Abstract:
A method and a system for resistance seam welding of a foil and at least one foil support of a fuel cell system. During welding, the thin foil, together with the thicker foil support, is moved relative to the roller electrode while resting on a flat support element. In a suitable welding system, a counter-electrode is designed as a flat support element, such as a welding strip, that is displaceable relative to the roller electrode, the roller electrode being in rolling contact with the foil support, but not with the foil. Depending on whether one foil is to be welded to one or two foil frames, the support element may be designed having a high or a low specific electric resistance.

Description:
Priority is claimed to German Patent Application No. DE 103 06 235.1, filed on Feb. 14, 2003, the entire disclosure of which is incorporated by reference herein. 
     BACKGROUND 
     The present invention relates to a method and a system for resistance seam welding of a foil and at least one foil support of a fuel cell system. 
     Diaphragm modules which contain metal separation diaphragms or foils are used in fuel cells which are operated using hydrogen which is extracted from hydrocarbons, methanol, for example. At low permeation rates metal separation foils function selectively and in a temperature-stable manner. In selective separation of hydrogen via a metal separation foil, the permeation rate depends on the foil material, the pressure, the temperature, and the foil thickness. The temperature range is generally between 250° C. and 450° C. No noteworthy hydrogen separation occurs below 250° C. Intermetallic structural transformations, which drastically deteriorate permeation, occur above 450° C. when palladium-copper alloys are used. The metal separation foils separate a high-pressure area from a low-pressure area in the diaphragm module. The higher the differential pressure between the two areas, the better the permeation rate. The differential pressure is limited by the strength of the foil. The efficiency of the fuel cell system decreases because a good deal of energy must be spent in producing high pressures. The thinner a foil, the higher the permeation rate. As a rule, metal separation foils have a thickness of less than 25 micrometer. The metal separation foils are framed for the use in diaphragm modules. In known systems, the metal separation foils are pressed with graphite seals or are sealingly joined with one another by spattering of blocking layers. The known systems have a carbon monoxide leak rate which is insufficient for commercial applications. If, in addition to hydrogen, carbon monoxide diffuses to the low-pressure side, then the required purity of the anode gas for the fuel cell system is not provided. 
     A hydrogen separator for a fuel cell reformer is described in Unexamined Patent Application DE 100 44 406 A1 in which a palladium foil having a thickness of 3–15 micrometer is applied to a mesh wire-shaped support structure via pressing or via rolling and pressing. The foil adjusts in part to the support structure, thereby increasing the effective surface, and the foil may expand and contract without ripping or forming creases. The wire mesh-shaped support structure has the disadvantage that, due to its waviness, the high-pressure and low-pressure sides of the fuel cell reformer cannot be operated reliably sealed from one another. 
     In principle, welding methods are to be considered for joining thin workpieces. Two workpieces are melted in each welding process, both workpieces being liquefied along a weld seam. Since, as a rule, the melting energy is supplied from one source such as, for example, a gas burner, an electric arc, or a laser light source, sufficient energy must be made available for both workpieces. If a foil and one or two foil supports of a fuel cell system are to be welded together, the problem arises that the joined pieces have different mass or thickness so that the foil would initially melt and run without being joined with the unmolten foil frame. Mechanically pressing the two pieces together is not an option in gas melt methods, electric arc methods, or in the use of laser light since the pre-stress produced must be in the welding area and thus molten, i.e., welded. 
     A thin separator foil is applied to a porous metal body in the hydrogen separator described in JP 08-215 551 A. A peripheral support frame is inserted in the edge area of the metal body to compensate for unevenness. The construction made up of separator foil, support frame, and metal body is joined with a bracket by hermetic welding, thereby creating a separation between a high-pressure area and a low-pressure area. Hermetic welding is carried out using a laser beam or an electron beam, a sealing weld seam being produced on the front side of the construction and on the bracket. The danger of defects exists due to the different thicknesses of the materials used, in particular in the area of the very thin separator foil. The reliability of the seal is affected. Furthermore, seam welding may be considered in joining thin workpieces. These methods are resistance welding methods. The workpieces to be joined are passed between two rollers. While the rollers rotate, they transfer force and current to the workpieces. A continuous linear seam is formed under constant current. The workpieces have electric resistance. Due to the specific resistance of the joining parts, heat is generated by the current flow, so that the necessary melting energy is released. In workpieces having different thicknesses, no melting away of the thinner workpiece occurs, since the workpieces are pressed together by the rollers. The rollers themselves are made of a metal having a high melting point and are actively cooled so that there is no fusion with the workpiece. Very thin workpieces cannot be welded to relatively thick workpieces using conventional seam welding methods, since the danger exists that the thin workpiece is damaged. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to develop a method and a system for resistance seam welding of a foil and at least one foil support of a fuel cell system which reduce the leak rate for carbon monoxide during use in a diaphragm module and improve on the reliability of the fuel cell system. 
     The present invention provides a method for resistance seam welding of a foil and at least one foil support of a fuel cell system. According to the method, the thicker foil support is paired with the foil, and the foil support and the foil are joined gas-tight by resistance heating by using a roller electrode and an electric power supply. During welding, the foil ( 3 ,  8 ) and the foil support ( 2 ,  6 ,  7 ) are moved together relative to the roller electrode ( 20 ) while resting on a flat support element ( 12 ,  13 ). The invention also provides a system for resistance seam welding of a foil and at least one foil support of a fuel cell system, having a roller electrode and a counter-electrode, the foil and the foil support being pressed between them, the roller electrode being movable relative to the foil and the foil support, and having an electric power supply whose terminals are connected to the roller electrode and the counter-electrode. The counter-electrode is designed as a flat support element ( 13 ) which is displaceable relative to the roller electrode ( 20 ), and the roller electrode ( 20 ) is in rolling contact with the foil support ( 2 ,  6 ) 
     According to the present invention, in seam welding of a foil and at least one foil support, foil and foil support are moved together relative to a roller electrode while resting on a flat support element. Prior to welding, the foil is mechanically fixed on the support element using one or two foil carriers or foil frames. A positioner having a welding strip is preferably provided as the support element. The foil does not come in contact with the roller electrode during welding. The roller electrode rolls on the relatively thick foil frame located on top, the foil remaining flat on the positioner or fixed between the two foil frames. This makes it impossible for the pressing and the rolling movement of the roller electrode to damage the thin foil. The pressing forces are absorbed by the foil frame which is in contact with the roller electrode. During welding, the support element or the positioner may be moved relative to the stationary axis of the roller electrode, or the roller electrode is moved along the stationary support element. If multiple parallel linear seams are to be produced, a corresponding number of roller electrodes arranged in parallel may be provided and simultaneously supplied with current. In order to avoid creases and buckles in the foil, the welded workpieces may be subjected to a heat treatment. For increasing the permeation rate, annealing in vacuum or under an inert gas or under pure hydrogen may follow the welding process. Foil supports and foils welded in this way allow for the lightweight design of hydrogen separation modules. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is explained in greater detail in the following on the basis of exemplary embodiments and with reference to the drawings, in which: 
         FIG. 1  shows a schematic representation of a welding group composed of one foil frame and one foil in two views; 
         FIG. 2  shows a schematic representation of a welding group composed of two foil frames and one foil; 
         FIG. 3  shows a schematic representation of a welding device for manufacturing the welding group according to  FIG. 1 ; 
         FIG. 4  shows an orthogonal view of the welding device according to  FIG. 3 ; and 
         FIG. 5  shows a schematic representation of a welding device for manufacturing the welding group according to  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
     The schematic representation of a welding group  1  in  FIG. 1  shows, in top view and sectional view, a foil frame or foil support  2  and a foil  3  which are welded together for use in a fuel cell system. Weld seam  4  of width b runs peripherally and concentrically to foil frame  2  which has the same outer dimensions as foil  3 . Foil frame  2  has a thickness d 1 =200 μm and is made of a ferritic material such as austenitic steel or nickel. Foil  3  has the thickness d 2 =10 μm. Thickness d 2  of foil  3  may vary depending on the application. Further practical thicknesses are 18 μm or 25 μm. Foil  3  is suited for hydrogen separation in a reformer module of the fuel cell system and is made, for example, of a palladium- and copper-containing alloy. Other alloys are also possible. Weld seam  4  connects foil  3  and foil frame  2  so tight together that there is a leak rate of smaller than 10 −7  mbar*liter/second or that less than 40 ppm of carbon monoxide is present in the anode gas flow. 
       FIG. 2  shows a welding group  5  including two foil frames  6 ,  7  of the same thickness and the same size and a foil  8  positioned between them. The comment made in connection with  FIG. 1  applies with regard to material, dimensions, and leak tightness. Welding group  5  has a peripheral weld seam  9  including two melting areas  10 ,  11  situated between foil  8  and foil frames  6 ,  7 , respectively. 
       FIGS. 3 and 4  are schematic representations of a welding device for manufacturing the welding group according to  FIG. 1 . Foil frame  2  situated on top and foil  3  situated beneath it are flush on top of each other, mechanically pre-fixed, and positioned on a positioner  12 . A welding strip  13  is embedded, slightly protruding, in positioner  12  on the side facing foil  3 . Guide strips  14  of a longitudinal guide are situated at the bottom of positioner  12 . In addition, the longitudinal guide includes a stationary guide bar  15  and roll bodies  16 . Positioner  12  is displaceable in forward feed direction  17  using the longitudinal guide. A working cylinder  18  which is connected to a pneumatic control unit  19  is provided to displace positioner  12 . Any positioning mechanism may be used instead of working cylinder  18 . One side of foil frame  2  and welding strip  13  are positioned parallel to the forward feed direction  17 . A roller electrode  20  which is rotatably held in a stationary bearing  21  contacts foil frame  2  at the center of the leg of foil frame  2 . Using force F, roller electrode  20  presses welding group  1  against welding strip  13  in positioner  12 . Roller electrode  20  is connected to a contact piece  22  which is connected to a terminal of a power supply  24  by a line  23 . Line  25  connects the other terminal of power supply  24  to a contact piece  26  which feeds the welding current to welding strip  13 . Contact pieces  22 ,  26  are suited for carrying high welding currents. 
     Similarly to  FIG. 4 ,  FIG. 5  shows the design of the welding system for manufacturing the welding group according to  FIG. 2 . In contrast to  FIG. 4 , two foil frames  6 ,  7  and a foil  8  positioned between them are held mechanically pre-fixed on positioner  12 . Roller electrode  20  is in contact with foil frame  6  which is positioned on top, while foil frame  7  which is positioned on the bottom is in contact with welding strip  13 . All other elements shown in  FIG. 5  have the functions described in  FIGS. 3 and 4 . 
     The manufacture of welding groups  1 ,  5  using the systems according to  FIGS. 3 through 5  is described in the following: 
     Using working cylinder  18 , positioner  12  is moved in the forward feed direction  17  at uniform speed for permanent and leak-proof joining of foils  3  or  8  with foil support  2  or  6 ,  7 , respectively. Power supply  24  generates a current I, the circuit being formed by lines  23 ,  25 , contact pieces  22 ,  26 , roller electrode  20 , welding strip  13 , and the particular welding group  1  or  5 . According to Ohm&#39;s law, the welding power is determined by the resistance and the current I. The resistance at the welding point is composed of the individual resistances of each workpiece. The individual resistance of a workpiece results from the product of the specific resistance and the quotient formed by the length and cross-sectional area of the current path. It is disregarded that a weld seam is not exactly defined and that several current paths may run in parallel. The specific resistance is material-specific. The length of the current path corresponds to the material thickness of a workpiece, and the cross-sectional area corresponds to the contact surface of roller electrode  20  on foil frame  2  or  6 , situated on top. The contact surface is kept constant due to the constant pressing forces of roller electrode  20 . Thus, the melting heat in the material results as a function of the material thickness and welding current I. That is, the thinner the workpiece, the less heat is released in it and accordingly less melting heat is necessary. To keep the contact resistance low, foil frames  2 ,  6 ,  7  and foils  3 ,  8  are manufactured with a small peak-to-valley height, preferably in the range of 1–2 μm, which is easily achieved via rolling. 
     In the system according to  FIGS. 3 and 4 , resistance heat (or meltig heat J ) is generated only in the one metallic foil frame  2  situated on top. Since foil  3  rests on welding strip  13  and too little heat is generated within foil  3  itself during welding, welding strip  13  must have a high specific resistance in order to make heat available which is transferred onto thin foil  3 . To keep the welding conditions constant, welding strip  13  and roller electrode  20  are actively cooled in this case. Otherwise, several welding processes would cause heating of welding strip  13  and roller electrode  20  which would result in a change in the welding parameters. In this system, welding strip  13  is advantageously made of a tungsten-copper alloy which has extreme endurance. The material of roller electrode  20  has a low specific resistance. A suitable material having extreme endurance is a copper-beryllium alloy or a tungsten-copper alloy. 
     In the system according to  FIG. 5 , melting heat J is released in both foil frames  6 ,  7 . The melting heat in melting areas  10 ,  11  is sufficient to tightly join together foil frames  6 ,  7  and foil  8 . In this case, welding strip  13  is made of a material having a low specific resistance and extreme endurance which is provided by a copper-beryllium alloy or a tungsten-copper alloy, for example. 
     Since seam welding involves a relative displacement between roller electrode  20  and foil frames  2 ,  6 , small displacements between foil frames  2 ,  6  and foil  3 ,  8  occur during welding. This creates creases in foil  3 ,  8 . During use of foil  3 ,  8  in the reformer module of a fuel cell system, foil  3 ,  8  is subject to a differential pressure which presses the creases together. This inevitably results in folding and buckling of foil  3 ,  8 . If this folding and buckling occurs in the area of weld seam  9 - 11  or several bucklings intersect in foil  3 ,  8 , then microcracks may occur in the foil material which result in leaks. In order to avoid this, it is advantageous if welding groups  1 ,  5  undergo an aftertreatment which is described below. 
     If foils  3 ,  8  are manufactured by rolling, the individual metal bodies are stretched and strained. The strain-related increase in tension also increases the hardness of the material. This increase in hardness is undesirable, and therefore welding groups  1 ,  5  undergo a heat treatment after welding. During this process, foils  3  or  8  together with foil frames  2  or  6 ,  7  are heated to 425° C. in a controlled, slow heating, then kept at this temperature for one hour, and subsequently cooled down to 70° C. This heat treatment causes recrystallization in foils  3 ,  8 . After this treatment, foils  3 ,  8  contain no more undesirable tensions. Foils  3  or  8  themselves are stretched in foil frames  2  or  6 ,  7 . Possible residual creases in foils  3 ,  8  may be removed by using geometric holding devices. 
     The permeation rate of a foil  3 ,  8  is determined by the inner structure of the foil material. Depending on the type of first-time operation of a foil  3 ,  8 , different permeation rates may occur. In order to approximate the permeation to the theoretically possible value in the first place, it is of advantage to anneal foils  3  or  8  together with foil frames  2  or  6 ,  7  in vacuum. It is also possible to anneal welding groups  1 ,  5  under an inert gas, or under pure hydrogen. 
     The heat treatment may include slowly heating the foil to a first temperature value during a first time period, keeping the foil at the first temperature value during a second time period and, decreasing the temperature of the foil to a second temperature value during a third time period. The foil may contain at least one of palladium and copper and a ratio of the first to the second time period to the third time period may be essentially 5:2:1. For example, the first time period may last 2.5 hours, the second time period 1 hour, and the third time period 0.5 hour. 
     References to specific values, such as time or temperature values, are to be understood as referring to approximate values, i.e. to a range of values that approximate the named value.