JPS6323461B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a solar heat absorber used in water heaters and the like that utilize solar heat. Conventionally, materials used as solar heat absorbers include those made of metal base materials such as copper or iron coated with black paint, and those made of plastic mixed with black pigments such as carbon black. These conventional solar heat absorbers have a high energy absorption rate in the solar energy spectrum (approximately 96% of a black body), but they also emit a large amount of heat when the temperature increases by absorbing solar energy (95% of a black body). %), the overall absorption efficiency was poor, which caused problems especially when it was desired to obtain hot water at a high temperature. As an improvement on this, the surface of the base material exhibits high absorption in the solar energy spectrum range (wavelengths of 0.3 to 2.5 microns) and thermal radiation range (wavelengths of 0.3 to 2.5 microns).
(2.5 to 30 microns), solar heat absorbers with selective absorption films that exhibit low emissivity are in practical use. Such solar heat absorbers include those in which a semiconductor oxide or sulfide layer is formed on the surface by chemical conversion treatment of a base material made of copper or copper alloy, and those in which a layer of semiconductor oxide or sulfide is formed on the surface of a metal such as stainless steel. There are some that have an oxide film of an appropriate thickness that acts as an interference film. All of these have an absorption rate of about 90% in the solar energy spectrum range and a thermal emissivity of about 10 to 15%, so they have a high overall absorption efficiency. By the way, in the selective absorption film, the composition, film thickness, and microstructure of the oxides and sulfides that make up the film have a great influence on its absorption efficiency, but when using a solar heat absorber, the selective absorption film is Since it is exposed to extremely harsh atmospheric conditions, it is not possible to maintain the composition, film thickness, and microstructure in a desirable state, and it is common for the absorption efficiency to gradually decrease. That is, for example, as mentioned above, if copper or copper alloy is used as the base material and a selective absorption film made of copper compounds such as copper oxide or copper sulfide is formed by chemical conversion treatment, copper will be removed during use. As the oxidation of copper oxide progresses, a new layer of copper oxide develops under the selective absorption film, which has the disadvantage that the thickness of the selective absorption film gradually increases, and the thermal emissivity increases accordingly. In addition, there is a strong tendency for the absorption efficiency to decrease particularly in industrial areas and salt-affected areas where concentrations of SO ₂ gas, Cl ₂ gas, etc. are high, and in these cases, the metal base material itself is often corroded. Therefore, in order to maintain the selective absorption characteristics at a high level, the surface of the selective absorption membrane is sometimes coated with fluorine resin or silicone resin, but even in this case, there are some problems with heat resistance, and the absorption efficiency gradually decreases. decreased. This invention was made in view of the above circumstances, and by selecting the material of the base material, it is possible to provide a highly durable solar heat absorber. This will be explained below. The solar heat absorber according to the present invention has a base material made of a non-copper-based chemically and thermally stable material, and a thin layer made of at least one of copper and a copper alloy on the surface of the base material. It is characterized by being equipped with a selective absorption film made of a copper compound that has a high energy absorption rate in the solar energy spectrum region and a low emissivity in the thermal radiation region, which is created by chemical conversion treatment of the entire body. There is. Next, this will be explained in detail. The base material may be other than copper or copper alloy,
In other words, a non-copper-based material must be selected. Moreover, the base material must be chemically and thermally stable. Therefore, stainless steel plates, nickel-plated steel plates, chrome-plated steel plates, etc. are suitable. A selective absorption film made of a copper compound such as copper oxide or copper sulfide is formed on the surface of the base material. It can be said that the formation method and type of the selective absorption film do not matter as long as it has a high energy absorption rate in the solar energy spectrum range and a low emissivity in the thermal radiation range. However, it is usually created as follows. That is, the surface of the stable base material is first plated,
A thin layer of copper or copper alloy is formed using a method such as vacuum evaporation or sputtering. The thickness of this thin layer is suitably between 0.1 and 1 micron (μm). Next, the entire thin layer is oxidized or sulfurized by chemical conversion treatment, preferably to form a selective absorption film made of cupric oxide (CuO). In this case, the crystal form of cupric oxide is fibrous or leaf-like when viewed under an electron microscope (×20,000). The length of this crystal is 0.4 ~
Preferably it is 2 microns. Depending on chemical conversion treatment conditions, cuprous oxide (Cu ₂ O)
may be generated. When this cuprous oxide is exposed to high temperatures (approximately 200°C) during use, it becomes oxidized into a second oxide.
It turns into copper, but the secondary oxide produced in this way
When copper is viewed under an electron microscope (x20,000), its crystal form is not the fibrous or leaf-like shape described above, but granular crystals. Although fibrous or leaf-like crystals have a high absorption rate in the solar energy spectrum region, granular crystals have a low absorption rate and are therefore not very desirable in practical terms. Once a selective absorption film made of cupric oxide is obtained,
If necessary, the surface is coated with a transparent protective film such as silicone resin, fluororesin, bismaleimide/triazine resin, or inorganic silica. When forming a silicone resin film as a protective film, a commercially available silicone resin (for example, KR-177N manufactured by Shin-Etsu Chemical Co., Ltd.) was diluted with xylene to a solid content of about 10% by weight, and the film was immersed in the solution. Afterwards, the coating is heated at about 100°C for about 5 minutes to harden the coating. In the case of fluororesin, it is better to dilute it with toluene. In either case, the formation of the coating film is preferably repeated two or more times. When forming a siliceous film as a protective film, an aqueous solution of a silicate such as lithium silicate, potassium silicate, or thorium silicate is applied onto the selective absorption film, dried, and then baked. A protective film made of a film-forming material containing bismaleimide triazine (BT) resin may be formed on the surface of the selective absorption film. As for the bismaleimide triazine resin, BT2170 manufactured by Mitsubishi Gas Chemical Co., Ltd. is known as a commercially available product. In this case, the film-forming material is this bismaleimide.
In addition to being made by mixing triazine resin with commonly used compounding raw materials for film-forming materials such as acrylic resin and silicone resin, it can also be made from bismaleimide triazine resin alone. At that time, known solvents such as ethyl acetoacetate are used as the solvent for forming a paint. The bismaleimide triazine resin itself is brown to dark brown, but when the protective film made of the film-forming material in which it is blended is a thin film of 2.0 microns or less, it is almost transparent. If the thickness of this protective coating is made thinner than 0.1 micron, sufficient protective performance cannot be expected in terms of corrosion resistance, etc. On the other hand, if it is thicker than 2.0 micron, absorption in the infrared region will occur and the thermal emissivity will increase. This problem tends to occur. Therefore, the thickness of the protective coating is preferably 0.1 to 2 microns. When the protective coating is made of other materials, the thickness is also
Preferably, the thickness is 0.1 to 2 microns. In the solar heat absorber obtained in this way, the base material is composed of a stable material, and the entire thin layer of copper or copper alloy formed on the surface serves as a selective absorption film, so it is difficult to absorb heat over time. It is not susceptible to damage and has excellent durability. Moreover, the selective absorption membrane is produced by oxidizing or sulfurizing it by chemical conversion treatment in an alkaline atmosphere. The structure of the membrane is different from that of a selective absorption membrane, which is made by treating a metal layer, which has a greater tendency to ionize than copper, with an ammoniacal solution containing a copper compound. That is, the selective absorption film obtained by the above chemical conversion treatment method for obtaining the solar heat absorber of the present invention is
For example, copper oxide, which is once formed on the surface of a thin film of copper or copper alloy, is eluted into the reaction solution in the form of Cu(OH) ₂ and then precipitated on the surface of the substrate again due to the supersaturation state caused by the solubility product. The second
As schematically shown in the figure, fibrous or leaf-shaped crystals 2 are formed as independent clusters on the surface of the base material 1. On the other hand, a selective absorption film obtained by electrolytic oxidation treatment, for example, has a continuous layer 3 of a copper compound on the surface of a base material 1, as shown schematically in FIG. …
Each of these occurs independently. The selective absorption membrane of this invention has the above structure,
Because its structure itself exhibits a type of selective absorption, it has a high energy absorption rate in the solar energy spectrum region, making it an excellent material. Next, examples of the solar heat absorber according to the present invention will be given, and their performance will be shown in Table 1 below, together with comparative examples. [Example 1] Using a stainless steel plate with a thickness of 0.5 mm as the base material,
After nickel strike plating was applied to the surface, copper plating with a thickness of 0.6 microns was applied. After degreasing the surface, it was immersed in a 2%-HNO ₃ aqueous solution for 1.5 minutes at room temperature to activate the surface.
After that, wash thoroughly and use 50% caustic soda.
g, potassium persulfate, 10 g, and water (1 g) for 2 minutes at 100° C. to form a selective absorption membrane.
After thoroughly washing it with water, it was dried with hot air at 80°C for 10 minutes. The material of the obtained selective absorption film was examined by X-ray diffraction and was found to be CuO. Further, as a result of observation using an electron microscope (×20,000), the crystal shape was not fibrous and leaf-like. Silicone resin KC223 manufactured by Shin-Etsu Chemical Co., Ltd. was diluted 5 times with ethanol and applied twice by dipping onto the surface of the selective absorption surface. The thickness of the coating film was 0.5 microns. Thereafter, it was heated at 120°C for 10 minutes to obtain a solar heat absorber. [Example 2] A 0.2 micron thick copper layer was formed on the surface of a 0.5 mm thick stainless steel plate by vapor deposition. After degreasing the surface, it was immersed in a 1%-HNO ₃ aqueous solution for 25 seconds at room temperature to activate the surface. After washing this thoroughly with water, add 90% sodium chlorite.
g/, 80% in a treatment solution consisting of 15g/ of caustic soda.
A selective absorption membrane was formed by immersion at ℃ for 6 minutes. Then, after thoroughly washing with water, it was dried with hot air at 70°C for 20 minutes. The selective absorption membrane obtained had a component of CuO and a fibrous or leaf-like crystal shape. On the surface of the selective absorption membrane, a fluororesin JX900 manufactured by Sumitomo 3M Co., Ltd. diluted with toluene to a solid content of 4% by weight was applied (dip coating). This application is 3
Repeated times. The thickness of the coating film was 1.1 microns. This was heated at 100°C for 10 minutes to obtain a solar heat absorber. [Example 3] Copper was deposited to a thickness of 1 micron on the surface of a 0.8 mm thick nickel-plated steel plate (nickel layer thickness 15 microns) by sputtering. After degreasing this, it was immersed in a 3%-HCl aqueous solution for 1 minute at room temperature to activate the surface. After thorough washing with water, it was immersed for 4 minutes at 100°C in a treatment solution consisting of 15 g of potassium persulfate, 40 g of caustic soda, and 1 part of water to form a selective absorption membrane. The material of the selective absorption membrane was CuO, and its crystal form was fibrous or leaf-like. The same silicone resin liquid as that used in Example 1 was applied three times to the surface of the obtained selective absorption membrane. The thickness of the coating film was 1.4 microns. This was heated at 120°C for 10 minutes to obtain a solar heat absorber. [Comparative Example 1] The same procedure as in Example 1 was carried out except that a 0.5 mm thick copper plate was used as the base. [Comparative Example 2] In Example 1, instead of forming copper oxide by chemical conversion treatment, a selective absorption film made of copper oxide was formed by heating at 250° C. for 100 hours. The material of the obtained selective absorption film was confirmed to be cupric oxide from the results of X-ray diffraction. Furthermore, the crystal form was found to be granular crystals when observed using an electron microscope. [Comparative Example 3] A 1% by weight aqueous solution of sodium hydroxide was placed in a polyethylene container, and the polyethylene container was heated to 35% by weight.
The solution was placed in a hot water bath whose temperature was controlled at 35°C, and the temperature of the aqueous sodium hydroxide solution was maintained at 35±1°C. Next, in this sodium hydroxide aqueous solution, the thickness
A 0.5 mm metal copper plate was used as an anode and a platinum plate was used as a cathode, and a voltage of 25 V was applied for 60 seconds from a constant voltage power supply between the two electrodes to perform electrolytic oxidation treatment. At this time, the distance between both electrodes was 1.5 cm. By this electrolytic oxidation treatment, a black film was formed on the surface of the metal copper. After the treatment, hot air drying was performed at 80°C for 10 minutes to obtain a sample. The material of the selective absorption film formed on the sample surface is
It was confirmed from the line diffraction results that it was cupric oxide. Furthermore, when observed with an electron microscope (×20,000), the surface of the structure is fibrous or leaf-like, which is the same as in Examples 1 to 3;
It was very short, about 0.5 μm, whereas it was more than 5 μm. In addition, the thickness of the selective absorption membrane was measured by cutting out a part of the sample using a galvanostatic reduction method, and found that the average thickness was 0.4 μm when the voids between the fibers or leaves were completely filled. . Calculating backwards from this value, it was considered that the selective absorption film in this sample had a structure in which the short fibrous or leaf-like crystals were formed on a thin continuous film of copper oxide. A coating film of fluororesin (manufactured by Sumitomo 3M, JX-900) was formed on the surface of the sample in the same manner as in Example 2 to obtain a solar heat absorber. The optical properties of the thus obtained solar heat absorbers and the solar heat absorbers of Examples 1 to 3 were measured. The results are shown in Figure 1. As shown in the figure, Examples 1 to 3 (solid line in the figure), which are solar heat absorbers of the present invention, showed uniform absorption in the near-infrared region, whereas Comparative Example 3 (broken line in the figure) , the absorption rate in the near-infrared region with wavelengths of 0.6 μm or more became poor.

【table】

[Table] The test method in Table 1 was as follows. Thickness of protective coating: The cross section was observed under a microscope, and the coating weight was calculated along with this. Absorption rate: α＝∫ ^2.5 / _0.3 α〓・I〓dλ／∫ ^2.5 / _0.3 I
〓dλ (∫ ^2.5 _0.3 means that the spectral range of sunlight is 0.3~
(Based on 95% presence at 2.5μ) Where α: Absorption rate (relative to total solar energy) d〓: Absorption rate at wavelength λ I〓: Radiation intensity at wavelength λ of sunlight Emissivity: ε=∫ ¹⁵ / _2.5 S〓 _T=150・ε〓dλ／∫ ^15/2 _.
5 S〓 _T=150 dλ Where ε; Emissivity (relative to the total energy radiated by the black body) S〓 _T=150 ; Radiation intensity at wavelength λ from a black body at 150℃ ε〓; Emissivity at wavelength λ (relative to the black body) ) The reflectance P〓 in the infrared region was measured using an infrared spectrophotometer, and it was set as ε〓=1−P〓. SO ₂ gas test: One cycle consisted of filling the bottom of a desiccator with water to a depth of 30 mm, injecting 4000 ppm SO ₂ gas for 10 minutes, displacing the air, sealing the desiccator, and leaving it for 24 hours.

[Brief explanation of the drawing]

FIG. 1 is a graph showing one example of the measurement results of the absorption characteristics of the solar heat absorbers of Examples 1 to 4 and Comparative Example 3, FIG. 2 is a schematic cross-sectional view of the solar heat absorber according to the present invention, and FIG. is a schematic cross-sectional view of a conventional solar heat absorber. 1...Base material, 2,2'...Crystal.

Claims (1)

[Claims] 1. The base material is made of a non-copper-based chemically and thermally stable material, and a thin layer made of at least one of copper and a copper alloy is formed on the surface of the base material, The entire thin layer is oxidized or sulfurized by a chemical conversion treatment in an alkaline atmosphere, and the surface of the substrate is made of a copper compound that has a high energy absorption rate in the solar energy spectrum region and a low emissivity in the thermal radiation region. A solar heat absorber equipped with a selective absorption film. 2. The solar heat absorber according to claim 1, wherein the selective absorption film is made of cupric oxide crystals exhibiting a fibrous or leaf shape. 3. The solar heat absorber according to claim 1 or 2, wherein the surface of the selective absorption film is covered with a transparent protective film.