JPS5845443A - Solar heat abosrbing body - Google Patents

Abstract

PURPOSE:To obtain the solar heat abosrbing body excellent in the selective absorption characteristics but stabilized in the color tone thereof by a method wherein a selective absorption film, consisting of at least one of black Cr and black Ni, is formed on the surface of a Ni base of which surface is not flat. CONSTITUTION:The solar heat abosrbing body is consisting of the selective absorption film, consisting of at least one of the black Cr and the black Ni, formed on the surface of the base, of which surface is made of Ni at least and is not flat. The surface of the base is consisting of a nickel film 1 having a leafy or fibrous crytal form, in which a length in an average longer diametral direction is 0.5-2.0mu and the average distance between tip ends of the crystal is 0.5-2.5mu. The surface of the base is consisting of a nickel film 2 having granular crystal form, in which the average graular diameter is 0.1-1mu and the average distance between grains is 0.1-0.4mu. The thickness of the selective absorption film is 0.05-1mu.

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, black nickel-based and oxidation-based selective absorption films, and selective absorption films obtained by subjecting the surface of stainless steel to oxidation treatment. Among these conventional selective absorption membranes, copper oxide-based selective absorption membranes have drawbacks in heat resistance and corrosion resistance, and stainless steel-based selective absorption membranes have problems in thermal conductivity and processability. Both have major practical drawbacks. On the other hand, black chromium-based materials are said to have problems in terms of cost and pollution, and black nickel-based materials are said to have problems in moisture resistance and UV resistance. However, regardless of these problems, black chromium films and black nickel films are superior to other selective absorption films, and are expected to be used as selective absorption films for high-temperature collectors and the like.

A major practical drawback of black chromium films and black nickel films is that the color tone is not stable. Unstable color tone means that the black chromium film or black nickel film partially takes on a slight yellow, green, or red color tone.

A black chromium film or a black nickel film that has such a color tone does not have stable selective absorption characteristics. That is, within a single selective absorption film, there may be a portion where the absorption rate of sunlight is particularly low or a portion where the emissivity is particularly high. Furthermore, the selective absorption characteristics of each selective absorption membrane tend to be different, making it difficult to obtain membranes of the same quality. Furthermore, the IC selective absorption characteristics may deteriorate over time.

This instability in color tone is thought to be caused by slight changes in the thickness of the black chrome film or black nickel film.
It is very difficult to make the film thickness exactly the same, and it can be said to be virtually impossible.

The present invention was made in view of the above circumstances, and it is an object of the present invention to provide a solar heat absorber that is made of black chromium or black nickel and has a stable color tone and has excellent selective absorption characteristics. This will be explained below.

The solar heat absorber according to the present invention has at least a surface made of nickel, and a selective absorption film made of at least one of black chromium and black nickel is formed on the surface of a base material whose surface is not flat. It is characterized by being done.

The base material is made entirely of nickel, or
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 of the selective absorption membrane. The purpose of its use is
This is to improve the corrosion resistance and heat resistance of the solar heat absorber. 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 using a solar heat absorber, if it is not exposed to sunlight without conducting heat medium, the temperature may rise to 200°C.
High temperatures can reach as high as 1, which causes oxidation of the base material.When the base material is oxidized, the absorption characteristic α of the selective absorption film and the irradiation sensitivity deteriorate.

In this respect, it is preferable to use Nikel, which is chemically and thermally stable, 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, which is the same as in the prior art. However, the characteristic feature is that the underlying nickel surface is not flat. If the surface force f of the nickel base is smooth,
The base itself has 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 black nickel or black chromium is formed on this nickel base, a mutual effect is produced between them, and the absorption characteristics are further improved. This is thought to be because sunlight in the visible and near-infrared regions causes multiple reflections on the surface of this nickel base, making it easier to absorb.

The nickel base preferably has one of the two crystal forms described above, one of which is a leaf-like or fibrous crystal form and the other a granular crystal form. be.

When the nickel base, which acts as a kind of selective absorption film, is formed as a film on the core material, microscopically it is not a single continuous film, but a film of clusters of the above crystal particles. It can be said that it is a thing. Crystals grow in the form of successive crystal pieces growing from one crystal piece. If so, the result will be as shown in Figures 1 and 2.The first object shows crystal particles that are leaf-like or fiber-like, and the second object shows crystal particles that are granular.The size of the crystals, It is preferable that the crystal spacing is uniform.The average length of the longest part of the leaf-like or fibrous crystal grains l in FIG. It is preferably in the range of 2.0 microns (10 m, hereinafter abbreviated as "μ"), and the average distance between the tips of the crystal grains (indicated by l in Figure 3) is 0.5
It is preferably in the range of ~2.5μ. 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. The average grain size of the granular crystal grains 2 in FIG. It is preferable that the average distance separating them is in the range of 0.1 to 0.4 μ.

The nickel base with the above crystals causes multiple reflections on the surface of visible light and near-infrared light, so
It absorbs visible light and near-infrared light well, but the wavelength is 2.5μ.
It is thought that it acts as a flat surface for the above infrared light weight, reflects infrared light well, and therefore has a low emissivity.

The nickel base may contain a small amount of impurities such as a small amount of oxide due to surface oxidation.

In addition, such a nickel film as a base 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 consisting of leaf-like or fibrous crystals, the most stable manufacturing method is to use electrolytic plating as described below.

That is, as a plating solution, 50 to 30% of nickel chloride is used.
0f/j (preferably 150 to 25of/j), boric acid 20 to 6of/l<30 to 50f/j), and a liquid temperature of 2o to 70'C (preferably 55 to 50f/j).
65°C), current density of 0.5 to 4 A/dm" (preferably 1 to 3 A/am"), and plating time of 0.5 to 8 minutes (preferably 1 to 2 minutes). It is.

As the core material, various metal materials such as iron plate, m plate, copper plate, etc. can be used. When performing such nickel plating on the surface of a core material made of an iron or copper plate, it is generally necessary to perform pretreatment such as degreasing (electrolytic degreasing if necessary) and acid precipitation. When a stainless steel copper plate is used as the core material and the plate is plated with nickel, the following pretreatment is usually required, for example.

That is, after degreasing with ordinary chemicals and electrolytic degreasing, activation treatment is performed by cathodic electrolysis using 5 to 30% sulfuric acid, and then nickel strike plating is performed under the following conditions.

(Treatment liquid composition) Nickel chloride 50-300 f/1
Hydrochloric acid 50-2009/1 (Processing conditions) Liquid temperature: Room temperature Current density: 5-10 A/dm" Processing time: 0.5-5 minutes In this way, the core material subjected to the necessary pretreatment is plated. If nickel plating is applied by this method, a blackish-gray or grayish nickel plating layer will be formed on the surface of the core material.At that time, the thickness of the nickel plating layer should be within the above range of 0.1 to 3μ. It is preferable to select the plating processing conditions based on the above.As mentioned above, nickel plating consists of clusters of crystals, so it actually has a complex unevenness and fine structure.Therefore, it is usually Since it is measured by a thickness measurement method, the above-mentioned film thickness is calculated based on a state in which these irregularities are smoothed out and there are no voids.

Next, by further forming a selective absorption film of black chromium or black nickel on the nickel base formed in this way, it is possible to obtain a highly practical solar heat absorber with extremely excellent selective absorption characteristics. can. In other words, although solar heat absorbers equipped with black chromium or black nickel selective absorption films have very good selective absorption characteristics, they have the disadvantage of poor heat resistance and corrosion resistance, and that they gradually deteriorate after long-term use. However, by forming a selective absorption film made of black chromium or black nickel on a base metal layer made of nickel, durability is further improved because the base metal layer is thermally and chemically stable. Because it does. Such a selective absorption film made of black chromium or black nickel is formed, for example, by a plating method on a gold base. In this case, the above-mentioned nickel base with an uneven surface has extremely good adhesion to the black chromium or black nickel selective absorption film formed thereon, and has the effect of firmly holding the black chromium or black nickel layer. have This is because the nickel base has microscopically extremely fine uneven surfaces, and these uneven surfaces and black chromium or black nickel crystals intertwine in a complex manner, resulting in a strong bond between them. Conceivable. When the underlying metal layer is made of nickel film,
It is preferable that the thickness is about 0.1 to 3 μm.

This is because if the thickness is outside this range, the adhesion of the black chromium or black nickel layer formed on the top layer tends to decrease.

Next, a specific example of a plating method for forming a selective absorption film made of black chromium or black nickel is listed below.

Black chromium is a mixture of Crew and Cr, and one of its plating methods employs the following conditions.

(Plating solution composition) Oxidation 9 o A (W) (Crys) 200
~450 f/1 Sodium hydroxide (Mail()
60-70 y/l hexafluorosilicic acid (
HJiF@) 0.5~11/1 sucrose
2-3 f/1 (processing conditions) Current density 20-25 A/d-liquid
Temperature: 15-20°C Black nickel is a mixture of NiS and ZmS, and one of its plating methods has the following conditions.

(Plating solution composition) Nickel sulfate 75-901/1 Ammonium sulfate 40-501/1 Zinc sulfate
35-20 f/1 Sodium thiocyanide 15-20 1/1 (processing conditions) Current density 0.3-IA/dm" Liquid temperature 20-25°C The thickness of the selective absorption membrane obtained here is 0.05- Preferably, the thickness is 1 μ.The selective absorption film thickness is measured by a conventional plating thickness measurement method.

A transparent coating of silicone resin or other material may be formed on the selective absorption membrane to protect it.

The solar heat absorber thus obtained has excellent selective absorption characteristics. Figure 133 shows a conventional solar heat absorber (A)
It is a graph for comparing the selective absorption characteristics of the solar heat absorber (B) and the solar heat absorber (B) according to the present invention. The solar heat absorber (A) is placed on a 1μ thick nickel base with a flat and shiny surface.
It is plated with black chrome plating with a thickness of 0.2μ.

The solar heat absorber (B) has a 0.2'μ thick black chromium plated on a 1μ thick surface or non-flat nickel base. Referring to Figure 3, (A), (B
), it can be seen that (B) has a higher absorption rate than (A) in both the visible sunlight region and the near-infrared region.

FIG. 4 is a graph qualitatively showing the relationship between the thickness of the selective absorption film and the absorption rate and emissivity.

In the figure, the solid line shows the selective absorption characteristics of the solar heat absorber, which has a smooth nickel surface and is plated with black chromium or black nickel, and the broken line shows the solar heat absorber according to the present invention, that is, the surface is The selective absorption characteristics of the solar heat absorber (B'), which is made of black chromium or nickel plating on a non-flat nickel base, are shown below. The upper solid line and broken line indicate the absorption rate -, and the lower solid line and broken line indicate the emissivity -. As shown in the figure, in both solar heat absorbers (A') I (B'), as the thickness of the selective absorption film increases, the emissivity 6 increases, so it is better to make the film thickness as thin as possible. On the other hand, the absorption rate α decreases rapidly when the film thickness becomes thinner than a certain level.
Such a situation is undesirable, and therefore, it is required to ensure a certain level of film thickness.

Therefore, the film thickness at which the emissivity C and the absorption coefficient α are balanced is the point at which the film thickness is 1 in (A') and the point at which the film thickness is 1 in (B'). Film thickness is film J! ＃' Kojiku than a,
Therefore, the solar heat absorber (B') according to the present invention is
It can be seen that the balance is better where the selective absorption membrane is thinner than in (A'). Therefore, according to this invention,
By making the film thinner, the emissivity can be lowered.

Next, regarding color stability, in the solar heat absorber according to the present invention, since the surface of the nickel base is not flat, the base itself exhibits a black-gray or gray color. Therefore, if a selective absorption film made of black chromium or black nickel is formed on top of this, even if the thickness of this selective absorption film varies to some extent, the resulting color tone will be almost invisible to the naked eye. Therefore, the selective absorption characteristics become stable. On the other hand, in the case of conventional solar heat absorbers with flat underlying surfaces, slight yellow, green, or
This may be due to the high reflectance in some parts of colors such as red and in the near-infrared region, but such color unevenness becomes even more noticeable after a heat resistance test. This means that in the case of conventional solar heat absorbers, their selective absorption properties are not stable and degrade when exposed to sunlight, etc.
] It means that there is a possibility.

As described above, the solar heat absorber according to the present invention has excellent selective absorption characteristics and is stable compared to conventional solar heat absorbers.

Next, examples and comparative examples of the present invention will be described.

The solar heat absorbers of Examples and Comparative Examples were made as follows.

(Example 1) A copper plate with a thickness of 0.5 mm was used as a core material, and after being subjected to alkaline degreasing and pickling with 3% -1 cI,
Nickel chloride 200171. Using a plating solution consisting of boric acid 40 f/1, solution temperature 60°C, current density 2, -A/d.
Nickel plating was carried out under the conditions of 3 minutes of plating time.The resulting nickel film had a lli locus of +!1μ. When observed with an electron microscope (7,600x magnification), it was found that it was fruit-like or fibrous. It was confirmed that crystals of the shape were formed.The average length of the crystals in the major axis direction was 0.8 μ.

By further applying black chromium plating to the base material having the nickel film obtained as above on its surface under the following conditions, a solar heat absorber with a black chromium film thickness of approximately 0.5μ was obtained. Ta.

(Plating solution composition) Chromic acid 4001/1 Sodium hydroxide (caustic soda) 70 fl/IH
, SiF, 0.
5 f/1 glaze 21
/1 (processing conditions) Current density 25 A/dm” Liquid temperature
20°C Plating time: 1 minute (Example 2) A 0.5 mm thick copper plate was used as the core material, and after degreasing with alkali and pickling with 2% HCl,
Using a plating solution consisting of nickel chloride 400 f/l and boric acid 451/1, the solution temperature was 55°C and the current density was 1.9 A/l.
Nickel plating was carried out under the conditions of 3 minutes of plating time.The thickness of the obtained nickel film was approximately 0.9μ.When observed with an electron microscope (7,600x magnification), granular crystals were observed. It was confirmed that the average grain size of these crystals was O55μ, and the average spacing between particles was 0.2μ.
Met.

The base material having the nickel film obtained as described above on its surface was plated with black nickel under the following conditions to produce a solar heat absorber.

(Plating solution composition) Nickel sulfate 751/1 Ammonium sulfate 401/1 Zinc sulfate
37 f/1 sodium thiocyanide
15 f/1 (processing conditions) Liquid density 0.6 A/dm” liquid
! pH of the 25°C solution, s /, + time 0.5 minutes (Example 3) Using a stainless steel plate with a thickness of 0.3 mm as the core material,
After applying alkaline degreasing and electrolytic degreasing to this, 5%
3 using sulfuric acid at a current density of 10 A/dm” at room temperature.
Pretreatment was performed by cathodic electrolysis for 0 seconds. Next, using a plating solution consisting of nickel chloride 20 (1//) and hydrochloric acid 50 ml/l, a current density toA/dm" and a solution temperature of 20 to 2
Primary nickel plating was performed at 5°C and plating time of 0.5 minutes to form a primary nickel film on Table 1 of the base material. Continuing on top of this plating layer, nickel chloride 30 (1
Secondary nickel plating was performed using a plating solution consisting of #7 and boric acid 501/l at a current density of 2 A/dm, a liquid temperature of 60°C, and a plating time of 2 minutes.The above primary nickel plating This is to improve adhesion to stainless copper, and the secondary nickel plating is to avoid forming a nickel base film consisting of leaf-like or fibrous crystals.Thickness of the resulting nickel film was 1.1μ.The average length of the crystal in the major axis direction was approximately 1.5μ.
, the average spacing of the tips was about 0.9μ.

The base material having the nickel film obtained as described above on its surface was subsequently plated with black chromium under conditions similar to plating. As a result, a solar heat absorber having a black chromium film thickness of 0.15 μm was obtained.

(Plating solution composition) Chromium oxide (Vl) (Cr03) 250
f/1 amidosulfonic acid (NHzSOsH)
20 fl/1 boric acid (HsBOa)
10'I/1 (processing conditions) First-class density 30 A/dm" liquid
Temperature: 25° C. (Example 4) A base material having a nickel film on the surface was prepared in the same manner as in Example 3, and black nickel plating was applied to this base material under conditions similar to that of a rainbow. This reduces the thickness of the black nickel to 0.
A 2μ solar heat absorber was obtained.

(Plating solution composition) Nickel sulfate 54 f/1 Nickel ammonium sulfate 771/1 Zinc sulfate
29 f/1 Sodium thiocyanide 5 f/1 (processing conditions) First class density o, 5A/dm! liquid
Temperature: 22°C (Comparative Example 1) In Example 1, instead of the base nickel plating,
It has been given a bright nickel plating similar to that of a tsuki. That is, a substrate with a nickel film having a glossy surface was plated using a plating solution containing nickel nitrate as a main component. The thickness of this nickel film was 1-μ as in Example 1. The base material obtained as described above was plated with black chrome in the same manner as in Example 1. As a result, a solar heat absorber having a black chromium film thickness of approximately 0.5 μm was obtained.

(Comparative Example 2) A selective absorption film was prepared by performing the same treatment as in Example 2 on a layer material having a bright nickel surface obtained in the same manner as in Comparative Example 1.

(Comparative Example 3) A selective absorption film was prepared by performing the same treatment as in Example 3 on a substrate having a bright nickel surface obtained in the same manner as in Comparative Example 1.

Absorption rate and emissivity of each solar heat absorber obtained as described above were measured, and a color unevenness test was also conducted. The results are shown in Table 1.

$X Table Selective absorption membranes with the same type and thickness, that is, Example 1 and Comparative Example 1. Example 2 and Comparative Example 2. Comparing Example 3 and Comparative Example 3, the absorption rate is higher in both Examples. And the emissivity is the same. On the other hand, regarding color unevenness, all the comparative examples have partial unevenness, whereas all the examples have almost no unevenness. From this, it can be seen that the solar heat absorbers of the examples are all superior to the comparative examples.

The absorption rate, emissivity, and color unevenness in Table 1 are as follows, where α
; Absorption rate (relative to total solar energy) αλ; Absorption rate at wavelength λ lλ; Radiation intensity of sunlight at wavelength λ, where 1; Emissivity (relative to total black body radiant energy) Sλt=tse; Black body at 150°C radiant intensity of wavelength λ from
And so.

Color unevenness: A 150 x 300 mm test piece was used and visually judged.

[Brief explanation of drawings]

Figure 1 is a cross-sectional schematic diagram of a leaf-like or fibrous nickel crystal, iJ2 diagram is a cross-sectional diagram of a granular nickel crystal, and s3 diagram shows the relationship between the wavelength of sunlight and the optical characteristics of a solar heat absorber. Figure 4 is a graph showing the relationship between the selective absorption film and optical properties of a solar heat absorber. l... Leaf-shaped or fibrous nickel crystal 2... Granular nickel crystal Patent applicant: Matsushita Electric Works Co., Ltd. Representative Patent attorney: Takehiko Matsumoto Figure 1 Figure 2

Claims (5)

[Claims] (1) A solar heat absorber in which a selective absorption film made of at least one of black chromium and black nickel is formed on the surface of a base material whose surface is made of nickel and whose surface is not flat. . (2) The surface of the base material is made of a nickel film having a leaf-like or fibrous crystal form, and the average length of the crystals in the longitudinal direction is 0.
5 to 2.0 microns, and the average spacing between the tips of the crystals is 0.5 to 2.5 microns.
Solar heat absorber as described in section. (3) The surface of the base material is made of a nickel film with a granular crystal shape, and the average grain size of the crystals is 0.1 to 1 micron, and the average interval between particles is 0.1 to 0.4 micron. The solar heat absorber according to claim 1. (4) The solar heat absorber according to claim 2 or 3, wherein the nickel film has a thickness of 0.1 to 3.0 microns. (5) The solar heat absorber according to any one of claims 1 to 4, wherein the selective absorption film has a thickness of 0.05 to i microns.