Excellent Thermochromic Properties Research Articles

Perovskite thermochromic smart window: Advanced optical properties and low transition temperature

Windows are one of the most inefficient components in buildings. Common thermochromic smart windows using VO2 can mitigate such energy loss. However, they suffer from several problems, namely, low solar modulation ability, high transition temperature (i.e. 68 °C) and low luminous transmittance. In this study, we propose a perovskite thermochromic smart window towards achieving high solar modulation ability whilst maintaining a high luminous transmittance and a low transition temperature. Perovskite material shows a significant thermochromism in the visible and ultraviolet region. Since half of the photons lie in this spectral region, a high solar modulation can be achieved by perovskites. The material was optimized by varying the spin speed in the fabrication process as well as the mixing ratio between precursors. The optimized sample exhibits a solar modulation ability of 25.5% with luminous transmittance of 34.3% and higher than 85% in the hot (80 °C) and cold (25 °C) states, respectively, making this material suitable for practical device applications. The hysteresis loop, the transition temperature as well as transition time in relation to the relative humidity of a perovskite smart window during the heating and cooling process are investigated in this study. From field tests results, the perovskite smart window can help reduce the indoor air temperature by about 2.5 °C compared to a normal window. Overall, based on the results obtained in this study, the perovskite thermochromic smart window has potential to achieve excellent thermochromic properties, providing an alternative to alleviate the high energy consumed in buildings.

Surface modification-assisted solvent annealing to prepare high quality M-phase VO2 nanocrystals for flexible thermochromic films

Well-crystalline and stoichiometric M-phase vanadium dioxide (VO2 (M)) ultrafine nanoparticles are desired in fabricating high-efficient VO2-based flexible thermochromic films. In this study, a novel solvent annealing technique is developed to avoid growth and agglomeration of VO2 (M) nanoparticles in the post-annealing process. Oxygen-deficient VO2 (D) nanoparticles (~36 nm) are firstly prepared by a facial hydrothermal method. A double-layered reactor is designed to separate the surfactant-modified VO2 (D) nanoparticles physically from H2O2 which serves as the oxidant. The unique reactor enables the formation of a moderate oxygenation environment in the solvent annealing process, making the transformation of oxygen-deficient VO2 to VO2 (M) more controllable. The growth and agglomeration of VO2 nanoparticles are effectively suppressed due to the surface modification by polyvinyl pyrrolidone, liquid environment and low temperature. The obtained VO2 (M) nanoparticles have an average particle size of ~41 nm. The flexible film prepared with the VO2 (M) nanoparticles exhibit excellent thermochromic properties with ΔTsol reaching 18.03% and Tlum up to 58.33%. This amazing performance is attributed to the high quality of VO2 (M) nanoparticles and ultrafine particle size that makes the surface plasmon absorption take effect in the solar light region. This work demonstrates the proposed solvent annealing technique is an effective approach to prepare high-quality ultrafine VO2 (M) nanoparticles for achieving efficient thermochromic performance of the flexible smart windows.

Nanomaterials for infrared shielding smart coatings

Synthesis and characterization of functional nanoparticles and their applications for smart window are reviewed. Various kinds of nanomaterials and their composites are successfully synthesized by solvothermal process. Thin films on glass substrate can be fabricated by doctor blade method using nanoparticles as starting materials. The mixed valence state tungsten-based homogeneous nanomaterials possess excellent infrared (IR) light shielding properties, implying their potential applications for the heat ray shielding and indoor energy saving effect in summer. On the other hand, VO2 monoclinic-based nanoparticles possess smart and excellent thermochromic properties, show excellent heat ray shielding effect in summer and heating effect of winter indoors. Also, multifunctionality of thin films can be realized by fabricating the composites with various functional components, implying their potential applications for IR shielding, photocatalysis and self-cleaning at the same time. The design of composites and structure of thin films might result in the enhancement or properties of practice applications on window materials.

Low-temperature rf sputtered VO2 thin films as thermochromic coatings for smart glazing systems

VO2 is a material of great interest due to its excellent thermochromic properties. In this work, Vanadium dioxide (VO2) films were deposited on commercial SnO2-coated glass by the rf magnetron sputtering technique, at the low deposition temperature of Tsub = 300 °C. The effect of thickness and oxygen content in Ar–O2 plasma on the thermochromic properties were studied. In specific, it was found that the critical transition temperature (Tc) remained constant, while the width of transmittance hysteresis loop (ΔTC) was slightly decreased as thickness was increased from 35 nm to 260 nm. In addition, solar/transmittance modulation (ΔTrsol) was increased by about 6%, while luminous transmittance (Trlum) was decreased almost 30%, with thickness. Furthermore, as the oxygen content in the plasma ranged from 2% to 4%, both TC and ΔTC increased about 4 °C, while ΔTrsol and Trlum showed a slight increase of 1% and 3%, respectively. Finally, ZnO antireflective coatings of thicknesses from 22 to 100 nm were deposited on VO2 thermochromic films by dc magnetron sputtering technique at room temperature, in an effort to enhance the luminous transmittance. As a result, an increase in Trlum of over 8%, with a 30 nm ZnO film was observed, while ΔTrsol was increased by about 1%, independent of the ZnO thickness, while both TC and were unaffected by the presence of ZnO antireflective coating.

Effect of Fe doping on thermochromic properties of VO2 films

Vanadium dioxide is a well-known phase transition material which has the potential to be applied in the smart windows. In this work, Fe-doped vanadium dioxide films were successfully prepared on glass substrates at 450 °C annealing temperature by direct current magnetron sputtering. These films were characterized by XRD, Raman shift, XPS and SEM tests. It can be found that Fe doping affects both the surface morphology and the thermochromic properties of the films. It is of great interest to note that the TC drastically reduces which is attributed to the substitution of V4+ by Fe3+ in the VO2 crystal structure. Particularly, when Fe content increases to 9.8%, the lowest TC is 33.5 °C (34.5 °C lower than bulk). Moreover, the luminous transmittance enhances and the solar modulation ability remains at 13.7%, greater than 10%, which is difficult to be achieved by adding other dopants. Such excellent thermochromic properties are good enough for practical application. These features indicate that Fe doping provides a new idea of applying VO2 to smart windows.

Hexagonal VO2 particles: synthesis, mechanism and thermochromic properties.

Monoclinic vanadium dioxide VO2 (M) with hexagonal structure is synthesized by hydrothermal method, and the phase evolution is evidenced. Interestingly, the hexagonal morphology comes into being as a result of the low-energy coherent interfaces, (211̄)1//(21̄1̄)2 and (21̄1̄)1//(020)2. The size of hexagonal particles is well controlled by changing the concentration of precursor solutions. Hexagonal particles exhibit excellent thermochromic properties with a narrow hysteresis of 5.9 °C and high stability. In addition, the phase transition temperature can be substantially reduced down to 28 °C by simply W doping.

Optimized Atmospheric-Pressure Chemical Vapor Deposition Thermochromic VO2 Thin Films for Intelligent Window Applications.

Monoclinic vanadium(IV) oxide (VO2) has been widely studied for energy-efficient glazing applications because of its thermochromic properties, displaying a large change in transmission of near-IR wavelengths between the hot and cold states. The optimization of the reaction between VCl4 and ethyl acetate via atmospheric-pressure chemical vapor deposition (APCVD) was shown to produce thin films of monoclinic VO2 with excellent thermochromic properties (ΔTsol = 12%). The tailoring of the thermochromic and visible light transmission was shown to be possible by altering the density and morphology of the deposited films. The films were characterized by X-ray diffraction, atomic-force microscopy, scanning electron microscopy, ellipsometry, and UV–vis spectrometry. This article provides useful design rules for the synthesis of high-quality VO2 thin films by APCVD.

A High-Effective Method to Prepare VO2(M) Nanopowders and Thin Films with Excellent Thermochromic Properties

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.

Hydrothermal growth of VO2 nanoplate thermochromic films on glass with high visible transmittance.

The preparation of thermochromic vanadium dioxide (VO2) films in an economical way is of interest to realizing the application of smart windows. Here, we reported a successful preparation of self-assembly VO2 nanoplate films on TiO2-buffered glass by a facile hydrothermal process. The VO2 films composed of triangle-shaped plates standing on substrates exhibit a self-generated porous structure, which favors the transmission of solar light. The porosity of films is easily controlled by changing the concentration of precursor solutions. Excellent thermochromic properties are observed with visible light transmittance as high as 70.3% and solar modulating efficiency up to 9.3% in a VO2 film with porosity of ~35.9%. This work demonstrates a promising technique to promote the commercial utilization of VO2 in smart windows.

Enhanced Thermochromic Properties and Solar-Heat Shielding Ability of W(x)V(1-x)O2 Thin Films with Ag Nanowires Capping Layers.

Considerable efforts have been made to shift the phase transition temperature of metal-doped vanadium dioxide (VO2) films nearer the ambient temperature while maintain the excellent thermochromic properties simultaneously. Here, we describe a facile and economic solution-based method to fabricate W-doped VO2 (V(1-x)W(x)O2) thin films with excellent thermochromic properties for the application of smart windows. The substitutional doping of tungsten atoms notably reduces the phase transition temperature to the ambient temperature and retains the excellent thermochromic property. Furthermore, Ag nanowires (NWs) are employed as capping layers to effectively decrease the thermal emissivity from 0.833 to 0.603, while the original near infrared region (NIR) modulation ability is not severely affected. Besides, the Ag NWs layers further depress the phase transition temperature as well as the hysteresis loop width, which is important to the fenestration application. These solution-grown Ag NWs/V(1-x)W(x)O2 thin films exhibit excellent solar modulation ability, narrowed hysteresis loop width as well as low thermal emissivity, which provide a promising perspective into the practical application of VO2-based smart windows.

Enhancement of VO2 thermochromic properties by Si doping

Silicon doped vanadium dioxide films were successfully prepared on indium tin oxide coated glass substrates at 255°C annealing temperature by direct current magnetron sputtering. Maintaining spheroidal grains, the size of VO2 films nanoparticles decreased with silicon doping ratios increasing. Transmittance spectra indicated a 46nm blue-shift of absorption edge and improved the samples' integrated luminous transmittances (from 28.4% at Si/V=0 to 36.1% at Si/V=0.17) with similar thickness of 90nm. The samples presented excellent thermochromic properties that possessed higher than 40% near-infrared switching efficiency at 2000nm and showed at least 9.2% solar modulation efficiency. Moreover, the metal–semiconductor phase transition temperature for heavily doped VO2 (Si/V=0.17) decreased to 46.1°C (22°C lower than bulk). These features suggest the practical application of VO2 to smart windows.

A cost-effective method to fabricate VO2 (M) nanoparticles and films with excellent thermochromic properties

In this paper, high crystallinity and pure phase VO2 (M) powder is synthesized by a novel and facile method. Aiding by additional manual grinding and etching process, 22nm high-quality VO2 (M) nanoparticles can be obtained. The structure and properties of the VO2 (M) particles were characterized by X-ray diffraction analysis, transmission electron microscopy, differential scanning calorimetry and UV–vis–NIR spectrophotometer. After mixing VO2 (M) nanoparticles with transparent polymer, thin films prepared by grinded VO2 nanoparticles show excellent thermochromic properties. The solar modulation ability is up to 12.4% with luminous transmittance of 62.7%. Moreover, The haze of films prepared by grinded VO2 (M) nanoparticles is down to 1.9%, which is far less than that of films prepared by original VO2 (Haze=8.5%) and etched VO2 particles (Haze=4.6%). Dramatical improvement of thermochromic property and definition indicate that it is a promising method to prepare large-scale VO2 nanoparticles and cost-effective smart window.

Solid-state-reaction synthesis of VO2 nanoparticles with low phase transition temperature, enhanced chemical stability and excellent thermochromic properties

Solid-state-reaction synthesis of VO2 nanoparticles at 500 °C with different time.

Crystallised mesoporous TiO2(A)–VO2(M/R) nanocomposite films with self-cleaning and excellent thermochromic properties

A “two-step” method was developed for the preparation of crystallised TiO2(A)–VO2(M/R) nanocomposite mesoporous films with self-cleaning properties and excellent thermochromic performance.

Effect of Oxide Buffer Layer on the Thermochromic Properties of VO2 Thin Films

VO2 thin films were deposited on soda lime glass substrates with ZnO, TiO2, SnO2, and CeO2 thin films applied as buffer layers between the VO2 films and the substrates in order to investigate the effect of buffer layer on the formation and the thermochromic properties of VO2 film. Buffer layers with thicknesses over 50 nm were found to affect the formation of VO2 film, which was confirmed by XRD spectra. By using ZnO, TiO2, and SnO2 buffer layers, monoclinic VO2 (VO2(M)) film was successfully fabricated on soda lime glass at 370 °C. On the contrary, films of VO2(B), which is known to have no phase transition near room temperature, were formed rather than VO2(M) when the film was deposited on CeO2 buffer layer at the same film deposition temperature. The excellent thermochromic properties of the films deposited on ZnO, TiO2, and SnO2 buffer layers were confirmed from the temperature dependence of electrical resistivity from room temperature to 80 °C. Especially, due to the tendency of ZnO thin film to grow with a high degree of preferred orientation on soda lime glass at low temperature, the VO2 film deposited on ZnO buffer layer exhibits the best thermochromic properties compared to those on other buffer layer materials used in this study. These results suggest that deposition of VO2 films on soda lime glass at low temperature with excellent thermochromic properties can be achieved by considering the buffer layer material having structural similarity with VO2. Moreover, the degree of crystallization of buffer layer is also related with that of VO2 film, and thus ZnO can be one of the most effective buffer layer materials.

Preparation and Characterization of Self-Supporting Thermochromic Films Composed of VO2(M)@SiO2 Nanofibers

Nanofibers of VO2(A) with the diameter and length averagely at 100 nm and 10-20 μm were prepared via a facile one-step hydrothermal method by reducing NH4VO3 with 1,3-propylene glycol in an acidic solution. The obtained VO2(A) was coated by SiO2 to form VO2(A)@SiO2 core-shell nanocomposites, which were then transformed into VO2(M)@SiO2 by annealing under nitrogen atmosphere. The resulted composites maintained the original fibrous morphology, particularly with a large amount of pores emerging inside the fiber due to the volume shrinkage during the phase transition, which may improve its thermal insulation ability in real applications. The VO2(M)@SiO2 nanofibers were arranged into a self-supporting film by filtration, which shows excellent thermochromic properties.

F-doped VO2 nanoparticles for thermochromic energy-saving foils with modified color and enhanced solar-heat shielding ability

F-doped VO2 (M1) nanoparticles were prepared via one-pot hydrothermal synthesis. The F-doping can minimise the size of the VO2 (M1) nanoparticles, induce a homogeneous size distribution and effectively decrease the phase transition temperature to 35 °C at 2.93% F in VO2. VO2 smart glass foils obtained by casting these nanoparticles exhibit excellent thermochromism in the near-infrared region, which suggests that these foils can be used for energy-efficient glass. Compared to a pure VO2 foil, the 2.93% F-doped VO2 foil exhibits an increased solar-heat shielding ability (35.1%) and a modified comfortable colour, while still retaining an excellent solar modulation ability (10.7%) and an appropriate visible transmittance (48.7%). The F-doped VO2 foils are the first to simultaneously meet the requirements of a reduced phase transition temperature, diluted colour and excellent thermochromic properties, and these properties make the further improved F-doped VO2 foils suitable for commercial applications in energy efficient glass.

Enhancing thermochromic performance of VO2 films via increased microroughness by phase separation

Application of VO2 to smart windows calls for an enhancement of both luminous transmittance (Tlum) and solar transmittance modulation (ΔTsol). These two properties are related to optical constants, microstructure and film thickness. In this article, VO2-based films with increased microroughness are prepared by polymer assisted deposition, and ZnCl2 is added to the precursor solution to control the microstructure of the films. Addition of ZnCl2 increases the microroughness of films through phase separation, which largely decreases the refraction index and the reflectivity of the films, resulting in enhanced luminous transmittance and solar transmittance modulation; two films with balanced optical performance show Tlum=50.5% and ΔTsol=11.3%, and Tlum=59.3% and ΔTsol=10.5%, respectively. Moreover, this kind of microstructure control has no influence on doping VO2, which is proven by the deposition and characterization of W-doped VO2-based films. By the formation of a W-doped VO2/F:SnO2/glass multilayer stack, a film shows excellent thermochromic properties and low emissivities; typically, a film with a phase transition temperature, Tlum, ΔTsol of 43.4°C, 30.0%, 8.82%, respectively, and emissivity of 0.20 for monoclinic and 0.33 for rutile phase is successfully prepared.

High contrast thermochromic switching in vanadium dioxide (VO2) thin films deposited on indium tin oxide substrates

Vanadium dioxide thin films with excellent thermochromic switching properties were deposited on indium tin oxide (ITO)-coated glass substrates with the RF magnetron sputtering technique. Reversible transmission switching of these films from as much as 65% to near zero at 2500nm, with contrast ratios of more than 3000, was observed. These films compare favorably to those deposited on glass slides, and, in particular, show a resistivity drop of nearly 2 orders of magnitude upon switching to the metallic state. Inducing the metal–insulator transition by electrical current through the ITO layer lowers the transition temperature by 4–7°C, as compared to traditional heating of the sample by a heating plate. The presence of an ITO sublayer also seems to result in smaller grain size and slightly broader hysteresis in VO2 films.

Enhanced chemical stability of VO2 nanoparticles by the formation of SiO2/VO2 core/shell structures and the application to transparent and flexible VO2-based composite foils with excellent thermochromic properties for solar heat control

Vanadium dioxide is a key material for thermochromic smart windows that can respond to environmental temperature and modulate near infrared irradiation by changing from a transparent state at low temperature to a more reflective state at high temperature, while maintaining visible transmittance. VO2 thermochromism is commonly used in films on glass that function as smart windows. Flexible VO2 nanocomposite foils are able to combine the intrinsic properties of VO2 nanoparticles with the added functionalities contributed by nanoscale and interface effects, such as increased visible transparency and infrared modulation ability. These foils are promising for applications in construction and automotive glasses to increase energy efficiency. However, VO2 nanoparticles may be unstable, and they are difficult to prepare in stable dispersive suspensions. In this paper, we report a novel all-solution process that can be used to prepare transparent, stable and flexible VO2-based composite films. These films exhibit UV-shielding properties and an excellent temperature-responsive thermochromism in the near infrared region. A typical film has a solar modulation efficiency of 13.6%, which is the highest value for VO2 thermochromic films with comparable visible transmittance. Coating the VO2 nanoparticles with a thin SiO2 shell significantly improved their anti-oxidation and anti-acid abilities. This result represents an important breakthrough in VO2 thermochromism, and it may have applications for near infrared modulation of glass used in construction or cars.