chaPter 7 nanomaterials: Properties
218
The heat capacity and coefficient of thermal expansion of nanomaterials have been much less studied. However, nanocrystalline iron
was found to exhibit an enhanced heat capacity relative to coarsegrained polycrystalline iron. This effect has been attributed to the
entropy contribution to the heat capacity as a result of the large
fraction of grain boundaries. On the other hand, a study of nano
zinc oxide flakes showed that the heat capacity is lower than that of
coarse Zn oxide in the temperature range between 83°K to 103°K,
whereas above 103°K the opposite is true. Again, the increase in
the configuration and vibration entropy of grain boundaries could
explain the behavior above 103°K. However, for temperatures below
103°K, the results are difficult to explain. In terms of coefficient of
thermal expansion, nanoparticles of silver with 3.2 nm average size
embedded in glass showed an increase of the expansion parameter
with temperature, compared with bulk silver. However, for larger
silver particles with 5.1 nm average size, no changes in the coefficient of thermal expansion were observed with respect to bulk
material. This overall behavior is claimed to be due to the higher
surface-to-volume ratio of smaller particles and the bonds created
across the particle-glass interface. In the case of single-walled carbon
nanotubes, it has been shown that the coefficient of thermal expansion is very low. This is due to in-plane expansion, bond stretching,
and bond bending effects that cancel each other out. Therefore, a
nanocomposite of aluminum reinforced with 15% volume fraction of single-walled carbon nanotubes exhibited a coefficient of
thermal expansion that is one-third that of pure nano aluminum.
7.3 electrical ProPerties
In Section 4.5, we discussed the electronic conduction of electrons
in systems considered large in size compared with the nanoscale.
In this case, the conduction of electrons is delocalized, that is, electrons can move freely in all dimensions. As they travel their paths,
the electrons are primarily scattered by various mechanisms, such
as phonons, impurities, and interfaces, resembling a random walk
process. However, as the system length scale is reduced to the nanoscale, two effects are of importance: (1) the quantum effect, where
due to electron confinement the energy bands are replaced by discreet energy states, leading to cases where conducting materials can
behave as semiconductors or insulators, and (2) the classical effect,
where the mean-free path for inelastic scattering becomes comparable with the size of the system, leading to a reduction in scattering
events.
218
The heat capacity and coefficient of thermal expansion of nanomaterials have been much less studied. However, nanocrystalline iron
was found to exhibit an enhanced heat capacity relative to coarsegrained polycrystalline iron. This effect has been attributed to the
entropy contribution to the heat capacity as a result of the large
fraction of grain boundaries. On the other hand, a study of nano
zinc oxide flakes showed that the heat capacity is lower than that of
coarse Zn oxide in the temperature range between 83°K to 103°K,
whereas above 103°K the opposite is true. Again, the increase in
the configuration and vibration entropy of grain boundaries could
explain the behavior above 103°K. However, for temperatures below
103°K, the results are difficult to explain. In terms of coefficient of
thermal expansion, nanoparticles of silver with 3.2 nm average size
embedded in glass showed an increase of the expansion parameter
with temperature, compared with bulk silver. However, for larger
silver particles with 5.1 nm average size, no changes in the coefficient of thermal expansion were observed with respect to bulk
material. This overall behavior is claimed to be due to the higher
surface-to-volume ratio of smaller particles and the bonds created
across the particle-glass interface. In the case of single-walled carbon
nanotubes, it has been shown that the coefficient of thermal expansion is very low. This is due to in-plane expansion, bond stretching,
and bond bending effects that cancel each other out. Therefore, a
nanocomposite of aluminum reinforced with 15% volume fraction of single-walled carbon nanotubes exhibited a coefficient of
thermal expansion that is one-third that of pure nano aluminum.
7.3 electrical ProPerties
In Section 4.5, we discussed the electronic conduction of electrons
in systems considered large in size compared with the nanoscale.
In this case, the conduction of electrons is delocalized, that is, electrons can move freely in all dimensions. As they travel their paths,
the electrons are primarily scattered by various mechanisms, such
as phonons, impurities, and interfaces, resembling a random walk
process. However, as the system length scale is reduced to the nanoscale, two effects are of importance: (1) the quantum effect, where
due to electron confinement the energy bands are replaced by discreet energy states, leading to cases where conducting materials can
behave as semiconductors or insulators, and (2) the classical effect,
where the mean-free path for inelastic scattering becomes comparable with the size of the system, leading to a reduction in scattering
events.