Deposition and characterization of TiAlN/TiAlON/Si 3N 4 tandem absorbers prepared using reactive direct current magnetron sputtering

Abstract

Spectrally selective TiAlN/TiAlON/Si 3N 4 tandem absorber was deposited on copper, glass, stainless steel, nickel and nimonic substrates using a reactive direct current magnetron sputtering system. In this tandem absorber, TiAlN acts as the main absorber layer, Si 3N 4 acts as an antireflection coating and TiAlON acts as a semi-absorber layer. The tandem absorber was characterized using solar spectrum reflectometer and emissometer, cross-sectional transmission electron microscopy (XTEM), selected area diffraction, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy (XPS) and phase-modulated ellipsometry techniques. The compositions and thicknesses of the individual component layers were optimized to achieve high solar absorptance and low thermal emittance. The XPS data indicated that, for the TiAlN and Si 3N 4 layers, nitrogen was attached to Ti, Al and Si, whereas prominent peaks pertaining to TiO 2 were observed in the case of TiAlON. The XTEM data indicated that both the TiAlN and TiAlON layers were nanocrystalline, whereas Si 3N 4 was amorphous in nature. The optimized tandem absorber deposited on copper substrate exhibited an absorptance of 0.958 and an emittance of 0.07. The high solar absorptance of the tandem absorber was achieved by increasing the refractive index from the surface to the substrate. The high thermal stability of the component layers indicates the importance of TiAlN/TiAlON/Si 3N 4 tandem absorber for high temperature solar selective applications.