A Comparative Study on the Switching Kinetics of W/VO2 Micron-Sized Powders and Nanoparticles
Lavinia Calvi1,2, Luc Leufkens3,4, Cindy P. K. Yeung3,4, Roberto Habets3,4, Romy van Geijn3,4, Janique Hupperetz3,4, Daniel Mann3,4, Ken Elen1,2, An Hardy1,2, Marlies K. Van Bael1,2, Pascal Buskens1,3,4,5
1Hasselt University, Institute for Materials Research (IMO), DESINe Group, Hasselt, Belgium; 2IMEC Vzw, IMOMEC Associated Laboratory, Diepenbeek, Belgium; 3The Netherlands Organisation for Applied Scientific Research (TNO), Eindhoven, the Netherlands; 4Brightlands Materials Center, Geleen, the Netherlands; 5Zuyd University of Applied Sciences, Heerlen, the Netherlands.
Monoclinic vanadium dioxide VO2 (M) is a thermochromic material based on its reversible metal-insulator transition. The underlying structural phase transition (SPT) involves a transition from VO2 (M) to rutile VO2 (R) and vice versa. To apply VO2 (M) in thermochromic energy efficient glazing, dopants are required to lower the transition temperature, T0, from 68°C to 20-30°C. Employing nanoparticles in this application is interesting as it may reduce Mie scattering. We investigated the switching kinetics of undoped and W-doped VO2 (M) micron-sized powders and nanoparticles. These studies were carried out using data obtained from differential scanning calorimetry and isoconversional methods for the kinetic analysis. Undoped and W-doped VO2 (M) powders were prepared via solution-phase reduction of V2O5 with oxalic acid followed by thermal annealing. Nanoparticles were obtained through bead milling of the powders in 2-propanol.
The results show that the activation energy of the SPT is temperature dependent and decreases when the difference between the material temperature and T0 increases. Additionally, the activation barrier for both VO2 (M) to VO2 (R) and vice versa is similar for W-doped VO2 (M) powders, and ranges between 138-563 kJ·mol-1. Indicating that similar defects are involved in the SPT. Wet bead milling effectively decreased the VO2 particle size from 0.5-10 μm in powders to 50-500 nm for nanoparticles dispersed in 2-propanol. This leads to a decrease in switching enthalpy from 47 to 28 J·g-1, amongst others caused by partial loss of crystallinity during bead milling. The activation energy decreases with decreasing particle size. Simultaneously the asymmetry in the switching kinetics increases. This suggests that the defects introduced into the crystal structure by bead milling affect the transitions VO2 (M) to VO2 (R) and vice versa differently. In conclusion, nanoparticles provide additional benefits from the switching kinetics point of view, whereas the switching thermodynamics rather need to be compromised on.