JPS6029867B2 - Solar heat selective absorption material and its manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a selective solar heat absorbing material, and particularly to a selective solar heat absorbing material having an aluminum base material on which a selective solar heat absorbing film is formed by electrolytic coloring, and a method for producing the same.

In this specification, the term "aluminum" is used to include aluminum-based alloys.

Electrolytic coloring is known as the most common method for forming a selective absorption film with good selective absorption characteristics and durability on the surface of an aluminum material, and has been widely used in recent years.

This method involves anodizing aluminum material in an acidic bath such as sulfuric acid, phosphoric acid, or oxalic acid to form a porous anodic oxide film on its surface, which is then coated with metal such as nickel salt or cobalt salt. Electrolytic treatment is performed in an aqueous solution containing a salt to reduce and precipitate the metal salt in the fine pores of the oxide film to turn it black, thereby obtaining the desired selective absorption film. By the way, the performance of a selective absorption film is generally evaluated based on the relative relationship between the absorption rate Q in the sunlight wavelength range and the emissivity in the long wavelength range.
The closer to 1, the better the quality.

Here, in order to improve the absorption rate Q, it is necessary to make the film good in colorability, and on the other hand, in order to lower the emissivity, it is necessary to make the film thickness permanently thin. It is necessary to do so. However, in the above electrolytic coloring method, in order to obtain a film with good colorability, if the thickness of the anodic oxide film is increased to increase the amount of precipitated metal,
The emissivity tends to increase with the absorption rate Q. On the other hand, if the thickness of the anodic oxide film is made thinner, the emissivity decreases, but the reduction and precipitation reaction of metal salts on the film increases, resulting in poor coloration and poor absorption Q. There is a dilemma of not being able to get something. In view of the fact that the selective absorption performance of solar heat selective absorption films made by electrolytic coloring is greatly influenced by the film structure produced by anodizing treatment, this invention aims to: This was done with the intention of improving the normal reduction and precipitation of metals and (2) increasing the absorption rate with as little amount of precipitated metal as possible.

As a result of intensive research aimed at overcoming the above technical problems, the present inventor has developed an ideal solution that satisfies the above requirements Obtained the knowledge that it is possible to form a film,
completed this invention. One aspect of the present invention is to provide a selective solar heat absorbing material having the above-mentioned anodic oxide film structure, which includes an anodic oxide film on the surface of an aluminum material and an electrochemical absorber in the micropores of the anodic oxide film. In a solar heat selective absorption film formed with precipitated and filled metal particles, the anodic oxide film is formed in two layers: an upper layer film with a coarse distribution of micropores and a similarly dense lower layer film. It is characterized by the fact that

Another aspect of the present invention relates to a method for producing a selective solar heat absorbing material as described above, which selectively absorbs solar heat by anodizing an aluminum material and then electrolytically treating it in an aqueous solution containing a metal salt. In the method for manufacturing a solar heat selective absorber that forms a film, the anodizing treatment is performed in two stages, a first and a second stage, and the anodizing voltage during the second stage treatment is equal to that of the first stage. This method is characterized in that the chemical formation voltage is lowered more rapidly than the chemical formation voltage during the treatment.

Through this two-stage anodizing process, in which the anodizing voltage is rapidly lowered between the first stage and the second stage, the upper and lower layers with different film structures are formed on the aluminum material surface. A two-layer anodic oxide film is produced, and this film structure can achieve the desired effect of increasing absorption rate and reducing emissivity. The type of treatment stage in which the anodizing treatment is performed is not particularly limited in both the first stage and the second stage, and an aqueous solution of phosphoric acid, sulfuric acid, oxalic acid, etc. can be used for 30 to 300 minutes.

Further, it is desirable that the temperature of the liquid for the treatment is about 15 to 35 qo. The first stage electrolytic treatment in the anodizing treatment is preferably performed by applying a voltage of 3 to 30 V and for a formation time of about 2 to 10 minutes.

When these are less than 3 V and less than 2 minutes, respectively, it is difficult to obtain a film with good metal reduction and precipitation reaction.
On the other hand, when the voltage exceeds 30 V and the formation time exceeds 10 minutes, the thickness of the upper layer film increases, leading to an increase in emissivity. The second electrolytic treatment performed after the first stage treatment is performed by rapidly dropping the voltage from the first stage formation voltage, and this causes the current recovery phenomenon, that is, as shown in Figure 1. As shown, the current does not flow immediately, but begins to flow gradually after several seconds to several minutes, and reaches a steady state even more slowly. , which also produces a lower layer film with a higher density of micropores.

Here, the second stage formation voltage E2 is in the range of 1 to 20 V, and the voltage drop ratio (E2/E,) with respect to the first stage formation voltage E, is 2/3 or less, especially 1/3. It is desirable to set the value to a value in the range of ~2'3. If this voltage drop ratio is smaller than 1'3, the absorption rate Q will be low,
On the other hand, if it is higher than 2/3, it will be difficult to achieve the desired effect of the present invention. Further, the formation time of the second stage electrolytic treatment is preferably about 5 to 20 minutes. The performance of the selective absorption membrane is greatly influenced by the formation time of this second stage, especially the formation time after current recovery.If this is less than 2 minutes, the absorption rate Q will be low, and if it is more than 10 minutes, the formation time will be significantly affected. When this happens, the film becomes thicker, resulting in an increase in emissivity. As a result of the two-stage anodic oxidation treatment described above, as shown in Figure 2, the surface of the aluminum material 1 is coated with an upper layer film with a relatively coarse distribution of fine pores chemically formed in the first stage electrolytic treatment. Underneath 2a, a lower layer film 2b with a relatively dense distribution of micropores is formed by the second-stage electrolytic treatment.
is generated, and an anodic oxide film 2 consisting of these two layers is formed.
is obtained.

The two-layer structure of the film 2 makes it possible to form an ideal solar heat selective absorption film that increases the absorption rate Q without increasing the emissivity. When considering the reason for this in comparison with a conventional selective absorption film having a single-layer structure anodic oxide film 3 as shown in Fig. 3, in the case of this conventional product, sunlight A incident on the film 3 is In contrast, in the case of a two-layer film as shown in FIG. 2 according to the present invention, each layer 2a, 2b
Since the film structures of the layers are different, differences naturally arise in the optical properties of each layer, and there is an opportunity for light that is not absorbed by the upper layer film 2a to be absorbed by the lower layer film 2b, resulting in an overall absorption rate Q of sunlight A. is expected to increase. Note that the conditions of the electrolytic coloring treatment in an aqueous solution containing a metal salt performed after the above-mentioned anodic oxidation treatment are not limited in any way in the present invention, and any conventional conventional treatment may be employed.

For example, in an aqueous solution containing metal salts such as nickel salts, cobalt salts, copper salts, tin salts, etc., at a voltage of 10 to 3 volts and a current density of o. 5
~2. Electrolytic treatment is performed by applying a current such as direct current, alternating current, or a waveform similar to the above, to precipitate and fill the fine pores of the anodic oxide film, thereby achieving the desired blackening of the film. achieve. In accordance with the above, the present invention has a selective absorption film on the surface of an aluminum material, which has a relatively thin thickness, does not increase the emissivity, and exhibits a high absorption rate Q with a small amount of metal precipitation. Therefore, it is possible to obtain a solar heat selective absorption material that has excellent performance in terms of solar heat selective absorption characteristics.

Next, specific examples of the present invention will be shown in comparison with comparative examples.

Example: Aluminum materials with a thickness of 1.5 and having a purity of JIS AI 050 were used, and after being subjected to the usual pretreatment, each of the first According to the electrolytic conditions shown in the table, two
Anodization treatment using direct current electrolysis was performed in stages.

The anodic oxide film thus formed on the surface of the aluminum material had a two-layer structure consisting of a coarse upper layer film with fine pores and a dense lower layer film. Then, this aluminum material after the anodizing treatment was subjected to a coloring electrolytic treatment using alternating current in an aqueous solution containing nickel sulfate 30 t/Z and boric acid 30 t/Z under the electrolytic conditions shown in Table 1. A solar heat selective absorbing material having the desired solar heat energy selective absorption film on the surface of an aluminum material was obtained.
Comparative Example Using the same aluminum material and the same electrolytic solution as in the above example, an anodizing treatment consisting of only a single step and then a coloring electrolytic treatment using alternating current were performed sequentially under the electrolytic conditions shown in Table 1. A selective solar energy absorption film was formed on the surface of the aluminum material.

In Comparative Examples 1 and 2, the anodic oxidation treatment was performed by DC electrolysis, and in Comparative Examples 3 and 4, AC electrolysis was performed. Table 1 When the absorption coefficient Q and emissivity of the selective absorption film of the solar heat selective absorption materials obtained in the above Examples and Comparative Examples were measured, the results shown in Table 2 were obtained.

Table 2

[Brief explanation of the drawing]

Fig. 1 is a graph showing changes in electrolytic voltage and current over time due to anodizing treatment in this invention, Fig. 2 is a cross-sectional view schematically showing the state of the anodic oxide film according to this invention, and Fig. 3 is a graph showing a conventional solar heat selective absorption method. FIG. 3 is a cross-sectional view schematically showing the state of the anodic oxide film in the film. 1... Aluminum material, 2... Anodic oxide film, 2a...
... Upper layer film, 2b... Lower layer film. Figure 1 Figure 2 Figure 3

Claims (1)

[Scope of Claims] 1. A solar heat selective absorption film comprising an anodic oxide film and metal particles electrochemically deposited and filled in the micropores of the aluminum material is formed on the surface of the aluminum material, wherein the anode A solar heat selective absorption material characterized in that an oxide film is formed in two layers: an upper film with coarse micropores and a lower film with dense pores. 2. In a method for producing a solar heat selective absorption material in which an aluminum material is anodized and then subjected to electrolytic treatment in an aqueous solution containing a metal salt to form a solar heat selective absorption film, the anodization treatment is performed in two stages, a first and a second stage. A method for producing a selective solar heat absorbing material, characterized in that the formation voltage in the second stage of treatment is lowered more rapidly than the formation voltage in the first stage of treatment. 3 The formation voltage during the first stage of anodizing treatment is 3 to 30V, and the formation voltage during the second stage is 1 to 30V.
3. The method for manufacturing a solar heat selective absorber according to claim 2, wherein the method is carried out at a voltage of 20 V and a voltage drop ratio of 2/3 or less.