A study on the change of VO₂ thin-film coating behavior according to the droplet size using ultrasonic spray

Global environmental regulations have accelerated the research on smart windows that can make efficient use of energy. Vanadium dioxide (VO₂) is a material whose crystal structure changes depending on the external environment and can reflect part of the sunlight. We manufactured smart window thin film through solution process using VO₂ nanoink instead of traditional dry process such as CVD and sputtering. VO₂ thin film was manufactured by ultrasonic spray coating onto PEN (polyethylene naphthalate) and the coating behavior of the thin film was studied by controlling the deflector flow rate, which is considered to have the greatest influence on thin-film formation among various process variables. The solvent contained in the droplet is affected by the air from the deflector, and as the deflector flow increases, the droplet size decreases due to the acceleration of solvent evaporation. The behavior of the thin-film formation depends on the fluidity of the droplets remaining on the substrate. If the fluidity of the droplets is high, the leveling effect will form a uniform thin film; otherwise the adjacent droplets may only combine to form a non-uniform thin film. When the deflector flow rate was 6 L/min, it was impossible to form a uniform thin film due to lack of fluidity.