We investigate the effect of pore size and shape on the thermal conductivity of a series of idealized metal-organic frameworks (MOFs) containing adsorbed gas using molecular simulations.

Whether the presence of adsorbates increases or decreases thermal conductivity in metalorganic frameworks (MOFs) has been an open question. Here we report observations of thermal transport in the metal-organic framework HKUST-1 in the presence of various liquid adsorbates: water, methanol, and ethanol. Experimental thermoreflectance measurements were performed on single crystals and thin films, and theoretical predictions were made using molecular dynamics simulations. We find that the thermal conductivity of HKUST-1 decreases by 40-80% depending on the adsorbate, a result that cannot be explained by effective medium approximations. Our findings demonstrate that adsorbates introduce additional phonon scattering in HKUST-1, which particularly shortens the lifetimes of lowfrequency phonon modes. As a result, the system thermal conductivity is lowered to a greater extent than the increase expected by the creation of additional heat transfer channels. Finally, we show that thermal diffusivity is even more greatly reduced than thermal conductivity by adsorption.

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We have studied the mechanisms of heat transfer in a porous crystal-gas mixture system, motivated by the not insignificant challenge of quickly dissipating heat generated in metal-organic frameworks (MOFs) due to gas adsorption. Our study reveals that the thermal conductance of the system (crystal and gas) is dominated by lattice thermal conductivity in the crystal, and that conductance is reduced as the concentration of gas in the pores increases. This mechanism was observed from classical molecular simulations of a monatomic gas in an idealized porous crystal structure. We show that the decreased conductivity associated with increased gas concentration is due to phonon scattering in the crystal due to interactions with gas molecules. Calculations of scattering rates for two phonon modes reveal that scattering of the lowest frequency mode scales linearly with gas density. This result suggests that the probability of a phonon-gas collision is simply proportional to the number of gas molecules in the pore.

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We have calculated the semi-classical thermoelectric power factor of suspended single-layer (SL)- MoS2 utilizing electron relaxation times derived from ab initio calculations. Measurements of the thermoelectric power factor of SL-MoS2 on substrates reveal poor power factors. In contrast, we find the thermoelectric power factor of suspended SL-MoS2 to peak at ∼2.8 × 104 μW/m K2 at 300 K, at an electron concentration of 1012 cm−2. This figure is higher than that in bulk Bi2Te3, for example. Given its relatively high thermal conductivity, suspended SL-MoS2 may hold promise for in-plane thin-film Peltier coolers, provided reasonable mobilities can be realized.

The virial stress tensor-based instantaneous heat flux, which is used by LAMMPS, is only valid for the small subset of simulations that contain only pairwise interactions. For systems that contain many-body interactions using 3-or 4-body potentials, a more complete derivation is required. We have created a software patch to LAMMPS that implements the correct heat flux calculation approach for 3-and 4body potentials, based on the derivation by Torii et al. (J. Chem. Phys. 2008, 128, 044504) Using two example systems, the error in the uncorrected code for many-body potential heat flux is shown to be significant and reaches nearly 100% of the manybody potential heat flux for the systems we studied; hence, the error of the total heat flux calculation is proportional to the fraction of the total heat flux transferred through the many-body potentials. This error may have consequences for thermal conductivities calculated using the Green−Kubo method or any NEMD method that uses the instantaneous heat flux. We recommend that all researchers using LAMMPS for heat flux calculations where significant heat is transferred via the many-body potentials adopt the corrected code.

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