Abstract |
Vanadium dioxide is one of the most promising thermochromic solid-state materials. It has potential application in the field of smart windows and glazings, due to its unique optical properties related to its inherent and reversible structural transition at a critical temperature of Tc
= 68 oC. In this thesis we present the motivations, challenges and results of our project “Thermochromic Vanadium Dioxide for energy saving applications: A study on solution-based synthetic approach”. In our hydrothermal synthesis approach, we started with reagents V2O5 as the vanadium source and oxalic acid as its
reducing agent. Then a study of the effects of different parameters on the solution-based synthesis
of VO2 was conducted by focusing on each parameter and characterizing the resulting products. We preferred to experiment with the chemical parameters of our synthesis, such as reagents concentration (molar ratio of precursors and V2O5
density in solution), synthesis volume (20 vs 40 mL), synthesis pH (acidic to basic), effect of additives (H2SO4 and thiourea) and effect of dopants (W and
Mg). The as obtained products were characterized by x-ray diffraction (XRD), differential scanning
calorimetry (DSC), field emission scanning electron microscopy (SEM), energy dispersive x-ray
(EDX) and thermogravimetric analysis (TGA). During the hydrothermal reduction, V2O5 was reduced into different vanadium oxides and also into different VO2 crystalline polymorphs. Then, in order to achieve better crystallinity and to fully convert the products to the desired thermochromic polymorph, our samples were annealed in a furnace under constant nitrogen gas flow, to avoid the oxidation of our samples. The final products were also characterized by the same techniques and results were discussed in comparison to the as obtained data and the synthesis parameter changes. We achieved to produce powders of high crystallinity pure thermochromic VO2 in high yields, and with good thermochromic properties with Tc ranging from 59 oC to 67 oC depending from the synthesis parameters configuration used. An optimized hydrothermal route for achieving better crystallinity products in high yields is proposed, and a possible explanation of the mechanism behind our synthesis is discussed.
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