Preparation And Thermochromic Properties Of Acid Solution Processed VO₂-Based Composite Films

Smart windows are an important way to save energy in buildings.The commonly used coating material is a typical thermochromic phase change material,vanadium dioxide（VO₂）,which can automatically adjust the near-infrared light transmittance according to changes in ambient temperature and intelligently adjust the indoor temperature.A lot of work has been done since VO₂with phase-change properties were used in smart window films,but the films have not been applied to architectural glass on a large scale.This is mainly because VO₂-based films are difficult to meet the requirements of smart windows:high visible light transmittance(T_lum),strong near-infrared light modulation rate(ΔT_NIR)and good durability.To solve these problems,Mg O-VO₂、Zn₂V₂O₇-VO₂、Zr O₂-VO₂and AZO-VO₂were prepared in this paper by combining magnetron sputtering method and acid solution process,utilizing the good durability of magnetron sputtering film,as well as the easily controlled structure of acid solution processd film.The structure of the composite film was regulated by preparation process parameters and the thermochromic properties of the composite film were also studied.The main findings are as follows:（1）The effects of sputtering process parameters and acid solution processing time on the thermochromic properties of pure VO₂films and Mg O-VO₂composite films were studied.Keeping the acid solution treatment time unchanged for 5 s,it was found that with the Mg O sputtering power increasing from 60 W to 120 W,the T_lumof the composite film increased from 47.1%to 79.3%,and the solar light modulation rate(ΔT_sol)decreased from 8.7%to 5.7%.Such results were caused by the partly consumed VO₂due to the acid solution process.With the increasing time of the acid solution processing,the T_lumof the film enhanced significantly,and theΔT_solgradually decreased.（2）The influence of the sputtering process on the thermochromic properties of the Zn₂V₂O₇-VO₂and AZO-VO₂composite films was studied.As the sputtering power of the target Zn O increased from 60 W to 120 W,T_lumincreased from 41%to72.1%,andΔT_solstabilized at around 10%.The thickness of the film can be controlled by adjusting the sputtering power and the targeting time of Zn O,and the T_lumcan be adjusted according to the acid solution processing time.The obtained porous film had excellent thermochromic properties:T_lumwas as high as 72%whileΔT_solwas 10.7%.On the other hand,due to the introduction of Al element in the AZO target,the phase transition temperature of the AZO-VO₂composite film was changed into 56°C,which was lower than that of the Zn₂V₂O₇-VO₂composite film（64.5°C）.and the thermal hysteresis loop width of the AZO-VO₂composite film was 22°C,which was lower than 23°C of the Zn₂V₂O₇-VO₂composite film.In addition,the T_lumof the AZO-VO₂composite membrane after the acid solution process reached 66.7%,and theΔT_solcould be maintained at 11.5%.（3）The effect of sputtering process on the thermochromic properties of Zr O₂-VO₂composite films was studied.The films were prepared by the DC and RF dual target method.The thickness of the films can be controlled between 146-193 nm by adjusting the sputtering power and time of the Zr O₂target head.As the targeting power increased from 15 W to 120 W,the T_lumof the Zr O₂-VO₂composite film decreased from 44.2%to 31.2%,and theΔT_solfirstly increased from 13.2%to 14.3%and then decreased to 9.2%.It was assumed that the introduction of Zr can improve the crystallinity of the VO₂film and thus increase theΔT_solof the film.However,too much introduction of Zr was not conducive to the crystal growth process of VO₂.When the Zr O₂sputtering power was 90 W,Zr O₂-VO₂composite film exhibited best optical performance withΔT_solof 12.6%and T_lumbeing up to 45%.