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

Abstract

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.

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“…In addition, the overlap energy between C4 and H5 and that between C5 and H6 have the largest values, followed by those between O1 and C1, O2 and C2, and C1 and H3, which indicates that energy is localized at these positions and that long-distance coherence cannot be established. 32 Figure 5 also shows that the overlap energy between the central C atoms and four bonded atoms is almost the same, which indicates that the energy flowing through the central atom is equally distributed along the four bonds. Compared with NPG molecules, the materials with fewer atomic species, such as graphene and carbon nanotubes, which have only one atomic species, exhibit much higher thermal conductivity.…”

“…However, the less phonon coupling between O atoms and H atoms caused by mass mismatching hinders the energy transfer in NPG molecules. In addition, the overlap energy between C4 and H5 and that between C5 and H6 have the largest values, followed by those between O1 and C1, O2 and C2, and C1 and H3, which indicates that energy is localized at these positions and that long-distance coherence cannot be established Figure also shows that the overlap energy between the central C atoms and four bonded atoms is almost the same, which indicates that the energy flowing through the central atom is equally distributed along the four bonds.…”

“…Compared with NPG molecules, the materials with fewer atomic species, such as graphene and carbon nanotubes, which have only one atomic species, exhibit much higher thermal conductivity. This can be attributed to stronger vibrational match and less energy localization in these materials. ,, …”

“…Then, the accumulated spectral thermal conductivity can be computed by integrating f κ, i (ν) over the frequency range: where V and T are the volume and temperature of the system, respectively, and k B is the Boltzmann constant. The VDOS of atoms in group β was computed by taking the Fourier transform of their velocity autocorrelation function: , where c β is the number fraction of atomic group β, ν is the frequency, and γ β is the velocity autocorrelation function: where v i β is the velocity of the i th atom in group β.…”

“…where V and T are the volume and temperature of the system, respectively, and k B is the Boltzmann constant. The VDOS of atoms in group β was computed by taking the Fourier transform of their velocity autocorrelation function: 32,33…”