Abstract
The thermal stability of ferritic stainless steel conversion coatings for selective solar absorbers was studied in air and under low oxygen pressure (partial vacuum 2 × 10 -2 Pa) using optical techniques, X-ray diffraction, reflection high energy electron diffraction and secondary ion mass spectrometry analyses. Stability of optical selectivity was demonstrated up to 450°C in air and 500°C under vacuum. However, changes in the coating composition were observed in this temperature domain: the oxygen diffuses into the coating and the oxidation of the substituted magnetite phase (which is the main coating component) leads to α-Fe 2O 3 and (Fe 1− x Cr x ) 2 O 3 in air and to FeCr 2O 4 under low oxygen pressures. At higher temperatures substantial degradation of the optical properties was observed: the solar absorptance decreased and the emittance increased, causing high radiance losses. This phenomenon is related to substrate oxidation (which produces an increase in the coating thickness) and to large chemical modifications induced by chromium diffusion into the coating from the substrate.
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