UCLA

The urgent need to reduce the energy demand for space heating and cooling necessitates the development of new building materials. This dissertation focuses on designing porous silica nanoarchitectures for thermal insulation and radiative cooling applications. Using sol-gel methods, a variety of ambiently-dried mesoporous silica materials, also known as ambigels or aerogels, were synthesized. Their structural evolution under various synthetic conditions was examined and correlated to their final optical transmission, infrared absorption, thermal insulation, and mechanical properties. The findings provide valuable insights for optimizing window coatings, scaling manufacturing processes, and designing passive daytime radiative cooling materials.The first two sections focus on the development of nanoparticle-based ambigels (Chapters 2 and 3), and molecular-based ambigels (Chapters 4-6), both for optically transparent window coatings. In nanoparticle-based systems, the effect of pH, silica particle size, and concentration on colloidal aggregation was investigated, enabling control over the pore structure and optical transparency of the resulting ambigels. Hollow silica spheres were next explored as alternative building blocks to introduce additional surfaces for scattering of heat carriers, further reducing thermal conductivity. Sub-20 nm hollow nanoparticles were successfully synthesized using polymer templating and assembled into free-standing monoliths. For the work on molecular-based systems, the influence of surface modification on mechanical properties was first investigated. The significant flexibility of these molecular-based ambigels makes them suitable for large-scale window manufacturing. Finally, scalability of the molecular-based ambigel production process was explored by monitoring structural changes during drying, with a goal of reducing drying time and preventing crack formation. The introduction of forced gas flow accelerated ambigel drying but increased pore sizes, while heptane-saturated forced gas flow minimized drying stresses, enabling production of large, crack-free ambigels. The final part of this dissertation (Chapter 7) focuses on creating a daytime radiative cooling material. The study examined silica ambigels with large silica nanoparticles embedded in the matrix to effectively backscatter solar radiation, coupled with high emissivity in the IR atmospheric transparency window. The effects of particle size, concentration, and ambigel thickness on pore structure, optical properties, and emissivity were systematically studied to establish the design space for highly emissive and solar-reflective radiative cooling materials.