Chemistry: electrical and wave energy – Processes and products
Patent
1981-06-29
1983-12-20
Williams, Howard S.
Chemistry: electrical and wave energy
Processes and products
126901, 148 627, 427160, C23F 706, C25D 1104
Patent
active
044216122
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention pertains to a process for the preparation of a dark-coloured, wave-length selective oxide film on aluminium.
Providing a dark-coloured, wave-length selective oxide film on aluminium results in products which are suitable for use, inter alia, as thermal solar panels or, due to their characteristic appearance, for decorative purposes.
In the ensuing discussion the term "spectral selectivity" will first of all be described in connection therewith that aluminium, obtained by the process according to the present invention, will be essentially perceived to the eye as having a black colour. Thereafter processes for the black-colouring of aluminium will be described, in order to show that the invention is founded on other principles than those previously known. Finally, the results from optical measurements which have been carried out, as well as some illustrative Examples, will be presented.
As is known, an absolutely black surface will absorb most radiation, but it will, according to Kirchhoff's equation, also have the highest loss of heat through radiation.
Collectors for solar heat should have a high absorption coefficient for the frequencies which the main portion of the energy of the sun rays have, at the same time as the radiation of heat should be as low as possible. The heat radiation has a much lower frequency than the main portion of the solar radiation. Thus, it is desirable that the absorption coefficient vary with the frequency of the radiation, so that the absorption coefficient will be high for high frequency and low for low frequency, i.e., a spectral selectivity which is suitable for thermal solar panels.
In addition to the classic division into black, grey, white, reflecting and bright surfaces, there exist also surfaces having a spectrally selective film. The appearance of a surface having a spectrally selective film will vary with the wave-length for the change from low to high reflection, and for solar panels it is desired that the change is to occur at a higher wave-length than is perceived by the eye. A surface of a spectrally selective film for solar panels will usually be conceived by the eye as nearly black.
The reduced radiation of long-wave infrared radiation is important for a solar panel, as it thereby is possible to obtain a higher temperature than for an absolutely black surface. Whereas an absolutely black surface will attain equilibrium between incoming and emitted radiation at about 80.degree. C. at about 60.degree. latitude, it is not unrealistic to expect temperatures of over 200.degree. C. for equilibrium for radiation equilibrium for aluminium treated with the process according to the present invention. The increased temperature is of decisive importance for the possibilities of economical storage of solar heat.
The "classic" example of a surface having a spectrally selective absorbing film is so-called "black chromium" (chromium black) reported some years ago by G. E. McDonald (Sol. Energy 17, 119 (1975)). The correct explanation of the phenomenon was found by Fan et Spura (Appl. Phys. Lett. 30, 511 (1977)) who stated that these layers consisted of polycrystalline Cr.sub.2 O.sub.3 admixed with extremely fine-grained chromium metal.
As aluminium treated by the process according to the present invention is given a nearly black appearance, the main features of known processes for black-colouring of aluminium will be described. A series of mutually related processes is based on the precipitation of black salts on the surface and another series of processes is to apply a porous oxide film on aluminium by anodization and fill dyestuff into the pores. To illustrate the first group of processes, it is to begin with referred to an example where also the thermal characteristics are noted.
According to Dutch Patent No. 21 066 76 copper-containing aluminium alloys can be black-coloured by means of a two-stage chemical process wherein the first bath consists of water to which NiCl.sub.2, NH.sub.4 Cl and KCNS have been added, and the nex
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patent: 4145234 (1979-03-01), Meissner
patent: 4145462 (1979-03-01), Kuwabara et al.
J. R. Rairden et al., A Chemically Formed Oxide Coating on Aluminum, Electrochemical Technology, vol. 4, No. 1-2 (1966), pp. 63-69.
Institutt for Energiteknikk
Leader William
Williams Howard S.
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