Ultralong mean free path phonons in HKUST-1 and their scattering by water adsorbates

HZ Fan and C Yang and YG Zhou, PHYSICAL REVIEW B, 106, 085417 (2022).

DOI: 10.1103/PhysRevB.106.085417

Metal-organic frameworks (MOFs) have shown big potential in energy storage and thermal management, which are closely related to their thermal properties. It has been believed for some time that the vibrational mean free path in MOFs is relatively small and the resulting thermal conductivity has weak size dependence. By studying one of the typical MOFs, i.e., Cu-3(BTC)(2) where BTC is benzene-1,3,5-tricarboxylate, also referred to as HKUST-1, we computationally prove that the mean free path of these small wave vectors or equivalently low-frequency vibrations in HKUST-1 can be as large as similar to 120 nm and the corresponding thermal conductivity is strongly size dependent, which is attributed to its good crystallinity. We further find that these long mean free path phonons are strongly scattered by the adsorbed water molecules and the thermal conductivity is decreased by similar to 51.3% when the water molecules are adsorbed. Consequently, the thermal conductivity of HKUST-1 with water molecules is weakly size dependent. Two pathways for the thermal energy exchange, e.g., the phonons and the water molecules, exist in HKUST-1 with water molecules. The thermal conductivity of the adsorbed HKUST-1 is found to decrease and then increase with the quantity of the adsorbates owing to the competition between the two thermal pathways. Our study here provides a fundamental understanding on the thermal transport mechanisms in the metal-organic framework with consideration of adsorbates, which offers useful guidance for thermal management design in these thermal- related applications such as adsorption-driven heat pumps.

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