Thermochromic glass laminates comprising W/VO2 nanoparticles obtained by wet bead milling: An in-depth study of the switching performance

Abstract

The switching performance of W/VO2 nanoparticles in thermochromic glass laminates was investigated. W/VO2 powder was prepared, and displayed a phase transition temperature and switching enthalpy of 20.9 degrees C and 37.5 +/- 0.2 J g- 1, respectively. Using wet bead milling, the particle size was reduced from 24 +/- 2 mu m to 120 +/- 10 nm. In the same process, the switching enthalpy decreased to 18.2 +/- 0.6 J g-1 due to partial loss of crystallinity. The kinetics of the structural phase transition were studied using Friedman's differential isoconversional method. This demonstrated that the activation energy |E alpha| was inversely proportional to the square of the difference between the material's temperature and the critical switching temperature T0, pointing out that nucleation kinetics were determining the rate. Furthermore, |E alpha| decreased upon milling, and kinetic asymmetry was induced. The milled nanoparticles were compounded with PVB to produce thermo-chromic films, which were applied for laminating glass plates. The impact of nanoparticle size and concentration on the resulting optical properties of the laminate, viz. solar transmission and solar modulation, was studied in detail. The highest solar modulation obtained was 9.4%. The results obtained in this study are of direct importance for the application in smart windows, showing that (i) the W/VO2 particle size needs to be <= 100 nm to avoid excessive haze, (ii) both powder production and bead milling require further process optimization to minimize functional performance losses, and (iii) T0 should be set about 3 degrees C lower to ensure a sufficiently fast switch at the temperature of choice.