Radiation Damage Effects Of ZnO/TiO₂/SiO₂ Powders With Different Micro-nano Structures And Thermal Control Coatings Based On Them

With the continuous development of modern technology and aerospace technology,higher and higher requirements are put forward for the design of a new generation of aerospace.This includes:requirement to increase the life of the spacecraft;the emergence of new trends in spacecraft designs（creation of unsealed structures,small spacecraft,unified space platforms）;equipping spacecraft with new equipment;the complexity of hardware systems;onboard equipment sensitivity increase;development of new orbits and spacecraft launching methods.This puts forward higher and higher protection requirements for spacecraft operating in complex space environments,especially the protection of space radiation environments.As the functional material on the outer surface of the spacecraft,the thermal control coating is the first line of defense for the spacecraft’s thermal control system,and it plays an important role in maintaining the normal temperature level of the spacecraft.Paint-type thermal control coatings are one of the widely used thermal control materials on spacecraft,and the pigments they choose are oxide powders with high stability to space environmental factors.Under the action of various types of radiation（solar ultraviolet rays,protons and electrons）,photolysis and radiation decomposition of the lattice occur in the pigment,and various types of defects are formed.This structural damage will lead to the formation of a color center,which reduces the reflectance of the pigment in the entire spectrum from the main absorption edge to 2500nm,and causes the solar absorption ratio of the coating to increase,which leads to the failure of thermal control.At present,certain achievements have been made in the development of thermal control coatings that are resistant to different space environmental factors.However,when designing a spacecraft with a life span of 15-20 years,the stability problem of the thermal control coating still exists.The emergence of nanomaterials with special physical and chemical properties provides an important improvement idea for solving the radiation stability problems of general materials,especially the space environment stability of thermal control coatings.In this dissertation a new conception has been proposed for increasing the radiation stability of thermal control coatings of the class solar reflectors,which is consists in use of ZnO,SiO₂ and TiO₂ hollow particles as pigments,since hollow particles have a high specific surface area,which is promote relaxation defects formed under irradiation.Coatings based on hollow particles have low ionization loss and low weight,most of the color centers will be in the deep of the material without affecting to the optical properties.Studies the optical properties and radiation damage mechanisms of ZnO,SiO₂ and TiO₂ powders（including hollow microspheres）with different micro-nano structures and their thermal control coatings under the irradiation by protons and electrons with sub-threshold energy.Synthesized zinc oxide micro-nano powders and hollow zinc oxide microspheres with different shapes.Hollow microspheres of titanium dioxide and silicon dioxide with different particle sizes were synthesized.The radiation stability of ZnO,TiO₂,and SiO₂ powders with micro-nano powders and hollow microspheres structures under irradiation of protons and electrons with subthreshold energy was studied.The influence of size effect and surface morphology on the stability of the optical properties of the above-mentioned oxide powders after irradiation was analyzed.The defect energy level structure,diffuse reflectance spectrum and photo-luminescence spectrum of ZnO,TiO₂ and SiO₂ powders with micro-nano powders and hollow microspheres structures due to radiation have been studied.The synthetic conditions of ZnO,TiO₂ and SiO₂ hollow microspheres have been determined to affect their optical properties,defect energy level structure caused by radiation,and radiation stability.The"annealing effect"of the changes in the optical properties of ZnO,TiO₂ and SiO₂ micro-nano powders and hollow microspheres under electron irradiation is analyzed.The effect of nanopowders modification on the optical properties and radiation stability of thermal control coatings prepared on based of the hollow microspheres was studied.The research results of this dissertation indicates that:Under the conditions of proton and electron irradiation with sub-threshold energy,the irradiation stability of ZnO nanopowder is significantly lower than that of traditional micron-sized ZnO powder.This is because ZnO nanopowders not only have more primary intrinsic defects,but also produce relatively higher concentrations of cationic sublattice radiation-induced defects under irradiation,including:V_Zn"（3.05e V）,V_Zn’（2.95e V）,Zn_i^·（2.8e V）and V_O^··（2.67e V）.These cationic sublattice radiation-induced defects（color centers）will cause the diffuse reflectance spectrum of ZnO nanopowders to form an obvious absorption band in the visible light region.Under the conditions of proton and electron irradiation with sub-threshold energy,the irradiation stability of SiO₂ nanopowder is also significantly lower than that of traditional micron-sized SiO₂ powder.This is also because SiO₂ nanopowders not only have more primary intrinsic defects,but also produce relatively higher concentrations of radiation-induced defects under irradiation.Radiation-induced defects（color centers）in amorphous SiO₂ powder include:E_s1’/E_s3’（≡Si·）（4.58e V）,E_s1’/E_s3’（≡Si·）（4.19e V）,≡Si─O─O─Si≡（3.84e V）,Е_γ’（3.46e V）and（≡Si─O）₂Si（O₂）（2.94e V）.Which will also cause the diffuse reflectance spectrum of SiO₂ nanopowders to form an obvious absorption band in the visible light region.The opposite phenomenon was found in TiO₂ powder.Under the conditions of proton and electron irradiation with sub-threshold energy,the radiation stability of TiO₂nanopowder is significantly higher than that of traditional micron-sized TiO₂powder.This is due to the small size effect and tunneling effect,the TiO₂nanopowder can withstand higher charge states and discharge changes of free carriers.As a result,the cationic sublattice radiation-induced defects in the TiO₂nanopowders are more localized on the surface,and it is difficult to generate higher-charged cationic sublattice radiation-induced defects.However,the cationic sublattice radiation-induced defects in the micron-sized TiO₂ powder are more generated inside the powder particles,and more cationic sublattice radiation-induced defects with higher charge states can be formed.These highly charged cationic sublattice radiation-induced defects include:Ti_i˙˙˙˙（1.71e V）,V_Ti’’’（1.44e V）,V_Ti’’’（1.15e V）and V_O^X（0.87e V）.The will cause the micron-sized TiO₂ powder to produce obvious absorption bands in the near-infrared range of the diffuse reflectance spectrum.Under the experimental conditions of this dissertation,the radiation stability of micron-sized ZnO and SiO₂ powders is higher than that of nano-powders.For TiO₂ powder,the irradiation stability of nano powder is higher than that of micron powder.Under irradiation with proton and electron,the types of radiation-induced defects that may be produced in micro-and nano-powders are the same.The differences in the atomic binding energy,lattice integrity,and relaxation of radiation-induced defects of micro-and nano-powders are the main reasons for the obvious differences in the degree of degradation of their optical properties.Under proton irradiation with sub-threshold energy,the irradiation stability of ZnO,TiO₂ and SiO₂ hollow microspheres is slightly higher than that of traditional micron-sized powders.Under the condition of electron irradiation with sub-threshold energy,the irradiation stability of ZnO,TiO₂ and SiO₂ hollow microspheres is significantly improved.Among all the test samples studied,SiO₂hollow microspheres have the highest radiation stability.Compared with traditional micron-sized powders,within the same surface layer thickness range,the concentration of radiation-induced defects in hollow microspheres is significantly lower.This is consistent with the simulation calculation results.The results of microscopic analysis proved that within the same volume range,the types of radiation-induced defect absorption centers produced in hollow microspheres remained unchanged,but the concentration was significantly lower.This shows that it is feasible to synthesize ZnO,TiO₂ and SiO₂ powders with hollow microsphere structure to improve its optical performance and radiation stability.Using ZnO,TiO₂ and SiO₂ nanoparticles to modify epoxy resin and silicone resin can effectively improve its radiation stability.The optimal doping concentration of the modified nanoparticles is 1 to 2%by weight.Among them,SiO₂ nanoparticles have the best modification effect,which can increase the radiation stability of silicone resin by 80%and the radiation stability of epoxy resin by about 55%.Under proton irradiation conditions,the radiation stability of ZnO,TiO₂ and SiO₂ hollow microsphere-based thermal control coatings is significantly higher than that of the corresponding traditional thermal control coatings,up to more than 30%.This is not only related to the higher radiation stability of ZnO,TiO₂ and SiO₂ hollow microspheres than the corresponding traditional micron powders,but also closely related to the nanostructure of the shell layer of ZnO,TiO₂and SiO₂ hollow microspheres.The nanostructure of the shell layer of ZnO,TiO₂and SiO₂ hollow microspheres can be used as reaction sites to form a complex structure with the organic polymer binder（resin）and promote the formation of a cross-linked grid structure of molecules.It is conducive to the relaxation of radiation-induced defects and the migration of free carriers,thereby further improving the overall radiation stability of the coating.