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.
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 [
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 radiation, in addition to high energy charged particles like electrons and protons [
Inorganic polymers, or “geopolymers” (the word being formed from “geologic polymers”) as they are commonly known, are generally based on aluminosilicate powders. They have been studied for literally thousands of years as the replacements for traditional cements because of their lower densities and easier curing conditions. Generally, geopolymers have fewer effluent species than their organic counterparts (due to the lack of organic volatiles used in making the organic polymers). They cure at lower temperatures than epoxies, reducing the production costs and increasing the categories/choice of materials with which they can be bonded (since the other materials do not have to get exposed to higher temperature environments). Geopolymers have another advantage over the organic polymers in space optics applications; they have a low baseline CTE. The Air Force Research Laboratory, Materials and Manufacturing Directorate has been working on geopolymers over a decade for space optics applications, since geopolymers are promising materials as an adhesive or as a structural material [
UV and high energy particle resistance of geopolymers have not been adequately studied under space or simulated space conditions. The most significant related research involving UV exposure of geopolymers has been conducted by the New Jersey Department of Transportation where the UV resilience of a geopolymer coating on a test strip of highway retainer wall was studied [
Therefore, there is a need to determine initial investigation of the performance of metakaolin under radiation conditions as experienced in space environments before these materials can be used in space systems applications. The present paper presents the effects of the space environment on metakaolin, specifically the changes in optical parameter performance after exposure to ultraviolet and high energy electrons. This information is needed before its real-life applications in space optics systems.
Metakaolin is a processed form of the kaolinite mineral. The classical chemical formula for kaolinite is Al2Si2O5(OH)4. To process kaolinite to metakaolin, endothermic dehydroxylation (i.e., dehydration) is performed by raising the temperature of the kaolinite as high as 550–600°C to produce disordered metakaolin, Al2Si2O7 [
Simultaneous UV and high energy electron exposure were performed in Space Combined Effects Primary Test and Research (SCEPTRE) Facility located at the Air Force Research Laboratory. Exposure test in the facility was performed in accordance with the guidelines of ASTM E 512-94, Standard Practice for Combined, Simulated Space Environment Testing of Thermal Control Materials with Electromagnetic and Particulate Radiation, established by the American Society for Testing and Materials [
For the UV exposure test, the SCEPTRE Facility was configured to hold up to eighteen 2.5 cm diameter disks. The sample’s nominal size was 2.5 cm diameter and 0.3 cm thickness. These samples were mounted on a central mounting wheel. The samples were placed on the holders using wire braids to keep the samples in place. The sample mounting wheel was rotated to ensure equal exposure to the source over the test period. Each disk was weighed and photographed before and after exposure. There were also unexposed baseline (or control) samples, which were kept outside of the chamber. These were used to compare with the exposed samples. The SCEPTRE chamber details for this test are listed in Table
SCEPTRE test conditions.
Time of exposure | 1096 hrs |
Solar environment | |
UV source (2500 W xenon arc lamp) | 2.75 EUVS, (200–400 nm); 3014 ESH |
VUV source (150 W deuterium lamp) | 20 EuVS (115–400 nm); 2800 ESH |
Electron flux | |
1 keV electron flux |
|
10 keV electron flux |
|
Electron fluence after exposure time |
|
Other exposure test environmental values | |
Inner specimens | 75–120°C |
Outer specimens | 110–160°C |
Vacuum pressure |
|
After SCEPTRE exposure, each sample was immediately weighed, as soon as it was taken out of the chamber while it was still attached to its metal housing. The mass measurement of each sample was taken in similar way before chamber exposure. As seen in Table
Mass measurements.
Sample | Preexposure mass (g) | Postexposure mass (g) | % mass loss |
---|---|---|---|
Metakaolin 1 | 22.51 | 22.32 | 0.8 |
Metakaolin 2 | 21.72 | 21.60 | 0.5 |
Metakaolin 3 | 22.54 | 22.45 | 0.4 |
Metakaolin 4 | 22.82 | 22.58 | 1.1 |
Samples were photographed before and after SCEPTRE exposure with a standard digital camera for visual evidence of color changes and any obvious signs of damage mechanisms. The photometric response of the samples was also taken before and after exposure with a Perkin-Elmer Lambda 950 UV-Vis-NIR double beam spectrophotometer. Photometric signatures were recorded from the UV through IR wavelengths. The signatures in the form of reflectivity measurements for pre- and postexposure samples were averaged by the wavelength. Since the samples were still mounted to aluminum mounting pucks and held with thin aluminum wire braiding, consistent distances from the sample to the spectrophotometer sensor aperture could not be ascertained that it was consistent. Therefore, the photometric signatures were normalized by setting the peak reflectivity amplitude to unity (1) for each individual sample. So in presenting the data, the curves are shown with reflectivity as a percentage of the maximum reflectivity value for that signature. Since the objective of the study is whether the general behavior of the specimen changed with exposure in terms of its spectral signature, this technique allowed the analysis of the changes. The amplitude of the signature may change as its distance to the sensor changes; that is, it is related to the inverse of the square of distance (
Figure
Photospectroscopic signature of single metakaolin samples at pre- and postexposure.
Photospectroscopic signature of metakaolin samples at pre- and postexposure, normalized.
Figure
The reduction in reflectivity in the visible wavelengths (400–700 nm) was consistent with the darkening of the samples as seen in the visible light photographs due to additional oxidation occurring on the surface, typical of many oxide compounds. Reflectance spectroscopy is sensitive to subtle changes in crystal structure or chemistry, and these results seem to confirm a surface chemistry or polymeric structure change following UV and high energy electron exposure. It is also important to note, however, that oxides (which geopolymers are) darken when oxygen atoms are knocked out of the polymer matrix due to UV exposure [
Ultraviolet exposure in combination with high energy electron exposure did affect the metakaolin’s optical properties. Photospectroscopic analysis showed a change in behavior in the visible and mid-IR regions. While visible changes were noticeable, they were not unexpected as they are consistent with long understood oxide darkening mechanisms. If left uncoated or untreated and exposed to the space environment, the IR reflectivity changes in the metakaolin may complicate heat management design of a spacecraft.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The views expressed in this paper are those of the authors and do not reflect the official policy or position of the United States Air Force, Department of Defense, or the U.S. Government. The lead author would like to thank Mr. Cliff Cerbus from the University of Dayton Research Institute who operated and maintained the SCEPTRE Facility. Without his assistance and guidance during the exposure testing, there would have been many more problems without solutions over the time that was available due to paucity of funds. Finally, the authors would also like to acknowledge the support of the Materials and Manufacturing Directorate at Wright-Patterson Air Force Base.