Preparation Of Doped Vanadium Oxide Thin Films By Sol-gel Method And Study Of Thermochromic Properties

In recent years,with the rapid development of the world economy,people begin to pay attention to energy-saving materials.Air conditioning accounts for a large part of the total energy consumption.If the smart window is used,it can block the transmission of infrared rays when the temperature is high,reduce the indoor temperature and reduce the use of air conditioners.VO₂has a variety of crystal structures,among which M1 phase VO₂（monocline,space group:P2₁/C）is an insulator at room temperature and has a high optical transmittance to the infrared band.When the temperature rises beyond its phase transformation temperature T_c,it turns into R phase（tetragonal,space group:P4₂/mnm）,showing metal properties,reduces the transmittance of infrared band.This process is reversible and the transmittance of visible light is basically unchanged before and after MIT transformation,so it is suitable for application in the field of smart windows.However,there are three main problems in the direct commercial application of VO₂in smart windows,that is,the transformation temperature T_cis high,the visible light transmittance T_lumand the near-infrared regulation efficiency（?）T_IRare low.To solve these three problems,in this paper,the conditions of sol-gel preparation of VO₂film were optimized,and the method of element doping was adopted to improve the thermochromic optical properties of VO₂film.The main research contents and results of this paper are as follows:1.VO₂films with different thermochromic properties were prepared on quartz glass by sol-gel method with fixed annealing velocity of 100 m L/min and annealing temperature of550℃,555℃,560℃and 580℃for 1 h respectively.The effects of annealing temperature on the phase,crystallinity,visible transmittance T_lumand near infrared control efficiency（?）T_IRof the film were investigated.The annealing temperature is 550℃and 555℃,the T_lumis lower（32.5%and 35.4%）,but the（?）T_IRis higher（12.0%and 15.6%）.With further increasing the temperature,the crystallinity of VO₂films continued to increase,but（?）T_IRcontinued to decrease;when the temperature rises to 580℃,the T_lumof the prepared film is 47.6%,the（?）T_IRdecreases to 0.1%,and V₂O₅heterophase appears in the film.Therefore,we fixed the annealing temperature at 550℃and hope to prepare thin film samples with better thermochromic properties by adjusting the annealing Ar flow rate to 100 m L/min,150m L/min,175 m L/min,225 m L/min and 250 m L/min.When the Ar flow rate is low,the T_lumand（?）T_IRof the films are low.When the Ar flow rate is 225 ml/min,the T_lumand（?）T_IRreach55.5%and 20.2%,and the comprehensive performance is the best.2.In order to improve the thermochromic optical properties of VO₂film,W,Mo and Sn elements are selected to doping VO₂film,and the effects of different elements doping on the film phase,crystallinity,phase transition temperature T_c,visible light transmittance T_lumand near-infrared control efficiency（?）T_IRare studied.The results show that the T_cof VO₂films can be effectively reduced by doping W and Mo.The T_cof VO₂films doped with 1%W and1%Mo decreases by 16.0℃and 12.6℃,respectively,and the phase transition temperature drops to 46.2℃and 46.3℃,respectively,when the doping amount is 3%.Meanwhile,with the increase of doping content,（?）T_IRcontinues to decline,while T_lumchanges little,both greater than 60%.In addition,Sn doping can effectively improve the visible light transmittance of VO₂film:With the increase of Sn content,the optical band gap of the film increases from 2.43 e V to 2.63 e V,and the T_lumfirst increases and then decreases,and the（?）T_IRcontinues to rise.The film has the best comprehensive performance when Sn doping amount is 5%(T_lum=70.7%,（?）T_IR=17.5%).3.In order to reduce the T_cof VO₂film while maintaining the T_lumand（?）T_IRof the film,we choose to prepare Sn-W co-doped VO₂film.The results show that Sn-W co-doping has a synergistic effect and can improve the properties of Sn and W single-doped VO₂films simultaneously.Compared with pure VO₂film(T_lum=69.0%,（?）T_IR=13.5%,T_c=74.3℃),5%Sn-3%W co-doped VO₂film(T_lum=76.0%,（?）T_IR=13.4%,T_c=46.7℃)has the best comprehensive performance.It has higher visible light transmittance T_lum,lower T_c,while maintaining a certain（?）T_IR,which is conducive to application in the field of smart windows.