Sol-gel derived CuCoMnOx spinel coatings for solar absorbers: Structural and optical properties

A novel black coloured coating with the composition CuCoMnOx was prepared u sing sol-gel synthesis. The coatings were deposited using the dip-coating t echnique from alcoholic sols based on Mn-acetate and Co- and Cu-chloride pr ecursors. Thermogravimetric analysis showed that xerogels become crystallin e at 316 degreesC while X-ray diffraction analysis revealed that the coatin gs and powders correspond predominantly to CuCoMnOx spinels. Rutherford bac k scattering (RBS) and transmission electron microscopic (TEM) studies comb ined with energy dispersive Xray spectroscopy (EDXS) measurements confirmed that Cu, Mn and Co are present in the films in stoichiometric ratios close to that in the initial sols. IR spectroscopy has been employed to study th e formation of sols by following the changes in the vibrational bands of th e acetate groups during both thermal hydrolysis and the ageing of sols to x erogels. It was found that ageing of xerogels was accompanied by the format ion of -COO- bridging units, which at 250 degreesC are no longer visible in the IR spectra but substituted by the vibrational modes characteristic for CuCoMnOx. The solar absorptance (a(s)) and thermal emittance (e(T)) of the coatings when deposited on an Al-substrate are a(s) = 0.9 and e(T) = 0.05, which rank deposited black sol-gel CuCoMnOx spinels among the promising ca ndidates for spectrally selective absorber coatings for solar collectors an d solar facades. (C) 2001 Elsevier Science B.V. All rights reserved.