Understand Lattice Thermal Transport in Hierarchical Structures

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Ma, Tengfei

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2022

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Dissertation

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Thermal transport in materials and devices has been a critical subject for multiple modern technologies. Particularly, thermoelectric devices and thermal barrier coatings request the minimization of thermal conductivity, while the fast-growing power density of electronic devices demands aggressive heat dissipation technologies, for which materials with higher heat transfer coefficient is urgently needed. Many efforts have been put into the design of novel nano-engineered materials and structures to achieve desired thermal properties, such as alloys, nanowires, polymers and their composites, etc. The materials with hierarchical structures, for example, polymers and superlattices, have attracted intense research attention over recent years, due to the unique thermal properties they may possess. Therefore, we conduct detailed studies on phonon transport in polymers and superlattices using various simulation and modeling approaches. First of all, the phonon properties of single-chain polyvinylidene fluoride (PVDF) is rigorously investigated by using first-principles density functional theory calculations. Due to its unique characteristics such as low cost, lightweight, and outstanding ferroelectric and electrocaloric properties, PVDF and its copolymers/composites have been widely used in many areas including energy storage, actuators, and solid-state cooling. In this work, we find from our density functional theory calculations that the thermal conductivity for a single-chain PVDF decreases significantly under tensile strain, which is contrary to the previous studies on bulk amorphous polymers. The detailed mode-wise phonon analysis indicates that the decrease in thermal conductivity results from the reduced group velocity of the acoustic phonon modes. In addition, the dominant phonon branch that contributes most to the overall thermal conductivity is found to transit from the longitudinal acoustic (LA) branch to the transverse acoustic (TA) branch as the tensile strain increases to 5\%. This transition is attributed to the strain-induced flattening of the LA branch and reduced anharmonic phonon scattering of TA modes.For superlattice structures, we conduct research on two distinct yet related topics. Firstly, the dimensionality effect on phonon localization in two-dimensional superlattices is investigated. Specifically, we perform molecular dynamics simulations on graphene/hexagonal boron nitride superlattices and random multilayers (i.e., aperiodic superlattices). In addition, we investigate the effect of the third dimension (i.e., the out-of-plane atomic displacement) on phonon transport and localization by freezing the out-of-plane atomic motion. Much more significant coherent phonon transport and localization can be observed in two-dimensional (2D) systems compared with their three-dimension (3D) counterparts. The detailed phonon transmission calculations and scattering phase space analyses demonstrate that the strong anharmonic phonon-phonon scattering in 3D systems restricts the coherent phonon transport and thus hinders the observation of the phonon localization phenomenon. In addition, further phonon participation ratio calculations reveal that the flexural modes that are related to the out-of-plane atomic displacement are rather extended while the in-plane modes can be mostly localized in random multilayers. These two factors lead to difficulty in observing phonon localization in 3D systems. In addition to the study on 2D superlattices, we propose a novel strategy to enhance thermal transport in 3D superlattice devices. By applying appropriate heat baths, we can achieve ex situ enhancement of thermal conductivity for superlattice devices without 'touching' the device itself. In this work, we perform non-equilibrium molecular dynamics (NEMD) simulations to calculate the effective thermal conductivity of superlattice devices using superlattice heat baths (or coherent heat baths) and pure-material heat baths (or incoherent heat baths). When the incoherent heat baths are applied, the coherent-incoherent phonon nonequilibrium between the heat baths and device is expected and further proved by our phonon transmission calculation, due to the mismatch between the phonon spectrum of the heat bath and that of the device. When the different kinds of heat baths are applied, the coherent-incoherent phonon nonequilibrium in the incoherent bath cases can lead to ~400% difference in the effective thermal conductivity of conceptual Lennard-Jones superlattice device and $\sim$40\% difference in that of more realistic silicon/germanium superlattice device. A coherent-incoherent phonon transport mode is then proposed to further understand the phonon nonequilibrium at the bath-device interfaces.

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