JPS6021228B2 - Two-step process for creating a darkened surface on aluminum - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Solar energy can be utilized in a very simple manner using solar collectors without polluting the environment.

The collector consists of an absorbing surface, in which the sunlight is converted into heat, which is then delivered to an associated tubing system, in which the heat is absorbed by the circulating liquid and also to the user. The sun has near infrared rays and visible light, so approximately 0.2 French m~2
．． It provides the energy of 5 Buddha wavelengths to the collector.
A good absorber should absorb this energy quantitatively (absorption coefficient Q=1), ie it should be black in this wavelength range. Normally, a black body also has good emissivity (emission coefficient = 1), so Q/
The ratio is equal to 1, resulting in high thermal radiation losses. The term "selective absorber" means one that suppresses this thermal fin radiation loss so that the Q/co ratio is greater than 1. A surface with a Q' ratio of ≧5 is a highly selective surface. Aluminum can be blackened by a series of known methods, such as anodic, chromic acid and chemical coloring methods; all these methods yield a surface with a ratio of approximately 1, and the blackening is It is co-suitable in that it is not selective. Surprisingly, however, the oxide layer produced by chemical oxidation in an alkaline bath containing chromic acid is selectively dyed black if the layer thickness is kept small, i.e. at most 2.5 mm. It has been found.

The composition of the dye bath, ie the concentration and the mutual proportions of the salts, can be chosen practically arbitrarily. However, the pH value of the solution is 1
．． Must be between 5 and 4.5. The layers produced by chemical oxidation are colorless to pale gray in this case and are unsuitable as absorption layers, and dyeing is extremely difficult in the case of the thin layer thicknesses according to the invention. Dyeing is achieved in a second step in a potassium permanganate-containing solution, in which case blackening is only possible by adding cobalt and/or copper salts. The process is detailed below with some examples: 1) Degreased and briefly soaked aluminum in a solution heated to 95° C. consisting of 50 t/d of sodium carbonate and 15 t/d of sodium chromate. Process for 2 minutes.

A pale gray oxide layer with a thickness of 0.5 mm is formed. After thorough washing, blackening is carried out for 5 minutes at a pH value of 2 in a dyeing bath consisting of 10 μl of potassium permanganate and 20 μl of cobalt nitrate. The temperature of the pot was 90:00, and the pH value was adjusted with nitric acid. The absorber layer has an absorption coefficient Q=0.90 and q/Q is 7.4. 2) After suitably cleaning the aluminum, treat it for 1 minute in a solution heated to 100 qo consisting of 45 t/d sodium carbonate and 15 t/d sodium chromate.

The resulting oxide layer, after thorough washing, is dyed black in a solution heated to 90°C, consisting of 100 parts potassium permanganate, 10 parts cobalt nitrate and 4 parts nitric acid. . The surface layer has Q=0.85 and the Q' ratio is 7.4. 3) Oxidize the washed and soaked aluminum in a solution consisting of 55 parts/Z of sodium carbonate and 20 parts/Z of sodium chromate for 5 minutes.

If this layer is dyed in a solution heated to 90°C containing 10 parts of potassium permanganate, 10 parts of cobalt nitrate and 4 parts of acetic acid in one solution, this layer will have a Q
It has an absorption coefficient of =0.83. 4) Aluminum oxidized to a layer thickness of 2 mm in an alkaline chromic acid bath was mixed with potassium permanganate 10 mm/h and copper nitrate 100 mm.
Consisting of evening/evening, adjusted to 1.5 with nitric acid, 90
When dyeing in a solution heated to °C, the absorption coefficient Q = 0
．． 9 and a selective blackening with a Q/ratio of 3.2 is obtained.

5) Sodium carbonate 50/day and potassium chromate 2
Oxidation of aluminum in the heated solution for 1 minute yields an oxide layer 0.5 mm thick.

Claims (1)

[Claims] 1. In a two-step process for providing a darkened surface on aluminum with selective absorption properties, especially for solar collectors, the aluminum is chemically treated in the first step in an alkaline chromic acid bath in the shortest possible time. (layer thickness: up to 2.5 μm) and dyeing this oxide layer black in a second step in a hot solution consisting of potassium permanganate and cobalt nitrate or copper nitrate. A two-step process for providing a darkened surface on aluminum. 2 The dyeing tank contains 1 to 200 g/l of potassium permanganate,
2. A method according to claim 1, which advantageously contains between 5 and 30 g/l. 3. Process according to claim 1 or 2, in which the dye bath contains 1 to 100 g/l, preferably 5 to 25 g/l of cobalt nitrate. 4 The dyeing bath contains 1 to 100 g/l of copper nitrate, preferably 5 to 2 g/l.
4. A method according to any one of claims 1 to 3, comprising 5 g/l. 5. A method according to any one of claims 1 to 4, wherein the dye bath has a pH of 0.5 to 5.0, preferably 2 to 3. 6. The method according to any one of claims 1 to 5, wherein the pH value is adjusted with nitric acid. 7. The method according to any one of claims 1 to 6, wherein the pH value is adjusted with acetic acid.