Device and method for drying a liquid film

Abstract

The invention relates to a device and a method for drying a liquid film (5) on a substrate. The device and the method are characterized by the following features: - A first and a second air knife (1, 2) with slot nozzles, for providing a first and a second laminar gas flow with flow directions each predetermined by the slot nozzles of the air knives (1, 2), and wherein the slot nozzles are arranged parallel to one another and the air knives (1, 2) are pivotably arranged in the device while maintaining the parallelism of the slot nozzles. - A bracket (7, 7') for the pivoting arrangement of the air knives. - A first and a second translatable side wall (3, 4) for at least partially blocking superficial gas flows when the laminar gas flows are provided by the air knives (1, 2), and wherein the side walls (3, 4) enclose the laminar gas flows. - A support for the arrangement and translation of the side walls in the device. - The device is at least partially open in the direction opposite to the flow directions. During operation of the device, the side walls (3, 4) follow the pivoting of the air knives.

Description

[0001] The present invention relates to a device and a method for drying a liquid film. The device and method are used in particular for forming layers (films) after applying liquids to a surface.

[0002] Absorber materials in solar cells, such as perovskites, are often applied using techniques that use solutions of the absorbers and can be described as liquid coatings. The properties of, for example, metal halide perovskite, in a layer formed from a deposited liquid in a solar cell, depend on parameters such as crystallinity, size of the crystallites in the layer, defect structures, thickness of the layer, and homogeneity, which also have a significant influence on the efficiency and lifetime of a solar cell. The review article by H.S. Jung and N.-G. Park (Perovskite Solar Cells: From Materials to Devices, small, Vol. 11, No. 1, 2015, pp. 10-25) discusses some aspects of the quality, processing, and use in solar cells of organometallic halide perovskites of the general formula AMX3, where, for example, A = methylammonium CH3 NH3 or formamidinium H NCHNH 22, M = Pb or Sn, and X = Cl, Br, or I have been discussed in detail. Such multihalide systems are particularly difficult to dry homogeneously after deposition (application) as a liquid. Crystallinity and its homogeneity are of utmost importance for the perovskite when used as an absorber material, since inhomogeneities, especially with regard to crystallinity, impair the functionality of the material as an absorber or, in the worst case, even prevent it altogether.

[0003] The processes that have been used to date for the deposition of absorber materials, and in particular perovskite layers, include slot-die coating, inkjet printing, spray coating, spin coating, blade-knife coating (also called doctor blade process) and other liquid (wet film) coating methods. All processes can be carried out with a quenching step that acts on the deposited liquid using an air knife. The air knife delivers a controlled laminar, i.e. uniform, single-plane gas stream in an elongated jet from a slot nozzle, i.e. a jet with a profile that is broadened perpendicular to the flow direction, primarily in a direction predetermined by the slot nozzle.The slot nozzles in the sense of the invention are also provided by a linear sequence of individual nozzles, which also results in a laminar gas flow that is primarily broadened in a direction perpendicular to the gas flow. Quenching takes place after the deposition of the solution and primarily serves to dry the deposited solution. The gas used in a so-called air knife can be, for example, nitrogen, argon, air, dry air and many others. The quenching step with the air knife promotes the evaporation of the solvent in the solution and thus the drying, which in turn drives the crystallization of the material in the deposit. The latter influences the formation of the crystallites in the resulting layer and thus has a direct influence on the quality of the final layers, such as perovskite layers. The article by Y. Yue et al.(A Review on Gas-Quenching Technique for Efficient Perovskite Solar Cells, solar rrl, Vol. 5, 2021, 2100386 - 2-15) provides an overview of various applications of air knives in different methods of perovskite layer deposition. Air knives are usually tools designed to blow away impurities or unwanted liquids on a surface, i.e., to clean them. Within the meaning of the invention and the described use in the production of, for example, perovskite layers, the air knife is used to improve drying. An air knife within the meaning of the invention is any device that provides a laminar gas flow. To obtain an air knife suitable for the purpose of drying, the parameters, in particular the air flow, must be checked and adapted for specific applications if necessary.An air knife for the intended application within the meaning of the invention is preferably equipped with a slotted nozzle, preferably also with so-called "shims" (for shaping the flow), which provide a laminar gas flow. An arrangement of several small nozzles in a row is possible, but not advisable due to the resulting inhomogeneities. The slotted nozzle determines the flow direction of a laminar gas flow, which is to be generated by the air knife. The air knife is also equipped with a supply for compressed gas. The pressurization should be designed such that sufficient compressed gas is supplied to form an air flow that supports the drying of a liquid but is always below a pressure that would be sufficient to blow the liquid away. The ranges depend heavily on the nature of the liquid (see also below) and are therefore experimental in nature, i.e.In case of doubt, they should be determined experimentally. The gas used in the air knife can be supplied at ambient temperature, cooled, or heated. The air knife is usually also equipped to allow different angles of incidence of the air stream on a surface, as well as different distances from the surface. To achieve different positions on a surface, the air knife and the surface are moved against each other, regardless of the relationship to each other. A special application of the air knife is in the so-called roll-to-roll coating process. Air in the sense of "air knives" does not always include only air, but also in particular pure nitrogen and other inert gases, as well as any type of gas mixture, including mixtures with chemical vapors, such as an antisolvent.The term "air" in the context of air knives is historically determined and is used here to refer to the correct systems. However, since air knives within the meaning of the invention usually do not use air, but rather other gases, particularly inert ones such as nitrogen, the term "gas knife" is to be understood as a full synonym and completely interchangeable with "air knife" in the context of this application.

[0004] To improve the homogeneity of the drying process, L.L. Gao et al. (Large-area high-efficiency perovskite solar cells based on perovskite films dried by the multi-flow air knife method in air, Journal of Materials Chemistry A, Vol. 5, 2017, 1548–1557) propose an arrangement of several air knives arranged one behind the other in the coating direction, which is referred to as the "multi-flow air knife" method. This is not to be confused with multi-slit air knives, in which the slots are stacked in the direction of the air flow and thus laminated in this direction, as disclosed, for example, by J.R. McDermid et al. (Modeling and Measurement of Novel Air-Knife Designs, 106th Galvanizers Association Meeting October 5–8, 2014, Jackson, MS, USA).

[0005] In the publication by Y. Zhang et al. (Influence of Air-Knife Wiping on Coating Thickness in Hot-Dip Galvanizing, Journal of Iron and Steel Research, International Vol. 19(6), 2012, 70-78), several parameters of an air knife equipped with a slot nozzle, which is used in this case in hot-dip galvanizing, are discussed and presented. According to this paper, a laminar gas flow, as provided by the slot nozzle, is deflected laterally (i.e., redirected to the side) upon impacting a surface to be dried and subsequently flows predominantly parallel to the surface in a flow that can also be described as laminar, and thus has an effective area that is many times larger than that given by the area of ​​the slot nozzle of the air knife itself.The displacement of the surface to be dried relative to the laminar gas flow provided by the air knife additionally has an asymmetric effect on the deflection of the laminar gas flow upon impact with the surface, in that the deflection is more pronounced in the direction of the (at least possibly relative) displacement of the surface.

[0006] Another issue in the production of solar cells, for example, is the scalability of the processes to industrial mass production. All steps optimized at the laboratory scale should also be applicable to the production of large solar cells and solar cell modules in an automated production facility.

[0007] The terms "liquid film", "layer to be dried" and "deposited / applied liquid" refer hereinafter to a material containing water (other than water of crystallization) or another liquid, such as a solvent. The term encompasses ranges of the ratio of liquid to dissolved or suspended material, encompassing liquids in the usual sense up to a suspension or slurry with a liquid volume that is small to very small compared to the volume of the solid / crystallite. The process of removing the solvent / liquid, i.e., drying, determines, among other parameters, the formation of the crystal structure. The solvents / liquids can be of very different types and depend on the materials to be formed.For the formation of metal halide perovskites, dimethylformamide (DMF), N-methyl-2-pyrrolidone (NMP), dimethyl sulfoxide (DMSO), 2-methoxyethanol (2-ME) or isopropanol (IPA) can be used as solvents.

[0008] Due to the established importance of drying conditions for the crystallization process during the deposition of solutions or other liquids to form layers, the object of the present invention is to provide an apparatus and a method that enable better control over the drying process of a liquid film on a surface, which at the same time reduces costs.

[0009] The object of the invention is solved by the subject matter of claims 1 and 5. Advantageous embodiments are the subject matter of the subclaims.

[0010] The object of the invention is achieved by a device and a method in which a gas flow that flows predominantly parallel to a surface of a liquid film to be dried is at least partially blocked. The predominantly parallel gas flow to a surface of a liquid film to be dried results from a laminar gas flow impinging on the surface of a liquid film to be dried, in which the gas flow is deflected laterally. In this context, predominantly parallel to a surface means that the flow in question flows largely (approx. > 70%) parallel to the surface and, to a lesser extent, also has components in the flow direction that are not oriented parallel to the surface. In other words, a representation of such a gas flow in streamlines, as used in fluid mechanics, is characterized by streamlines parallel to the surface.This gas flow, which is predominantly parallel to the surface of the liquid film to be dried, is referred to below as the superficial gas flow. The blocking of this superficial gas flow occurs in such a way that it is at least reduced or, ideally, even eliminated, and the blocked part of this superficial gas flow is at least partially deflected away from the surface to be dried, i.e., is directed back (with respect to a flow direction of the laminar gas flows). In this way, the drying front is controlled, which, as shown by the invention, is of great importance for the drying process and the quality of the layers to be formed, in particular with regard to the homogeneity of the density and smoothness of the surface of the dried layer. The inventive type of drying control also means that the drying process, although arranged spatially and temporally close to the process step of liquid coating, can take place.carried out, is limited in its influence on the liquid coating, in which the spatial range of action of a laminar gas flow from an air knife is limited or weakened.

[0011] A first aspect of the invention relates to a device comprising at least the components described below.

[0012] The device according to the invention comprises at least a first and a second air knife. The air knives serve in the device to provide a first laminar gas flow from the first air knife and a second laminar gas flow from the second air knife. The laminar gas flows each have a flow direction predetermined by the air knives. Following the purpose of drying a liquid film, the two flow directions are aligned such that, in each case with respect to the individual laminar gas flow, a single surface can be flowed over simultaneously. The latter, in other words, means that the two flow directions are not opposite to each other and do not intersect before impinging on the surface to be dried when the device is in operation.The two laminar gas flows provided by the two air knives are further characterized by the fact that they are each widened in directions perpendicular to the flow directions predetermined by the slot nozzles of the air knives. The slot nozzles of the air knives are also arranged parallel to each other. According to the invention, the air knives are pivotably mounted in the device, with the pivoting being configured to maintain the parallelism of the slot nozzles.

[0013] For the pivotable arrangement of the air knives, the device comprises at least one mount. In a preferred embodiment, the mount is provided by a circular guide rail on which the air knives are arranged such that the provided laminar gas flows are aligned tangentially to the circular guide rail, and the air knives can be moved within the guide rail while maintaining their tangential alignment. The pivotable arrangement is also achieved, for example, by arranging the air knives on rotary axes. The pivoting can be driven manually or by an electric motor. The latter can also be advantageously software-controlled.The pivoting occurs under the condition that the two flow directions of the laminar gas streams are not opposite to each other and do not intersect before impinging on the liquid film to be dried, so that the two laminar gas streams, provided by the two air knives, can simultaneously flow onto one and the same (a single) surface. Advantageously, the pivoting of the air knives occurs synchronously with respect to a surface normal of a liquid film to be dried, or the air knives are arranged mirror-symmetrically to each other. If necessary, the device must be aligned accordingly with a surface of a liquid film to be dried, also with respect to the simultaneous impingement of the two provided laminar gas streams.During operation, the device must also be arranged in relation to a liquid film to be dried in such a way that the slot nozzles are aligned parallel to the surface of the liquid film.

[0014] For the pivotable arrangement of the air knives, the device, in one embodiment, comprises at least one mount with guide rails arranged biconcavely relative to one another, each forming a partial circle. The concave guide rails are connected to one another via webs. In particular, two such mounts are provided in a mirror-image arrangement, framing the air knives from two sides. The air knives are guided in the concave guides, with an imaginary center line through the air knives always forming a secant to the full circles of the partial circles.

[0015] The device further comprises at least a first and a second translatable side wall. The side walls are for at least partially blocking superficial gas streams which, during operation of the device, flow predominantly parallel to a surface of the liquid film to be dried, and which occur when the laminar gas streams are provided by the air knives when the laminar gas streams impinge on the surface of the liquid film to be dried and are subsequently deflected laterally. As a result of the blocking, the superficial gas streams are directed back, i.e. away from the surface. The gas streams thus directed back can leave the device on the side facing away from the surface of the liquid film, a side to be referred to as the rear. For this purpose, the device is at least partially open at the rear.Advantageously, the rear of the device is not covered by more than 20% (≤ 20%), i.e. more than 80% (≥ 80%) open. The opening area to be considered is defined by the area delimited by the side walls. In a preferred embodiment, an extraction system is arranged on this rear side of the device, with which the backward-directed gas streams can be extracted, thereby increasing the drying performance. The two side walls are arranged with respect to the laminar gas streams in such a way that they enclose them. The laminar gas streams therefore lie between the side walls. Both side walls also have at least one edge each, which is arranged parallel to the slot nozzles of the air knives and which determines the end of the side walls, which extends into the surface gas streams for the purpose of at least partially blocking them.The side walls are also advantageously arranged so that they enclose an angle in their imaginary extension. During operation, the device is to be aligned so that a virtual vertex of this angle lies below or on the surface of the liquid film to be dried. The two side walls are also advantageously of the same size and arranged mirror-symmetrically to each other.

[0016] Also included is a holder for the arrangement and translation of the side walls in the device. The holder is advantageously provided as a guide rail, which enables translation of the side walls perpendicular to both edges of the two side walls and thus also perpendicular to the slot nozzles. Such translation also means translation of the side walls during operation of the device parallel to the surface of the liquid film to be dried. By providing the translation in the manner described (parallel to a surface), the side walls can be guided according to a pivoting of the air knives with respect to a concomitant translation of the impact of the laminar gas streams on a surface of a liquid film to be dried. The side walls can be translated in such a way that they do not protrude into the laminar gas streams themselves, but always exclusively into the superficial gas streams.The advantage of blocking the surface gas streams is thus also maintained when the air knives are pivoted. By pivoting the air knives, an expanded area on the surface of the liquid film to be dried can be reached, which then dries under improved conditions, i.e. more homogeneously and more completely in relation to the area. In one embodiment, the side walls are arranged rotatably on the guide rail. The translation and rotation can be carried out manually or by an electric motor, in particular software-controlled. In another embodiment, the translation of the side walls is directly coupled to the pivoting of the air knives by connecting each side wall to an opposite air knife, e.g. via a connecting rod.The alignment of the side walls to the laminar gas flows is to be optimized in such a way that a virtual vertex of an angle formed by an imaginary extension of the side walls and an imaginary extension of the laminar gas flows lies on or below the surface of the liquid film to be dried, so that the surface gas flows flow over a distance as short as possible from their point of origin, the impact of the laminar gas flow, to their blockage, in a manner to be optimized.

[0017] In one embodiment, the support for the arrangement and translation of the side walls is provided by guide rails that engage the two edges of the side walls that protrude into the surface flows to block them, thus making the side walls translatable over the surface.

[0018] The side walls are advantageously rectangular and are advantageously made as a plate or sheet.

[0019] All translations and rotations are motorizable.

[0020] All supports and guides are suspended or attached to a reference system outside the fixture itself. For this purpose, a frame or stand, for example, must be provided.

[0021] The individual embodiments of the device, as described, can be combined with each other in their individual aspects.

[0022] The method according to the invention for drying a liquid film is given by the method steps described below.

[0023] Providing a first and a second pivotable laminar gas stream. The laminar gas streams are arranged as mirror images of a plane between them and are directed toward the liquid film to be dried.

[0024] Furthermore, a first and a second translatable sidewall are provided. The sidewalls each have at least one edge, each oriented parallel to the laminar gas streams and defining the respective end of the sidewalls that protrudes into the surface gas streams to block them. The sidewalls are further arranged to enclose the laminar gas streams.

[0025] The arrangement of the translatable side walls in relation to the pivotable laminar gas flows is such that the translatable side walls at least partially block superficial gas flows on the surface of the liquid film to be dried.

[0026] During the process, the translatable side walls continue to track changes in the position of the laminar gas streams on the surface of the liquid film to be dried during their pivoting. This is done in such a way that the blocking of surface gas streams is maintained.

[0027] All features of the method according to the invention and of the device for drying a liquid film as described above can be combined with one another and can be used in addition to the characterization in the respective other aspect (method and device) of the invention.

[0028] The main advantage of the invention is that a film (layer) dried using a device according to the invention for drying a liquid film or using a method using the blocking mechanism according to the invention is less rough than a layer dried without the inventive idea, i.e. it is more homogeneous, which is highly desirable in many applications and in particular in the production of solar cells, for example perovskite solar cells. The inventive arrangement and guidance of the air knives also enables drying right up to the edges of a coated substrate. Furthermore, the inventive solution promotes homogeneous crystallization of layers of crystallites such as perovskite layers, which can thus be applied by liquid coating with reduced defects.This is achieved by better controlling the area of ​​action of a laminar gas flow on the surface of a liquid film to be dried, by at least partially blocking the laterally deflected gas flows that arise upon impact. Furthermore, the advantage of the invention is simple and cost-effective to achieve. Examples

[0029] The invention is explained in more detail using two examples and four figures.

[0030] The figures show: Fig. 1: Schematic view of the blocking mechanism according to the invention. Fig. 2: Schematic side view of a first example of a device according to the invention in three different operating states (a), b), c)). Fig. 3: Schematic side view of a second example of a device according to the invention. Fig. 4: Schematic plan view of the second example of a device according to the invention. Fig. 5: Schematic side view of a third example of a device according to the invention.

[0031] In Fig. 1 shows a schematic view of the blocking mechanism according to the invention. The illustration is highly simplified and only shows very selective gas flows for illustrative purposes. During operation of the device according to the invention or during implementation of the method according to the invention, a first laminar gas flow “1a” from a first air knife 1 (see subsequent figures) is directed onto the surface of a liquid film 5 to be dried. By striking the surface 5a, parts of the laminar gas flow “1a” are deflected laterally and subsequently flow superficially, which is represented by the dotted arrows (·····>). The liquid film 5 to be dried is applied to a substrate 6. The superficial gas flow, which results from the lateral deflection of the first laminar gas flow “1a”, is blocked in the flow direction by a first side wall 3.An edge 3a defines one end of the first side wall, which protrudes into the surface gas flow to block it. The surface gas flow blocked by the side wall 3 is subsequently directed away from the surface of the liquid film 5a to be dried and flows toward the rear of the device. Because the device is predominantly open in this rear direction, the rear side (see above), the gas flow flowing toward the rear can leave the device or even be sucked away from the rear. The gas flow deflected toward the rear by the blocking according to the invention is represented by dashed arrows (--->).

[0032] In the Fig. Figure 2 shows a first embodiment of a device for drying a liquid film, schematically illustrated in a side view for three different operating states. The three operating states are achieved by different pivoting of the first and second air knives 1, 2. The laminar gas flows from the air knives 1, 2 are represented by arrows that depict the flow direction of the laminar gas flows. The directions of the laminar gas flows run perpendicular to the plane of the drawing, where they are primarily broadened, which is predetermined by the slot nozzles of the air knives 1, 2. The slot nozzles are arranged parallel to one another. A first translatable side wall 3 and a second translatable side wall 4, each with edges (3a, not separately designated with reference numerals here for reasons of clarity), enclose the laminar gas flows. The edges of the side walls 3, 4 are oriented parallel to the laminar gas flows.The side walls 3, 4 are fastened to a guide rail (not shown) for translation and arrangement in the device, which guide rail allows translation of the side walls 3, 4 parallel to the surface of the liquid film 5 to be dried. In the exemplary embodiment, the side walls 3, 4 are arranged such that their imaginary extensions (dash-dotted lines in a)) enclose an angle whose apex SP, in the exemplary embodiment, lies below the surface of the liquid film 5 to be dried. The air knives 1, 2 can be pivoted while maintaining their parallelism. The different pivoting in the . Fig. 1 a), b) and c) results in a different location of impact of the laminar gas streams on the surface of the liquid film 5 to be dried. The side walls 3, 4 are or can be adjusted to this change in location by translation, so that the blocking mechanism is maintained in the different operating states of the device. The effect of the blocking mechanism to direct gas streams backwards is shown in the figure by dashed arrows (--->). A rectangular liquid film of corresponding size can be dried homogeneously, right up to the edge regions, using the device or method according to the invention, which is a further advantage of the invention. The liquid film 5 to be dried is applied to a substrate 6. The liquid film can, for example,The coating can be applied by slot-die coating, inkjet printing, spray coating, spin coating, blade knife coating (also called doctor blade coating or doctor blade process), or other liquid (wet film) coating methods. Drying with the device or method according to the invention advantageously takes place shortly after the application of the liquid film. Fig. 1 a) also shows an extraction system A which is arranged at the rear of the device and with which gas streams directed backwards can be extracted, thereby increasing the drying performance of the device.

[0033] In the Fig. Figure 3 shows a second embodiment of a device for drying a liquid film, schematically shown in a side view for two different operating states (a), b). The two operating states a) and b) are achieved by different pivoting of the first and second air knives 1, 2. Compared to the first embodiment, as shown in Fig. 1, the second embodiment differs in that the air knives 1, 2 are arranged for pivoting on two circular 7, 7' guide rails. Fig. 2 only one of the guide rails 7 can be seen through the projection, since the second 7' is covered by the first.

[0034] In the Fig. Figure 4 shows a schematic plan view of the second embodiment of a device for drying a liquid film. The two circular guide rails 7, 7', on which the air knives 1, 2 are arranged, are distinguishable in the plan view. In the plan view in the Fig. 4 also shows the side walls 3, 4. The top view clearly shows that the device is open at the rear, relative to the flow direction of the air knives 1, 2, so that gas flows directed backward (away from the surface of the liquid film 5 to be dried) can also exit the device at the rear or can even be sucked off at the rear, which leads to improved drying performance.

[0035] In the Fig. Figure 5 shows a third embodiment of a device for drying a liquid film, schematically shown in a side view for four different operating states, represented by different dash-dotted lines. The air knives 1, 2 are only indicated by rectangles. The reference numerals are arranged near the associated features / components, and for reasons of clarity, no connecting lines are shown. In this example, the air knives 1, 2 are arranged for pivoting on two concave guide rails, which are connected via webs and thereby provide the holder 7. Fig.5 only one of the holders 7 can be seen through the projection, since the second holder 7' is hidden by the first in this view. The side walls 3, 4 are guided on guide rails 8, 9 as holders for translation, which are arranged laterally to the substrate 6, on which the liquid film 5 to be dried is applied. The translation of the side walls 3, 4 is to be synchronized with the rotation or pivoting of the air knives, which can be easily adjusted mathematically, in particular with computer-aided control. This example of one of the embodiments of the invention has all the advantages of the other examples and embodiments and is in particular also at least partially open at the rear.

Claims

[1] Device for drying a liquid film (5), comprising at least: - a first and a second air knife (1, 2) with slot nozzles, for providing a first and a second laminar gas flow with flow directions predetermined by the slot nozzles of the air knives (1, 2), and wherein the slot nozzles are arranged parallel to one another and the air knives (1, 2) are arranged pivotably in the device while maintaining the parallelism of the slot nozzles, - a holder (7, 7') for the pivotable arrangement of the air knives, - a first and a second translatable side wall (3, 4) for at least partially blocking superficial gas flows when the laminar gas flows are provided by the air knives (1, 2), and wherein the side walls (3, 4) enclose the laminar gas flows, - a support for the arrangement and translation of the side walls in the device, - and wherein the device is at least partially open in the direction opposite to the flow directions. [2] Device for drying a liquid film (5) according to claim 1, characterized by that the side walls (3, 4) are rotatably arranged in the holder for the arrangement and translation of the side walls (3, 4) in the device. [3] Device for drying a liquid film (5) according to claim 1 or 2, characterized by that the holder (7, 7') for the pivotable arrangement of the air knives (1, 2) comprises a circular guide rail. [4] Device for drying a liquid film (5) according to claim 1 or 2, characterized by that the holder (7, 7') for the pivotable arrangement of the air knives (1, 2) comprises two concave guide rails. [5] Device for drying a liquid film according to one of claims 1 to 3, characterized by that the device comprises a suction device (A). [6] Method for drying a liquid film (5), comprising at least the following steps: a. Providing a first and a second pivotable laminar gas stream, wherein the extension directions of the laminar gas streams are arranged parallel to each other and the laminar gas streams are directed towards the liquid film (5) to be dried, b. Providing a first and a second translatable side wall 3, 4 each having an edge, wherein the edges are arranged parallel to the extension directions of the laminar gas flows and wherein the side walls 3, 4 enclose the laminar gas flows, c. Arrangement of the translatable side walls 3, 4 to the pivotable laminar gas flows, so that the translatable side walls block superficial gas flows, d. Swiveling of the laminar gas flows and e. Tracking of the translatable side walls 3, 4 with respect to a change in the position of the laminar gas flows during their pivoting.

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