Preparation And Modification Of M-phase Vanadium Dioxide Powder By Hydrothermal Method And Its Application In Smart Window

Vanadium dioxide（VO₂）is a typical metal oxide with thermochromic characteristics at near room temperature,which has great application potential in the field of smart windows.At present,there are many researches on the direct preparation of inorganic VO₂coated glass by means of deposition,sputtering,evaporation,etc.However,due to their complicated technology,high cost and small film forming area,it is not suitable for large-scale production.The coating technology based on VO₂ nano powder can effectively overcome the above defects,and the nanocomposite film has advantages in solar energy regulation and visible light transmittance,which has a broad application prospect.However,there are few reports about the VO₂ nanocomposite films,and the commercial application has not been realized yet.There exist three main limiting factors:First,pure VO₂（M/R）ultrafine powder is difficult to be industrialized;Second,the phase transition temperature of VO₂ is around 68℃,which is higher than the environmental temperature required for normal human life;Third,VO₂exposed to air and water for a long time is easy to be gradually oxidized into V₂O₅,thus losing its ability of phase transformation.Aiming at the above problems,this paper has completed the research of three systems,the main research contents,methods and results are as follows:（1）Preparation and properties of M-phase VO₂ ultrafine powder.In this paper,one-step hydrothermal method was used to prepare M-phase VO₂ultrafine powder.Vanadium pentoxide was used as vanadium source and hydrazine monohydrochloride was used as reducing agent,the precursor suspension was prepared by controlling the dropping amount of hydrochloric acid and ammonia.The effects of hydrothermal temperature,hydrothermal time on the crystal phase and morphology of the product were explored,and the VO₂ powder was prepared into slurry and then coated on glass substrate.The slurry formulation and coating process were optimized,the obtained VO₂ film has excellent optical properties such as high transmittance in visible light area and strong solar energy adjustment ability.The samples were characterized by XRD、SEM、TEM、DSC and UV-3600.The results showed that the hydrothermal reaction temperature and time will affect the composition and crystal structure of the powder.The optimum synthesis condition was determined to be240℃,the hydrothermal reaction time was 24 h,and the filling degree was controlled at50%.At this time,M-phase VO₂ powder can be directly synthesized.The powder has a high purity and uniform spherical morphology with an average particle size of 23.2 nm.The thermochromic properties of the powder and the film are evaluated by DSC,UV-3600,DSC curve shows that the phase transition temperature of VO₂ powder is 54.6℃.The transmittance spectrum shows that the visible light transmittance of the film at low temperature is 57.59%,the dimming efficiency（NIRSE）at 1500 nm is 33.30%,and the solar energy regulation ability is 14.17%,which has excellent thermochromic performance.By adding the powder into different coatings,they all show certain temperature control ability.（2）Preparation and properties of tungsten doped VO₂ powders and thin films.On the basis of（1）,powders and films with different tungsten doping contents were prepared with ammonium tungstate as doping agent.The effects of different W-doped contents on the crystal structure,morphology and phase transition temperature of VO₂powders were investigated.The effects of different W-doped contents on the optical properties and thermal insulation effect of VO₂ films were also studied.The results show that the doping content of tungsten atom does not change the crystalline phase and morphology of VO₂powder in a certain doping level.With the increase of tungsten source content,the actual doping ratio will decrease,and the doping efficiency will increase first and then decrease.The phase transition temperature was reduced by an average of 11 K for every 1 at%of tungsten atom doped（molar ratio to vanadium）.When the tungsten doping amount is 1.5 at%,the phase transition temperature of vanadium dioxide powder can be reduced to near room temperature,tungsten doping has little effect on the visible transmittance of the film.With the increase of doping content,the solar modulation efficiency of the film will be weaken,and the width of the thermal hysteresis loop will decrease.The thermal insulation effect of the film was evaluated by energy-saving tests.After a period of exposure,the insulation temperature on the lower side of all coated glass will rise and then gradually stabilize.The higher the tungsten doping content was,the shorter the time for the platform temperature to appear,and the smaller the final thermal insulation temperature difference was.The comprehensive performance of the films with tungsten doping content of 0.5 at%is the best.The visible light transmittance at low temperature is 43.33%,the nirse at 1500nm is 30.95%,the solar energy adjustment ability is 9.46%,the phase transition temperature T_c=31.9℃,and the insulation temperature difference is 7.8℃.（3）In order to improve the visible light transmittance of VO₂ coating,the surface modification and properties of VO₂ nanoparticles were studied.In this paper,storber method was used to coat SiO₂on the surface of VO₂ powder with TEOS as raw material,and the abtained VO₂@SiO₂ powder was prepared into thin film.The results showed that the visible light transmittance of VO₂@SiO₂ composite film increased from 57.59%of the pure VO₂ film to 69.04%,with an increase of 19.9%,and the excellent solar energy regulation efficiency(ΔT_sol=12.73%)was still maintained.By coating a certain thickness of SiO₂ on the surface of VO₂ powder,the oxidation resistance of VO₂ can be greatly improved.Finally,the surface grafting modification of VO₂@SiO₂ with Triethoxy-1H,1H,2H,2H-tridecafluoro-n-octylsilane was attempted,the obtained VO₂@SiO₂-F powder and film possess both stability and superhydrophobic characteristics.