6.1 General Considerations Concerning Nanocrystalline Deposits
187
Reverse pulse plating is also often applied to achieve nanocrystalline deposits.
The concept of reverse pulse plating is that the dissolution pulse can further decrease
the crystallite size of the deposit simply by the consumption of the already existing
grains and then allowing the nucleation of new grains during the subsequent pulse.
While reverse pulse plating obviously does not impact the composition of the deposits
of a single metallic element, the composition of nanocrystalline alloys is sensitive
to the reverse pulse parameters. Due to the selective nature of dissolution, reverse
pulse plating leads to alloys enriched in the MN component since the LN component
usually dissolves selectively. It is to be noted that a selective dissolution may lead to
dealloying, hence producing a porous structure. Such deposits will be dealt with in
Chap. 8.
For both pulse and reverse pulse plating, the pulse length is practically limited
by the capacitive damping effect. This defines a practical lower limit of the pulse
lengths at about 1 ms, below which the actual Faraday current gets very close to the
d.c. plating (as averaged for the entire cycle). The practical upper limit of the pulse
length is when the thickness of the pulsating diffusion layer becomes comparable to
that prevailing during d.c. plating. The latter case is rather an intermittent d.c. plating
in which the distinct features of pulse plating can no longer be exploited.
6.1.4 Properties of Nanocrystalline Materials
Volume ratio of the grain boundary region. As the crystallite size of a material is
decreased, the fraction of the atoms situated in the grain boundary region increases.
Hence, one encounters the enhancement of the role of the more or less disordered
segments of the material where material constants valid for well-ordered crystals are
no longer valid.
Figure 6.1 shows the volume fraction dependence of the regions far from the bulk
zones for a boundary region thickness of 1 nm (for a different boundary region thickness, the exact volume fraction values may vary but the character of the dependencies
is unchanged).
As can be seen from Fig. 6.1, the volume fraction of the intercrystalline regions
drastically increases with the transition from microcrystalline to nanocrystalline
materials. It is particularly interesting that the volume fraction of the triple-junction
regions precedes that of the grain boundaries as the grains are refined. This is because
triple junction is defined as a region where the boundary regions of various adjacent
crystal pairs overlap. The saturation of the total volume fraction of the intergranular
regions at small grain size indicates a virtually smooth transient from a nanocrystalline to a non-crystalline (amorphous) structure. The smooth transient clearly shows
that the definition of amorphicity cannot be related to a sudden disappearance of
the atomic-scale ordering; rather, it is usually related to the diffraction pattern of a
material, and the term “X-ray amorphous” is used very often.
Structure of the grain boundary. Not only is the number of atoms being present
in the grain boundaries is important but the structure of the grain boundary may
187
Reverse pulse plating is also often applied to achieve nanocrystalline deposits.
The concept of reverse pulse plating is that the dissolution pulse can further decrease
the crystallite size of the deposit simply by the consumption of the already existing
grains and then allowing the nucleation of new grains during the subsequent pulse.
While reverse pulse plating obviously does not impact the composition of the deposits
of a single metallic element, the composition of nanocrystalline alloys is sensitive
to the reverse pulse parameters. Due to the selective nature of dissolution, reverse
pulse plating leads to alloys enriched in the MN component since the LN component
usually dissolves selectively. It is to be noted that a selective dissolution may lead to
dealloying, hence producing a porous structure. Such deposits will be dealt with in
Chap. 8.
For both pulse and reverse pulse plating, the pulse length is practically limited
by the capacitive damping effect. This defines a practical lower limit of the pulse
lengths at about 1 ms, below which the actual Faraday current gets very close to the
d.c. plating (as averaged for the entire cycle). The practical upper limit of the pulse
length is when the thickness of the pulsating diffusion layer becomes comparable to
that prevailing during d.c. plating. The latter case is rather an intermittent d.c. plating
in which the distinct features of pulse plating can no longer be exploited.
6.1.4 Properties of Nanocrystalline Materials
Volume ratio of the grain boundary region. As the crystallite size of a material is
decreased, the fraction of the atoms situated in the grain boundary region increases.
Hence, one encounters the enhancement of the role of the more or less disordered
segments of the material where material constants valid for well-ordered crystals are
no longer valid.
Figure 6.1 shows the volume fraction dependence of the regions far from the bulk
zones for a boundary region thickness of 1 nm (for a different boundary region thickness, the exact volume fraction values may vary but the character of the dependencies
is unchanged).
As can be seen from Fig. 6.1, the volume fraction of the intercrystalline regions
drastically increases with the transition from microcrystalline to nanocrystalline
materials. It is particularly interesting that the volume fraction of the triple-junction
regions precedes that of the grain boundaries as the grains are refined. This is because
triple junction is defined as a region where the boundary regions of various adjacent
crystal pairs overlap. The saturation of the total volume fraction of the intergranular
regions at small grain size indicates a virtually smooth transient from a nanocrystalline to a non-crystalline (amorphous) structure. The smooth transient clearly shows
that the definition of amorphicity cannot be related to a sudden disappearance of
the atomic-scale ordering; rather, it is usually related to the diffraction pattern of a
material, and the term “X-ray amorphous” is used very often.
Structure of the grain boundary. Not only is the number of atoms being present
in the grain boundaries is important but the structure of the grain boundary may