WO2005042805A1

WO2005042805A1 - Method of depositing selectively absorbent film on a metal substrate

Info

Publication number: WO2005042805A1
Application number: PCT/TR2003/000081
Authority: WO
Inventors: Figen Kadirgan
Original assignee: Figen Kadirgan
Priority date: 2003-10-31
Filing date: 2003-10-31
Publication date: 2005-05-12
Also published as: AU2003283949A1; TR200602074T1

Abstract

This invention relates to a method of coating metal substrates, especially solar collectors, with selectively absorbent films and a product obtained thereof. In this invention the metal substrate panel, which is cleaned, degreased and brightened, is coated with a layer of nickel at the first step and an outermost layer of absorbent nickel oxide-nickel zinc sulphide by electrochemical deposition at relatively low temperatures at the second step.

Description

METHOD OF DEPOSITING SELECTIVELY ABSORBENT FILM ON A METAL SUBSTRATE

Technical field

This invention relates to a method of coating metal substrates, especially solar collectors, with selectively absorbent films and a product obtained thereof.

Background of the invention

Spectrally selective metal surfaces absorb light in the ultraviolet and near infrared regions of the spectrum, whereas they emit light in the far infrared region. It is important for solar collectors that the absorbance rate is high and the emittance rate is low in the respective regions mentioned; because, high absorbance rate ensures that most of solar energy is converted to thermal energy in the metal substrate whereas the low emittance rate prevents loss of the gained thermal energy by radiation from the surface in the far infrared region of the spectrum. Metal substrates having an absorbance to emittance ratio higher than 4 can be fabricated to provide that the heat absorbed is transferred to fluid conduits in the metal body for subsequent use in heating and cooling operations.

Aluminium, copper, low carbon steel (or the alloys of these metals) or other metals can be used as the metal substrate in high efficiency solar absorbers. Coated aluminium absorbers are especially valuable for heat exchange units because of the lightness of aluminium and the resultant decrease in the complexity and weight of structural elements required for support thereof. Moreover, the easy machining and fabrication of them add to their value. Coated copper absorbers are also very valuable for heat exchange units and more resistant to the corrosion, h order to overcome the weight problem, it is possible to use very thin layers of copper as substrate. On the other hand, coated low carbon steel absorbers are interesting from the cost point of view. Black nickel coatings are known as solar selective surfaces for the conversion of solar radiation into useful heat (J. Jurisson, et al, J. Vac. Sci. Technol. 12: 1975, 1010; H.Tabur, Low Temperature Engineering Application to Solar Energy, ed.R.C. Jordan, New York: 1969, p. 41; R.B. Pettit, R.R. Sowell, J.Vac.Sci. Technd. 13: 1976, 596). These coatings possess good optical properties but the durability shown is often poor. They degrade completely after exposure to 200°C. Selective coatings may be prepared by chemical conversion, vacuum techniques or electrochemical ways.

Such processes are well-known form prior art documents. For instance, the invention disclosed in US patent no. 2,917,817 consists in a receiver for solar heaters, which is basically a composite body comprising a metal base and a selectively absorbent thin coating applied to the said base. The nickel base is immersed into an aqueous electrolytical bath containing nickel sulphate, zinc sulphate, ammonium sulphate, ammonium tiocyanate, and citric acid. The bath temperature is about 30°C. When the base is aluminium, it is first covered with an oxide layer; then immersed in a solution of copper nitrate, nitric acid, and potassium permanganate at 85-90°C; and finally dried and heated to 450°C so that the surface colour becomes almost black.

US patent no. 4,177,325 discloses a panel for selectively absorbing solar energy and a method of producing thereof. This panel comprises an aluminium substrate, a layer of zinc thereon, a layer of nickel over the zinc layer, and a layer of nickel oxide. Copper substrate may well be used for the same purpose. The chemical oxidation of the nickel layer is performed at temperatures as high as 900-950°F.

US patent no. 3,920,413 discloses that an aluminium metal substrate can be cleaned, prepared to receive a brightening layer, coated with the brightening layer and further coated with a very thin solar thermal energy absorbing coating of black nickel. The bath temperature is in the range from about 115° to about 140°F. European patent no. 0029257 explains a black, selectively absorbing layer consisting of finely comminuted nickel, nickel-zinc or nickel-lead on a substrate, particularly a solar collector plate, which is protected from corrosion by a thin passivating chromic-oxide layer. Corrosion resistance in this invention is mainly achieved by the application of the chromate-layer.

However, it is mentioned in the literature that nickel black selective surfaces deteriorate in time and loose their optical properties under heat and moisture. Especially, surfaces heated up to 200°C loose their selectivity completely by the oxidation of copper or other metals. These effects considerably reduce the service lives of solar collectors.

Summary of the invention

The object of the present invention is to obtain a highly efficient, corrosion resistant solar collector with a long service life and low costs.

For practical purposes, a selective absorber should have good optical properties, good thermal stability, low cost and ease of large-scale production. Keeping these considerations in view, in the present invention electrochemical methods are proposed for the preparation of nickel black absorber surfaces, at two stages.

In this invention the metal substrate panel, which is cleaned, degreased and brightened, is coated with a layer of nickel at the first step and an outermost layer of absorbent nickel oxide-nickel zinc sulphide mixture by electrochemical deposition at relatively low temperatures at the second step. Copper, low carbon steel or steel substrates are treated according to this procedure. However aluminium substrates are first coated with a layer of zinc before the nickel layer, and the nickel layer is applied thereon. The solar absorbance and emittance of the resulting collectors were found to be 0.95and 0.08, respectively. Description of the invention

All the metallic substrates must be clean before the electrochemical deposition processes. They could cleaned by the following steps:

• Degreasing in an appropriate aqueous solution of alkaline cleaner containing NaOH, sodium silicates or sodium carbonates at 60-90°C for 10 minutes;

• rmmersing in an appropriate brighter dip solution such as phosphoric acid 50%, nitric acid 10% and sulphuric acid 40% at 90-95°C;

• Soaking at 50-70°C for 30 seconds in an alkaline cleaner;

• Dipping in a diluted acid solution (e.g. 40-55 % nitric acid) at room temperature. During the cleaning process, the metal substrates are washed with deionized water between each of the steps.

Having cleaned, the metal substrates are treated according to the following method:

• Nickel plating with a solution containing nickel sulfamate, sulphate, phosphate or chloride at 1-20 A/dm² at 40-60°C for 10 - 30 minutes.

• Absorbent nickel oxide-sulphide plating with a solution containing nickel sulfamate, sulphate, chloride, phosphate or nitrate of the following composition: - nickel salt 75-150 g/1, ammonium chloride, sulphate, phosphate or nitrate 11- 35 g/1, - zinc chloride, sulphate, phosphate or nitrate 8 -35 g/1, sodium or potassium tiociyanide or sulphide 3-18 g/1, Electroplating is performed for 0.1-15 min., at 20-35°C, under a potential difference of 1-20 A dm². The resulting coating is deep blue-black in color and the absoφtivity of the specimens is 0.93-0.97 whereas the emissivity is 0.05-0.12. If the metal substrate is aluminium, between the cleaning and nickel plating steps, a zincating step is performed.

The invention will now further be explained with reference to the following specific examples for a clearer understanding of this invention, which are merely illustrative and are not to be understood as limiting the scope and underlying principles of this invention in any way.

Example 1 : Aluminium substrates or its alloys are converted to selective absorbers for solar energy by the following steps: • degreasing in an alkaline solution for 10 minutes at 60-70°C • neutralising with 40-50%o nitric acid at room temperature • immersing in bright dip solution at 90-95°C for 8-10 minutes • immersing in alkaline solution at 50-70°C for 30 seconds • neutralising with 40 or 50%> nitric acid at room temperature • zincating with a solution containing zinc oxide (30-60 g/1), sodium cyanide (40- 80 g/1) and sodium hydroxide (60-90 g/1) at 25-50 °C • nickel plating with a solution containing nickel sulfamate, sulphate, phosphate or chloride of following composition: nickel content 65-90 g/1, - boric acid 30 - 45 g/1, - anti-pitting agent 0.5-1 g/1, Electroplating is done at 1-20 A/dm² at 40-60°C for 10-30 minutes. • Absorbent nickel plating with a solution containing nickel sulfamate, sulphate, chloride, phosphate or nitrate of the following composition: - nickel salt 75-150 g/1, - ammonium chloride, sulphate, phosphate or nitrate 11-35 g/1, - zinc chloride, sulphate, phosphate or nitrate 8 -35 g/1, - sodium or potassium tiociyanide or sulphide 3-18 g/1, Electroplating is done at 20-35°C, under 1-20 A/dm².

The resulting selective coating is deep blue-black in color and the absorptivity of the specimens is 0.93-0.97 and the emissivity is 0.05-0.12.

Example 2:

Copper substrates or its alloys are converted to selective absorbers for solar energy by the following steps: • degreasing in an alkaline solution for 10 minutes at 60-70°C • neutralising with 40-50% nitric acid at room temperature • immersing in bright dip solution at 90-95°C for 8-10 minutes • immersing in alkaline solution at 50-70°C for 30 seconds • neutralising with 40 or 50% nitric acid at room temperature • nickel plating with a solution containing nickel sulfamate, sulphate, phosphate or chloride of following composition: nickel content 65-90 g/1, - boric acid 30 - 45 g/1, anti-pitting agent 0.5-1 g/1, Electroplating is done at 1-20 A/dm² at 40-60°C for 10 - 30 minutes. • Absorbent nickel plating with a solution containing nickel sulfamate, sulphate, chloride, phosphate or nitrate of the following composition: - nickel salt 75 - 150 g/1, ammonium chloride, sulphate, phosphate or nitrate 11-35 g/1, - zinc chloride, sulphate, phosphate or nitrate 8 -35 g/1, sodium or potassium tiociyanide or sulphide 3-18 g/1, Electroplating is done at 20-35°C, under 1-20 A/dm². The selective coating is deep blue-black in colour and the absoφtivity of the specimens is 0.93-0.97 and the emissivity is 0.05-0.12.

Example 3: Low carbon steel or steel substrates or its alloys are converted to selective absorbers for solar energy by the following steps: • degreasing in an alkaline solution for 10 minutes at 60-70°C • neutralising with 40-50% nitric acid at room temperature • immersing in bright dip solution at 90-95°C for 8-10 minutes • immersing in alkaline solution at 50-70°C for 30 seconds • neutralising with 40 or 50% nitric acid at room temperature • nickel plating with a solution containing nickel sulfamate, sulphate,phosphate or chloride of following composition: - nickel content 65-90 g/1, - boric acid 30 - 45 g/1, - anti-pitting agent 0.5-1 g/1, Electroplating is done at 1-20 A/dm² at 40-60°C for 10 - 30 minutes. • nickel plate with a solution containing nickel sulfamate, sulphate, chloride, phosphate or nitrate of the following composition: nickel salt 75 - 150 g/1, ammonium chloride, sulphate, phosphate or nitrate 11- 35 g/1, zinc chloride, sulphate, phosphate or nitrate 8 -35 g/1, sodium or potassium tiociyanide or sulphide 3-18 g/1, Electroplating is done at 20-35°C, under 1-20 A/dm².

The selective coating is deep blue-black in color and the absoφtivity of the specimens is 0.93-0.97 and the emissivity is 0.05-0.12.

Claims

1. A method of depositing a selectively absorbent film on a metal substrate panel, comprising the steps of a. cleaning, degreasing and brightening the metal substrate b. coating with a first layer of nickel and characterised in that the method further includes the step of c. coating with an outermost layer of absorbent nickel oxide-nickel zinc sulphide layer by electrochemical deposition at relatively low temperatures.

2. The method of Claim 1, characterised in that the metal substrate is selected from the group consisting of copper, low carbon steel, steel and their alloys.

3. The method of Claim 1, characterised in that the metal substrate is aluminium.

4. The method of Claim 3, characterised in that the substrate is coated with a layer of zinc before the nickel plating step.

5. The method of any of the Claims above, characterised in that the resulting coating is deep blue-black in color and the absoφtivity of the specimens is 0.93- 0.97 whereas the emissivity is 0.05-0.12.

6. The method according to any of the claims above, characterised in that the cleaning step comprises the steps of a. Degreasing in an appropriate aqueous solution of alkaline cleaner containing NaOH, sodium silicates or sodium carbonates at 60-90°C for 10 minutes; b. Immersing in an appropriate brighter dip solution such as phosphoric acid 50%, nitric acid 10% and sulphuric acid 40% at 90-95°C; c. Soaking at 50-70°C for 30 seconds in an alkaline cleaner; d. Dipping in a diluted acid solution (e.g. 40-50 % nitric acid) at room temperature.

7. The method of Claim 6 above characterised in that the metal substrates are washed with deionized water between each of the steps of the cleaning process.

8. The method of any of the claims above, characterised in that the nickel plating is performed with a solution containing nickel sulfamate, sulphate, phosphate or chloride at 1-20 A/dm² at 40-60°C for 10 - 30 minutes.

9. The method of any of the claims above, characterised in that the absorbent nickel oxide-nickel sulphide plating is performed with a solution containing nickel sulfamate, sulphate, chloride, phosphate or nitrate of the following composition: a. nickel salt 75-150 g/1, b. ammonium chloride, sulphate, phosphate or nitrate 11-35 g/1, c. zinc chloride, sulphate, phosphate or nitrate 8 -35 g/1, d. sodium or potassium tiociyanide or sulphide 3-18 g/1, wherein the electroplating is performed for 0.1-15 min., at 20-35°C, under a potential difference of 1-20 A dm .