Study On The Preparation And Mechanism Of Vanadium Dioxide (B/D) Phase Into M Phase

VO₂（M）has a reversible phase transition at 68℃,from low temperature insulating phase（M）to high temperature metallic phase（R）,accompanied by a sudden change in optical properties that it has a high transmittance for visible and infrared light before phase transition,low transmittance for infrared light after phase transition,and almost no effect on the visible light transmittance.Therefore,VO₂（M）can adjust and control the infrared radiation exchange between indoor and outdoor according to the temperature change,so as to realize the purpose of intelligent adjustment of indoor temperature.And it becomes one of the most potential materials in the field of intelligent energy-saving window.There are many methods to prepare VO₂（M）,including physical method and chemical method.Among them,hydrothermal method has become a research hotspot because of its simple preparation process and controllable nano particle size.At present,there are two ways to prepare VO₂（M）powder by hydrothermal method:one is to prepare VO₂（M）powder by one-step method,which usually requires higher reaction temperature and longer reaction time,with lower efficiency and higher energy consumption;The other is a two-step method,which first prepares the metastable phase,and then prepares VO₂（M）through the metastable phase.Compared with the one-step method,the two-step method has the advantages of low reaction temperature,short reaction time and energy saving.The main contents of this paper are as follows:firstly,the metastable VO₂（B/D）was prepared by hydrothermal method,and the transformation mechanism of metastable VO₂（B/D）into VO₂（M）by annealing and ball milling was explored.According to the characteristics of VO₂ metastable phase,the process was optimized to prepare VO₂（M）nano powder with excellent performance.The research contents and related results are as follows.（1）VO₂（B）was prepared by hydrothermal method,and then VO₂（B）was transformed into VO₂（M）by annealing method and ball milling method.For annealing method,annealing temperature and annealing time determine the phase transition process.Increasing annealing temperature can greatly reduce the phase transition time,but the VO₂（M）prepared by annealing has lower solar light modulation ability,which is mainly due to its larger particle size.For ball milling,the transformation from VO₂（B）to VO₂（M）is induced and the particle size is reduced.By optimizing the process,VO₂（M）nanoparticles with small particle size,good dispersion and high crystallinity were prepared under the conditions of ball to material ratio of10:1 and ball milling time of 3 h.The VO₂（M）nanoparticles showed excellent optical properties.When the visible light transmittance(T_lum)was 41%,the solar light modulation ability was improved(ΔT_sol)can reach 14.03%.In addition,VO₂（A）phase appears during ball milling,but not during annealing,which indicates that the mechanism of promoting phase transformation in these two ways is different.By changing the size of the ball and then changing the impact pressure of the ball,the pure VO₂（A）was successfully prepared when the ball with a diameter of 5 mm was selected,which indicated that the phase transformation induced by ball milling was caused by the impact pressure of the ball.（2）VO₂（D）was prepared by hydrothermal method,then VO₂（D）was transformed into VO₂（M）by annealing and ball milling,and the transformation mechanism was explored.The synthesized VO₂（D）was annealed at 350-500℃for 1-5 h.XRD data analysis showed that VO₂（D-M）transition temperature should be higher than 350℃,VO₂（D）to VO₂（M）is the result of temperature and time control.Increasing the temperature greatly reduces the conversion time.After optimization,the optical properties of the products heat treated at 450℃for 2 h are better.When the T_lum is 74%,61%,49%,respectively,the optical properties of the products are better,hisΔT_sol was 9.16%,13.87%and 17.43%respectively.For ball milling method,by optimizing the ball to material ratio and ball milling time,VO₂（M）nanoparticles with small particle size,good dispersion and high crystallinity were prepared when the ball to material ratio was 10:1 and ball milling time was 2 h.When the T_lum was 56%,VO₂（M）nanoparticles with small particle size,good dispersion and high crystallinity were prepared,ΔT_sol can reach 6.14%.