Optical Degradation Effects And Mechanisms On ZnO/Silicone White Paint Irradiated By Charged Particls

ZnO/Silicone white paint is an important type of thermal control coatings used in spacecraft. Under the exposure of space charged particles, the degradation in optical properties occurs for the ZnO/Silicone, which would affect the lifetime and reliability of thermal control systems or even spacecraft. It is of significance to characterize the damage effects and mechanisms caused by space radiation environment. In this study, the effects of <200keV protons and electrons on degradation in optical properties of the ZnO/Silicone are investigated separately and combinedly, using a ground-based simulation equipment for a large amount of charged particles with lower energies existing in the Earth radiation belts. The formation mechanisms of the radiation-induced defects are revealed in terms of various microscopic analyses, such as the photoluminescence and X-ray photoelectron spectroscopy, and a kinetic model for the concentration of colour centers is established through the analyses of absorption bands in the reflective spectra.It is indicated that the responses of the ZnO/Silicone white paint are different to the protons, the electrons and the protons plus electrons, which have energies less than 200 keV. Under the proton exposure, the degradation in optical properties occurs mainly in the visible region, while in the near-infrared region primarily for the electrons irradiation. Under the simultaneous irradiation of protons and electrons, the degradation in the visible region is mainly controlled by the protons, and that in the near-infrared one primarily by the electrons. The effects caused by the sequential and simultaneous irradiation are similar, and the combined exposure of protons and electrons shows a synergistic effect on the degradation in optical properties of ZnO/Silicone white paint. Under lower fluences, the combined exposure makes the degradation stronger; otherwise, the situation is the opposite. Air exposure leads to an obvious recovery of the degradation in optical properties. The recovery magnitude is the largest after electron irradiation, the next after the simultaneous irradiation and the lowest after the proton one. In the case of the range less than the optical absorption layer for the ZnO/Silicone, the protons with higher energies lead to more obvious degradation, whereas the electrons with higher energies are not benefit to the formation of colour centers in the visible region under larger flunces.The X-ray photoelectron spectroscopy shows that with increasing proton fluence, both the amount of the lattice oxygen in ZnO and the Zn3d peak area decrease, demonstrating that the oxygen vacancies increase during the exposure. The photoluminescence spectrum analyses indicate that the singly-ionized oxygen vacancies gradually become the dominant radiation-induced defects with increasing proton fluence. This result is also demonstrated by the analysis of slow positron annihilation spectra. The analyses of Fourier transform infrared spectroscopy show that under the exposure, the degraded product of the silicone binder would be changed from the cyclotrisiloxane(D3 structure) into Si-C=O groups, due to the existence of ZnO. The formation of Si-C=O groups causes a red-shift in the ?ρλspectrum of the ZnO/Silicone. SRIM analysis illustrates that the damage of the organic silicone binder enhances the degradation of ZnO/Silicone white paint.Based on the analyses of absorption bands in reflective spectra, it is revealed that under the proton and electron exposures, 9-types of colour centers can be generated in the ZnO/Silicone, including: the recombined emission bands, VZn, p-, V-, VO??, Frenkel, F+, F and F-. Also, the proton irradiation could cause the degradation of the silicone binder to form the Si-C=O groups, resulting in the n→π* jumps. Under the exposure of <200keV protons, the kinetics of cumulation for colour centers in the ZnO/Silicone show a general trend that rapidly increases first and then tends to increase slowly. The cumulation rates of various colour centerts are different. The concentrations of the colour centers related with oxygen vacancies increase faster, and that for the n→π* jump type of colour centers rises slowly. Based on the evolution kinetics of colour centers, a quantitive expression for the change in concentration of colour centers with proton fluence is established, which is in good agreement with the change in ?αs.