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 VO 2 (M) to rutile VO 2 (R) and vice versa were studied for particulate materials obtained by bead milling of VO 2 (M) powder. Using wet bead milling, we decreased the particle size of VO 2 (M) powder from ∼1 μm 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 | E α | decreases with increasing difference between the actual temperature of the material and its switching temperature ( T 0 ). Furthermore, | E α | decreases with progressive milling, and kinetic asymmetry is induced. For milled particulate materials, | E α | is lower for the switch from VO 2 (R) to VO 2 (M) than for the opposite switch. For hydrothermally synthesized nanoparticles, | E α | 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 VO 2 particles have potential for use in energy efficient thermochromic windows. • Switching enthalpy of VO 2 particulate material decreases with progressive milling. • Activation energy of the structural phase transition decreases with particle size. • Progressive milling induces asymmetry in switching kinetics of particulate material. • Hydrothermally synthesized particles display inverse asymmetry in kinetics. • Milled VO 2 particulate materials are suited for use in thermochromic windows.