Optical Response of Metakaolin after Ultraviolet and High Energy Electron Exposure

Metakaolin, which is part of a class of inorganic polymers called geopolymers, is being tested currently for its use as a lightweight mirror material in spacecraft applications. Metakaolin, as with most geopolymers, has the advantages of low initial coefficient of thermal expansion, easy preparation at room temperature and pressure, and high specific strength. Even though metakaolin has been known as a structural material for millennia, it has not been properly vetted for use as a material in spacecraft applications, especially with respect to exposure to its environments. This research highlights one particular aspect of response to the space environment; that is, how do the optical properties of metakaolin change after subjugation to bombardment by ultraviolet and high energy electron radiation? These two radiation sources are common in low earth orbit and a primary cause of degradation of organic polymers in space. Photospectroscopic analysis showed that ultraviolet in combination with high energy electrons causes changes in the metakaolin which need to be accounted for due to their potential impacts on the thermal management of a spacecraft and during application in composite mirror structures. 1. Introduction The primary choice of mirror material for spacecraft imaging optics, since the beginning of the space age, has been monolithic glass. Monolithic glass mirrors have enabled spacecraft designers to achieve mirror diameters of over 1？m, and they are well understood in terms of mechanical and thermal performance as monolithic glass variants have been one of the first man-made construction materials [1]. However, material performance requirements for the future space mirrors for advanced imaging missions necessitate a lower areal density than glass with similar if not superior mechanical strength. Additionally, any material chosen must also be able to withstand the unique environment of low earth orbit, namely the near-vacuum conditions, radiation environment and interaction with atomic oxygen. The space environment poses unique hazards for materials. There is a specific concern with any material, including geopolymers, and it is about the behavior under the radiation environment encountered in orbit. Previous investigations, both on the ground and in-flight experiments, have shown that significant degradation of the organic polymer strength occurs due to the increased cross-linking of polymer networks after absorbing the radiation emitted from the sun or deep space. The typical radiation environment includes exposure to ultraviolet and gamma