New composite absorbers were developed on aluminium and cooper substrates by spray pyrolysis technique. To enhance the heat-transfer process a thin (0.5 mm) sheet of high-thermal-conductivity material (Al, Cu) was used as substrate. For increasing the adherence properties, the aluminium surface was subjected to chemical rinsing in alkaline solutions (10-15 g/L NaOH, 30-50 g/L Na2CO3, 30-50 g/L Na3PO4). Afterwards the samples were anodized in nitric acid solution for 10 minutes at 3A. The copper substrate was mechanically polished with sandpaper No 800, and then washed with deionised water before each deposition.

Due to atmosphere and/or thermal oxidation, the copper substrate surface can actually be described as Cu/CuOx, while the aluminium substrate consists of a thin alumina layer on aluminium, Al/Al2O3. The black nickel was deposited on Al/Al2O3 and Cu/CuOx (samples of 31.5 x 34 cm) in the Thin Film Laboratory from the Centre: Product Design for Sustainable Development. Thus, the absorber plates have the following structures:

(1) Cu/CuOx/NiO;

(2) Al/Al2O3/NiO.

Aqueous solution of Ni(CH3COO)24H2O (99% Across Organics) was used as precursors. Low concentration (50 ppm) of maleic anhydride copolymers (Petru Poni Institute of Macromolecular Chemistry, Romania) were used to tailor the surface morphology. In order to minimize the reflection losses a TiO2 thin layer was deposited with the same technique (SPD).The optimized deposition parameters were previously reported [7-9]. The solar absorptance (aS) and the thermal emittance (sT) of the I. R. component layers and full absorbers, determined from the reflectance spectra [10] are presented in Table 1.

The addition of the antireflexive coating has no major influence on the selective coatings, still it was used because the environment resistance is enhanced by the titania film.

In developing the layered selective coatings the reciprocal infiltration is targeted. Thus, the first (oxidized) layer has a very high roughness and the second (and third) SPD deposited layers are actually filling a mezzo-porous surface and develop a smooth, regular morphology. The surface roughness of the absorber effectively increases the thermal emittance, inducing radiation reflection and scattering compared to a smooth surface [11], Table 1. The solar absorber (2) is smoother then (1), thus better collector’s efficiency of the solar collector is expected.

Considering the results, one can conclude that an average roughness of 30 nm is enough to insure convenient emittance values, below 0.1.

7th to 10th October, Lisbon — Portugal *