The Study On The Preparation Technology And Metal Doping Of VO₂ Films

VO₂ can undergo a reversible phase transition near 68?,achieving free transition between metallic states and insulating states and accompanied by large changes in optical and electrical properties.This makes it having great potential applications in many fields,especially for smart windows.However,there are some challenges in practical applications.There are many types of oxides for V element and a variety of crystal structures of VO₂.However,the reversible metal-insulator phase transition only exists between the monoclinic phases and rutile phases.These make it difficult to prepare a pure monoclinic phase of VO₂.In the field of smart windows,the properties of main focus are the average luminous transmittance,the solar modulation efficient and the infrared modulation efficient(2500 nm).For VO₂ films,the average luminous transmittance and the modulation efficient of solar and infrared are incompatible.Therefore,it is a great challenge to simultaneously improve above properties.A lot of studies have been done in this area.However,these often require complex fabrication processes such as multilayer film design or compositing with other materials with different dielectric constants,which leads to the high-cost preparation.For the above problems,the research adopted a sol-gel method to achieve low-cost preparation of high-quality VO₂ films with outstanding optical properties.By adjusting the ratio of V₂O₅ powder to diamide hydrochloride,a tetravalent vanadium solution is generated.After spin coating and annealing,optical films with outstanding thermochromic properties were synthesized.The structure,morphology and optical properties of VO₂ films were characterized by related techniques,such as XRD diffraction,scanning electron microscope,atomic force microscope and UV-visible-NIR spectrophotometer,etc.Subsequently,for the pure film,the effects of different thicknesses and annealing times on optical properties were studied systematically.And through Mg doping and W doping,the adjustment of average luminous transmittance and phase transition temperature were achieved.The specific research contents and conclusions are as follows:(1)On the preparation process of pure VO₂ films,the uniform VO₂ films with monoclinic phase has successfully been prepared.Comparing with different spin coating methods,it was found that a very uniform and complete surface of sample would be achieved via the appropriate spin coating parameters and annealing after the sol was pre-coated on the substrate.In the coating process,the film thickness could be adjusted from 70 nm-150 nm by changing the vanadium ion concentration of the sol,the spin speed and the PVP mass fraction.If the annealing time is short,it was difficult to ensure sufficient decomposition of PVP,and then more residual carbon in the film would inhibit the crystallization of VO₂,resulting in poor optical properties or even no thermochromic properties.However,too long annealing time would lead to special morphology with island-like growth,which would also inhibit the average luminous transmittance,the solar modulation efficient and the infrared modulation efficient of VO₂ film.In addition,under the same thickness,the annealing time would have an effect on the average luminous transmittance.On the premise of the decomposition of PVP,the film presented a porous morphology if the annealing time was short,which would help to improve the average luminous transmittance(43.1%)and keep the high solar modulation efficient(11.5%)and the high infrared modulation efficient(65.1%).Furthermore,during annealing process,the flow rate of the protective gas also played an important role in the synthesis of monoclinic VO₂ film.With the flow of the protective gas in the tube,the temperature of different areas in the tube would be also different.(2)Through Mg doping and W doping,for VO₂ film,the improvement of the average luminous transmittance and the reduction of the phase transition temperature have been realized.For Mg doping,it was found that Mg atom entered into the film.However,it was not doped into the lattice of VO₂ and existed as an amorphous oxide,which was difficult to expand the band gap of VO₂.When the Mg doping concentration was high(6%),it could improve the average luminous transmittance(41.1%)and maintain a high infrared modulation efficient(2500 nm,60.0%).For Mg and W doping,it required the long annealing time to prepare the monoclinic VO₂.The higher doping concentration,the longer annealing time was required.According to the characterization of W doping,it was found that W atoms replaced V atoms in VO₂lattice.Under the low W doping concentration(1%),VO₂ film obtained a low phase transition temperature(47?)and high average luminous transmittance(43.3%).With W doping concentration increasing,the phase transition temperature of VO₂film was difficult to confirm and its microwave transmittance varied linearly with temperature.