5.3 The Single-Bath Method
151
the same role in the two cases [68]. The verification of the theoretical description
of the cell was experimentally confirmed for steady-state cell operation [69]. The
mathematical description was given for high-amplitude modulation of the flow rate
including non-linear terms [70], which was found in agreement with the experiments
concerning the limiting current of a single electrode reaction [71].
The UIC was used for depositing Ni–Fe CMA coatings by modulating the flow
rate [72]. Both galvanostatic and potentiostatic conditions were tested, leading to the
obvious result that the free (i.e., non-regulated) electrical parameter varied as the flow
rate is varied between two discrete values. The decay of the non-regulated electrical
parameter was comparable to the flow pulse length applied; hence, it can be assumed
that the interface between the layers deposited at particular solution flow rates is not
sharp. The composition modulation wavelength (Λ) was inversely proportional to
the flow rate modulation frequency (ν) at a fixed current density, in accord with the
formula below:
Λ =
V M,av. η |j|
2 F ν
(5.1)
where the z = 2 substitution is due to the Me
2+ form of the ion of both constituent
metals, η is the current efficiency and V M,av. is the average molar volume of the
deposit. The smallest modulation wavelength achieved was 55 nm. The periodicity
of the flow-modulated deposits was checked with a stripping analysis, but neither
in-depth composition analysis nor TEM observation of the layered structure was
given.
The rotating ring-disc electrode (RRDE) configuration has also been tested for
the deposition of CMA structures [73, 74]. In both studies, a sinusoidal rotation
rate modulation was applied, for which the complete theoretical background was
published later [75]. In the composition modulation, the frequency of the rotation rate
modulation is dominant, while the higher-order terms have relatively little weight.
In the experimental studies [73, 74], Ni–Fe deposition was tested with rotation rate
oscillation frequency of a few hundred millihertz. The smallest composition oscillation wavelength observed with potentiostatic stripping analysis was around 7 nm.
Electron microscopic images and space-resolved EDX analysis was given for much
larger wavelengths only. The 7-nm minimum wavelength observation was only partly
due to the limited resolution capability of the stripping analysis method. The codeposition characteristics of Ni and Fe also led to decreasing mole fraction oscillation
amplitudes as the modulation periodicity decreased, which made it nearly impossible
to resolve the composition oscillation for rotation rate modulation frequency above
0.2 Hz. CMAs other than Ni–Fe were not tested with any flow modulation technique.
Instead of modulating the solution flow (where the electrode configuration is
crucial for achieving a laterally even deposit), the transport rate can be modulated
with the periodic operation of an ultrasonicator. If the sonication head of the ultrasonic device provides uniform energy dissipation in the solution, the transport rate
151
the same role in the two cases [68]. The verification of the theoretical description
of the cell was experimentally confirmed for steady-state cell operation [69]. The
mathematical description was given for high-amplitude modulation of the flow rate
including non-linear terms [70], which was found in agreement with the experiments
concerning the limiting current of a single electrode reaction [71].
The UIC was used for depositing Ni–Fe CMA coatings by modulating the flow
rate [72]. Both galvanostatic and potentiostatic conditions were tested, leading to the
obvious result that the free (i.e., non-regulated) electrical parameter varied as the flow
rate is varied between two discrete values. The decay of the non-regulated electrical
parameter was comparable to the flow pulse length applied; hence, it can be assumed
that the interface between the layers deposited at particular solution flow rates is not
sharp. The composition modulation wavelength (Λ) was inversely proportional to
the flow rate modulation frequency (ν) at a fixed current density, in accord with the
formula below:
Λ =
V M,av. η |j|
2 F ν
(5.1)
where the z = 2 substitution is due to the Me
2+ form of the ion of both constituent
metals, η is the current efficiency and V M,av. is the average molar volume of the
deposit. The smallest modulation wavelength achieved was 55 nm. The periodicity
of the flow-modulated deposits was checked with a stripping analysis, but neither
in-depth composition analysis nor TEM observation of the layered structure was
given.
The rotating ring-disc electrode (RRDE) configuration has also been tested for
the deposition of CMA structures [73, 74]. In both studies, a sinusoidal rotation
rate modulation was applied, for which the complete theoretical background was
published later [75]. In the composition modulation, the frequency of the rotation rate
modulation is dominant, while the higher-order terms have relatively little weight.
In the experimental studies [73, 74], Ni–Fe deposition was tested with rotation rate
oscillation frequency of a few hundred millihertz. The smallest composition oscillation wavelength observed with potentiostatic stripping analysis was around 7 nm.
Electron microscopic images and space-resolved EDX analysis was given for much
larger wavelengths only. The 7-nm minimum wavelength observation was only partly
due to the limited resolution capability of the stripping analysis method. The codeposition characteristics of Ni and Fe also led to decreasing mole fraction oscillation
amplitudes as the modulation periodicity decreased, which made it nearly impossible
to resolve the composition oscillation for rotation rate modulation frequency above
0.2 Hz. CMAs other than Ni–Fe were not tested with any flow modulation technique.
Instead of modulating the solution flow (where the electrode configuration is
crucial for achieving a laterally even deposit), the transport rate can be modulated
with the periodic operation of an ultrasonicator. If the sonication head of the ultrasonic device provides uniform energy dissipation in the solution, the transport rate