Understanding Thermal Transport in "Breathing" Porous Crystals

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Porous materials are used widely in modern society, from air and water filters, to battery electrodes and catalysts for manufacturing chemicals. One particular application of porous materials is to better store gases; highly porous materials are able to soak up gases just like a bath sponge soaks up water. For example, porous materials can be used to store large amounts of oxygen in portable containers or to store natural gas in vehicles. The most effective materials for storing gases are the ones whose pores are of nanoscopic size, where each pore is just large enough to store a few molecules of gas. However, there have been two challenges that researchers have faced in developing better materials for gas storage: (1) gas getting stuck in the pores (referred to as 'stranded' gas), and (2) the pores getting excessively hot due to the heat that is generated when gases enter and bind to their surfaces. This project aims to shed light on both challenges by looking at the thermal properties of a special class of materials called 'breathing' porous crystals. The pores of these materials are able to open when gases enter, and close when gases leave, thus squeezing out any residual gas; not unlike how our lungs work. For this reason, breathing porous materials are very promising for revolutionizing industrial gas storage and separations. However, we currently have no understanding of how heat dissipates in pores that open and close in response to the presence of gases. By illuminating these thermal properties, this project will help facilitate the practical implementation of these promising breathing porous crystals. The objective of this research is to study heat transfer phenomena in flexible porous crystals. This project will systematically study a series of idealized flexible porous crystal structures to help build a fundamental understanding of the factors that play a role in their thermal transport properties. The guiding hypothesis is that the ratio of thermal conductivity for expanded versus contracted pores can be as much as an order of magnitude. Under vacuum conditions, the high ratio might be considered an obvious outcome based on the relative difference in crystal densities, but in the presence of gas, which would be the relevant application condition, there are corresponding changes in gas density that could counteract thermal conductivity changes. In addition to answering these questions directly, our modeling efforts will uncover structure-property relationships that could add further insight into thermal transport behavior in porous systems. Working with experimental collaborators, there will be an effort to subsequently validate the modeling results with empirical measurements.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.