Preparation And Properties For Low Emissivity VO₂/AgNWs Based Energy Efficient Smart Coatings

Thermochromic vanadium dioxide（VO₂） materials can intelligently modulate the near-infrared transmittance in response to environmental temperature, Which are considered to be one of the most attractive materials for applications in energy efficient fields. However, the emissivity for VO₂ based glass coatings are too high for practical applications. Composition with low emissivity transparent conductive（TC） materials are considered to be an effective route for achieving high energy efficiency, although severe reduction in solar modulation efficient is observed because of the plasma resonance absorption in near-infrared region for the TC layers.Development of novel low-emissivity structure with broad band optical modulation properties（e.g., high reflection in middle-far infrared region） and high transmittance in visible to near infrared region is demanded for VO₂ based smart windows. This work is focused on the preparation of VO₂ energy-saving coating and the improvement of its emissivity. So, pure monoclinic VO₂ powders were synthesized via a low temperature molten salt method which were beneficial for large scale production, and quasicontinous Ag NW films were preparaed, which were composited with VO₂-PU energy-saving foils to investigate the optical properties,and also a layer of IGZO were deposited on the VO₂-PU/Ag NWs coating by magnetron sputtering. The main content and results were shown as follows:（1） The preparation and property of VO₂ powders by a molten salt method. Pure rutile structure VO₂ powders were obtained by a Li Cl-KCl molten salt assisted annealing process at temperature of above 400 °C in N2 atmosphere. The influence of annealing temperature and the incorporation of Sb3+ dopant were investigated.Investigation indicated that the incorporation of Sb3+ could effectively decrease the irreversible phase transformation temperature from VO₂（B） to VO₂（M）. Pure thermochromic VO₂ powders with average size of several tens of nanometers could be obtained when the precursor were annealied at 380 °C for 30 min. Thin films directly deposition from these VO₂ powders（with Sb3+doping content of 3%）exhibited a reduced phase of 65 °C and a distinct reduction of 30.6% for transmittance at 2000 nm can be obtained.（2） Preparation and properties for Ag NWs based low emissivity coatings. Silver nanowies with an average diameter of 76.8 nm and high aspect ratio of 700 were synthesized by a polyol method at 170 °C. By dispersion and coating process, these Ag NWs can be easily coated onto PET substrate, to form a quasicontinous thin film.These thin films exhibited high transmittance in visible to near-infrared range, while low thermal emission properties were also observed. For one sample, a transmittance of 77.1% at 550 nm and emissivity of 0.43 can be obtained. Further investigation by a finite-difference time-domain（FDTD） method for rigid silver micromesh structure confirmed the waveband regulated optical properties as observed in our investigations, which may prompt the improvement of optical properties for VO₂ based low emissivity and smart windows.（3） Preparation and properties for VO₂-PU/Ag NWs/IGZO composite smart coatings. By dispersion and coating process, VO₂ nanopowders dipersed in polyurethane（PU） were coated on PET substrate, then different layers silver nanowires were deposite on VO₂-PU coating by spin coating method. Indium gallium zinc oxide（IGZO） toplayer can improve the adhesion and abate the oxidation of Ag-nanowires. Results indicated that the incorporation of silver layers gradually decrease the emissivity of the composite films, although insignificant influence on solar modulation efficiency as compared with reported continous layers of VO₂/FTO and Pt/VO₂ were observed. For samples with Ag NWs deposition layers of 4, a reduced emissivity from 0.94 to 0.30 and a solar modulation efficiency of6.93% were obtained.