Transition Temperature Of VO2 Research Articles

Enhanced Pd/a-WO3 /VO2 Hydrogen Gas Sensor Based on VO2 Phase Transition Layer.

The utilization of clean hydrogen energy is becoming more feasible for the sustainable development of this society. Considering the safety issues in the hydrogen production, storage, and utilization, a sensitive hydrogen sensor for reliable detection is essential and highly important. Though various gas sensor devices are developed based on tin oxide, tungsten trioxide, or other oxides, the relatively high working temperature, unsatisfactory response time, and detection limitation still affect the extensive applications. In the current study, an amorphous tungsten trioxide (a-WO3 ) layer is deposited on a phase-change vanadium dioxide film to fabricate a phase transition controlled Pd/a-WO3 /VO2 hydrogen sensor for hydrogen detection. Results show that both the response time and sensitivity of the hydrogen sensor are improved greatly if increasing the working temperature over the transition temperature of VO2 . Theoretical calculations also reveal that the charge transfer at VO2 /a-WO3 interface becomes more pronounced once the VO2 layer transforms to the metal state, which will affect the migration barrier of H atoms in a-WO3 layer and thus improve the sensor performance. The current study not only realizes a hydrogen sensor with ultrahigh performance based on VO2 layer, but also provides some clues for designing other gas sensors with phase-change material.

Tuning Phase Transition and Thermochromic Properties of Vanadium Dioxide Thin Films via Cobalt Doping.

Vanadium dioxide (VO2) featuring a distinct thermally triggered phase transition is regarded as the most attractive thermochromic material for smart window applications. However, the high transition temperature (∼67 °C) and moderate luminous transmittance (<50%) of the pristine VO2 circumvent room temperature applications. In this work, epitaxial cobalt-doped VO2 thin films were fabricated to tailor the electric and optical properties on a c-plane sapphire substrate. At the highest doping concentration of 10%, the transition temperature of VO2 is reduced to 44 °C, accompanied by a high luminous transmittance of 79% for single-element Co-doped VO2. The roles of cobalt doping and detailed band variation are fully explained experimentally and by modeling (DFT calculation), respectively. Furthermore, the dramatically increased carrier concentration in cobalt-doped VO2 underscores the promising future of cobalt-doped VO2 unveiled by temperature-dependent Hall effect measurement.

Determination of the effective thermal conductivity of particulate composites based on VO2 and SiO2

The effective thermal conductivity of composites made up of VO2 (SiO2) spherical particles randomly distributed and embedded in a SiO2 (VO2) matrix are numerically studied in a range of temperatures around the metal-insulator transition of VO2. This is done by means of three-dimensional finite element simulations for different concentrations and sizes of the particles as well as various interface thermal resistances. Our results are validated against the Mori-Tanaka analytical model. In addition, we develop a numerical method to calculate the heat storage capacity for composites with VO2 particles dispersed into a SiO2 matrix. It is shown that: i) The effective thermal conductivity of VO2/SiO2 composites increases with the VO2 particles' size, while the one of SiO2/VO2 composites is pretty much independent of the SiO2 particles' radius. ii) At the VO2 transition temperature (342.5 K), the effective thermal conductivity of VO2/SiO2 composites increases significantly at a rate of 2.7 × 10−3 Wm−1K−2, such that its value doubles up the SiO2 matrix thermal conductivity at the particle concentration of 40.2%. By contrast, the effective thermal conductivity of SiO2/VO2 composites decreases at a rate of 8.6 × 10−3 Wm−1K−2. iii) The effective thermal conductivity is strongly affected by the thermal resistance in VO2/SiO2 composites, by contrast the resistance effect does not play an important role for particle volume fractions of SiO2 up to 34.1% in SiO2/VO2 composites. The Mori-Tanaka model and our simulations predict the same trend of the effective thermal conductivity values of VO2/SiO2 composites. However, the analytical model fails when the matrix is made up of VO2 and the volumetric fraction of SiO2 exceeds 34.1%. The latent heat storage capacity of VO2/SiO2 composites increases with the VO2 particles’ concentration, such that at 40.2%, it takes the value of 24553 J kg−1 (486.7 cal mol−1), which is about half that of the pure VO2.

Temperature dependence of reconfigurable bandstop filters using vanadium dioxide switches

In this Letter, we report and investigate the temperature dependency of various radio frequency (RF) parameters for a fabricated reconfigurable bandstop filter with vanadium dioxide (VO2) switches measured up to 55 GHz. Here, the insulator-to-metal (ITM) and metal-to-insulator transition (MIT) hysteresis of the VO2 thin film influence on the RF characteristics of the filters is analyzed from 25 °C and 120 °C in heating and cooling. The resonance frequency and maximum insertion loss (IL) stability and sensitivity with temperature variations are explored. It is noticed that increasing the temperature to 50 °C from 25 °C (or decreasing it to 50 °C from 120 °C) will result in a less than 1% fractional frequency shift with respect to the off and on resonance frequencies. The sharp DC conductivity level variations of the VO2 thin film around the transition temperatures translate into sharp effects on the resonance characteristics of the filters. On the contrary, the maximum IL levels are less sensitive to the sharp conductivity changes of DC films around the VO2 transition temperature. A unique behavior is reported when successively heating-up and cooling-down, over and below, respectively, the transition temperature of VO2: the fabricated filter exhibits completely different resonance frequencies. This suggests that in the temperature dependence of the VO2 RF design, the practical use of reconfigurable RF functions has to take into account the history of thermal effects and increase or decrease in the device temperature when crossing the IMT/MIT transition point.

Thermally-switchable spectrally-selective infrared metamaterial absorber/emitter by tuning magnetic polariton with a phase-change VO2 layer

A metamaterial made of submicron aluminum disks on phase-change vanadium dioxide (VO2) thin film, which is synthesized by furnace oxidation method, has been fabricated by metal deposition, photolithography, and lift-off processes. By varying over-exposure time during the photolithography process with a stepper, metamaterials with submicron disk diameters down to 0.55 μm were successfully fabricated. Characterized by a Fourier transform infrared microscope, the metamaterial exhibits an absorption peak close to unity at the wavelength around 7 μm at room temperature as a selective absorber, while it is highly reflective once VO2 becomes metallic after phase transition at higher temperatures. Elucidated by numerical simulation, the spectral absorption peak is attributed to the excitation of magnetic polariton (MP) within the insulating VO2 film. Once the metamaterial is heated above the transition temperature of VO2, MP cannot be excited within the metallic VO2 film, resulting in disappearance or “switch-off” of the absorption peak. The switchable behavior is further explained by an equivalent inductor-capacitor circuit model, which predicted absorption peak wavelengths in excellent agreement with experimentally observed ones of fabricated metamaterials with different disk diameters. This thermally-switchable spectrally-selective metamaterial could facilitate applications in dynamic infrared camouflage and active radiative thermal management.

Gram scale synthesis of monoclinic VO2 microcrystals by hydrothermal and argon annealing treatment

We report gram scale synthesis of 100% phase fraction of VO2 (M) monoclinic in powder form with reversible phase transition by combining hydrothermal method and Ar annealing at high temperature. Optimization of single phase VO2 (M) growth and its phase transition characteristics have been analysed systematically by varying growth parameters such as time, synthesis temperature and post growth annealing conditions. Argon annealing of hydrothermally grown VO2 powders at 800 °C found to play key role in obtaining VO2 (M) phase in gram scale with characteristic phase transition temperature of 68 °C. In-situ TEM has been performed to investigate the microstructure and phase change across the annealing temperature. Detailed characterizations have been carried out to correlate the phases, microstructure and transition temperature of VO2 with respect to growth parameters.

Synthesis of Thermochromic Mo-Doped VO&lt;sub&gt;2&lt;/sub&gt; Particles

Mo-doped vanadium dioxide particles were produced by simple solution-based, rapid microwave assisted, and calcination in an inert gas method respectively. The precursors and the heat-treated final powders were characterized by X-ray diffractometer (XRD), differential scanning calorimeter (DSC), scanning electron microscope (SEM), and transmission electron microscope (TEM). The Mo dopant influenced the crystal structure and phase transition of VO2 (M) particles. The XRD pattern indicated that all Mo-doped VO2 particles were monoclinic crystal. The DSC results showed that phase transformation temperature of VO2 from monoclinic to tetragonal or rutile, which normally occurred at 67-68 oC was reduced to 43.18 oC, whereas over doping of Mo led to the increased transition temperature. The results suggested a significant effect of Mo doping content on the transition temperature of VO2. Doping with an optimum amount of Mo into VO2 structure can achieve the lowest phase transformation temperature of VO2 powders.

Effect of W addition on the electrical switching of VO2 thin films

Vanadium Oxide has been a frontrunner in the field of oxide electronics because of its metal-insulator transition (MIT). The interplay of different structures of VO2 has played a crucial role in deciding the magnitude of the first order MIT. Substitution doping has been found to introduce different polymorphs of VO2. Hence the role of substitution doping in stabilizing the competing phases of VO2 in the thin film form remains underexplored. Consequently there have been reports both discounting and approving such a stabilization of competing phases in VO2. It is reported in the literature that the bandwidth of the hysteresis and transition temperature of VO2 can be tuned by substitutional doping of VO2 with W. In this work, we have adopted a novel technique called, Ultrasonic Nebulized Spray Pyrolysis of Aqueous Combustion Mixture (UNSPACM) to deposit VO2 and W- doped VO2 as thin films. XRD and Raman spectroscopy were used to investigate the role of tungsten on the structure of VO2 thin films. Morphology of the thin films was found to be consisting of globular and porous nanoparticles of size ∼ 20nm. Transition temperature decreased with the addition of W. We found that for 2.0 at % W doping in VO2, the transition temperature has reduced from 68 o C to 25 o C. It is noted that W-doping in the process of reducing the transition temperature, alters the local structure and also increases room temperature carrier concentration.

Smart optically active VO2 nanostructured layers applied in roof-type ceramic tiles for energy efficiency

The capability to control in an smart way the infrared reflectance to environmental temperature variations can be achieved with thermochromic materials like VO2. In this paper, we report the application of VO2 on ceramic tiles aiming to control the reflected infrared radiation on smart roofs and thus improving the energy efficiency, which is associated to the reduction of the carbon dioxide emissions. The VO2 nanoparticles have been produced by hydrothermal synthesis assisted by microwave irradiation, providing a new, quicker and cleaner production route. Afterwards, the VO2 nanoparticles were transferred to the surface of ceramic glassy tiles, which were prepared through dispersing the nanopowders in water and followed by spray coating with posterior annealing treatments in order to promote the surface adhesion as well as the stabilization of the material monoclinic phase. The VO2 transition temperature was controlled by doping with tungsten and a reduction from 69°C to 49°C was achieved. The superior thermochromic characteristics of VO2 nanoparticles in conjunction with this new application to a smart roof offers a great potential to regulate the energy in an intelligent way.

Phase transition characteristics of vanadium oxide films under the CO2 laser irradiation

Vanadium oxide is a type of thermochromic material. At a certain temperature it undergoes a phase transition, The phase transition temperature of VO2 is about 68°C.Using the magnetron sputtering method, vanadium oxide thin films were prepared on glass substrates. Heat treatment was carried out. The film was analysed using X-ray diffraction (XRD) spectrum. The result shows that the main constituents of the film are VO2 and V2O3. A beam of CO2 laser was directed to the film. A laser power meter was used to measure the specular reflection power from the film. The test results showed that the temperature of the film and its base rose fast when the laser irradiated on it and a semiconductor-metal phase transition occurred in the film. As a result, the film's reflectivity grew high. The specular reflectivity of 10.6 μm laser from the film rose 76.4% after the phase transition.

Combinatorial Atmospheric Pressure Chemical Vapor Deposition of Graded TiO2–VO2 Mixed-Phase Composites and Their Dual Functional Property as Self-Cleaning and Photochromic Window Coatings

A combinatorial film with a phase gradient from V:TiO₂ (V: Ti ≥ 0.08), through a range of TiO₂-VO₂ composites, to a vanadium-rich composite (V: Ti = 1.81) was grown by combinatorial atmospheric pressure chemical vapor deposition (cAPCVD). The film was grown from the reaction of TiCl₄, VCl₄, ethyl acetate (EtAc), and H₂O at 550 °C on glass. The gradient in gas mixtures across the reactor induced compositional film growth, producing a single film with numerous phases and compositions at different positions. Seventeen unique positions distributed evenly along a central horizontal strip were investigated. The physical properties were characterized by wavelength dispersive X-ray (WDX) analysis, X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), and UV-visible spectroscopy. The functional properties examined included the degree of photoinduced hydrophilicity (PIH), UVC-photocatalysis, and thermochromism. Superhydrophilic contact angles could be achieved at all positions, even within a highly VO₂-rich composite (V: Ti = 1.81). A maximum level of UVC photocatalysis was observed at a position bordering the solubility limit of V:TiO₂ (V: Ti ≈ 0.21) and fragmentation into a mixed-phase composite. Within the mixed-phase TiO₂: VO₂ composition region (V: Ti = 1.09 to 1.81) a decrease in the semiconductor-to-metal transition temperature of VO₂ from 68 to 51 °C was observed.