Synthesis And Property Investigation Of Thermochromic Vanadium Dioxide Porous Films

Thermochromic VO2 thin films, which can intelligently control the amount of solar thermal irradiation in response to ambient environment, are ideal materials used as “smart glass” coatings. It can regulate the indoor temperature automatically and keeps the building warm in winter and cool in summer, this is very important for architecture energy saving. However, VO2-based smart windows remain a grand challenge for practical application due to its native dark yellow color and associated low visible transmittance. How can improve the visible transmittance of VO2 thin films without effecting its solar modulating ability has become a hot topic now. Also, the high requirement for equipment, complex process and high cost are common problems in present fabrication methods, which restrict its widely industrial application.In this dissertation, an easy-to-handle and cost-effective inorganic Sol-gel method had been used to prepare VO2 thin films. We successfully prepared a good film-forming vanadium pentoxide sol, and through a NH3 reduction, one step annealing, the excellent thermochromic porous VO2 thin films were fabricated. First, XRD and TG thermal analysis were used to study the component and structure of the precursor gel, then based on the obtained thermochromic VO2 porous films, XRD, Raman, FE-SEM and UV-Vis-NIR spectrophotometer were utilized to characterize the film's crystal phase, microstructure and optical properties. Last, the effect of heat treatment on the thermochromic properties of VO2 thin films were further studied and the best thermochromic sample was obtained. The forming mechanism and speciality of the prepared porous structure were intensively explored in this dissertation. The simultaneous TG-DSC and MS(evolved gas detection) analyses were performed to investigate the effect of reducing atmosphere on the annealing of the precursor gel. The main results and conclusions were as follows.(1) The component of the precursor gel prepared by V2O5+H2O2 sol-gel method was V2O5?1.6H2 O, there was layer-like V-O octahedral which preferred orientated along the Z axis in the gel's structure.(2) The obtained thin films were VO2 porous films, the size of the pores was 30-70 nm. The special porous structure was formed by the escaping of interlayer water of the precursor gel in the annealing process.(3) The effect of heat treatment on the thermochromic properties of thin films were further studied. The thermochromic phenomenon began to appear when the annealing temperature rose to 500?, with the increase of temperature, a more pure VO2 M phase and crystalline property can be obtained. The best thermochromic sample with a visible transmittance Tlum=60.5%, solar modulating ability ?Tsol=8.5%, was prepared under NH3 reduction, 550 and 30 min one step annealing.?(4) The effect of reducing atmosphere on annealing temperature were confirmed by simultaneous heating process under various sorts of reducing air(10%H2+90%Ar, 10%CO+90%Ar, Ar, N2 and 10%NH3+90%Ar). Systematic analysis indicated that the effective reduction temperature(about 550?) decided by reducing capability of NH3 played a key role on the formation of porous VO2 thin films.