Research Projects

Smart (insulated/energy saving) windows play an important role in minimizing the heat loss (lowering the thermal budget) in a heating- and cooling-intensive building without hindering the visible light transmission into the building. Similarly, these windows could play a major role in reducing the thermal budget (air conditioning cost) of offices and cabins positioned near the metallurgical furnaces, kilns where the ambient temperature could be very high. In another application, the external body of the spacecraft experiences a variation in temperature from +150ºC to -150ºC, whereas the onboard electronics can only operate reliably within a narrow tolerable limit, leading to the development of a cost-effective, lightweight and reliable passive smart radiator device (PSRD) paramount for economical space missions. The solution could be to use thin thermochromic coatings, which can change their optical properties based on the external environment. To this end, VO2 is an oxygen-deficient metastable oxide with a monoclinic crystal structure at room temperature (RT). Upon heating this oxide above ~68oC, this relatively low symmetric monoclinic, semiconducting phase transforms to a high symmetric rutile (R), metallic phase (heat reflector). However, to make VO2 applicable as a heat reflector for various applications, both the structural and semiconductor-to-metal transition temperature (Tc) need to be lowered to near RT, which could be achieved by appropriately straining the monoclinic to obtain rutile crystal structure. Here, the proposed project will search for the exact strain condition (generated chemically, mechanically or through combination of both the types) in VO2 which could bring down Tc to near RT, thereby stabilizing rutile phase at relatively lower temperature. As rutile crystal structure in VO2 is metallic in nature, the coating would then act as a hear reflector, reflecting outside heat and thereby keeping the building/spacecraft cooler. In this regard, first thermodynamic calculations will be carried out using Miedema’s semi-empirical approach, through which the total Gibbs free energy of VO2 on glass would be estimated by considering the Bulk Gibbs free energy of oxide, its surface and interface energy. A range of possible dopants of various sizes and valence states (W+6, Mo+6, Ta+5, Nb+5) would be used to induce chemical strain in VO2. Mechanical strain would be induced in VO2 by use of appropriate substrate with rutile crystal structure. Comparison between these energetics would give information on the choice of a dopant, its concentration, substrate type and/or combination of these factors that holds potential to reduce Tc close to RT. These optimized parameters would then be adopted experimentally to fabricate thin (chemically doped or undoped) VO2 film on appropriate substrate using RF magnetron sputtering. A series of characterization techniques would be used to ascertain Tc, reversibility, electrical and optical properties.