JOURNAL
OF M A T E R I A L S
SCIENCE
LETTERS 5 (I986) 715-716
Characteristics of a selective coating obtained by chemical conversion of an aluminium substrate S. A B I S , M. L. LORENZETTI Alumina SpA - Istituto Sperimentale Metalli Leggeri, via G. Fauser 4, 28100 Novara, Italy
Selective films are commonly used in the exploitation of solar energy to meet better performances of passive solar collectors. They are mainly characterized by a high value of the ratio as/5 between the solar absorptivity, a~, and the thermal emittance, 5, by which it is possible to obtain an high collecting efficiency of the solar radiation energy in the visible range (high a~), and a low loss of energy by thermal radiation in the near infrared (low value of 5). Selective stacks can be produced by several techniques (for a review, see [1]), but to be of industrial interest three important criteria must be fulfilled: good performance, good durability, low cost. The last of these must reflect, in particular, the important aspect of pollution resulting, for example, from the use of treatment baths containing dangerous metallic ions. A new kind of selective film has been recently proposed based on the chemical conversion of an aluminium substrate [2]. The particular bath required for deposition makes this film particularly interesting for its low cost (very few problems of pollution) and good durability. In this letter we describe some characteristics of the stack based on optical and Auger measurements. The sample preparation is mainly based on chemical conversion of an aluminium alloy achieved by immersion of the alloy in a treatment bath for times varying from 30 to 180 rain at a temperature of 90 to 120° C; the water-based bath contains 1.7 to 1.9 gl -~ of clay pellets normally used to resolve problems of thermal insulation in buildings (Fig. 1). Following this method, a series of samples has been prepared, starting from conditions indicated in Table I, by using rectangular specimens (50 x 100 x 2mm)obtained from extruded strips of AA 6060 aluminium alloy.
T A B L E I Composition of the bath and parameters used in obtaining the deposition of the selective coating A m o u n t of pellets Time of immersion Tempeature pH
1.9gl 120 min 98 ° C 8.5*
*The value of pH is kept constant by the action of pellets until exhaustionof the bath. A typical high-angle X-ray analysis of powders obtained from pellets is presented in Fig. 2. Diffraction peaks show the presence of calcium, sodium, potassium and aluminium salts, S i O 2 a s cristobalite and an amorphous component attributed to silicates at low 20 angles. The optical response of the stack has been detected by using a Perkin Elmer DK2 in the range of wavelength 0.36 to 2.7 #m (MgO as reference) and a Perkin Elmer IR21 in the range 2.5 to 10pm. A typical reflectivity curve is reported in Fig. 3. The curve shows the typical trend of a selective stack, with the drop in reflectivity (at a value of - 50%) near 2.5 #m; by an integration of the curve, it obtains a value of as of 98% following the formula fo [1 - R(2)] A(2) d2 a s
fo A(2) d2 where A(2) is the solar spectrum at sea level [3]. An estimation of the emittance e obtained by integrating the whole experimental R(2) curve, following the formula
fo [1 - R(2)] W(2) d2 f ? W(2) d)~
H
Figure 1 Typical aspect of a clay pellet used in aqueous bath to obtain the selective film.
0261-8028/86 $03.00 + .12 © 1986 Chapman and Hall Ltd.
(W(A) is the Planck spectrum at 70 ° C) gives a value of 7%. Auger ESCA examinations have been performed on two representative samples obtaining the depth distribution of the constituent elements; the thickness has been estimated as 0.13 #m. Table II reports the results of the XPS analysis, and depth profiles are shown in Fig. 4. As can be seen, the film contains essentially aluminium, silicon, oxygen and calcium, as A1203, SiO2 and CaO; SiO2 and CaO have been probably trapped in the oxide bulk during the growth of the stack. In conclusion, selective coatings deposited on aluminium substrates by using low-cost water-based baths containing clay pellets show good optical 71 5
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Figure 3 Reflectivity curve of the selective coating. The curve of reflectivity of aluminium is reported too, as calculated by using values tabulated in [4] in the visible range.
Figure 5 Comparison between the experimental R(2) curve the calculated values for a layer of AI203 0.13 #m thick and the aluminium substrate.
properties for applications in aluminium passive solar collectors. The selectivity characteristics of the coating should be attributed, probably, to a strong thin-film interference effect. Reflectivity calculations, performed on a singlelayer model comprising a stack of A12030.13 #m thick on an aluminium substrate, confirm this hypothesis. Results of calculations carried out by assuming a value of two for the dielectric permeability of A1203 (irrespective of wavelength) as suggested in [5] and optical characteristics for the aluminium substrate tabulated in [4] are shown in Fig. 5. A r.m.s, value of
T A B L E I I XPS analysis performed on the selective film
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At (x0.5)
Element
Composition (at %)
Oxygen Aluminium Silicon Calcium
64.2 16.8 (as A13+) 9.8 (as Si4+ ) 9.2 (as Ca 2+ )
roughness of 0.04 has been used, as suggested in [6], to take into account the variations in thickness of the layer. The process of deposition of the stack is similar to the aluminium conversion described previously [7], but a simpler and more economical process is required for production.
References
100_
1. W. F. BOGAERTS and C. M. LAMPERT, J. Mater. Sci. 18 (1983) 2847. 2. P. FIORINI, et al., Italian Patent request n. 234 62 A/82 (1982). 3. K. L. COULSON, "Solar and Terrestrial Radiation Methods and Measurements" (Academic, New York, 1975). 4. H. EHRENREICH et al., Phys. Rev. 132 (1963) 1918. 5. A. ANDERSSON et al., J. Appl. Phys. 51 (1980) 754. 6. H. DAVIES, Proc. IEEE 101 (1954) 209. 7. ALCOA, US Patent no. 397 1674 (1976).
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Time(min) Figure 4 Auger-depth profiles of the components of the film.
716
Received 23 December 1985 and accepted 8 January 1986
