JPS5939666B2 - Method for forming solar energy selective absorption film - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for forming a selective absorption film that efficiently absorbs solar energy and causes little radiation loss of the absorbed energy.

The surface temperature of the sun is about 6000°, and most of the radiated solar energy has a spectrum in the wavelength range of 0.3-2μ.

However, the temperature of objects on the ground (such as solar collector plates) heated by solar heat is usually 60 to 100 degrees Celsius.
In many cases, even solar collectors with special structures
It rarely exceeds 0°C.

Therefore, the energy emitted from these objects falls into the near-infrared region and is distributed over a wavelength range of about 2 to 20 microns.

The absorption rate α of solar heat using this wavelength range shift is high,
Various methods have been proposed for producing so-called selective absorption films that have a low emissivity ε from the object itself.

As one method for producing a selective absorption membrane, aluminum or an aluminum alloy is anodized in an acid or alkaline solution capable of forming a porous oxide film, and then electrolytically colored in a solution containing a metal salt. There is a method of depositing metal in the pores of the film.

Usually, anodizing treatment is performed in a solution of sulfuric acid, phosphoric acid, oxalic acid, chromic acid, or a mixture of two or more of these, and the metals deposited in the pores include Ni msn,
Examples include Co and Fe.

In order to increase the absorption rate α of solar energy, it is desirable to increase the amount of precipitated metal to make it appear black.

In order to increase the amount of precipitated metal, it is easier to increase the thickness of the anodic oxide film, but as the film thickness increases, the emissivity ε in the infrared region increases.

Furthermore, when a metal is precipitated, the emissivity ε tends to increase depending on the thickness of the precipitated metal itself, so the metal does not need to be deposited to the full depth of the pore from the bottom of the pore to the opening surface.
A thickness of 0.2 to 0.3 microns is often sufficient.

Therefore, it is desirable to reduce the thickness of the anodic oxide film to the minimum necessary to lower the emissivity.

However, if the thickness is less than the thickness of the oxide film, it is usually 0.
．． If the value is less than 4 to 0.5μ, it becomes difficult to precipitate a sufficient amount of metal uniformly within the pores. Conventionally, due to this restriction, a sufficiently large absorption rate α and a sufficiently small emissivity ε were used. It was not possible to form a selective absorption film having the following properties on the surface of an aluminum material.

The present invention skillfully overcomes the above-mentioned limitations and forms a solar energy selective absorption film with excellent performance on the surface of an aluminum material.

That is, the method of the present invention will be explained based on a schematic enlarged cross-sectional view of an aluminum material. An anodic oxide film consisting of a thick porous layer 2 and pores 3 is formed on the aluminum base material 1 (the first
Figure A), then a suitable electrolytic coloring treatment is carried out to deposit the metal 4 sufficiently and uniformly within the pores (Figure 1B).

Thereafter, this aluminum material (including alloys, the same shall apply hereinafter) is immersed in an acid or alkaline solution capable of dissolving the oxide film to form an anodic oxide film without dissolving the metal 4 deposited in the pores 3. In order to dissolve the surface layer of (Fig. 1C),
This reduces the emissivity ε and improves the selective absorption of solar energy.

In the anodizing process, the thickness of the oxide film is 0.2 to obtain sufficient and uniform metal deposition in the next electrolytic coloring process.
The electrolytic solution and electrolytic conditions should be selected to form a film with a thickness of ~1.0μ, preferably about 0.4~0.7μ. This is carried out using one or more kinds or an alkaline solution such as sodium phosphate or sodium hydroxide.

At this time, the standard current is direct current, but it is not limited to this.

Next, the electrolytic coloring treatment can be carried out under the usual AC electrolytic coloring treatment conditions, but it is preferable to increase the amount of precipitated metal as much as possible to make it appear black in order to increase the absorption rate, and depending on the bath, the DC method may also be effective. be.

Further, it is appropriate that the thickness of the deposited layer be about 1/2 of the thickness of the oxide film, that is, about 0.2 to 0.3 μm.

In the immersion process of aluminum material after electrolytic coloring, the metal precipitated in the pores will not dissolve, and acid or alkaline solutions may be used as long as the porous layer above the precipitated layer is dissolved, and the degree of dissolution will depend on the precipitated metal. to the extent that it does not fall off easily,
The area with virtually no precipitated metal (anodized film) is 0.0
It is good if 5 to 0.2μ remains.

At that time, the type, concentration, temperature, etc. of the acid or alkali should be set at a time that allows the dissolution rate to be easily controlled industrially, for example, about several minutes.
, about 1 to 3 is preferable.

Further, as the alkali, sodium hydroxide can be used, and as the acid, one or more of sulfuric acid, chromic anhydride, phosphoric acid, and oxalic acid can be used.

Therefore, as the acid or alkaline solution, the solution used as the anodic oxidation bath may be used as it is, which has the advantage of facilitating process control.

In addition, in order to reduce the elution of precipitated metals, the above-mentioned dipping treatment may be performed after electrolytic coloring and sealing treatment.
Although the emissivity ε increases somewhat, it maintains good quasi-selective absorption,
In addition, corrosion resistance and weather resistance can be imparted to the film.

Next, a sealing treatment may be performed after the dipping treatment (although in this case as well, the emissivity ε will increase somewhat, but a similarly good selective absorption film can be obtained).

According to the method of the present invention, it is possible to form an excellent solar heat selective absorption film on the surface of an aluminum material, which was difficult to achieve using only the conventional electrolytic coloring method. It is something that contributes.

Hereinafter, the method of the present invention will be explained in more detail with reference to Examples.

Example 1 Aluminum extrusion material A6063 (flat bar) was pretreated (including alkali etching) by a conventional method, and then heated to 0.28A in a 6 (H'/l) phosphoric acid solution at 30°C.
Anodic oxidation was performed by direct current for 13 minutes at a current density of /di to obtain an anodic oxide film with a thickness of about 0.4 μm.

This extruded material was mixed with nickel sulfate BOY/l and boric acid 40
It was electrolytically colored at 160 cycles at 30° C. for 7 minutes at 15 V AC in an electrolytic coloring bath having a composition of f/l, sulfuric acid magnetic screw + 7A30r/11, and a surface close to black was obtained.

Next, this material was mixed with 25% of chromic anhydride. /13°phosphoric acid (85%) 35rrll/l phosphoric acid-chromic acid bath p
Immerse it in H0.5 (room temperature), change the immersion time, observe the change in the surface, and determine the absorption rate α from the spectra.
, and the emissivity ε was determined by a thermal method using an emissivity meter.

The results are shown in Table 1. As shown in Table 1, after 60 minutes of immersion, α did not decrease at all, and ε increased from 24% to 1
The ratio decreased to 7%, and the α/ε ratio improved from 3.8 to 5.4.

However, after 24 hours of immersion, the oxide film was completely lost.
The precipitated nickel became easily desorbed.

Therefore, the processing time in this case is preferably about 1 hour.

Example 2 A black aluminum material (A6063) obtained by surface treatment and electrolytic coloring in the same manner as in Example 1 was immersed in the same phosphoric acid-chromic acid solution heated at 40°C as in Example 1. , the results shown in Table 2 below were obtained.

Heating the bath is effective in shortening the immersion treatment time, and
It was possible to significantly reduce (improve) the emissivity without significantly impairing the absorption rate.

After immersing the sample for 10 minutes, the sample was sealed for 15 minutes with boiling water containing a commercially available nickel salt sealant, and ε=
Although it increased slightly to 20 (%), the effect of the immersion treatment was still recognized.

Example 3 An aluminum material that had been surface-treated and electrolytically colored black in the same manner as in Examples 1 and 2 was immersed at 35°C in a phosphoric acid bath with a pH of 1.5 having the same composition as that used for anodizing. , the following table 3
I got the result.

As shown in Table 3, by immersion for up to about 5 minutes, ε could be significantly reduced (improved) without impairing α.

However, decolorization was observed after 10 minutes. In addition, the sample after electrolytic coloring was subjected to sealing treatment for 15 minutes in boiling water containing a commercially available nickel-chlorine sealant to obtain a sample with ε = 32 (%). ε=
17 (%), which resulted in a slightly lighter color, but the degree of this was lighter than in the case where the pores were not sealed.

A similar effect was observed in the immersion treatment after sealing.

Example 4 Aluminum plate (AIlo) pretreated according to the standard method
o) trisodium phosphate (decahydrate) 100 ? /1
The sample was anodized in a DC bath (30° C.) at 2 A/d m for 2.2 minutes, and then electrolytically colored under the same conditions as in Example 1 to obtain a black surface.

The emissivity of this product was ε = 22 (%), but when immersed for 30 seconds in the bath used for anodizing (pH 12), the emissivity decreased (improved) to ε = 17 (%), although the appearance did not change. I was able to do that.

[Brief explanation of the drawing]

FIGS. 1A, B, and C are schematic enlarged cross-sectional views of the aluminum material after anodizing treatment, electrolytic coloring treatment, and surface layer dissolution of the oxide film, respectively. DESCRIPTION OF SYMBOLS 1... Aluminum base material, 2... Porous layer, 3... Pores, 4... Precipitated metal.

Claims (1)

[Scope of Claims] 1. After anodizing aluminum or an aluminum alloy to form a porous film on the surface, and then depositing metal in the pores of the film by metal salt electrolytic coloring treatment,
A method for forming a solar energy selective absorption film, which comprises immersing the film in an acid or alkaline solution that dissolves the film to dissolve the surface layer of the film. 2. The method of forming a solar energy selective absorption film according to claim 1, wherein the film-soluble acid or alkaline solution is the same as the anodizing bath.