<title>Thermal and chemical metallic-dielectric transitions of TiNxOy-Cu absorber tandems</title>

Abstract

Reliability and structural properties of selective TiNO-Cu tandem absorbers for solar applications at high temperatures were investigated. A tempering process which improves thermal emittance of the coatings was observed. Degradation is monitored by directional total thermal emittance. Four steps of degradation have been separated and confirmed by XRD, EDAX, REM and optical spectroscopy. Coatings show crystalline phases and transitions to amorphous due to degradation and tempering. They are susceptible to oxidation and to reactions with the substrate.

Review of Mid- to High-Temperature Solar Selective Absorber Materials

“…This destroys the selective properties and mechanical stability of the film. The final fourth step is the reconstruction of a chemically stable amorphous film of titanium oxides and crystalline copper oxides with 1-µm craters located at previous pore sites [106]. However, the formation of copper oxide is operative only at elevated temperatures; in the operational range of 350°C, the outer surface is nitrogen rich, not oxygen rich, contrary to the previous literature, and Ti ions are progressively replaced by Cu diffusing from the substrate [107].…”

“…[104,105]. These coatings are stable in high vacuum, but degrade (mainly the emittance) quickly when exposed to air, of even low partial pressure, at elevated temperatures [106]. After examination of the degradation mechanisms, improvements have been made in the α/ε(100°C)=0.94/0.08-0.12 by replacing the Cu with Al and adding an SiO 2 AR layer, but the construction still needs to be optimized [105].…”

Optical Properties and Microstructure of TiNxOy and TiN Thin Films before and after Annealing at Different Conditions

“…Different types of SSA, such as metal-dielectric composite, multilayer absorbers and semiconductor-metal tandems have been developed due to their higher thermal absorbance, higher thermal stability and low IR emittance [3][4][5][6]. In recent years, many types of selective solar absorber coatings for mid-to-high-temperature applications were studied, but only a few of them have been successfully commercialized.…”

Optical properties of titanium oxynitride thin films and application in selective solar absorber

“…This is because N is replaced by O in TiNxOy due to the high reactivity between Ti and O atoms. When the annealing temperature is larger than 450 C, the TiNxOy thin film becomes unstable even in a low pressure atmosphere, and the detailed degradation process has been reported by H. Schellinger [130]. To reduce the convective loss, the heat collecting tube is normally encapsulated in a glass tube, and the chamber between the two tubes is vacuumed [131].…”

“…As the absorbing layer in the SSA, TiNxOy thin film has a lot of advantages, such as extraordinary optical properties, high hardness, good chemical and thermal stability, and environmental friendliness [130,126,24]. Furthermore, the absorption property of TiNxOy can be tuned by changing the N/O atomic ratio [134,177]; for example, it was reported that the optical properties of the TiNxOy films fabricated by reactive mid-frequency magnetron sputtering from TiN target can be controlled in terms of the reactive gas flow rate [24].…”