First-principles study of thermal transport in highly anharmonic materials and complex crystalline polymers

Abstract

Lattice thermal conductivity ?_? plays a crucial role in determining properties of materials for applications such as thermal management of electronics and thermoelectric devices. However, accurately describing the lattice dynamics and microscopic mechanism of thermal transport in materials with strong anharmonicity continues to be a long-standing challenge in condensed state physics. The development of a comprehensive understanding of anharmonic properties and heat transport of crystalline compounds with strong anharmonicity is not only fundamental interesting but also helps to the rational design of new materials. Additionally, crystalline polymers recently received great attention due to their many merits, however, their thermal transport behaviour remains unclear or unknown because of their complex structure. Therefore, it is potentially useful to investigate the microscopic mechanism of thermal transport in complex molecular crystals. In this thesis, we first explore the microscopic mechanism of thermal transport in simple but highly anharmonic perovskite BaZrO₃. The anharmonic lattice dynamics and thermal transport in BaZrO₃ were investigated by combining the first-principles-based self-consistent phonon theory and a unified theory of thermal transport. Good agreements between the calculations and experiments indicate considering the anharmonic phonon renormalization is essential for accurate prediction of phonon dispersion and lattice thermal conductivity ?_? in BaZrO₃. Specifically, the lattice anharmonicity induces significant low-frequency optical phonon hardening at elevated temperature, thereby resulting in enhancement in phonon thermal transport. Moreover, although the coherent thermal transport is minor at low temperatures, it becomes non-negligible at elevated temperatures and may contribute up to 11% of ?_? at 1500 K. We next examine the effect of high-order anharmonicity on lattice dynamics and thermal transport in complex skutterudite YbFe₄Sb₁₂ by coupling a state-of-the-art first-principles-based anharmonic phonon renormalization technique and a unified theory of thermal transport. We show that the unusual lattice thermal conductivity ?_? in YbFe₄Sb₁₂ can be accurately predicted by considering anharmonic phonon renormalization and coherence contribution. Specifically, the anharmonicity-induced phonon stiffening of the low-lying flat modes significantly enhances the ?_? ^? of particle-like phonons, resulting in much-improved agreement with experiments. By further including the coherence contribution, the predicted ?_? increases by ~22% throughout the entire temperature range, well reproducing the experimental values in both magnitude and temperature dependence. Finally, we investigated the lattice vibrational properties and phonon transport properties of poly (pphenylene vinylene) (PPV), poly (p-phenylene oxide) (PPO), and poly (p-phenylene sulfide) (PPS) by combining the first-principles-based framework of lattice dynamics with the Peierls-Boltzmann transport equation for phonons. We found that the phonon lifetimes in different polymers are quite similar while the group velocity, which is correlated with the bond strength, is the dominant factor that determines the thermal conductivities in different polymers. Furthermore, we find that the complexity of crystalline structure is a roadblock to achieving the high axial ?_? in complex polymers by comparing the physical properties of those three-type of complex polymers with that of bulk polyethylene (PE). Our results in this thesis highlight the importance of anharmonic phonon renormalization and coherent thermal transport channel in predicting lattice thermal conductivity ?_? of highly anharmonic compounds. Furthermore, the first-principles approach based on density functional theory (DFT) coupling with the conventional PBTE method is needed to be extended for materials with strong anharmonicity, i.e., further including temperature effect, non-diagonal term of heat flux operators. In addition, our results also show that the DFT-based PBTE method provides an efficient tool to unravel the correlation between the crystal structure and microscopic mechanism of thermal transport in complex crystalline polymers.

Date of Award	2022
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Baoling HUANG (Supervisor)