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.

Enhanced thermal stability of high yttria concentration YSZ aerogels

“…32 Prior work on yttriaand ytterbia-stabilized zirconia in the range of 0-15 mol% MO 1.5 demonstrated improvements to the thermal stability upon doping of zirconia aerogels to 1000°C. 33 In all works, higher dopant levels maintained higher surface areas and smaller particle sizes, although still shy of acceptable values for use as effective insulation. Beyond these studies, there remains much to be understood in terms of the thermodynamic and kinetic contributions to the microstructural evolution of porous structures.…”

“…15YSZ crystallized into either the cubic or tetragonal phase, with higher temperatures favoring the formation of the cubic phase as observed previously. 33 Given the very small crystallite size, it was impossible to definitively distinguish between the cubic and tetragonal phases of YSZ using laboratory-scale XRD, especially given the peak broadening from small crystallite sizes. However, based on the diffraction profile and phase diagram, 30 and 50YSZ are expected to be cubic and 15YSZ lies near the phase boundary.…”

“…The stabilizing effect of yttria in YSZ aerogels was demonstrated in studies utilizing up to 8.7 mol% YO 1.5 31 and 15 mol% YO 1.5 32 . Prior work on yttria‐ and ytterbia‐stabilized zirconia in the range of 0–15 mol% MO 1.5 demonstrated improvements to the thermal stability upon doping of zirconia aerogels to 1000°C 33 . In all works, higher dopant levels maintained higher surface areas and smaller particle sizes, although still shy of acceptable values for use as effective insulation.…”

Design, Synthesis, and Use of High Temperature Resistant Aerogels Exceeding 800 oC

“…The performance of zirconia aerogels can be remarkably increased by doping hybrid atoms (variable atoms and doping degree) or introducing nanofibrous building blocks. [116] Because of the existence of oxygen vacancies in defect structures, ytterbia-doped zirconia aerogels prepared by Hurwitz et al [116] had a low thermal conductivity (2 W/mK) at extremely high temperature (1000 to 1200 ℃). Doping process enabled better phase stability and lower thermal diffusivity.…”

Phase Composition and Stability, Sintering and Thermal Conductivity of Gd2O3 and Yb2O3 Co-Doped YSZ

“…YSZ TBCs have been successfully used for several decades [9,10], but they have become less and less reliable with the development of aero-engines [11][12][13][14]. One crucial aspect in this is issue of the phase decomposition and phase transformation [1,15,16]. As is known to all, YSZ TBCs are usually prepared either by electron beam physical vapor deposition (EB-PVD) [17,18] or by air plasma spraying (APS) [19][20][21].…”