Degradation of the optical properties of ZnO-based thermal control coatings in simulated space environment

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Abstract
Optical absorption and photoluminescence spectroscopy have been used to investigate the physical changes induced by proton and electron irradiation in selected thermal control coatings. This study focused on a white paint and on its two components, a polydimethylsiloxane resin and zinc oxide powder. Samples were irradiated by either 45 keV protons (fluence up to 1016 protons cm-2) or 400 keV electrons (fluence up to 6×1015 electrons cm-2). In situ reflectance measurements were made during the test and were complemented after air introduction by photoluminescence characterization. The optical properties of the paint are essentially those of the ZnO pigment. The optical degradation of the material appears to be correlated with the density of defects created by ionization in a zone close to the surface of the paint and called the optical thickness of the material. Two degradation regions of the reflectance properties in the wavelength range 250-2500 nm have been identified: one in the infrared and one in the visible blue region. The degradation in the IR region disappears on exposure to air so that no complementary technique could be used for identifying its origin. The point defects responsible for the optical degradation of the paint in the blue region are singly ionized oxygen vacancies (F+ centres) either initially present in the material or induced by irradiation. Irradiation quenches the green photoluminescence emission. The latter would be excited by the recombination of doubly ionized oxygen vacancies with photoformed electrons of the conduction band.