Influence of the gradient distribution of VO2 particles on the performance of thermochromic smart window films.

Smart windows play an important role in regulating solar radiation and reducing building energy consumption. Thermochromic smart windows based on VO₂ nanoparticles offer great potential for large-scale applications. However, non-uniform distributions of particle size and volume fraction along the film thickness direction are often present in VO₂ films, which not only affect optical modulation but also induce uneven local phase transitions, further impacting thermal regulation and response efficiency. In this study, a multiscale coupled model was developed using the finite-difference time-domain (FDTD) method and the finite element method (FEM) to investigate how these non-uniformities influence the optical and thermal behavior of VO₂ nanoparticle films. When the particle size gradient increased from 30-40-50 to 30-50-100 nm, τ_lum dropped by 8.29% and Δτ_sol by 7.43%, with similar trends observed across different film thicknesses. In 15 µm films, the peak temperature of the film with a uniform particle size distribution is 2.31°C higher than that of the non-uniform film, which promotes more complete and more synchronous phase transition. The influence of non-uniform volume fraction on optical performance was limited, but its effect on local temperature response was pronounced. In 5 µm films, a downward-f_v distribution increased the peak temperature by 0.75°C. When the particle size variation is within 10 nm, a uniform model remains effective; however, with larger gradients or thicker films, fine modeling is required to ensure predictive accuracy. Optimizing particle size and volume fraction distributions can improve the optical-thermal response of VO₂ films and lower the required transition temperature.