7.1 Composite Preparation by Codeposition of Metals
219
were formed with continuously varying composition. The collateral advantages of
the application of the complexing agent include the diminished overall deposition
rate and the decrease in the difference in the deposition potentials of the constituent.
The latter change is important in the design of the alloy composition because the
composition vs. deposition potential function changes much less abruptly than in the
absence of the complexing agent.
It is common that pulse electrodeposition is used to obtain granular materials
or their precursor metastable alloys. The reason for applying pulse plating is not to
achieve layered deposits, in contrast to the cases presented in Chap. 5.4, and the pulse
lengths are mostly not even appropriate for this purpose. The verification of pulse
plating can be one of the following arguments. First, if the LN metal is codeposited
with MN one, the deposition of the MN metal takes place with diffusion-limited rate,
which often leads to dendritic growth. In order to avoid the dendrite formation, it
is worth of operating the bath at a current density lower than the diffusion-limited
current density of the MN metal in the low-current pulse. This can lead to a deposit
with a relatively small roughness. The current density is to be increased only for
the short periods when the LN metal is deposited, but this usually leads to a negligible change in the surface roughness. The pulse lengths can be easily chosen to
deposit submonolayer quantities during each pulse, hence obtaining a homogeneous
alloy also in the composition range where the constituent metals might segregate.
This approach was elaborated for Cu–Ni codeposition and called by the authors as
“precision electrodeposition” [5].
Secondly, the application of pulses can be useful if the nucleation barrier of the
LN metal onto the MN one is large. This makes it necessary to apply a large negative
potential to deposit the LN metal at all; however, the long-term maintenance of this
large negative potential can lead to a continuous growth of the LN metal at a high
rate. This leads to that the available mole fraction range is not favourable since
a matrix composed of the MN metal with only a small amount of the LN metal
becomes impossible. The start of the codeposition of the LN metal turns the system
immediately from MN rich to LN rich. Hence, the application of current or potential
pulses makes it possible to tune the deposit composition with a larger precision than
the application of constant potential or current density.
Many granular deposits were investigated because of their magnetic properties. In
such cases, the small magnetic particles have to be accommodated in a non-magnetic
matrix. If the particle size is at most a few tens of nanometres, such systems have
special magnetic properties and also show GMR (for the definition and the background related to GMR, see Chap. 5.4.6). The background of the GMR in granular
systems is essentially the same as for layered materials. The major difference is that
in granular materials, the random distribution of the particles does not allow the
occurrence of an oscillatory magnetic coupling as a function of the distance of the
magnetic entities since it always has a statistical distribution. The two major types
of composites with non-magnetic matrix and magnetic particles will be summarized
in the following chapters, and then other metallic composites will be discussed.
219
were formed with continuously varying composition. The collateral advantages of
the application of the complexing agent include the diminished overall deposition
rate and the decrease in the difference in the deposition potentials of the constituent.
The latter change is important in the design of the alloy composition because the
composition vs. deposition potential function changes much less abruptly than in the
absence of the complexing agent.
It is common that pulse electrodeposition is used to obtain granular materials
or their precursor metastable alloys. The reason for applying pulse plating is not to
achieve layered deposits, in contrast to the cases presented in Chap. 5.4, and the pulse
lengths are mostly not even appropriate for this purpose. The verification of pulse
plating can be one of the following arguments. First, if the LN metal is codeposited
with MN one, the deposition of the MN metal takes place with diffusion-limited rate,
which often leads to dendritic growth. In order to avoid the dendrite formation, it
is worth of operating the bath at a current density lower than the diffusion-limited
current density of the MN metal in the low-current pulse. This can lead to a deposit
with a relatively small roughness. The current density is to be increased only for
the short periods when the LN metal is deposited, but this usually leads to a negligible change in the surface roughness. The pulse lengths can be easily chosen to
deposit submonolayer quantities during each pulse, hence obtaining a homogeneous
alloy also in the composition range where the constituent metals might segregate.
This approach was elaborated for Cu–Ni codeposition and called by the authors as
“precision electrodeposition” [5].
Secondly, the application of pulses can be useful if the nucleation barrier of the
LN metal onto the MN one is large. This makes it necessary to apply a large negative
potential to deposit the LN metal at all; however, the long-term maintenance of this
large negative potential can lead to a continuous growth of the LN metal at a high
rate. This leads to that the available mole fraction range is not favourable since
a matrix composed of the MN metal with only a small amount of the LN metal
becomes impossible. The start of the codeposition of the LN metal turns the system
immediately from MN rich to LN rich. Hence, the application of current or potential
pulses makes it possible to tune the deposit composition with a larger precision than
the application of constant potential or current density.
Many granular deposits were investigated because of their magnetic properties. In
such cases, the small magnetic particles have to be accommodated in a non-magnetic
matrix. If the particle size is at most a few tens of nanometres, such systems have
special magnetic properties and also show GMR (for the definition and the background related to GMR, see Chap. 5.4.6). The background of the GMR in granular
systems is essentially the same as for layered materials. The major difference is that
in granular materials, the random distribution of the particles does not allow the
occurrence of an oscillatory magnetic coupling as a function of the distance of the
magnetic entities since it always has a statistical distribution. The two major types
of composites with non-magnetic matrix and magnetic particles will be summarized
in the following chapters, and then other metallic composites will be discussed.