DE102012107398A1 - Solar absorber comprises carrier substrate and solar absorber layer system that is arranged on side of substrate and comprises absorber layer and cover layer, where absorber layer includes sub-layers - Google Patents

Abstract

The solar absorber comprises a carrier substrate and a solar absorber layer system arranged on a side of the substrate. The solar absorber layer system comprises an absorber layer and a cover layer. The absorber layer includes sub-layers. One of the sub-layers is a chromium nitride or chromium oxide layer and other sub-layer is made of chromium oxynitride. The solar absorber comprises a carrier substrate and a solar absorber layer system arranged on a side of the substrate. The solar absorber layer system comprises an absorber layer and a cover layer. The absorber layer includes sub-layers. On of the sub-layers is a chromium nitride or chromium oxide layer and other sub-layer is made of chromium oxynitride (CrO xN y), where x is 1.3-1.48 and y is 0-0.5. An independent claim is included for a method for manufacturing a solar absorber.

Description

The invention relates to a method for producing solar absorber layer systems which are based on chromium oxide and are deposited by means of magnetron sputtering.

Solar absorbers regularly consist of a composite material, which on a, usually metallic, carrier material at least one solar absorbing layer, also referred to as absorber layer, and have one or more non-reflective layers. Further complementary layers between substrate and absorber layer e.g. to improve the adhesion and / or IR reflection increase or for the mechanical and chemical protection of the layer system are possible.

The aim of the absorber layer systems is to selectively absorb the incident solar radiation, which essentially comprises visible and near infrared wavelengths (0.3 μm-2.5 μm), in order to absorb heat to a maximum and to a transmission means thermally connected to the absorber, to discharge and at the same time emit as little infrared radiation in the wavelength range> 2.5μm as self-emission. Because of the equivalence of absorption and emissivity and the statement valid for non-transparent materials that the sum of the absorption and reflectance always equals 1, the coating system should absorb the solar radiation to a high degree and reflect it as low as possible Range of infrared radiation (> 2.5 microns) show a opposite absorption and reflection behavior.

Some of the known solar absorber coating systems use chromium oxide-containing materials as absorbers, it also being possible for the absorber to consist of several partial layers which, in addition to oxygen compounds of chromium, may also have nitrogen and / or carbon compounds thereof. In order to increase the achievable absorption and to maintain this for as long periods as possible, more and more complex systems are being developed.

For example, in the DE 20 2009 015 334 U1 describes an absorber layer consisting of three sub-layers. As the "real" absorber is a chromium compound of the composition CrO _x N _y C _z will be described there, on both sides of the protective layers adjacent to suppress of aging processes. These protective layers consist of Cr ₂ O ₃ and CrN.

As a conventional method of depositing CrO _x , reactive magnetron sputtering is used. In this case, an Ar-oxygen mixture is used as the process gas.

Furthermore, the production of chromium oxynitride layers, eg for antimigration layers in transparent solar control layer systems, by means of magnetron sputtering is known by using a mixture of Ar, O ₂ , N ₂ and NO ₂ as the process gas of the reactive sputtering process.

The known method uses for generating the working gas mixture by mass flow controller held constant partial flows of the process gas components Ar, O ₂ , N ₂ , NO ₂ or pre-made mixtures. However, it has the disadvantage that certain CrO _x N _y stoichiometries, ie specific values for x and y, of the layers deposited by sputtering can not be achieved because a change in the sputtering target with oxide layers results in a reduction of the sputtering yield and subsequently in a Decrease of the oxygen consumption and thus a shift of the operating point of the reactively guided process to a mode of operation with oxide coverage of the sputtering target takes place. The latter is characterized by a low sputtering rate and oxygen-rich stoichiometry of the deposited layer, the skilled person calls this an oxidic process.

For others, stably be produced by the known method stoichiometries CrO _x N _y due to the autogenous oxide coating of the sputtering target, the sputtering rate available, thereby limiting the productivity of the process. In particular, for layers having a stoichiometric parameter x in the range of 1.1 <x <1.6, the achievable layer thickness per magnetron target with a sputtering power of 15 W per mm sputtering target length and a substrate transport speed of 1 m / min is below the values using the known method 16nm limited.

The production of a CrO _x N _y layer with a thickness of 32 nm at a substrate transport speed of 10 m / min is therefore required when using the known method, for example 20 pieces in the process flow direction successively arranged magnetron sputter sources. The coating system required for this is very large and expensive. Furthermore, when using the known method, certain advantageous for the absorption behavior of the overall layer stack optical properties, in particular the absorption coefficient is not stable or stable adjustable within narrow limits.

The invention is therefore based on the task, a solar absorber with a carrier substrate and a reactive sputtering process for the deposition of its solar absorber layer system indicate which, with high solar absorption, allows to overcome the stated limitations in stoichiometry, stability of the process and achievable layer thicknesses or deposition rates.

According to the invention, the actual absorber layer, which was deposited reactively by means of magnetron sputtering, is divided into two partial layers, of which one partial layer is a chromium nitride or chromium oxide layer and the other partial layer is a chromoxinitride layer of the composition CrO _x N _y . According to the invention, the chromium oxynitride partial layer has fractions of oxygen and nitrogen with mixing ratios in the range of 1.1 <x <1.6 and 0 <y <0.5, preferably in the range of 1.3 <x <1.48 and 0 < y <0.5. These mixing ratios can be produced stably with the claimed method and have, in conjunction with a further partial layer, a higher solar absorption than can be achieved with the absorber layer systems produced according to the prior art.

A solar absorber is produced with such a chromoxynitride partial layer in that, at least during the deposition of this partial layer, the partial pressure of oxygen is measured and regulated with a control loop using the amount of oxygen supplied to the sputtering process as a manipulated variable. In contrast to the prior art, in which constant gas flows are used and the adjustment of the operating point via the constantly supplied gas mixture and the power, the oxygen flow is used as an active control variable for controlling the oxygen partial pressure. Thus, in the complex interaction with the acting as oxygen sink reactive sputtering process, the oxygen wegtransportierenden vacuum pumps and influenced by the oxygen partial pressure oxide occupancy of the target, which in turn affects the effectiveness of the reactive sputtering process, other process working points, determined by oxygen partial pressure and target oxide, accessible than at the state of the art operation with constant gas flows. Only by measuring and active regulation of the oxygen partial pressure is it possible to deposit CrOxNy layers with mixing ratios in the range 1.1 <x <1.6 and 0 <y <0.5, preferably at 1.3 <x <1.48 and 0 <y <0.5 with high sputtering rate. Thus, with a sputtering power of 15 W per mm of target length and a substrate transport speed of 1 m / min, layer thicknesses of more than> 20 nm per magnetron target are possible.

According to one embodiment of the method, it has proved to be advantageous if the desired value of the oxygen partial pressure in the deposition of the chromoxynitride partial layer with the named mixing ratio is controlled to a value in the range 3 × 10 ^-5 mbar to 6 × 10 ^-4 mbar.

To regulate the oxygen partial pressure, a lambda sensor or a mass spectrometer can be used as a transmitter according to various embodiments of the method. Both measuring methods are suitable for the absolute or relative determination of the partial pressure of oxygen in a gas mixture.

According to one embodiment of the method, a constant flow of nitrogen can be supplied to the sputtering process in addition to the regulated oxygen flow, whereby the control effort for maintaining the mixing ratios according to the invention is reduced and the stabilization of the operating point is improved.

For the production of an absorber according to the invention is on a substrate support, for example, consists of an aluminum strip, optionally on a the aluminum tape protective and smoothing and adhesion of the other layers improving intermediate layer, eg Al ₂ O ₃ , first deposited a infrared radiation reflecting layer. As IR-reflecting layers, for example, noble metal layers are known, as well as chromium, aluminum or molybdenum. Due to their high IR reflection they have a low emissivity in the wavelength range, which reduces the radiation of heat and thus supports the dissipation of heat into the transmission medium. The IR-reflecting layer is optional.

The intermediate layer is e.g. can be produced by anodizing the aluminum strip, but can also be pronounced in the form of a thin oxide skin formed by oxidation in ambient air. A metallic IR reflective layer is made by magnetron sputtering from a metallic target of the coating material.

Over the IR-reflecting layer follows the absorber layer, which consists in the embodiment of a partial layer of CrN and a sub-layer of CrO _x N _y with 1.3 <x <1.48 and 0 <y <0.5. The deposition of these two layers takes place by reactive magnetron sputtering while supplying a constant flow of nitrogen to the argon as working gas. For the CrO _x N _y sublayer oxygen is also supplied, the partial pressure is maintained by a suitable control in the range of 3 x 10 ^-5 mbar to 6 x 10 ^-4 mbar. The sputtering takes place for both partial layers of Cr targets.

About the absorber layer are one or more SiO _2- containing anti-reflective coatings, also by reactive Magnetron sputtering, deposited. Alternatively, a change layer system with alternating high and low refractive indices can also be arranged for antireflection coating.

The deposition of the SiO 2 -containing top layer can also be effected by other coating methods, for example electron beam vapor deposition or plasma-enhanced chemical vapor deposition (PECVD).

As substrate material in addition to aluminum and copper, steel or polymers can be used.

QUOTES INCLUDE IN THE DESCRIPTION

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Cited patent literature

• DE 202009015334 U1 [0005]

Claims (6)

Solar absorber comprising a carrier substrate and having a solar absorber layer system arranged on one side of the carrier substrate, which has an absorber layer and a cover layer from the carrier substrate, characterized in that the absorber layer comprises at least two partial layers, of which a partial layer comprises a chromium nitride or chromium oxide layer. Layer is and the other is a chromium oxynitride layer of the composition CrO _x N _y , where 1.1 <x <1.6 and 0 <y <0.5. Solar absorber with a carrier substrate according to claim 1, characterized in that preferably 1.3 <x <1.48 and 0 <y <0.5. A method for producing a solar absorber with a solar absorber layer system deposited on a carrier substrate, wherein an absorber layer and above a cover layer are deposited on the carrier substrate by means of magnetron sputtering, characterized in that the absorber layer is reactively deposited from at least two partial layers, wherein a partial layer as a chromoxynitride of the composition CrO _x N _y with 1.1 <x <1.6 and 0 <y <0.5 is deposited by measuring the oxygen partial pressure at least in the deposition of the chromium oxynitride sublayer and with a control loop using the amount of the sputtering process supplied oxygen is controlled as a manipulated variable. A method for producing a solar absorber according to claim 3, characterized in that the sputtering process, a constant nitrogen flow is supplied. Process for the preparation of a solar absorber according to one of claims 3 or 4, characterized in that the oxygen partial pressure for the deposition of the chromoxynitride partial layer is controlled to a value in the range 3 × 10 ^-5 mbar to 6 × 10 ^-4 mbar. Process for producing a solar absorber according to one of Claims 3 to 5, characterized in that a lambda sensor or a mass spectrometer is used as the sensor for regulating the oxygen partial pressure.