Quantifying Thermal Transport Channels in Complex Materials

Abstract

Thermal conductivity, quantifying heat dissipation capability, emerges from vibrational mode transport mechanisms including modal propagating and intermode coherence. This dissertation quantifies these channels in complex materials, establishing a comprehensive understanding from mode-level behaviors to macroscopic conductivity.

Initially, the research focuses on the modal thermal transport behavior of quasilocalized vibrational modes (QVMs) in glassy solids. QVMs concentrate energy within their localized regions, and thus they are hypothesized to be inefficient heat carriers. We here show that the modal thermal conductivity of QVMs is comparable to that of delocalized vibrational modes, which demonstrates QVMs can be efficient heat carriers similar to delocalized vibrational modes. Further analysis shows that the mutual coherence between QVMs and other modes explains their high thermal exchange performance.

Then, the study explores how different thermal transport channels collaborate to affect macroscopic thermal conductivity. We systematically study the thermal transport properties of HfO₂ in the range of 300-2000 K, which undergoes a phase transition between monoclinic and tetragonal phases at ∼1765 K. The thermal conductivity of HfO₂ decreases from 11.95 to 1.72 W/mK over this range, which is collaboratively contributed by propagating and coherence thermal transport channels. The contribution of coherence increases with temperature-induced scattering, and it accounts for ∼30% of total thermal conductivity above 1500 K. Four-phonon scattering is found to be significant for the thermal transport in tetragonal HfO₂, which can result in the thermal conductivity reduction of ∼50%. The strong four-phonon scattering in tetragonal HfO₂ mainly arises from the large weighted phase space caused by high temperature.

Finally, this dissertation reveals the effect of structural complexity on thermal conductivity through its impacts on thermal transport channels, which can be regarded as a potential way to manipulate thermal conductivity. We take rich-lithium materials (LiSi and Li₁₅Si₄) as examples. The more complex structure of Li₁₅Si₄ compared to LiSi reduces propagating thermal conductivity for low-frequency vibrational modes by decreasing their specific heat. However, more optical branches are generated in high-frequency regime of Li₁₅Si₄ due to its more complex structure than LiSi. Compared to LiSi, the denser branches lead to smaller frequency differences between modes in Li₁₅Si₄ and increase coherence thermal conductivity of high-frequency modes. The compensation of thermal transport between low-and high-frequency modes results in comparable thermal conductivities of LiSi and Li₁₅Si₄, which deviates from the Slack model’s prediction of strongly negative correlation between thermal conductivity and structural complexity.

Date of Award	2025
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Simen Zhou (Supervisor)