Thermal transport in layered materials for thermoelectrics and thermal management

Bo Qui, Purdue University

Abstract

Atomic level thermal transport in layered materials, namely, Bi 2Te3 and graphene is investigated using first principles calculations, lattice dynamics (LD) calculations, molecular dynamics simulations, spectral phonon analysis and empirical modeling. These materials resemble geometrically while differ significantly in the nature of thermal transport. Because of their uniquely low/high thermal conductivities, they are of great interest in thermoelectrics and thermal management applications, respectively. Besides Bi2Te3 and graphene, many other materials in the family of layered materials also exhibit great promises for various applications in thermoelectrics, thermal management, and electronics. In order to investigate the thermal properties of general layered materials, we explore the use of tight-binding molecular dynamics (TBMD) approach, which neither relies on the availability of classical potentials nor demands significant computational resources as ab initio MD approach does. In addition, a general model for the effective phonon group velocities, which is relevant for the lattice thermal transport in general few-layer materials, is developed. First of all, two-body interatomic potentials in the Morse potential form have been developed for bismuth telluride. The density functional theory with local-density approximations is first used to calculate the total energies for many artificially distorted Bi2Te3 configurations to produce the energy surface. Then by fitting to this energy surface and other experimental data, the Morse potential form is parameterized. The fitted empirical interatomic potentials are shown to reproduce the elastic and phonon data well. With the classical interatomic potentials developed, molecular dynamics simulations are performed to predict the thermal conductivity of bulk Bi2Te3 at different temperatures, and the results agree with experimental data well. To facilitate phonon-engineering, we predict the thermal conductivity of Bi2Te3 nanowires with diameters ranging from 3 to 30 nm with both smooth and rough surfaces. It is found that when the nanowire diameter decreases to the molecular scale, the thermal conductivity shows significant reduction as compared to bulk value. On the other hand, the thermal conductivity for the 30-nm-diam nanowire only shows less than 20% reduction, in agreement with recent experimental data. Also, the thermal conductivity of nanowires shows a weaker temperature dependence than the typical T –1 trend, consistent with experimental observations. This is attributed to the strong boundary scattering of phonons. Being motivated by the recent experimental exfoliation of atomically-thin Bi2Te3 flakes, thermal conductivity of perfect and nanoporous few-quintuple Bi2Te3 thin films as well as nanoribbons with perfect and zig-zag edges is investigated using molecular dynamics (MD) simulations with Green-Kubo method. We find minimum thermal conductivity of perfect Bi2Te3 thin films with three quintuple layers (QLs) at room temperature. Nanoporous films and nanoribbons are studied for additional phonon scattering channels in suppressing thermal conductivity. So far we have studied the lattice thermal conductivity of Bi2Te 3 bulk and important nanostructures. To understand the detailed phonon transport, dominant phonon modes and how various nanostructures can efficiently alter the thermal conductivity in Bi2Te3 through phonon scattering, we further study the mode-wise phonon properties in Bi2Te 3 using spectral energy density approach (SED). The anharmonic phonon dispersions and relaxation times along Γ – Z direction are extracted. To validate the as-obtained spectral properties, the lattice thermal conductivity in the cross-plane direction is obtained under isotropic approximation, which is found to agree with Green-Kubo predictions well. (Abstract shortened by UMI.)

Degree

Ph.D.

Advisors

Ruan, Purdue University.

Subject Area

Mechanical engineering|Materials science

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