JPS621513B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a solar heat absorber used in solar water heaters and the like. Solar heat absorbers are known to have a selective absorption film formed on the surface of a base material made of metal, which has a high absorption rate in the visible range of sunlight and a low emissivity in the infrared range. It is being Such selective absorption films include black chromium-based and copper oxide-based selective absorption films, as well as selective absorption films obtained by subjecting the surface of stainless steel to oxidation treatment. Selective absorption membranes based on copper oxide have problems in terms of economic efficiency, copper oxide based selective absorption membranes are highly economical but have drawbacks in heat resistance and corrosion resistance, and stainless steel selective absorption membranes have poor thermal conductivity. All of them had major practical drawbacks, such as problems with processability and processability. To further discuss copper oxide-based selective absorption films, there are two known types: cuprous oxide (Cu ₂ O) film and cupric oxide (CuO) film, but both have the disadvantages described below. It had a In other words, when a cuprous oxide film is used as a selective absorption film, it absorbs solar energy well, but on the other hand,
Since the heat radiation when heated is large, the overall efficiency cannot be said to be sufficient. Moreover, when the cupric oxide film is used as a selective absorption film, on the contrary, although the heat radiation is small, the absorption of solar energy is also somewhat small, so the efficiency as a whole is still insufficient. Copper oxide-based selective absorption membranes have various problems as mentioned above, but considering economic efficiency and other aspects, they are excellent in practical use if the above problems can be solved. I can say that there is.
Therefore, it was desired to solve these problems. This invention was made in view of the above circumstances, and by forming a selective absorption film by combining the cuprous oxide film and the cupric oxide film, and by selecting the film thickness within a specific range. , to provide an efficient and durable solar heat absorber. The solar heat absorber according to the present invention has at least a surface made of nickel, and a selective absorption film made of cupric oxide and cuprous oxide is formed on the surface of a base material whose surface is not flat, The selective absorption film is characterized in that the total thickness is 0.05 to 0.50 microns, and that at least 0.005 microns and less than 0.03 microns is occupied by cuprous oxide. This will be explained in detail above. The base material is made entirely of nickel,
Alternatively, a core material (base metal) made of metal or the like with a nickel film coated on the surface is used. It has already been proposed by the inventors that nickel is preferably used as the base for the selective absorption membrane. The purpose of its use is to improve the corrosion resistance and heat resistance of solar heat absorbers. That is, since black chromium or black nickel-based selective absorption films are very thin and porous, it is difficult to prevent corrosion of the base material. Furthermore, the base material may be oxidized if it is heated in the atmosphere. For example, when a solar heat absorber is used and exposed to sunlight without conducting a heat medium, the temperature can sometimes reach as high as 200°C, which causes oxidation of the base material.
When the base material is oxidized, the absorption characteristics α and radiation characteristics ε of the selective absorption film deteriorate. In this respect, nickel, which is chemically and thermally stable, is preferably used as the base of the selective absorption membrane because the base material will not be corroded or oxidized. The solar heat absorber according to the present invention uses nickel as the base of the selective absorption film, as in the prior art. However, the characteristic feature is that the underlying nickel surface is not flat. When the surface of the nickel base is smooth, the base itself has almost no selective absorption properties. On the other hand, if the surface of the nickel base is not flat because it is made of leaf-like, fibrous, or granular crystals, the nickel base itself has selective absorption properties. When a selective absorption film of copper oxide is formed on this nickel substrate, a mutual effect appears between them, and the absorption characteristics are further improved. This is thought to be because sunlight in the visible and near-infrared regions undergoes multiple reflections on the surface of this nickel base and is easily absorbed. Preferably, the nickel substrate has one of the two crystalline forms described above. That is, one of them is a leaf-like or fibrous crystal form, and the other is a granular crystal form. When the nickel base, which acts as a kind of selective absorption membrane, is formed as a membrane on the core material,
Microscopically, it is not a single continuous film,
It can be said that the above-mentioned crystal particles are clustered in a film shape. Crystals may grow in such a way that one crystal piece successively grows another crystal piece. Figures 1 and 2 schematically show what happens when it takes the simplest form. FIG. 1 shows crystal particles exhibiting a leaf-like or fiber-like shape, and FIG. 2 shows crystal particles exhibiting a granular shape. The crystal size and crystal spacing are preferably as follows. The average longitudinal length of the leaf-like or fibrous crystal grains 1 in FIG. 1 (the length of the longest part, represented by t in FIG. 1) is
0.5 to 2.0 microns (10 ^-6 m, hereinafter abbreviated as "μ")
The average distance between the tips of the crystal grains (represented by l in Figure 1) is preferably in the range of 0.5 to 2.5μ.
It is preferable that it is in the range of . Regarding fibrous or foliar crystals, as mentioned above, one fibrous or foliar crystal may grow another fibrous or foliar crystal. Therefore, in such a case, the length in the major axis direction will be determined for each individual crystal. Second
The average grain size of granular crystal particles 2 in the figure is 0.1 to 1
It is preferably in the range of μ, and the average spacing between crystal grains (the average distance separating adjacent crystal grains from each other, as represented by l' in Figure 2) is 0.1.
It is preferably in the range of ~0.4μ. The nickel substrate with the above crystals causes multiple reflections on the surface of visible and near-infrared light, so it absorbs visible light and near-infrared light well, but it absorbs visible light and near-infrared light well, but it absorbs visible light and near-infrared light well, but it absorbs visible light and near-infrared light well. It seems to act as a flat surface against external light, and reflects infrared light well.
Therefore, the emissivity is small. The nickel base may contain small amounts of impurities such as trace amounts of oxides due to surface oxidation. Next, the nickel film that serves as the base is
For example, it can be formed by various methods such as electrolytic plating, electroless plating, vacuum evaporation, and sputtering, and the method is not limited. However, when producing a nickel film made of leaf-like or fibrous crystals, the most stable manufacturing method is to use electrolytic plating as described below. That is, as a plating liquid, a liquid having a composition of nickel chloride 50 to 300 g/(preferably 150 to 250 g/) and boric acid 20 to 60 g/(preferably 30 to 50 g/) is used, and the liquid temperature is 20
~70℃ (preferably 55-65℃), current density 0.5~
4A/ ^dm2 (preferably 1-3A/ ^dm2 ), plating time 0.5-8 minutes (preferably 1-2 minutes)
The plating is done under these conditions. Various metal materials such as iron plate, steel plate, copper plate, etc. can be used as the core material. When performing such a nickel plating treatment on the surface of a core material made of an iron plate or a copper plate, it is generally necessary to perform pretreatment such as degreasing (electrolytic degreasing if necessary) and pickling. When using a stainless steel plate as the core material and applying the above-mentioned nickel plating to it, the following careful pretreatment is usually required, for example.
That is, after degreasing with ordinary chemicals and electrolytic degreasing, an activation treatment is carried out by cathodic electrolysis using 5 to 30% sulfuric acid, and then nickel strike plating is carried out under the following conditions. (Processing liquid composition) Nickel chloride 50-300g/Hydrochloric acid 50-200g/ (Processing conditions) Liquid temperature: Room temperature Current density: 5-10A/ ^dm2 Processing time: 0.5-5 minutes In this way, the necessary pretreatment is performed. When the core material is nickel-plated by the plating method described above, a black-gray or grayish nickel plating layer is formed on the surface of the core material. At that time, it is preferable to select plating conditions within the above range so that the thickness of the nickel plating layer is 0.1 to 3 microns. As mentioned above, the Nickelmetsuki film is made up of clusters of crystals, so it actually has complex irregularities,
It has a fine structure. Therefore, since it is usually measured by plating thickness measurement method, etc., the film thickness mentioned above is expressed as a value converted to a state in which these unevenness is smoothed out and there are no voids. . Next, by forming a copper oxide selective absorption film as described later on the nickel base thus formed, a highly practical solar heat absorber with excellent selective absorption properties can be obtained. can.
That is, although solar heat absorbers equipped with copper oxide-based selective absorption films generally have excellent selective absorption properties, they have the disadvantage of poor heat resistance and corrosion resistance, and gradually deteriorate after long-term use. If the selective absorption film made of copper oxide is formed on the base metal layer made of nickel, the durability is further improved because the base metal layer is thermally and chemically stable. Such a selective absorption film made of copper oxide can be produced by, for example, forming a copper thin film layer on a nickel base by a plating method, sputtering method, vacuum evaporation method, etc., and then subjecting it to oxidation treatment by chemical conversion treatment. It is formed as a result. In this case, the nickel base having the uneven surface has extremely good adhesion to the copper oxide selective absorption film formed thereon, and also has the function of firmly holding the copper oxide layer. This is because the above-mentioned nickel base has extremely fine uneven surfaces when viewed microscopically.
It is thought that the uneven portions and the copper oxide crystals are intricately intertwined and their mutual bond becomes strong. When such a base metal layer is made of a nickel film,
The thickness is preferably about 0.1 to 3 μm. This is because, if it is outside this range, the adhesion of the copper oxide layer formed thereon tends to decrease. The copper oxide layer consists of cuprous oxide (Cu ₂ O) and cupric oxide (CuO) having a leaf-like or fibrous crystal form. Although both components may be mixed with each other, they are generally separated into two layers: a cuprous oxide layer and a cupric oxide layer. The total thickness of the copper oxide layer serving as the selective absorption film needs to be 0.05 to 0.50μ. This is because if the film thickness exceeds 0.50μ, thermal radiation increases, and if it is less than 0.05μ, sufficient blackness cannot be obtained and the absorption rate decreases. The reason why the selective absorption film is made of a combination of two types of copper oxides is as follows. That is, when cupric oxide is used alone, the sunlight absorption rate is slightly low, which is bad, but when cuprous oxide is added to this, this point is improved. Figure 3 shows A in which a selective absorption film consisting of cupric oxide and a trace amount of cuprous oxide is formed on the same base material.
This is a graph showing the selective absorption characteristics of the device B in which a selective absorption film made only of cupric oxide is formed, but from this point on, it can be seen that when cupric oxide is mixed with a very small amount of cuprous oxide, Near infrared region (wavelength
It can be seen that the absorption rate becomes high at 0.7 to 2.5μ). However, the amount of cuprous oxide mixed should not be too large. A selective absorption film is required to be stable even when kept at about 200℃ for a long time, but if cuprous oxide is left at such high temperatures, it will form a crystalline form (granular) that has poor solar energy absorption. This is because it changes to cupric oxide. Therefore, in terms of film thickness, it is necessary that the amount of cuprous oxide be kept at a very small amount, not exceeding 0.03μ. If it is too small, there will be no effect, so it needs to be in the range of 0.005μ or more and less than 0.03μ. If the amount is as small as this, the absorption rate will hardly change over time. Therefore, it has good heat resistance stability. Regarding thermal emissivity, the amount of cuprous oxide mixed is 0.005 μ or more and 0.03
As long as it is less than μ, the combined system is as good as the cupric oxide single system. The preferred film thickness is 0.008 to 0.01μ for cuprous oxide and 0.06 to 0.12μ for cupric oxide.
This is the range. The selective absorption film and the cuprous oxide layer contained therein are generally formed as a layer with many voids or a layer with many irregularities, but since the thickness of these films is usually measured by a constant current reduction method, etc. All thicknesses mentioned here are values calculated as a dense film with no voids or a flat film with no irregularities. In order to form the above-mentioned selective absorption film, as described above, a method of oxidizing the copper-containing surface layer formed on the base material is practically adopted. This oxidation treatment includes, for example, immersing the substrate in an alkaline aqueous solution of an oxidizing agent such as sodium chlorite, potassium chlorite, or potassium persulfate, or immersing it in a mixed solution of potassium permanganate and copper nitrate or copper sulfate. There is a process in which the base material is immersed. The respective thicknesses of the cuprous oxide layer and the cupric oxide layer formed by such oxidation treatment vary depending on the temperature, composition, concentration, treatment time, etc. of the treatment bath, so the optimal conditions may be determined depending on the purpose. All you have to do is choose. A transparent coating such as silicone resin may be formed on the selective absorption membrane to protect it. The solar heat absorber thus obtained has excellent selective absorption properties (sunlight absorption properties, thermal radiation properties), not only in the initial properties but also in the properties after the heat resistance test. Example 1 A copper plate with a thickness of 0.5 mm was used as a core material, and after degreasing with alkaline and pickling with 3% HCl, a plating solution consisting of 200 g of nickel chloride and 40 g of boric acid was used, and the liquid temperature was 60°C. °C, current density
Nickel plating was carried out under the conditions of 2 A/dm ² and a plating time of 3 minutes. The thickness of the obtained nickel film was 1 μm. When this was observed using an electron microscope (7600x magnification), it was confirmed that leaf-like or fibrous crystals were formed. The average length of this crystal in the major axis direction was 0.8 μ. Copper was plated on the obtained nickel film to a thickness of 0.15μ using a copper sulfate plating solution. Add 80 g of sodium chlorite and 60 g of caustic soda to this copper plating layer.
Oxidation treatment was carried out for 5 minutes with a treatment solution consisting of
A selective absorption film of copper and cupric oxide was formed. Example 2 A stainless steel plate with a thickness of 0.3 mm was used as the core material, and after being subjected to alkaline degreasing and electrolytic degreasing, the current density was reduced at room temperature using 5% sulfuric acid.
Pretreatment was carried out by cathodic electrolysis at 10 A/dm ² for 30 seconds. Next, nickel chloride 200g/, hydrochloric acid
Using a plating solution consisting of 50 ml, the current density
Primary nickel plating was performed under the conditions of 10 A/dm ² , liquid temperature of 20 to 25°C, and plating time of 0.5 minutes to form a primary nickel film on the surface of the core material. Continuing on top of this plating layer, nickel chloride 300g/, boric acid
Using a plating liquid consisting of 50 g/m, the current density
Secondary nickel plating was performed under the conditions of 2 A/dm ² , liquid temperature of 60° C., and plating time of 2 minutes. The primary nickel plating is for improving adhesion to stainless steel, and the secondary nickel plating is for forming a nickel film made of the leaf-like or fibrous crystals. The thickness of the obtained nickel film was 1.1μ, and the surface microstructure was fibrous or leaf-like. On the obtained Ni film, the thickness was
0.10μ copper plating was performed. This sample was mixed with 100g of sodium chlorite and 100g of caustic soda.
A black selective absorption membrane was obtained by treatment with a chemical conversion treatment solution (solution temperature: 50°C) for 8 minutes. Example 3 A stainless steel plate (SUS 430) with a thickness of 0.4 mm was used as the core material, and it was subjected to alkaline degreasing, electrolytic degreasing,
After sequentially performing cathodic electrolysis using 5% sulfuric acid, nickel strike plating for stainless steel was performed under the following conditions. (Strike plating liquid composition) Nickel chloride 200g/ Hydrochloric acid 100g/ (Treatment conditions) Temperature: 25℃ Current density: 5A/dm ² hours: 3 minutes On this surface, apply nickel chloride 250g/, boric acid
Using a plating liquid containing 30g/, the current density is 3A/
Nickel plating was performed under the conditions of dm ² , plating liquid temperature of 40° C., and plating time of 2.5 minutes to form a nickel plating layer with a thickness of 0.7 μm. Its fine structure was fibrous. Next, copper plating was performed on the surface of this nickel plating layer with a copper sulfate plating solution at a current density of 1A/dm ² and a plating time of 1.5 minutes to form a copper plating layer with a thickness of 0.20 μm. Oxidation treatment was performed using a chemical conversion treatment solution containing 90 g of soda at a chemical conversion temperature of 70° C. for a treatment time of 3 minutes, thereby converting the copper plating layer into a copper oxide layer. Example 4 A copper plate with a thickness of 0.5 mm was used as a core material, and after degreasing with alkaline and pickling with 2%-HCl, 400 g of nickel chloride and 45 g of boric acid were used.
Nickel plating was carried out using a plating solution consisting of 50.0 g/g/g/g/g/g under the conditions of a solution temperature of 55 DEG C., a current density of 2 A/ ^dm.sup.2 , and a plating time of 3 minutes. Note that during the plating process, the plating solution was sufficiently stirred. The thickness of the obtained Ni film was about 1 μm. When this was observed using an electron microscope (7,600x magnification), it was confirmed that granular crystals were formed. After completing this Ni plating, copper was further plated to a thickness of 0.1μ using a copper sulfate based plating solution. Next, in order to oxidize this copper plating layer, an oxidation treatment was performed for 5 minutes at a liquid temperature of 60°C using a chemical conversion treatment solution consisting of 45 g of sodium chlorite and 70 g of caustic soda. A selective absorption film of cupric oxide consisting of black crystals was used. The composition and film thickness of the obtained copper oxide are Cu ₂ O 0.008
μ, CuO was 0.08μ. Example 5 A stainless steel (SUS 304) plate with a thickness of 3 mm was used as the core material, and after being subjected to alkaline degreasing and electrolytic degreasing, it was cathode-treated with 5% sulfuric acid at a current density of 10 A/dm ² for 30 seconds at room temperature. Pretreatment by electrolysis was performed. Next, nickel chloride 200g/,
Using a plating solution consisting of 50 ml of hydrochloric acid, the current density
First-order nickel plating was carried out under the conditions of 10 A/dm ² , liquid temperature of 20 to 25° C., and plating time of 0.5 minutes. On top of this plating layer, nickel chloride 360g/, boric acid 40g
Secondary plating was carried out using a plating solution consisting of 2.5 g/g/g/g/g/g of plating solution at a solution temperature of 55° C., a current density of 2 A/dm ² , and a plating time of 2 minutes. The thickness of the obtained nickel film was 1.1μ, and the fine structure was granular. On the obtained Ni film, the thickness was
Copper plating with a thickness of 0.15 μm was applied, and the sample was treated with a chemical conversion solution (liquid temperature: 60° C.) containing 40 g of sodium chlorite and 60 g of caustic soda for 6 minutes to obtain a black selective absorption membrane. Comparative Example 1 Ni plating (leaf-like or fibrous crystals) was performed in the same manner as in Example 1, and then copper was plated to a thickness of 0.19 μm using a copper sulfate plating solution. Sodium chlorite 65g/, caustic soda 95g on this copper plating layer
The oxidation treatment was carried out for 10 minutes with a treatment solution consisting of
A copper selective absorption film was formed. Comparative Example 2 Using a base material plated with Ni (granular crystals) in the same manner as in Example 5, copper plating with a thickness of 0.18μ was applied to this, and this copper plating layer was coated with 70% sodium chlorite.
An oxidation treatment was carried out for 8 minutes using a treatment solution containing 90 g/g of caustic soda and 90 g/g of caustic soda at a temperature of 90° C. to form a cupric oxide selective absorption film consisting of fibrous black crystals. Comparative Example 3 Ni plating (leaf-like or fibrous crystals) was performed in the same manner as in Example 1, and copper was then plated to a thickness of 0.14 μm using a copper sulfate plating solution. Sodium chlorite 95g/, caustic soda 60g on this copper plating layer
Oxidation treatment was carried out for 5 minutes with a treatment solution consisting of
A selective absorption film of copper and cupric oxide was formed. Cuprous oxide and dibasic oxide in the selective absorption films of the solar heat absorber samples obtained in the above examples and comparative examples.
In addition to measuring the thickness of each copper film, the initial optical properties (absorption rate α, emissivity ε) and
After a heat resistance test of exposing the material to a 200°C atmosphere for 300 hours, the optical properties (absorption rate α, emissivity ε) were measured, and the results are shown in Table 1. As is clear from Table 1, the solar heat absorber samples obtained in Examples 1 to 5 are the same as those of Comparative Examples 1 and 2.
It had excellent initial optical properties compared to the sample. Moreover, the optical properties of the samples of Examples 1 to 5 hardly deteriorated after the heat resistance test, and the initial excellent optical properties were maintained. On the other hand, the sample of Comparative Example 3, which has a cuprous oxide film thickness exceeding 0.03μ,
A decrease was observed in both absorption rate and emissivity.

〔Test method〕

Film thickness: Film thickness was calculated from the amount of electricity required for reduction using a constant current reduction method. Therefore, this film thickness is expressed as the thickness of a dense film with no voids or a flat film with no irregularities. When determining the thickness of the first copper layer and the second copper layer,
X-ray diffraction method was also used. Absorption rate:

[Formula] (∫ ^{2.5 0.3} _means that the spectral range _of sunlight is 0.3 to 2.5
to μ
where α: Absorption rate (with respect to solar energy) α: Absorption rate at wavelength λ I: Radiant intensity at wavelength λ of sunlight Emissivity:

[Formula] 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°C ε: Emissivity at wavelength λ (relative to black body) The reflectance P〓 in the infrared region was measured using an infrared spectrophotometer, and it was set as ε〓=1−P〓.

[Brief explanation of the drawing]

Figure 1 is a schematic cross-sectional view of a leaf-like or fibrous nickel crystal, Figure 2 is a cross-sectional view of a granular nickel crystal, and Figure 3 shows the wavelength of sunlight and the solar heat absorber (selected). 3 is a graph showing the relationship between the optical characteristics of absorbing films with and without cuprous oxide mixed therein. 1... Foliate or fibrous nickel crystal, 2...
Granular nickel crystal.

Claims (1)

[Claims] 1. A selective absorption film made of cupric oxide and cuprous oxide is formed on the surface of a base material whose surface is made of nickel and whose surface is not flat. The total thickness of the absorption membrane is 0.05 to 0.50 microns, and 0.005 microns or more of that
A solar heat absorber characterized in that less than 0.03 microns is occupied by cuprous oxide. 2 The surface of the base material is made of a nickel film with a leaf-like or fibrous crystal shape, and the average length of the crystals in the longitudinal direction is 0.5 to 2.0 microns, and the average distance between the tips of the crystals is 0.5 to 2.5 microns. A solar heat absorber according to claim 1. 3. Claim 1, wherein the surface of the base material is made of a nickel film having a granular crystal shape, the average grain size of the crystals is 0.1 to 1 micron, and the average spacing between the grains is 0.1 to 0.4 micron. Solar heat absorber as described in section. 4. The solar heat absorber according to claim 2 or 3, wherein the nickel film has a thickness of 0.1 to 3 microns.