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

Abstract

Optical absorption and photoluminescence spectroscopy have been usedto investigate the physical changes induced by proton and electron irradiationin selected thermal control coatings. This study focused on a white paint andon its two components, a polydimethylsiloxane resin and zinc oxide powder.Samples were irradiated by either 45 keV protons (fluence up to1016 protons cm-2) or 400 keV electrons (fluence up to6×1015 electrons cm-2).In situ reflectance measurements were made during the test and werecomplemented after air introduction by photoluminescence characterization. Theoptical properties of the paint are essentially those of the ZnO pigment. Theoptical degradation of the material appears to be correlated with the densityof defects created by ionization in a zone close to the surface of the paintand called the optical thickness of the material.Two degradation regions of the reflectance properties in the wavelength range250-2500 nm have been identified: one in the infrared and one in thevisible blue region. The degradation in the IR region disappears on exposureto air so that no complementary technique could be used for identifying itsorigin. The point defects responsible for the optical degradation of the paintin the blue region are singly ionized oxygen vacancies (F+ centres) eitherinitially present in the material or induced by irradiation. Irradiationquenches the green photoluminescence emission. The latter would be excited bythe recombination of doubly ionized oxygen vacancies with photoformedelectrons of the conduction band.