JPH0220909B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION OBJECTS OF THE INVENTION Industrial Application Field The present invention relates to solar heat collectors, and in particular to selective absorption surfaces thereof.

Conventional technology The absorption surface of a solar heat collector has a high energy absorption rate in the solar radiation wavelength band (wavelength 0.3 to 2.5 μm), and has a blackbody radiation wavelength band (with the same temperature as the collector operating temperature). For example, when the operating temperature is 100°C, a spectral characteristic that provides a low emissivity at a wavelength of 3 to 50 μm is required (first
(see curve in figure). Absorption surfaces with such spectral characteristics are generally called selective absorption surfaces, and absorption surfaces coated with a copper oxide film and absorption surfaces coated with a black nickel plating film are being put into practical use, but these are not durable. It has shortcomings in properties, heat resistance, adhesion, etc., and is also economically problematic.

Problems to be Solved On the other hand, rustless steel has excellent heat resistance and corrosion resistance, and the present invention chemically colors black stainless steel with such excellent heat resistance and corrosion resistance. The purpose is to use this as a selective absorption surface for solar heat collectors.

[Structure of the invention]

Means for Solving the Problems The solar heat collector of the present invention, which can achieve the above-mentioned objects, is produced by activating the surface of ferritic or austenitic stainless steel, and then applying acid black oxidation or alkaline black oxidation. It can be obtained by using a selective absorption surface colored black by an oxidation method as an absorption surface of a solar heat collector.

For blackening treatment of rustless steel, acidic black oxidation method,
Alkaline black oxidation method, sulfur oxidation method, molten salt bath method,
Various treatment methods are known, such as electrolytic melting treatment. Such black chemical conversion treatment methods are described, for example, in Stainless Steel Handbook (Nikkan Kogyo Shimbun), pp. 858-859, and New Edition Surface Treatment Handbook (Sangyo Tosho Co., Ltd.), p. 486.
Although described in pages 1 to 488, only the acidic black oxidation method and the alkaline black oxidation method are suitable for the purpose of the present invention.

The acidic black oxidation method used in the present invention is
An aqueous solution containing sulfuric acid and an oxidizing agent such as dichromate, nitric acid or nitrate, vanadate.
It refers to a method of blackening the surface of stainless steel by heating it to 90°C or higher and immersing it in the same.Also, the alkaline black oxidation method used in the present invention is a method of blackening the surface of stainless steel by heating it to 90°C or higher and immersing it in the same. and an aqueous solution containing subcomponents of inorganic acid salts such as sulfates, phosphates, nitrates, and nitrites, and promoting components of oxidizing metal compounds such as lead peroxide and ferric hydroxide, at a temperature of 90°C or higher. A method of blackening the surface of stainless steel by heating it and immersing it in it.

When a film is formed by chemical conversion treatment on stainless steel as a selective absorption surface for a solar heat collector, the film formed by the sulfurization oxidation method is unstable in the atmosphere, and the molten salt bath method and electrolytic treatment method are There are many problems with temperature and processing equipment. On the other hand, the oxide films produced by the acidic black oxidation method and the alkaline black oxidation method are very stable and pose no problems during treatment.

Further, in performing such a blackening treatment, an activation treatment is required as a pretreatment. The activation treatment used in the present invention is to remove oxide films and contaminants from the surface of stainless steel, and to obtain a heat-absorbing surface that is uniform and has good selectivity. A method of cleaning the surface of stainless steel using an aqueous solution containing an inorganic acid such as nitric acid or perchloric acid. Methods for activating the surface of stainless steel include immersion in a mixture of 1 volume of nitric acid and 1 volume of water for about 1 hour;
2-3 in an aqueous solution of 30% perchloric acid and 1% potassium chloride.
A chemical activation treatment method such as a partial immersion method is suitable. Generally, when using the alkaline chemical conversion method, only degreasing is performed and no activation treatment is performed, but in the present invention, the activation treatment is also performed when using the alkaline black oxidation method. Also, points that require attention in the activation treatment are the concentration and treatment time. If the stainless steel surface is excessively roughened, the absorption range of the selective absorption surface will be at wavelength 3.
It extends to the infrared region of ~8 μm and is spectrally unfavorable. According to the present invention, a good selective absorption surface can be obtained by subjecting stainless steel to an appropriate activation treatment and then subjecting it to acidic black oxidation or alkaline black oxidation.

Rustless steels are broadly classified into martensitic, ferritic, and austenitic steels based on their heat treatment characteristics and microstructures. Among these, martensitic stainless steels have problems in weldability, and are therefore not recommended as a selective absorption surface for solar heat collectors. It's inappropriate.

Treatment Example 1 The surfaces of ferritic and austenitic stainless steels were activated with a nitric acid mixture, and then black oxidized under the following bath composition and treatment conditions.

Sodium dichromate 100g/Sulfuric acid 400g/Immersion treatment at 106-108℃ for 30-35 minutes The spectral reflection characteristics of the absorption surface thus obtained are shown in Figure 1 in comparison with absorption surfaces obtained by other methods. . In the figure, the curve shows the spectral reflection characteristics of the absorption surface of the ferritic stainless steel according to the present invention with an oxide coating, and the curve shows the spectral reflection characteristics of the absorption surface of the austenitic stainless steel according to the invention with an oxide coating. The curve shows the spectral reflection characteristics of an absorption surface made of copper coated with an oxide film using the alkaline black oxidation method, and the curve shows the spectral reflection characteristics of an absorption surface made of iron with appropriate strike plating and then black nickel plating. The curve is the ideal spectral reflection characteristic of the selective absorption surface of the collector with an operating temperature of 100℃.

The absorption surface coated with a copper oxide film exhibits a very high reflectance at long wavelengths of 4 μm or more, but in the solar radiation wavelength range (0.3 to 2.5 μm), including the diffuse reflectance, It exhibits a reflectance 3 to % higher than that of an absorption surface (curved and curved) with an oxide film applied to a stainless steel surface. On the other hand, with the selective absorption surface made of ferritic stainless steel shown by the curve according to the present invention, the reflectance is almost negligible at wavelengths of 2.0 μm or less, and the reflectance is quite high at wavelengths above that. Even when compared with a selective absorption surface made of copper, a good selective absorption surface with no significant difference can be obtained. Furthermore, as shown by the curve, the selective absorption surface made of austenitic stainless steel has a slightly lower reflectance in the wavelength band of blackbody radiation at the same temperature as the collector operating temperature, compared to the selective absorption surface made of ferritic stainless steel. , the excellent corrosion resistance of austenitic stainless steel, although its spectral properties as a selective absorption surface are somewhat inferior;
Considering weldability, it has sufficient value for commercialization as a selective absorption surface for solar heat collectors.

Treatment Example 2 The surface of ferritic stainless steel was treated in the same manner as in Treatment Example 1, and then black oxidized using the following bath composition and treatment conditions.

Sodium hydroxide 190g/trisodium phosphate 38g/sodium nitrite 25g/ferric hydroxide 1g/lead peroxide 25g/Immersion treatment at 103~106°C for 25~30 minutes The absorbing surface thus obtained was treated in Treatment Example 1. It showed the same spectral reflection characteristics as in the case of , and was found to be an excellent selective absorption surface.

In this way, the selective absorption surface obtained by activating the surface of ferritic or austenitic stainless steel and then coloring it black using acidic black oxidation or alkali black oxidation has good spectral properties and is unique to stainless steel. Since it can maintain its corrosion resistance and heat resistance, it can not only be used advantageously as a selective absorption surface for solar heat collectors, but its oxide film is a uniform and stable film that does not reduce the inherent corrosion resistance of stainless steel. .

FIG. 2 is a sectional view showing the structure of a preferred solar heat collector using a selective absorption surface made of ferritic or austenitic stainless steel according to the present invention.

As shown by the arrow in the figure, the sunlight entering from above is covered with a transparent glass or alkali or other synthetic resin transparent material 1, which is used to retain heat and prevent exposure to the atmosphere.
(consisting of one to three transparent bodies), passes through the air layer 2, is absorbed by the black oxide film 3 formed on the surface of the ferritic or austenitic stainless steel according to the present invention, and is converted into heat. The converted heat passes through a ferritic or austenitic stainless steel 4 with a black oxide film adhered to one side and another stainless steel 5 joined to this by pressure welding or diffusion bonding, and is then transferred to air, water, etc. is conducted to the heat medium. 6 is an air layer for heat retention, and 7 is a heat insulating layer using glass wool, asbestos, or a honeycomb structure.

〔Effect of the invention〕

A solar heat collector using a selective absorption surface obtained by chemically coloring ferritic or austenitic stainless steel black according to the present invention has an excellent heat collecting effect.

[Brief explanation of drawings]

FIG. 1 is a graph showing a comparison of the spectral reflection characteristics of the selective absorption surface according to the present invention and other selective absorption surfaces, and FIG. 2 is a graph showing a preferred solar heat collector using the selective absorption surface according to the present invention. FIG. 3 is a cross-sectional view of a specific example. 1... Glass or synthetic resin transparent body, 2... Air layer, 3... Black oxide film, 4... Rustless steel, 5...
...Rustless steel, 6... Air layer, 7... Heat insulation layer, 8...
Heat medium passage.

Claims (1)

[Claims] 1 After activating the surface of ferritic or austenitic stainless steel using an aqueous solution of an acid such as nitric acid or perchloric acid in which the generated metal salt is highly soluble in water, the surface of the ferritic or austenitic stainless steel is activated. Obtained by blackening by a black coloring method selected from either an acidic black oxidation method using a hot aqueous solution or an alkaline black oxidation method using a hot aqueous solution containing an alkali metal hydroxide, an inorganic salt, and an oxidizing metal compound. A selective absorption surface whose spectral reflectance value is from wavelength 4 to
A solar heat collector characterized in that a light absorption surface exhibiting a change rate of 20%/μm or more within a 6μm range is used.