Development of thermally stable aerogels for aerospace applications

Olson, Nathaniel Scott

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https://hdl.handle.net/2142/120211 Copy

Description

Title

Development of thermally stable aerogels for aerospace applications

Author(s)

Olson, Nathaniel Scott

Issue Date

2023-03-17

Director of Research (if dissertation) or Advisor (if thesis)

Krogstad, Jessica A

Doctoral Committee Chair(s)

Krogstad, Jessica A

Committee Member(s)

• Murphy, Catherine J
• Hurwitz, Frances I
• Perry, Nicola H
• Shoemaker, Daniel P

Department of Study

Materials Science & Engineerng

Discipline

Materials Science & Engr

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• aerogel
• porous ceramic
• yttria-stabilized zirconia
• thermal protection system
• zirconia
• sol-gel

Abstract

The extreme environments of space exploration have pushed scientists to create revolutionary materials that have filtered back to terrestrial life. A key challenge is lightweight insulation to replace dense ceramics for aerospace applications, where every kilogram is worth $5,000 to $20,000 to just reach low Earth orbit. A promising form of insulation is aerogels. The high porosities and low densities of ceramic aerogels offer outstanding insulative performance in applications where weight is a critical factor. The high surface-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. The purpose of this work was the establishment of a design framework for thermally stable, highly porous materials. Design considerations including composition, synthetic parameters, and post-synthetic modification were evaluated via sol-gel synthesis and characterization of metal oxide aerogels. Zirconia aerogels doped with varying concentrations of yttrium, ytterbium, gadolinium, cerium, and calcium were studied to 1200 °C. For yttria-stabilized zirconia (YSZ) aerogels, increased yttria content was associated with increased thermal stability and retention of porosity to 1200 °C. The improvements in thermal stability with increased yttria content are hypothesized to be the result of a reduction in surface energy and cation diffusivity, thereby reducing the driving force for densification and slowing transport processes that result in densification, respectively. When the broader set of dopants were analyzed, increased dopant concentration was found to reduce rate of densification for all materials. Gadolinium and yttrium were the most effective dopants for improving thermal stability. Evaluation of material property – thermal stability relationships was challenging because of a lack of material property data for the compositions studied. A material property database for the aerogels studied would serve as a useful tool for future analysis of design considerations for thermally stable aerogels and furthering the understanding of properties and mechanisms controlling the densification of highly porous structures. The synthetic parameters of solids loading and water content were used to modify the starting structure of the aerogel, including its specific surface area, mesopore volume, and mesopore size. The structural evolution of these materials was studied at elevated temperatures. The differences in starting structure had negligible impact on the thermal stability of the aerogel. This result serves as an important control for the study of composition, where choice of dopant and dopant concentration influence both starting structure and thermal stability. Post-synthetic modification of YSZ aerogels with coatings of silica was performed in an effort to further improve thermal stability. Silica coatings significantly improved thermal stability to 1000 °C but underwent rapid densification beyond this temperature, which was attributed to viscous sintering of silica. This result points towards a strong motivation to study YSZ aerogels with coatings of zirconia, titania, and alumina to identify a coating that offers improved thermal stability to temperatures beyond 1000 °C by avoidance of viscous sintering. The colloidal stability of yttria was evaluated in an effort to develop a colloidal synthesis for YSZ aerogels. The development of a colloidal synthesis for YSZ aerogels will enable the study of aerogel thermal stability in context of structural motifs and how these motifs are assembled. Ultrasonication of yttria colloids in aqueous citric acid solutions or in ethanol were found to reduce the particle size and improve colloidal stability, an important first step in developing a colloidal YSZ synthesis. The colloidal synthesis of alumina-zirconia as an alternative composition to YSZ was also proposed and preliminary work identified precursors and possible synthetic routes. Overall, important design considerations were identified for thermally stable metal oxide aerogels, including surface energy and cation diffusivity. Surface energy is expected to be especially important, given the extremely high specific surface energies of aerogels. There are certainly more design rules and considerations to be identified. This work suggests promising future routes of study including the establishment of material property databases in conjunction with thermal stability measurements, the use of non-densifying coatings, and the development of novel colloidal syntheses for thermally stable aerogel compositions. Thermally stable aerogels can be implemented into flexible reinforcements such as metal oxide felts, papers, and weaves to form composite insulation. The addition of aerogel will reduce the thermal conductivity and permeability of the composite, permitting higher operating temperatures and pressures while avoiding the transport of heat and gas through the insulation. This work identified metal oxide aerogels, including zirconia doped with yttrium or gadolinium as well as silica coated aerogels, that offer promise as components in insulative composites for aerospace thermal protection systems.

Graduation Semester

2023-05

Type of Resource

Thesis

Copyright and License Information

Copyright 2023 Nathaniel S. Olson

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