Efficient VO2(M) synthesis to develop thermochromic cement-based materials for smart building envelopes

Abstract

The buildings constructions up to now have been shown to require a high energy consumption in their everyday functioning. In this sense, thermochromic materials are being studied to reduce the energy demands by improving the buildings energy efficiency and minimizing the heat island effects. They are smart materials that change their optical properties with temperature. The working principle for this application is based on thermochromic materials that promote the solar reflection in the building envelope in warm weather to limit the building overheating, and to facilitate the absorption in cold weather to transfer heat into the building. For this purpose, VO2(M), which has already been applied for smart windows, is being investigated to be applied for the thermochromic functionalization of buildings opaque components. This paper aims to study the more adequate VO2(M) production to be applied in cement-based materials for building envelopes. The synthesis of VO2(M) powder through hydrothermal synthesis routes is studied considering the most efficient and sustainable conditions. For this purpose, the VO2(M) one-step hydrothermal synthesis, starting from V2O5 and H2C2O4 precursors and performed at 240 °C is considered. The major objective is to provide a clearer definition of the hydrothermal synthesis parameters to produce VO2(M) with those conditions. The effect of the molar ratio of the precursors, their solid concentration and the synthesis time was analysed. Moreover, the two-step synthesis through VO2(D) transformation was also reproduced to compare the morphology and properties of the obtained products. X-ray diffraction and differential scanning calorimetry were done in the synthesized products to study the structural phase and the thermochromic transition, respectively. Results showed that the molar ratio of the precursors was shown to be a determining factor in the VO2(M) one-step synthesis, and that the obtained powder presented lower agglomeration in the one-step as compared with the two-step synthesis.