DE102011012044B4 - Method for producing a reflective layer system - Google Patents

Abstract

Method for producing a reflection layer system of a rear mirror, in which at least one reflection layer and, moreover, a lacquer system are deposited on a substrate, characterized in that a method step for annealing the layer structure and increasing the total solar reflectivity of said reflection layer (R1, R2) in comparison to untreated reflective layer (R1, R2) by means of storage of said coated with said reflective layer (R1, R2) coated substrate (S) at room temperature over a period of at least 24 hours before deposition of the paint system.

Description

The invention relates to a method for the production of a reflective layer system for solar applications with a solar-spectrum highly reflective layer which is deposited on a substrate.

Reflection coating systems have been used in many areas of our lives since time immemorial, but they are becoming increasingly important nowadays, e.g. for mirrors in solving the energy issue too. While mirrors for normal indoor applications "only" need to reflect the visible parts of the light spectrum, for the new solar applications they have to reflect the entire range of the solar spectrum in the best possible way.

In the case of mirrors, a distinction is always made between front and rear side mirrors, depending on which side of the substrate causes the main reflection.

Frequently, reflective coating systems for indoor and outdoor applications, e.g. solar applications (CSP - Concentrated Solar Power), by using a wet-chemical method to apply a reflective coating to a substrate, e.g. Glass is deposited. For example, these are mirrors in which the reflective coating is either on the back of the substrate (e.g., rear mirror) or on its front (e.g., front mirror). Depending on the specific application and the associated requirements for the mechanical, chemical and environmental resistance, the reflection coating of the rear-view mirrors is protected on the atmosphere side with single or multi-layer paint systems. Most widely used are paint systems of three different paint layers, the individual paint layers have different tasks, such as adhesion, corrosion and UV protection or mechanical protection to meet.

The decisive factor for the quality of a reflective layer system is, among other things, the value of its total solar reflectivity (TSR), ie its ability to reflect the solar radiation. This value is determined, in addition to the absorption losses by the substrate itself, primarily by the reflectivity of its coating. In order to achieve the highest possible reflection, silver is preferably used as the reflective layer and a particularly low-absorption and highly transparent substrate, e.g. so-called white glass or solar glass used. On the reverse side, the silver layer is then closed by a copper layer, which also serves as an interface layer for the subsequent lacquer coating.

The manufacturing process of such reflective layer systems usually looks like this. After an appropriate pretreatment, cutting to the required shape, grinding e.g. the grinding of the substrate edges, the bending and / or tempering of the flat or already bent substrates and may include other steps, they are optionally once again polished and washed.

Still wet, they are then provided with an adhesion-promoting tin dichloride solution for activation. Thereafter, the disk passes successively through coating stations, where it is wet-chemically coated with silver and immediately thereafter with copper. This is immediately followed by the coating of the paint or the various paints of the multi-stage paint system. Subsequently, the entire coating is then dried at 150 ° C-200 ° C. By producing and drying the lacquer layer, the morphological structure of the reflective layer system is frozen as it were.

Depending on the absorption properties of the substrate and its thickness, mirrors with a TSR of 93% -94% can be produced by means of the described method, for example with a solar glass thickness of 4 mm. This value is below the achievable values which would result, for example, in simulation calculations with correspondingly tabulated optical data for silver.

To optimize the properties it is also known to anneal reflection layers ( DE 695 26 191 T2 . US 2006/0068227 A1 . DE 694 08 933 T2 ). To improve the reflective properties of bodies, the coating of these with a reflective layer system comprising a plurality of thin layers of a highly reflective material, for. B. from the US 2009/0220802 A1 known.

It is therefore an object to provide a reflective coating system for solar applications and a method for its production, with which higher TSR values can be achieved.

The object is achieved by a method according to claim 1. Embodiments of the method are described in the respective dependent claims 2 to 8.

With the method according to the invention for depositing the reflection layer system, it is possible to form a layer structure for the reflection layer, regardless of whether it consists of a layer or, according to one embodiment, of several partial layers, which brings about an increase in the reflection. in the Method an additional process step is carried out, which allows a "healing" of the layer structures. The annealing of crystalline structures is generally known to heal structural defects that otherwise degrade the layer properties, in this case the reflectivity. Due to the annealing of the reflection layer, the crystal structures obtainable by the method according to the invention are optimal for high reflectivity.

In addition, reflection-reducing effects that occur through further process steps after the deposition can be reduced or even avoided. Thus, in particular with the deposition of paint systems above the reflective layer system occurring layer stresses can be avoided.

According to the invention, the annealing step in a rear mirror in the storage of the reflective layer system at about room temperature. Again, the above effects were achieved, the storage time is at least 24 hours before deposition of the paint system, but is dependent on the structure and the material of the layer system. By simple experiments, the optimum storage time for the particular reflective layer system must be determined. The method described can be used for various reflection materials. Particularly suitable are silver, aluminum, gold, platinum or an alloy of at least one of these metals. The metals mentioned all have a comparatively high solar reflectance, optionally for certain wavelengths such as gold and platinum, and are therefore suitable for the reflection layer system. The necessary minimum layer thicknesses can be determined as a function of the material of the metallic, reflective functional layer, if appropriate by means of experiments or by simulation, so that the maximum reflection can be achieved.

If, in a further embodiment of the method, the reflection layer is deposited from at least two partial layers, a first metallic reflection layer and a second metallic reflection layer, the second metallic reflection layer can be made of copper, nickel, chromium, stainless steel, silicon, tin, zinc or an alloy of at least one consist of these metals. With these materials, the reflective properties can be linked with mechanical and / or chemical protection.

The deposition of the reflection layer into two partial layers offers the possibility of carrying out a supplementary annealing step after the deposition of the first partial layer. The additional process step causes a reduction of diffusion processes in the silver layer, in particular between the silver and the copper layer. Such diffusion processes also reduce the reflectivity of the reflection layer system.

According to alternative embodiments of the invention, the deposition of the reflective layer system may alternatively be carried out by wet-chemical coating method or by PVD methods, e.g. Sputtering, done. The above-described annealing step leads to the described effects in both options. Consequently, the advantages associated with the respective method can be utilized for the various applications without disadvantages for the reflectivity of the layer system.

According to an embodiment of the method, the surface of the substrate facing the reflective layer system is pretreated before the deposition of said reflective layer. As pretreatments of the surface of the substrate facing the reflective layer system prior to the deposition of said reflective layer, the deposition of an adhesion-promoting and diffusion-barrier layer (HS) with a thickness in the range of less than 5 nm or a plasma treatment under vacuum may be considered.

The adhesion-promoting and diffusion-barrier layer on the substrate can be deposited very thinly, so that it has little or no optically absorbing effect. This layer does not necessarily have to form a closed layer or surface on the substrate and can therefore also be regarded as a so-called seed layer. For this reason, very small layer thicknesses are sufficient here. They are usually below 5 nm. However, layer thicknesses of less than 1 nm and more preferably less than 0.3 nm are preferred.

Alternatively, a plasma treatment, for example by means of glow discharge carried out under vacuum. The reflective layer systems produced in this way fulfill the necessary requirements with regard to the chemical and thermal resistance as well as the adhesive strength, which are tested by various standardized tests. The mentioned pretreatment steps can also be combined with each other. This also includes that, in addition to depositing an adhesion-promoting and diffusion-barrier layer, a plasma treatment may be performed. As a plasma treatment, for example, a glow discharge can take place. For this purpose, usually in a dilute gas atmosphere, which may contain Ar, O ₂ , N ₂ , CDA (Compressed Dry Air) or any mixtures thereof, at a pressure of 2-5 10-2 mbar, a direct current (DC) or medium frequency (MF) glow discharge, which is exposed to the later to be coated side of the substrate.

With one or a combination of several pretreatments can be dispensed with the chemical activation of the substrates by means of a tin-dichloride solution, as used in the wet-chemical coating according to the prior art. There are no toxic wastewaters.

To improve and specifically adjust the optical properties, an embodiment of the method comprises an alternating-layer system which comprises at least one layer sequence with a high-index dielectric layer and a low-index dielectric layer. Such a shift layer system is e.g. suitable to increase the reflection. Due to this function, the alternating layer system is arranged on the side of the reflective layer system facing the incident light with respect to its reflection layer. Consequently, the alternating layer system is disposed at a back mirror between the substrate and the reflective layer.

In the case of a dielectric reflection-increasing alternating layer system applied directly to the substrate, the abovementioned adhesion-promoting and diffusion-barrier layer is deposited directly on the low-refractive-index layer of the alternating layer system instead of on the substrate, so that it superficially serves as a barrier layer. Independently of this, the alternative pretreatment mentioned above is carried out directly on the substrate by means of plasma treatment.

Different materials can also be used for the dielectric layers of the alternating-layer system, the refractive indices being considered to be high or low-refractive relative to one another. In addition to titanium oxide, for example niobium oxide (Nb ₂ O ₅ ) can also be used as the high-index layer. For example, silica can be replaced by alumina (Al ₂ O ₃ ) or magnesium fluoride (MgF ₂ ).

The invention will be explained in more detail with reference to an embodiment. The accompanying drawing shows in

1 an embodiment of a reflective layer system of a rear mirror produced by the method according to the invention.

The reflective layer system according to 1 comprises a substrate S which faces the light. The incidence of light is symbolized in both figures by three arrows. As substrate S all common materials can be used, eg glass, plastic or metals, also flexible materials.

• 1.) Haftvermittlungs- und Diffusionsbarriereschicht HS, 0.1nm, aus Titanoxid (TiO₂)
• 2.) erste Reflexionsschicht R1, 75nm, aus Silber (Ag);
• 3.) zweite Reflexionsschicht R2, 45nm, aus Kupfer (Cu).

Directly on a polished, washed and dried substrate made of solar glass, which has the lowest possible absorption, ie the highest possible transmission, successive layers are deposited by wet chemical deposition or alternatively by magnetron sputtering without further pretreatment and the following layer thicknesses:

• 1.) Bonding and Diffusion Barrier Layer HS, 0.1nm, Titanium Oxide (TiO ₂ )
• 2.) first reflective layer R1, 75nm, of silver (Ag);
• 3.) second reflection layer R2, 45nm, made of copper (Cu).

The pretreated surface O of the substrate S is produced in this example by the deposition of the adhesion-promoting and diffusion-barrier layer HS. The incidence of light takes place in 1 through the substrate S, so that the first reflection layer R1 faces the incident light, in comparison to the second reflection layer R2. Since, in general terms, the characterization of a reflection layer system for rear or front mirror is always defined by the incidence of light on the substrate S, the order of the deposition is likewise based on the substrate S, depending on the predefined functionality as a rear or front mirror.

After the deposition of the two reflection layers R1, R2, the coated substrate is stored more than 24 hours before deposition of the paint system.

Alternatively, both reflection layers are annealed by a non-inventive heat treatment at temperatures in the range between 60 ° C and 150 ° C, preferably between 80 ° C and 100 ° C.

The layer system according to 1 was on the side facing away from the light incident with a paint system, which in the exemplary embodiment has three paint layers L1, L2, L3, coated outside the vacuum system and then dried. Alternatively, other paint systems are possible.

Depending on the glass quality (absorption) and glass thickness can be produced over the conventional coating technology by means of the described inventive method, for example at a solar glass thickness of 4mm, mirror with a TSR of greater than 94.8%, ie at least plus 0.8% to well over 1%.

Claims (8)

Method for producing a reflection layer system of a rear mirror, in which at least one reflection layer and, moreover, a paint system are deposited on a substrate characterized in that a method step for annealing the layer structure and for increasing the total solar reflectivity of said reflection layer (R1, R2) in comparison to the untreated reflection layer (R1, R2) by means of storage of said reflective layer (R1, R2) coated substrate ( S) at room temperature for a period of at least 24 hours prior to deposition of the paint system. Method for producing a reflection layer system according to Claim 1, characterized in that the reflection layer (R1, R2) is deposited from at least two partial layers, a first reflection layer (R1) and a second reflection layer (R2). A method for producing a reflection layer system according to claim 2, characterized in that an annealing step takes place after the deposition of the first part-layer. Method for producing a reflection layer system according to one of the preceding claims, characterized in that the deposition is carried out by a wet-chemical coating method or by PVD method. Method for producing a reflection layer system according to one of the preceding claims, characterized in that the surface of the substrate (S) facing the reflection layer system is pretreated before the deposition of said reflection layer (R1, R2). Process for producing a reflection layer system according to claim 5, characterized in that the pretreatment by means of deposition of a bonding and diffusion barrier layer (HS) takes place with a thickness in the range of less than 5 nm. Process for producing a reflection layer system according to claim 5 or 6, characterized in that the pretreatment by means of plasma treatment takes place under vacuum. Method for producing a reflection layer system according to one of the preceding claims, characterized in that an alternating layer system (WS) is deposited which comprises at least one layer sequence with a high-refractive and a low-refractive dielectric layer and with respect to the reflection layer (R1, R2) on the predefined one Light incident side of the reflective layer system is arranged.

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