Phase development and pore stability of yttria‐ and ytterbia‐stabilized zirconia aerogels

Abstract 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°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°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°C. Largest shrinkage and loss of pore volume occurred on crystallization from the amorphous phase, with further loss of pores at temperatures above 1000°C attributable to changes in lattice parameters.