2020
In this article, we developed a robust thermochromic coating for smart windows: a material that changes how it handles solar infrared light as temperature changes, while staying clear and durable on ordinary architectural glass. The active material is vanadium dioxide, or VO₂. Around its phase-transition temperature, VO₂ shifts from a semiconducting to a more metallic state, allowing it to regulate heat-carrying infrared radiation. This could lower cooling demand, but conventional VO₂ coatings often struggle with visible transmission, haze, durability and compatibility with float glass.
We addressed these constraints by combining VO₂ with silica in a sol-gel coating on silica-coated float glass. The silica barrier protects against interaction with the soda-lime glass, while the mixed coating controls the size and distribution of the VO₂ domains. By varying the VO₂-to-silica composition, thickness and curing conditions, we investigated how the microstructure could balance thermochromic switching with optical quality.
The process had to respect the limits of float glass. After dip coating, we treated the samples first in air to remove organic material and then in inert gas to crystallise VO₂. The crystallisation temperature is limited by the glass softening temperature, so process control is crucial. The resulting films remained optically homogeneous with very low haze and glare—essential conditions for a coating that has to look like a usable window, not a laboratory sample.
We found that phase separation of VO₂ and SiO₂ creates a particularly useful architecture. The silica-rich matrix allows small, well-dispersed VO₂ domains, while the barrier layer helps protect the system. The coatings combine strong solar modulation with high visible transmittance, two properties that are normally difficult to achieve together. We also tested mechanical and environmental robustness rather than considering optical performance alone.
For us, the importance of this work is that it moves a promising material closer to real glazing. Buildings need to limit heat gains without giving up daylight, clarity or façade durability. A passive coating that responds to temperature could help achieve that without sensors, wiring or active control. Our results show a route to transparent, resilient smart windows compatible with widely used architectural glass.