Air-based sputtering deposition of TiNxOy films for solar selective absorber coatings applications

Titanium oxynitride (TiNxOy) thin films with tunable electrical and optical properties were produced by sputtering using air as a reactive gas at a high base pressure of 1.3 × 10−2 Pa. Pure N2 and O2 with a fixed ratio of 79/21 was also used as a reference reactive gas (denoted as S-air) at a low base pressure of 6.7 × 10−4 Pa to investigate the influence of trace components in air on the characteristics of the films. By merely changing either the air/Ar or the S-air/Ar flow ratio within suitable processing windows, crystal phases, microstructures, electrical resistivities, and optical properties could be tailored. In practice, the TiNxOy thin films produced by such air-based sputtering could turn from conductive, semiconductive, to dielectric and would correspondingly change from opaque, translucent, to transparent. For solar selective absorber (SSA) coatings evaluations, bilayer and gradient TiNxOy-based SSA structures were tested, respectively. The gradient TiNxOy SSA coatings exhibited the best performance with solar absorptance of 86.3% and thermal emittance of 11.3%. This facile air-based sputtering technique with such a gradient coating design has great potential for solar selective absorber coatings applications.