Sub-Micrometer Phonon Mean Free Paths in Metal-Organic Frameworks Revealed by Machine Learning Molecular Dynamics Simulations

PH Ying and T Liang and K Xu and J Zhang and JB Xu and Z Zhong and ZY Fan, ACS APPLIED MATERIALS & INTERFACES, 15, 36412-36422 (2023).

DOI: 10.1021/acsami.3c07770

Metal-organic frameworks (MOFs) are a family ofmaterialsthat have high porosity and structural tunability and hold great potentialin various applications, many of which require a proper understandingof the thermal transport properties. Molecular dynamics (MD) simulationsplay an important role in characterizing the thermal transport propertiesof various materials. However, due to the complexity of the structures,it is difficult to construct accurate empirical interatomic potentialsfor reliable MD simulations of MOFs. To this end, we develop a setof accurate yet highly efficient machine-learned potentials for threetypical MOFs, including MOF-5, HKUST-1, and ZIF-8, using the neuroevolutionpotential approach as implemented in the GPUMD package, and performextensive MD simulations to study thermal transport in the three MOFs.Although the lattice thermal conductivity values of the three MOFsare all predicted to be smaller than 1 W/(m K) at room temperature,the phonon mean free paths (MFPs) are found to reach the sub-micrometerscale in the low-frequency region. As a consequence, the apparentthermal conductivity only converges to the diffusive limit for micrometersingle crystals, which means that the thermal conductivity is heavilyreduced in nanocrystalline MOFs. The sub-micrometer phonon MFPs arealso found to be correlated with a moderate temperature dependenceof thermal conductivity between those in typical crystalline and amorphousmaterials. Both the large phonon MFPs and the moderate temperaturedependence of thermal conductivity fundamentally change our understandingof thermal transport in MOFs.

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