FRONT SURFACE MIRROR FOR REFLECTING SUNLIGHT, METHOD FOR MANUFACTURING THE MIRROR AND USE OF THE MIRROR

BACKGROUND OF THE INVENTION

1. Field of the Invention This invention relates to a front surface mirror (FSM) for reflecting sunlight and a method for manufacturing the mirror. Moreover a use of the front surface mirror is provided.

2. Description of the Related Art

In general front surface mirrors for reflecting sunlight are known for the use in sun field power plants. A base FSM comprises a multi layer stack on a substrate surface of a substrate with following stacking sequence: An adhesion layer with Chromium (Cr) , a binding layer with a Nickel Chromium alloy (NiCr) , a reflection layer with Silver (Ag) and a passivation layer with Zink Sulfide (ZnS) or Zink Oxide (ZnO) . Scratch resistance of the front surface mirror for an application in a sun field power plant application is one of the crucial features of the mirror.

Concerning the scratch resistance, an overcoat with Aluminum Oxide (AI ₂ O3) on the base FSM deposited by an Electron Beam Gun (E.B. gun) method, shows good results. But a deposition rate of this method is restricted in order to avoid a very high heat load of the front surface mirror. That makes productive and profitable manufacturing of a front surface mirror with such an overcoat very problematic. On the other hand increasing deposition rate would result in a front surface mirror with low reflectivity and poor durability. Such a mirror is not qualified for the use for a sun field power plant. SUMMARY OF THE INVENTION

It is an object of the invention to provide a front surface mirror with a high reflectivity concerning sunlight. Moreover the front surface mirror should be durable in matters of different environmental conditions.

It is another object of the invention to provide a method for manufacturing the front surface mirror. The method should be implementable in a high production machine.

It is a further object of the invention to provide a use of the front surface mirror.

These objects are achieved by the inventions specified in the claims .

A front surface mirror for reflecting sunlight is disclosed, wherein the mirror comprises a multi layer stack on a substrate surface of a substrate, the multi layer stack comprising following stacking sequence: At least one adhesion layer arranged on the substrate surface of the substrate; at least one intermediate binding layer; at least one reflective layer with at least one reflective material for the reflecting the sunlight; at least one passivation layer that makes the reflective material inert to at least one outdoor environmental condition; and at least one scratch resist protection layer. The passivation layer combined with the scratch resist layer is used for environmental protection of the multi layer stack.

The layers are directly stacked to one another. Adjacent layers of the stack cover each other at least partially. In particular a layer covers the subjacent layer completely. Inert to at least one outdoor environmental condition means that no or nearly no degradation process of the reflective material of the reflective layer occurs while the front surface mirror is exposed to the outdoor environmental condition. For instance, the reflective material is Silver and the outdoor environmental condition is an Oxygen partial pressure. A probability for the appearance of the degradation process (an oxidation of Silver) is reduced.

Beside the front surface mirror a method for manufacturing the front surface mirror is described. The method comprises following steps: a) providing a substrate with a substrate surface; b) arranging the adhesion layer on the substrate surface; c) arranging the intermediate binding layer on the adhesion layer; d) arranging the reflective layer on the intermediate binding layer; e) arranging the passivation layer on the reflective layer; and f) arranging the scratch resist protection layer on the passivation layer.

Finally a use of the front surface mirror in a power plant for converting solar energy into electrical energy is disclosed, wherein a concentrating of sunlight by the front surface mirror is carried out. The front surface mirror, e.g. a parabolic front surface mirror, is a part of a sunlight collecting device of the power plant. By the mirror sunlight is focused to a tube which is filled with a heat transfer fluid in order to absorb the focused (concentrated) sunlight. The heat transfer fluid is a thermo-oil or a thermo-liquid salt.

A front (first) surface mirror (also commonly abbreviated FS mirror) is a mirror with a reflective surface being above a backing, as opposed to a conventional, second surface mirror with the reflective surface behind a substrate which is transparent for sunlight. For example, the transparent substrate consists of a glass or an acrylic glass. The front surface mirror leads to a strict reflection without a ghosting effect. Sunlight in the sense of this invention means in particular electromagnetic radiation of the visible spectrum of the sunlight (about 350 nm to about 780 nm) and electromagnetic radiation of the infrared spectrum. Especially radiation of the near infrared spectrum of the sunlight (about 780 nm to 2.500 nm) is meant. But radiation with longer wavelengths is possible, too.

In general every kind of substrate is possible. A substrate material can be an organic material like polyethylene

terephthalate (PET) or an inorganic material like glass. Here usual glass, e.g. window glass, or hardened glass is used. A further possible substrate material is a metal. For instance, the metal is Aluminum (Al) or stainless steel. The surface of the substrate can be flat or bended. Additionally the surface can be polished and/or lacquered.

For the depositing one or more layers of the multilayer stack nanotechnology can be used. But in a preferred embodiment for the arranging of the different layers (steps b to f) a thin film deposition technique is carried out. The thin film deposition technique is preferably selected from the group consisting of atomic layer deposition (ALD) , chemical vapor deposition (CVD) and physical vapor deposition (PVD) . In particular the physical vapor deposition is sputtering. Thereby in a preferred embodiment a reactive sputtering is used. It is advantageous to use the same thin film deposition technique for all the steps b) to f ) . But it is also possible, to use different thin film techniques for the different steps. While, before and/or after at least one of the steps a) to f) a heat treatment of the substrate or the substrate with the arranged layers is carried out. The heat treatment comprises a heating to a temperature selected from the range between 50°C to 350°C and particularly selected from the range between 50°C to 250°C. In a preferred embodiment the heat treatment comprises a plasma glow discharge treatment. The heat treatment and in particular the plasma glow discharge treatment is carried out in the presence of Argon, Nitrogen and/or Oxygen. The heat in general and the plasma glow discharge treatment in particular can be necessary for the used thin film technique. Moreover by the heat treatment a cleaning of the substrate and/or the arranged layers can be carried out. The adhesion layer can consist of different materials. In a preferred embodiment the adhesion layer comprises at least one adhesion material selected from the group consisting of Chromium (Cr) and Nickel (Ni) . By the intermediate binding layer the adhesion layer (e.g. with Chromium) and the reflective layer (e.g. with Silver) are not attached directly to each other. The intermediate binding layer leads to a smearing the galvanic potential. By that a galvanic corrosion effect is reduced.

It is same for the intermediate binding layer: In particular the intermediate binding layer comprises at least one intermediate material selected from the group consisting of Nickel and Chromium.

In a further preferred embodiment the adhesion layer comprises an adhesion layer thickness selected from the range between 0,5 nm and 20 nm, particularly selected from the range between 1 nm and 15 nm and more particularly selected from the range between 5 nm and 10 nm. In contrast to that, the intermediate binding layer comprises preferably an intermediate layer thickness selected from the range between 5 nm and 80 nm, particularly selected from the range between 10 nm and 60 nm and more particularly selected from the range between 30 nm and 50 nm.

For the reflective layer any reflective material like a metal like Aluminium or Copper is possible. In a preferred embodiment the reflective material is Silver. A reflective layer with such a reflective material show high reflectivity for the sunlight. The reflective layer consists of Silver or a Silver alloy. By the passivation layer which covers the reflective layer the Silver of the reflective layer is protected against severe outdoor reactivity, e.g. oxidation of Silver.

In a preferred embodiment the reflective layer comprises a reflective layer thickness selected from the range between 50 nm and 250 nm, particularly selected from the range between 70 nm and 200 nm and more particularly selected from the range between 100 nm and 150 nm. With these reflective layer thicknesses a high reflectivity is ensured. Additionally in view of the used thin film technique the thickness is economically manageable.

In a further preferred embodiment at least one transition zone layer is arranged between the adhesion layer and the intermediate binding layer and the transition zone layer comprises both the adhesion material of the adhesion layer and the intermediate material of the intermediate binding layer. Via the transition layer an intersection of the compositions of the adhesion layer and the intermediate binding layer is given. For instance, the adhesion layer consists of Chromium. The intermediate binding layer consists of an alloy of Nickel and Chromium. The transition zone layer consists of an excess of Chromium compared to the composition of the intermediate binding layer. Moreover it is advantageous that at least one further transition zone layer is arranged between the intermediate binding layer and the reflective layer. Thereby the further transition zone layer comprises both the intermediate layer material of the

intermediate binding layer and the reflective material of the reflective layer. For instance, the reflective material is Silver. The further transition zone layer contains Silver. These transition zone layers are quite thin. In a preferred embodiment the transition zone layer and/or the further transition zone layer comprise a transition layer thickness selected from the range between 0,5 nm and 20 nm, particularly selected from the range between 1 nm and 15 nm and more particularly selected from the range between 5 nm and 10 nm.

The passivation layer has the function of protection of the reflective layer and the reflective material, respectively. For instance, the reflective material is Silver. By the passivation layer a reactivity of Silver should be inhibited. Different proper passivation materials are possible. But in a preferred embodiment the passivation layer comprises at least one passivation material selected from the group consisting of Zink Oxide (ZnO), Zink Selenide (ZnSe) and Zink Sulfide (ZnS) . Theses passivation materials show very good characteristics concerning the protection function.

Using a proper passivation material like Zink Oxide, Zink Selenide or Zink Sulfide, the thickness of the passivation layer can be very low. In a preferred embodiment the passivation layer comprises a passivation layer thickness selected from the range between 0,1 nm and 20 nm, particularly selected from the range between 0,5 nm and 15 nm and more particularly selected from the range between 1,0 nm and 10 nm. ZnS can be sputtered only by using RF power supply. It means that the width of the machine, for the best, is limited to -2.5 m. To overcome that limitation one can evaporate the sublimatic ZnS material downwards by using a long thermal evaporating boat. Alternatively pulse DC generator is used to sputter conductive ZnO as substitutional protective material.

In a preferred embodiment the scratch resist protection layer comprises Silicon Oxide as a scratch resist protection material. Other components can be contained, e.g. Aluminium Oxide. Silicon Oxide (Si0 ₂ ) has a good scratch resistivity. Moreover Silicon Oxide can be sputtered. Using reactive sputtering of Si to deposit the S1O ₂ layer, enough heating power is possible which effects and satisfies interfacial reaction between Silver of the reflective layer and the S1O ₂ layer. The reactive sputtering of S1O ₂ can be implemented on large scale production machine. The using of reactive sputtering with a target comprising Si and Al leads a scratch protection layer with these elements showing very good results. The amount of Al is selected from the range between 1 wt% to 20 wt% and preferably from the range between 5 wt% and 15 wt%. An amount of 10 wt% of Al (and 90 wt% of Si) is preferred from the durability point of view, because the resulting scratch protection layer (comprising Si and Al) is almost free of mechanical surface stress.

In particular the protection layer comprises a protection layer thickness selected from the range between 200 nm and 3000 nm, particularly selected from the range between 500 nm and 2500 nm and more particularly selected from the range between 1000 nm and 2000 nm. Thicknesses below (e.g. 100 nm) or above (e.g. 4000 nm) the mentioned ranges are possible, too. In a further preferred embodiment at least one enhancement layer is arranged between the passivation layer and the scratch resist protection layer. Two or more enhancement layers are possible, too. The enhancement layer comprises at least one enhancement material selected from the group consisting of Silicon Oxide, Titanium Oxide (Ti0 ₂ ) and Zirconium Oxide (Zr0 ₂ ) . More than one enhancement material is possible, too. The enhancement layer leads to a higher reflectivity. Very good results are achieved by an enhancement layer which comprises an enhancement layer thickness selected from the range between 10 nm and 150 nm, particularly selected from the range between 20 nm and 100 nm and more particularly selected from the range between 30 nm and 80 nm. For instance, there are two enhancement layers with S1O ₂ (70 nm) and T1O ₂ (30 nm) . These enhancement layers are attached to each other.

In order to test characteristics the front surface mirror is arranged in a box. Thereby the mirror is arranged in the box such that sunlight can be reflected by the mirror. For instance the box is a weather chamber or a salt fog chamber. Such a box is used for an investigation of the durability of the front surface mirror under different and defined environmental conditions. For that, the box can be designed such that the mirror is hermetically (completely) insulated from the environment of the box. A partially insulation is possible, too. For instance, the box is designed such that the covering of the mirror by dust is prevented, but an exposure of the mirror by humidity is possible.

The box consists preferably of a box material with high quality steel. But depending on the environmental conditions a box with other metal alloys or even with a pure (or nearly pure) metal is possible, too. For the investigation of an optical characteristic of the front surface mirror (e.g. reflectivity) the box comprises a window through which the mirror can be irradiated by the sunlight or by a range of the sunlight. The window is transparent for the sunlight or for the range of the sunlight. For instance the box comprises a cover with glass or acrylic glass. Sunlight with special wavelengths passes the cover of the box and attains a reflective surface of mirror. In summary following advantages are connected to the inventions:

- The front surface mirror shows a high reflectivity (96% - 98%) and a high durability (more than 20 years) against different environmental conditions.

- The complete multi layer stack can be manufactured by sputtering. The use of different thin film deposition techniques is not necessary. - The sputtering can easily be implemented in a high production machine .

BIEF DESCRIPTION OF THE DRAWING Further features and advantages of the invention are produced from the description of exemplary embodiments with reference to the drawing.

Figure shows a cross section of a front surface mirror.

DETAILED DESCRIPTION OF THE INVENTION

The front surface mirror 1 has a multi layer stack 10 arranged on a substrate surface 111 of a substrate 11. The multi layer stack comprises following stacking sequence: at least adhesion layer (101) arranged on the substrate surface of the substrate; at least one intermediate binding layer (102) ; at least one reflective layer (103) for the reflecting the sunlight; at least one passivation layer (104) that makes Ag inert to severe outdoor environmental conditions; and at least one scratch resist protection layer (105) for scratch resist and environmental protection the multi layer stack.

The substrate is a window glass. The substrate surface 111 is either flat or bended. I one embodiment the bended substrate surface is parabolic. The adhesion layer comprises Chromium. The adhesion layer thickness is about 10 nm. The intermediate binding layer comprises an alloy of Nickel and Chromium. The intermediate layer thickness is about 40 nm. Between the adhesion layer 101 and the intermediate binding layer 102 a transition zone layer 112 is arranged. This transition zone layer comprises an alloy of Nickel and Chromium, too. But the content of Chromium of the transition zone layer is higher than the content for Chromium of the intermediate binding layer. The transition layer thickness is about 10 nm.

The reflective layer 103 comprises Silver as reflective material. The reflective layer thickness is about 150 nm. Between the intermediate binding layer 102 and the reflective layer 103 a further transition zone layer 123 is arranged. This further transition zone layer comprises an alloy of Nickel and Chromium. Additionally comprises the further transition zone layer the reflective material of the reflective layer, namely Silver. Over the reflective layer a passivation layer with Zink Oxide as passivation material is arranged. In an alternative embodiment the passivation material is Zink Oxide. The passivation layer thickness is about 5 nm.

After that two enhancement layers arranged: An enhancement layer 145 with Silicon Oxide (thickness about 80 nm) and

an additional enhancement layer 154 with Titanium Oxide

(thickness about 50 nm) .

Competed is the multi layer stack by a scratch resist protection layer with the scratch resist protection material Silicon Oxide. The protection layer thickness of the scratch resist protection layer is about 2000 nm.

For the manufacturing of the front surface mirror following steps are carried out: a) providing a substrate with a substrate surface; b) arranging the adhesion layer on the substrate surface; c) arranging the intermediate binding layer on the adhesion layer; d) arranging the reflective layer on the intermediate binding layer; e) arranging the passivation layer on the reflective layer; and f) arranging the protection layer on the passivation layer. For all the steps b) to f) sputtering is used. For the sputtering a reactive sputtering is used.

The front surface mirror is used in a power plant for converting solar energy into electrical energy. Thereby a concentrating of sunlight by the front surface mirror is carried out. By bended mirrors to parabolic shape, sunlight is focused to a tube which is filled with a heat transfer fluid. The heat transfer fluid is a thermo-oil or a melted thermo-salt. By a heat transfer exchanger steam is produced for operating a turbine.