Patent Application
20120263885
This application claims priority of German application no. 10 2011 007 500.3 filed on Apr. 15, 2011, and of German application no. 10 2011 080 961.9 filed on Aug. 15, 2011, the entire contents of these applications being hereby incorporated herein.
The invention generally refers to a method for the manufacture of a reflective layer system for back surface mirrors, which is deposited on a substrate, and which comprises at least one reflective layer as well as at least one silicon oxide containing layer. The invention refers in particular to a method for the manufacture of such a layer system for solar applications.
Reflective layer systems have always been used in many different areas of life; however they have become increasingly more important today, e.g. for mirrors that aid the solution concerning energy. While mirrors for common interior use, ‘only’ need to reflect the visible components of the light spectrum, they must reflect the entire spectrum of sun light for new solar applications, preferably, wavelengths within a range of approximately 300 to approximately 2500 nm.
A reflective layer system comprises a reflective layer or several reflective layers, which combined contribute to the desired high reflection. For solar applications, this is mostly a combination of a silver and a copper layer, wherein the silver layer faces the incident light and the copper layer acts as a protective layer of the silver, however given the layer thickness of the silver does not contribute to the reflection. Depending on the application, other highly reflective materials may be considered such as aluminium, gold, silver, chrome, platinum or molybdenum.
Mirrors fundamentally can be distinguished between front surface and back surface mirrors, depending on which surface of the substrate in relation to the direction of the incident light produces the main reflection. Back surface mirrors, therefore, are mirrors that have the reflective coating on the back surface of the substrate facing away from the incident light.
The quality of a reflective layer system, in particular regarding solar applications, is determined also by the value of its Total Solar Reflectivity (TSR); that is, its capacity to reflect solar radiation. This value is determined mainly by the reflection capacity of its coating in addition to the loss of absorption through the substrate itself. For achieving a highest possible reflection, silver mainly as a reflective layer and a substrate particularly low in absorption and highly transparent, e.g. so called white glass or solar glass are used. On the back surface, the silver layer is then finished with a copper layer, which at the same time serves as an interface layer for a possible subsequent lacquer coating.
The manufacturing process of such reflective layer systems on back surface mirrors can be described as follows. Subsequent to a suitable prior necessary processing, which can comprise the cutting into a required shape, the grinding of the edges of the substrate, their bending and/or tempering of the flat or already bended substrate and other steps, they are, if necessary, polished and washed again. Still wet, they are then activated through an adhesive-promoting tin chloride solution. Subsequently, the plate moves through coating stations one-by-one, where it is coated with silver in a wet-chemical manner, and directly following, coated with copper.
Directly following, there can be a coating with a lacquer or with different lacquers of a multi-layered lacquer system. Subsequently, the entire coat is then dried at a temperature of 150° C.-200° C. Through manufacturing and drying the lacquer layer, the morphological structure of the reflective layer system is in a way frozen.
Depending on the absorbing characteristics of the substrate and its thickness, mirrors with a TSR of for instance 93%-94% and a solar glass thickness of 4 mm can be manufactured using the described method. This value is below the obtainable values which could be determined through simulation calculations using corresponding tabulated optical data for silver.
For improving and specific adjustment of the optical characteristics, a reflective layer system frequently can comprise one, mostly several reflection-enhancing layers that consist of dielectric, low absorptive material. Thus, the experts knows of double and multi-layered alternating layer systems on glass substrates which comprise at least one series of layers containing a dielectric layer with a high refractive index, which is facing the incident light, and a transparent dielectric layer with a low refractive index. Because of such a function, the alternating layer system is arranged on the incident light facing side of the reflective layer system. Having a high refractive index in regards to solar applications means a material with a refractive index of greater than 2.0, and having a low refractive index means a refractive index of less than 1.8, preferably less than 1.65.
One object of the invention is to present a method for the manufacture of a reflective layer system through which a higher reflection can be achieved in a cost-effective way.
A method is described, which makes use of known and tested wet-chemical methods for the deposition of the reflective layer or the reflective layers that have very good reflective characteristics, and which combines these layers with at least one dielectric, transparent and silicon oxide containing layer. The latter, in particular, is preferred as a component of the reflective layer system because of its chemical and mechanical resistance. Additionally, their optical characteristics can be adjusted very easily via the deposition procedure and/or their proportion of reactive gas if the deposition takes place using PVD, preferably using sputtering, so that the substrate or an already on the substrate deposited stack of layers of several transparent layers is covered through the use of a silicon oxide containing layer for the dielectric, transparent layer by a material, which has the preferred optical, chemical and mechanical characteristics comparable to the characteristics of the glass substrate.
In addition, the use of a silicon oxide containing layer as an under-layer proves itself as advantageous, as it presents a cover layer, which can serve temporarily as a preliminary product that is coated using PVD, and as it enables the transfer out of vacuum for further coating.
Thus, a preliminary product is manufactured, which is very flexibly applicable in regard to further processing, so that the following procedural steps are largely disconnected from prior steps. The covering of the uncoated or coated substrate with a silicon oxide containing layer permits the subsequent wet-chemical deposition, under normal pressure, of differently structured reflective layers or layer systems as it is known from the direct coating of substrates. In particular, it was found that a subsequently wet-chemically deposited silver containing reflective layer has good adhesive characteristics in contrast to silver layers deposited using PVD.
A comparison of TSR values of a reflective layer system manufactured using the method according to the invention to a merely wet-chemically deposited system has shown better results for the layer system, which for its deposition combines PVD and wet-chemical deposition methods. Total Solar Reflectivity values within the range of up to 95% were obtained.
The method according to the invention permits that both the second and first, in a vacuum conducted, portions of the method can largely be varied and optimized separately. The variation refers, in particular, in this case to the amount and order of the individual layers or optional pre-treatments, e.g. the materials used, the addition of adhesive-promoting layers in several necessary or beneficial locations, or a preferred order of the transparent, dielectric layers. The optimization refers, in particular, to the procedural parameters, so that beneficial or pre-defined characteristics can be adjusted. For instance, it is known that the refractive index of the silicon oxide can be manipulated via the oxygen and nitrogen levels or via the regulation of the procedure.
Also, the method according to the invention permits an intermediate storage between both basic stages of the method. Depending on the layers, which are deposited on the silicon oxide containing layer, and thus, depending, in particular, on the reflective layer, various pre-treatments of the preliminary product can take place. This is possible prior to and/or following the outward transfer from the vacuum of the preliminary product. For instance, the known chemical activation of the preliminary product can be used prior to the wet-chemical deposition, e.g. using a tin chloride solution or another suitable solution for a subsequent deposition of the silver. Alternatively, and prior to the outward transfer of the substrate, the silicon oxide containing layer can be coated with an adhesive layer using PVD methods. Only a small thickness of such an adhesive layer within a range of 5 mn is necessary.
In addition, the use of a silicon oxide containing layer has the advantage that this layer can be part of a reflection-enhancing, transparent alternating layer system, which comprises, according to one embodiment of the invention, at least one dielectric layer with a high refractive index facing the incident light and the silicon oxide containing layer as a layer with a low refractive index. Also, other sequences of an alternating layer system with a silicon oxide containing layer as a finishing layer are possible.
In the following, the invention shall be described further using an embodiment. The associated drawing presents an embodiment of a reflective layer system of a back surface mirror.
The reflective layer system according to the FIGURE comprises a substrate S, which faces the incident light. The incident light is represented by three arrows. For the substrate S, all common materials can be used, e.g. glass or plastic, also flexible materials.
Directly on a polished, washed and dried substrate made of solar glass, which has the smallest possible absorption, i.e. highest possible transmission, the following layers are deposited one-by-one through magnetron sputtering without any further pre-treatments in a vacuum and of the following thicknesses:
In the presented embodiments, the deposition of these four layers takes place always through magnetron sputtering.
The adhesive-promoting layers HS are manufactured either from a ceramic target with or without an additional inlet of oxygen in DC or MF mode, or from a metallic target in a fully reactive mode with an inlet of oxygen in MF mode. For a reactive coating from the metallic target in MF mode, the sputtering process is operated in an oxidic mode. As a result, particular intensive plasma combined with small sputtering rates is realized. For the deposition directly on the substrate S, this results in an improved distance of the constantly to the substrate surface bound water and an optimized realization of a sufficiently thin first adhesive-promoting layer HS. Additionally, carbonaceous contaminations, which usually have a negative effect on the adhesiveness, are oxidized to gaseous CO2, which can be evacuated via the vacuum pump. As long as the dielectric, transparent layer with a high refractive index of the alternating layer system WS consistent with this embodiment consists of TiO2 or of comparably well adhering material, the first adhesive-promoting layer HS can be omitted.
The layers of the alternating layer system WS are deposited in one embodiment of the method in a reactive MF mode. As a result, the dielectric layer with a high refractive index is deposited from a metallic target in a fully reactive MF mode with an inlet of oxygen. Alternatively, it can also be deposited from a ceramic target with a small, additional inlet of oxygen in MF mode.
For a reactive coating from a metallic target in MF mode, the sputtering process can be, on the one hand, operated in an oxidic mode. Alternatively, for a reactive coating from a metallic target in MF mode, the sputtering process can be operated within the transitional range between oxidic and metallic range in a so called transition mode in a controlled manner. This range is characteristic of no or low absorption at significantly higher coating rates through a suitable choice of an operating point in comparison to the oxidic mode.
As a result, the dielectric SiO2-layer with a low refractive index is deposited either from the metallic target in fully reactive MF mode with an inlet of oxygen, or in voltage regulated transition mode. The operating point of the sputtering process is here adjusted using the voltage of the process and arranged above the inlet of oxygen. In this way, significantly higher coating rates are obtained at a smaller partial pressure of the oxygen than in the fully reactive oxidic mode.
The alternating layer system is complemented by a further thin adhesive-promoting layer HS, which is deposited from the ceramic target without or with only a small, additional inlet of oxygen in DC or MF mode. The layer created in this way serves as an adhesive-promoting layer between the dielectric SiO2 and the reflective layer R that is to be deposited subsequently. It is not necessary that this layer has a closed surface. It can be understood as a so called seed layer. For this reason, very small layer thicknesses are sufficient in this case. They are usually below 5 nm, preferably smaller than 1 nm.
For the deposition of the reflective layer R, the coated substrate is transferred out of the vacuum subsequently, and using a wet-chemical method, the following layers are deposited one-by-one:
The light incidence takes place in the Figure through the substrate S, so that the metallic, reflective layer R is facing the light incidence in comparison to the metallic, reflective functional layer F.
The layer system according to the Figure is coated on the side facing away from the incident light with a lacquer, which, in the embodiment, has three lacquer layers L1, L2, L3, outside the vacuum system, and subsequently is dried. Alternatively, individual lacquer layers or other lacquer systems are possible too.
In the described embodiment, a pre-treated surface O of the substrate is produced through the deposition of the first adhesive-promoting layer HS. A pre-treatment of the surface of the substrate S that is to be coated can take place alternatively also through a direct current (DC) or medium frequency (MF) glow discharge, which mostly is ignited in a rarefied gas atmosphere, that can contain Ar, O2, N2, CDA (Compressed Dry Air) or any combination of these, at a pressure of 2-5×10−2.
The first adhesive-promoting layer HS or a pre-treatment can also be omitted, so that the silicon oxide containing layer or the alternating layer system as described in the above embodiment are directly deposited on the substrate S. This is possible, for instance if the first layer that is to be deposited on the substrate S of the alternating layer system WS is made of titanium oxide or a comparably well adhering material.
Also, the second adhesive-promoting layer HS is optional as the silicon oxide containing layer SiOS that is to finish the alternating layer system WS has a good mechanical and chemical resistance, and thus, is suitable for the subsequent outward transfer and further treatment of the substrate taking place in atmospheric conditions.
Another embodiment therefore can have the following layer structure of the reflective layer system on a washed glass as the substrate:
Also, the adhesive-promoting layer HS can be omitted in this reflective layer system.
Alternatively, a chemical activation of the surface of the silicon oxide containing layer SiOS through the known method using a solution acting as an adhesive agent, e.g. tin chloride, can take place as it is known from purely wet-chemically executed methods.
As a result for instance, the following layer structure of the reflective layer system on a washed glass as the substrate is obtained:
The ZAO adhesive-promoting layer HS can be omitted also in this reflective layer system, in this case the first adhesive-promoting layer HS.
For each of the above described optional layer systems, further processing is possible using wet-chemical depositing of one or several subsequent reflective layers in atmospheric conditions. The processing can follow directly after the outward transfer from vacuum or after any storage of the coated substrates S.
Also, the materials used for the reflective layer R and the reflective, functional layer F can deviate from the silver or copper as stated in this case. For the reflective layer R, other metals can be used such as aluminium, gold, platinum or an alloy, which contains at least one of the mentioned metals. The mentioned metals all have a comparably high, in particular, solar reflection, if necessary for certain wavelengths such as for gold and platinum, and thus, are suitable for the reflective layer system.
For the metallic, reflective functional layer F, materials such as copper, nickel, chrome, stainless steel, silicon, tin, zinc or an alloy, which contains at least one of the mentioned metals, are considered. Through these materials, the reflective characteristics can be combined with mechanical and/or chemical protection.
For other solar applications, other reflective materials are considered without having a direct influence on prior treatment and coating processes.
Also, for the dielectric layer with a high refractive index of the alternating layer system, various materials can be used, e.g. also niobium oxide (Nb2O5).
For the adhesive-promoting layer HS, other materials can be used as an alternative, e.g. materials made of a group of oxides comprising ZnOx, SiOx, SnOx, TiOx or ZrOx, wherein x≦2.
Depending on the quality of the glass (absorption) and thickness, mirrors can be manufactured with a TSR according to ISO 9050:2003 of up to 95% using the layer system according to the invention, e.g. according to the above embodiments and the method of its manufacture for instance with a solar glass thickness of 4 mm.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2011 007 500.3 | Apr 2011 | DE | national |
| 10 2011 080 961.9 | Aug 2011 | DE | national |
