Identifying the Bottleneck for Heat Transport in Metal–Organic Frameworks

Abstract

Controlling the transport of thermal energy is key to most applications of metal-organic frameworks (MOFs). Analyzing the evolution of the effective local temperature, the interfaces between the metal nodes and the organic linkers are identified as the primary bottlenecks for heat conduction. Consequently, changing the bonding strength at that node-linker interface and the mass of the metal atoms can be exploited to tune the thermal conductivity. This insight is generated employing molecular dynamics simulations in conjunction with advanced, ab initio parameterized force fields. The focus of the present study is on MOF-5 as a prototypical example of an isoreticular MOF. However, the key findings prevail for different node structures and node-linker bonding chemistries. The presented results lay the foundation for developing detailed structure-to-property relationships for thermal transport in MOFs with the goal of devising strategies for the application-specific optimization of heat conduction. The structure of a metal-organic framework (MOF) is characterized by an open framework of metal ions interconnected by organic linkers. Its porous structure is particularly interesting for various applications including catalysis [1] and the capture,

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

“…For the actual MOFs, supercells containing 8 to 24 cubic unit cells in the heat transport direction were used. This corresponds to cell lengths of 200-1000 Å. Perpendicular to the heat transport direction, we observed convergence for cross sections consisting of 2 × 2 cubic unit cells, which is larger than the 1 × 1 cells employed in [50]; convergence issues in these earlier simulations are discussed in the Supplementary Materials Section S3.3. Similar cell sizes were used for the model systems, ensuring convergence with the simulation cell length parallel and perpendicular to the heat transport direction (see Supplementary Materials Section S5.2).…”

“…The impact of such chemical details has already been addressed for IRMOF-1 in Ref. [50] by varying the metal atoms in the nodes (in this way changing node masses and node-linker bonding strengths). Additional details motivating the choice of the model systems, the parameters used in the model, and their impact on the most relevant investigated quantities, are described in the Supplementary Materials Section S5.1.…”

“…Aiming at a better understanding of the specific mechanisms of heat transport in MOFs, we found in a previous investigation that the primary bottlenecks for thermal transport are the interfaces between the inorganic nodes and the organic linkers [50]. This suggests that increasing the length of the linker could lead to an improved thermal conductivity, as it decreases the density of interfaces in the heat-transport direction.…”

Insights into the thermal conductivity of MOF-5 from first principles

“…Such ultralow κ in MOFs, according to force-field-based molecular dynamics (MD) simulations, has been attributed to the short phonon mean free path, 16 the mass mismatch between metal and oxygen atoms, 17 and the interface thermal resistance between metal nodes and organic linkers. 18 Only a limited number of parameter-free investigations exist, and hence first-principles calculations are welcome in order to provide insights into the underlying mechanism responsible for the ultralow κ in MOFs. Besides, κ in MOFs is intimately related to the structural (such as pore size 19 and pore shape 20 ) and chemical (such as the type of the functional group 21 ) properties.…”

Understanding the Origin of the Particularly Small and Anisotropic Thermal Expansion of MOF‐74

“…[ 62 ] These considerations emphasize that studying thermal expansion is also an important first step toward understanding the anharmonic character of phonon modes in MOFs, which is relevant also for other processes like thermal conduction. [ 62–67 ] Particularly relevant quantities in this context are the so‐called mode Grüneisen parameters, which describe the relative changes of the frequencies of individual phonon modes with the unit‐cell volume (or the applied strain). These mode Grüneisen parameters are directly connected to higher‐order force constants beyond the harmonic approximation.…”