Thermally insulating, optically transparent silica aerogels are promising materials for window applications to reduce the energy consumption required for space heating and cooling. However, the manufacturing of window-sized aerogels is limited by the size and cost of supercritical drying typically used in aerogel synthesis. Recent work by our group has demonstrated the ability to dry aerogels under ambient pressure by exchanging the pore solvent from water to heptane. Using this method, we have prepared optically transparent aerogels with pores smaller than 20 nm and porosities greater than 80%. Drying conditions that produce the best combination of high porosity, high optical clarity and fast drying for scalable applications have not yet been determined. Fast-drying is best for manufacturing, but we have found that it increases the haze of the material. To better understand the effect of different drying rates on the nanoscale architecture of our material, we employ <i>in situ </i>synchrotron-based small-angle X-ray scattering to study how the silica gel nanostructure evolves during the drying process. From the diffuse scattering pattern, we can retrieve the radius of gyration, which is indicative of the size of the pores that act as scattering centers, and connect the changes we see in size over time to the final pore structure of the dried gel measured from nitrogen adsorption porosimetry. We have found that when drying without flow, the condition that results in the smallest pore sizes and the greatest optical transparency, the material reaches a steady-state small radius of gyration for an extended period of time during the constant rate period when solvent evaporates from the surface of the wet gel. This does not occur when drying more quickly. The additional time at small size appears to be crucial for small pores and optical clarity, as hydroxyl groups at the surface of the material are brought into close proximity and given enough time to condense. As a result, the gel's ability to spring back to large pores during the falling rate period when the pores begin to empty of solvent is limited. Understanding this mechanism allows us to reduce overall drying time without sacrificing pore size by changing the flow rate at different points in the drying process and ultimately achieve good optical transmittance of the final gel.

Using <i>in situ </i>small-angle X-ray scattering to examine structural changes of silica aerogels dried under directional flow at ambient pressure for window insulation applications

ACS Spring 2022

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Using in situ small-angle X-ray scattering to examine structural changes of silica aerogels dried under directional flow at ambient pressure for window insulation applications