The impact of bead milling on the thermodynamics and kinetics of the structural phase transition of VO2 particulate materials and their potential for use in thermochromic glazing

Abstract

The thermodynamics and kinetics of the structural phase transition from monoclinic VO2 (M) to rutile VO2 (R) and vice versa were studied for particulate materials obtained by bead milling of VO2 (M) powder. Using wet bead milling, we decreased the particle size of VO2 (M) powder from similar to 1 mu m to 129 nm. With progressive milling, the switching enthalpy decreased from 47 J g(-1) to 29 J g(-1) due to a loss of crystallinity. The switching kinetics were studied using Friedman's differential isoconversional method. The activation energy vertical bar E-alpha vertical bar decreases with increasing difference between the actual temperature of the material and its switching temperature (T-0). Furthermore, vertical bar E-alpha vertical bar decreases with progressive milling, and kinetic asymmetry is induced. For milled particulate materials, vertical bar E-alpha vertical bar is lower for the switch from VO2 (R) to VO2 (M) than for the opposite switch. For hydrothermally synthesized nanoparticles, vertical bar E-alpha vertical bar is in the same order of magnitude, albeit with inverse switching asymmetry. Latter may result from different defects that are introduced during both preparation techniques. Applying layers of milled particulate material to glass sheets yielded thermochromic coatings with luminous transmission of 40.7% and solar modulation of 8.3%. This demonstrates that milled VO2 particles have potential for use in energy efficient thermochromic windows.