Thermal transport spectroscopy in porous and hybrid crystals

Abstract

Thermal engineering to optimize the thermal transport process is crucial for improving the performance and accelerating the practical adoption of many emerging energy applications. New materials developed for these emerging energy applications usually have imperfect heat transport capabilities. A comprehensive understanding of heat transfer mechanisms and the development of effective techniques to manipulate thermal transport in such functional materials are therefore of significant practical importance. This thesis presents combined experimental and simulation studies of thermal transport in two energy-related functional materials: metal-organic frameworks and organic-inorganic hybrid perovskites.

Using the molecular dynamics simulations, the heat transfer of a typical metal-organic framework, HKUST-1, was first studied. A pronounced size effect in thermal conductivity and a relatively long phonon mean free path were observed in HKUST-1. Upon the water adsorption, two competing effects—phonon scattering by the framework and enhanced overall thermal transport—were found to dominate, leading to an initial decrease followed by an increase in thermal conductivity. Furthermore, the experimental measurements and simulations demonstrated that the cross interface thermal transport between Au and water adsorbed MOFs was found largely enhanced by water adsorbates. The enhancement is caused by the bridge effect of water which could activate the high frequency phonon and introducing additional heat transfer channel of water/solid introduced by water. This study provide a novel strategy for improving interfacial thermal transport .

External mechanical stress commonly occurs in practical applications. The influences of mechanical strains on heat transfer of HKUST-1 was also studied. An anomalous strain dependent thermal conductivity was obtained. The phonon dispersion and transport properties of phonons in HKUST-1 was strongly influenced by strains, primarily due to increased anharmonicity under compressive strain and decreased anharmonicity under tensile strain. Another practically relevant factor is the gas adsorption. The influences of three different gases (hydrogen, methane, and ethane) on thermal transport were comprehensively studied. Similar phonon scattering effects were seen in gas adsorbed systems. But much higher contribution from hydrogen than methane and ethane to thermal conductivity was verified at high adsorptions.

Finally, by mixing the halide atoms in organic-inorganic hybrid perovskites, we found that the thermal conductivity of hybrid perovskites were effectively reduced with a minimum at middle mixing ratio. The suppressed phonon mean free path and tuned group velocity are responsible for reduced thermal conductivity. The alteration in local potential landscape and halide alloying effects were attributed as the origins of such changes.

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