposition condition for a bulk deposit. No detailed methodical
papers are available on whether the change in the deposition conditions from one
layer type to the other leads to any unwanted composition change (i.e., a partial dealloying/dissolution when the high-current pulse ends and a low-current pulse begins).
Hence, the choice of the electrical parameter used for the deposition seems to be less
crucial for the anomalous and the induced deposition mode than for the regular one.
5.3.2 Multilayer Formation with Single-Pulse Plating
and Displacement
The principle of the displacement-combined single-pulse method is essentially
the same as for the SLRR process. The driving force of both the SLRR and the
displacement-combined single-pulse deposition method is the difference in the standard potential of two metal ion/metal systems, which results in that the oxidized
form of the more positive system can displace the reduced form of the more negative redox pair. The important differences are that during the multilayer formation,
the sacrificial layer is much thicker than a single atomic layer, its deposition during
the current pulse is not surface area-limited, the sacrificial layer is displaced by the
other metal only partly so that this layer conserves its integrity when thinned by the
displacing metal, and the thickness of the layer formed from the displacing metal is
not limited to a monolayer either.
The concentration ratio of the ions of the LN and MN metals is similar to that
applied in the SLRR processes. However, the concentration values themselves are
larger. This is because a continuous layer from the LN metal can form in a short time
only in the case when the concentration of its ions is high enough. The high concentration of the salt of the LN metal is necessary also because during its deposition, the
mass transport control and the concomitant inclination to the formation of dendrite
are undesired. For this reason, the concentration of the salt of the LN metal ranges
from a few tenths of mol/litre to a nearly-saturated solution of its salt. In contrast, for
achieving both a moderate rate of displacement and a small proportion of the MN
metal codeposited together with the LN one, the solution is rather dilute with respect
5 Compositionally Modulated and Multilayered Deposits
a potentiostat/galvanostat device that was suitable for switching the operation mode
within a usual instrumental transient time (at most a few tens of microseconds) [45].
Concerning the other deposition modes (i.e., anomalous or induced), the polarization curves usually show no characteristic features (i.e., plateau) which could
be indicative of the deposition rate of either of the components. Since the alloying
elements in these composition modes are codeposited in the entire range of current
density in which a coating can be produced at all, the measurement of the composition–current density relationship has to be determined experimentally. It has to be
emphasized that, as it will be described in Sect. 5.4.1, a transient may also take place
when the current density (or electrode potential) is modified, which is expected give
rise to a difference between the composition of a nanolayer and that measured at
otherwise identical deposition condition for a bulk deposit. No detailed methodical
papers are available on whether the change in the deposition conditions from one
layer type to the other leads to any unwanted composition change (i.e., a partial dealloying/dissolution when the high-current pulse ends and a low-current pulse begins).
Hence, the choice of the electrical parameter used for the deposition seems to be less
crucial for the anomalous and the induced deposition mode than for the regular one.
5.3.2 Multilayer Formation with Single-Pulse Plating
and Displacement
The principle of the displacement-combined single-pulse method is essentially
the same as for the SLRR process. The driving force of both the SLRR and the
displacement-combined single-pulse deposition method is the difference in the standard potential of two metal ion/metal systems, which results in that the oxidized
form of the more positive system can displace the reduced form of the more negative redox pair. The important differences are that during the multilayer formation,
the sacrificial layer is much thicker than a single atomic layer, its deposition during
the current pulse is not surface area-limited, the sacrificial layer is displaced by the
other metal only partly so that this layer conserves its integrity when thinned by the
displacing metal, and the thickness of the layer formed from the displacing metal is
not limited to a monolayer either.
The concentration ratio of the ions of the LN and MN metals is similar to that
applied in the SLRR processes. However, the concentration values themselves are
larger. This is because a continuous layer from the LN metal can form in a short time
only in the case when the concentration of its ions is high enough. The high concentration of the salt of the LN metal is necessary also because during its deposition, the
mass transport control and the concomitant inclination to the formation of dendrite
are undesired. For this reason, the concentration of the salt of the LN metal ranges
from a few tenths of mol/litre to a nearly-saturated solution of its salt. In contrast, for
achieving both a moderate rate of displacement and a small proportion of the MN
metal codeposited together with the LN one, the solution is rather dilute with respect