GB2039865A

GB2039865A - Production of spectrally selective coatings on enamelled metal surfaces

Info

Publication number: GB2039865A
Application number: GB8000823A
Authority: GB
Original assignee: Nederlandse Organisatie voor Toegepast Natuurwetenschappelijk Onderzoek TNO
Current assignee: Nederlandse Organisatie voor Toegepast Natuurwetenschappelijk Onderzoek TNO
Priority date: 1979-01-11
Filing date: 1980-01-10
Publication date: 1980-08-20
Also published as: IT1128454B; FR2446328B1; FR2446328A1; DE3000729A1; IT8067029A0; BE881040A; GB2039865B

Abstract

A spectrally selective coating is formed on a dark coloured enamelled metal surface by forming on the enamel layer a coating of a solution comprising one or more organic tin compounds capable of being thermally decomposed to form spectrally selective tin oxide, and trifluoroacetic acid in glacial acetic acid, and heating the coated surface to decompose the compound(s). Coated plates so produced are useful as solar heat collectors.

Description

SPECIFICATION Production of spectrally selective coatings on enamelled metal surfaces This invention is concerned with the formation of spectrally selective coatings on metal surfaces, particular metal plates, having a dark coloured enamel layer thereon and with the use of the resulting products as solar heat collectors.

U.S. Patent 4,105,822 described a method of forming a spectrally selective coating on a metal plate which comprises providing a black enamel layer on the metal plate and then spraying a compound which can be decomposed to form a semi-conducting material, such as indium chloride, on to the enamel layer while the latter is at a temperature above 500"C so as to form a layer of semi-conducting material, such as indium oxide. The indium oxide layer is preferably coated with a layer of silicon oxide by atomizing silicon chloride on to the coating immediately after forming the indium oxide layer. The resulting plate absorbs solar radiation strongly and reflects infrared radiation to only a low degree.

U.S. Patent 4,105,822 also describes a solar heat collector comprising an enamelled metal plate having a semi-conductive coating. Such a solar heat collector may comprise a substantially closed housing with a window permeable to radiation, a metal plate in the housing behind the window forming an air cavity with the window and forming part of a fluid reservoir that is isolated from a backwall of the housing and has an inlet and outlet for the fluid. The metal plate in the housing has a coating comprising a black enamel layer adjacent to the plate and a thin layer of semi-conducting material on the black enamel layer.

In an article of H.J.J. Van Boort en R. Groth in Philips Technisch Tijdschrift, 29, Nr. 1, (1968), pages 17 and 18 it is suggested that heat reflecting filters of tin oxide and indium oxide should be applied to the inner surface of the glass envelope forming a discharge tube for sodium illumination. Such oxide coatings are formed, for example, by spraying tin tetrachloride or indium chloride, dissolved in an organic solvent, such as butyl acetate, on to the glass of the envelope while the latter is hot.

These known processes have several disadvantages. When metal chlorides are used to form the oxides, chlorine and/or hydrogen chloride is/are formed, both of which attack the enamel layer upon the coating which is formed. As a result, the final product sometimes shows a white haze, which can only be removed with difficulty or cannot be removed at all and which can affect the spectrally selective properties in certain respects. Moreover, much of the metal which is to form the oxide is lost because only a small percentage of the solution as sprayed contributes to the formation of the tin oxide or indium oxide layer upon the enamel.

This is particularly disadvantageous in the case of indium because of its high cost. Finally, it is not desirable to spray organic solutions on to hot surfaces because noxious vapours and/or decomposition products are likely to be formed which can give rise to environmental pollution.

The last-mentioned difficulties are also encountered when using the process of U.S. Patent 2,567,331, according to which a solution of an organic tin compound, such as dibutyl tin diacetate, in an organic solvent is sprayed on to a glass surface, and when using the process of published Dutch patent application 7212532, in which process a solution of an organic tin fluoroester, for example the stannous salt of trifluoroacetic acid, in a solvent, such as methylethyl ketone, is sprayed on to a heated ceramic material, such as glass.

When applying an electroconductive tin oxide film to glass, the formation of haze on the surface of the coated glass is prevented according to U.S. Patent 3,107,177 by spraying a solution containing ammonium hydrogen fluoride or hydrogen fluoride, ammonia or ammonium acetate and ethanol and, optionally, glacial acetic acid, the solution also containing or being accompanied by, for example, an organic tin compound.

The use of a solution of dibutyl tin oxide in glacial acetic acid and n-propanol without a fluoro compound is also described, but this solution is stated to be less desirable because the electroconductivity of the resulting layer is increased by the presence of fluorine. However, acceptable results are not obtained by forming tin oxide layers upon dark coloured enamel in the way described in U.S. Patent 3,107,177. The presence of hydrogen fluoride gives rise to the formation of an opaque tin oxide layer which reduces the absorption of solar radiation by the dark enamel layer. When ammonium hydrogen fluoride is used it decomposes rapidly so that the fluorine, which is required as a dopant for the tin oxide layer, disappears and undesirable hydrogen fluoride is formed.Moreover, the tin compounds are only soluble in the solutions described to a limited extent, which necessitates the use of low concentrations in order to prevent clogging of the spraying orifice by crystallization or precipitation of the tin compound.

As an example of a solar heat collector made by the above-mentioned known processes, the solar heat collector described in U.S. Patent 3,981,293 may be mentioned. This solar heat collector comprises a solar radiation absorbing plate and a cover plate mounted in spaced relation to the absorption plate. The solar radiation absorbing plate consists of a heat conductive material, such as copper or aluminium, having either a selective or a non-selective surface. A selective surface, for example a copper oxide surface, has an absorptivity coefficient for solar radiation which is substantially different from the emissivity coefficient for infrared radiation. A non-selective surface, such as a black surface, has an absorptivity coefficient for solar radiation that is equal to the emissivity coefficient for infrared radiation.The cover plate, for example a glass plate, has a surface which is coated with, for example, tin oxide or indium oxide.

Finally, it is known from British Specification 1,459,995 to improve the hardness of vitreous enamel coatings on a substrate by spraying on to the vitreous enamel at a temperature of 400"C to 8000C, a solution, dispersion or emulsion of, for example, tin chloride or an alkyl stannate. The tin penetrates into the enamel surface to a depth of from 5 to 10 ism.

We have now developed an improved process for forming a spectrally selective tin oxide layer doped with fluorine on a dark coloured enamelled metal surface, which process is free from or less subject to the disadvantages inherent in known processes.

According to the present invention, there is provided a process for forming a spectrally selective coating on a metal surface having a dark coloured enamel layer thereon, which comprises forming on the enamel layer a coating of a solution comprising one or more organic tin compounds capable of being thermally decomposed to form spectrally selective tin oxide without forming chlorine and/or hydrogen chloride, and trifluoroacetic acid in glacial acetic acid, and heating the coated surface to decompose the tin compound(s) to form tin oxide.

In this specification the term "dark coloured enamel" is used to mean an optically black enamel having an absorption coefficient for solar radiation (wave length 0.3-2.5 itm) of at least 0.9.

Suitable organic tin compounds are, for example, those that contain, in addition to tin, only carbon and hydrogen and, optionally, oxygen atoms. Preferred compounds are, for example, dialkyl tin oxides and bis(trialkyltin) oxides, in which the alkyl groups contain 1 to 6 carbon atoms. Dibutyl tin oxide is particularly preferred for application to enamel layers which are at a temperature of at least 550"C.

The amount of organic tin compound in the glacial acetic acid solution is maintained as high as possible and is, in general, from 25 to 60% by weight, depending on the particular tin compound used.

The trifluoroacetic acid serves as a source of fluoride dopant for the tin oxide or cassiterite lattice and the proportion of the acid used should be such as to provide the desired level of dopant to obtain a spectrally selective layer. For reasonable spectral selectivity, an electron number density of about 3 x 1026 m-3 and an electron mobility of at least 4.0 x 10-3 m2Ns are desirable.

In generai, 0.5 to 3.0, preferably about 1.5, atoms of fluorine per 100 atoms of tin are desirable in the tin oxide coating. This means, in practice, that the concentration of trifluoroacetic acid in the glacial acetic acid solution should be from 10 to 50% by weight, preferably 20 to 30% by weight.

When using dibutyl tin oxide, this means that the weight ratio of dibutyl tin oxide to trifluoroacetic acid is preferably from 2:1 to 1:2.

By using a solution of dibutyl tin oxide and trifluoroacetic acid in glacial acetic acid and a black enamel having an original absorptivity coefficient for perpendicularly incident solar radiation (wave length 0.3-2.5 clam) of 0.95, coated plates are obtained having an absorptivity coefficient for perpendicularly incident solar radiation (wave length 0.3-2.51lm) of from 0.90 to 0.93 and a reflectance coefficient for perpendicularly incident heat radiation (wave length above 2 lim) of 0.84 to 0.89.The latter coefficient corresponds to that of a layer having a hemispherical emissivity coefficient for heat radiation of 0.19 to 0.14 at a surface temperature of 120"C. The reflectance coefficient for heat radiation appears to be substantially independent of the temperature up to a temperature of 300"C.

The thickness of the final spectrally selective coating is preferably 0.3 ym or more. The upper thickness limit is not critical, but, in practice, the thickness of the coating is preferably not more than 1 lim.

In general the temperature of the enamel layer on to which the glacial acetic acid solution is sprayed is preferably at least 550"C, more preferably 750-850"C. It is preferred to spray the glacial acetic acid solution containing the tin compound during cooling of the enamel layer applied on the metal plate following the formation of the enamel layer. In this way, the spraying process is combined with the cooling procedure of the enamel layer.

A great number of metals can be used for the metal plate, for example iron, steel and aluminium.

The enamelled metal plates having a spectraily selective coating as obtained according to the invention may be used in solar heat collectors, for example flat solar heat collectors as described in U.S. Patent 4,105,822, and also tubular solar heat collectors in which the solar radiation is focussed by means of mirrors.

The coated metal plates obtained according to the invention can also be used for wall decoration, the colour of which may be selected from a great number of different colours. Because the material reflects heat radiation, it may contribute to fuel saving.

In order that the invention may be more fully understood, the following examples are given by way of illustration only: Example I Two steel plates of 15 x 15 cm were coated in a furnace at a temperature of 820"C with a layer of black enamel (manufacturer Ferro Holland, code number G 303), the enamelled plates were rapidly removed from the furnace and sprayed with a solution containing 500 g dibutyl tin oxide, 330 g trifluoroacetic acid and 330 g glacial acetic acid, by means of a spray gun. Spraying was effected for four seconds or eight seconds respectively and the capacity of the spray gun was 8 litres per hour.

After the plates had been cooled, the absorptivity coefficient for perpendicularly incident solar radiation (a), the reflectance coefficient for perpendicularly incident infrared radiation (r), the hemispherical emissivity coefficient (E) and the thickness of the spectrally selective coating (d) were determined. The following results were obtained: a r e d spraying period4sec.: 0.91 0.88 0.15 0.41lm spraying period 8 sec.: 0.91 0.89 0.14 0.81lem.

Example Il A steel plate of 15 x 15 cm was first coated with a priming enamel and then with an optically black coating enamel, having a dark blue colour. After the enamelling process, the plate was heated in a furnace having a temperature of 820 C. After a residence time of 1 minute in the furnace, the plate was rapidly removed therefrom and sprayed with the same solution as used in Example I in the same way as described in that example for 6 seconds. The properties of the resulting plate were as follows: a = 0.9, r = 0.87, E = 0.15, and d = 0.6 ijm.

Example 111 A steel plate of 15 x 50 cm was first coated with a priming layer and thereafter with a coating of black enamel (manufacturer Ferro Holland, code number M 353). After the enamelling process, the enamelled plate having a temperature of 820"C was drawn from the furnace at a uniform speed of 0.1 m/sec. At the outlet of the furnace, the plate was sprayed with the same solution and with the same spray gun as described in Example I. The properties of the final plate were as follows: a = 0.90, r = 0.87, E = 0.16, and d = 0.35 lim.

Claims

1. A process for forming a spectrally selective coating on a metal surface having a dark coloured enamel layer thereon, which comprises forming on the enamel layer a coating of a solution comprising one or more organic tin compounds capable of being thermally decomposed to form spectrally selective tin oxide without forming chlorine and/or hydrogen chloride, and trifluoroacetic acid in glacial acetic acid, and heating the coated surface to decompose the tin compound(s) to form tin oxide.

2. A process according to claim 1, in which the organic tin compound contains, in addition to tin, only carbon and hydrogen and, optionally, oxygen atoms.

3. A process according to claim 1 or 2, in which the organic tin compound is a dialkyl tin oxide or a bis(trialkyl tin) oxide, in which the alkyl groups contain from 1 to 6 carbon atoms.

4. A process according to any of claims 1 to 3, in which the organic tin compound is dibutyl tin oxide.

5. A process according to any of claims 1 to 4, in which the solution contains 25 to 60% by weight of the organic tin compound(s).

6. A process according to any of claims 1 to 5, in which the solution contains 10 to 50% by weight of trifluoroacetic acid.

7. A process according to any of claims 1 to 6, in which the solution contains 20 to 30% by weight of trifluoroacetic acid.

8. A process according to any of claims 1 to 7, in which the solution comprises dibutyl tin oxide and trifluoroacetic acid in glacial acetic acid, the weight ratio of dibutyl tin oxide to trifluoroacetic acid being from 2:1 to 1:2.

9. A process according to any of claims 1 to 8, in which the solution is applied to form a coating having a final thickness, after heating, of 0.3 to 1 Fm.

10. A process according to any of claims 1 to 9, in which the solution is sprayed on to the enamelled surface to form the coating while the surface is at a temperature of at least 550"C.

11. A process according to claim 10, in which the enamelled surface is at a temperature of 750 to 850"C.

12. A process according to claim 10 or 11, in which the solution is sprayed on to the enamelled surface during the cooling of the enamel layer following its formation.

13. A process according to any of claims 1 to 12, in which the metal surface is an iron, steel or aluminium plate.

14. A process for forming a spectrally selective coating on a metal surface having a dark coloured enamel layer thereon, substantially as herein described in any of the Examples.

15. Dark coloured enamelled metal plate having a spectrally selective coating thereon when made by the process claimed in any of the preceding claims.

16. A solar heat collector comprising a dark coloured enamelled metal plate having a spectrally selective coating thereon which has been made bythe process claimed in any of claims 1 to 14.