Infrared optical properties of LaCoO3 thin films for thermal regulation in solar collectors

Abstract

The perovskite LaCoO₃ can be used as a solar-selective layer in flat solar absorbers with an optimal thickness of approximately 600 nm. This thermochromic material is particularly well-suited for thermal regulation, with a significant increase in infrared emissivity occurring below the transition temperature. The optical properties of LaCoO₃ thin films deposited by magnetron sputtering were analyzed using Fourier Transform Infrared (FTIR) spectroscopy, measuring transmittance and reflectance variations across the mid-infrared spectral range. The transmittance and reflectance variations were plotted at a wavelength of 8 μm as a function of temperature. By identifying an inflection point, the derivative pinpointed the maximum thermochromic effect at approximately 300 °C. This methodology also facilitated comparison with VO₂-based selective coatings. Subsequently, a thermal camera was then employed to examine films of three different thicknesses, evaluated using two distinct methods. The first method consisted of plotting the infrared temperature measured by a thermal camera against the actual temperature. This technique highlighted the temperature dependence of the thermochromic effect in relation to film thickness. The second method, which accounted for all radiation contributions within the measurement environment, allowed the calculation of emissivity across Band III (7.5–13 μm). This approach revealed the onset of the thermochromic effect at approximately 80 °C for a film thickness of 551 nm.