Structural and IR spectroscopic analysis of sol-gel processed CuFeMnO4 spinel and CuFeMnO4/silica films for solar absorbers

Black colored CuFeMnO4 spinel powders and films were prepared using sol-gel process from Mn-acetate and Fe- and Cu-chloride precursors. Films were deposited by dip-coating technique and heat-treated at 500 degreesC. For CuFeMnO4/silica films 3-aminopropyl-triethoxysilane (3-APTES) or tetraethoxysilane (TEOS) were used in molar proportion (Mn : Cu : Fe) : silica = 1 : 1. Films and powders were prepared by heating at 500 degreesC. IR spectroscopic measurements were employed to follow the hydrolysis-condensation reactions in (Mn : Cu : Fe)/3-APTES sols hydrolysed with water, and (Mn : Cu : Fe)/TEOS sols hydrolysed with (NH3)(a)q (Stober processing). The resulting coatings were examined with transmission electron microscopy (TEM) combined with electron dif-fraction analyses, Rutherford back scattering (RBS) and proton induced X-ray emission (PIXE) techniques. Results revealed that (Mn : Cu : Fe)/3-APTES films had a composite structure consisting of the upper Cu1.4Mn1.6O4 spinel and the lower amorphous SiO2 layer. RBS measurements confirmed the composite structure, showing also that the composition of the film was Mn : Cu : Fe = 1 : 0.96 : 0.29, i.e. close to the precursors ratio Mn : Cu : Fe = 3 : 3 : 1. (Mn : Cu : Fe)/TEOS films prepared from sols which were catalysed with (NH3)(a)q consisted of amorphous monodispersed spherical SiO2 particles with a size of about 400-420 nm. Solar absorbance (a(s)) and thermal emittance (e(T)) values of CuFeMnO4 (500 degreesC) and (Mn : Cu : Fe)/TEOS films (500 degreesC) showed that CuFeMnO4 films could be used as potential selective coatings for solar absorbers in solar collector systems.