JPS6140897B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing a solar heat absorber used in solar water heaters and the like. As an absorber of solar energy, it has a high absorption rate for electromagnetic waves in the visible light region and near-infrared region.
Moreover, it is required to have low emissivity in the infrared region. Conventionally, various solar heat absorbers have been devised that have coatings on their surfaces that absorb solar energy well and emit little heat in the infrared region (generally called selective absorption coatings). As a selective absorption film, a cuprous oxide (CU ₂ O) film or a cupric oxide film is used.
Copper oxide coatings such as copper (CUO) coatings are widely known. However, conventional solar heat absorbers in which a selective absorption film made of copper oxide is formed on the surface of a base material generally have poor heat resistance, so that they may be exposed to sunlight without conducting a heat medium during use. As a result, temperatures can sometimes reach temperatures as high as 200°C, causing deterioration. Therefore, near-infrared region (wavelength
0.7 to 2.5 microns), the absorption rate decreases, and
Since the underlying copper component is oxidized and the thickness of the selective absorption film gradually increases, problems such as increased heat radiation in the infrared region often occur. Among these problems, in order to solve the problem of increased film thickness, the inventors formed a thin film layer of copper or copper alloy on the surface of a chemically and thermally stable metal base material. We devised a method of forming a selective absorption film by oxidizing the layer. This is because if all the copper components in the copper or copper alloy thin film layer are oxidized, the underlying layer is stable, so the film thickness will no longer increase, and heat radiation in the infrared region will not increase. The inventors decided to use a base material whose entire or only surface is made of nickel as a stable metal base material, and conducted repeated research to obtain a solar heat absorber with even better performance. In the process, they discovered that the fine structure of the nickel surface increases the absorption rate of solar heat, and as a result of searching for a method to effectively create such a fine structure, they finally arrived at this invention. This invention was made in view of the above circumstances, and provides a manufacturing method that can effectively produce a solar heat absorber that has a high solar heat absorption rate and excellent heat resistance. This will be explained below. The method for manufacturing a solar heat absorber according to the present invention includes etching the surface of the base material and depositing copper on the surface by contacting an acidic solution containing copper ions with at least the surface of a base material made of nickel. After that, the copper plating layer on the surface of the base material is subjected to an oxidation treatment to form a selective absorption film of cupric oxide. In this manufacturing method, the metal substrate is made entirely of nickel or has a thin nickel film on its surface;
That is, a base material whose at least the surface is made of nickel is used. The core material of the base material having the nickel thin film may be any material, but it is preferably a material with high thermal conductivity such as metal. The nickel thin film is formed using methods such as plating, vacuum evaporation, and sputtering. The above-mentioned nickel base material (including a base material having a nickel thin film) is electrolessly plated with copper and further subjected to a chemical conversion treatment to form the copper plating layer on the surface of the base material into a selective absorption film made of cupric oxide. As described above, the present invention is characterized in that the thin film layer of copper or copper alloy, which becomes the selective absorption film, is formed by electroless plating of copper. In the copper plating step, the nickel base material is immersed in an acidic solution containing copper ions to bring the surface of the base material into contact with the acidic solution containing copper ions. Before that, the surface of the nickel base material is pretreated.
Of course, the nickel surface may be activated with a dilute acid such as dilute nitric acid. When an acidic solution containing copper comes into contact with the surface of the base material,
Based on the difference in ionization tendency between Ni and Cu, the nickel surface melts, forming a fine uneven structure. Along with this (roughening), copper is deposited on the surface of the base material, forming a thin film layer containing copper as the main component. The acidic solution containing copper ions used here includes at least one copper salt selected from copper sulfate CuSO ₄ , copper nitrate Cu(NO ₃ ) ₂ or copper chloride CuCl ₂ and has a pH of 1.0 or less. Most preferably, it is used. The immersion time must be appropriately determined depending on the properties of the acidic solution, etc. so that the nickel surface is sufficiently roughened. In this way, by roughening the surface of the nickel base material, the optical properties of the resulting solar heat absorber are significantly changed and its performance is improved. This will be explained using FIG. FIG. 1 is a graph showing the optical characteristics of nickel base material C and samples A, B, and D obtained by subjecting it to various treatments. Nickel base material C was made as follows. A cold-rolled steel plate was used as the core material, and nickel plating was applied to the core material to a thickness of 5 microns (10 ^-6 m, hereinafter abbreviated as "μ"). The composition of the plating solution used is shown below. Nickel sulfate 240g / Nickel chloride 45g / Boric acid 30g / Satucharin 0.5-1.5g / Formalin 1-2 c.c. / Sample D was coated on the surface of nickel base material C with a thickness of 0.18
A cupric oxide thin film of μ is formed. This sample D was made as follows. Using a plating solution containing copper cyanide as the main component, nickel base material C
Copper plating was applied to a thickness of 0.2μ. here,
The surface of the nickel base material C is not roughened. Further, oxidation treatment was carried out by immersing it at 90° C. for 5 minutes in a chemical conversion treatment solution consisting of 40 g of sodium chlorite (NaClO ₂ ), 90 g of sodium hydroxide (NaOH), and 1000 g of water. Sample B also has a thickness of 0.18μ on the surface of nickel base material C.
In this case, the surface of the nickel base material C was roughened. In other words, nickel base material C is diluted with 5% nitric acid (at room temperature) for 1
Activate the surface by soaking for a minute, then add 50g/
The sample was immersed in an aqueous solution containing copper chloride (pH 1.0 or less, room temperature) for 5 minutes. In this way, a copper thin film with a thickness of 0.2 μm was formed, and then oxidation treatment was performed in the same manner as Sample D. Sample A was prepared as follows. That is, sample B was immersed in 5% by weight hydrochloric acid (at room temperature) for 1 minute to dissolve and remove cupric oxide on the surface of nickel base material C. Here, care was taken to prevent the eluted cupric oxide from re-precipitating as copper on the surface of the substrate, and the immersion time was not made too long. Sample A has a nickel surface like nickel base material C, but has a much higher absorption rate in the solar heat absorption region than nickel base material C. This is because the nickel surface was roughened during copper plating. Sample B, which has such a roughened nickel surface as its base, has a much higher solar heat absorption rate than sample D, which has a non-roughened nickel surface as its base. In this way, when the surface of the nickel base material is roughened and has fine irregularities, the absorption rate of the solar heat absorber increases. The oxidation treatment of the thin film layer containing copper as a main component is preferably carried out by the following chemical conversion treatment. That is, this chemical conversion treatment liquid for oxidation treatment is a mixed aqueous solution of an oxidizing agent and an alkaline additive, or a mixed aqueous solution to which copper ions are added, and as the oxidizing agent,
Sodium chlorite (NaClO ₂ ), sodium hypochlorite (NaClO), potassium persulfate (K ₂ S ₂ O ₈ ),
Chlorites, hypochlorites, or persulfates such as sodium persulfate (Na ₂ S ₂ O ₈ ) and ammonium persulfate [(NH ₄ ) ₂ S ₂ O ₈ ] are used, and as alkaline additives, Sodium hydroxide (NaOH), potassium hydroxide (KOH), etc. are used. _NaClO2 as oxidizing agent and NaOH as alkaline additive
It is practically best to use The content of oxidizing agent and alkaline additive in the chemical treatment solution is
Assuming that the amounts added to 1000g are xg of the alkaline additive and yg of the oxidizing agent, the amounts should be such that they simultaneously satisfy the following three equations. x≦125 y≧15 y≦25/24x-5 It is effective to set the copper ion concentration in the chemical conversion treatment liquid to 500 ppm or less. Although it is not necessarily necessary to include copper ions, it has been experimentally confirmed that the presence of copper ions in the above concentration range improves the optical properties of the CuO selective absorption film. The method of adding copper ions to the chemical conversion treatment solution was free. For example, it may be possible to add a trace amount of an aqueous solution of copper salt such as copper sulfate (CuSO ₄ ), copper nitrate (Cu(NO ₃ ) ₂ ), or copper chloride (CuCl ₂ ), or by adding metallic copper to the chemical conversion treatment solution. Alternatively, the copper ions may be increased by immersing the material in water and treating it with this solution. If oxidation treatment is performed using such a chemical conversion treatment solution, the thin film layer made of copper or copper alloy will have a length in the long axis direction (the length of the longest part) of approximately
The selective absorption film is made of cupric oxide crystals having a diameter of 0.6 to 2.0 μm and approximately fibrous or leaf-like. Regarding fibrous or foliar crystals, other fibrous or foliar crystals may grow from one fibrous or foliar crystal. Therefore, in such a case,
The length in the major axis direction is determined for each individual crystal. A selective absorption film having such a length in the long axis direction of the crystal has a high absorption rate. The reason is as follows. In general, a selective absorption film made of cupric oxide crystals has a reduced absorption rate in the so-called near-infrared region with a wavelength of 0.7μ or more, but the degree of this reduction depends on the thickness and microstructure of the selective absorption film. greatly influenced,
The decrease is the least when the length of the crystal in the major axis direction is 0.6μ or more. This is in the near-infrared region (wavelength 0.7 to 2.5
If the length of the crystal is 0.3μ or less, it will be a geometrically flat surface, but if it is 0.6μ or more, multiple reflections will occur and the absorption rate will improve. Conceivable. In addition, according to the experimental results, when the crystal length is in the range of 0.6 to 2.0μ,
The absorption rate did not improve any further. Copper (Cu) or cuprous oxide (Cu ₂ O) in the selective absorption film
If it remains, it will deteriorate during use, so the above oxidation treatment must be carried out sufficiently. However, even if a small amount remains, it will not have a major effect. Regarding the case where Cu ₂ O is present in the CuO layer, the initial characteristics are very good in this case.
This is because the presence of Cu ₂ O increases the absorption rate in the near-infrared region. However, when subjected to heat (150 to 200°C), this Cu ₂ O changes to CuO, and in this case, it becomes granular crystals instead of fibrous or leaf-like, and the deterioration becomes severe. Therefore, the absorption rate in the near-infrared region ends up being worse than when it is made of only fibrous or leaf-like CuO from the beginning.
If Cu remains in the CuO layer, not only the absorption rate in the near-infrared region will decrease, but also the emissivity will increase due to the increase in film thickness. The thickness of the selective absorption membrane is preferably 0.03 to 1.0μ. Since this film thickness is usually determined by the electrolytic cyclic method, it is expressed in terms of the thickness of a flat layer without voids or irregularities.
If the thickness of the selective absorption film is less than 0.03μ, the absorption rate of solar heat will be small. On the other hand, if it is thicker than 1.0μ, the emissivity will increase, similar to black paint, and the overall absorption efficiency will decrease. Next, examples and comparative examples of the present invention will be described. [Example and Comparative Example] 300 g of NiSO ₄ 7H ₂ O/,
Using a Watts bath containing NiCl ₂ 6H ₂ O 50g/, H ₃ BO ₃ 50g/, current density 3A/dm ₂ , plating time
Nickel plating was applied for 15 minutes to form a nickel plating layer with a thickness of 5 μm. Next, 5wt of this nickel-plated base material
Activated by immersing in %-HNO ₃ (room temperature) for 1 minute,
A copper plating layer was formed on the surface of the nickel plating layer under the conditions shown in Table 1. At this time, the nickel surface was not roughened in Comparative Example 1, and was not roughened in Comparative Example 2 as well. After that, the obtained copper plating base material was treated with 70g of NaClO ₂ , 90g of NaOH,
The substrate was oxidized by immersion in a chemical conversion solution containing 1000 g of water at a temperature of 95°C for 10 minutes, and a selective absorption film made of cupric oxide (CuO) as shown in Table 2 was provided on the surface of the substrate. Obtained a solar heat absorber. The optical properties of each solar heat absorber were as shown in Table 3.

【table】

【table】

【table】

[Table] (Note) The thickness test method is described later.

[Table] (Note) The test methods for absorption rate and emissivity are described later.As can be seen from Table 3, all of the examples have higher absorption rates in their initial characteristics than the comparative examples. In addition, because it has excellent heat resistance, it maintains a high absorption rate and does not show any deterioration. As is clear from the above description, according to the method for manufacturing a solar heat absorber according to the present invention, it is possible to successfully produce a solar heat absorber with good solar heat absorption efficiency and excellent heat resistance. The test methods in Tables 2 and 3 are as follows. (Test method) Copper plating thickness: An electrolytic film thickness gauge manufactured by Chuo Seisakusho was used. CuO film thickness: Constant current ring method was used. Crystal length: At the early stage of CuO formation (when CuO crystals are mottled), the length in the major axis direction was measured using an electron micrograph. CuO crystal is
This method may be used because it is determined by the composition of the chemical conversion treatment liquid regardless of time and humidity. (∫ ^2.5 _0.3 means that the spectral range of sunlight is 0.3 to 2
.Five
Based on 95% presence in μ) where α: Absorption rate (relative to total solar energy) α〓: Absorption rate at wavelength λ I〓: Radiation intensity at wavelength λ of sunlight Here, ε: Emissivity (relative to the total energy of black body radiation) S〓 _T = 150; Radiation intensity at wavelength λ from a black body at 150℃ ε = Emissivity at wavelength λ (relative to black body) In addition, infrared spectroscopy The reflectance P〓 in the infrared region was measured with a photometer, and it was set as ε〓=1−Pλ.

[Brief explanation of the drawing]

FIG. 1 is a graph for explaining the influence of the surface condition of a nickel base material on optical properties.

Claims (1)

[Claims] 1. An acidic solution containing copper ions is brought into contact with the surface of a base material at least the surface of which is made of nickel, etching the surface of the base material and depositing copper on this surface, and then . A method for producing a solar heat absorber, characterized in that a copper plating layer on the surface of a base material is subjected to oxidation treatment to form a selective absorption film of cupric oxide. 2. The acidic solution containing copper ions contains at least one copper salt selected from copper sulfate, copper nitrate, and copper chloride, and has a pH of 1.0 or less. Manufacturing method for solar heat absorbers. 3 Oxidation treatment for the copper plating layer is carried out using an oxidizing agent consisting of a salt selected from chlorite, hypochlorite and persulfate, sodium hydroxide and potassium hydroxide as a chemical conversion treatment liquid. Alkaline additives consisting of selected hydroxides and 0-
Use an aqueous solution that contains 500 ppm of copper ions, and where x and y are within a range that simultaneously satisfies the following three formulas, where xg is the amount of alkaline additive added and yg is the amount of oxidizing agent added to 1000 g of water. A method for manufacturing a solar heat absorber according to claim 1 or 2. x≦125 y≧15 y≦25/24x-5