Thermal transport in metal-organic frameworks from first principle calculations

Metal-organic frameworks (MOFs) have been intensively studied during the last years due to their role numerous possible applications, for example, in gas absorption and catalysis. In spite of their importance, their heat transport properties of MOFs are rarely investigated, although they are crucial for many applications (e.g. heat dissipation, thermoelectricity). The presence of very heavy and light atoms in the unit cell can lead to low-frequency optical phonon modes that typically have a high contribution to the thermal conductivity, which complicates the situation.
Moreover, the ability to change the constituents of MOFs opens the possibility to design materials with tailor-made thermal transport properties; by anisotropic linker structures also preferred avenues for heat transport can potentially be realized. This calls for achieving an in depth understanding of the heat transport in MOFs. To achieve that, we present a study dealing with the influences of different MOF constituents on the thermal conductivity, to deduce reliable structure-to-property relationships : Starting from MOF-5, a systematic variation of (i) the metal nodes (Zn, Mg, Ca) and (ii) the organic linkers (different aromatic molecules, functional substituents, alkynes) is performed. In order to account for quantum effects at low temperatures, atomistic simulations are carried out in the framework of density functional theory (DFT) and density functional tight binding (DFTB). The thermal conductivity tensors for various MOFs are calculated using the model proposed by Bjerg et al.1 relying on the Boltzmann transport equation (BTE) within quasiharmonic lattice dynamics.

1 Lasse Bjerg, Bo B. Iversen, and Georg K. H. Madsen. Modeling the thermal conductivities of the zinc antimonides ZnSb and Zn4Sb3 . Phys. Rev. B , 89:024304, Jan 2014.