The application temperature and exposure time of lightweight, high performance aerogel insulation is limited by the thermal stability of the aerogel system used. Elevated temperatures cause rapid densification of the porous structures accompanied by increases in thermal conductivity and density. Previous studies have demonstrated the importance of doping concentration to thermal stability in doped metal oxide aerogel systems. The compositional route remains insufficient for stabilizing the pore structures of yttria-stabilized zirconia aerogels at elevated temperatures above 1100 °C. Non-compositionally, modifying synthetic parameters in the aerogel synthesis have been known to change the as dried pore structures. However, few studies have investigated the microstructure evolution of these pore structures under heat treatment. The current work investigates YSZ aerogels prepared via a sol-gel method at 30 mol

Maintaining high surface area and porosity at high temperatures is important when considering aerogels for use in thermal management systems. The mesoporous structure of aerogels results in extremely low thermal conductivity, making them lightweight, high performance insulating materials. Maintaining these properties requires innovative routes to suppress sintering and pore collapse as use temperatures rise. The current work aims to improve the pore structure stability of yttria-stabilized zirconia aerogels by the addition of a SiO2 coating. The functionalization of surface hydroxyl groups by the addition of SiO2 is hypothesized to mitigate condensation reactions, which are a driving force for shrinkage and pore structure collapse. Zirconia aerogels with 0, 10, and 30 mol% yttria (YO1.5) additions were coated in a tetraethyl orthosilicate (TEOS) solution and exposed to temperatures up to 1200 degrees C. Crystal structure, pore structure, and aerogel morphology were investigated to understand changes in aerogel thermal stability. The SiO2 coating exhibited a greater influence on pore stability over yttria concentration, with specific surface area of the coated aerogels being twice that of the uncoated aerogels up to 1000 degrees C. However, the presence of the SiO2 coating promoted rapid sintering and densification at 1200 degrees C, establishing an upper use temperature for SiO2 and a need to develop other coating chemistries.

Numerous applications from insulation to catalytic supports and fuel cells can benefit from lightweight, high surface area, mesoporous materials which maintain their mesoporous structure to temperatures of 600 to 1200 °C. Polymeric aerogels are limited to temperatures of nominally 400 °C due to thermal degradation of organic groups. Silica aerogels begin to densify by 700 °C. A number of aerogel systems show stability at higher temperatures, including alumina, alumina silicates, yttrium-doped alumina, and zirconia and yttria-stabilized zirconia aerogels. Within a given chemical composition, the morphology and textural stability of a mesoporous structure is dependent upon the synthesis method used. Other important considerations in choosing an aerogel composition include the time at temperature required for a given application, phase transformations inherent in a given system, and approaches to phase stabilization such as introduction of dopants into the backbone structure. Reinforcement of the aerogel through a composite approach also is addressed.

The high porosities and low densities of ceramic aerogels offer outstanding insulative performance in applications where weight is a critical factor. The high surface area-to-volume ratios and specific surface areas provide extremely low thermal conductivity, but also contribute to rapid densification of the pore structure at elevated temperatures. This densification diminishes their favorable properties and inhibits use of aerogels in high-temperature applications. This work contributes to a design framework for thermally stable aerogels via the study of dopant chemistry (Y, Yb, Gd, Ca, Ce) in zirconia aerogels. The structural evolution was studied through 1200 & DEG;C using nitrogen physisorption, scanning electron microscopy, and X-ray diffraction. The role of dopant identity and concentration in thermal stability was elucidated. In the context of the design framework, dopant chemistry is an aggregate for many closely related material properties, each of which may contribute to aerogel structural evolution. To develop a truly predictive design framework for ceramic-based aerogels, systematic and comprehensive evaluation of thermodynamic and kinetic properties must be performed in conjunction with studies on structural evolution.

Aerogels are a promising class of materials for lightweight, high-performance insulation. However, their high specific surface area contributes to rapid densification of the structure at elevated temperatures. Upon densification, the favorable properties of low thermal conductivity and low density are lost. Investigation of doped metal oxide systems presents a route to stabilization of porous structures at high temperatures and a platform to study parameters conducive to thermal stability. Our work focuses on yttria-stabilized zirconia (YSZ) aerogels prepared via a sol-gel method and supercritically dried. Yttria concentrations were studied from 0 to 50 mol% YO1.5 to stabilize porosity to temperatures of 1200 degrees C and develop an understanding of properties contributing to improved stability. Increased yttria content improved the thermal stability of the pore structure by reducing densification and suppressing crystallite growth, resulting in retention of the mesoporous structure to 1200 degrees C. The improvement in thermal stability is related to associated reductions in specific surface energy and cation diffusivity at higher yttria concentrations. This work demonstrates that tuning thermodynamic and kinetic factors is a viable route to improved thermal stability in highly porous structures for use as insulation in extreme environments.

High-porosity yttria- and ytterbia-stabilized zirconia aerogels offer the potential of extremely low thermal conductivity materials for high-temperature applications. Yttria- and ytterbia-doped zirconia aerogels were synthesized using a sol-gel approach over the dopant range of 0-20 atomic percent. Surface area, pore volume, and morphology of the as-dried aerogels and materials thermally exposed for short periods of time to temperatures up to 1200 degrees C were characterized by nitrogen physisorption, scanning and transmission electron microscopy, and X-ray diffraction. The aerogels as supercritically dried all were X-ray amorphous. At a 5% dopant level, a tetragonal structure with a smaller monoclinic phase developed on thermal exposure. Mixed tetragonal and cubic phases or predominantly cubic materials were observed at higher dopant levels, depending on the dopant level, temperature and exposure time. The formation of crystalline phases was accompanied by loss of surface area and pore volume, although some mesoporous structure was maintained on short-term exposure to 1000 degrees C. Incorporation of the smaller Yb atom into the lattice structure resulted in smaller lattice dimensions on crystallization than was seen with Y doping and favored a more highly equiaxed structure. Aerogels synthesized with 15% Y maintained the smallest particle size without evidence of sintering at 1100 degrees C. Largest shrinkage and loss of pore volume occurred on crystallization from the amorphous phase, with further loss of pores at temperatures above 1000 degrees C attributable to changes in lattice parameters.

The isomerization of glucose to fructose is an important step in the conversion of biomass to valuable fuels and chemicals. A key challenge for the isomerization reaction is achieving high selectivity towards fructose using recyclable and inexpensive catalysts. In this study, the isomerization of glucose to fructose is investigated through using imogolite nanotubes as a catalyst. Imogolite is a single-walled aluminosilicate nanotube characterized by surface areas of 200-400 m(2)/g and pore widths near 1 nm. Imogolite is a highly tunable structure and can be modified through substitution of Si with Ge or through functionalization of methyl groups to the inner surface. These modifications change the surface properties of the nanotubes and enable tuning of the catalytic performance. Imogolite nanotubes are successfully used as a heterogeneous catalyst for the isomerization of glucose to fructose. Of the compositions tested, aluminosilicate imogolite is the most active material for the conversion of glucose, achieving a glucose conversion of 30% and a fructose selectivity of 45%. Catalyst recycling experiments reveals that organic content accumulates on the nanotubes that results in a minor reduction in conversion while maintaining similar catalytic selectivity. The catalyst can be washed with aqueous ammonia, allowing the productivity of fructose to be recovered. Overall, imogolite nanotubes are an active and tunable catalytic platform with moderate selectivity for the isomerization of glucose to fructose.