Exploring the Impact of the Linker Length on Heat Transport in Metal–Organic Frameworks

Abstract

Metal–organic frameworks (MOFs) are a highly versatile group of porous materials suitable for a broad range of applications, which often crucially depend on the MOFs’ heat transport properties. Nevertheless, detailed relationships between the chemical structure of MOFs and their thermal conductivities are still largely missing. To lay the foundations for developing such relationships, we performed non-equilibrium molecular dynamics simulations to analyze heat transport in a selected set of materials. In particular, we focus on the impact of organic linkers, the inorganic nodes and the interfaces between them. To obtain reliable data, great care was taken to generate and thoroughly benchmark system-specific force fields building on ab-initio-based reference data. To systematically separate the different factors arising from the complex structures of MOF, we also studied a series of suitably designed model systems. Notably, besides the expected trend that longer linkers lead to a reduction in thermal conductivity due to an increase in porosity, they also cause an increase in the interface resistance between the different building blocks of the MOFs. This is relevant insofar as the interface resistance dominates the total thermal resistance of the MOF. Employing suitably designed model systems, it can be shown that this dominance of the interface resistance is not the consequence of the specific, potentially weak, chemical interactions between nodes and linkers. Rather, it is inherent to the framework structures of the MOFs. These findings improve our understanding of heat transport in MOFs and will help in tailoring the thermal conductivities of MOFs for specific applications.

Sub-Micrometer Phonon Mean Free Paths in Metal–Organic Frameworks Revealed by Machine Learning Molecular Dynamics Simulations

“…Using a MOF-FF potential, Wieser et al 23,26 observed that the thermal conductivity of MOF-5 not converged up to 20 nm in NEMD simulations, which is consistent with our results. This means that the large phonon MFPs in MOFs are not sensitive to the details of the interatomic potential.…”

“…In the last two decades, due to their ultra-high porosity and structural tunability, metal–organic frameworks (MOFs) have shown great potential in various applications, such as gas storage and separation, water harvesting, electronic devices, and heterogeneous catalysis . Lattice thermal conductivity is a critical parameter for MOFs in the context of thermal energy conversion, thermal management, and thermal stability and has attracted extensive experimental − and theoretical − studies.…”

Polarization-dependent plasmonic heating in epitaxially grown multilayered metal–organic framework thin films embedded with Ag nanoparticles

“…The unique anisotropic optical property was discussed as originating from differences in electron-phonon interactions attributed to anisotropy in the thermal conductivity of the oriented MOF lattice surrounding AgNPs. 32,[51][52][53] Plasmonic metal nanoparticles have been intensively employed in a broad range of applications in the fields of biomedical, energy, and information technologies due to their unique optical properties arising from their interactions with an incident light. 54,55 In addition to the unique optical properties, the plasmonic nanoparticles efficiently and quickly convert light into heat, which leads to lattice heating, heating of the environment and the generation of hot electrons.…”

“…The unique anisotropic optical property was discussed as originating from differences in electron–phonon interactions attributed to anisotropy in the thermal conductivity of the oriented MOF lattice surrounding AgNPs. 32,51–53…”

Heat Transfer in Nanostructured Materials

“…However, this phenomenon was not observed in bulk silicon. Wieser et al [ 5 ] examined the impact of organic linkers, inorganic nodes, and interfaces on metal-organic frameworks’ (MOFs) thermal conductivity using non-equilibrium molecular dynamics calculations. The study found that the dominance of interface resistance derived from intrinsic framework structures of MOFs, rather than chemical interactions between nodes and linkers.…”