Application-oriented VO2 thermochromic coatings with composite structures: Optimized optical performance and robust fatigue properties

For decades, VO2-based smart windows have aroused much interest in energy efficient materials. To meet the practical requirements, VO2-based thin films should exhibit a high quality performance in equilibrating the solar modulation ability (ΔTsol) and the luminous transmittance (Tlum) simultaneously. More importantly, durability and fatigue performance, which determine the life-time in actual use, have rarely been properly explored and obviously cannot be neglected. Herein, SiNx/VO2, VO2/SiNx and SiNx/VO2/SiNx multilayer structures were proposed and prepared by reactive magnetron sputtering in this work. The amorphous SiNx not only acts as the antireflection layer for promoting optical performance, but also serves as the ions diffusion barriers or passivation layers on the bottom and/or top of VO2. As expected, the optimized composite structures successfully enhanced the solar modulation ability (from 10.8% to 14.5%, 34% maximum increase rate of ΔTsol) and luminous transmittance (from 36.1% to 40.4%, 12%maximum increase rate Tlum-L) compared with single-layer VO2, in accordance with our optical modeling. More significantly, fatigue property had been unprecedentedly proposed and measured in about 2 years, demonstrating an ultra-long heating-cooling cycling times (104 cycles) in ambient atmosphere for SiNx/VO2/SiNx structure, exhibiting a dramatic life-time of 27 years, while the single layer VO2 could only provide the life-time of 4 years (1.6 × 103 cycles). Moreover, SiNx/VO2/SiNx structure also presented excellent durability on hot and wet condition, which is three times as the VO2 film without SiNx. Complementally, the improvement of the hydrophobicity with SiNx surface was found by contact angle test, which would make it possible to form an unobstructed view on rainy day. In conclusion, our works can provide facile structures and novel assessment method on fatigue performance for VO2-based films, and promote the industrial application for smart windows.