Vanadium Dioxide-based Smart Window Metamaterial Simulation And Experimental Research

Vanadium dioxide?VO₂?has a phase transition temperature close to room temperature and a reversible metal-insulator phase transition?MIT?characteristic.When the temperature is higher than the phase transition temperature,its crystal structure has changed from a monoclinic system in the semiconductor state?M phase?to a rutile tetragonal system in the metallic state?R phase?with great changes in optical characteristics,it is an ideal material for smart windows.VO₂-based smart window application research can effectively save energy consumption and use resources reasonably,but its practical application is limited by low visible light transmittance(T_lum)or low solar spectrum modulation capability(?T_sol),Liu et al.pointed out in 2013 that in practical applications its T_lumum is at least 40%and?T_solol is at least 20%.In addition,the VO₂-based metamaterial structure exhibits very flexible characteristics.It can artificially design and adjust structural parameters to achieve the required functions.At the same time,it is also considered as a strong competitor and excellent candidate for smart window applications.Aiming at the problems faced by VO₂-based smart window applications,this paper proposes a new solution.That is,a VO₂-based metamaterial structure is introduced on top of the VO₂-based smart window structure to improve both T_lumum and?T_sol.Through a specific metamaterial structure,?T_solol is improved while maintaining a high T_lumum to achieve application requirements,which is,T_lumum is greater than 40%and?T_solol is at least 20%.Toward this goal,this article has done the following work:1.The anti-reflection principle of TiO₂/VO₂ double-layer smart window structure based on VO₂ film was studied.The structure was simulated using CST simulation software and Drude model of VO₂ material,and its transmittance in the 380-2500nm waveband was obtained.Through structural optimization,the T_lum,M(T_lumum when VO₂is M phase),T_lum,R(T_lumum when VO₂ is R phase),and?T_solol of the TiO₂/VO₂double-layered structure were 59.4%,51.1%,and 9.7%,respectively.This structure has a high T_lumum but a low?T_solol and it is not suitable for practical applications.2.A metamaterial structure based on VO₂?VO₂?ms?,where"ms"is the abbreviation of metamaterial structure?is designed to form a VO₂?ms?/TiO₂/VO₂three-layer structure.The metamaterial layer is a VO₂ film with uniformly distributed round holes with a specific radius and pitch.Through simulation and optimization,excellent performance is obtained.Its T_lum,Mum,M is 50.4%,T_lum,Rum,R is 41.8%,and the modulation amplitude at 2.5?m is 63.8%,and?T_solol is as high as 22.8%,achieving the research goal.The analysis shows that the introduction of the VO₂?ms?layer has a blocking effect on visible light,which makes T_lumum lower than the TiO₂/VO₂double-layer structure by about 9%at high and low temperatures,respectively.At the same time,the formation of MIM resonators with the VO₂ phase changed to a metallic state brought about a significant increase in the near-infrared modulation amplitude,which caused?T_solol to increase from 9.7%to 22.8%.In addition,the analysis shows that an increase in the distance D between round holes will lead to a decrease in the performance of the structure.3.Samples of VO₂?ms?/TiO₂/VO₂ structure were experimentally prepared by magnetron sputtering and polystyrene?PS?microsphere-assisted preparation methods.Good performance was obtained through experiments,with?T_solol of 20.8%,T_lum,Mum,M of46.5%,T_lum,Rum,R of 41.2%,and the actual spacing D of this sample was 1381nm.Combining experiments and simulations,it is concluded that they have similar trends,and the negative effect of the increase of the spacing D on the performance of the sample was confirmed.