Thermal Conductivity of 3D Boron-Based Covalent Organic Frameworks from Molecular Dynamics Simulations

Covalent organic frameworks (COFs) have been widely investigated for use in gas storage and separation, while their thermal properties have been scarcely studied. In the study reported in this paper, the thermal conductivities of 3D boron-based COFs were investigated for the first time using molecular dynamics simulations (MD) employing the Green-Kubo method. The predicted thermal conductivities of COF-102, COF-103, COF-105, and COF-108 were on the order of 0.1 W/(m.K) at 300 K. The thermal conductivity decreased by up to 47% with the increase in temperature from 200 to 500 K. This resulting low thermal conductivity was due to the short mean free path of the phonon in the COFs, which was deduced to be 2.7-9.2 nm. The low-frequency phonon modes below 50 THz contributed mostly to heat conduction. By analyzing the phonon vibrational density of states and overlap energy between per two bonded atoms, it was revealed that the connection between phenylene rings in COF-102 and COF-103 weakens the phonon coupling and then harms the energy flow, as did the connection between phenylene and triphenylene rings in COF-105 and COF-108. In addition, COF-105 had the lowest total overlap energy between all atoms, leading to the minimum thermal conductivity in the four COFs. This study provided a quantitative prediction of the thermal conductivities of COFs and microscopic insight into the mechanism of heat transfer.