866 research outputs found
Phonon heat conduction in layered anisotropic crystals
The thermal properties of anisotropic crystals are of both fundamental and
practical interest, but transport phenomena in anisotropic materials such as
graphite remain poorly understood because solutions of the Boltzmann equation
often assume isotropy. Here, we extend an analytical solution of the Boltzmann
equation to highly anisotropic solids and examine its predictions for graphite.
We show that the phonon mean free paths in the cross-plane direction can be
comparable to those in the in-plane direction despite the low cross-plane
thermal conductivity, which instead arises primarily from the differences in
group velocities and phonon frequencies supported along each direction.
Additionally, we demonstrate a method to reconstruct the anisotropic mean free
path spectrum of crystals with arbitrary dispersion relations without any prior
knowledge of their harmonic or anharmonic properties using observations of
quasiballistic heat conduction
Multidimensional quasiballistic thermal transport in transient grating spectroscopy
Transient grating spectroscopy has emerged as a useful technique to study
thermal phonon transport because of its ability to perform thermal measurements
over length scales comparable to phonon mean free paths (MFPs). While several
prior works have performed theoretical studies of quasiballistic heat
conduction in transient grating, the analysis methods are either restricted to
one spatial dimension or require phenomenological fitting parameters. Here, we
analyze quasiballistic transport in a two-dimensional transient grating
experiment, in which heat conduction can occur both in- and cross-plane, using
an analytic Green's function of the Boltzmann equation we recently reported
that is free of fitting parameters. We demonstrate a method by which phonon
MFPs can be extracted from these measurements, thereby extending the MFP
spectroscopy technique using transient grating to opaque bulk materials
Towards a microscopic understanding of phonon heat conduction
Heat conduction by phonons is a ubiquitous process that incorporates a wide
range of physics and plays an essential role in applications ranging from space
power generation to LED lighting. Heat conduction has been studied for over two
hundred years, yet many microscopic aspects of heat conduction have remained
unclear in most crystalline solids, including which phonons carry heat and how
natural and artificial structures scatter specific phonons. Fortunately, recent
advances in both computation and experiment are enabling an unprecedented
microscopic view of thermal transport by phonons. In this topical review, we
provide an overview of these methods, the insights they are providing, and
their impact on the science and engineering of heat conduction
Thermal phonon boundary scattering in anisotropic thin films
Boundary scattering of thermal phonons in thin solid films is typically
analyzed using Fuchs-Sondheimer theory, which provides a simple equation to
calculate the reduction of thermal conductivity as a function of the film
thickness. However, this widely-used equation is not applicable to highly
anisotropic solids like graphite because it assumes the phonon dispersion is
isotropic. Here, we derive a generalization of the Fuchs-Sondheimer equation
for solids with arbitrary dispersion relations and examine its predictions for
graphite. We find that the isotropic equation vastly overestimates the boundary
scattering that occurs in thin graphite films due to the highly anisotropic
group velocity, and that graphite can maintain its high in-plane thermal
conductivity even in thin films with thicknesses as small as ten nanometers
Cross-plane heat conduction in thin solid films
Cross-plane heat transport in thin films with thickness comparable to the
phonon mean free paths is of both fundamental and practical interest. However,
physical insight is difficult to obtain for the cross-plane geometry due to the
challenge of solving the Boltzmann equation in a finite domain. Here, we
present a semi-analytical series expansion method to solve the transient,
frequency-dependent Boltzmann transport equation that is valid from the
diffusive to ballistic transport regimes and rigorously includes
frequency-dependence of phonon properties. Further, our method is more than
three orders of magnitude faster than prior numerical methods and provides a
simple analytical expression for the thermal conductivity as a function of film
thickness. Our result enables a more accurate understanding of heat conduction
in thin films
Importance of frequency-dependent grain boundary scattering in nanocrystalline silicon and silicon-germanium thermoelectrics
Nanocrystalline silicon and silicon-germanium alloys are promising
thermoelectric materials that have achieved substantially improved figure of
merits compared to their bulk counterparts. This enhancement is typically
attributed to a reduction in lattice thermal conductivity by phonon scattering
at grain boundaries. However, further improvements are difficult to achieve
because grain boundary scattering is poorly understood, with recent
experimental observations suggesting that the phonon transmissivity may depend
on phonon frequency rather than being constant as in the commonly used gray
model. Here, we examine the impact of frequency-dependent grain boundary
scattering in nanocrystalline silicon and silicon-germanium alloys in a
realistic 3D geometry using frequency-dependent variance-reduced Monte Carlo
simulations. We find that the grain boundary may not be as effective as
predicted by the gray model in scattering certain phonons, with a substantial
amount of heat being carried by low frequency phonons with mean free paths
longer than the grain size. Our result will help guide the design of more
efficient thermoelectrics
Heat conduction in multifunctional nanotrusses studied using Boltzmann transport equation
Materials that possess low density, low thermal conductivity, and high
stiffness are desirable for engineering applications, but most materials cannot
realize these properties simultaneously due to the coupling between them.
Nanotrusses, which consist of hollow nanoscale beams architected into a
periodic truss structure, can potentially break these couplings due to their
lattice architecture and nanoscale features. In this work, we study heat
conduction in the exact nanotruss geometry by solving the frequency-dependent
Boltzmann transport equation using a variance-reduced Monte Carlo algorithm. We
show that their thermal conductivity can be described with only two parameters,
solid fraction and wall thickness. Our simulations predict that nanotrusses can
realize unique combinations of mechanical and thermal properties that are
challenging to achieve in typical materials
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