Preparation Of VO₂ Films By Rapid Thermal Annealing And Optimization Of Transition Properties

In recent years,in response to the increasing global energy demand and environmental pollution,a thermochromic smart window concept has been proposed to save the energy consumed by buildings for air conditioning cooling and heating to regulate indoor temperature.Thermal smart windows can increase the amount of solar radiation entering through the window when the room temperature is low and isolate the solar radiation entering when the room temperature is high,thus effectively reducing the energy consumed by people to regulate the room temperature.Vanadium dioxide(VO₂)is a thermochromic phase transition material with a critical temperature close to room temperature.Vanadium dioxide can undergo a reversible phase transition between the low temperature monoclinic phase and the high temperature rutile phase near 68�C.The optical,electrical and magnetic properties of the material change dramatically before and after the phase transition,and this property makes VO₂ very suitable for use as a functional coating for smart windows.In order to make VO₂ applied to smart windows,there are still several problems to be solved in the current research,such as: complicated preparation process,low visible light transmittance and solar modulation efficiency,and critical temperature slightly higher than room temperature,etc.Therefore,it is an important issue to investigate the influence of preparation parameters on the composition,crystallinity and transition properties of VO₂ films and to optimize the transition properties of VO₂ films.To solve these problems,this experiment uses magnetron sputtering to prepare metallic vanadium films followed by rapid thermal annealing in oxygen to prepare VO₂ films.This method separates the sputtering process from the film oxidation process,reducing the preparation requirements,and also has the advantages of high repeatability and good controllability.Few studies have been conducted on this preparation method.In our experiments,we have purposefully regulated the properties of VO₂ films by controlling the rapid thermal annealing time,annealing oxygen flow rate and the thickness of the films during magnetron sputtering to produce high-quality VO₂ films.In addition,in order to further optimize the thermotropic phase change properties of VO₂ films,we also tried to prepare films with V/Ti stacked structure on the basis of single-layer VO₂ films in this experiment,so that the properties of the films were effectively improved.The study of this experiment mainly includes the following three aspects:(1)Firstly,the effects of annealing time and oxygen pressure on the properties of VO₂ films,such as physical phase,surface morphology,transmittance spectrum and phase transition temperature,were investigated during rapid thermal annealing.The annealing process parameters for the preparation of VO₂ films on glass substrates were experimentally derived.(2)A series of VO₂ films with different thicknesses were prepared by magnetron sputtering on glass substrates under selected annealing process parameters.The results show that the visible transmittance and solar modulation efficiency of the films with thickness of 150 nm achieved a balance,and the films were pure phase with good thermal phase change performance.The visible transmittance of the films was 34.9%,the solar modulation efficiency was 7.1%,and the phase transition temperature was60.3�C.(3)VO₂ films with V/Ti structure were prepared by ion beam sputtering and magnetron sputtering,and the effects of the new structure on the elemental content,crystal structure,surface morphology,and transition properties of VO₂ films were tested.The results showed that the transition temperature of the V/Ti structured VO₂ films was significantly lowered to 48.4? compared with the monolayer films prepared under the same conditions,while the visible light transmittance and solar modulation efficiency were increased to 36.3% and 7.4%,respectively.