Proprietes Thermochromcs des Couches Minces de VO2 Adjointes au Titane Metallique
Abstract
As a result of a structural phase transition near the critical temperature of 68 ° C, crystalline vanadium dioxide (VO2) undergoes a rearrangement of its electronic structure manifested as a change in its optical, electrical and magnetic properties. The observed change of these parameters is often very pronounced and occurs abruptly while crossing the critical temperature, making this material interesting not only from a theoretical viewpoint but also in terms of its utility for switchable device design. In particular, since VO2 thin films switch from transparent to reflective in the infrared region, it is well suited as a coating for solar heating management. This work asse11s radio-frequency sputtering with post-annealing as a method of depositing this substance onto metallic titanium which, in tum, is the material of choice to cover the satellite surface. Spectrophotometry and ellipsometry with near TR, visible and UV wavelengths (i.e. the solar irradiation spectrum) demonstrate the presence of pronounced interference effects that, along with specific thicknesses ( 400 nm), they can produce large changes in reflectance and thus in absorption as well. Likewise, the characterization of emissivity by Fourier transform infrared spectroscopy (FTIR) in the 1 - 25 mum spectral range indicates that the system in question has a relatively low radiative cooling rate change but that it is comparable to other sources found in literature. Measurements of the optical properties, with fixed wavelengths, as well as those of the sheet resistance, obtained by direct current 4-point probe, as a function of temperature adopt a hysteresis profile during a heating and cooling cycle. A variation in the width of this hysteresis and the corresponding critical temperatures is observed from one sample to another. Atomic force microscopy (AFM) shows that this effect is associated with the morphology of microstructured VO2 films, but it is confirmed that it is also influenced by the presence of the metallic substrate. Specifically, the hysteresis width is systematically narrowed by the influence of Ti. However, the control of this phenomenon proved difficult because of the poor reproducibility of the morphological characteristics. X-ray photoelectron spectroscopy (XPS) indicates a possible correlation between this problem and VO2 stoichiometry. Attributed to the exposure to air, this technique also demonstrates that the degree of oxidation of VO2 increases over time at least at its surface. Despite these moderate difficulties, it seems that VO2 thin films coupled with metallic Ti has sufficiently good characteristics to be considered as a solar heating management solution. Technical improvements along with optimizations can maximize, among other things, the efficiency of the change of the visible absorption as well as infrared emissivity. On the other hand, XPS and X-ray diffraction depth analysis of the films allows to study in detail the oxidation by air as well as the behavior of Ti at the contact interface. In addition, the influence of arbitrary metallic substrates seems like a fruitful avenue not only for thermal systems for satellites but also for optical switching devices in general.
- Publication:
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Masters Thesis
- Pub Date:
- 2019
- Bibcode:
- 2019MsT.........10C
- Keywords:
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- Physical chemistry;Condensed matter physics;Materials science