WO1996009502A1

WO1996009502A1 - Spectrally selective collector coating and process for its production

Info

Publication number: WO1996009502A1
Application number: PCT/EP1995/003764
Authority: WO
Inventors: Lothar Herlitze; Wolfgang Graf
Original assignee: Interpane
Priority date: 1994-09-22
Filing date: 1995-09-22
Publication date: 1996-03-28
Also published as: DE4433863A1

Abstract

The present invention relates to a device for heating a transport medium by solar radiation, especially a solar collector, with a spectrally selective layer system, the layers of which are arranged on a heat-conducting substrate in heat-conducting contact with the transport medium to be heated. The layers (3, 4, 6a, 6b, 6c, 6d) of the spectrally selective layer system contain dielectric and absorbent materials which can be applied to the substrate (1) by sputtering. The present invention also relates to a heat-conducting substrate (1) in which the layers (3, 4, 6a, 6b, 6c, 6d) of the spectrally selective layer system are applied successively to the heat-conducting substrate (1) by sputtering.

Description

Spectral selective collector coating and process for its production

The present invention relates to a device for heating a transport medium by means of solar radiation, in particular a solar collector, with a spectrally selective layer system, the layers of which are arranged on a heat-conducting substrate which is in heat-conductive contact with the transport medium to be heated.

Solar panels are used to heat a transport medium, such as. B. a liquid or a gas, with the help of solar radiation. In order to ensure efficient conversion of solar radiation into heat, a highly absorbent layer system is applied to a heat-conducting substrate.

Loss of heat on the side of the solar collector facing away from the sun is reduced by opaque insulation, whereas losses on the side facing the sun are avoided by thermal insulation that is transparent to solar radiation.

Copper, aluminum, steel or stainless steel are used as substrates for reasons of heat conduction and for reasons of cost. There are a number of different types of collectors, such as. B. flat collectors, vacuum and tube collectors, etc.

A distinction is made between low-temperature collectors that work at an operating temperature of up to 100 ° C and high-temperature collectors whose working temperature is up to 300 ° C. These collectors show very different designs and requirement profiles for their respective components.

In order to achieve the highest possible collector efficiency, selectively coated absorbers are used. The aim is to absorb as much as possible the total radiation power of the sun, ie to strive for a spectral absorption capacity oL ⁽ Z depending on the wavelength • * of the solar radiation of 100%. To keep the heat losses low, the emissivity £ () im Radiation range of the black body at the corresponding collector or working temperature must be minimal, so a spectrally selective collector coating is characterized by a high spectral Absorbance c in the area of solar radiation and a low emissivity € in the radiation area of the black body of the respective working temperature.

Since the mostly used metal substrates have no transmission in the solar range, a vanishing reflection R (X) is aimed for in order to ensure a high absorption capacity.

The ideal absorber therefore has a reflectivity R (X) = 0 below a wavelength of - = 2500 nm and R (X) = 1 above 2500 nm. To implement such a principle, various absorber concepts are used, e.g. B. intrinsically selective materials, gradient layers, cermet layers and anti-reflective layers. Usually several of these concepts are used at the same time.

For industrial use, electro-technically manufactured systems based on chrome-chromium-oxide-Ceimet, as well as layers using metal-aluminum oxide mixed layers, have become established through * g ».

The selectivity properties that can be achieved depend on the composition of the electrolyte used, the working conditions and the material of the substrate. As such, copper, aluminum, stainless steel and galvanized or non-galvanized steel are used. An intermediate layer of nickel is often applied to the substrates. It serves to protect against corrosion.

Chromium-chromium oxide cermet systems are among the selective

Black chrome coatings that can be produced in different ways. In addition to electroplating processes with different bath compositions, vacuum processes are also used. The term "chrome black" has a superordinate meaning and is not restricted to a specific product.

Black chrome is considered stable in the working area of the low-temperature collectors and is more resistant to moisture. The general disadvantage of many electroplated selective layers is that their properties deteriorate due to aging and in the environmentally hazardous use of toxic salt solutions in the manufacture.

The manufacturing process of nickel structure layers on aluminum substrates is comparable to the two-step process known from anodizing technology. The too Coating aluminum absorbers are degreased, pickled and decapitated. This is followed by the creation of an anodized layer with a defined pore pattern and the deposition of the nickel into the pores.

For high-temperature vacuum absorber layers for focusing systems, e.g. Vacuum tube collectors for solar power plants, selective layers based on high-frequency sputtered aluminum oxide with embedded metal particles, e.g. Molybdenum. Although these layers have good temperature resistance, they have insufficient corrosion resistance to moisture.

The present invention is therefore based on the object of providing a spectrally selective coating for a solar collector and a method for producing such which have a high thermal resistance up to the standstill temperature, i.e. H. if the collector is operated without transport medium and has a high chemical resistance to environmental influences such as Ensure gases, dust, water vapor, etc. Further properties worth striving for include high resistance to temperature changes, high resistance to UV radiation, long service life, large-scale producibility and low manufacturing costs.

The object is achieved by a device according to the preamble of claim 1, which is characterized in that the layers of the spectrally selective layer system contain dielectric and absorbent materials which can be applied to the substrate by sputtering.

Furthermore, the above-mentioned object is achieved according to claim 8 by a method for producing a spectrally selective layer system on a thermally conductive substrate, which is characterized in that the layers of the spectrally selective layer system are applied in succession to the thermally conductive substrate by sputtering.

In order to achieve optimal layer properties, both the absorber substrate and the coating components must be considered in their individual properties and in their interactions.

Metals, metal foils, plastics (PTFE, EPDM), glass, rubber and the like can be used as materials for the heat-conducting substrate. Because of the heat conduction properties, it is usually limited to metals such as copper, Stainless steel or aluminum, the use of copper and aluminum or nickel being advantageous because of their inherently low emissivity in the heat radiation range.

The surface quality (roughness characteristics) of the substrate sheets used is of great importance for the resistance behavior of the absorbers. Surfaces that are as smooth as possible and have low roughness depths, without deep groove formation, that is, gently undulating surfaces, have proven to be advantageous.

A further possibility for improving the corrosion properties consists in the use of tin-plated or nickel-plated sheets, the layer thickness of the tin or nickel layer being greater than or equal to 1 μm.

The substrate sheets are pretreated either by degreasing in appropriate cleaning baths or preferably by a flame treatment, a silicon-containing compound, preferably tetraethoxylan-TEOS, being added to the combustion gas, which leads to the build-up of a very thin SiO _x layer on the substrate sheet with an anti-corrosion effect.

There are two ways to guarantee a low emissivity of the substrate. The first option is to use substrates with a low emissivity, e.g. B. copper, aluminum, nickel or the like. The other possibility is to apply a suitable layer with a low emissivity on the substrate. Tin, nickel, molybdenum, gold, platinum metals or low-emitting compounds such as nitrides, in particular the nitrides of the subgroup elements such as titanium nitride or zirconium nitride, can be used as materials for these layers. The layer thickness can be between 10 nm and a few μm.

The spectrally selective layer system, which contains layers of dielectric and absorbent materials, is then applied to the low-emitting surface. It has been found that materials produced by sputtering processes, such as. B. the DC magnetron or HF sputtering method can be applied to the heat-conducting substrate, are characterized by a high thermal and chemical resistance and a high resistance to temperature changes. Furthermore, the resistance to UV radiation and the long service life of the materials that can be applied by sputtering are to be emphasized. Further advantages are the excellent layer uniformity, which is well manageable in terms of process technology Process control and the possibility of large-area coating of semi-finished substrate products and ready-made absorbers. The manufacturing process can be designed particularly economically, in particular by the DC magnetron sputtering method. The method is advantageously suitable for coil coating, coating using the inline method and batch coating.

Two different variants have proven to be particularly favorable for the construction of the layers of the spectrally selective layer system.

In the first variant, the dielectric and the absorbent layers are deposited one above the other in a sandwich-like structure. It therefore contains at least one layer of a dielectric material and at least one further layer of an absorbent material, an absorbing layer and a layer containing a dielectric material being arranged alternately. The individual layer thicknesses are between 1 nm and 100 nm. They are determined by the refractive index and the absorption capacity of the layers in such a way that a high absorption for solar radiation is achieved with a low emissivity of the entire layer system.

Carbides, preferably silicon carbide, or oxides of the 4th main group or subgroup elements, preferably niobium oxide, zirconium oxide and tantalum oxide, are used as dielectric materials. Nitrides or oxynitrides of silicon, boron or the subgroup elements are also suitable.

As absorbent materials, absorbent reactive layers, and in particular nitrides, preferably chromium nitride or nitrides of chromium alloys such as, for. B. Ni / Cr used. Molybdenum and elemental carbon have also proven to be cheap.

A cermet structure is achieved by clustering the absorbent materials.

It has proven to be particularly advantageous to produce a gradient layer system in which the concentration of the absorbent material increases from the light incidence side to the substrate.

The second variant for the construction of the spectrally selective layer system is that the layers are a material mixture of absorbent and dielectric Contain material portions, the stoichiometry changes from layer to layer so that the absorbent material portion of the material mixture increases towards the substrate.

It is possible to manufacture the layer system in such a way that it is only produced from a basic material. The system CrO _x represents an example. By a suitable choice of the reaction conditions, as described further below, x can be varied such that values 0 <x <1, 5 are reached, with x increasing from the substrate to the light incidence side.

In principle, all subgroup elements, their alloys, and in particular Ni / Cr alloys, can be considered as materials.

However, an additional absorbent material can also be built into the layers. All subgroup elements, their alloys, in particular Ni / Cr alloys, but also molybdenum or carbon can in turn be used as materials for this.

In addition to or instead of the stoichiometry change due to an increase in the absorbent material portion of the material mixture towards the substrate, the concentration of the additional absorbent material towards the substrate can also increase.

Furthermore, in both variants of the layer system structure, an anti-reflective layer with a low refractive index can be arranged on the light incidence side of the layer system.

In addition, a layer with a low emissivity can be arranged between the heat-conducting substrate and the spectrally selective layer system. This layer can also function as a corrosion protection.

As already stated above, sputtering processes are particularly suitable for producing spectrally selective layer systems. In the sandwich-like variant of the layer system structure, the nitride layers are sputtered from the metal target, e.g. B. Chrome target. Silicon carbide can be produced by sputtering from the silicon target in a carbon-containing atmosphere or preferably by sputtering from the silicon carbide target. The carbon layers are produced by sputtering from the graphite target. Oxide layers are created by reactive sputtering in an oxygen atmosphere. In all cases, the working pressure is approximately 5 x 10 ^ bar, the sputtering power densities being between 1 and 10 W / cm ^ target surface.

In the production of spectral systems according to the second variant, in which the absorbent material content changes continuously, the difficulty lies in finding stable working points for the coating process during the sputtering. In particular, it is difficult to stabilize states in the transition area between the "metallic" and the "compound" mode.

According to the present invention, this problem is solved in that the stoichiometry of the material mixture from layer to layer is changed step by step or continuously by a time-modulated change in the reactive gas content in the sputtering gas.

The other possibility is to change the stoichiometry of the material mixture from layer to layer step by step or continuously by changing the sputtering power in a time-modulated manner.

There is a brief transition between the "metallic" and the "compound" mode. Through a specific setting of the dwell times t1 and t2 in the respective state, a mixed layer can be generated and the layer composition can thereby vary within wide limits. The corresponding frequencies can be between 10 ^" * Hz and 10 ^ Hz, a state change frequency of 1 - 10 Hz has proven to be particularly favorable.

The required parameter changes are made on the flow controller or the setpoint specification units for the power supplies of the sputtering system.

The layer systems according to the invention and the layer systems produced according to the invention are distinguished by a high solar absorption capacity of o = 90% to 96% and a low emissivity € = 3% to 10%. The silicon carbide layers in particular show very good mechanical resistance and are smudge-proof and easy to handle, properties such as scratch resistance also being influenced by the substrate material.

Furthermore, the layers mentioned above, in particular the silicon carbide systems, show very good stability in the climate change test at temperatures up to 90 ° C. and in saturated water vapor. The reason for this can be seen in the fact that electrochemical corrosion processes are effectively prevented by the deposition of non-metallic layers which also contain hardly any metal impurities.

In addition, the layers are hydrophobic, so they are poorly wetted by water.

Finally, the layers mentioned above, in particular the SiC / CrN _x systems in the

Vacuum up to temperatures of 400 ° C and more stable and therefore also suitable for use in vacuum collectors.

It is also conceivable to use the layer systems according to the invention not only for collector coatings, but also as sun protection or as heat protection coatings on glass substrates.

The present invention is described in more detail below by means of preferred exemplary embodiments with reference to the accompanying drawings. Show it:

1 schematically shows the sandwich-like variant of the layer structure of a spectrally selective layer system, FIG. 2 schematically shows the variant of the layer structure with changes in stoichiometry, FIG. 3 shows the dependence of the sputter voltage on the reactive gas flow, FIG. 4 shows a time-modulated change in the sputtering power, FIG. 5 a time-modulated change in the reactive gas content in the sputtering gas.

1 shows a first variant of a spectrally selective layer system according to the invention, in which the dielectric and absorbent materials that can be applied to the substrate 1 by sputtering have a sandwich-like structure. At least one layer 3 contains a dielectric material and at least one further layer 4 contains an absorbent material. The layers 3 with dielectric material and the layers 4 with absorbent material are arranged alternately.

Materials with a high thermal conductivity are used for the substrate, in particular metals such as copper, stainless steel, aluminum or nickel.

A layer 2 with a low emissivity can be arranged between the heat-conducting substrate and the spectrally selective layer system. Tin, nickel, molybdenum, gold, platinum metals or low-emitting compounds such as nitrides, in particular nitrides, can be used as the material for layer 2 Sub-group elements such as titanium nitride or zirconium nitride are used. The layer thickness of layer 2 is between 10 nm and a few μm.

An anti-reflective layer 5 with a low refractive index can also be arranged on the light incidence side of the spectrally selective layer system. This anti-reflective layer 5 serves to lower the reflection in the visible region of the light.

The concentration of the absorbent material in the absorbent layers 4 increases from the light incidence side towards the substrate according to a certain function.

2 shows the structure of a spectrally selective layer system, the layers 6 of which contain dielectric and absorbing materials which can be applied to the substrate 1 by sputtering. The layers 6a, 6b, 6c, 6d contain a material mixture of absorbent and dielectric material portions, the stoichiometry changing from layer to layer such that the absorbent material portion of the material mixture increases towards the substrate 1.

An example of such a system is the CrO _{x system} . The layer 6a consists z. B. from CrO _x ι, the layer 6b from CrOχ2, the layer 6c from CrO _x 3 and the layer

6d from CrO _x 4- The following applies: 0 <xl <x2 <x3 <x4 <. 1.5, so that the

Stoichiometry changes from layer to layer in such a way that the absorbent material portion of the material mixture CrO _x increases towards the substrate.

Instead of Cr, all subgroup elements and their alloys, in particular Ni / Cr alloys, are suitable as materials.

Furthermore, absorbent material can also be built into the layers. All subgroup elements and their alloys, in particular Ni / Cr alloys, but also elemental carbon or molybdenum, are also suitable as materials for this. The concentration of this additional absorbent material can also increase towards the substrate.

In this second variant of the layer structure, an anti-reflective layer 5 with a low refractive index can also be applied to the light incidence side in order to reduce the reflection in the visible region of the solar radiation. Furthermore, a layer with a low emissivity can also be arranged here between the substrate 1 and the spectrally selective layer system. In the production of spectrally selective layer systems on a heat-conducting substrate, the desired properties described above can be achieved if the layers are applied to the heat-conducting substrate in succession by sputtering. The DC magnetron sputtering method is particularly preferred. For the production of layers which contain a material mixture of absorbent and dielectric material portions, the stoichiometry changing from layer to layer such that the absorbent material portion of the material mixture increases towards the substrate, either the reactive gas content in the sputtering gas or the sputtering power can be time-modulated be changed. 3 illustrates these methods. The diagram shown shows the sputtering voltage as a function of the reactive gas flow, the two curves P1 and P2 showing two different sputtering powers. To generate a mixed state, a brief transition is carried out between different working points AI, A2 and A3. The arrows between the working points AI and A3 indicate the change in the sputtering performance during which the reactive gas flow remains constant.

4 shows a diagram of the time-modulated change in the sputtering power. The one working or operating point is held during a time period t1 and the other working or operating point is held for a time period t2, so that the time-modulated change in the sputtering power is equivalent to a step function.

The transition between the operating points AI and A2, which is illustrated by arrows in FIG. 3, characterizes the change in the reactive gas flow with constant sputtering power. 5 shows a diagram with the time-modulated change in the reactive gas content. One operating point is in turn held for a time t1 and the other for a time t2, so that the time-modulated change in the reactive gas content also presents itself as a step function.

In both methods, the alternating frequencies between the respective working or operating points can be between 10 "! And 10 ^ Hz. It has proven to be particularly favorable if the frequencies are between 1 - 10 Hz. The function type of the time-modulated change is not dependent on It is conceivable, in particular, to combine the two methods described above of the time-modulated change, the sputtering power and the time-modulated change of the reactive gas content.

Claims

Expectations

1. Device for heating a transport medium by means of solar radiation, with a spectrally selective layer system, the layers of which are arranged on a heat-conducting substrate which is in heat-conducting contact with the transport medium to be heated, characterized in that the layers (3, 4, 6a, 6b , 6c, 6d) contain dielectric and absorbent materials which can be applied to the substrate (1) by sputtering.

2. Device according to claim 1, characterized in that at least one layer (3) is a dielectric material and at least one further

Layer (4) contains an absorbent material, wherein an absorbent and a dielectric material-containing layer are alternately arranged.

3. Device according to claim 1, characterized in that the layers (6a, 6b, 6c, 6d,) contain a material mixture of absorbent and dielectric material components, the stoichiometry changing from layer to layer such that the absorbent material component of the material mixture Substrate (1) increases.

4. The device according to claim 3, characterized in that an additional absorbent material in the layers (3, 4, 6a, 6b, 6c, 6d) is installed.

5. The device according to claim 2 or 4, characterized in that the concentration of the absorbent material to the substrate (1) increases.

6. Device according to one of the preceding claims, characterized in that between the heat-conducting substrate (1) and the spectrally selective layer system, a layer (2) with a low emissivity is arranged.

7. Device according to one of the preceding claims, characterized in that an anti-reflective layer (5) with a low refractive index is arranged on the light incidence side of the spectrally selective layer system.

8. A method for producing a spectrally selective layer system on a thermally conductive substrate, characterized in that the layers (3, 4, 6a, 6b, 6c, 6d) of the spectrally selective layer system are applied one after the other by sputtering onto the thermally conductive substrate (1).

9. The method according to claim 8, characterized in that the layers (3, 4, 6a, 6b, 6c, 6d) contain a material mixture of absorbent and dielectric material components, the stoichiometry of the material mixture from layer to layer by a time-modulated change in the reactive gas content is gradually or continuously changed in the sputter gas.

10. The method according to claim 8, characterized in that the layers (6a, 6b, 6c, 6d) contain a material mixture of absorbent and dielectric material portions, the stoichiometry of

Material mixture from layer to layer by a time-modulated change in the

Sputter performance is changed gradually or continuously.

11. The method according to claim 9 or 10, characterized in that the time-modulated change takes place between two operating points, the one operating point being held for a time t1 and the other operating point being held for a time t2.

12. The method according to claim 11, characterized in that the alternating frequency between the two operating points is 1-10 Hz.