Elucidating the Degradation of Multi-Component Silica Aerogel

Silica aerogels are a material to insulate concentrating solar thermal (CST) receivers because of their high solar transparency and strong insulating properties. Although prior research shows silica aerogels can improve the solar collection efficiency at moderate temperatures (<550C), there is an incomplete understanding of thermal stability at temperatures necessary for next-generation CST (>700 °C). While viscous sintering is commonly thought to be the primary degradation pathway, the mechanism for which porous silica degrades is not fully understood. In this study, we report progress made towards elucidating the degradation mechanism of silica aerogels by studying how varying surface modifications and pore structures affect aerogel densification and surface-mediated particle aggregation. To test these, we deposit ultrathin films of refractory oxides onto aerogels via atomic layer deposition (ALD) and anneal the aerogels in air at temperatures >700C. We vary the surface coverage, film thickness, and chemistry, and characterize the aerogels with linear shrinkage measurements and x-ray spectroscopy (XRD, EDX, and XPS). Specifically, we show that particle size and aerogel chemistry have a significant impact of the nano-restructure and degradation of silica aerogels, but each to varying extents. Our work here demonstrates an improved understanding of the various structural changes in aerogels at high temperatures and provides a framework to improve the thermal stability.