The principle of thermochromism mainly adjusts the input of solar radiation (wavelength concentrated in 190-3000nm) energy and the output of blackbody energy through windows based on the ambient temperature. Thermochromic materials will change their transparency, absorbance, and color when the temperature changes. Thermochromic can be used as a passive design strategy to adjust near-infrared transmittance while maintaining visible light transmittance, without the need for external energy or manual operation. Therefore, thermochromic smart windows have become a hot research topic in building energy-saving windows due to their simple structure and broad application prospects.
Vanadium dioxide (VO2) is a thermally induced phase change material that has attracted much attention in the field of smart windows. The main characteristic of VO2 is that a reversible metal to insulator transition (MIT) occurs at T ≈ 67 ℃, which can be used directly as a thermochromic film material. The transition from metal to insulator in VO2 thin film is accompanied by a sudden change in near-infrared transmittance: T>67 ℃, in the state of tetragonal rutile metal, the infrared transmittance decreases significantly and becomes semi transparent; T ＜ 67 ℃, monoclinic semiconductor state, high infrared transmittance, transparent [12-13]. About 50% of the solar radiation energy is concentrated in the near-infrared band, just within the range of VO2 regulating the solar transmittance before and after the phase transition. Ideally, in the cold winter, VO2 based smart windows can allow most of the solar radiation to enter the room; In the hot summer, VO2 undergoes a phase transition and near-infrared light is mainly reflected outdoors to prevent indoor temperature rise, as shown in Figure 2c. In order to apply it to intelligent energy-saving windows, the phase transition temperature of VO2 can be reduced to room temperature or even lower through doping, microstructure control, composite, hybridization and other methods. Due to the phase transition temperature of inorganic VO2 being closest to room temperature, and the rapid and reversible phase transition with good repeatability, VO2 thin films are the preferred choice for the next generation of intelligent dimming coatings.
The preparation processes of VO2 thin films mainly include physical magnetron sputtering, sol-gel, chemical vapor deposition, hydrothermal and pulsed laser deposition. Researchers at home and abroad have done a lot of work and achieved fruitful results in the preparation, phase transition mechanism, and improvement of dimming efficiency of VO2 thin films. However, there are still bottlenecks and challenges in transitioning from laboratory to practical applications, such as high VO2 phase transition temperature, low efficiency of solar light regulation, low visible light transmittance, poor weather stability, and unattractive color (brownish yellow). The research on VO modification includes element doping, multi-layer film structure design, microstructure design, etc. The Jin Pingshi research group at the Shanghai Institute of Ceramics, Chinese Academy of Sciences, a representative research institution in China, has done a lot of systematic work in this area: doping of VO2 based smart windows, microstructure design and regulation, multi-layer composite, multifunctionality, working performance, low-temperature flexible preparation, color regulation, skin comfort, antibacterial and environmental friendliness, etc.
In addition to VO2 smart materials, at present, many new thermal responsive materials, including hydrogels, ionic liquids, perovskite and metamaterials, are also used in the research of thermochromic smart windows.
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