218
7 Composites
hindrance of the crystallization that reduces the grain size. The X-ray diffraction
pattern of mechanical mixtures shows the diffraction lines of both constituents in the
entire range of mean composition that is available with electrodeposition. Since no
impurity incorporation takes place, the position of the diffraction lines is the same as
for the pure elements. Many examples for such systems can be found in Watanabe’s
work [1] (e.g., Sn–Zn, Cu–Pb, Cd–Zn, Cd–Sn, Ag–Cu, Ag–Co) and elsewhere (e.g.,
Co–Pb [2]). Electrodeposited mechanical mixtures can be nanogranular materials in
their as-received state. In exceptional cases when the crystallite size of both metals
is smaller than about 10 nm, a single diffraction peak can only be observed [3].
In other cases, the electrodeposition process results in a metastable alloy. This
is the intermediate of the granular material that can be obtained with an annealing
treatment. The formation of a metastable solid solution-type alloy is possible when
the crystalline forms of the constituents are structurally related to each other, e.g.,
they have the same crystal structure with nearly the same lattice constant, even though
the equilibrium miscibility is nearly zero. The Cu–Co system is a prominent example
of a metal pair that is prone to produce metastable alloys. Cu and Co both can form
fcc crystals (although for Co it is stable only above 422 °C), but the Cu impurity
easily induces its formation also at room temperature. The lattice mismatch of pure
Cu and Co is about 2%.
It should be noted that the formation of either mechanical mixtures or metastable
alloys can be regulated to some extent with the bath chemistry. As it was discussed
in Chap. 2.11, the difference in the deposition potentials of the constituents affects
their ability for alloy formation. While a high difference in the deposition potentials
favours the formation of segregated crystals, nearly equal deposition potentials are
beneficial for the formation of an alloy. Even if an alloy cannot form, a decrease
in the difference in the deposition potentials leads to a reduction of the grain size,
hence improving the nanogranular nature of the deposit. A change of the deposition
potentials can be achieved by using a suitable complexing agent. The obvious goal
is a large shift of the deposition potential of the MN metal to the negative direction,
while the deposition potential of the LN metal should not be affected in the ideal
case (or can decrease to a much smaller extent than that of the MN metal).
As an example of the impact of the complexing agent on the deposition of a
mechanical mixture, a systematic study on the Cu–Ag system can be given here
[4]. In the absence of a complexing agent, the growth of the Cu–Ag mixture leads
to a dendritic coating, in agreement with the general trend that the deposition of
metals with high exchange current densities leads to dendrite formation. This is
particularly true for mixtures of metals with a large difference in the deposition
potentials, where the MN metal is deposited with a diffusion-limited rate in the
potential regime of the LN metal codeposition. In the absence of complexing agents,
the lattice distances of Cu and Ag appear separately in the diffractograms. The deposit
is already nanogranular under such deposition conditions since the immiscible metals
mutually hinder the growth of each other, leading to a 10–30 nm grain size. Upon the
application of a complexing agent (which was thiourea in the study cited above), the
grain size decreased further down to 5 nm. However, the main effect is much beyond
the grain size reduction itself since the deposit shows a single phase as if an alloy
7 Composites
hindrance of the crystallization that reduces the grain size. The X-ray diffraction
pattern of mechanical mixtures shows the diffraction lines of both constituents in the
entire range of mean composition that is available with electrodeposition. Since no
impurity incorporation takes place, the position of the diffraction lines is the same as
for the pure elements. Many examples for such systems can be found in Watanabe’s
work [1] (e.g., Sn–Zn, Cu–Pb, Cd–Zn, Cd–Sn, Ag–Cu, Ag–Co) and elsewhere (e.g.,
Co–Pb [2]). Electrodeposited mechanical mixtures can be nanogranular materials in
their as-received state. In exceptional cases when the crystallite size of both metals
is smaller than about 10 nm, a single diffraction peak can only be observed [3].
In other cases, the electrodeposition process results in a metastable alloy. This
is the intermediate of the granular material that can be obtained with an annealing
treatment. The formation of a metastable solid solution-type alloy is possible when
the crystalline forms of the constituents are structurally related to each other, e.g.,
they have the same crystal structure with nearly the same lattice constant, even though
the equilibrium miscibility is nearly zero. The Cu–Co system is a prominent example
of a metal pair that is prone to produce metastable alloys. Cu and Co both can form
fcc crystals (although for Co it is stable only above 422 °C), but the Cu impurity
easily induces its formation also at room temperature. The lattice mismatch of pure
Cu and Co is about 2%.
It should be noted that the formation of either mechanical mixtures or metastable
alloys can be regulated to some extent with the bath chemistry. As it was discussed
in Chap. 2.11, the difference in the deposition potentials of the constituents affects
their ability for alloy formation. While a high difference in the deposition potentials
favours the formation of segregated crystals, nearly equal deposition potentials are
beneficial for the formation of an alloy. Even if an alloy cannot form, a decrease
in the difference in the deposition potentials leads to a reduction of the grain size,
hence improving the nanogranular nature of the deposit. A change of the deposition
potentials can be achieved by using a suitable complexing agent. The obvious goal
is a large shift of the deposition potential of the MN metal to the negative direction,
while the deposition potential of the LN metal should not be affected in the ideal
case (or can decrease to a much smaller extent than that of the MN metal).
As an example of the impact of the complexing agent on the deposition of a
mechanical mixture, a systematic study on the Cu–Ag system can be given here
[4]. In the absence of a complexing agent, the growth of the Cu–Ag mixture leads
to a dendritic coating, in agreement with the general trend that the deposition of
metals with high exchange current densities leads to dendrite formation. This is
particularly true for mixtures of metals with a large difference in the deposition
potentials, where the MN metal is deposited with a diffusion-limited rate in the
potential regime of the LN metal codeposition. In the absence of complexing agents,
the lattice distances of Cu and Ag appear separately in the diffractograms. The deposit
is already nanogranular under such deposition conditions since the immiscible metals
mutually hinder the growth of each other, leading to a 10–30 nm grain size. Upon the
application of a complexing agent (which was thiourea in the study cited above), the
grain size decreased further down to 5 nm. However, the main effect is much beyond
the grain size reduction itself since the deposit shows a single phase as if an alloy