7.2 Composite Deposition from Particle Suspensions
237
optimal ultrasonic power needed to achieve a deposit with the desired properties
[88].
Particle size distribution of the dispersion. The common pursuit is that researchers
use monodisperse particle samples for the electrodeposition of composites. The relative width of the size distribution of the particles is usually quite large and it may
depend on the preparation mode. Samples of ground particle usually exhibit larger
distribution width than those produced by precipitation and sol-gel methods where
auxiliary components can be used to regulate the upper limit of the particle size. It
is customary that 95% of the particles falls in the ±25% range of mean particle size
(as calculated by the number of the particles).
In special cases, it may occur that particles with bimodal size distribution are used
purposefully [81, 89]. In these cases, the particle fractions of larger and smaller sizes
can be associated with different advantageous new properties of the deposit.
Polarization behaviour of the dispersion coating systems. Cyclic voltammetry is
an essential element of the tool set of electrochemistry; hence, it is often applied
to characterize the plating systems used to obtain dispersion coating. The comparison of the voltammograms obtained in both the absence and presence of suspended
particles led to interesting experience. Both current increase and decrease at a particular potential may occur, and the direction of the current change cannot be directly
correlated with the electrical conductivity of the particles.
For large and elongated conducting particles like carbon nanotubes, it is not
surprising at all that the addition of nanoparticles to the plating bath can increase the
cathodic current at a particular potential [76, 90–92]. The reason is the enhancement
of the electrode surface area, which results in an enhanced current density despite
the nucleation of metals on carbonaceous materials is usually much hindered. The
current density enhancement for CNT-containing bath can be as high as 100%. In
contrast, codeposition of small isotropic metallic particles may show either a similar
or an opposite trend. For the deposition of Sn–Cu(Ag), the addition of the Ag particles to the metal plating bath resulted in an enhanced current density [93]. However,
for a Zn–Cu(Ag) deposition system, the reduction of the current was found upon
the addition of the nanoparticles [94]. Ag nanoparticles were thought to hinder both
the Cu and Zn–Cu deposition, leading to smaller currents. The explanation of the
inhibition of the deposition is likely the nucleation barrier effect of the Ag particles
for Cu and Zn that exhibit crystal structure with very dissimilar nearest neighbour
atomic distances.
Concerning non-conducting particles, the diminished current density can be elucidated on the basis of the reduction of the active surface area [95]. However, several
examples can be found for the opposite trend, too; namely, a moderate [71, 87] or
even twofold [96] increase of the current density can be seen upon the addition of
the nanoparticles to the metal plating bath. Since this experience is highly collateral
beside the properties of the deposits, no systematic study can be found for correlating
the current increase with other bath properties. If we recall the codeposition model
conditions setup by Fransaer et al. [68], it is apparent that adsorbed metal ions are
also considered to be reactive at the cathode surface.
237
optimal ultrasonic power needed to achieve a deposit with the desired properties
[88].
Particle size distribution of the dispersion. The common pursuit is that researchers
use monodisperse particle samples for the electrodeposition of composites. The relative width of the size distribution of the particles is usually quite large and it may
depend on the preparation mode. Samples of ground particle usually exhibit larger
distribution width than those produced by precipitation and sol-gel methods where
auxiliary components can be used to regulate the upper limit of the particle size. It
is customary that 95% of the particles falls in the ±25% range of mean particle size
(as calculated by the number of the particles).
In special cases, it may occur that particles with bimodal size distribution are used
purposefully [81, 89]. In these cases, the particle fractions of larger and smaller sizes
can be associated with different advantageous new properties of the deposit.
Polarization behaviour of the dispersion coating systems. Cyclic voltammetry is
an essential element of the tool set of electrochemistry; hence, it is often applied
to characterize the plating systems used to obtain dispersion coating. The comparison of the voltammograms obtained in both the absence and presence of suspended
particles led to interesting experience. Both current increase and decrease at a particular potential may occur, and the direction of the current change cannot be directly
correlated with the electrical conductivity of the particles.
For large and elongated conducting particles like carbon nanotubes, it is not
surprising at all that the addition of nanoparticles to the plating bath can increase the
cathodic current at a particular potential [76, 90–92]. The reason is the enhancement
of the electrode surface area, which results in an enhanced current density despite
the nucleation of metals on carbonaceous materials is usually much hindered. The
current density enhancement for CNT-containing bath can be as high as 100%. In
contrast, codeposition of small isotropic metallic particles may show either a similar
or an opposite trend. For the deposition of Sn–Cu(Ag), the addition of the Ag particles to the metal plating bath resulted in an enhanced current density [93]. However,
for a Zn–Cu(Ag) deposition system, the reduction of the current was found upon
the addition of the nanoparticles [94]. Ag nanoparticles were thought to hinder both
the Cu and Zn–Cu deposition, leading to smaller currents. The explanation of the
inhibition of the deposition is likely the nucleation barrier effect of the Ag particles
for Cu and Zn that exhibit crystal structure with very dissimilar nearest neighbour
atomic distances.
Concerning non-conducting particles, the diminished current density can be elucidated on the basis of the reduction of the active surface area [95]. However, several
examples can be found for the opposite trend, too; namely, a moderate [71, 87] or
even twofold [96] increase of the current density can be seen upon the addition of
the nanoparticles to the metal plating bath. Since this experience is highly collateral
beside the properties of the deposits, no systematic study can be found for correlating
the current increase with other bath properties. If we recall the codeposition model
conditions setup by Fransaer et al. [68], it is apparent that adsorbed metal ions are
also considered to be reactive at the cathode surface.