<title>Optimization of SiO2-TiNxOy-Cu interference absorbers: numerical and experimental results</title>

Abstract

SiO<SUB>2</SUB> - TiN<SUB>x</SUB>O<SUB>y</SUB>-Cu absorbers were prepared with activated reactive evaporation (ARE). The deposition parameters for the ARE process were adjusted according to the results of the numerical optimizations by a genetic algorithm. We present spectral reflectance, calorimetric and grazing incidence X-ray reflection (GXR) measurements. Best coatings for applications as selective absorber in the range of T equals 100 (DOT)(DOT)(DOT) 200 degree(s)C exhibit a solar absorptance of 0.94 and a near normal emittance of 0.044 at 100 degree(s)C. This emittance is correlated with the hemispherical emittance of 0.061 obtained from calorimetric measurements at 200 degree(s)C. First results on lifetime studies show that the coatings are thermally stable under vacuum up to 400 degree(s)C. The SiO<SUB>2</SUB> film passivates the absorber, a substantial slow down of degradation in dry air is observed. Our tests demonstrate that the coating will withstand break down in cooling fluid and vacuum if mounted in an evacuated collector.