Resistant transition-metal-nitrides based coatings for solar energy conversion

A four-layered Cr/CrAlN/CrAlON/CrAlO tandem coating was annealed at 500 °C in air for as long as 1000 h. It was interesting to find that the absorptance was increased from 0.90 to 0.92 while the emittance was decreased from 0.20 to 0.14 after the annealing treatment. The microstructural analysis suggested that the enhanced selectivity was caused by the formation of AlN, CrN and Cr2N nanocrystallites in the CrAlN and CrAlON amorphous matrices and Al2O3 and Cr2O3 nanocrystals in the CrAlO antireflection layer after annealing. Furthermore, the excellent thermal tolerance was primarily ascribed to the self-passivation of Al atoms, which would partially oxidize during annealing in air and thus result in the formation of aluminum oxide layer covering these nanoparticles. These results suggest that incorporating an amorphous-phase structure consisting of transition-metal-nitrides embedded in amorphous matrices is an attractive strategy to obtain high performance cermet-based solar absorbers for photo-thermal conversion.