arXiv subjects

Laura de Sousa Oliveira

Publications and source records attributed to Laura de Sousa Oliveira.

10 recordsLinked to original sources

arXiv2024

Super-suppression of long wavelength phonons in constricted nanoporous geometries

In a typical semiconductor material, the majority of heat is carried by long wavelength, long mean-free-path phonons. Nanostructuring strategies to reduce thermal conductivity, a promising direction in the field of thermoelectrics, place scattering centers of size and spatial separation comparable to the mean-free-paths of the dominant phonons to selectively scatter them. The resultant thermal conductivity is in most cases well predicted using Matthiessens rule. In general, however, long wavelength phonons are not as effectively scattered as the rest of the phonon spectrum. In this work, using large-scale Molecular Dynamics simulations, Non-Equilibrium Greens Function simulations, and Monte Carlo simulations, we show that specific nanoporous geometries, which create narrow constrictions in the passage of phonons, lead to anticorrelated heat currents in the phonon spectrum. This results in super-suppression of long-wavelength phonons due to heat trapping, and reductions in the thermal conductivity well below what is predicted by Matthiessens rule.

cond-mat.mes-hall↗

arXiv2022

Super-Suppression of Long Phonon Mean-Free-Paths in Nano-engineered Si due to Heat Current Anticorrelations

The ability to minimize the thermal conductivity of dielectrics with minimal structural intervention that could affect electrical properties is an important capability for engineering thermoelectric efficiency in low-cost materials such as Si. We recently reported the discovery of special arrangements for nanoscale pores in Si that produce a particularly large reduction in thermal conductivity accompanied by strongly anticorrelated heat current fluctuations, a phenomenon that is missed by the diffuse adiabatic boundary conditions conventionally used in numerical Boltzmann transport models. This manuscript presents the results of molecular dynamics simulations and a Monte Carlo ray tracing model that teases apart this phenomenon to reveal that special pore layouts elastically backscatter long-wavelength heat-carrying phonons. This means that heat carriage by a phonon before scattering is undone by the scattered phonon, resulting in an effective mean-free-path that is significantly shorter than the geometric line-of-sight to the pores. This effect is particularly noticeable for the long-wavelength, long mean-free-path phonons whose transport is impeded drastically more than is expected purely from the usual considerations of scattering defined by the distance between defects. This super-suppression of the mean-free-path below the characteristic length scale of the nanostructuring offers a route for minimizing thermal conductivity with minimal structural impact, while the stronger impact on long wavelengths offers possibilities for the design of band-pass phonon filtering. Moreover, the ray tracing model developed in this paper shows that different forms of correlated scattering imprint a unique signature in the heat current autocorrelation function that could be used as a diagnostic in other nanostructured systems.

cond-mat.mtrl-sci↗

arXiv2020

Hierarchically nanostructured thermoelectric materials: Challenges and opportunities for improved power factors

The field of thermoelectric materials has undergone a revolutionary transformation over the last couple of decades as a result of the ability to nanostructure and synthesize myriads of materials and their alloys. The ZT figure of merit, which quantifies the performance of a thermoelectric material has more than doubled after decades of inactivity, reaching values larger than two, consistently across materials and temperatures. Central to this ZT improvement is the drastic reduction in the material thermal conductivity due to the scattering of phonons on the numerous interfaces, boundaries, dislocations, point defects, phases, etc., which are purposely included. In these new generation of nanostructured materials, phonon scattering centers of different sizes and geometrical configurations (atomic, nano- and macro-scale) are formed, which are able to scatter phonons of mean-free-paths across the spectrum. Beyond thermal conductivity reductions, ideas are beginning to emerge on how to use similar hierarchical nanostructuring to achieve power factor improvements. Ways that relax the adverse interdependence of the electrical conductivity and Seebeck coefficient are targeted, which allows power factor improvements. For this, elegant designs are required, that utilize for instance non-uniformities in the underlying nanostructured geometry, non-uniformities in the dopant distribution, or potential barriers that form at boundaries between materials. A few recent reports, both theoretical and experimental, indicate that extremely high power factor values can be achieved, even for the same geometries that also provide ultra-low thermal conductivities. Despite the experimental complications that can arise in having the required control in nanostructure realization, in this colloquium, we aim to demonstrate, mostly theoretically, that it is a very promising path worth exploring.

cond-mat.mtrl-sci↗

arXiv2020

Heat current anticorrelation effects leading to thermal conductivity reduction in nanoporous Si

Prevailing nanostructuring strategies focus on increasing phonon scattering and reducing the mean-free-path of phonons across the spectrum. In nanoporous Si materials, for example, boundary scattering reduces thermal conductivity drastically. In this work, we identify an unusual anticorrelated specular phonon scattering effect which can result in additional reductions in thermal conductivity of up to ~ 80% for specific nanoporous geometries. We further find evidence that this effect has its origin in heat trapping between large pores with narrow necks. As the heat becomes trapped between the pores, phonons undergo multiple specular reflections such that their contribution to the thermal conductivity is partly undone. We find this effect to be wave-vector dependent at low temperatures. We use large-scale molecular dynamics simulations, wave packet analysis, as well as an analytical model to illustrate the anticorrelation effect, evaluate its impact on thermal conductivity, and detail how it can be controlled to manipulate phonon transport in nanoporous materials.

cond-mat.mes-hall↗

arXiv2020

Effect of wave versus particle phonon nature in thermal transport through nanostructures

Comprehensive understanding of thermal transport in nanostructured materials needs large scale simulations bridging length scales dictated by different physics related to the wave versus particle nature of phonons. Yet, available computational approaches implicitly treat phonons as either just waves or as particles. In this work, using a full wave-based Non-Equilibrium Green's Function (NEGF) method, and a particle-based ray-tracing Monte Carlo (MC) approach, we investigate the qualitative differences in the wave and particle-based phonon transport at the vicinity of nanoscale features. For the simple example of a nanoporous geometry, we show that phonon transmission agrees very well for both methods with an error margin of approximately 15%, across phonon wavelengths even for features with sizes down to 3-4 nm. For cases where phonons need to squeeze in smaller regions to propagate, we find that MC underestimates the transmission of long wavelength phonons whereas wave treatment within NEGF indicates that those long wavelength phonons can propagate more easily. We also find that particle-based simulation methods are somewhat more sensitive to structural variations compared to the wave-based NEGF method. The insight extracted from comparing wave and particle methods can be used to provide a better and more complete understanding of phonon transport in nanomaterials.

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arXiv2019

Modelling thermoelectric performance in nanoporous nanocrystalline silicon

Introducing hierarchical disorder from multiple defects into materials through nanostructuring is one of the most promising directions to achieve extremely low thermal conductivities and thus improve thermoelectric performance. The success of nanostructuring relies on charge carriers having shorter mean-free-paths than phonons so that the latter can be selectively scattered. Nevertheless, introducing disorder into a material often comes at the expense of scattering charge carriers as well as phonons. In order to determine the tradeoff between the degradation of the lattice thermal conductivity and of the power factor due to this, we perform a theoretical investigation of both phonon and electron transport in nanocrystalline, nanoporous Si geometries. We use molecular dynamics for phonon transport calculations and the non-equilibrium Green's function method for electronic transport. We report on the engineering tradeoff that the porosity (number of pores and their in-between distance) has on the overall thermoelectric performance for the material optimization. We indeed find that the reduction in thermal conductivity is stronger compared to the reduction in the power factor, for the low porosities considered in this study (up to 5 %), and that the ZT figure of merit can experience a large increase, especially when grain boundaries are included, compared to just nanoporosity.

cond-mat.mtrl-sci↗

arXiv2019

Large-scale molecular dynamics investigation of geometrical features in nanoporous Si

Nanoporous materials are of broad interest for various applications, in particular advanced thermoelectric materials. The introduction of nanoscale porosity, even at modest levels, has been known to drastically reduce a materials thermal conductivity, in some cases even below its amorphous limit, thereby significantly increasing its thermoelectric figure of merit, ZT. The details of the important attributes that drive these large reductions, however, are not yet clear. In this work, we employ large-scale equilibrium molecular dynamics to perform an exhaustive atomistic-scale investigation of the effect of porosity on thermal transport in nanoporous bulk silicon. Thermal transport is computed for over 50 different geometries, spanning a large number of geometrical degrees of freedom, such as cylindrical pores and voids, different porosities, diameters, neck sizes, pore/void numbers, and surface-to-volume ratios, placed in ordered fashion, or fully disordered. We thus quantify and compare the most important parameters that determine the thermal conductivity reductions in nanoporous materials. Ultimately, we find that, even at the nanoscale, the effect of merely reducing the line-of-sight of phonons, i.e. the clear pathways that phonons can utilize during transport, plays the most crucial role in reducing the thermal conductivity in nanoporous materials, beyond other metrics such as porosity and surface/boundary scattering.

cond-mat.mes-hall↗

arXiv2019

Transport simulations in hierarchically disordered nanostructures for thermoelectric material design

Hierarchically nanostructured materials, where disorder is introduced in various length scales (at the atomic scale, the nanoscale, and the mesoscale) is one of the most promising directions to achieve extremely low thermal conductivities and improve thermoelectric performance. Here we theoretically investigate one such system, a nanocrystalline material with nanopores that are introduced between the crystalline regions. We use the Nonequilibrium Greens Function method for electronic transport and Molecular Dynamics for phonon transport.

cond-mat.mtrl-sci↗

arXiv2019

Enhanced Phonon Boundary Scattering at High Temperatures in Hierarchically Disordered Nanostructures

Boundary scattering in hierarchically disordered nanomaterials is an effective way to reduce the thermal conductivity of thermoelectric materials and increase their performance. In this work we investigate thermal transport in silicon based nanostructured materials in the presence of nanocrystallinity and nanopores at the range of 300 K to 900 K using a Monte Carlo simulation approach. The thermal conductivity in the presence of nanocrystallinity follows the same reduction trend as in the pristine material. We show, however, that the relative reduction is stronger with temperature in the presence of nanocrystallinity, a consequence of the wavevector dependent (q dependent) nature of phonon scattering on the domain boundaries. In particular, as the temperature is raised the proportion of large wavevector phonons increases. Since these phonons are more susceptible to boundary scattering, we show that compared to the wavevector independent thermal conductivity value this q dependent surface scattering could account for even a approx. 40 per cent reduction in the thermal conductivity of nanocrystalline Si. Introduction of nanopores with randomized positions enhances this effect, which suggests that hierarchical nanostructuring is actually more effective at high temperatures than previously thought.

cond-mat.mtrl-sci↗

arXiv2018

Spin phonon interactions and magneto-thermal transport behavior in p-Si

The spin-phonon interaction is the dominant process for spin relaxation in Si, and as thermal transport in Si is dominated by phonons, one would expect spin polarization to influence Si's thermal conductivity. Here we report the experimental evidence of just such a coupling. We have performed concurrent measurements of spin, charge, and phonon transport in p-doped Si across a wide range of temperatures. In an experimental system of a freestanding two um p-Si beam coated on one side with a thin (25 nm) ferromagnetic spin injection layer, we use the self-heating 3 omega method to measure changes in electrical and thermal conductivity under the influence of a magnetic field. These magneto-thermal transport measurements reveal signatures in the variation of electrical and thermal transport that are consistent with spin-phonon interaction. Raman spectroscopy measurements and first principle's calculations support that these variations are due to spin-phonon interaction. Spin polarization leads to softening of phonon modes, a reduction in the group velocity of acoustic modes, and a subsequent decrease in thermal conductivity at room temperature. Moreover, magneto-thermal transport measurements as a function of temperature indicate a change in the spin-phonon relaxation behavior at low temperature.

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