Light modulation in phase change disordered metamaterial - A smart cermet concept

Cermet coatings are popular solar selective absorbers as they allow capturing most of the solar energy while minimising radiative losses. Embedded metallic nanoparticles in dielectric matrices promote multiple internal reflection of light and provide an overall low emissivity. VO2 in the metamaterial state is regarded in this study as a responsive mixed phase comprising metallic rutile VO2 inclusions in semiconducting monoclinic VO2 phase mimicking cermet. The smart cermet responds to thermal stimuli by modulating the size of the metallic inclusions and thereby enabling the manipulation of their interaction with light. The highly reliable and reproducible response of the smart cermet corroborates with the observed ramp reversal memory effect in VO2. We demonstrate a thermally controlled 85% emissivity switch taking advantage of the narrow hysteresis and tuning abilities of the disordered metamaterial

Tunable thermochromic properties of V 2 O 5 coatings

Thermochromic Di vanadium pentaoxide (V2O5) coatings displaying a variety of colours were synthesised. Tuning of thermochromic behaviour was achieved via a controlled oxidative annealing under ambient air of the as-grown VOx films. Adjusting the oxygen deficiency in V2O5, allows tuning the colour of the films and as a consequence its thermochromic behaviour. Non oxygen deficient V2O5 did not feature any measurable thermochromis

Vanadium Oxide as a Key Constituent in Reconfigurable Metamaterials

Tunable materials are paving the way towards improved functionality of metamaterials. Vanadium oxide (VO2) with its prototypical near-room-temperature transition between phases featuring greatly contrasting electrical and optical behavior is an appealing candidate as an active component in metamaterials. However, it is seldom known that VO2 in itself has metamaterial characteristics. VO2 under certain temperature conditions demonstrates a phase coexistence enabling highly tunable electrical and optical properties. In this chapter, we describe how VO2 in its hysteretic region behaves as a smart responsive Metasurface with cutting edge applications

Visible thermochromism in vanadium pentoxide coatings

Although di-vanadium pentoxide (V2O5) has been a candidate of extensive research for over half a century, its intrinsic thermochromism has not been reported so far. Films of V2O5 grown on silicon, glass, and metal substrates by metal organic chemical vapor deposition in this study exhibit a thermally induced perceptible color change from bright yellow to deep orange. Temperature-dependent UV− vis spectroscopy and X-ray diffraction allow the correlation between the reversible continuous red shift of the absorption and the anisotropic thermal expansion along the (001) direction, that is, perpendicular to the sheets constituting the layered structure. Furthermore, the possibility of tuning the thermochromic behavior was demonstrated via a chemical doping with chromiu

Electrical Switching in Semiconductor-Metal Self-Assembled VO2 Disordered Metamaterial Coatings

As a strongly correlated metal oxide, VO2 inspires several highly technological applications. The challenging reliable wafer-scale synthesis of high quality polycrystalline VO2 coatings is demonstrated on 4” Si taking advantage of the oxidative sintering of chemically vapor deposited VO2 films. This approach results in films with a semiconductor-metal transition (SMT) quality approaching that of the epitaxial counterpart. SMT occurs with an abrupt electrical resistivity change exceeding three orders of magnitude with a narrow hysteresis width. Spatially resolved infrared and Raman analyses evidence the self-assembly of VO2 disordered metamaterial, compresing monoclinic (M1 and M2) and rutile (R)domains, at the transition temperature region. The M2 mediation of the M1-R transition is spatially confined and related to the localized strain-stabilization of the M2 phase. The presence of the M2 phase is supposed to play a role as a minor semiconducting phase far above the SMT temperature. In terms of application, we show that the VO2 disordered self-assembly of M and R phases is highly stable and can be thermally triggered with high precision using short heating or cooling pulses with adjusted strengths. Such a control enables an accurate and tunable thermal control of the electrical switching

Synthesis of vanadium oxide films with controlled morphologies: Impact on the metal–insulator transition behaviour

Precise control over the growth of VO2 films with different morphologies is achieved by varying the deposition parameters in the DLI-MOCVD process such as temperature, pressure, concentration of precursor and time of deposition. In this study, thin films of VO2 with wide range of morphologies having Metal to Insulator Transition (MIT) temperature of (τc) ∼ 52 °C were deposited. Adjusting the process parameters has allowed the growth of highly porous nanocrystalline films and dense microcrystalline films with controlled crystallite size up to several hundred nanometres. Vanadium (V) oxy tri-isopropoxide was used in this study as a single source precursor. Porous films lead to a diffuse change in resistivity across the transition temperature while the crystalline films have sharp and high resistivity drop (Δρ). This enabled a qualitative study of the MIT behaviour with respect to the microstructure of the films and correlates the effect of deposition conditions to the obtained morphologies. Fine control over the morphology without additional doping or post deposition process provides the ability to tailor VO2 thin films for their respective applications. Scanning electron microscopy, Raman spectroscopy and X-ray diffraction were used to characterize the microstructure of the films while electrical resistivity measurements were carried out to observe the MIT behaviour of the films

Kinetics of methane combustion over CVD-made cobalt oxide catalysts

Bahlawane N. Kinetics of methane combustion over CVD-made cobalt oxide catalysts. APPLIED CATALYSIS B-ENVIRONMENTAL. 2006;67(3-4):168-176.The present investigation provides the required kinetic parameters to evaluate and to predict the rate of the catalytic combustion of methane over cobalt oxide. For this purpose, monolithic cordierites with low specific surface area were uniformly coated with cobalt oxide thin films of controlled thickness using the chemical vapor deposition (CVD) process. The obtained catalysts were tested in the catalytic combustion of methane in oxygen-deficient and -rich conditions. Catalysts with loadings above 0.46 wt.% are active starting at a temperature of 250 degrees C and completely convert methane to CO2 below 550 degrees C where the conversion rate reaches 35 mu mol (CH4)/g(cat) s. The involvement of the bulk-oxide-ions in the catalytic reaction was supported by the constant value of the normalized reaction rate to the weight of deposited cobalt oxide. The experimental data fit well to the Mars-Van Krevelen redox model and can be approximated with a power rate law in oxygen-rich mixtures. The resulting activation energies and frequency factors allow the identification of the rate-limiting step and accurately reproduce the effect of the temperature and partial pressure of the reactants on the specific reaction rate. (c) 2006 Elsevier B.V. All rights reserved

Systematic microstructural investigation of alumina deposited by liquid fuel combustion chemical vapor deposition

Bahlawane N. Systematic microstructural investigation of alumina deposited by liquid fuel combustion chemical vapor deposition. SURFACE AND COATINGS TECHNOLOGY. 2006;200(12-13):4097-4103

The growth of nanoscale ZnO films by pulsed-spray evaporation chemical vapor deposition and their structural, electric and optical properties

Jiang Y, Bahlawane N. The growth of nanoscale ZnO films by pulsed-spray evaporation chemical vapor deposition and their structural, electric and optical properties. Thin Solid Films. 2010;519(1):284-288.Great interest in nanoscale thin films (sub-100 nm) has been stimulated by the developing demands of functional devices. In this paper, nanoscale zinc oxide (ZnO) thin films were deposited on glass substrates at 300 degrees C by pulsed-spray evaporation chemical vapor deposition. Scanning electron micrographs indicate uniform surface morphologies composed of nanometer-sized spherical particles. The growth kinetics and growth mode are studied and the relationship between the film thickness and the electric properties with respect to the growth mode is interpreted. X-ray diffraction shows that all ZnO films grown by this process were crystallized in a hexagonal structure and highly oriented with their c-axes perpendicular to the plane of the substrate. Optical measurements show transparencies above 85% in the visible spectral range for all films. The absorbance in the UV spectral range respects well the Beer-Lambert law, enabling an accurate optical thickness measurement, and the absorption coefficient was measured for a selected wavelength. The measured band gap energies exhibit an almost constant value of 3.41 eV for all films with different thicknesses, which attributed to the thickness-independent crystallite size. (C) 2010 Elsevier B.V. All rights reserved

Effect of Nucleation and Growth Kinetics on the Electrical and Optical Properties of Undoped ZnO Films

Jiang Y, Bahlawane N. Effect of Nucleation and Growth Kinetics on the Electrical and Optical Properties of Undoped ZnO Films. JOURNAL OF PHYSICAL CHEMISTRY C. 2010;114(11):5121-5125.Physical properties of ZnO thin films are of great importance for their potential application. In this paper, we reported the electrical and optical properties of undoped ZnO films with a high dependence oil the nucleation and growth kinetics. These ZnO films were deposited on glass substrates within the temperature range of 250-350 degrees C by pulsed-spray evaporation chemical vapor deposition (PSE-CVD). X-ray diffraction (XRD) shows that ZnO films grow in the c-axis orientated hexagonal structure. The inspection of the surface morphology with scanning electron micrographs (SEM) shows a substantial effect of the growth temperature, which was directly correlated with the nucleation kinetics. A comprehensive nucleation and growth process was also proposed for the interpretation of changeable film morphology and corresponding electrical conductivity. The films deposited at intermediate growth temperatures of 310-340 degrees C show compact structures and consequent high electrical conductivity, Which Was Still improved with increase of film thickness. Ill contrast to the electrical conductivity, the optical hand gal) rather shows a correlation with the size of the crystals forming the films rather than with their arrangement. The band gap energy decreases from 3.56 to 3.42 eV toward the bulk value Upon the increase of the crystallite size from 15 to 30 nm