Process for coating solar collectors

The present invention relates to a method for coating a base or a substrate for producing a solar collector and a solar collector with an optically selective coating.

In order to achieve a high degree of efficiency in the thermal conversion of solar energy with solar collectors, a solar selective coating of the collectors is essential. This selective surface coating is intended to ensure a high absorption of the incident sunlight and at the same time a small emission of thermal radiation. Since the wavelengths of the electromagnetic waves from the sun and from the thermal radiation are different, the desired selectivity can be achieved by a special course of the reflectivity of the surface. The reflectivity must be as small as possible in the wavelength range of sunlight between approximately 0.3 and 2 µm and as close as possible to 100% in the range of infrared radiation above approximately 2 µm. This reflection profile is usually realized by a thin coating (thickness between 0.1 and 0.2 µm) of a compact, metallic base (usually steel, copper or aluminum). The coating acts as an anti-reflective layer of the reflective base up to wavelengths of 2 µm and is essentially transparent for longer wavelengths, as a result of which the high reflection of the base is retained.

In order to meet the optical requirements for the coating, it is advantageous to design the structure of the coating in such a way that it also has metallic properties directly on the metallic base and becomes increasingly dielectric, ie more insulating, as the thickness increases. For this reason, materials that consist of a mixture of an insulator and a metal are suitable for the coating. In the so-called cermet, desired optical properties can be set via the proportions of the insulating and the metallic component. The entire coating can then be built up either from different layers with a composition that is changed step by step or from a single layer with a continuous course of the concentration.

So far, the selective coatings have mostly been applied galvanically in the commercially available collectors. However, the wet chemical methods have the disadvantage that acid residues which are harmful to the environment can occur during production and the reproducibility of the layer properties is not always guaranteed. These disadvantages can be avoided with more modern vacuum separation methods, such as cathode sputtering. In conventional sputtering, an inert gas is supplied to a vacuum chamber, which is equipped with a cathode, an anode and the substrate to be coated, and a certain working pressure (usually between 0.1 Pa and 10 Pa) is set. By applying a direct voltage (DC) or a high-frequency alternating voltage (RF) between the cathode and the anode, a plasma ignites, from which ions are extracted and accelerated to the cathode. The energetic ions hitting the cathode knock (sputter) atoms out of the cathode surface, which is provided with a target made of a specific material. The sputtered atoms finally form a coating on the substrate. If this method is expanded, reactive sputtering, a reactive gas (e.g. O₂, N₂ or H₂) fed. Chemical reactions can then take place between components of the reactive gas and the target material and compounds can be built into the layer. Magnets are often attached to the cathode so that the plasma is enclosed in the magnetic field and concentrated in front of the target. In this case one speaks of magnetron sputtering.

G. Harding et al, Method of and apparatus for reactively sputtering a graded surface coating onto a substrate, United States Patent 4,309,261 dated January 5, 1982, describes a reactive DC sputter deposit method and system for the evacuated tube collectors with a Coating insulator-metal mixture and continuous concentration curve. Using amorphous hydrocarbon as the insulator and stainless steel as the metal, these tube collectors are manufactured by the company Shiroky, Japan and are commercially sold, for example, by the company Cuenod, Satigny, GE.

Regarding the state of the art, reference is made to the following references:<ul id="ul0001" list-style="dash"><li>RA Haefer, Surface and Thin Film Technology, Part I Coatings of Surfaces, Springer-Verlag, Berlin, Heidelberg, New York, 1987.</li><li>CP Klages, R. Memming, Microstructure and physical properties of metal-containing hydrogenated carbon films, in Properties and characterization of amorphous carbon films, Materials Science Forum Vols. 52 &amp; 53 (1989), Trans. Tech. Publications, Aedermannsdorf, pages 609-644.</li><li>B. Carlsson et al., Accelarated Life Testing of Solar Energy Materials, Case study of some selective solar absorber coating materials for DHW Systems, A Report of Task X Solar Materials Research and Development, February 1994, IEA International Energy Agency.</li></ul>

For the rest, reference is made to various brochures from solar collector companies.

Since the price of evacuated tube collectors is high and their maintenance is complicated, it is desirable to also use coatings produced by vacuum technology in the cheaper and simple flat collectors. In the case of flat-plate collectors, however, in addition to the requirements for optical selectivity, there is also a very high level of aging resistance, because these are subject to very corrosive environmental conditions. The collector coating must withstand both high temperatures (typically around 60 ° C, peak values ​​up to 200 ° C) and moisture in air so that there is no intolerable decrease in efficiency within a period of at least 25 years. The coating material produced by the Harding method is not sufficiently stable under these conditions. But even the stability of the layer material alone is not enough. In combination with the collector plate, additional aging mechanisms occur that limit the lifespan of a collector. For example, the metal from which the collector plate is made can diffuse through the coating, reach the surface and oxidize. The risk of such diffusion is particularly great in the case of rolled sheets, which are common in industrial applications. On these there are up to a few µm deep rolling grooves, the edges of which are only incompletely covered by the coating. Diffusion therefore preferably takes place at these edges.

It is therefore an object of the present invention to create vacuum-coated collectors with sufficient durability and high optical selectivity.

The object of the invention is achieved by means of a method according to the wording of claim 1.

A method for coating a substrate for the production of a solar collector is proposed, in that a process from RF plasma-activated gas phase deposition of a hydrocarbon gas or. fluorinated hydrocarbon gas is combined with the sputtering of a metal target. It is important for the aging resistance of the layers that the substrate or the substrate is kept at a negative electrical potential (bias) during the deposition. In addition, it proves to be particularly advantageous for stability at high temperatures in air if the substrate or the coating is heated during the manufacturing process.

Further preferred embodiment variants of the method according to the invention are characterized in the dependent claims 2 to 10.

The solar collector produced according to the invention with a selective coating is characterized by the wording according to claim 11.

The coating of the solar collector produced according to the invention contains amorphous hydrocarbon, respectively. fluorinated amorphous hydrocarbon and metal sputtered from a target, the coating consisting of different layers with gradually changing composition and / or of at least one layer with an at least almost continuous course of the concentration, the metal concentration in the coating close to the base or the substrate surface is higher than close to the surface of the coating or the solar collector. It is also possible for individual layers to act as diffusion barriers and / or adhesion promoters. Such a diffusion barrier or adhesion promoter layer can be the first, metal-rich layer lying directly on the base. It is technologically advantageous that for the creation of these diffusion barriers or Adhesion promoter layers no additional coating methods are necessary. In the case of different layers with different layer compositions, it can also be advantageous if the composition at the respective layer boundary layers does not change abruptly but continuously.

Further preferred embodiment variants of the solar collector proposed according to the invention are characterized in the dependent claims 12 to 17.

The invention will now be explained in more detail with reference to the accompanying figures and exemplary embodiments.

It shows:<dl id="dl0001"><dt>Fig. 1</dt><dd>for example, a trained deposition facility, while</dd><dt>Fig. 2</dt><dd>shows the time course of deposition parameters with reference to embodiment 3.</dd></dl>

A deposition system with its essential components is shown schematically and in a highly simplified manner in FIG. 1 is the vacuum housing of the system and 2 is the pump system necessary to maintain the vacuum. 3 and 4 provide the supply lines for the inert gas, respectively. Hydrocarbon gas resp. fluorinated hydrocarbon gas, the inflow of which is controlled by suitable control and measuring devices. 5 is the power supply for the high-frequency AC voltage, which is transmitted to the cathode 6 via capacitive or inductive coupling. It is possible, but not absolutely necessary, to design the cathode as a magnetron. The metal target 7 is located on the cathode, which can consist of pure metals but also metal alloys. 8 represents the base to be coated, usually the collector plate. The substrate is heated with a heater 9 during the coating process and is kept at a negative electrical potential (bias) via a DC voltage source 10. In this arrangement, the vacuum chamber walls represent the anode.

In the case of the substrate to be coated, however, instead of a metal collector plate, for example, a suitable polymer material can also be selected, which for example has a metal coating or at least a coating with metallic properties or a coating whose optical effect corresponds to that of a metallic coating.

Examples of possible combinations of hydrocarbon gases, fluorinated hydrocarbon gas and metal targets are listed in Table 1 below: <tables id="tabl0001" num="0001"><table frame="all"><title>Table 1</title><tgroup cols="2" colsep="1" rowsep="1"><colspec colnum="1" colname="col1" colwidth="78.75mm" /><colspec colnum="2" colname="col2" colwidth="78.75mm" /><thead valign="top"><row><entry namest="col1" nameend="col2" align="center">For example, possible material combinations of hydrocarbon gas, fluorinated hydrocarbon gas and metal target.</entry></row><row><entry namest="col1" nameend="col1" align="center">Hydrocarbon gas, respectively. fluorinated hydrocarbon gas</entry><entry namest="col2" nameend="col2" align="center">Metal target</entry></row></thead><tbody valign="top"><row><entry namest="col1" nameend="col1" align="left">methane</entry><entry namest="col2" nameend="col2" align="left">chrome</entry></row><row><entry namest="col1" nameend="col1" align="left">acetylene</entry><entry namest="col2" nameend="col2" align="left">chrome</entry></row><row><entry namest="col1" nameend="col1" align="left">methane</entry><entry namest="col2" nameend="col2" align="left">titanium</entry></row><row><entry namest="col1" nameend="col1" align="left">acetylene</entry><entry namest="col2" nameend="col2" align="left">titanium</entry></row><row><entry namest="col1" nameend="col1" align="left">Difluoromethane</entry><entry namest="col2" nameend="col2" align="left">chrome</entry></row><row><entry namest="col1" nameend="col1" align="left">Difluoromethane</entry><entry namest="col2" nameend="col2" align="left">titanium</entry></row><row><entry namest="col1" nameend="col1" align="left">Tetrafluoromethane</entry><entry namest="col2" nameend="col2" align="left">chrome</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="left">Tetrafluoromethane</entry><entry namest="col2" nameend="col2" align="left">titanium</entry></row></tbody></tgroup></table></tables>

The material that is deposited in this coating process consists of amorphous hydrocarbon (also called aC: H or diamond-like carbon in the literature) or fluorinated hydrocarbon (aC: H: F) and metal sputtered from the target. If the metal consists of a carbide former, it is up to very high metal concentrations (typically more than 50 atomic percent) in the form of metal carbide particles in the amorphous hydrocarbon matrix or. fluorinated hydrocarbon matrix installed. About the proportions of hydrocarbon gas, respectively. fluorinated hydrocarbon gas and inert gas, the metal content of a deposited layer can be adjusted. The aging stability of the coating depends on the stability of the metal carbide and the amorphous hydrocarbon matrix respectively. fluorinated hydrocarbon matrix. It is also important that no undesired diffusion processes of substrate material in and through the coating nor of coating material take place in the substrate. Especially in a humid atmosphere, the adhesion of the coating to the substrate proves to be an essential parameter for the aging resistance.

Due to the negative electrical potential of the base, the maximum energy with which ions extracted from the plasma hit the growing layer can be set. As is also known for pure amorphous hydrocarbon films, ion bombardment leads to a densification of the layer. This allows the hardness and density of the layers to be increased and the formation of pores to be restricted. Thus, with a suitable bias voltage or suitable, negative, electrical potential significantly improve the aging resistance of the coating. By varying the negative electrical potential or the bias voltage of the substrate or the substrate, the composition of the coating can also be influenced, as an increase in the metal concentration in the coating can be achieved with an increased, negative, electrical potential. By heating the underlay, the incorporation of weakly bound hydrogen into the amorphous hydrocarbon matrix, respectively. fluorinated hydrocarbon matrix reduced.

The coating, consisting of the various layer layers, or having an essentially continuous course of the concentration, is selected such that a selective reflection profile is achieved and, in addition, individual layers or areas of the coating act as diffusion barriers and adhesion promoters.

On the basis of concrete examples, it will be shown how optically selective coatings with high aging resistance can be produced using the described method. In the following examples, the layers can be further improved with regard to their optical characteristics by an optimal choice of layer thickness and composition. The vacuum apparatus in which the sheets have been placed is at a final pressure of approximately 1 × 10 before coating^-4Pa has been evacuated. We worked with methane and argon as process gas and a chrome target. The target with a diameter of 90mm was attached to a magnetron and was powered by a capacitive coupling from an RF power supply (13.56MHz and 200W). During the coating, the sheet was kept at an electrical potential of -200V. The other coating parameters are listed in the individual examples.

In order to characterize the resistance to aging, the coatings were accelerated aged by the SPF solar energy testing and research institute at the Intercantonal Technical Center in Rapperswil in artificial environments that simulate a multiple of the natural load.

Example 1:

Process for producing a two-layer layer:

A two-ply layer was made using the parameters listed in Table 2. Technical copper sheet with a typical average roughness (RMS roughness) of about 0.10 µm was used as a base. The first layer shows directly the substrate or the copper sheet, covering a metal content of 54 atomic percent chromium, while layer 2, forming the surface of the collector, has a metal content of 32 atomic percent chromium. It is therefore essential for the selective coating to design the metal content of the layer in such a way that it has a high metal concentration in the range of approx. 40-60 atom percent at the border to the metallic base or the substrate and a small metal content on the surface of the collector owns. This means that the metal on the substrate surface is almost completely carbidized and the proportion of the hydrocarbon matrix is ​​small; On the other hand, the hydrocarbon matrix is ​​almost exclusively present on the layer surface of the collector. The first, metal-rich layer, consisting almost entirely of chromium carbide, further serves as an adhesion promoter between the coating and the metallic base to be coated.

The two-layer layer produced in accordance with Table 2 was accelerated aged in air at 150 ° C. for 150 hours in order to characterize the stability at high temperatures in air. A solar absorption capacity of 66.5% and a thermal emissivity of 10.9% (at an assumed collector temperature of 100 ° C) were determined from the reflection curve before the aging treatment. After the thermal treatment, the solar absorptivity even improved to 71.6% and the thermal emissivity deteriorated only slightly and rose to a value of 12.0%. Since both these integral key figures and the reflection profile of the collector coating remained essentially constant during accelerated aging at 300 ° C in air, it can be concluded that the coating would have a very long service life at the operating temperatures usual in flat collectors. The great stability of the collector surface has two causes. On the one hand, each of the individual layers is very stable and hard. If a layer is deposited on a silicon single crystal surface, it cannot be scratched with a steel tip. On the other hand, the first layer, which is very rich in metal, i.e. has a high proportion of chromium carbide, acts as a barrier against the diffusion of copper. Atomic force microscopic examinations of layers produced under these conditions showed that the surfaces are very flat and have no pores. As a result, no copper can diffuse from the substrate along pores to the layer surface. In addition, since the chromium carbide crystallites have diameters of only a few nanometers, the situation is quasi amorphous. In contrast to fine crystalline layers, there is no grain boundary diffusion. The thickness of the first layer was adapted to the roughness of the copper sheet used so that the edges of the rolling grooves are adequately covered.<tables id="tabl0002" num="0002"><table frame="all"><title>Table 2</title><tgroup cols="3" colsep="1" rowsep="1"><colspec colnum="1" colname="col1" colwidth="52.50mm" /><colspec colnum="2" colname="col2" colwidth="52.50mm" /><colspec colnum="3" colname="col3" colwidth="52.50mm" /><thead valign="top"><row><entry namest="col1" nameend="col3" align="center">Coating parameters for the individual layers of the selective coating according to embodiment 1.</entry></row><row><entry namest="col1" nameend="col1" align="left">Coating parameters</entry><entry namest="col2" nameend="col2" align="right">Location 1</entry><entry namest="col3" nameend="col3" align="right">Location 2</entry></row></thead><tbody valign="top"><row><entry namest="col1" nameend="col1" align="left">Argon volume flow [sccm]</entry><entry namest="col2" nameend="col2" align="char" char=",">40</entry><entry namest="col3" nameend="col3" align="char" char=",">40</entry></row><row><entry namest="col1" nameend="col1" align="left">Methane volume flow [sccm]</entry><entry namest="col2" nameend="col2" align="char" char=",">1,0</entry><entry namest="col3" nameend="col3" align="char" char=",">1,5</entry></row><row><entry namest="col1" nameend="col1" align="left">Pressure [Pa]</entry><entry namest="col2" nameend="col2" align="char" char=",">0,8</entry><entry namest="col3" nameend="col3" align="char" char=",">2,6</entry></row><row><entry namest="col1" nameend="col1" align="left">Substrate temperature [° C]</entry><entry namest="col2" nameend="col2" align="char" char=",">200</entry><entry namest="col3" nameend="col3" align="char" char=",">200</entry></row><row><entry namest="col1" nameend="col1" align="left">Metal content [atomic percent chromium]</entry><entry namest="col2" nameend="col2" align="char" char=",">54</entry><entry namest="col3" nameend="col3" align="char" char=",">32</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="left">Thickness [µm]</entry><entry namest="col2" nameend="col2" align="char" char=",">0,056</entry><entry namest="col3" nameend="col3" align="char" char=",">0,055</entry></row></tbody></tgroup></table></tables>

Example 2:

Production of a three-layer layer:

The optical efficiency of the collector coating can be further improved by an additional third layer, which has an even lower metal content of &lt;10 atomic percent metal. Table 3 lists the parameters with which aC: H / Cr three-layer layers were produced, which had a solar absorptivity α before the aging treatments_S of about 89% and a thermal emissivity ε_{100 ° C} of about 14% (see Table 4). Technical copper sheet with a typical mean roughness (RMS roughness) of 0.10 µm was used as the backing.<tables id="tabl0003" num="0003"><table frame="all"><title>Table 3</title><tgroup cols="4" colsep="1" rowsep="1"><colspec colnum="1" colname="col1" colwidth="39.37mm" /><colspec colnum="2" colname="col2" colwidth="39.37mm" /><colspec colnum="3" colname="col3" colwidth="39.37mm" /><colspec colnum="4" colname="col4" colwidth="39.37mm" /><thead valign="top"><row><entry namest="col1" nameend="col4" align="center">Parameters for the individual layers of the selective coating according to embodiment 2.</entry></row><row><entry namest="col1" nameend="col1" align="left">parameter</entry><entry namest="col2" nameend="col2" align="left">Location 1</entry><entry namest="col3" nameend="col3" align="left">Location 2</entry><entry namest="col4" nameend="col4" align="center">Location 3</entry></row></thead><tbody valign="top"><row><entry namest="col1" nameend="col1" align="left">Argon volume flow [sccm]</entry><entry namest="col2" nameend="col2" align="char" char=",">40</entry><entry namest="col3" nameend="col3" align="char" char=",">40</entry><entry namest="col4" nameend="col4" align="char" char=",">40</entry></row><row><entry namest="col1" nameend="col1" align="left">Methane volume flow [sccm]</entry><entry namest="col2" nameend="col2" align="char" char=",">1,0</entry><entry namest="col3" nameend="col3" align="char" char=",">1,75</entry><entry namest="col4" nameend="col4" align="char" char=",">4,0</entry></row><row><entry namest="col1" nameend="col1" align="left">Pressure [Pa]</entry><entry namest="col2" nameend="col2" align="char" char=",">0,8</entry><entry namest="col3" nameend="col3" align="char" char=",">2,8</entry><entry namest="col4" nameend="col4" align="char" char=",">3,2</entry></row><row><entry namest="col1" nameend="col1" align="left">Substrate temperature [° C]</entry><entry namest="col2" nameend="col2" align="char" char=",">200</entry><entry namest="col3" nameend="col3" align="char" char=",">200</entry><entry namest="col4" nameend="col4" align="char" char=",">200</entry></row><row><entry namest="col1" nameend="col1" align="left">Metal content [atomic percent chrome]</entry><entry namest="col2" nameend="col2" align="char" char=",">54</entry><entry namest="col3" nameend="col3" align="char" char=",">24</entry><entry namest="col4" nameend="col4" align="char" char=",">5</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="left">Thickness [µm]</entry><entry namest="col2" nameend="col2" align="char" char=",">0,056</entry><entry namest="col3" nameend="col3" align="char" char=",">0,080</entry><entry namest="col4" nameend="col4" align="char" char=",">0,050</entry></row></tbody></tgroup></table></tables>

Accelerated aging tests were carried out on these three-layer layers in a humid atmosphere (samples 1 and 2) and under UV radiation (samples 3 and 4). As shown in Table 4, none of the treatments resulted in a significant deterioration in the optical indicators. The moisture treatments are characterized by the duration of the treatment and the air temperature / relative humidity / sample temperature. Even during 70 hours in the extremely corrosive conditions of an 85 ° C hot air atmosphere with 95% relative humidity and a sample temperature of 80 ° C, the key figures of sample 2 did not deteriorate. Since the temperature of the samples is below the air temperature, the samples are completely covered with condensation water during the aging treatment. In addition to the low porosity of the layers and thus low permeability to water, the fact that the properties of these coatings are not deteriorated by the influence of moisture is due to the adhesion imparted by the first, metal-rich layer.

The radiation tests can be used to check whether the aC: H / Cr collector coating is sensitive to UV radiation. (A UV400F lamp from Dr. Hönle was used for the UV test. With this lamp, 3.5 times the UV-A radiation and 6.5 times the UV-B radiation were compared to the samples the daylight spectrum D65). During the UV resistance test, sample 3 was cooled to 50 ° C and sample 4 was not cooled. The latter warmed up to a temperature of 75 ° C. The solar UV radiation is reduced by the glass cover that is common in flat-plate collectors, but it could not be excluded from the outset that photochemical reactions adversely affect the optical properties of the aC: H / Cr coatings. However, the investigations show that there is no significant change in the optical properties as a result of UV radiation. It can therefore be concluded that a significant degradation of the selective properties is not to be expected under either condensation or UV exposure. <tables id="tabl0004" num="0004"><table frame="all"><title>Table 4</title><tgroup cols="6" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="26.25mm" /><colspec colnum="2" colname="col2" colwidth="26.25mm" /><colspec colnum="3" colname="col3" colwidth="26.25mm" /><colspec colnum="4" colname="col4" colwidth="26.25mm" /><colspec colnum="5" colname="col5" colwidth="26.25mm" /><colspec colnum="6" colname="col6" colwidth="26.25mm" /><thead valign="top"><row rowsep="1"><entry namest="col1" nameend="col6" align="center">Solar absorption level α_S and thermal emissivity ε_{100 ° C} of three-layer layers according to embodiment 2 before and after various aging treatments</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" rowsep="0" align="center">sample</entry><entry namest="col2" nameend="col3" align="left">unaged</entry><entry namest="col4" nameend="col4" rowsep="0" align="center">treatment</entry><entry namest="col5" nameend="col6" align="center">aged</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" /><entry namest="col2" nameend="col2" align="left">α_S</entry><entry namest="col3" nameend="col3" align="center">ε_{100 ° C}</entry><entry namest="col4" nameend="col4" /><entry namest="col5" nameend="col5" align="center">α_S</entry><entry namest="col6" nameend="col6" align="center">ε_{100 ° C}</entry></row></thead><tbody valign="top"><row><entry namest="col1" nameend="col1" align="right">1</entry><entry namest="col2" nameend="col2" align="char" char=",">89,3%</entry><entry namest="col3" nameend="col3" align="char" char=",">14,8%</entry><entry namest="col4" nameend="col4" align="center">200h at 60 ° C / 95% / 55 ° C</entry><entry namest="col5" nameend="col5" align="char" char=",">88,7%</entry><entry namest="col6" nameend="col6" align="char" char=",">13,3%</entry></row><row><entry namest="col1" nameend="col1" align="right">2</entry><entry namest="col2" nameend="col2" align="char" char=",">89,2%</entry><entry namest="col3" nameend="col3" align="char" char=",">14,4%</entry><entry namest="col4" nameend="col4" align="center">70h at 85 ° C / 95% / 80 ° C</entry><entry namest="col5" nameend="col5" align="char" char=",">89,4%</entry><entry namest="col6" nameend="col6" align="char" char=",">13,1%</entry></row><row><entry namest="col1" nameend="col1" align="right">3</entry><entry namest="col2" nameend="col2" align="char" char=",">88,7%</entry><entry namest="col3" nameend="col3" align="char" char=",">13,2%</entry><entry namest="col4" nameend="col4" align="center">86h UV at 50 ° C</entry><entry namest="col5" nameend="col5" align="char" char=",">89,0%</entry><entry namest="col6" nameend="col6" align="char" char=",">13,1%</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="right">4</entry><entry namest="col2" nameend="col2" align="char" char=",">88,9%</entry><entry namest="col3" nameend="col3" align="char" char=",">13,0%</entry><entry namest="col4" nameend="col4" align="center">86h UV at 75 ° C</entry><entry namest="col5" nameend="col5" align="char" char=",">88,9%</entry><entry namest="col6" nameend="col6" align="char" char=",">13,1%</entry></row></tbody></tgroup></table></tables>

Example 3:

Production of quasi-continuous three-layer layers

In order to further increase the degree of solar absorption, aC: H / Cr three-layer coatings were produced in which the metal content of the individual layers merges almost continuously. In the exemplary embodiments listed above, the target was conditioned to the new parameters for several minutes between the deposition of the individual layers without coating the substrate during this time. In contrast, with quasi-continuous three-layer coating, the substrate is also coated immediately after a parameter change. Since the target only returns to a stable state after a change in parameters, in which the deposition and re-sputtering of hydrocarbon on the target surface is balanced, the layer composition does not change suddenly, but continuously. The exact time course of the deposition parameters methane volume flow and pressure is shown in FIG. 2. The vertical dashed line at a time of 14 min indicates a conditioning phase (10 min duration). During the entire deposition, the argon volume flow is 40 sccm and the substrate temperature is 300 ° C. The structure of the resulting coating can be roughly described as a sequence of three homogeneous layers. The first layer consists of an approximately 0.028 µm thick chromium carbide-rich intermediate layer (54 atom percent chromium), which acts as a diffusion barrier and an adhesion promoter. Given the low surface roughness of the mechanically polished copper substrates used in these samples, this thickness is sufficient to suppress preferential diffusion along the rolling grooves of the sheet (RMS roughness of 0.07 µm). The second layer consists of an approximately 0.060 µm thick aC: H / Cr layer with 15 atomic percent chromium and the last layer consists of a 0.072 µm thick, pure aC: H cover layer. The degree of solar absorption of the quasi-continuous three-layer layers is about 92%, the thermal emissivity is 8.6% (see Table 5).

Using three samples (samples 5 to 7), which were produced according to the profile in FIG. 2, it was estimated whether the aC: H / Cr coatings could meet the qualification criteria of the International Energy Agency IEA (see list of references). A collector coating is considered qualified if, within 25 years, the optical key figures α in a standard collector_S and ε_{100 ° C} expected to change so little that the criterion<maths id="math0001" num=""><math display="block"><mrow><msub><mrow><mtext>-Δ α</mtext></mrow><mrow><mtext>S</mtext></mrow></msub><msub><mrow><mtext> + 0.25 Δε</mtext></mrow><mrow><mtext>100 ° C</mtext></mrow></msub><mtext> ≦ 5%</mtext></mrow></math><img file="EP0736612A2_D0001.tif" /></maths> remains fulfilled. In order to investigate the kinematics of the aging processes, the samples were annealed at various elevated temperatures until they failed, that is, until the qualification criterion was no longer met. Table 5 shows the key figures of samples 5 to 7 and the lifetimes when tempering in air.<tables id="tabl0005" num="0005"><table frame="all"><title>Table 5</title><tgroup cols="5" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="31.50mm" /><colspec colnum="2" colname="col2" colwidth="31.50mm" /><colspec colnum="3" colname="col3" colwidth="31.50mm" /><colspec colnum="4" colname="col4" colwidth="31.50mm" /><colspec colnum="5" colname="col5" colwidth="31.50mm" /><thead valign="top"><row rowsep="1"><entry namest="col1" nameend="col5" align="center">Solar absorption level α_S and thermal emissivity ε_{100 ° C} quasi-continuous three-layer layers according to embodiment 3 before the annealing treatment and their service life at elevated temperature</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="center">sample</entry><entry namest="col2" nameend="col2" align="center">α_S</entry><entry namest="col3" nameend="col3" align="center">ε_{100 ° C}</entry><entry namest="col4" nameend="col4" align="center">temperature</entry><entry namest="col5" nameend="col5" align="center">lifespan</entry></row></thead><tbody valign="top"><row><entry namest="col1" nameend="col1" align="right">5</entry><entry namest="col2" nameend="col2" align="char" char=",">91,8%</entry><entry namest="col3" nameend="col3" align="char" char=",">8,4%</entry><entry namest="col4" nameend="col4" align="right">250 ° C</entry><entry namest="col5" nameend="col5" align="right">80 .... 120h</entry></row><row><entry namest="col1" nameend="col1" align="right">6</entry><entry namest="col2" nameend="col2" align="char" char=",">91,8%</entry><entry namest="col3" nameend="col3" align="char" char=",">8,8%</entry><entry namest="col4" nameend="col4" align="right">300 ° C</entry><entry namest="col5" nameend="col5" align="right">&lt;20h</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="right">7</entry><entry namest="col2" nameend="col2" align="char" char=",">91,8%</entry><entry namest="col3" nameend="col3" align="char" char=",">8,6%</entry><entry namest="col4" nameend="col4" align="right">220 ° C</entry><entry namest="col5" nameend="col5" align="right">&gt; 280h</entry></row></tbody></tgroup></table></tables>

The lifespan of the quasi-continuous three-layer layers is between 80 and 120h at 250 ° C, less than 20h at 300 ° C and more than 280h at 220 ° C. Based on time transformation functions, it can be concluded that the service life under the thermal loads in a standard collector should be at least 25 years. The coating thus fulfills the qualification criteria of the IEA.

The deposition system shown for example in FIG. 1 and the exemplary embodiments 1, 2 and 3 are examples in order to explain the invention in more detail and to make it understandable. Of course, the invention is not limited to the system shown and the exemplary embodiments. So instead of methane other hydrocarbon gases resp. fluorinated hydrocarbon gases are used, and other metals can be used in place of chromium to be included in the coating. Finally, a copper sheet does not necessarily have to be used as a substrate or base, but other metals or metal alloys can also be used, or non-metallic substrates which have a metal coating or a coating with an optical effect corresponding to that of a metallic surface.

It is essential to the invention that plasma-activated gas phase deposition is combined with an atomization process and negative substrate potential when coating a substrate or a base for producing a solar collector.