The effect of thermochromic coatings of VO2 on the fire performance of windows

The effect of thermochromic coatings of vanadium dioxide (VO₂) on the fire performance of windows was experimentally tested. Prototypes were subjected to radiant heat and the radiation transmitted through the specimens was measured as a function of time. The results indicate that windows coated with VO₂ can reduce radiative heat transfer from fires and thereby also reduce or prevent fire spread. The results clearly show that VO₂ coatings on BK7 substrates hinder approximately 30% of the transmission of radiation from fire sources when compared with the performance of uncoated windows. It is expected that VO₂ will not be solely implemented for the purpose of increasing fire performance of windows, but it will rather provide a secondary positive effect if such windows are realized for energy‐saving purposes.     

Preparation of VO2/Al-O core-shell structure with enhanced weathering resistance for smart window

VO₂/Al-O core-shell structure (V/AO) was synthesized by a facile method and its application for thermochromic film on smart window was investigated. Comparisons of thermochromic properties and stabilities in different environments between VO₂ and annealed V/AO nanoparticles were made. As a result, VO₂ nanoparticle was easily oxidized and lost thermochromic property. In contrast, with the protection provided by Al-O-based shell, the VO₂ core remained stable at a high temperature (350 °C in air) and in H₂O₂ solution. Especially, thermochromic films made by annealed V/AO nanoparticles almost kept thermochromic performance in damp heating environment (T=60 °C, RH=90%) for 20 days while VO₂ films lost their thermochromic properties after 3 days. Besides, the degradation progress of VO₂ and V/AO nanoparticles in damp heating environment was also explored. Results indicate that Al-O-based shell provides a good protection for the VO₂ core originated from the compact structure of shell, which effectively prevents oxygen and water from eroding VO₂. In summary, the VO₂ nanoparticles coated with Al-O-based shell exhibit a great potential in the application of smart window.     

Black ceramic coatings on Ti alloy with enhanced high absorptivity and high emissivity by plasma electrolytic oxidation

A black ceramic coating with high absorptivity and emissivity was successfully prepared on TA7 (Ti-5Al-2.5Sn) in a hybrid electrolyte solution by plasma electrolytic oxidation for improving the imaging precision of optical system. The influence of electrolyte components and technical parameters on the composition, structure, and optical properties was investigated. The results show the coatings with typically porous structure are mainly composed of O, P, Si, Ti, V, Fe, and Ni. The corresponding amorhous oxide in the outer layer endows the coating with strong absorption in the visible light and infrared areas, and the crystallized TiO₂ indwelling the inner layer contributes to the strong UV absorption property. In addition, the micropores of the coatings have different size ranges corresponding to the wavelengths, facilitating the increase of absorptivity and emissivity in some degree. The absorptivity and emissivity can be adjusted by electrolyte components and technical parameters. The coating presents the best absorptivity of 0.962 and emissivity of 0.950 in the electrolyte solution of 3 g/L NH₄VO₃, 5 g/L FeSO₄, and 5 g/L C₄H₆O₄Ni under 400 V for 10 minutes.     

Citrate-assisted hydrothermal synthesis of vanadium dioxide textured films with metal-insulator transition and infrared thermochromic properties

The present study reports the fast synthesis of VO₂ (М₁) oriented films on the r-Al₂O₃(012) substrates by the hydrothermal method using citric acid and vanadium (V) oxide as precursors with post deposition annealing in inert atmosphere. The effects of synthesis parameters (solution concentration and autoclave filling factor) on the films composition, morphology and electric properties are considered using XRD, Raman spectroscopy, AFM and SEM. The obtaining VO₂(M₁)/r-Al₂O₃ films show a metal-insulator transition with resistivity change of ~3.5 orders of magnitude and excellent thermochromic properties in the long-wavelength IR region. This work demonstrates a promising technique to promote the commercial implementation of VO₂ as material for design of infrared (IR) switch devices.     

Sub‐micrometre coatings as an infrared mirror: A new route to flame retardancy

Most of the polymeric materials used are easy to ignite and show extensive flame spread along their surfaces. Apart from extensive heat release rates, their short time to ignition (t_ig), in particular, is a key fire hazard. Preventing ignition eliminates fire hazards completely. Protection layers that shift t_ig by more than an order of magnitude are powerful flame retardancy approaches presenting an alternative to the usual flame retardancy concepts. Coatings are proposed that consist of a three‐layer system to ensure adhesion to the substrate, acting as an infrared (IR) mirror and protecting against oxidation. The IR‐mirror layer stack is realised by physical vapour deposition in the sub‐micrometre (<1 µm) range, reducing heat absorption by up to an order of magnitude. Not only is the ease of ignition diminished (t_ig is increased by several minutes), the flame spread and fire growth indices are also remarkably reduced to as little as 1/10 of the values of the uncoated polymers open for further optimization. Sub‐micrometre thin IR‐mirror coatings yielding surface absorptivity <0.1 are proposed as a novel and innovative flame retardancy approach. Copyright © 2011 John Wiley & Sons, Ltd.     

Multistep synthesis of VO2 (M) nanoparticles and their application to thermochromic hybrid films for IR modulation

Vanadium dioxide (VO₂) is widely known as one of the excellent thermochromic materials based on a reversible insulator-to-metal phase transition upon temperature change. In this study, VO₂ (M) powder was initially synthesized through a hydrothermal method and a subsequent post-annealing treatment. Additionally, a particle size of the VO₂ (M) powder was reduced and uniformized by introducing a ball-milling process. The resultant VO₂ (M) nanoparticles (NPs) were dispersed in ethanol with the addition of polyvinylpyrrolidone (PVP). The ethanolic dispersion was then coated on a transparent heater used as a substrate by spin-coating to produce VO₂ (M)/PVP composite films. We have attained an exact temperature control of the films by applying voltages to the heater for the assessment of their thermochromic performance such as the solar and the infrared modulation ability. For example, the film temperature could be raised from room temperature to 85.5°C within 180 s at a low voltage of 11 V, which was enough for inducing the phase transition of the VO₂ (M) NPs showing the infrared modulation ability of 19.3%. The combination of the composite films and the heater was thus proved to be a promising way for realizing transparent thermochromic devices.     

The novel preparation method of thermochromic VO2 films with a low phase transition temperature by thermal oxidation of V–Mo cosputtered alloy films

Vanadium dioxide (VO₂) has been studied extensively for its unique insulator-metal transition characteristics and potential applications in thermochromic smart windows, switching devices, and infrared detectors. However, how to balance the metal-insulator transition temperature, luminous transmittance (T_lum) and solar modulation ability (ΔT_sol) of VO₂ thin films remains a challenge. In this work, high-quality thermochromic VO₂ thin films were prepared by a two-step method of magnetron sputtering and thermal oxidation annealing. Metallic and alloyed V–Mo layers were first deposited by direct-current reactive magnetron sputtering, and then a thermal oxidation annealing process was used to obtain pure and Mo-doped VO₂ thin films. The Mo content in the films was regulated by changing the sputtering power of the vanadium target, and the effect of Mo doping on the crystallinity, microstructure, phase transition temperature and optical properties of VO₂ thin films was studied. The shift of the VO₂(011) peak to a lower 2θ angle in the XRD patterns showed that Mo was successfully diffused into vanadium dioxide films. The phase transition temperatures were decreased continuously from 57.4 to 32.7 °C by decreasing the sputtering power of vanadium. The thinner Mo-doped VO₂ thin films showed higher luminous transmittance and lower transition temperature. Our results were shown to be an innovative preparation method to fabricate thermochromic VO₂ films with a low phase transition temperature, balanced luminous transmittance and solar modulation ability by thermal oxidation of V–Mo cosputtered alloy films.     

Tuning thermochromic performance of VOx-based multilayer films by controlling annealing pressure

In this work, the nominal SiN_x/NiCr/NiCrO_x/VO_x/NiCrO_x/NiCr/SiN_x multilayer films deposited on glass were obtained by magnetron sputtering and subsequent rapid thermal annealing (RTA) in N₂ at 0.5–2000Pa pressure. Annealing pressure influenced the radiation thermal resistance, which can directly determine the crystallization and thermochromic performance of the VOx-based multilayer films. The VO₂ (M) film is very difficult to form under low annealing pressure because of insufficient oxygen and low-valence metal oxides in NiCrOx film. Therefore, the thermochromic performance is seriously deteriorated by the drastic diffusion between NiCrOx and VOx. Based on the phase transition temperature (T_c) and good crystallization, the wider process window of 80 Pa–300 Pa has been obtained successfully. Additionally, the films exhibit the optimal thermochromic performance with the solar regulation (ΔT_sol) of 18.4% and the luminous transmittance (T_lum) of 40.5%. Moreover, the T_c can be reduced to 54 °C by regulating annealing pressure without element doping. The results provide guidance for commercial production and quality consistency of VOx-based smart glass in building energy conservation.     

Absorptivity and its dependence on heat source temperature and degree of thermal breakdown

The spectral absorptivity of 62 products has been measured in the wavelength region of 0.3–20 µm. Effective absorptivity for fire‐induced heat radiation typically lies in the range of 0.75–0.95. It was found that the effective absorptivity varies significantly with the temperature of the heat source. This has implications on the heating of a surface. The effect is more important when the absorptivity is used as input for calculations of ignition temperature and thermal inertia. It was also found that the absorptivity of radiation from fires for products exposed to irradiation in many cases decreased with increased exposure time. This is surprising since, for example, wood that is darkened when exposed to heat obviously has a higher absorptivity in the visual part of the spectrum than fresh non‐darkened wood. The reason that was identified for this is because the absorptivity in the IR drops, and measurement results are given which clearly illustrate this. Copyright © 2010 John Wiley & Sons, Ltd.     

Tungsten doped M-phase VO2 mesoporous nanocrystals with enhanced comprehensive thermochromic properties for smart windows

The phase transition temperature (~68?°C) of M-VO₂ film can be lowered significantly by tungsten (W) doping into the crystal lattice of VO₂ due to the reduction of the strength of V-V pair interaction. However, W doping was always coupled with a serious weakening of luminous transmittance and solar modulation efficiency because W dopants can increase the electron concentration of VO₂ film. Herein, the simultaneous introduction of W dopants and mesopores into M-VO₂ nanocrystals was employed to prepare VO₂ film. Interestingly, the obtained 0.4?at%?W-doped mesoporous VO₂ nanocrystals based composite films exhibited enhanced comprehensive thermochromic performance with excellent solar modulation efficiency (ΔT_sol = 11.4%), suitable luminous transmittance (T_lum = 61.6%) and low phase transition temperature around 43?°C, much lower than 65.3?°C of undoped VO₂. It was demonstrated that the lower phase transition temperature of VO₂ can be primarily attributed to abundant lattice distortion after W doping, whereas the mesoporous structure can facilitate the uniform distribution of W dopants in VO₂ nanocrystals, enhance the luminous transmittance and guarantee enough VO₂ nanocrystals in the composite film to keep relatively high solar modulation efficiency. Therefore, this work can provide a new way to balance the three important parameters for the thermochromic performance of VO₂ film (ΔT_sol, T_lum and T_c) and probably promote the application of VO₂ nanocrystals in the energy efficient windows.     

VO2-ZnO composite films with enhanced thermochromic properties for smart windows

VO₂ film is a promising thermochromic material in smart windows due to its reversible metal to insulator transition (MIT) accompanied with an abrupt change of transmittance in near-infrared region at around 68 °C (T>68 °C, translucent; T < 68 °C, transparent), but which has not been widely applied because its low luminous transmittance (<60%) and solar modulation efficiency (<10%) are difficult to be improved simultaneously. In order to solve this problem, the ZnO-VO₂ composite film was prepared by a facile method, in which commercial ZnO nanoparticles (NPs) and VO₂ micro-particles were mixed by ball milling method to form the composites. By introducing ZnO NPs into the composite film, the luminous transmittance (T_lum) of the composite film was increased by 16.9% (from 54.9% to 63.9%) and the solar modulation efficiency (ΔT_sol) was increased by 14.1% (from 9.9% to 11.3%) compared to the pure VO₂ composite film. This was because ZnO NPs not only played the role of antireflection, but also prevented VO₂ particles from agglomeration by dispersing around VO₂ particles. Furthermore, the two-layered film based on ZnO-VO₂ composites exhibited an astonishing ΔT_sol of 18.8%, while maintaining excellent T_lum of 54.3%. This work could provide a simple and novel idea for us to improve the thermochromic properties of VO₂ films and simultaneously to promote their practical application.     

Boron doped M-phase VO2 nanoparticles with low metal-insulator phase transition temperature for smart windows

Vanadium dioxide (VO₂) is considered to be a promising candidate for energy-efficient smart windows because of its special reversible Metal-Insulator Transition (MIT) near the ambient temperature. However, its use is constrained by its high transition temperature (T_C) relative to the room temperature. In this paper, VO₂ doped by boron, could achieve an outstanding metal-insulator phase transition property with a low T_C (28.1 °C) close to the room temperature. This enhancement strongly contributes to the studies of the VO₂-based smart windows. A limit doping level of around 9.0 at% is observed for the boron-doped VO₂. Moreover, the particle size is getting smaller and more uniform and the particle distribution becomes more equal and compact with the continued increase in the doping content. Such uniform grain size and grain boundary conditions suppress the extension of the hysteresis loop (ΔT decreases from 25 °C to 7 °C). In addition, the T_C first declines with the increase in the boron content and it starts to increase after reaching its minima of 28.1 °C at 6.0 at% doping level. This feature is the consequence of the competition between the inhibition on the phase transition caused by the V⁵⁺ and the promotion on the phase transition caused by the heterogeneous defect-nucleation sites. VO₂ doped with 6.0 at% boron exhibits a favorable thermochromic performance with ΔT_sol of 12.5% and T_lum up to 54.3%, which is promising for the smart windows.     

Fabrication of oxide-based near infrared-shielding coatings for a smart window to prevent infrared-induced photoaging in human skin

Oxide-based near infrared (IR)-shielding coatings consisting of quarter‐wave stacks of oxygen-deficient tantalum oxide (Ta₂O_5?x) and silicon oxide (SiO₂) multilayers and tin-doped indium oxide (In₂O₃) (ITO) films with the thicknesses of 200–600 nm can block the passage of IR-A (wavelength: 760–1400 nm) and IR-B (wavelength: 1400–3000 nm) radiation, respectively. In this study, the optical properties and microstructure of these oxide-based IR-shielding coatings were investigated. Transmission electron microscopy images indicated that amorphous Ta₂O_5?x/amorphous SiO₂ multilayers were uniform and dense. ITO films were found to be highly crystalline and show carrier concentrations of up to 7.1 × 10²⁰ cm^?3, resulting in the strong IR-B optical absorption due to the plasma excitation of the free carriers. Oxide-based IR-shielding coatings with an ITO thickness of 420 nm were found to have near-IR shielding rates of >90% and an average visible light transmittance of >70%. The effects of IR on human keratinocytes were studied to evaluate the IR-induced photoaging in human skin. It was found that the downregulation of cellular proliferation and the enhancement of senescence-associated β-galactosidase activity induced by IR irradiation were significantly inhibited by oxide-based IR-shielding coatings. Thus, this study provides a facile method for the development of coatings for smart windows with high IR-shielding ability and high visible light transmittance.     

纳米掺锡氧化铟涂料的制备及其红外特性分析

采用超声波分散和物理分散相结合,制备了掺锡氧化铟纳米涂层并用电子扫描显微镜对涂层中填料的分散状态进行了表征,结果表明该涂料已达到纳米级分散,填料颗粒平均粒度小于100 nm;运用傅立叶红外吸收光谱、紫外-可见光-近红外分光光度计及红外发射率测试仪等对涂层的光谱性能进行了测试,结果显示该涂层具有屏蔽紫外线、反射可见光和近红外、中远红外强吸收等特点,可作为一种多功能红外隐身及隔热材料.     

Synthesis and study of Y0.9Ln0.1VO4 nanophosphors and Y0.9Ln0.1VO4@SiO2 luminescent nanocomposites with Ln=Eu,Dy, Er

Y_0.9Ln_0.1VO₄ nanophosphors with Ln=Eu, Dy and Er samples were prepared by hydrothermal synthesis and covered with silica (SiO₂) by the Stöber method using different reaction times. The synthesized samples were investigated by X-ray diffraction (XRD), Fourier-transform infrared (FTIR) spectroscopy, transmission electron microscopy (TEM) and photoluminescence spectroscopy (PL). XRD patterns show diffraction maxima compatible with a zircon-type structure where more intense reflexions are observed in the XRD pattern of Y_0.9Er_0.1VO₄ sample. FTIR spectra of Y_0.9Ln_0.1VO₄@SiO₂ samples reveal absorption bands which can be attributed to Si-O-Si and O-Si-O stretching bands and Si-O bending vibration. In all cases, TEM micrographs of Y_0.9Ln_0.1VO₄ phosphors show spherical nanoparticles. Nanoparticles agglomerates covered by a silica were found in TEM micrographs of Y_0.9Ln_0.1VO₄@SiO₂ samples. The thickness of the silica coating is dependent on the reaction time. PL measurements show variation of the emission intensity related to the different thickness of the silica coating.     

Phase evolution and thermochromism of vanadium oxide thin films grown at low substrate temperatures during magnetron sputtering

Vanadium oxides (VO_x) have been studied extensively for applications in thermochromic materials, electrochomics, and infrared detectors due to their unique phase transition characteristics. However, various vanadium oxide phases usually occur under different deposition conditions due to their particularly complex vanadium-oxygen system. In this research, V₃O₇, VO₂(B), VO₂(M), and V₂O₅ thin films were obtained as pure or mixed phases by controlling the substrate temperatures between 250 °C and 400 °C during magnetron sputtering. The microstructure and phase composition of vanadium oxide thin films were characterized and analyzed using X-ray diffraction and Raman spectroscopy. The phase evolution was dependent on the substrate temperature and could be clarified. Metastable V₃O₇ and VO₂(B) phases were obtained at substrate temperatures of 250–300 °C, while stable VO₂ and V₂O₅ phases were obtained at 350–400 °C. The surface morphology and optical properties of vanadium oxide thin films with different substrate temperatures were investigated in detail. Our results provide methods for transforming vanadium oxide phases under well controlled substrate temperatures.     

Tetragonal pencil-like VO2(R) as electrode materials for high-performance redox activities

Pencil-like tetragonal vanadium dioxide has been synthesized via one-step hydrothermal treatment. The compounds were analyzed through X-ray powder diffraction; scanning electron microscope and X-ray photoelectron spectroscopy (XPS). The optical properties of the as-synthesized material were studied by UV–visible diffuse reflection spectroscopy and room temperature photoluminescence. Thin films of VO₂(R) deposited on ITO substrates were electrochemically characterized by cyclic voltammetry (CV). The voltammograms show a reversible redox behavior with a doping/dedoping process corresponding to reversible cation intercalation/de-intercalation into the crystal lattice of the pencil. This process is easier in propylene carbonate than in aqueous solvent. It is also easier for the small Li⁺ cation than larger ones, Na⁺ and K⁺. This is attributed to a probable presence of one tunnel cavities in the structure of VO₂(R). The good electrochemical property of the VO₂(R) is attributed to its unique ultralong nanopencils structure with a good structural stability.     

Luminescence and Quantum Efficiencies of Eu3+‐Doped Vanadate Garnets

Nondoped and 5.0 mol% Eu³⁺‐doped vanadate garnets Ca₅Mg₄(VO₄)₆, NaCa₂Mg₂[VO₄]₃, KCa₂Mg₂[VO₄]₃, and NaSr₂Mg₂[VO₄]₃ were synthesized by solid‐state reactions. The formation of single‐phase compound with garnet structure is confirmed by X‐ray diffraction. The photoluminescence (PL) and PL excitation (PLE) spectra are investigated together with color coordinates. The luminescence process is discussed on the charge‐transfer transitions in [VO₄]^3? ions and the crystal structure. The PL quantum efficiencies (QE) are measured for nondoped and Eu³⁺‐doped samples. The Eu³⁺‐doped samples have higher QEs than the corresponding nondoped ones although the energy transfer occurs from [VO₄]^3? to Eu³⁺. Broad emission band due to [VO₄]^3? with intense sharp lines due to Eu³⁺, which gives white color, is observed in Eu³⁺‐doped NaCa₂Mg₂[VO₄]₃ and NaSr₂Mg₂[VO₄]₃ under excitation with UV light. These materials are suggested to be useful for lighting under the excitation with near‐UV LED.     

Ca2+‐Doped LaCrO3: A Novel Energy‐Saving Material with High Infrared Emissivity

The present study describes the successful synthesis of a Ca²⁺‐doped LaCrO₃ ceramic with high infrared (IR) emissivity, which is important for high‐temperature applications for significant energy saving. It is demonstrated that 20 mol% Ca²⁺‐doped LaCrO₃, i.e., La_0.8Ca_0.2CrO₃, exhibited an IR emissivity as high as 0.95 in the spectral region of 3–5 μm, which was 33.8% higher than that of LaCrO₃. By using La_0.8Ca_0.2CrO₃ as IR radiation agent in surface coating of heating unit, the radiative heat transfer could be enhanced significantly. The mechanism of the high IR emissivity of La_0.8Ca_0.2CrO₃ was attributed to the following aspects: Ca²⁺ doping introduced an impurity energy level of Cr⁴⁺ into LaCrO₃ and increased the hole carrier concentration, enhancing both impurity absorption and hole carrier absorption in the IR region; moreover, the doping caused lattice distortion enhanced the lattice vibration absorption. This novel high IR emissivity ceramic shows a promising future in high‐temperature applications for the purpose of energy‐saving.     

Near room temperature tunability of thermochromic VO2 thin films prepared via thermal oxidation method,influence of CO2:N2 gas flux ratios

In this study, we have investigated the thermochromic characterizations of VO₂ thin films synthesized by the thermal oxidation method. The oxidation process of a DC sputtered metallic vanadium layer on glass substrates, at 450 °C for 1 h, was carried out in the presence of CO₂:N₂ gases with different flux ratios of 30:70, 40:60, 50:50, 60:40, and 70:30, respectively. Using CO₂, as the oxidizing gas, provides an easy control route for obtaining the VO₂ phase among various vanadium oxide phases. The layers were characterized by FESEM, XRD & Raman spectra, sheet resistance vs. temperature (20–120 °C), and UV–Vis–NIR spectra at 25 and 90 °C. The XRD and Raman spectra confirmed all prepared layers have a VO₂ polycrystalline structure in monoclinic phase. We found among the studied samples CN30-70 and CN50-50 having desirable optical characteristics of peak visible transmittance of about 55%, with low transition temperatures (T_cr) of ∼42 and 31 °C, and also relatively high amounts of ΔT_1700nm, ΔT_sol and T_lum,av are good candidates for thermochromic smart windows, both from optical properties and economical fabrication method points of view.