6.1 General Considerations Concerning Nanocrystalline Deposits
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much larger mean value than that in the original specimen. However, when chemical segregation can also take place during annealing and it creates heterogeneous
zones with significantly different diffusion characteristics, the grain size distribution
may become bimodal. In this case, the large-grained zones exhibit a low density of
segregation and a relatively unhindered diffusion characteristic. The fraction with
small-sized grains, however, exhibits a relatively large segregation density which
has a kinetic limiting effect on grain growth.
Electrical resistivity. As the grain size decreases, the resistivity of a metal increases
approximately with the reciprocal grain size. This is due to the electron scattering at
the grain boundaries. The grain boundary resistivity contribution, ρ GB [22], can be
taken as an additive factor to the resistivity originating from the point-like defects in
the crystals and the thermal excitation. Interestingly, the grain boundary resistivity
contribution depends on the nature of the grain boundary, i.e., low-angle grain boundaries do not acts as scattering centres. This is why grain size as determined from either
EBDS or TEM can be used for the calculations, while grain size obtained from XRD
yields to incorrect resistivity result. The measurement of the electrical resistivity is a
sensitive and simple test of the grain size of metallic elements [23]. However, it must
be kept in mind that metallic elements of relatively large self-diffusion coefficient
(like Cu and Ag) tend to undergo self-annealing, and their grain size can drastically
grow after the synthesis of the as-received nanocrystalline specimens, which can
be easily detected by resistivity measurements. In contrast to elementary metals,
the measurement of the electrical resistivity is not a proper test of the grain size of
alloys since the chemical disorder itself usually causes about an order of magnitude
resistivity enhancement as compared to the microcrystalline form of the constituent
elements which hides the impact of the grain size on the resistivity.
Magnetic properties. Many early reports on consolidated nanocrystalline
magnetic materials showed a drastic decrease in saturation magnetization as the
grain size was diminished. Later on, it turned out that the consolidated nanocrystalline specimens studied were not fully compact and the magnetization loss was
strongly related to the contact with reactive gases (also with air). It was shown
first for electrodeposited nickel [24] that the grain size dependence of the saturation magnetization is nearly negligible, which was later confirmed for properly
treated consolidated nc-Ni [25]. Numerical calculation showed for nickel [26] that
the disorder occurring at the grain boundaries is responsible for at most 15% loss in
saturation magnetization contribution of the atoms in the boundary layer. Nowadays,
the general view is that the grain size itself does not impact significantly the saturation magnetization of magnetic metals. Studies on the saturation magnetization
of electrodeposited nickel as a function of grain size show a slight but systematic
decrease of saturation magnetization with decreasing grain size, which may occur
as a synergetic effect of the grain boundary disorder and the preferential impurity
incorporation at the grain boundaries. Since the presence of alloying elements itself
and the chemical disorder due to alloying has a much larger effect on the saturation
magnetization, the assessment of the structure effect alone is possible for elements
only.
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much larger mean value than that in the original specimen. However, when chemical segregation can also take place during annealing and it creates heterogeneous
zones with significantly different diffusion characteristics, the grain size distribution
may become bimodal. In this case, the large-grained zones exhibit a low density of
segregation and a relatively unhindered diffusion characteristic. The fraction with
small-sized grains, however, exhibits a relatively large segregation density which
has a kinetic limiting effect on grain growth.
Electrical resistivity. As the grain size decreases, the resistivity of a metal increases
approximately with the reciprocal grain size. This is due to the electron scattering at
the grain boundaries. The grain boundary resistivity contribution, ρ GB [22], can be
taken as an additive factor to the resistivity originating from the point-like defects in
the crystals and the thermal excitation. Interestingly, the grain boundary resistivity
contribution depends on the nature of the grain boundary, i.e., low-angle grain boundaries do not acts as scattering centres. This is why grain size as determined from either
EBDS or TEM can be used for the calculations, while grain size obtained from XRD
yields to incorrect resistivity result. The measurement of the electrical resistivity is a
sensitive and simple test of the grain size of metallic elements [23]. However, it must
be kept in mind that metallic elements of relatively large self-diffusion coefficient
(like Cu and Ag) tend to undergo self-annealing, and their grain size can drastically
grow after the synthesis of the as-received nanocrystalline specimens, which can
be easily detected by resistivity measurements. In contrast to elementary metals,
the measurement of the electrical resistivity is not a proper test of the grain size of
alloys since the chemical disorder itself usually causes about an order of magnitude
resistivity enhancement as compared to the microcrystalline form of the constituent
elements which hides the impact of the grain size on the resistivity.
Magnetic properties. Many early reports on consolidated nanocrystalline
magnetic materials showed a drastic decrease in saturation magnetization as the
grain size was diminished. Later on, it turned out that the consolidated nanocrystalline specimens studied were not fully compact and the magnetization loss was
strongly related to the contact with reactive gases (also with air). It was shown
first for electrodeposited nickel [24] that the grain size dependence of the saturation magnetization is nearly negligible, which was later confirmed for properly
treated consolidated nc-Ni [25]. Numerical calculation showed for nickel [26] that
the disorder occurring at the grain boundaries is responsible for at most 15% loss in
saturation magnetization contribution of the atoms in the boundary layer. Nowadays,
the general view is that the grain size itself does not impact significantly the saturation magnetization of magnetic metals. Studies on the saturation magnetization
of electrodeposited nickel as a function of grain size show a slight but systematic
decrease of saturation magnetization with decreasing grain size, which may occur
as a synergetic effect of the grain boundary disorder and the preferential impurity
incorporation at the grain boundaries. Since the presence of alloying elements itself
and the chemical disorder due to alloying has a much larger effect on the saturation
magnetization, the assessment of the structure effect alone is possible for elements
only.