This PhD thesis focuses on the mechanical properties and the process to synthesis novel multiscale and multifunctional mesoporous silica monoliths that are both optically transparent (not translucent) and thermally insulating to reduce energy losses through windows in commercial and residential buildings. Silica aerogels have attractive thermal insulating properties. However, conventional supercritically-dried silica aerogels are translucent due to their wide pore size distribution featuring nanopores larger than 30-40 nm resulting in a slightly blue haze. Furthermore, they are too fragile and stiff to sustain the window manufacturing process. Recent efforts have demonstrated the synthesis of ambiently-dried mesoporous silica monoliths - using sol-gel chemistry or preformed nanoparticles - with large porosity (> 70%), narrow and small pore size distribution (< 20 nm) so as to minimize simultaneously heat conduction, light scattering, and haze. The small slabs (2-3 cm in diameter) could be made flexible and hydrophobic using trimethychlorolsilane (TMCS) surface treatment. This dissertation aims to (1) experimentally measure the elastic and plastic mechanical properties of sol-gel and nanoparticle-based mesoporous silica monoliths; (2) to develop a continuous process to produce crack-free thick (1-3 mm), large, transparent and, flexible mesoporous monoliths; (3) to integrate these monoliths into glazing units; and (4) to assess quantitatively the glazing units’ thermal, optical, and acoustic performances and their durability.
First, the elastic and plastic mechanical properties were measured using nanoindentation to find the relationship between the effective Young’s modulus and hardness of the mesoporous silica monoliths and their effective density. The mesoporous silica monoliths featured porosity ranging from 46% to 92% achieved with different chemistries and drying conditions. The effective Young’s modulus and the hardness of the mesoporous slabs obeyed a power law with respect to their effective density. In addition, deposition of a hard alumina coating was demonstrated as a successful way to increase the hardness of the mesoporous silica monoliths. Second, 29 Si solid-state Magic Angle Spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy, structural characterization, and three-point bending test were used to elucidate the reason why silica aerogel monoliths became flexible upon TMCS surface modification treatment before drying. Third, a continuous and scalable process was developed to produce ambiently-dried flexible and transparent silica aerogel monoliths. The process combined gel casting and jet impingement drying methods to produce 1.2 mm � 14 cm � 14 cm silica aerogel monoliths in less than 72 hours. Fourth, ambiently-dried aerogel monoliths were adhered to soda-lime silica glass panes using an optically clear adhesive before integration into double-pane glazing units. Finally, the performances of the window solutions were measured using ASTM standardized testing protocols including (i) their acoustic transmission loss, (ii) their durability under ultraviolet light, moisture, and temperature gradient and cycling, while (iii) their U-value and condensation temperature were calculated using a widely used computer code.
