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The VO2/Cs0.32WO3 composite thermochromic films were designed to promote the practical application of the VO2-based smart windows. The addition of 0.9 wt% Cs0.32WO3 can significantly shorten the phase transition response time and improve the solar heat shielding capacity of VO2, while maintaining high visible light transmittance (49.6%) and good solar modulation ability (10.4%). Furthermore, the phase transition of the composite film can occur at indoor temperature of 30 °C, and the indoor temperature can be greatly reduced by 7.7 °C under the simulated solar irradiation. These excellent properties make the VO2/Cs0.32WO3 films promising as smart energy-saving windows.

The optical properties of thin films result from the transportation of light in the multilayer medium. According to the Fresnel equation, antireflection (AR) coatings are utilized to suppress Fresnel reflection between different media and help photons to enter the thin films. As for AR coatings on vanadium dioxide (VO2) thin films, the layer number of the AR coating is the film upper or under the VO2 thin film. Though a single-layer AR coating is confirmed to improve the luminous transmission and thermochromic properties, the luminous transmission is still low and the thermochromic properties are less than what is desired for practical application. Factually, there are several methods for suppressing the reflection: single-layer AR coatings, double-layer AR coatings, multilayer AR coatings, gradient AR coatings, and nanostructure AR coatings. Though a nanostructure AR design can vastly improve luminous transmission, solar modulation is generally sacrificed due to a reduction in the effective thickness of VO2 thin films.

This work developed novel transparent nanocomposite film with thermochromic and hydrophobic properties fabricated by electrospinning and hot-pressing approach. The homogeneous composite nanofibers mats were obtained via co-electrospinning the organic polymer polymethylmethacrylate (PMMA) and inorganic VO 2 (M) nanoparticles. The VO 2 (M) nanoparticles with average diameter of 30 nm were distributed in the electrospun nanofibers and isolated from air simultaneously, which was beneficial for antioxidation of the VO 2 (M). Additionally, the mechanical property of the nanocomposite films with different stacked layers was always improved significantly via hot-pressing treatment. Moreover, the thermochromic property of the film showed the solar modulation ability Δ T sol of 7.31% and the luminous transmittance of 29.6% at 550 nm. Thus, the study offered a new notion for the energy-saving material, and it could be a good promising substitute of smart glass.

The ordered nano-structure composite films based on VO2/SiO2 as the thermochromic “smart window” have been fabricated by combining the PS template and RTA process. The study showed that the ordered VO2/SiO2 composite films exhibited good thermochromic performance with the large solar modulation efficiency (ΔTsol) of 8.42% and high visible transmittance (Tlum) of 55.6%, which is superior to pure VO2 foils with ΔTsol of 3.03% and Tlum of 46.1%, due to simultaneous control of transmittance in visible and near infrared region for VO2/SiO2 composite films. Hence, it offers a novel method to boost remarkably thermochromic properties for the “smart window” based on VO2 film.

Highly purified VO2(M) nanopowders was prepared by sol-gel process combined with mild hydrothermal reaction. The results illustrated the VO2(M) nanopowders are uniform quasi-sperical single crystal with a particle size of similar to 25 nm. The DSC analysis proves that nanopowders show superior phase transition properties around 68 degrees C. A kind of clear VO2(M) slurry was prepared by adding appropriate dispersing agent, as well as ultrasonic dispersion and high speed stirring dispersion methods. The final VO2(M) thin film prepared by spin-coating process has higher visible transmission (67.7%), higher luminous transmittance (61.5%) and higher solar modulation ability (12.5%), as well as lower haze (5.8%). The nanopowders synthesis method and film forming technology are extremely important in promoting the VO2(M) thermochromic films material development and making it closing to practical application as smart windows coatings.

In this approach, the VO2 nanoparticles have been successfully fabricated via combusting the low-cost precursor solution consisted of NH4VO3, C2H6O2 and C2H5OH. By the XRD, TEM and XPS analysis, it can be found that the synthetic monoclinic VO2 is single crystal and no impurity is defined. After dispersing the VO2 nanoparticles into the polymer, the solar modulation of VO2-based composite film is up to 12.5% with luminous transmission and haze around 62.2% and 0.5%, respectively. In other words, the composite films show high performance of thermochromic properties. This could open an efficient way to fabricate low-cost and large-scale VO2 (M) nanoparticles and thermochromic films.

This paper describes a solution-based route to synthesize vanadium dioxide (VO2) thermochromic thin films on glass substrate by spin-coating technology followed by nitrogen-annealing with vanadiumpentoxide (V2O5) and oxalic acid (H2C2O4) as source material, which is fairly economical and practical. Surface morphologies indicate that the films obtained by this method are homogeneous and particulate, irregular prisms emerge as the annealing temperatures increase. X-ray diffractions show that films annealed at relatively low temperature are pure monoclinic phase with a preferred orientation of (011). NaV4O7 and NaV6O15 form along with raising the heating temperatures. VO2 films obtained exhibit excellent visible transparency and switching property at near-infrared wavelengths across the metal-semiconductor transition. Transmittance change at lambda = 2000 nm of VO2 thin film annealed at 450 degrees C attains as high as 41.5% and its solar modulation efficiency reaches up to 8.8%. The W-doped VO2 film at a doping level of 1 at.% exhibits a thermochromic switch at 37 degrees C with a narrow hysteresis, which will greatly favor the practical application of VO2-based smart windows. (c) 2013 Elsevier B.V. All rights reserved.

VO2 (M) STF through reduction of V2O5 STF was prepared. The results illustrate that V2O5 STF can be successfully obtained by oblique angle thermal evaporation technique. After annealing at 550 °C/3 min, the V2O5 STF deposited at 85° can be easily transformed into VO2 STF with slanted columnar structure and superior thermochromic properties. After deposition SiO2 antireflective layer, Tlum of VO2 STF is enhanced 26% and ΔTsol increases 60% compared with that of normal VO2 thin films. Due to the anisotropic microstructure of VO2 STF, angular selectivity transmission of VO2 STF is observed and the solar modulation ability is further improved from 7.2% to 8.7% when light is along columnar direction. Moreover, the phase transition temperature of VO2 STF can be depressed into 54.5 °C without doping. Considering the oblique incidence of sunlight on windows, VO2 STF is more beneficial for practical application as smart windows compared with normal homogenous VO2 thin films.