6.1 General Considerations Concerning Nanocrystalline Deposits
189
boundaries are formed as a result of the fragmentation of existing crystals, and hence,
such processes have no significant chemical aspect. In contrast, electrodeposition is
notorious of being influenced by all chemical species present at the surface of the
newly forming phase. As it was described in connection with the additive effect,
adsorbed chemical species (both molecules and ions) take part in the acceleration
of the nucleation, which finally leads to grain refinement. Since the additives (or
their decomposition products) may remain at the surface as adsorbate at a longer
time scale than the grain growth, they can still be present at the grain boundaries in
the electrodeposited nanocrystalline materials. There are only a few studies dealing
with the impurity effect in the stabilization of the nanocrystalline deposits. Since the
absolute concentration of the impurities is usually as low as a few tens or at most
a few hundreds of ppm, a conventional EDS analysis cannot reveal the relationship
between the impurity concentration and the grain size. Instead, analysis methods
applied in metallurgy for the quantification of non-metallic elements can be taken
advantage of for the detection of additive residues. However, the location of the
impurities can be revealed by a TEM analysis only.
Establishment of the grain size. The most common methods for the assessment
of the grain size, d, are application of the Scherrer equation for the full width at half
maximum of a peak seen in an X-ray diffractogram recorded in the Bragg–Brentano
geometry:
d = K λ/β cos
(6.2)
where λ is the wavelength of the diffracted radiation, is the Bragg angle, β is
the line broadening (full width at half maxima, corrected with the instrumental line
width). The dimensionless shape factor, K, is usually taken as 0.9.
However, the peak widening may have various reasons in addition to the grain
refinement. For instance, internal stress also has a contribution to XRD peak
widening. If the nanocrystalline deposit is weakly structured, the Williamson–Hall
method can be used for the grain size assessment. For materials composed of cubic
crystals, the full line profile analysis can be applied, even though this method requires
a high-sensitivity XRD instrument, which is by far not common even in advanced
physical laboratories. X-ray diffraction yields a grain size assessment in which the
neighbouring grains separated by a small-angle grain boundary are taken as different
crystals. This is because such boundaries break the coherence of the scattering.
In contrast, electron backscattered diffraction (EBSD) performed on well-polished
deposit surfaces is not sensitive to small-angle grain boundaries; hence, its result
is not fully comparable to the grain size assessment of XRD. Finally, TEM yields
a reliable basis for grain size assessment and statistics with the same constraint as
EBSD concerning the inability to identify the low-angle boundaries. When the line
profile analysis and TEM-based grain size assessment were used in parallel for electroplated metals, the results indicate that d TEM is larger than d XRD , as expected due
to the neglect of the low-angle boundaries in the former method [17–19].
189
boundaries are formed as a result of the fragmentation of existing crystals, and hence,
such processes have no significant chemical aspect. In contrast, electrodeposition is
notorious of being influenced by all chemical species present at the surface of the
newly forming phase. As it was described in connection with the additive effect,
adsorbed chemical species (both molecules and ions) take part in the acceleration
of the nucleation, which finally leads to grain refinement. Since the additives (or
their decomposition products) may remain at the surface as adsorbate at a longer
time scale than the grain growth, they can still be present at the grain boundaries in
the electrodeposited nanocrystalline materials. There are only a few studies dealing
with the impurity effect in the stabilization of the nanocrystalline deposits. Since the
absolute concentration of the impurities is usually as low as a few tens or at most
a few hundreds of ppm, a conventional EDS analysis cannot reveal the relationship
between the impurity concentration and the grain size. Instead, analysis methods
applied in metallurgy for the quantification of non-metallic elements can be taken
advantage of for the detection of additive residues. However, the location of the
impurities can be revealed by a TEM analysis only.
Establishment of the grain size. The most common methods for the assessment
of the grain size, d, are application of the Scherrer equation for the full width at half
maximum of a peak seen in an X-ray diffractogram recorded in the Bragg–Brentano
geometry:
d = K λ/β cos
(6.2)
where λ is the wavelength of the diffracted radiation, is the Bragg angle, β is
the line broadening (full width at half maxima, corrected with the instrumental line
width). The dimensionless shape factor, K, is usually taken as 0.9.
However, the peak widening may have various reasons in addition to the grain
refinement. For instance, internal stress also has a contribution to XRD peak
widening. If the nanocrystalline deposit is weakly structured, the Williamson–Hall
method can be used for the grain size assessment. For materials composed of cubic
crystals, the full line profile analysis can be applied, even though this method requires
a high-sensitivity XRD instrument, which is by far not common even in advanced
physical laboratories. X-ray diffraction yields a grain size assessment in which the
neighbouring grains separated by a small-angle grain boundary are taken as different
crystals. This is because such boundaries break the coherence of the scattering.
In contrast, electron backscattered diffraction (EBSD) performed on well-polished
deposit surfaces is not sensitive to small-angle grain boundaries; hence, its result
is not fully comparable to the grain size assessment of XRD. Finally, TEM yields
a reliable basis for grain size assessment and statistics with the same constraint as
EBSD concerning the inability to identify the low-angle boundaries. When the line
profile analysis and TEM-based grain size assessment were used in parallel for electroplated metals, the results indicate that d TEM is larger than d XRD , as expected due
to the neglect of the low-angle boundaries in the former method [17–19].