Low-energy Electron Exposure Effects on the Optical Properties of ZnO/K 2 SiO 3 Thermal Control Coating
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An experimental investigation was undertaken to study low-energy electron exposure effects upon ZnO/K2SiO3 thermal control coating. The specimens were exposed to 10-, 30-, 50-, and 70-keV electrons, respectively. The spectral reflectance of each specimen was measured in situ before and after electron exposures. The solar absorptance was calculated by assuming a Johnson solar spectral irradiance distribution. Electron paramagnetic resonance and photoluminescence measurements were made before and after electron exposures to explain physical changes induced by the electron exposures. All the specimens exhibited a reflectance decrease throughout the visible and the near-infrared regions, and also the induced optical degradation was found to be electron energy dependent, with increased damage for increasing electron energy. It is believed that the optical degradation of ZnO/K2SiO3 thermal control coating, induced by the low-energy electron exposures, is mainly due to the ionization process of the ZnO pigment.
I. INTRODUCTION
Over the past decades a great deal of work has been carried out in the field of spacecraft thermal control coatings development. The initial efforts were directed mainly at establishing engineering tolerances and design parameters. Now, however, the state-of-the-art demands that more detailed studies be performed to elucidate more definitely the damage mechanisms associated with the types of radiation encountered in space. Among the thermal control coating candidates investigated, zinc oxide has received more attention than any other. White ceramic coatings based on zinc oxide are used widely in space technology.1–7 They are characterized by high processing properties and satisfactory initial thermal radiation characteristics, i.e., solar absorptance ␣s ⳱ 0.15–0.2 and hemispherical emittance ⑀ 艌 0.9, and are relatively stable under the effect of solar electromagnetic radiation. However, as a result of an insufficient resistance to the energetic particles in orbits, they could be degraded. This degradation results in a solar absorptance increase of the coatings and thus reduces their effectiveness as passive-thermal-control materials. Look et al. have studied the high-energy electron irradiation effects on ZnO crystals and have presented electrical and optical data on ZnO crystals irradiated with a)
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J. Mater. Res., Vol. 17, No. 7, Jul 2002 Downloaded: 13 Mar 2015
1–2-MeV electrons.8,9 The main objective of the work reported here is to study the low-energy electron exposure effects on the optical properties of ZnO/K2SiO3 thermal control coating and to obtain further insight into the nature of this degradation. The electron energy range in this study is from 10 to 70 keV. These low-energy electrons are present in very high intensities in the earth’s trapped radiation belts. Understanding the degradation mechanism can guide the design of new thermal control materials. Moreover, ZnO has many ot
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