Thermochromic VO2 films for smart windows application (original) (raw)
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Thermochromic VO2 films for energy-efficient windows
Solar Energy Materials, 1987
VO 2 films were produced by reactive e-beam evaporation followed by annealing post-treatment. Electrical measurements demonstrated a semiconductor-metal transition at % -60 o C. Spectrophotometry showed that the near-infrared solar transmittance was reduced when % was exceeded while the luminous transmittance remained relatively unchanged. This thermochromism may be utilized for regulating the energy throughput of windows. Practical application hinges on improved transmittance and on re-depression. These goals can be accomplished to some extent by dielectric overlayers, as verified by measurements on SiO2-coated VO 2 films.
Materials Today: Proceedings, 2020
Thermochromic materials have been studied extensively for their thermal switching properties for smart window application, among this vanadium dioxide (VO 2) is extensively researched. In this study, acidic Chemical Bath Deposition (CBD) method has been used to deposit thermochromic vanadium dioxide (VO 2) thin film on microscope glass substrate modified by PVP K-30 surfactant and using NH 4 VO 3 as a source of vanadium. The thin films are synthesized by varying the deposition time (15 min, 30 min and 80 min.) and annealing temperature (450°C, 500°C, and 570°C) under nitrogen gas atmosphere. Thermal studies done by DSC shows that 68°C is the critical transition temperature of VO 2 thin film synthesized, an alternative route to identify the thermochromic characteristic other than the routine UV-Vis-near IR absorption spectroscopy. The DSC curve have also been used to notify crystallinity and purity of phases from Full Width Half Maxima (FWHM) and enthalpy of transition studies. The crystalline phases of the thin film were studied using XRD, which shows dominant peaks at 27.6°and 39.5°with their respective index of (0 1 1) and (0 2 0). The XRD characterization show that crystalline size and FWHM are directly proportional to the annealing temperature. In the present studies, it is clearly shown, the future potential of CBD synthesized VO 2 thin films for smart window applications compared to other synthesis techniques.
ACS Omega, 2017
Monoclinic vanadium(IV) oxide (VO 2) 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 VCl 4 and ethyl acetate via atmospheric-pressure chemical vapor deposition (APCVD) was shown to produce thin films of monoclinic VO 2 with excellent thermochromic properties (ΔT sol = 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 VO 2 thin films by APCVD.
Synthesis and characterization of VO2-based thermochromic thin films for energy-efficient windows
Nanoscale Research Letters, 2011
Thermochromic VO 2 thin films have successfully been grown on SiO 2-coated float glass by reactive DC and pulsed-DC magnetron sputtering. The influence of substitutional doping of V by higher valence cations, such as W, Mo, and Nb, and respective contents on the crystal structure of VO 2 is evaluated. Moreover, the effectiveness of each dopant element on the reduction of the intrinsic transition temperature and infrared modulation efficiency of VO 2 is discussed. In summary, all the dopant elements-regardless of the concentration, within the studied rangeformed a solid solution with VO 2 , which was the only compound observed by X-ray diffractometry. Nb showed a clear detrimental effect on the crystal structure of VO 2. The undoped films presented a marked thermochromic behavior, specially the one prepared by pulsed-DC sputtering. The dopants effectively decreased the transition of VO 2 to the proximity of room temperature. However, the IR modulation efficiency is markedly affected as a consequence of the increased metallic character of the semiconducting phase. Tungsten proved to be the most effective element on the reduction of the semiconducting-metal transition temperature, while Mo and Nb showed similar results with the latter being detrimental to the thermochromism.
VO 2 /TiO 2 bilayer films for energy efficient windows with multifunctional properties
Vanadium dioxide/titanium dioxide bilayer films have been investigated as a thermochromic coating for application as intelligent window glazings for buildings. The nanostructured VO 2 and TiO 2 films were deposited on fluorine doped tin oxide coated glass substrates using electric field assisted aerosol assisted chemical vapour deposition (ElFi-AACVD) and sol–gel spin coating, respectively. Their thermo-chromic properties were investigated using scanning electron microscopy, X-ray diffraction, variable temperature UV/Vis/NIR and X-ray photoelectron spectroscopies. Compared to bare VO 2 films, the VO 2 /TiO 2 bilayer films exhibit an increase in visible light transmittance of up to 30%, an increase in integrated luminous transmittance, with values up to 66%, and a higher transmittance modulation in the near infrared of up to 20% at the thermochromic transition temperature. The TiO 2 top layer not only enhances visible light transmittance, but also serves to protect the VO 2 bottom layer from oxidation. The bilayer films are shown to exhibit photo-induced super-hydrophilicity. These properties are affected by the morphology of the VO 2 under-layer.
Durability-enhanced vanadium dioxide thermochromic film for smart windows
Materials Today Physics, 2020
Vanadium dioxide (VO 2)ebased thermochromic films are of great interest for energy-saving smart windows as they can dynamically change the solar transmittance as the ambient temperature changes. However, VO 2 is thermodynamically unstable and could be easily oxidized by the oxygen and moisture in the ambient air. In this work, a durability-enhanced VO 2 nanoparticle-polymer thermochromic film was proposed and fabricated using the blade coating method where the cross-linked and highly entangled poly(methyl methacrylate) (PMMA) chains with a molecular weight (~950,000) was adopted to block gas diffusion in the polymer matrix. It was shown that the developed VO 2 nanoparticle film kept~30% of its solar modulation ability after~900 h of accelerated durability test in the aging environment with a temperature at 60 C and~95% relative humidity. This is~4 times of the lifetime of the VO 2 nanoparticles which are embedded in the nonecross-linked PMMA matrix with low molecular weight (~15,000). The cross-linked PMMA-VO 2 film also showed a high luminous transmittance of~50%, a high solar modulation ability of~17%, and a low haze of~11%. Our method provides an easy and effective strategy to improve the lifetime of VO 2 nanoparticles, showing a promising pathway toward environmentally stable and easily scalable thermochromic films for energy-efficient smart windows.
Submicronic VO2–PVP composites coatings for smart windows applications and solar heat management
Solar Energy, 2014
Large surface and chemically stable thermochromic composite coatings consisting of IR active sub-micron VO 2 particles embedded in a passive polymeric host matrix of Polyvinylphenol were synthesized by dip and spin coating process. The hybrid composite coatings are IR transparent and IR reflective below and above the switching temperature of 68°C.Althoughthesecosteffectivehybridcompositecoatingsaretwo−dimensionalmorphologically(1−3nonpercolatedsubmicronVO2crystalliteswithinthecoatingthickness),theyexhibitasignificantreversibleIRtransmissionmodulationwithtemperatureof68°C. Although these cost effective hybrid composite coatings are two-dimensional morphologically (1-3 nonpercolated submicron VO 2 crystallites within the coating thickness), they exhibit a significant reversible IR transmission modulation with temperature of 68°C.Althoughthesecosteffectivehybridcompositecoatingsaretwo−dimensionalmorphologically(1−3nonpercolatedsubmicronVO2crystalliteswithinthecoatingthickness),theyexhibitasignificantreversibleIRtransmissionmodulationwithtemperatureof45% between 25°C and 100°C at 1 mm. This satisfactory reproducible thermochromic optical switching characteristic could address the impediment to greater market penetration of thermochromic VO 2 based technology relatively to the high cost coatings engineered by vacuum technologies. From technological applications perspective, the global production of glass which could regulate solar heat by using such hybrid thermochromic coatings, could be part of 1 billion m 2 /year with 2525% for building sector and about 2511% for the automotive industry. Likewise, such cost effective hybrid thermochromic coatings could play a significant role in the minimization of air conditioning load and thus energy consumption in both building and automotive sectors. As hinted to in its 2012 annual report, the International Energy Council's predictions have estimated that with 2billionsm2ofcoatedwindowsworldwidewithsmartcoatings,energysavinginthebuildingandautomotiveeconomicfieldsisequivalentofCO2reductionbyabout2 billions m 2 of coated windows worldwide with smart coatings, energy saving in the building and automotive economic fields is equivalent of CO 2 reduction by about 2billionsm2ofcoatedwindowsworldwidewithsmartcoatings,energysavinginthebuildingandautomotiveeconomicfieldsisequivalentofCO2reductionbyabout100 millions of tons.
Optical Modeling and Measurements for Solar Energy Systems III, 2009
Overheating is a common problem both with the use of active and passive solar energy in thermal solar energy systems and in highly glazed buildings. In solar thermal collectors, the elevated temperatures occurring during stagnation result in reduced lifetime of the collector materials. Highly glazed building facades provide high solar gains in winter, but imply in most cases high energy needs for air conditioning in summer. A solution to such problems might be provided by "smart" thermochromic coatings. A durable inorganic thermochromic material is vanadium dioxide. At 68°C, VO 2 undergoes a reversible crystal structural phase transition accompanied by a strong variation in optical properties. By doping the material with tungsten, it is possible to lower the transition temperature making it suitable as a window coating. In order to simulate the optical behaviour of multilayered solar coatings, precise knowledge on the optical material properties is necessary. Experimental data reported in the literature are rare and controversial. We determined the complex dielectric function for VO 2 :W by spectroscopic UV-VIS-NIR ellipsometry above and below the transition temperature and subsequent point-bypoint analysis of the ellipsometric psi/delta data. For a validation, the solar reflectance, absorptance and transmittance were measured by spectrophotometry in the visible range and in the near infrared range up to 2500 nm. The experimental reflectance spectra have been compared with the computer simulations based on the determined optical material properties. Finally, we collected optical data in a more extended wavelength range by digital infrared imaging to detect the switch in thermal emissivity of VO 2 :W at around 45°C.
Thermochromic Oxide-Based Thin Films and Nanoparticle Composites for Energy-Efficient Glazings
Buildings
Today's advances in materials science and technology can lead to better buildings with improved energy efficiency and indoor conditions. Particular attention should be directed towards windows and glass facades-jointly known as "glazings"-since current practices often lead to huge energy expenditures related to excessive inflow or outflow of energy which need to be balanced by energy-intensive cooling or heating. This review article outlines recent progress in thermochromics, i.e., it deals with materials whose optical properties are strongly dependent on temperature. In particular, we discuss oxide-based thin surface coatings (thin films) and nanoparticle composites which can be deposited onto glass and are able to regulate the throughput of solar energy while the luminous (visible) properties remain more or less unaltered. Another implementation embodies lamination materials incorporating thermochromic (TC) nanoparticles. The thin films and nanocomposites are based on vanadium dioxide (VO 2), which is able to change its properties within a narrow temperature range in the vicinity of room temperature and either reflects or absorbs infrared light at elevated temperatures, whereas the reflectance or absorptance is much smaller at lower temperatures. The review outlines the state of the art for these thin films and nanocomposites with particular attention to recent developments that have taken place in laboratories worldwide. Specifically, we first set the scene by discussing environmental challenges and their relationship with TC glazings. Then enters VO 2 and we present its key properties in thin-film form and as nanoparticles. The next part of the article gives perspectives on the manufacturing of these films and particles. We point out that the properties of pure VO 2 may not be fully adequate for buildings and we elaborate how additives, antireflection layers, nanostructuring and protective over-coatings can be employed to yield improved performance and durability that make TC glazings of considerable interest for building-related applications. Finally, we briefly describe recent developments towards TC light scattering and draw some final conclusions.