Study On The Preparation Of Microstructured VO₂ Thermochromic Film And Its Smart Window Performance

Light and heat are basis for human beings survival and development.The seasonal,regional and nycterohemeral distribution of sunlight have caused obvious temperature differences on the earth.And human beings have to consume a lot of energy to adjust temperature differences.Building energy consumption accounts for30%-40%energy consumption in modern society.Smart window can effectively reduce the energy consumption of buildings by regulating the sunshine indoor,which will improve the living quality.Vanadium dioxide（VO₂）is a kind of phase transition materials,and it changes drastically from opaque to transparent in near-infrared wavelength,while its transmittance does not change in visible wavelength before and after phase transition,which makes it an ideal material for smart window.However,there are three key problems for VO₂ thermochromic smart window:Poor smart window performance can not meet the demand for temperature adjusting and daylighting;High phase transition temperature is not suitable for applicatoin;Weak cycle stability is not afford long-term application.In view of the above problems,this paper is mainly focus on the following research:For the poor VO₂ smart window performance:First,the double-sided VO₂（M）films were prepared by inorganic sol method.It can be achieved ultrahigh luminous transmittance（75.5%）and good solar modulate ability（7.7%）by optimizing the thickness of VO₂（M）film.Furthermore,it has been confirmed that the double-sided microstructure VO₂（M）film is superior to single-sided microstructure on smart window performance,which not only enhances luminous transmittance effectively,but also improves solar modulate ability 3.5 times.Based on the intrinsic properties of variable refraction index for VO₂（M）films,it seems that doube-sided have an advantage on enhancing the smart window performance of VO₂（M）films:the solar modulate ability can be improved by the larged transmittance variation before and after phase transition,which is caused by double-enhancement on transmittance at M phase and double absorption at R phase.And then,It has been found that the V₂O₅ transforms to VO₂（M）by an intermediate phase VO₂（B）.And the surface morphology changes from a flat surface to icecrystal like microstructure following by microparticles.And the variations change along with PVP decomposing.At last,based on the evolution of phase and morphology for VO₂（M）films,doube-sided micoparticle VO₂（M）films have been fabricated by optimazing the thickness,annealing temperature and time.And the transmittance in near infrared wavelengh can be reduced by the surface plasmon resonance at high temperature,where is closed to the maximum wavelength of solar radiation.Especially,based on the double enhancement on transmittance at room temperature and double absorption by surface plasmon resonance at high temperature,the luminous transmittance and solar modulate ability for double-sided microparticle VO₂（M）film can be achieved68%and 11.7%,respectively.For the high phase transition temperature:the W doped VO₂（M）film can be prepared by adjusting the inorganic sol method to lower phase transition temperature.And the W source is introduced by pouring melt V₂O₅ to ammonium tungstate solution.The evolution of phase,velence of surface element and morphology have been investigated at different annealing temperatures.The W doped VO₂（M）films also can be obtained by the transformation from V₂O₅ to VO₂（M）via VO₂（B）except for three phases coexistance.And the annealing temperature for W doped VO₂（M）film increases to 600℃compared with pure films.Besides,it is not isolated for the morphology changing to mircoparticle in the W doped VO₂（M）film.It indicates that the V element gradually changes from V⁵⁺to V³⁺at different annealling temperatures without only V⁴⁺existance.The varaition of N element suggests that the phase,V valence and morphology are mainly effected by PVP decomposing.And the valence of W element does not change throughout the annealling process.For the W doped VO₂（M）films annealled at 600℃,The solar modulate ability can reach 8.1%and the phase transition temperature lower to 57℃as well as the hysteresis loops width narrow to 6℃.It seems the solar modulate ability can be affected by crystallinity for W doped VO₂（M）films.The different W doped VO₂（M）films can be obtained by modifying the Ar flow at annealling process,which leads to crystallinity variation.It indicates that the crystallinity is more important than the microstructure on the smart window performance for W doped VO₂（M）fillms.And the phase transition properties is affected by crystallinity and microstructure,collectively.For the weak cycle stability of VO₂（M）films:different concerntrations W doped VO₂（M）films have been prepared.It seems that the crystallinity attenuation and the microstructure varaition are depressed by the increasing W doped concerntration after 550 heating and cooling cycles.It suggests that decreasing efficiency is 16.5 K/at.%in the W-doped VO₂（M）films.However,this decreasing efficiency achieves to 16.9 K/at.%after 550 cycles.Besides,the narrowing efficiency changes from 1.9 K/at.%to 2.2 K/at.%.It seems that the W doped VO₂（M）film not only lower phase temperature and narrow hystersis loops width,but also enhance cycle stability.Besides,the transmittance variation at near infrared wavelength is almost not attenuation for W doped VO₂（M）films after 550 cycles,Especially,it is enhanced by the slight variation in microstructure.In conclusion,double-sided microstructure VO₂（M）films can obvious enhance smart window perofrmance.And the W doped VO₂（M）film not only lower phase transition temperature and narrow hysteresis loops width,but also enhance the cycle stability.The research is beneficial to promote the application of thermochromic intelligent window.