Investigation into suitability of geopolymers (illite & metakaolin) for the space environment.

摘要

The United States has utilized high resolution imaging platforms for national defense since the beginning of the space age. In order to improve the resolution and swath width of imaging satellites, the primary restriction in optical hardware is the mirror size, specifically mirror diameter and mirror mass. This research addresses one of these concerns, reducing the mass of a spacecraft mirror by the use of innovative materials.;In contemporary imagery satellites, monolithic glass is the material of choice to produce large aperture mirrors that can survive the space environment. However, material performance requirements for future imaging mission mirrors necessitate a lower areal density than glass with similar if not superior mechanical strength. Additionally, any material chosen must also be able to deal with the unique environment of low earth orbit, namely the near-vacuum conditions, radiation environment and interaction with atomic oxygen.;This research focuses on investigation of a class of inorganic polymers known as geopolymers for use in the space environment. Geopolymers are based on aluminosilicate chemistry and have advantages of high specific strength combined with low densities, tailorable coefficients of thermal expansion, and easier curing processes than traditional space qualified epoxies. Geopolymers have a long history for use in terrestrial applications, but empirical data is not available addressing their suitability for the space environment.;This research focused on determining whether the geopolymer as a bulk material will respond favorably to environmental conditions as experienced during typical spaceflight operations. Two different formulations of geopolymer were investigated, one based on metakaolin chemistry, and the other based on illite chemistry. Three primary objectives were identified for assessing whether geopolymers could survive the space environment: could the materials be processed to minimize curing shrinkage, characterizing the outgassing performance, and analyzing the materials for damage following exposure to typical radiation and atomic oxygen levels seen in a short duration LEO mission.;The results of the test campaign showed promising results. Curing shrinkage was controlled using pressure as a primary variable, but a cracking failure mode was present due to thermal shock effects. Neither geopolymer emitted significant organic volatiles during outgassing, but water emission and re-absorption during test, qualification, and pre-launch storage associated with launch cycles may be an issue. Finally, space environmental exposure of ultraviolet radiation, high energy particles, and atomic oxygen bombardment resulted in only minor surface changes to the bulk material with structural performance maintained throughout the rest of the material. Overall, geopolymers show great promise as a spacecraft material due to their resistance to space environment effects, but specific caveats need to be considered for application development.