Anisotropic thermal transport in MOF-5 composites

Abstract

Metal-organic frameworks (MOFs) are a new class of porous, crystalline materials with applications in the capture, storage, and separation of gasses. Although much effort has been devoted to understanding the properties of MOFs in powder form, in a realistic system the MOF media will likely be employed as dense compacts, such as pucks or pellets, to maximize volumetric efficiency. In these applications efficient transport of the heat of adsorption/desorption is an important design consideration. Consequently, densified composites consisting of a physical mixture of a MOF and expanded natural graphite (ENG) have been proposed as a means to enhance the intrinsically low thermal conductivity of these materials. Here we demonstrate that the high-aspect ratio of ENG particles, combined with uni-axial compression, results in anisotropic microstructural and thermal transport properties in composite MOF-5/ENG pellets. Microscopy of pellet cross-sections revealed a textured microstructure with MOF particle boundaries and ENG orientations aligned perpendicular to the pressing direction. This anisotropy is manifested in the thermal conductivity, which is two to four times higher in directions perpendicular to the pressing direction. We further demonstrate that this anisotropy can be exploited using two processing techniques. First, a custom die and densification process allows for reorientation of the preferred heat flow pathway. Second, a layered MOF-5/ENG microstructure increases the thermal conductivity by an order of magnitude, with only minor ENG additions (5wt.%). These results reveal that anisotropic thermal transport in MOF composites can be tailored using a judicious combination of second phase additions and processing techniques.