Enhanced thermal stability of high yttria concentration YSZ aerogels

Abstract

A significant challenge in the field of aerospace materials is the development of lightweight, highly insulating materials that can survive in extreme environments. Materials with reduced thermal conductivity permit higher operating temperatures and improved insulative performance. Reduced weight mitigates cost and improves payload capacity. A promising class of lightweight, low thermal conductivity materials are aerogels. Aerogels are highly porous, extremely lightweight, and display extraordinarily low thermal conductivity. 1 The same highly porous structure is also the source of a critical engineering challenge: thermal stability. Upon thermal exposure, the aerogel's highly porous structure collapses, and the favorable properties of low thermal conductivity and low density are diminished. Aerogels with improved thermal stability must be developed to enable the use of aerogels as insulation in extreme environments and temperatures to 1200°C.Polymeric aerogels, which demonstrate excellent mechanical properties, are limited to temperatures below 500°C due to the polymer network's decomposition. 2,3 Silica aerogels, perhaps the most studied and well-known, undergo significant sintering and densification above 700°C. [4][5][6] Exploration of various metal oxides have led to mixed results. Zirconia aerogels lose much of their specific surface area (SSA) upon heating, from 282 m 2 /g as dried to

Thermal stability of MxOy‐doped zirconia aerogels (M = Y, Yb, Gd, Ce, Ca) studied through 1200°C

“…Looking at this wide range of metal oxides with varying properties and as‐dried SSAs, the difference in thermal stability was small. Doping alumina with yttria and zirconia with yttria and/or ytterbia improved thermal stability 8,12,13 . These doped aerogels offered improved thermal stability to 1000°C, but any advantage was consistently eliminated beyond this temperature.…”

“…Doping alumina with yttria and zirconia with yttria and/or ytterbia improved thermal stability. 8,12,13 These doped aerogels offered improved thermal stability to 1000 • C, but any advantage was consistently eliminated beyond this temperature.…”

“…This presents a challenge to a hypothesis proposed in our previous work on YSZ aerogels from 0 to 50 mol% YO 1.5 , where a similar trend was observed of coarser pore structures with increasing yttria content. 13 We hypothesized that with increasing yttria content, the concentration of nucleophile (Cl − ) for epoxide ring opening is higher as YCl 3 ⋅xH 2 O provides more chloride than ZrOCl 2 ⋅xH 2 O. This in turn increases the rate of gelation and results in structures that resemble agglomerated networks of precipitates rather than homogenous gels.…”

“…Previous work did this for yttria-stabilized zirconia (YSZ) aerogels: lower cation diffusivity and lower surface energy were identified as potential markers for improved thermal stability. 13,26,27 To further evaluate these hypotheses, study of a wider range of dopants and concentrations is necessary.…”

“…9 A variety of lanthanide oxides were found to lose most of their SSA after heating to 650 • C. 10 Zirconia aerogels experienced a reduction in SSA from 282 m 2 /g when dried to 92 m 2 /g after 2 h at 500 • C. 11 However, a study on doping zirconia aerogels with yttria and ytterbia up to 15 mol% MO 1.5 improved the thermal stability and later work demonstrated that yttria doping beyond 30 mol% YO 1.5 proved to be an even more effective means of improving the thermal stability of the pore structure. 12,13 The latter work hypothesized that a reduction in surface energy (driving force for densification) and cation diffusivity (rate of densification) with higher yttria content contributed to the improvement in thermal stability.…”