IntroductionZnO inorganic white paint is composed of ZnO pigment and a potassium silicate binder as a type of thermal control coating of spacecraft. It is commonly used for spacecraft antennas, cabin sections, and radiator surfaces. In general, the solar absorptance of ZnO inorganic white paint is lower than that of other thermal control coatings. ZnO mainly exists in the form of wurtzite structure, and its zinc oxygen ratio often deviates from stoichiometry, generating intrinsic defects internally. Oxygen deficient compounds are generally generated due to the easy escape of oxygen in zinc oxide. During the in-orbit service of spacecraft, ZnO inorganic white paint is subjected to space environmental effects, such as electrons, protons, ultraviolet (UV), and atomic oxygen (AO), resulting in its performance degradation and affecting the stable service of spacecraft in orbit. It is thus necessary to investigate the degradation and damage mechanism of ZnO inorganic white paint to support the optimization of material properties.MethodsZnO pigment sample was a circular flake powder, which was pressed into a grinding tool with an inner diameter of 25 mm using a machine. The pressure parameter is 10 kN and the pressure time is 5 min. A circular sample with potassium silicate-coated zinc oxide (KSZ) inorganic white paint with a diameter of 30 mm, composed of K₂SiO₃ binder and ZnO pigment was prepared on an aluminum substrate by a spray coating process.Space environmental irradiation experiments were conducted on ZnO pigments and KSZ coatings by environmental irradiation and low orbit AO equipment. The electron energy of ZnO pigment irradiation experiment is 50 keV, with a fluence of 1×10¹⁶ cm^-2, a proton energy is 50 keV, with a fluence 6×10¹⁵ p/cm² and the UV dose is 5000 ESH. The AO fluence is 1×10²¹ atoms/cm². The electron and proton energies of KSZ irradiation experiment are the same as those in the ZnO pigment experiment, with fluence of 2.25×10¹⁶ e/cm² and 3.15×10¹⁶ p/cm², respectively. The UV dose is 5000 ESH. The AO fluence is 2.0×10²² atoms/cm².The spectrum reflectance of ZnO pigment and KSZ at wavelengths of 300-2200 nm was determined by a model Lambda 950 UV-Vis-NIR spectrophotometer (Perkin Elmer Co., USA), and the solar absorptance of KSZ was calculated. The fluorescence spectra of ZnO pigment were characterized by a model RF-5301PC spectrometer (SHIMADZU Co., Japan). The excitation source was a xenon lamp with an 220 nm wavelength. The electron spin resonance on ZnO pigments at a microwave frequency of 9.435 GHz was measured by a model JES-FE3AX electron spin resonance spectrometer (JEOL Co., Japan).Results and discussionAfter environment irradiation experiment, the spectral reflectance coefficient of ZnO pigment increases sharply and reaches its maximum value in a wavelength range of 360-500 nm. The spectral reflectance coefficient decreases with increasing wavelength. The peak position of the optical absorption band of ZnO pigment is at 425 nm. Compared to other environments, the absorption peak changes most severely under proton irradiation.Under electron, proton, and UV irradiation, the inherent V_Zn²⁻ in ZnO pigments undergoes ionization, generating single ionized zinc vacancies (V'_Zn⁻). AO causes a very small portion of the inherent V_Zn²⁻ in ZnO pigments to be converted into V'_Zn⁻ due to its intense electron affinity and oxidizing properties, while the majority of V_Zn²⁻ is converted into V_Zn⁰.The initial value of the solar absorptance of KSZ coating is 0.12. After the electron, proton, and UV combined irradiation experiment, the solar absorptance of the coating increases with the increase of environmental test fluence. The solar absorptance of the coating increases to 0.27 at electron fluence of 2.25×10¹⁶ e/cm², proton fluence of 3.15×10¹⁶ p/cm², and UV dose of 5000 ESH. The solar absorptance of KSZ coating increases within 2000 ESH UV irradiation, and then the solar absorptance stabilizes at 0.15. The KSZ solar absorptance is increased by 0.02 after 2.0×10²² atoms/cm² AO irradiation.ConclusionsAfter space environment irradiation experiment, the spectral reflectance coefficient of ZnO pigment decreased, mainly concentrated in the visible and near-infrared regions, and the peak position of the optical absorption band was at 425 nm. The ionization of the inherent V_Zn²⁻ in ZnO pigments occurred, resulting in the formation V'_Zn⁻, which was the main mechanism for the degradation of the optical properties of ZnO pigments under electron, proton, and UV irradiation. AO caused a very small portion of the inherent V_Zn²⁻ in ZnO pigments to be converted into V'_Zn⁻ due to its intense electron affinity and oxidizing properties, and the majority was converted into V_Zn⁰. After the combined experiments of electron, proton, and UV (i.e., electron fluence of 2.25×10¹⁶ e/cm², proton fluence of 3.15×10¹⁶ p/cm², UV dose of 5000 ESH), the solar absorptance of KSZ coating increased from 0.12 to 0.27. After 5000 ESH UV irradiation, the KSZ solar absorptance reached to 0.15. After AO irradiation of 2.0×10²² atoms/cm², the KSZ solar absorptance was increased by 0.02. Compared to UV and AO, the electron and proton irradiation had a greater impact on the optical properties of KSZ inorganic white paint.