Optical properties and radiation stability of thermal control coatings based on doped zirconium dioxide powders
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The effect of the doping by compounds with bivalent and trivalent cations (BN, Al2O3, MgO + SiO2) on the diffuse reflectance spectra () and the integral solar absorptance (as) and their changes at irradiation (with 30 keV electrons, 3 keV protons, and electromagnetic radiation that imitates solar spectra) of the reflecting thermal control coatings based on ZrO2 powders is examined. The coating based on untreated ZrO2 pigment and the coatings based on the pigments treated with 1 and 3 mass% of Al2O3 nanopowder were subjected to simultaneous action of electrons, protons, and electromagnetic radiation that imitates action of these irradiations on geostationary satellite orbit. After that, the comparative investigations of the changes in spectral reflectance and solar absorptance of these coatings were carried out.
I. INTRODUCTION
Zirconium dioxide is a very promising pigment for spacecraft applications. Its wide band gap (Eg ⳱ 5.4 eV at 300 K)1 and sufficiently high refractive index, which equals 2.2,2 allow thermal control coatings (TCCs) to be fabricated on the basis of zirconium dioxide with an integral solar absorptance as of about 0.11. Theoretically, the photostability of zirconium dioxide may be higher than that of ZnO or TiO2 pigments widely used for spacecraft applications, as these pigments have shorter band gaps [3.2 and 3.05 eV for ZnO and TiO2 (rutile), respectively, at 300 K].3,4 An increase in Eg and a shift of the main absorption band edge of the ZrO2 pigment toward shorter wavelengths cause a part of the ultraviolet solar spectrum to be nonabsorbed, thereby increasing the photostability of the pigment. A disadvantage of ZrO2 pigments is their nonstoichiometry with respect to oxygen which causes a low stability of the lattice and the presence of anionic vacancies that form F and F+ color centers with free electrons created under the action of light or particles. This reduces the pigment reflectance and increases its solar absorptance as. One of the methods of stabilizing the optical properties of zirconium dioxide pigments is incorporation of elements with the valency lower than that of zirconium (Zr4+) into the lattice to optimize the cation– anion distance and to create trapping levels for electrons in the band gap. To this end, zirconium dioxide is doped with Cu2+ and Sr2+ cations to stabilize its cubic lattice. The current work is aimed at investigation of the effect of various dopants on the diffuse reflectance spectra , the solar absorptance as, and their stability under the effect of J. Mater. Res., Vol. 19, No. 2, Feb 2004
separate factors of the space environment (electrons and protons) and under the joint effect of these factors under conditions that imitate a geostationary satellite orbit (GSO).
II. EXPERIMENTAL
Zirconium dioxide pigment was treated with 1 mass% of BN, Al2O3, and MgO + SiO2 in equal parts. The choice of dopants was determined by the fact that the valency of B, Mg, and Al increases from 1 to 3, that is, the effect of cation valency on photo- and radiation stability o
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