Understanding Thermal Transport in "Breathing" Porous Crystals

多孔材料在现代社会中被广泛使用，从空气和水的过滤器，到电池电极和制造化学品的催化剂。多孔材料的一个特殊应用是更好地储存气体；高度多孔的材料能够吸收气体，就像沐浴海绵吸收水一样。例如，多孔材料可用于在便携式容器中储存大量氧气或在车辆中储存天然气。储存气体最有效的材料是那些具有纳米级孔隙的材料，其中每个孔隙的大小刚好足以储存几个气体分子。然而，在开发更好的储气材料的过程中，研究人员面临着两个挑战：(1)气体被卡在孔隙中（称为“滞留”气体）；(2)由于气体进入并与表面结合时产生的热量，孔隙变得过热。这个项目旨在通过研究一种被称为“呼吸”多孔晶体的特殊材料的热性能来阐明这两个挑战。这些材料的孔隙能够在气体进入时打开，在气体离开时关闭，从而将任何残余气体挤出；就像我们的肺一样。由于这个原因，呼吸多孔材料是非常有前途的革命工业气体储存和分离。然而，我们目前还不了解热量是如何在气孔中消散的，气孔会随着气体的存在而打开和关闭。通过阐明这些热特性，该项目将有助于促进这些有前途的呼吸多孔晶体的实际实施。本研究的目的是研究柔性多孔晶体中的传热现象。该项目将系统地研究一系列理想的柔性多孔晶体结构，以帮助建立对影响其热输运性质的因素的基本理解。指导性的假设是，膨胀孔与收缩孔的热导率之比可以高达一个数量级。在真空条件下，基于晶体密度的相对差异，高比值可能被认为是一个明显的结果，但在气体存在的情况下，这将是相关的应用条件，气体密度的相应变化可以抵消导热系数的变化。除了直接回答这些问题外，我们的建模工作还将揭示结构-性质关系，从而进一步了解多孔系统的热传输行为。与实验合作者合作，将努力随后用经验测量验证建模结果。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.