Structure and optical properties of pulsed sputter deposited CrxOy/Cr/Cr2O3 solar selective coatings

Spectrally selective Cr(x)O(y)/Cr/Cr(2)O(3) multilayer absorber coatings were deposited on copper (Cu) substrates using a pulsed sputtering system. The Cr targets were sputtered using asymmetric bipolar-pulsed dc generators in Ar+O(2) and Ar plasmas to deposit a Cr(x)O(y) (bottom layer)/Cr/Cr(2)O(3) (top layer) coating. The compositions and thicknesses of the individual component layers have been optimized to achieve high absorptance (0.899-0.912) and low emittance (0.05-0.06). The x-ray diffraction data in thin film mode showed that the Cr(x)O(y)/Cr/Cr(2)O(3) coating consists of an amorphous phase; the Raman data of the coating, however, showed the presence of A(1g) and E(g) modes, characteristic of Cr(2)O(3). The x-ray photoelectron spectroscopy (XPS) data from near-surface region of the absorber suggested that the chemical state of Cr was in the form of Cr(3+) and no phases of CrO(2) and CrO(3) were present. The experimental spectroscopic ellipsometric data have been fitted with theoretical models to derive the dispersion of the optical constants (n and k). The optical constants of the three layers indicate that the bottom two layers are the main absorber layers and the top Cr(2)O(3) layer, which has higher oxygen content, acts as an antireflection coating. In order to study the thermal stability of the Cr(x)O(y)/Cr/Cr(2)O(3) coatings, they were subjected to heat treatment (in air and vacuum) at different temperatures and durations. The coating deposited on Cu substrates exhibited high solar selectivity (alpha/epsilon) of 0.895/0.06 even after heat treatment in air up to 300 degrees C for 2 h. At higher temperatures, the solar selectivity decreased significantly (e.g., alpha/epsilon=0.855/0.24 at 350 degrees C in air), which is attributed to oxidation of Cr crystallites, increased surface roughness, and formation of CuO. The formation of CuO and the increase in Cr(3+) vacancies due to the outward diffusion of Cr at higher annealing temperatures were confirmed by XPS. In the case of vacuum annealing, for temperatures greater than 500 degrees C the outward diffusion of Cu was the dominating degradation mechanism. The microstructural stability of the absorber coatings heat treated in air (up to 325 degrees C) and vacuum (up to 600 degrees C) was confirmed by micro-Raman spectroscopy measurements. Studies on the accelerated aging tests indicated that the absorber coatings on Cu were stable in air up to 250 h at 250 degrees C with a solar selectivity of 0.898/0.11. (c) 2008 American Institute of Physics.