Study On The Preparation And Optical Performance Control Of Vanadium Dioxide Powders

Vanadium dioxide?VO₂?is a thermochromic material that can undergo a transition from semiconductor to metal at near room temperature.This phase change is accompanied by a marked change in optical and electrical performances.Due to these unique properties,VO₂ is now strongly impacting the development of smart windows,military camouflage,spacecraft thermal control,photoelectric switches and information storage.However,the application of VO₂ still faces many problems,such as the controllable preparation of single phase VO₂ nanopowders,the exploration of its mid-and far-infrared characteristics,the unpleasant color for application and the difficulty of achieving high resolution patterning.In this thesis,pure VO₂?M?powders have been successfully prepared via a single-step hydrothermal method.In addition the W-doped VO₂ powders and the VO₂/ZnS core-shell nanopowders were also obtained.The photoelectric properties of the as-prepared powders were investigated,and the mechanism was also explored.Furthermore,VO₂ inks were prepared and then printed as coatings on different substrates via inkjet printing technology,and the visible,near infrared,mid and far infrared characteristics of the printed coatings were studied.These studies provide important reference for its application in smart windows and military camouflage.The thesis is divided into four parts:?1?Study on the preparation of VO₂ powders.The pure VO₂ powders were prepared by one-step hydrothermal method using vanadium pentoxide as vanadium source and oxalic acid as reducing agent.The reaction temperature was controlled at 285°C and the reaction time was 48 h.The mechanism of hydrothermal preparation was investigated by examining the effect of precursor ratio,reaction temperature and reaction time on the composition and morphology of the products.The VO₂ nanopowders with average particle size of 65.7 nm were further successfully prepared using a strong reducing agent of hydrazine hydrochloride.The phase transition temperature can be effectively reduced by W doping.When the effective doping amount of W was 1.73%,the phase transition temperature decreased to 43.2°C.Finally,the reversible phase transition of VO₂ nanopowders was monitored via in-situ X-ray diffraction and in-situ Raman,and the mechanism of its transition was further investigated.DSC cycle tests showed that VO₂ nanopowders had good reversibility and stability of phase transition.?2?Study on the infrared optical properties of VO₂ nanopowders.The infrared properties of pure VO₂ nanopowders and W-doped VO₂ nanopowders were studied through temperature-dependent Fourier transform infrared spectroscopy?FTIR?.The results showed that the infrared emissivity of VO₂ changed abruptly before and after the phase transition,with a emissivity variation,??of 0.34 in the mid-infrared?3-5?m?region and??of 0.46 in far-infrared?8-14?m?region.The emissivity variation of 1 at.%W-doped VO₂ in the 3-5?m and 8-14?m bands were 0.27 and 0.30,respectively.The emissivity variation of 2 at.%W-doped VO2 in the 3-5?m and 8-14?m bands were0.17 and 0.28,respectively.And the infrared radiation temperature was further investigated by the infrared thermal cameras.When the actual temperature was 95°C,the radiation temperature difference between the VO₂ pellet and V₂O₅ reference sample reached to 17°C and 30°C.Therefore,VO₂ samples had an excellent adaptive infrared camouflage effect.The mechanism of emissivity change was studied by transmission electron microscopy combined with variable temperature resistance measurement.Finally,the environmental resistance of VO₂ nanopowders was investigated.The results showed that the nanopowders had good thermal stability,oxidation resistance and humidity resistance,which laid the foundation for its application.?3?Study on the preparation and optical properties of VO₂/ZnS core-shell nanopowders.The VO₂/ZnS core-shell nanoparticles were successfully prepared via a homogeneous precipitation method for the first time.The VO₂ nanoparticle core exhibited a remarkable infrared emissivity modulation ability and the ZnS shell was an infrared transparent material.The mechanism of formation was also discussed.The obtained VO₂/ZnS nanoparticles not only modified the color of VO₂ nanoparticle from unpleasant blue-black to gray-green that adapts to visible camouflage in the woodland background,but also had tunable emissivity properties,which its abrupt emissivity reached to 0.24 and0.28 in the 3-5?m and 8-14?m bands,respectively,before and after phase transition.In addition,ZnS shell can serve as a barrier layer for oxygen diffusion to enhance oxidation resistance.Therefore,the stability of VO₂/ZnS core-shell nanoparticles enhanced greatly,the temperature at which oxidation start temperature increased from 290°C to 385°C.?4?Study on the preparation and optical properties of inkjet printing inks based on vanadium dioxide nanopowders.VO₂ inks,with an average particle size of 77.23 nm,were successfully prepared through the design of formula.The viscosity and the surface tension of the as-prepared VO₂ inks were 8 mPa·s and 26.8 mN·m^-1,respectively.The inks were stored at room temperature for six months,and its physical and chemical properties were almost unchanged,indicating that as-prepared VO₂ inks had a dispersion stability to meet the requirements of inkjet printing.The inks were printed on a flexible substrate such as paper,polyethylene terephthalate?PET?and nanoporous polyethylene?NanoPE?by a commercial inkjet printer to obtain a digitized and patterned VO2 coating.The VO2coating had a clear edge and a high precision,and the nanoparticles were dispersed uniformly in the coating with little roughness.The VO₂ coatings printed on the PET substrate exhibited excellent thermochromic properties.The printed VO₂ coating with 3layers had a solar modulating ability?T_sol of 15.31%and a visible light transmittance T_lumum of 29.80%,which is superior to the vast majority of VO₂ coatings and films previously reported in the literature.The VO₂ coatings printed on the infrared transparent substrate NanoPE had good infrared switching characteristics.The changes of infrared transmittance before and after phase transition were 7.50%,9.82%,11.60%,14.68%and18.79%,respectively,for the coatings printed 1 time,2 times,3 times,4 times and 5 times.These results demonstrated the feasibility of large scale fabrication of VO₂ based optoelectronic devices by inkjet printing technology,and also provided guidance for large scale preparation of digitized and patterned VO2 smart materials.