Stability of Thermochromic Vanadium Dioxide Thin Films in Harsh Environments of Temperature and Humidity

Abstract

Stability of thermochromic VO₂ thin films against harsh environmental conditions, which is important if the films are to be used in practical smart windows, was investigated. The films were prepared on normal soda-lime glass substrates by reactive radio frequency magnetron sputtering of metallic vanadium target (99.95% purity) at a working pressure of mbar, argon and oxygen flow rates of 76 and 1.4 – 3.0 ml/min, respectively, substrate temperature of 400 ^°C and sputtering power of 150 watts. After deposition, the films were exposed to extreme temperatures and relative humidity at 100 °C, 400 °C and 89%, respectively, for different time durations. Upon exposure, the films structural properties were investigated using the transmission electron microscope and the atomic force microscope, whereas the UV/VIS/NIR spectrophotometer was used to investigate the films’ optical properties. The results showed some degradation in the first high-temperature exposure cycle (at t = ~ 100 ^°C) but were quite stable, and with sufficient thermochromism, after the subsequent cycles. Exposure of the films to extreme temperatures at 400 ^°C, resulted in complete loss of thermochromism. Exposure of the films to extreme humidity (RH ~ 89%) displayed a gradual increase in degradation with exposure time; however, the films retained sufficient thermochromism even after 3 weeks of exposure. Possible explanations for the degradation mechanism are discussed by correlating the observed effects in the structural and optical properties of the films.