Thermal stability of TiAlN/TiAlON/Si3N4 tandem absorbers prepared by reactive direct current magnetron sputtering

Spectrally selective TiAlN/TiAlON/Si3N4 tandem absorber was deposited on copper, stainless steel, and Nimonic substrates using a reactive direct current magnetron sputtering system. The absorptance and the emittance of the tandem absorbers were characterized using solar spectrum reflectometer and emissometer. The surface morphology of the tandem absorbers was studied using atomic force microscopy. The compositions and the thicknesses of the individual component layers have been optimized in such a way to achieve high absorptance (0.958) and low emittance (0.07 at 82 deg. C). In order to study the thermal stability of the tandem absorbers, they were subjected to heat treatment (in air and vacuum) at different temperatures and durations. The structural changes as a result of heating of the tandem absorbers were studied using micro-Raman spectroscopy. The tandem absorbers deposited on copper substrates exhibited high solar selectivity in the order of 9-10 even after heat treatment in air up to 600 deg. C for 2 h. These tandem absorbers also exhibited very high thermal stability (525 deg. C) in air for longer durations (50 h). The onset of oxidation for the tandem stack deposited on silicon substrates was 900 deg. C, indicating a very high oxidation resistance. The results of the present study indicate the importance of TiAlN/TiAlON/Si3N4 tandem absorber for high temperature solar selective applications