Title: Sub-micrometer phonon mean free paths in metal-organic frameworks revealed by machine-learning molecular dynamics simulations Show Chinese title

Title: 金属-有机框架中亚微米波恩平均自由程通过机器学习分子动力学模拟揭示

Abstract: Metal-organic frameworks (MOFs) are a family of materials that have high porosity and structural tunability and hold great potential in various applications, many of which requiring a proper understanding of the thermal transport properties. Molecular dynamics (MD) simulations play an important role in characterizing the thermal transport properties of various materials. However, due to the complexity of the structures, it is difficult to construct accurate empirical interatomic potentials for reliable MD simulations of MOFs. To this end, we develop a set of accurate yet highly efficient machine-learned potentials for three typical MOFs, including MOF-5, HKUST-1, and ZIF-8, using the neuroevolution potential approach as implemented in the GPUMD package, and perform extensive MD simulations to study thermal transport in the three MOFs. Although the lattice thermal conductivity (LTC) values of the three MOFs are all predicted to be smaller than 1 $\rm{W/(m\ K)}$ at room temperature, the phonon mean free paths (MFPs) are found to reach the sub-micrometer scale in the low-frequency region. As a consequence, the apparent LTC only converges to the diffusive limit for micrometer single crystals, which means that the LTC is heavily reduced in nanocrystalline MOFs. The sub-micrometer phonon MFPs are also found to be correlated with a moderate temperature dependence of LTC between those in typical crystalline and amorphous materials. Both the large phonon MFPs and the moderate temperature dependence of LTC fundamentally change our understanding of thermal transport in MOFs. Show Chinese abstract

Abstract: 金属-有机框架（MOFs）是一类具有高孔隙率和结构可调性的材料，在各种应用中具有巨大的潜力，其中许多应用需要对热传导特性有正确的理解。分子动力学（MD）模拟在表征各种材料的热传导特性方面起着重要作用。然而，由于结构的复杂性，很难构建准确的经验原子间势能函数，以进行MOFs的可靠MD模拟。为此，我们使用GPUMD包中实现的神经进化势方法，为三种典型的MOFs（包括MOF-5、HKUST-1和ZIF-8）开发了一组准确且高效的机器学习势能，并进行了广泛的MD模拟以研究这三种MOFs中的热传导。尽管三种MOFs的晶格热导率（LTC）值在室温下都被预测小于1 $\rm{W/(m\ K)}$，但在低频区域发现声子平均自由路径（MFPs）可以达到亚微米尺度。因此，表观LTC只有在微米单晶中才能收敛到扩散极限，这意味着在纳米晶体MOFs中LTC显著降低。亚微米声子MFPs还与LTC在典型晶体和非晶体材料之间的中等温度依赖性相关。声子MFPs较大以及LTC的中等温度依赖性从根本上改变了我们对MOFs中热传导的理解。