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

Optical absorption and photoluminescence spectroscopy have been used to inv estigate 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 10(16) pro tons cm(-2)) or 400 keV electrons (fluence up to 6 x 10(15) electrons cm(-2 )). In situ reflectance measurements were made during the test and were com plemented 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 d ensity of defects created by ionization in a zone close to the surface of t he paint and called the optical thickness of the material. Two degradation regions of the reflectance properties in the wavelength ran ge 250-2500 nm have been identified: one in the infrared and one in the vis ible blue region. The degradation in the IR region disappears on exposure t o air so that no complementary technique could be used for identifying its origin. The point defects responsible for the optical degradation of the pa int in the blue region are singly ionized oxygen vacancies (F+ centres) eit her initially present in the material or induced by irradiation. Irradiatio n quenches the green photoluminescence emission. The latter would be excite d by the recombination of doubly ionized oxygen vacancies with photoformed electrons of the conduction band.