Author: Xinjiang WangPublisher:
ISBN:
Category : Heat
Languages : en
Pages : 133
Book Description
First-principles Study of Phonon Transport in Low-dimensional Structures
Author: Xinjiang WangPhonons in Low Dimensional Structures
Author: Vasilios N. StavrouPublisher: BoD – Books on Demand
ISBN: 1789846269
Category : Science
Languages : en
Pages : 176
Book Description
The field of low-dimensional structures has been experiencing rapid development in both theoretical and experimental research. Phonons in Low Dimensional Structures is a collection of chapters related to the properties of solid-state structures dependent on lattice vibrations. The book is divided into two parts. In the first part, research topics such as interface phonons and polaron states, carrier-phonon non-equilibrium dynamics, directional projection of elastic waves in parallel array of N elastically coupled waveguides, collective dynamics for longitudinal and transverse phonon modes, and elastic properties for bulk metallic glasses are related to semiconductor devices and metallic glasses devices. The second part of the book contains, among others, topics related to superconductor, phononic crystal carbon nanotube devices such as phonon dispersion calculations using density functional theory for a range of superconducting materials, phononic crystal-based MEMS resonators, absorption of acoustic phonons in the hyper-sound regime in fluorine-modified carbon nanotubes and single-walled nanotubes, phonon transport in carbon nanotubes, quantization of phonon thermal conductance, and phonon Anderson localization.
Phonons in Low Dimensional Structures
Author: Vasilios N. StavrouPublisher:
ISBN: 9781789846270
Category : Physics
Languages : en
Pages : 174
Book Description
The field of low-dimensional structures has been experiencing rapid development in both theoretical and experimental research. Phonons in Low Dimensional Structures is a collection of chapters related to the properties of solid-state structures dependent on lattice vibrations. The book is divided into two parts. In the first part, research topics such as interface phonons and polaron states, carrier-phonon non-equilibrium dynamics, directional projection of elastic waves in parallel array of N elastically coupled waveguides, collective dynamics for longitudinal and transverse phonon modes, and elastic properties for bulk metallic glasses are related to semiconductor devices and metallic glasses devices. The second part of the book contains, among others, topics related to superconductor, phononic crystal carbon nanotube devices such as phonon dispersion calculations using density functional theory for a range of superconducting materials, phononic crystal-based MEMS resonators, absorption of acoustic phonons in the hyper-sound regime in fluorine-modified carbon nanotubes and single-walled nanotubes, phonon transport in carbon nanotubes, quantization of phonon thermal conductance, and phonon Anderson localization.
First-Principles Theoretical Investigation on Phonon Transport in Materials with Extreme Conductivity
Author: Huan WuPublisher:
ISBN:
Category :
Languages : en
Pages : 0
Book Description
Advanced materials with extreme thermal conductivity are critically important for various technological applications including energy conversion, storage, and thermal management. High thermal conductivity is desirable for efficient heat spreading in electronics, and low thermal conductivity is needed for thermal insulation and thermoelectric energy harvesting. However, practical application deployments are usually limited by the materials availability and understanding the fundamental origins for extreme thermal conductivity remains challenging. My PhD research focuses on applying and developing first-principles computations to understand the microscopic thermal transport mechanisms of the emerging materials and to discover new materials with ultrahigh and ultralow thermal conductivity. My dissertation is composed of three themes. The first theme is focused on understanding the fundamental origins and transport mechanisms for a group of high thermal conductivity semiconductors that were discovered recently by our group. In particular, boron phosphide (BP) and boron arsenide (BAs) crystals have been synthesized and measured with thermal conductivities of 460 and 1300 W/mK respectively, representing the best thermal conductor among common bulk metals and semiconductors. I have conducted ab initio calculations based on density functional theory to investigate phonon anharmonicity, size-dependent transport from diffusive to ballistic regime, as well as the effect from defect scattering. Our study shows that, unlike the commonly accepted rule for most materials near room temperature, high-order anharmonicity through the four-phonon process is significant in BA because of its unique band structure. In addition, I have performed multiscale Monte Carlo simulations to solve phonon Boltzmann transport equations to compute heat dissipation in three-dimensional practical measurement samples and electronic devices, which quantitively determines temperature distributed resulted by non-equilibrium phonon transport and underscores the promise of our developed BP and BAs for the next generation of thermal management technologies. The second theme of my thesis is to theoretical search for new ultra-high thermal conductivity materials, with the aim to push the limit of existing materials database. We have calculated the thermal conductivity of several B-C-X ternary compounds and found the R3m-BNC2 has ultrahigh thermal conductivity at ~2200 W/mK, which is comparable with the existing highest thermal conductivity materials, diamond. We also calculate the thermal conductivity of single-layer boron compounds in III-V group, and find high thermal conductivity of single-layer h-BAs at around 400 W/K. My computational studies enable atomistic understanding through their phonon band structures, scattering spaces, lifetimes, etc. The third theme of my thesis is to investigate phonon transport in ultralow thermal conductivity materials with a focus on tin selenide (SnSe). SnSe is a recently discovered high performance thermoelectric material, but its intrinsic low thermal conductivity remains debating in recent literature. In collaboration with my labmates, we combine phonon theory and experiments to investigate phonon softening physics. In particular, my calculated phonon frequencies of SnSe under varying temperatures indicate strong phonon renormalization due to higher-order anharmonicity. The comparison of my theory results with experiments indicates that the widely used harmonic model fails to descript the phonon renormalization and thus thermal conductivity of SnSe. Instead, I have developed self-consistent phonon theory to capture the higher order interactions and provided very good agreement with the experimentally measured ultralow thermal conductivity and thermophysical properties of SnSe.
Electron-phonon Interactions in Low-dimensional Structures
Author: Lawrence John ChallisPublisher:
ISBN: 9780198507321
Category : Science
Languages : en
Pages : 302
Book Description
The study of electrons and holes confined to two, one and even zero dimensions has uncovered a rich variety of new physics and applications. This book describes the interaction between these confined carriers and the optic and acoustic phonons within and around the confined regions. Phonons provide the principal channel of energy transfer between the carriers and their surroundings and also the main restriction to their room temperature mobility. But they have many other roles; they provide, for example, an essential feature of the operation of the quantum cascade laser. Since their momenta at relevant energies are well matched to those of electrons, they can also be used to probe electronic properties such as the confinement width of 2D electron gases and the dispersion curve of quasiparticles in the fractional quantum Hall effect. The book describes both the physics of the electron-phonon interaction in the different confined systems and the experimental and theoretical techniques that have been used in its investigation. The experimental methods include optical and transport techniques as well as techniques in which phonons are used as the experimental probe. The aim of the book is to provide an up to date review of the physics and its significance in device performance. It is also written to be explanatory and accessible to graduate students and others new to the field.
Molecular Dynamics Simulations of Phonon Thermal Transport in Low-dimensional Silicon Structures
Author: 李海鵬Publisher:
ISBN:
Category : Low-dimensional semiconductors
Languages : en
Pages : 256
Book Description
Predicting Phonon Transport in Two-Dimensional Materials Using First-Principles Calculations and the Boltzmann Transport Equation
Author: Armin TaheriPublisher:
ISBN:
Category :
Languages : en
Pages : 0
Book Description
The main objective of this thesis is to study phonon thermal transport in two-dimensional (2D) materials using first-principles density functional theory (DFT) calculations and the full solution of the Boltzmann transport equation (BTE). A wide range of 2D materials including graphene, 2D structures of group-VA, and recently emerged NX (X=P, As, Sb) compound monolayers are considered. Special attention is given to a mode-by-mode study of the thermal tunability via strain and functionalization. First, this thesis investigated the sensitivity of the DFT-calculated intrinsic thermal conductivity and phonon properties of 2D materials to the choice of exchange-correlation (XC) and pseudopotential (PP). It was found that the choice of the XC-PP combination results in significant discrepancies among predicted thermal conductivities of graphene at room temperature, in the range of 5442-8677 Wm^(-1)K^(-1). The LDA-NC and PBE-PAW combinations predicted the thermal conductivities in best agreement with available experimental data. This sensitivity analysis was an essential first step towards using DFT to engineer the phonon thermal transport in 2D systems. Next, DFT was used to systematically investigate the strain-dependent lattice thermal conductivity of -arsenene and -phosphorene, 2D monolayers of group-VA. The results showed that the thermal conductivity in both monolayers exhibits an up-and-down behavior when biaxial tensile strain is applied in the range from 0% to 9%. An interplay between phonon group velocities, heat capacities, and relaxation times, is found to be responsible for this behaviour. Finally, this project investigated the thermal conductivity of nitrogen functionalized - NX (X=P, As, Sb) monolayers. The results showed that the room-temperature thermal conductivities of -NP, -NAs, and -NSb are about 1.1, 5.5, and 34.0 times higher than those of their single-element -P, -As, and -Sb monolayers, respectively. The phonon transport analysis revealed that higher phonon group velocities, as well as higher phonon lifetimes were responsible for such an enhancement in the thermal conductivities of - NX compounds compared to single-element group-VA monolayers. Also, it was found that -NP has the minimum thermal conductivity among -NX monolayers, while it has the minimum average atomic mass. This thesis provides valuable insight into phonon physics and thermal transport in novel 2D materials using advanced DFT calculations.
First Principles Modeling of Phonon Heat Conduction in Nanoscale Crystalline Structures
Author: Publisher:
ISBN:
Category :
Languages : en
Pages :
Book Description
The inability to remove heat efficiently is currently one of the stumbling blocks toward further miniaturization and advancement of electronic, optoelectronic, and micro-electro-mechanical devices. In order to formulate better heat removal strategies and designs, it is first necessary to understand the fundamental mechanisms of heat transport in semiconductor thin films. Modeling techniques, based on first principles, can play the crucial role of filling gaps in our understanding by revealing information that experiments are incapable of. Heat conduction in crystalline semiconductor films occurs by lattice vibrations that result in the propagation of quanta of energy called phonons. If the mean free path of the traveling phonons is larger than the film thickness, thermodynamic equilibrium ceases to exist, and thus, the Fourier law of heat conduction is invalid. In this scenario, bulk thermal conductivity values, which are experimentally determined by inversion of the Fourier law itself, cannot be used for analysis. The Boltzmann Transport Equation (BTE) is a powerful tool to treat non-equilibrium heat transport in thin films. The BTE describes the evolution of the number density (or energy) distribution for phonons as a result of transport (or drift) and inter-phonon collisions. Drift causes the phonon energy distribution to deviate from equilibrium, while collisions tend to restore equilibrium. Prior to solution of the BTE, it is necessary to compute the lifetimes (or scattering rates) for phonons of all wave-vector and polarization. The lifetime of a phonon is the net result of its collisions with other phonons, which in turn is governed by the conservation of energy and momentum during the underlying collision processes. This research project contributed to the state-of-the-art in two ways: (1) by developing and demonstrating a calibration-free simple methodology to compute intrinsic phonon scattering (Normal and Umklapp processes) time scales with the inclusion of optical phonons, and (2) by developing a suite of numerical algorithms for solution of the BTE for phonons. The suite of numerical algorithms includes Monte Carlo techniques and deterministic techniques based on the Discrete Ordinates Method and the Ballistic-Diffusive approximation of the BTE. These methods were applied to calculation of thermal conductivity of silicon thin films, and to simulate heat conduction in multi-dimensional structures. In addition, thermal transport in silicon nanowires was investigated using two different first principles methods. One was to apply the Green-Kubo formulation to an equilibrium system. The other was to use Non-Equilibrium Molecular Dynamics (NEMD). Results of MD simulations showed that the nanowire cross-sectional shape and size significantly affects the thermal conductivity, as has been found experimentally. In summary, the project clarified the role of various phonon modes - in particular, optical phonon - in non-equilibrium transport in silicon. It laid the foundation for the solution of the BTE in complex three-dimensional structures using deterministic techniques, paving the way for the development of robust numerical tools that could be coupled to existing device simulation tools to enable coupled electro-thermal modeling of practical electronic/optoelectronic devices. Finally, it shed light on why the thermal conductivity of silicon nanowires is so sensitive to its cross-sectional shape.
Phonon Transport at Boundaries and Interfaces in Two-dimensional Materials
Author: Cameron FossPublisher:
ISBN:
Category :
Languages : en
Pages :
Book Description
A typical electronic or photonic device may consist of several materials each one potentially meeting at an interface or terminating with a free-surface boundary. As modern device dimensions reach deeper into the nanoscale regime, interfaces and boundaries become increasingly influential to both electrical and thermal energy transport. While a large majority of the device community focuses on the former, we focus here on the latter issue of thermal transport which is of great importance in implementing nanoscale devices as well as developing solutions for on-chip heat removal and waste heat scavenging. In this document we will discuss how modern performance enhancing techniques (strain, nanostructuring, alloying, etc.) affect thermal transport at boundaries and across interfaces through the avenue of three case studies. We use first-principles Density Functional Perturbation Theory to obtain the phonon spectrum of the materials of interest and then use the dispersion data as input to a phonon Boltzmann Transport model. First, we investigate the combined effects of strain and boundary scattering on the in-plane and cross-plane thermal conductivity of thin-film silicon and germanium. Second, we review a recently developed model for cross-dimensional (2D-3D) phonon transport and apply it to 3D-2D-3D stacked interfaces involving graphene and molybdenum disulfide 2D-layers. Third, we combine relevant models from earlier Chapters to study extrinsic effects, such as line edge roughness and substrate effects, on in-plane and through-plane thermal transport in 1H-phase transition metal dichalcogenide (TMD) alloys. Through these investigations we show that: (1) biaxial strain in Si and Ge thin-films can modulate cross-plane conductivity due to strong boundary scattering, (2) the thermal boundary conductance between 2D-3D materials can be enhanced in the presence of an encapsulating layer, and (3) the thermal conductivity of 1H-phase TMDs can be reduced by an order of magnitude through the combination of nanostructuring, alloying, and substrate effects.
Electron-phonon Interactions in Low-dimensional Structures
Author: Lawrence John ChallisPublisher:
ISBN: 9780191709319
Category : Electron-phonon interactions
Languages : en
Pages : 285
Book Description
The study of electrons and holes confined to two, one and even zero dimensions has uncovered a rich variety of new physics and applications. This book describes the interaction between these confined carriers and the optic and acoustic phonons within and around the confined regions.