215
one dimension. These confinement effects are similar for electron
transport in nanomaterials.
A good way to understand quantum confinement is to consider that
the presence of nearby surfaces in 0-D, 1-D, and 2-D nanostructures causes a change in the distribution of the phonon frequencies as a function of phonon wavelength as well as the appearance
of surface phonon modes. These processes lead to changes in the
velocity with which the variations in the shape of the wave’s amplitude propagate, the so-called group velocity. This is similar to a group
or ring of waves forming when a stone hits the surface of water. In
addition, the phonon lifetime is modified due to phonon-phonon
interaction and free surface and grain boundary scattering.
On this basis, a phonon bottleneck occurs in 0-D nanostructures.
One-dimensional nanomaterials behave as a phonon waveguide
similar to optical ones for light. For example, for carbon nanotubes,
several authors have predicted very high thermal conductivity along
the nanotubes, close to 3000 Wm
−1
K
−1
. Just as a comparison, we
should keep in mind that the thermal conductivity of copper is
approximately 400 Wm
−1 K
−1
. Despite these results, there are still
open questions about phonon transport in 1-D nanostructures,
particularly regarding the phonon/phonon interaction and the
role of defects. Ideally, in the future we would like to be able to
design nanowires and nanotubes with either high or low thermal
conductivities.
In the case of 2-D nanomaterials, there is also a great interest in
understanding the thermal properties of these materials because of
their wide application as components for handheld PCs and cellular phones; everyday home appliances and various modern
medical devices; coatings for radiation shielding, wear resistance,
and thermal barriers; and components for flat-panel display and
photovoltaic applications. In this regard we should distinguish
among 2-D single-layered nanomaterials with thicknesses at the
nanoscale, multilayered films composed of several nanoscale layers,
and thin films comprising a collection of nanostructured units.
These nanostructured thin film materials can be subdivided into
nanocrystalline materials and nanoporous materials that contain
nanovoids. In this regard, nanoporous materials are generally
selected as dielectric materials for the microelectronic industry
due to their low dielectric constants. However, their thermal conductivities are low, which is a problem.
Starting with single-layered nanoscale thin films, most results
show that the thermal conductivity is less than those of the
Thermal Properties of Nanomaterials
one dimension. These confinement effects are similar for electron
transport in nanomaterials.
A good way to understand quantum confinement is to consider that
the presence of nearby surfaces in 0-D, 1-D, and 2-D nanostructures causes a change in the distribution of the phonon frequencies as a function of phonon wavelength as well as the appearance
of surface phonon modes. These processes lead to changes in the
velocity with which the variations in the shape of the wave’s amplitude propagate, the so-called group velocity. This is similar to a group
or ring of waves forming when a stone hits the surface of water. In
addition, the phonon lifetime is modified due to phonon-phonon
interaction and free surface and grain boundary scattering.
On this basis, a phonon bottleneck occurs in 0-D nanostructures.
One-dimensional nanomaterials behave as a phonon waveguide
similar to optical ones for light. For example, for carbon nanotubes,
several authors have predicted very high thermal conductivity along
the nanotubes, close to 3000 Wm
−1
K
−1
. Just as a comparison, we
should keep in mind that the thermal conductivity of copper is
approximately 400 Wm
−1 K
−1
. Despite these results, there are still
open questions about phonon transport in 1-D nanostructures,
particularly regarding the phonon/phonon interaction and the
role of defects. Ideally, in the future we would like to be able to
design nanowires and nanotubes with either high or low thermal
conductivities.
In the case of 2-D nanomaterials, there is also a great interest in
understanding the thermal properties of these materials because of
their wide application as components for handheld PCs and cellular phones; everyday home appliances and various modern
medical devices; coatings for radiation shielding, wear resistance,
and thermal barriers; and components for flat-panel display and
photovoltaic applications. In this regard we should distinguish
among 2-D single-layered nanomaterials with thicknesses at the
nanoscale, multilayered films composed of several nanoscale layers,
and thin films comprising a collection of nanostructured units.
These nanostructured thin film materials can be subdivided into
nanocrystalline materials and nanoporous materials that contain
nanovoids. In this regard, nanoporous materials are generally
selected as dielectric materials for the microelectronic industry
due to their low dielectric constants. However, their thermal conductivities are low, which is a problem.
Starting with single-layered nanoscale thin films, most results
show that the thermal conductivity is less than those of the
Thermal Properties of Nanomaterials