Microstructure and thermal stability of Al/Ti0.5Al0.5N/Ti0.25Al0.75N/AlN solar selective coating

Solar selective coatings draw much attention due to its critical role in solar receiver employed in solar heat power generation. Al/Ti0.5Al0.5N/Ti0.25Al0.75N/AlN solar selective coating exhibits a high absorptance of 0.926–0.945 in solar spectrum range and low emittance of 0.04–0.06 (82 °C) in infrared spectrum range. The microstructure of the solar selective coatings by using Al as metal reflector layer had been analyzed by X-ray photoelectron spectroscopy (XPS), auger electron spectroscopy (AES), atomic force microscopy (AFM) and cross-sectional transmission electron microscopy (XTEM). After long time annealed in air, the coating is observed to lost effectiveness in solar spectrum adsorption. More Ti–N bonds rather than Ti–N–O bonds are observed in Ti0.5Al0.5N layer. The experimental results also indicate that the crystallization of the adsorption layers is not complete. It is believed that oxygen diffused into the Ti–Al–N layer to form Ti–Al–O–N structure, and the destruction of the four-layer structure, lead to the degradation of the coatings after long time (>192 h) annealed in air at high temperature (500 °C). Compared with the thermal stability in air, the coatings are much more stable in vacuum under high temperature, which makes it is more appropriate to be used in vacuum.

► Oxygen diffuses into Ti–Al–N layer forming Ti–Al–O–N layer at higher temperature.
► Four-layer structure is destroyed after 500 °C annealed in air.
► Solar selective coating is more appropriate to be used in vacuum.