A comparative study on the switching kinetics of W/VO2 powders and VO2 coatings and their implications for thermochromic glazing

Abstract

Monoclinic VO2 (M) displays thermochromic properties based on its reversible metal-insulator transition. We studied the kinetics of the underlying structural phase transition (SPT) from monoclinic VO2 (M) to rutile VO2 (R) and vice versa both in powders and coatings, using isoconversional kinetic analysis based on datasets obtained through differential scanning calorimetry and UV–vis–NIR spectrophotometry. For VO2 powders, prepared via solution-phase reduction of V2O5 with oxalic acid and subsequent thermal anneal, we show that the activation energy |Ea| of the SPT is temperature dependent, and decreases with increasing difference between the material's temperature and the critical switching temperature T0. |Ea| for both VO2 (M) to VO2 (R) and VO2 (R) to VO2 (M) is similar, and ranges between 138 and 563 kJ mol−1, depending on the temperature of the material. This indicates that similar defects play a key role in both SPTs. Upon doping with tungsten, T0 was lowered from 66.93 °C (0 at. % W) to −28.38 °C (3.5 at. % W). |Ea|, however, remained in the same range. Nonetheless, at W concentrations above 2 at. % a significant asymmetry was observed with higher |Ea| for the switch from VO2 (R) to VO2 (M). The SPT of VO2 (M) in coatings proceeded 4 times slower. This may result from the immobilization of the VO2 domains on the substrate surface and within the coating network, making the SPT more difficult to progress. The findings of this study have important implications for the application of VO2 (M) in energy efficient thermochromic glazing.