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Title Παραμετρική μελέτη υδροθερμικής σύνθεσης θερμοχρωμικού διοξειδίου του Βαναδίου VO2(M)
Author Δραγάνη, Θεοδώρα
Thesis advisor Μπίνας, Βασίλειος
Γκαγκαουδάκης, Εμμανουήλ
Reviewer Μακρής, Κωνσταντίνος
Απεραθίτης, Ηλίας
Abstract VO2 vanadium dioxide is widely used in various technological fields with that of energy efficient "smart" windows attracting intense interest due to the inherent and reversible structural transition, from single monoclinic to tetragonal rutile structure, which takes place at a critical transition temperature of T=68o C. This structural change is accompanied by changes in the electrical and optical properties of the material. In specific, at temperature higher than TC the material is converted from a semiconductor to a metal reflecting the infrared part of the solar radiation while at the same time a satisfactory percentage of visible light is permeable. In the present dissertation the development of thermochromic materials took place through the method of hydrothermal synthesis using V2O5 (vanadium pentoxide) being the source of vanadium while various acids were used to reduce V2O5 to vanadium dioxide. During hydrothermal reduction V2O5 was converted to different vanadium oxides as well as different crystalline polymorphs of VO2. The resulting materials were then subjected to thermal annealing at high temperatures (>500o C), in a suitable furnace under constant nitrogen flow to prevent oxidation, in order to obtain pure VO2 of high crystallinity. The present work was carried out in three axes. In the first part of experiments as mentioned above, various reducing agents were used to obtain VO2 and as a result two acids were finally selected as the subject of the study of the rest parameters of hydrothermal synthesis. The selection criteria were the formation of pure VO2(M) as well as the high yield of hydrothermal reaction, which results from the comparison of the quantity of the final product with the initial quantity of V2O5. For the two selected reducing agents, the effect of the additional urea and thiourea reagents as well as the annealing temperature on the structural, morphological and thermochromic characteristics of the samples was studied. The final products were characterized structural by X-Ray Diffraction (XRD) technique, morphologically using Scanning Electron Microscope (SEM), while the thermochromic characteristics were determined by Differential Scanning Calorimetry (DSC). Thus, pure VO2 in the form of powder, with high crystallinity and improved thermochromic properties achieving a low phase transition temperature of TC≈63.0o C was synthesized using hydrothermal reactions with high yields.
Language Greek
Issue date 2021-03-24
Collection   School/Department--School of Sciences and Engineering--Department of Physics--Graduate theses
  Type of Work--Graduate theses
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