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

Abstract

Monoclinic VO 2 (M) displays thermochromic properties based on its reversible metal-insulator transition. We studied the kinetics of the underlying structural phase transition (SPT) from monoclinic VO 2 (M) to rutile VO 2 (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 VO 2 powders, prepared via solution-phase reduction of V 2 O 5 with oxalic acid and subsequent thermal anneal, we show that the activation energy |E a | of the SPT is temperature dependent, and decreases with increasing difference between the material's temperature and the critical switching temperature T 0. |E a | for both VO 2 (M) to VO 2 (R) and VO 2 (R) to VO 2 (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, T 0 was lowered from 66.93 • C (0 at. % W) to 28.38 • C (3.5 at. % W). |E a |, however, remained in the same range. Nonetheless, at W concentrations above 2 at. % a significant asymmetry was observed with higher |E a | for the switch from VO 2 (R) to VO 2 (M). The SPT of VO 2 (M) in coatings proceeded 4 times slower. This may result from the immobilization of the VO 2 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 VO 2 (M) in energy efficient thermochromic glazing.