44 research outputs found
Experimental and numerical investigation of phonon mean free path distribution
Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2013.Cataloged from PDF version of thesis.Includes bibliographical references (p. 99-107).Knowledge of phonon mean free path (MFP) distribution is critically important to engineering size effects. Phenomenological models of phonon relaxation times can give us some sense about the mean free path distribution, but they are not accurate. Further improvement of thermoelectric performance requires the phonon MFP to be known. In this thesis, we improve recently developed thermal conductivity spectroscopy technique to experimentally measure MFPs using ultrafast transient thermoreflectance method. By optically heating lithographically patterned metallic nanodot arrays, we are able to probe heat transfer at length scales down to 100 nm, far below the diffraction limit for visible light. We demonstrate the new implementation by measuring MFPs in sapphire at room temperature. A multidimensional transport model based on the grey phonon Boltzmann equation is developed and solved to study the quasi-ballistic transport occurring in the spectroscopy experiments. To account for the nonlinear dispersion relation, we present a variance reduced Monte Carlo scheme to solve the full Boltzmann transport equation and compare the simulation results with experimental data on silicon.by Lingping Zeng.S.M
A Variational Approach to Extracting the Phonon Mean Free Path Distribution from the Spectral Boltzmann Transport Equation
The phonon Boltzmann transport equation (BTE) is a powerful tool for studying
non-diffusive thermal transport. Here, we develop a new universal variational
approach to solving the BTE that enables extraction of phonon mean free path
(MFP) distributions from experiments exploring non-diffusive transport. By
utilizing the known Fourier solution as a trial function, we present a direct
approach to calculating the effective thermal conductivity from the BTE. We
demonstrate this technique on the transient thermal grating (TTG) experiment,
which is a useful tool for studying non-diffusive thermal transport and probing
the mean free path (MFP) distribution of materials. We obtain a closed form
expression for a suppression function that is materials dependent, successfully
addressing the non-universality of the suppression function used in the past,
while providing a general approach to studying thermal properties in the
non-diffusive regime.Comment: 17 pages, 2 figure
Tailoring Thermal Conductivity of Single-stranded Carbon-chain Polymers through Atomic Mass Modification
Tailoring the thermal conductivity of polymers is central to enlarge their applications in the thermal management of flexible integrated circuits. Progress has been made over the past decade by fabricating materials with various nanostructures, but a clear relationship between various functional groups and thermal properties of polymers remains to be established. Here, we numerically study the thermal conductivity of single-stranded carbon-chain polymers with multiple substituents of hydrogen atoms through atomic mass modification. We find that their thermal conductivity can be tuned by atomic mass modifications as revealed through molecular dynamics simulations. The simulation results suggest that heavy homogeneous substituents do not assist heat transport and trace amounts of heavy substituents can in fact hinder heat transport substantially. Our analysis indicates that carbon chain has the biggest contribution (over 80%) to the thermal conduction in single-stranded carbon-chain polymers. We further demonstrate that atomic mass modifications influence the phonon bands of bonding carbon atoms, and the discrepancies of phonon bands between carbon atoms are responsible for the remarkable drops in thermal conductivity and large thermal resistances in carbon chains. Our study provides fundamental insight into how to tailor the thermal conductivity of polymers through variable substituents.National Science Council (China) (51376069)National Key Basic Research Program of China (2013CB228302
Thermal Conductivity of GaAs/Ge Nanostructures
Superlattices are promising low-dimensional nanomaterials for thermoelectric
technology that is capable of directly converting low-grade heat energy to
useful electrical power. In this work, the thermal conductivities of GaAs/Ge
superlattice nanostructures were investigated systematically in relation to
their morphologies and interfaces. Thermal conductivities were measured using
ultrafast time-domain thermoreflectance and were found to decrease with
increasing interface densities, consistent with our understanding of
microscopic phonon transport in the particle regime. Lower thermal
conductivities were observed in (GaAs)0.77(Ge2)0.23 alloys; transmission
electron microscopy study reveals phase separation in the alloys. These alloys
can be interpreted as fine nanostructures, with length scales comparable to the
periods of very thin superlattices. Our experimental findings help gain
fundamental insight into nanoscale thermal transport in superlattices and are
also useful for future improvement of thermoelectric performance using
superlattice nanostructures.Comment: 11 pages, 5 figure
Spectral mapping of thermal conductivity through nanoscale ballistic transport
Controlling thermal properties is central to many applications, such as thermoelectric energy conversion and the thermal management of integrated circuits. Progress has been made over the past decade by structuring materials at different length scales, but a clear relationship between structure size and thermal properties remains to be established. The main challenge comes from the unknown intrinsic spectral distribution of energy among heat carriers. Here, we experimentally measure this spectral distribution by probing quasi-ballistic transport near nanostructured heaters down to 30 nm using ultrafast optical spectroscopy. Our approach allows us to quantify up to 95% of the total spectral contribution to thermal conductivity from all phonon modes. The measurement agrees well with multiscale and first-principles-based simulations. We further demonstrate the direct construction of mean free path distributions. Our results provide a new fundamental understanding of thermal transport and will enable materials design in a rational way to achieve high performance
Disparate quasiballistic heat conduction regimes from periodic heat sources on a substrate
We report disparate quasiballistic heat conduction trends for periodic nanoscale line heaters deposited on a substrate, depending upon whether measurements are based on the peak temperature of the heaters or the temperature difference between the peak and the valley of two neighboring heaters. The degree of quasiballistic transport is characterized by the effective thermal conductivities of the substrate which are obtained by matching the diffusion solutions to the phonon Boltzmann transport equation results. We find that while the ballistic heat conduction effect based on the peak temperature diminishes as the two heaters become closer, it becomes stronger based on the peak-valley temperature difference. Our results also show that the collective behavior of closely spaced heaters can counteract the nonlocal effects caused by an isolated nanoscale hot spot. These results are relevant to thermal conductivity spectroscopy techniques under development and also have important implications for understanding nonlocal heat conduction in integrated circuits and carbon nanotube array thermal interface materials.United States. Dept. of Energy. Office of Science (Award DE-SC0001299/DE-FG02-09ER46577
Unifying first principle theoretical predictions and experimental measurements of size effects on thermal transport in SiGe alloys
In this work, we demonstrate the correspondence between first principle
calculations and experimental measurements of size effects on thermal transport
in SiGe alloys. Transient thermal grating (TTG) is used to measure the
effective thermal conductivity. The virtual crystal approximation under the
density functional theory (DFT) framework combined with impurity scattering is
used to determine the phonon properties for the exact alloy composition of the
measured samples. With these properties, classical size effects are calculated
for the experimental geometry of reflection mode TTG using the
recently-developed variational solution to the phonon Boltzmann transport
equation (BTE), which is verified against established Monte Carlo simulations.
We find agreement between theoretical predictions and experimental measurements
in the reduction of thermal conductivity (as much as 25\% of the bulk
value) across grating periods spanning one order of magnitude. This work
provides a framework for the tabletop study of size effects on thermal
transport
Monte Carlo study of non-diffusive relaxation of a transient thermal grating in thin membranes
The impact of boundary scattering on non-diffusive thermal relaxation of a transient grating in thin membranes is rigorously analyzed using the multidimensional phononBoltzmann equation. The gray Boltzmann simulation results indicate that approximating models derived from previously reported one-dimensional relaxation model and Fuchs-Sondheimer model fail to describe the thermal relaxation of membranes with thickness comparable with phonon mean free path. Effective thermal conductivities from spectral Boltzmann simulations free of any fitting parameters are shown to agree reasonably well with experimental results. These findings are important for improving our fundamental understanding of non-diffusive thermal transport in membranes and other nanostructures.United States. Dept. of Energy. Office of Science (Solid-State Solar-Thermal Energy Conversion Center Award DE-SC0001299/DE-FG02-09ER46577
Losses in plasmonics: from mitigating energy dissipation to embracing loss-enabled functionalities
Unlike conventional optics, plasmonics enables unrivalled concentration of
optical energy well beyond the diffraction limit of light. However, a
significant part of this energy is dissipated as heat. Plasmonic losses present
a major hurdle in the development of plasmonic devices and circuits that can
compete with other mature technologies. Until recently, they have largely kept
the use of plasmonics to a few niche areas where loss is not a key factor, such
as surface enhanced Raman scattering and biochemical sensing. Here, we discuss
the origin of plasmonic losses and various approaches to either minimize or
mitigate them based on understanding of fundamental processes underlying
surface plasmon modes excitation and decay. Along with the ongoing effort to
find and synthesize better plasmonic materials, optical designs that modify the
optical powerflow through plasmonic nanostructures can help in reducing both
radiative damping and dissipative losses of surface plasmons. Another strategy
relies on the development of hybrid photonic-plasmonic devices by coupling
plasmonic nanostructures to resonant optical elements. Hybrid integration not
only helps to reduce dissipative losses and radiative damping of surface
plasmons, but also makes possible passive radiative cooling of nano-devices.
Finally, we review emerging applications of thermoplasmonics that leverage
Ohmic losses to achieve new enhanced functionalities. The most successful
commercialized example of a loss-enabled novel application of plasmonics is
heat-assisted magnetic recording. Other promising technological directions
include thermal emission manipulation, cancer therapy, nanofabrication,
nano-manipulation, plasmon-enabled material spectroscopy and thermo-catalysis,
and solar water treatment.Comment: 43 pages, 18 figure