scholarly journals Thermal transport in nanocrystalline Si and SiGe by ab initio based Monte Carlo simulation

2017 ◽  
Vol 7 (1) ◽  
Author(s):  
Lina Yang ◽  
Austin J. Minnich
Keyword(s):  
Thermal Conductivity ◽  
Monte Carlo ◽  
Ab Initio ◽  
Grain Boundaries ◽  
Thermal Transport ◽  
Phonon Scattering ◽  
Phonon Dispersion ◽  
Computational Study ◽  
Phonon Transport ◽  
Nanocrystalline Si

Abstract Nanocrystalline thermoelectric materials based on Si have long been of interest because Si is earth-abundant, inexpensive, and non-toxic. However, a poor understanding of phonon grain boundary scattering and its effect on thermal conductivity has impeded efforts to improve the thermoelectric figure of merit. Here, we report an ab-initio based computational study of thermal transport in nanocrystalline Si-based materials using a variance-reduced Monte Carlo method with the full phonon dispersion and intrinsic lifetimes from first-principles as input. By fitting the transmission profile of grain boundaries, we obtain excellent agreement with experimental thermal conductivity of nanocrystalline Si [Wang et al. Nano Letters 11, 2206 (2011)]. Based on these calculations, we examine phonon transport in nanocrystalline SiGe alloys with ab-initio electron-phonon scattering rates. Our calculations show that low energy phonons still transport substantial amounts of heat in these materials, despite scattering by electron-phonon interactions, due to the high transmission of phonons at grain boundaries, and thus improvements in ZT are still possible by disrupting these modes. This work demonstrates the important insights into phonon transport that can be obtained using ab-initio based Monte Carlo simulations in complex nanostructured materials.

Spectral Detail of Phonon Conduction and Scattering in Graphene

2011 ◽  
Author(s):  
Dhruv Singh ◽  
Jayathi Y. Murthy ◽  
Timothy S. Fisher
Keyword(s):  
Thermal Conductivity ◽  
Phonon Scattering ◽  
Interatomic Potential ◽  
Phonon Dispersion ◽  
Transition Probabilities ◽  
Transition Probability ◽  
Phonon Transport ◽  
Temperature Perturbation ◽  
Boltzmann Transport ◽  
Wide Range

This paper examines the thermodynamic and thermal transport properties of the 2D graphene lattice. The interatomic interactions are modeled using the Tersoff interatomic potential and are used to evaluate phonon dispersion curves, density of states and thermodynamic properties of graphene as functions of temperature. Perturbation theory is applied to calculate the transition probabilities for three-phonon scattering. The matrix elements of the perturbing Hamiltonian are calculated using the anharmonic interatomic force constants obtained from the interatomic potential as well. An algorithm to accurately quantify the contours of energy balance for three-phonon scattering events is presented and applied to calculate the net transition probability from a given phonon mode. Under the linear approximation, the Boltzmann transport equation (BTE) is applied to compute the thermal conductivity of graphene, giving spectral and polarization-resolved information. Predictions of thermal conductivity for a wide range of parameters elucidate the behavior of diffusive phonon transport. The complete spectral detail of selection rules, important phonon scattering pathways, and phonon relaxation times in graphene are provided, contrasting graphene with other materials, along with implications for graphene electronics. We also highlight the specific scattering processes that are important in Raman spectroscopy based measurements of graphene thermal conductivity, and provide a plausible explanation for the observed dependence on laser spot size.

A first principles study of the phonon anharmonicity, electronic structure and optical characteristics of LaAlO3

2018 ◽  
Vol 1 (1) ◽  
pp. 161-180 ◽  
Author(s):  
Bahaa Ilyas ◽  
Badal Elias
Keyword(s):  
Thermal Conductivity ◽  
Electronic Structure ◽  
Ab Initio ◽  
Conversion Efficiency ◽  
First Principles ◽  
Phonon Scattering ◽  
Phonon Dispersion ◽  
Optical Spectra ◽  
Hybrid Functional ◽  
The Way

The way elementary excitations work together with their couplings and interact as condensed matter systems is very important when designing optimum energy-conversion devices. We investigated the electronic structure of LaAlO3, and we show that the bandgap insulator of LaAlO3 obtained theoretically by the hybrid functional HSE06 is an indirect 5.649eV that show a very good agreement with experimental data. The lattice constant is obtained exactly as experiment. In thermos-electric materials, the concept of conversion-efficiency (heat to electricity) is improved instantly by suppressing the phonon quasi-particles propagations that are responsible for draft macroscopic thermal transport. The material presented here for thermo-electric conversion-efficiency of cubic perovskite LaAlO3, show that it has an ultralow thermal-conductivity, while the formalism to its strong phonon scattering interactions resides mostly unclear. From the bases of Ab-initio simulations, the 4-dimensional phonon-dispersion surfaces of the cubic perovskite LaAlO3, have been mapped and we found that the origins of the ionic potential an-harmonicity being responsible for the unique behaviour and properties of LaAlO3. It is investigated that these phonon scattering arise solely from the LaAlO3 unstable electronic-structure, with its orbital interactions resulting to lattice instability similar to the ferroelectric instabilities. Our results show a microscopic insight bonding electronic-structure and phonon an-harmonicity in LaAlO3, and provides some new picture the way interactions happen between phonon–electron and phonon–phonon this lead to understand the concept of ultralow thermal-conductivity. Ab-initio calculations was performed on cubic perovskite LaAlO3 to obtain the phonon density of states (DOS) from 50 K to 5000 K, we find that the anharmonic behaviour starts around temperature limits of 500 K. The computed optical spectra were obtained using both the Beth Slapter Equation BSE and compared with the perturbed method using HSE06, optical spectra show that the inter-band transition occur precisely from the O-valence bands to the La-conduction bands throughout the low energy area. The energy-loss spectrum, optical conductivity and reflectivity and the refractive index are computed from first principles by using HSE06 hybrid functional. The optical band gap of material shows about 6.21 eV, which agrees with some cited experimental measurements.

scholarly journals The impact of tilt grain boundaries on the thermal transport in perovskite SrTiO3 layered nanostructures. A computational study

Nanoscale ◽  
2018 ◽  
Vol 10 (31) ◽  
pp. 15010-15022 ◽  
Author(s):  
Stephen R. Yeandel ◽  
Marco Molinari ◽  
Stephen C. Parker
Keyword(s):  
Thermal Conductivity ◽  
Grain Boundaries ◽  
Strontium Titanate ◽  
Thermal Transport ◽  
Lattice Thermal Conductivity ◽  
Computational Study ◽  
Thermoelectric Performance ◽  
Length Scales ◽  
Layered Nanostructures ◽  
The Impact

Stacking of interfaces at different length-scales affect the lattice thermal conductivity of strontium titanate layered nanostructures improving their thermoelectric performance.

Phonon Transport in SiGe-Based Nanocomposites and Nanowires for Thermoelectric Applications

2015 ◽  
Vol 1735 ◽  
Author(s):  
M. Upadhyaya ◽  
Z. Aksamija
Keyword(s):  
Thermal Conductivity ◽  
Silicon Germanium ◽  
Thermal Transport ◽  
Lattice Thermal Conductivity ◽  
Phonon Scattering ◽  
Voronoi Tessellation ◽  
Phonon Transport ◽  
Interface Properties ◽  
Boltzmann Transport ◽  
Rough Interfaces

ABSTRACTSilicon-germanium (SiGe) superlattices (SLs) have been proposed for application as efficient thermoelectrics because of their low thermal conductivity, below that of bulk SiGe alloys. However, the cost of growing SLs is prohibitive, so nanocomposites, made by a ball-milling and sintering, have been proposed as a cost-effective replacement with similar properties. Lattice thermal conductivity in SiGe SLs is reduced by scattering from the rough interfaces between layers. Therefore, it is expected that interface properties, such as roughness, orientation, and composition, will play a significant role in thermal transport in nanocomposites and offer many additional degrees of freedom to control the thermal conductivity in nanocomposites by tailoring grain size, shape, and crystal angle distributions. We previously demonstrated the sensitivity of the lattice thermal conductivity in SLs to the interface properties, based on solving the phonon Boltzmann transport equation under the relaxation time approximation. Here we adapt the model to a broad range of SiGe nanocomposites. We model nanocomposite structures using a Voronoi tessellation to mimic the grains and their distribution in the nanocomposite and show excellent agreement with experimentally observed structures, while for nanowires we use the Monte Carlo method to solve the phonon Boltzmann equation. In order to accurately treat phonon scattering from a series of atomically rough interfaces between the grains in the nanocomposite and at the boundaries of nanowires, we employ a momentum-dependent specularity parameter. Our results show thermal transport in SiGe nanocomposites and nanowires is reduced significantly below their bulk alloy counterparts.

Simulation of Sub-Micron Thermal Transport in a MOSFET Using a Hybrid Fourier-BTE Model

2010 ◽  
Author(s):  
James M. Loy ◽  
Dhruv Singh ◽  
Jayathi Y. Murthy
Keyword(s):  
Thermal Transport ◽  
Source Term ◽  
Phonon Scattering ◽  
Phonon Dispersion ◽  
Computational Cost ◽  
Phonon Transport ◽  
Boltzmann Transport ◽  
Hybrid Solver ◽  
Solution Methodology ◽  
Electron Phonon Scattering

Self-heating has emerged as a critical bottleneck to scaling in modern transistors. In simulating heat conduction in these devices, it is important to account for the granularity of phonon transport since electron-phonon scattering occurs preferentially to select phonon groups. However, a complete accounting for phonon dispersion, polarization and scattering is very expensive if the Boltzmann transport equation (BTE) is used. Moreover, difficulties with convergence are encountered when the phonon Knudsen number becomes small. In this paper we simulate a two-dimensional bulk MOSFET hotspot problem using a partially-implicit hybrid BTE-Fourier solver which is significantly less expensive than a full BTE solution, and which shows excellent convergence characteristics. Volumetric heat generation from electron-phonon collisions is taken from a Monte Carlo simulation of electron transport and serves as a heat source term in the governing transport equations. The hybrid solver is shown to perform well in this highly non-equilibrium situation, matching the solutions obtained from a pure all-BTE solution, but at significantly lower computational cost. The paper establishes that this new model and solution methodology are viable for the simulation of thermal transport in other emerging transistor designs and in other nanotechnology applications as well.

scholarly journals Simulation of Heat Transport in Graphene Nanoribbons Using the Ab-Initio Scattering Operator

2014 ◽  
Author(s):  
Colin D. Landon ◽  
Nicolas G. Hadjiconstantinou
Keyword(s):  
Ab Initio ◽  
Heat Transport ◽  
Density Functional ◽  
Phonon Scattering ◽  
Phonon Dispersion ◽  
Graphene Nanoribbons ◽  
Phonon Transport ◽  
Stochastic Algorithm ◽  
Boltzmann Transport ◽  
Scattering Operator

We present a deviational Monte Carlo method for simulating phonon transport in graphene using the ab initio 3-phonon scattering operator. This operator replaces the commonly used relaxation-time approximation, which is known to neglect, among other things, coupling between out of equilibrium states that are particularly important in graphene. Phonon dispersion relations and transition rates are obtained from density functional theory calculations. The proposed method provides, for the first time, means for obtaining solutions of the Boltzmann transport equation with ab initio scattering for time- and spatially-dependent problems. The deviational formulation ensures that simulations are computationally feasible for arbitrarily small temperature differences; within this formulation, the ab initio scattering operator is treated using an efficient stochastic algorithm which, in the limit of large number of states, outperforms the more traditional deterministic methods used in solutions of the homogeneous Boltzmann equation. We use the proposed method to study heat transport in graphene ribbons.

Monte Carlo Simulation of Thermal Conductivities of Silicon Nanowires

2005 ◽  
Author(s):  
Yunfei Chen ◽  
Deyu Li ◽  
Jennifer R. Lukes ◽  
Zhonghua Ni
Keyword(s):  
Thermal Conductivity ◽  
Monte Carlo Simulation ◽  
Monte Carlo ◽  
Dispersion Relation ◽  
Thermal Transport ◽  
Phonon Transport ◽  
Si Nanowires ◽  
Nanowire Diameter ◽  
Energy Conversion Devices ◽  
Tens Nanometer

One-dimensional (1D) materials such as various kinds of nanowires and nanotubes have attracted considerable attention due to their potential applications in electronic and energy conversion devices. The thermal transport phenomena in these nanowires and nanotubes could be significantly different from that in bulk material due to boundary scattering, phonon dispersion relation change, and quantum confinement. It is very important to understand the thermal transport phenomena in these materials so that we can apply them in the thermal design of microelectronic, photonic, and energy conversion devices. While intensive experimental efforts are being carried out to investigate the thermal transport in nanowires and nanotube, an accurate numerical prediction can help the understanding of phonon scattering mechanisms, which is of fundamental theoretical significance. A Monte Carlo simulation was developed and applied to investigate phonon transport in single crystalline Si nanowires. The Phonon-phonon Normal (N) and Umklapp (U) scattering processes were modeled with a genetic algorithm to satisfy both the energy and the momentum conservation. The scattering rates of N and U scattering processes were given from the first perturbation theory. Ballistic phonon transport was modeled with the code and the numerical results fit the theoretical prediction very well. The thermal conductivity of bulk Si was then simulated and good agreement was achieved with the experimental data. Si nanowire thermal conductivity was then studied and compared with some recent experimental results. In order to study the confinement effects on phonon transport in nanowires, two different phonon dispersions, one based on bulk Si and the other solved from the elastic wave theory for nanowires, were adopted in the simulation. The discrepancy from the simulations based on different phonon dispersions increases as the nanowire diameter decreases, which suggests that the confinement effect is significant when the nanowire diameter goes down to tens nanometer range. It was found that the U scattering probability engaged in Si nanowires was increased from that in bulk Si due to the decrease of the frequency gap between different modes and the reduced phonon group velocity. Simulation results suggest that the dispersion relation for nanowire solved from the elasticity theory should be used to evaluate nanowire thermal conductivity as the nanowire diameter reduced to tens nanometer.

scholarly journals Decoupling electron and phonon transport in single-nanowire hybrid materials for high-performance thermoelectrics

2021 ◽  
Vol 7 (20) ◽  
pp. eabe6000
Author(s):  
Lin Yang ◽  
Madeleine P. Gordon ◽  
Akanksha K. Menon ◽  
Alexandra Bruefach ◽  
Kyle Haas ◽  
...  
Keyword(s):  
Thermal Conductivity ◽  
Thermal Transport ◽  
Electrical Transport ◽  
High Performance ◽  
Figure Of Merit ◽  
Phonon Transport ◽  
Core Shell ◽  
Nanowire Diameter ◽  
High Performing ◽  
Polycrystalline Thin Films

Organic-inorganic hybrids have recently emerged as a class of high-performing thermoelectric materials that are lightweight and mechanically flexible. However, the fundamental electrical and thermal transport in these materials has remained elusive due to the heterogeneity of bulk, polycrystalline, thin films reported thus far. Here, we systematically investigate a model hybrid comprising a single core/shell nanowire of Te-PEDOT:PSS. We show that as the nanowire diameter is reduced, the electrical conductivity increases and the thermal conductivity decreases, while the Seebeck coefficient remains nearly constant—this collectively results in a figure of merit, ZT, of 0.54 at 400 K. The origin of the decoupling of charge and heat transport lies in the fact that electrical transport occurs through the organic shell, while thermal transport is driven by the inorganic core. This study establishes design principles for high-performing thermoelectrics that leverage the unique interactions occurring at the interfaces of hybrid nanowires.

scholarly journals Effect of ZnO and SnO2 Nanolayers at Grain Boundaries on Thermoelectric Properties of Polycrystalline Skutterudites

Nanomaterials ◽  
2020 ◽  
Vol 10 (11) ◽  
pp. 2270
Author(s):  
Sang-il Kim ◽  
Jiwoo An ◽  
Woo-Jae Lee ◽  
Se Kwon ◽  
Woo Nam ◽  
...  
Keyword(s):  
Thermal Conductivity ◽  
Atomic Layer Deposition ◽  
Grain Boundaries ◽  
Thermoelectric Properties ◽  
Phonon Scattering ◽  
Atomic Layer ◽  
Deposition Method ◽  
Plasma Sintering ◽  
Layer Deposition ◽  
Atomic Layer Deposition Method

Nanostructuring is considered one of the key approaches to achieve highly efficient thermoelectric alloys by reducing thermal conductivity. In this study, we investigated the effect of oxide (ZnO and SnO2) nanolayers at the grain boundaries of polycrystalline In0.2Yb0.1Co4Sb12 skutterudites on their electrical and thermal transport properties. Skutterudite powders with oxide nanolayers were prepared by atomic layer deposition method, and the number of deposition cycles was varied to control the coating thickness. The coated powders were consolidated by spark plasma sintering. With increasing number of deposition cycle, the electrical conductivity gradually decreased, while the Seebeck coefficient changed insignificantly; this indicates that the carrier mobility decreased due to the oxide nanolayers. In contrast, the lattice thermal conductivity increased with an increase in the number of deposition cycles, demonstrating the reduction in phonon scattering by grain boundaries owing to the oxide nanolayers. Thus, we could easily control the thermoelectric properties of skutterudite materials through adjusting the oxide nanolayer by atomic layer deposition method.

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