Synthesis and Thermokinetic Analysis of Thermochromic VO2 Particles for Smart Window Applications

Abstract

Vanadium dioxide (VO2) is often suggested as one of the most promising thermochromic materials for energy-efficient smart window applications due to its ability to undergo a reversible, structural phase transition (SPT) from monoclinic VO2(M) to tetragonal rutile VO2(R) at 68°C. This transition is accompanied by a distinct change in near-infrared (NIR) optical transmittance, enabling solar modulation without external power input. However, its practical implementation is hampered by complications such as high switching temperature, optical haze due to large particle size, and reduced functionality by doping and processing. In this work, we report on the scalable synthesis, processing, and thermokinetic characterization of undoped and W-doped VO2 particles, targeting their application as thermochromic pigments for smart windows. [1,2] Undoped and W-doped VO2 powders were synthesized via reduction of V2O5 using oxalic acid in aqueous medium, followed by a controlled two-step calcination process. Undoped VO2 exhibited a switching enthalpy close to the theoretical maximum (55 J·g⁻¹) and a narrow hysteresis. The introduction of W dopants in the VO2 crystal lattice lowered the SPT temperature at an average rate of 23°C per at% of W. Doped VO2 particles containing 2 at% of W exhibited an SPT temperature of 21ºC while maintaining a high switching enthalpy of 37.5 J·g⁻¹. The large initial particle size of 24 µm was successfully reduced by subsequent bead milling yielding sub-micron VO2 particles of approximately 120 nm, though partial amorphization led to a 30-40% crystallinity loss, effectively halving the switching enthalpy. Using the Friedman isoconversional method, the thermodynamic and kinetic profiles of the SPT were investigated as functions of particle size and doping percentage. The activation energy for the phase transition of VO2(M) to VO2(R) decreased from 1610 to 381 kJ·mol⁻¹ upon bead milling, correlating with a loss in enthalpy. This processing step also introduced a kinetic asymmetry in the reversible phase transition, where the transition of VO2(R) to VO2(M) demonstrates a lower activation energy compared to the opposite transition. W-doping introduced additional structural defects, further modulating switching kinetics. Below 2 at-% W, transitions remained quasi-symmetric. Above this threshold asymmetry emerged, with this time a lower activation energy for the transition of VO2(M) to VO2(R). These observations