Quality assurance in the production of stacks from solid oxide cells

Abstract

The invention relates to a solution for quality assurance in the production of stacks (1) made of solid oxide cells SOxC (21, 22, ... 2n), which are joined together in a specially designed oven (7) in a joining process under the influence of temperature and force using a solder inserted between them. Each stack (1) is monitored for undesirable deformations occurring in its geometry throughout the entire joining process by measuring the impedance at multiple measuring points (51, 52, ... 5n) distributed in pairs and spaced apart on the outer surfaces of the stack (1) using multichannel impedance spectroscopy (EIS) in a continuously repeating measurement cycle during the joining process. The impedance is measured at each of the measuring points (51, 52, ... 5n) distributed on the outer surfaces of the stack (1) using multichannel impedance spectroscopy (EIS). The values ​​at the measuring points (51, 52, ...5n) in the individual measurement cycles, geometric quantities describing the cell geometry of the SOxC (21, 22, ... 2n) provided with measuring points (51, 52, ... 5n) are calculated by means of a program application processed by a control and processing unit SVE of the furnace (7), which are used to control the joining process and / or to derive quality statements about the generated stack (1).

Description

The invention relates to a solution for securing quality in the production of stacks of solid oxide cells SOxC. More specifically, it relates to stacks of high temperature SOxC. Stacks of this type are produced in a special joining process under the action of a temperature and force on a plurality of solid oxide cells stacked one above the other using a solder introduced between the cells, which is melted during the joining process, typically a glass solder. With the aid of the invention, the effects of the force effect occurring during the joining process and associated deformation phenomena occurring at the cells can be taken into account with regard to the service life of the cells or of the stack joined from them. The invention relates to a method and a system suitably designed for carrying out this method.As already stated, in connection with the joining process that is carried out during the production of stacks from high-temperature SOxC, a force effect on the cells forming the stack is indispensable. The force effect required for the joining process takes place with the aid of at least one push rod, by means of which the force is applied to a central region of the uppermost or / and lowermost cell of the stack. Studies have shown that, by applying such a force, not only is a secure connection of the cells achieved by means of the introduced solder, but that this can also lead to deformation of the cells in an undesired manner. Deformations of this type which occur at the cells of the stack and thus also with respect to the stack geometry can in turn lead to degradation phenomena, that is to say to premature aging of the cells, during the later use of the stacks produced. This has a final adverse effect on the service life of the stacks. In the worst case, such undesired deformations of the cell and stack geometry can also lead to such a reduction in quality that a stack does not function at all or, on account of poor performance parameters, can be classified as scrap and thus cannot be used.It is obvious that due to the indispensibility of a force effect during the joining of the stacks, deformations at the cells and at the stack cannot be completely avoided. However, the tendency of the manufacturers to keep such deformations as low as possible, but at least to take them into account accordingly with regard to quality management.It is therefore an object of the invention to provide a solution for quality assurance in the production of stacks of solid oxide cells produced in a joining process under the action of temperature and force, with the aid of which stacks the quality requirements of consumers of stacks produced in this way can be taken into account in a suitable manner despite deformations at the cells and the stack geometry which may occur during the joining process. To this end, a method and a system suitable for carrying out this method are to be specified.The object is achieved by a method having the features of claim 1. A system which achieves the object and is suitable for carrying out the method is characterized by the first subject matter.According to the method proposed for achieving the object, stacks of solid oxide cells SOxC are also produced, as already known from the prior art, in a furnace designed for this purpose in a joining process under the action of a temperature and force with the aid of a solder introduced between the cells, preferably a glass solder. The force is exerted by means of at least one pressure on a central or central region of the push rod exerting the uppermost and / or lowermost SOxC of the stack. The SOx A stack are typically cuboidal, having a relatively low height extending in the stacking direction of the stack and a cross-sectional area which is square or rectangular with respect to the stacking direction and has edge lengths (circumferential lengths) which are many times greater than the height.According to the method, the stack is continuously monitored during the entire joining process with regard to undesired deformations occurring in its geometry. This is done by measuring the impedance at a plurality of measurement points distributed on the stack outer sides using a multichannel impedance spectroscopy EIS in a measurement cycle which continuously repeats during the joining process. In this case, the aforementioned measurement points are distributed on the outer sides of the log in such a way that they are arranged on a plurality of SOx The stack and in this case, with respect to a SOxC provided with them, in each case in pairs and spaced apart from one another.In this respect, an even number of measurement points is arranged on an SOxC of the stack to be provided with measurement points, wherein the measurement points of a measurement point pair are preferably (but not necessarily) formed on mutually opposite outer sides (on sides of the respective cell which are situated opposite one another on the circumference of the stack). In particular in the case of elongate cells, however, a pair of measurement points can optionally also be arranged at the ends (each measurement point at one end) of an outer longitudinal side-and therefore spaced apart from one another. The continuously repeated impedance measurement captures a time profile of the impedance that inevitably changes during the joining process for each of the measurement points of the stack, starting from a starting value measured for the impedance in the first measurement process.The impedances measured at the measurement points in the individual measurement cycles are respectively processed further by programming, in accordance with a physical-mathematical model, by a control and processing device of the furnace (hereinafter speaking, simply SVE). During the program-technical processing of the measured values, geometric variables are calculated by the SVE, which describe the cell geometry of the SOxC of the stack provided with a measurement point and thus ultimately also the stack geometry overall. The calculated geometric variables are each used directly to control the joining process for minimizing undesired deformations of the cells and thus of the stack, and / or to derive quality statements regarding a stack generated in each case based on their profile during the joining process.The proposed method assumes that, as has been shown in experiments, relationships exist between the changes in their geometry which inevitably occur at SOxC (cells of the stack) during the joining process and the likewise changing impedance of the cells forming the stack, which relationships can be described by a physical-mathematical model. In the most favorable case, corresponding changes in the impedance with respect to the individual measurement points fixed to the cells of the stack should show approximately the same time profile, which is essentially caused by shrinkage processes.If this is not the case, this can be attributed to changes in geometric variables describing the SOxC, which, viewed in relation to one another, make it possible to identify undesired deformations of the cells provided with measurement points (SOxC of the stack) and thus also the stack geometry overall. These are not geometric changes that typically occur in any case during the joining process, such as, for example, shrinkages, but rather geometric changes that cause the stack to get out of shape to some extent, i.e., for example, an inclination of the stack in the stacking direction develops. The findings or calculated values obtained in this respect over the course of the changing geometry of the cells and of the stack can be used in different ways, depending on the implementation of the method and equipment of the system with the OVE and the furnace serving for the temperature control of the stack during the joining process and having a push rod for applying force to the stack.Corresponding to a lower requirement for the manufacturing device or for the implementation of the method making the system, the geometric variables calculated in each case after a measurement process and describing the cell geometry and thus also the stack geometry, and the time curves resulting for these during the course of the joining process, are used to classify stacks manufactured using the method into different quality classes. In this case, stacks for which very strong uneven deformations were detected during the course of the joining process are then assigned to a less quality class. The reduced quality can have a disadvantageous effect on the life of a stack, for example. In addition, in practical use of such stacks, poorer efficiency must be assumed because of the lower quality of increasing operating costs. If the stacks are therefore sold by specifying a corresponding quality class, only a lower purchase price can be raised for them because of the possibly shorter service life and the typically poorer efficiency. In contrast, stacks which deform very uniformly during the joining process, namely almost exclusively due to the unavoidable shrinkage processes, are assigned to a higher quality class, to which typically a high service life to be expected, a higher efficiency and-in the case of a sale-a higher sales price can be coupled.In connection with this implementation form, or embodiment of the method, it should be pointed out that, for a reliable classification of the stacks, the profile of the geometric variables describing the cell geometry, which profile is to be drawn during the joining process, and not, for example, only their actual state at the end of the joining process, should actually be used. This is based on the fact that possibly also severe temporary deformations of a cell which occur in the course of the joining process are under certain circumstances also compensated again in other phases of the process, but can nevertheless leave damage which impairs the service life of the cell and thus of the stack.According to a preferred embodiment of the method, the findings obtained for the temporal course of the geometric relationships on the stack during the joining process by means of the EIS are used to act directly on the joining process and thereby keep the extent of undesired deformations as low as possible. In this method configuration, it is provided that after each of the measurement processes that are cyclically consecutive over the entire joining process and describe the geometry of the cells provided with the measurement points and thus also of the stack, be included as actual variables in a control function for adjusting the orientation of the push rod. In this case, the angle assumed by the push rod with respect to an imaginary straight line which is guided perpendicularly through the stack in the stacking direction is corrected slightly in accordance with the detected undesired deformations.During the joining process, therefore, each reaction is directly made to detected undesired deformations and these are continuously counteracted by a continuous correction of the angle of the push rod with respect to the perpendicular in the stacking direction of the stack. In this way, it is achieved that stacks produced according to this implementation of the method deform very uniformly and to the greatest extent only in a manner caused by the shrinkages occurring over the entire joining process.Nevertheless, this method design also almost inevitably leads, at least for a short time, to undesirable changes in the cell geometry, which can also have an influence on the service life of a respective cell if they are basically counteracted immediately by correcting the alignment of the push rod. Therefore, according to a likewise possible implementation of the method, both previously explained method configurations can also be implemented cumulatively. This means that, on the one hand, the alignment of the push rod is constantly corrected during the joining process, and on the other hand, the stacks that are produced are classified with respect to their quality while still taking into account undesirable deformations of their cells that occur during the joining process, or the stack geometry, after the completion of the joining process while taking into account such possible deformations.As stated at the beginning, in order to carry out the method, a plurality of measurement points are distributed on the outer surfaces of the stack and its SOxC forming it, and are arranged here in pairs on relevant cells. In order to obtain the most accurate possible statements about stack deformation, the corresponding measurement points should of course be arranged distributed approximately uniformly on the outer surfaces of the stack. As long as this is guaranteed, it is merely a question of the physical-mathematical model stored in the program application processed by the control device, with the aid of the impedances measured in each case at these measurement points and the time curves for the impedance resulting therefrom for these measurement points, to correctly calculate the geometric variables required for an informed statement about the time curve of the stack geometry.The model to be used as the basis for the calculation in this respect can be obtained by suitable experiments. For the practical implementation of the method, it has proven particularly advantageous in this case to arrange at least two measurement points which are respectively at the same distance from one another and are preferably opposite to one another on a plurality of SOxC, that is to say optionally, or preferably, on all SOxC, of the stack, which measurement points are each connected to a channel of an impedance measurement device used for impedance spectroscopy EIS. It has proven particularly expedient in this case to arrange at least four measurement points at each respective regular cuboid SOxC of the stack provided with measurement points, namely such that at least two measurement points are arranged on mutually opposite circumferential sides of the SOxC and at least two further measurement points are arranged on the two other circumferential sides of the respective SOxC which extend in each case orthogonally thereto and likewise mutually opposite circumferential sides. If, in the case of an imaginary coordinate system placed in the stack, the stacking direction of the stack is considered as a Z axis, then in this way, statements are obtained for the individual SOxC provided with measurement points in each case about the deformations occurring on them during the joining process along the X axis and along the Y axis with respect to the Z direction.In connection with the execution of tests for defining the respective physical-mathematical model to be used for the method, it has proven advantageous to limit the spectrum of the frequencies used for the EIS in a suitable manner. It has namely been shown that the correlation between the change in the geometric variables of the stack and the measured impedances is particularly clearly evident in specific frequency ranges of the measurement spectrum of the EIS. In order to achieve a high efficiency, in particular in connection with the regulation directed to a continuous correction of the alignment of the push rod, it is possible in this case to reduce the duration of the individual measurement processes to a certain extent by limiting the frequency range to a specific frequency window and to perform a larger number of measurement processes and calculations on the stack geometry over the entire joining process. The frequency range to be chosen in each case advantageously in this respect depends, inter alia, on the respective type of cells of the stack and on the basis of corresponding tests preceding the implementation of the method and the configuration, that is to say it is possible to define a test preceding the system on the basis of the device.A system that achieves the object and is suitable for carrying out the method, i.e. a system for a joining process carried out under the action of temperature and force for producing a stack of solid oxide cells SOxC, comprises a furnace for tempering the stack, at least one pressure exerted on a central region of the uppermost and / or lowermost SOxC of the push rod located in the furnace during the joining process, and a control and processing device SEV. The system is also equipped with a multichannel impedance measuring device for impedance spectroscopy EIS. During the joining process, a plurality of measurement channels of this impedance measurement device are connected to the stack. The connection of these multiple measurement channels to the stack is such that one of the measurement channels is connected to one of multiple measurement points distributed on the stack outer sides at multiple SOx The stack in each case in pairs and spaced apart from one another.The SEV is configured to operate during the joining processcontrolling the impedance measuring device to measure the impedance at all measuring points arranged on the stack in a continuously repeating measuring cycle during this joining process,calculating from the measured values of the impedance measuring device geometric variables correlating therewith, respectively, which describe the cell geometry of the SOxC provided with measuring points and thus the stack geometry,controlling the joining process with a view to minimizing undesired deformations of SOxC thus of the stack as a whole by means of the calculated geometric variables and / or deriving quality statements regarding the generated stack from the temporal profiles of these geometric variables.According to a first specific embodiment of the system characterized above with regard to its basic structure, its SEV is furthermore designed to assign a quality class in a manufactured stack, depending on the time curves determined for the geometric variables on the basis of its repeated calculation, after the termination of the joining process in an automated process. The assignment of the respective quality class by the SEV is effected on the basis of a schema stored in the SEV. Just as was explained for the method with regard to the relationship between the measured impedances and the geometric variables of an SOxC, the aforementioned scheme for classification is defined on the basis of the results of corresponding experiments. Such attempts can include, for example, artificial aging processes, with the aid of which relationships between deformations that have occurred at the SOxC of the stack and thus at the stack geometry during the joining process and the expected service life of a respective stack determined (in the course of artificial aging brought about on individual stacks for test purposes) can be recognized.According to a particularly preferred embodiment of the system, its SEV is designed alternatively to the last-described embodiment or additionally to reorient the push rod, by actuating an actuator, after each measurement cycle carried out by means of the impedance measurement device according to the principle of impedance spectroscopy EIS, with respect to the angle of its longitudinal axis to a perpendicular in the stacking direction of the stack. The SEV is designed in such a way that it carries out a control function for setting the aforementioned angle, i.e. for aligning the push rod, in which the geometric variables calculated from the measured values of a respective measurement cycle are included by the SEV as actual values.On the basis of drawings, some aspects of the invention will be explained again below in the manner of an exemplary embodiment. The drawings show in detail: FIG. 1 is a schematic illustration of a possible embodiment of the system according to the invention, FIG. 2 : shows an example of possible deformations occurring on a stack during the joining process, in a schematic illustration, FIG. 3 : a plan view of an SOx The stack with measurement points arranged thereon, FIG. 4 : shows an example of impedances measured at different measurement points of a SOxC.FIG. 1 shows a possible embodiment of the system according to the invention, including a stack 1 produced by means of the latter in a joining process, in a roughly schematic illustration. Essential components of the system are a furnace 7 for tempering a stack 1 of SOxC 2 1, 22,... arranged between an upper and a lower end plate during the joining process. 2" and a push rod 3 with an actuator 6 for its actuation and alignment. During the joining process, a pressure is exerted by means of the push rod 3 on a central region of the upper end plate and thus on the uppermost SOxC 2 1 of the stack 1 tempered in the furnace 7. To realize a defined heating regime, the furnace 7 has a plurality of heating elements 10 arranged with the formation of heating zones and cooling tubes 11 arranged around the stack.The system also includes an impedance measuring device operating on the principle of impedance spectroscopy EIS and a control and processing device SEV, which are not shown in detail in the drawing, however. With respect to the impedance measuring device, in FIG. 2 rather only their individual measuring points are 1, 52,... symbolically, each with a measuring point 5 arranged on the stack 1 or on an SOxC. 5 n connected measurement channels 4 1, 42,... 4 n. In a joining process lasting several hours or even several days, the stack 1 is joined by means of the push rod 3 under the action of temperature according to a predetermined temperature sequence scheme and a continuous application of force.During the entire joining process, the stack 1 becomes 1, 22,... with regard to the changing geometry of its SOxC2. 2 n. However, monitoring is not performed by means of optical sensors or the like, but using impedance spectroscopy EIS and the further processing of the measured values obtained by means of these to form geometric ones, the cell geometry of the measurement points 5 1, 52,... 5 n SOxC 2 1, 22,... 2 n. It should be noted in this regard that any undesirable deformations of the stack 1 that occur are typically so slight that they could hardly be detected by means of an optical system. In addition, the arrangement of such sensors or also other sensors, such as strain gauges and the like, on the stack 1 should be associated with a number of difficulties. This is true especially since very high temperatures prevail in the furnace 7 in which the joining process is carried out. Corresponding sensors would therefore also have to be highly temperature-resistant. Finally, it should also be pointed out that processes which occur in the volume of the cells (SOxC2 1, 22,... 2 n) processes, with sensors of the aforementioned type arranged on the stack outer sides, cannot be taken into account in principle.FIG. 2 shows, by way of example, likewise in a highly schematic illustration, possible on a stack 1 and its cells (SOxC2 1, 22,... 2 n) geometric deformations occurring during the joining process. The geometric conditions which are established on the stack 1 at any point in time according to the corresponding example are illustrated in the drawing with the furnace 7 surrounding the stack 1 omitted. As indicated in the drawing, the results on cells (SOxC2 1, 22,... 2 n) of the stack 1 and thus deformations occurring overall on the stack geometry from incorrect positions of the push rod 3, i.e. from deviations of its longitudinal axis 9 at an angle α from the perpendicular 8 of the stacking direction of the stack 3.In the drawing, however, these deviations, or the angle α, are shown greatly exaggerated for clarity. In reality, these deviations are so slight that they are hardly perceptible with optical means, but nevertheless too hardly perceptible deformations of the SOxC 2 1, 22,... that impair the quality and possibly the service life of the stack 1. 2 can carry n of the stack 1. Therefore, the stack 1 is monitored for the occurrence of such undesired deformations during the entire joining process. This is done in that, in a continuously repeating measurement cycle, a plurality of measurement cycles are arranged on the stack 1 and each have a channel 4 1, 42,... 4 2 of a measurement point 5 1, 52,... which is connected according to the principle of EIS. 5 n in each case the impedance is measured and geometric quantities expressing the deformations are calculated from the impedance measured values. In accordance with the deformations determined in this case, the alignment of the push rod 3 is constantly corrected with the aid of a regulating function and by actuating the actuator 6.In the example shown, 2 is 1, 22,... at each SOxC. 2 n on two opposite circumferential sides (outer sides) of a pair of measurement points 5 1, 52,... 5 n each of which has a channel (4 1, 42,... 4 n) of an impedance measuring device which operates according to the principle of EIS and is not shown. The impedance measurement values respectively obtained on these enable statements about the deformations occurring on a respective cell in the X direction. Preferably-not shown in FIG. 2-a measuring point connected to a channel of the impedance measuring device can also be arranged on the visible outer longitudinal side extending in the plane of the drawing and on the opposite (not visible) outer longitudinal side, wherein the deformations occurring at a respective cell in the Y direction can be determined at these measuring points.The latter is indicated in FIG. 3 (not to scale in relation to FIG. 2 ) in a plan view of an SOxC 2 1 of the stack 1 (otherwise, just like the furnace and push rod, not shown), wherein here the measurement points and the (measurement) channels connecting them to the impedance measurement device for detecting deformations in the Y direction are indicated only by a symbol and are not provided with a reference sign.FIG. 4 shows, by way of example, the different impedances Z measured at different measurement points at a specific point in time during the joining process with respect to the X direction. In the diagram shown in FIG. 4, the Z, unlike the X, therefore does not stand for the corresponding direction in space, but rather denotes the impedance.

Claims

Method for quality assurance during the production of stacks (1) from solid oxide cells SOxC 2 1, 22,... 2 n, which are connected to one another to form the stack (1) in a furnace (7) configured for this purpose under the action of a temperature and force with the aid of a solder introduced between them, wherein the action of force is effected by means of at least one push rod (3) which exerts pressure on a central region of the uppermost and / or lowermost SOxC of the stack (1), characterized in that a respective stack (1) is monitored during the entire joining process with regard to undesired deformations which occur in terms of its geometry, in that, in a measurement cycle which continuously repeats during the joining process, the impedance is measured in each case at a plurality of measurement points arranged in pairs and at a distance from one another on the stack outer sides of the stack with the aid of a multichannel impedance spectroscopy EIS, and therefore the impedance is measured in each case for each of the measurement points, starting from a starting value measured at the respective measuring point for the impedance in the first measuring cycle, a time profile of the impedance changing during the joining process is detected and from the impedances measured at the measuring points in the individual measuring cycles the cell geometry of the geometric variables describing SOxC provided with measuring points are calculated by means of a program application processed by a control and processing device SVE of the furnace and that these calculated geometric variables are each used directly for controlling the joining process for minimizing undesired deformations of SOxC and thus of the stack and / or quality statements regarding the generated stack are derived from the time profiles determined for these geometric variables by the repeated calculation during the joining process.Method according to Claim 1, characterized in that at least one pair of measurement points is arranged in each case on all SOxC of the stack, and each of these measurement points is exclusively connected to a channel of an impedance measurement device used for impedance spectroscopy EIS.Method according to Claim 1 or 2, characterized in that at least four measurement points are arranged in each case on a respective cuboid SOxC of the stack provided with measurement points and are connected in each case to a channel of the impedance measurement device, wherein at least one pair of measurement points is arranged on mutually opposite circumferential sides of the SOxC and at least one further pair is arranged on the two other circumferential sides of the respective SOxC which extend in each case orthogonally thereto and likewise mutually opposite.Method according to one of Claims 1 to 3, characterized in that a quality class is assigned to the stack in an automated process in accordance with a schema stored for this purpose in the program application processed by the SEV, said values describing the cell geometry of the SOxC provided with measurement points, said values being calculated in each case during the joining of the stack.Method according to one of Claims 1 to 4, characterized in that the geometric variables calculated in each case after a respective measurement cycle from the measurement values for the impedance are included as actual values in a control function which is executed by the SEV and according to which the orientation of the at least one push rod (3), with respect to the angle between its longitudinal axis and a solder precipitated in the stacking direction of the stack, is controlled by actuating an actuator.System for a joining process carried out under the action of temperature and force for producing a stack (1) from solid oxide cells SOxC (2 1, 22,... 2 n), with a furnace (7), with at least one push rod (3) which exerts pressure on a central region of the uppermost and / or lowermost SOxC of the stack (1) located in the furnace (7) during the joining process, and with a control and processing device SEV, characterized in that the system has a plurality of measurement channels (4 1, 42,... 4 n) to the impedance spectroscopy EIS, wherein several of the measurement channels (4 1, 42,... are provided during the joining process. 4 n) of the impedance measuring device are connected to the stack ( 1), namely in each case one of the measuring channels ( 4 1, 42,... 4 n) each having one of a plurality of SOxC (2 1, 22,... distributed over the stack outer sides. 2 n) of the stack ( 1) are each measurement points ( 5 1, 52,... arranged in pairs and spaced apart from one another. 5 n), and that the SEV is designed to control the impedance measuring device during the joining process in a continuously repeating measuring cycle at all measuring points (5 1, 52,... arranged on the stack (1). 5 n) to measure the impedance, from the measured values of the impedance measuring device, correlating therewith in each case the cell geometry of the measuring points (5 1, 52,... 5 n) of SOxC (2 1, 22,... 2 n) are used to calculate the joining process with a view to minimizing undesired deformations of SOxC(2 1, 22,... 2 n) and thus of the stack ( 1) and / or to derive quality statements regarding the generated stack ( 1) from the time curves of these geometric variables.System according to Claim 6, characterized in that the SEV is designed to assign a quality class to a manufactured stack (1) in an automated process as a function of the time curves determined on the basis of its repeated calculation for the geometric variables, after the end of the joining process, in accordance with a scheme stored for this purpose in the SEV.System according to Claim 6 or 7, characterized in that the SEV is designed to align the at least one push rod (3) by actuating an actuator (6) after each measurement cycle, with respect to the angle (α) of its longitudinal axis (9) to a perpendicular (8) in the stacking direction of the stack, in accordance with a control function in which the geometric variables calculated from the measurement values of the measurement cycle are included by the SEV as actual values.

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