Research On Variable Infrared Radiation Material Based On VO₂'s Thermochromic

With the power density of heat dissipation and the temperature control accuracy in a system of spacecraft is increasing,and there is a growing demand for a better performance of thermal radiative surface.Since the traditional thermal control coatings have a constant emissivity,they cannot adjust their thermal radiation characteristics to respond to the drastic fluctuations in heat flux.However,thermochromic materials can change their thermal radiation properties with the change of their own temperatures,as a consequence of a phase transition,which can achieve smart thermal control of the spacecraft.Therefore,it is important to study the thermally induced infrared radiation coating materials,and this work expect to promote the further research and development of new coating materials for smart thermal control applications.A design method of intelligent temperature-control coating based on therochromism of VO₂ core-shell structure has been proposed in this dissertation,which could realize the application of VO₂ in the form of coating in aerospace intelligent thermal control.The structural evolution mechanism and infrared radiation characteristic?(T)of the coating has been studied by coating preparation,computational simulation,structural characterization and performance testing.The research contents and results of this dissertation are as follows:(1)Since the emissivity of the VO₂ decreases with increasing temperature,it cannot be used in intelligent thermal control of aerospace.So,we proposed a thermally variable emissivity coating based on the VO₂ core-shell structure by simulation to realize the application.Based on the particle scattering theory and K-M theory,a theoretical calculation model of the emissivity of the coating was constructed.The relationship between the optical parameters of VO₂ core-shell particle,geometric structure and scattering characteristic were systematically analyzed,and the mechanism of thermally induced emissivity of the coating was explained.These results indicated that the core-shell structure design of the Ca F₂@VO₂ microspheres significantly alters the optical scattering and absorption of VO₂,resulting in the inversion of the emissivity of coatings containing these microspheres,and making the VO₂ coating can be used in intelligent thermal control of aerospace.(2)The Ca F₂@VO₂ core-shell microspheres with different morphologies and sizes were prepared by a solvent/hydrothermal-calcination method.In order to obtained the calcium fluoride microspheres with different sizes and the method of controlling the morphology and phase of the VO₂ shell,the effects of the various parameters and preparation process conditions have been systematically studied.Obtained results shown that the core-shell particle's phase structure and morphology strongly depended on the solvent,activator,annealing atmosphere and annealing temperature.Among them,the trisodium citrate and fluoborate played a crucial role in forming the hierarchical spheres,and size control of Ca F₂ spheres has been achieved by varying p H value,thus producing structure with a controllable edge length in the range 400nm?4?m.It was found that although dopamine and isopropanol would reduce VO₂ to V₂O₃ during the annealing process,but they were indispensable for the formation of Ca F₂@VO₂ core-shell particles with a good overcoating effect.Particularly,the conversion of V₂O₃ shell to VO₂(M)shell could be achieved by controlling oxygen flow ratio and oxidation temperature.(3)Based on theoretical calculations and experiments,the influence of various parameters on the infrared spectrum performance of the Ca F₂@VO₂coatings were systematically studied,and a design method to improve the modulation ability of the infrared spectrum of the coating was obtained.Obtained results shown that the coating's emissivity at different temperatures strongly depended on the VO₂ shell thickness,filler volume fractions and coating thickness.When the VO₂ content was controlled to 0.12,the filler volume fraction was 6%and coating thickness was 30?m,the total emittance of the coating could reversibly change from 0.47 at 30?to 0.83 at 90?with emittance variability of 0.36.(4)In order to solve the problem of high solar absorptivity of the thermally variable emissivity coating,the methods of adjusting and controlling the coating's spectral reflection characteristics were systematically studied,and a design method of the solar reflective coating was proposed.The effects of the adhesive and pigment particles on the spectral reflection characteristics of the coatings were systematically studied,and the preparation method of the composite thermally variable emissivity coating with a low solar absorption coefficient(0.35)was obtained,which realized the coupling control of solar light absorption and emissivity of the coating.Then,the relationship between the heat dissipation energy of the object,the temperature and material properties were established,the heat dissipation capacity of the thermally variable emissivity coating and the traditional thermal control coating in different areas were compared.Finally,the radiative heat transfer characteristics of the object in vacuum were analyzed through simulation,and the law of the influence of different parameters on the temperature control of the object was obtained.