Abstract:
In order to produce a seal arrangement for a fuel cell stack which comprises a plurality of fuel cell units that succeed one another in a stack direction wherein each of the fuel cell units comprises a housing incorporating at least one housing part consisting of a metallic material such that it exhibits an adequate electrically insulating effect and adequate mechanical rigidity even at a high operating temperature of the fuel cell stack, it is proposed that the seal arrangement comprises at least one housing part of a first fuel cell unit consisting of a metallic material which is provided with a coating of a ceramic material, wherein the housing part of the first fuel cell unit is brazed to a housing part of a second fuel cell unit by means of a metallic braze at least one position that is provided with the ceramic coating.

Description:
RELATED APPLICATION 
       [0001]    This application is a continuation application of PCT/EP 2006/009415 filed Sep. 28, 2006, the entire specification of which is incorporated herein by reference. 
     
    
     FIELD OF DISCLOSURE 
       [0002]    The present invention relates to a seal arrangement for a fuel cell stack which comprises a plurality of fuel cell units that succeed one another in a stack direction, wherein each of the fuel cell units comprises a housing incorporating at least one housing part consisting of a metallic material. 
       BACKGROUND 
       [0003]    For the purposes of setting the desired operating voltage, the necessary number of fuel cell units are arranged one upon the other in order to thereby obtain a fuel cell stack. In order to prevent an electrical short-circuit, the housings of the successive fuel cell units in the fuel cell stack must be electrically insulated from one another. Moreover, it is necessary to separate the fuel gas channels of the fuel cell stack from the oxidizing agent chambers of the fuel cell units in a gas-tight manner and to separate the oxidizing agent channels of the fuel cell stack from the fuel gas chambers of the fuel cell units in a gas-tight manner. 
         [0004]    In the case of known fuel cell stacks, sealing and insulating elements consisting of a glass solder or of ceramic sealing materials are used in order to produce the requisite electrically insulating effect and the requisite sealing effect. 
         [0005]    In the case of some of the usually used sealing materials, the electrical resistance at the operating temperature of a high temperature fuel cell unit (in the range of approximately 800° C. to approximately 900° C.) is no longer high enough for achieving a satisfactory insulating effect. Furthermore, some of the usually used sealing materials only exhibit a low level of stability for the changes in temperature (between the operating and quiescent phases) that frequently occur in a high temperature fuel cell unit. 
       SUMMARY OF THE INVENTION 
       [0006]    The object of the present invention is to produce a seal arrangement for a fuel cell stack of the type mentioned hereinabove which exhibits an adequate electrically insulating effect and adequate mechanical rigidity even at a high operating temperature of the fuel cell stack. 
         [0007]    In accordance with the invention, this object is achieved in the case of a seal arrangement comprising the features of the preamble of Claim  1  in that the seal arrangement comprises at least one housing part of a first fuel cell unit consisting of a metallic material which is provided with a coating of a ceramic material, wherein the housing part of the first fuel cell unit is brazed to a housing part of a second fuel cell unit by means of a metallic braze at least one position that is provided with the ceramic coating. 
         [0008]    By virtue of the solution in accordance with the invention, the housing part of the first fuel cell unit is brazed directly to the housing part of the second fuel cell unit without a separate intermediate element being arranged therebetween. A particularly simple structure for the fuel cell stack is thereby obtained. 
         [0009]    The metallic braze is solid at the operating temperature of the fuel cell stack. 
         [0010]    Likewise, the housing part of the second fuel cell unit, to which the housing part of the first fuel cell unit that is provided with the ceramic coating is brazed, is preferably formed from a metallic material. 
         [0011]    In addition, the housing part of the second fuel cell unit, to which the housing part of the first fuel cell unit that is provided with the ceramic coating is brazed, can be provided with an electrically insulating ceramic coating. 
         [0012]    In principle however, it is sufficient if one of the housing parts connected together by the brazing process comprises such an electrically insulating ceramic coating in order to ensure the electrical insulating effect of the seal arrangement. 
         [0013]    The ceramic coating is formed from a ceramic material which exhibits an electrical insulating effect at the operating temperature of the fuel cell stack so that the electrical insulation of the mutually successive fuel cell units in the fuel cell stack is ensured by this ceramic coating. 
         [0014]    Since the electrical insulation is already provided by the ceramic coating, a metallic braze which is stable at high temperatures and highly adaptable to changes in temperature can be used for the mechanical connections between the housings of successive fuel cell units and for the sealing of the fluid channels instead of a glass solder or a ceramic sealing material. 
         [0015]    Moreover, the concept in accordance with the invention permits a durable connection between the housings of the fuel cell units that are mutually successive in the stack direction to be produced in a simple manner so that the build-up of the fuel cell stack can be effected in a particularly simple and rapid manner by successively bonding the fuel cell units to one another. 
         [0016]    In a preferred embodiment of the invention, provision is made for the housing part of the first fuel cell unit to comprise at least one fuel gas passage opening. 
         [0017]    As an alternative or in addition thereto, provision may be made for the housing part of the first fuel cell unit to comprise at least one oxidizing agent passage opening. 
         [0018]    Furthermore, provision may be made for the housing part of the first fuel cell unit to comprise at least one passage opening through which, in the assembled state of the fuel cell stack, a cathode electrolyte anode unit of the first fuel cell unit is accessible for enabling electrical contact to be made by another fuel cell unit of the fuel cell stack. 
         [0019]    As an alternative or in addition thereto, provision may also be made for the housing part of the first fuel cell unit to comprise at least one contact element for the purposes of making electrical contact with a neighbouring cathode electrolyte anode unit. 
         [0020]    Furthermore, provision may be made for a cathode electrolyte anode unit of the fuel cell unit to be fixed, either directly or via a substrate of the cathode electrolyte anode unit, to the housing part of the first fuel cell unit, for example, by brazing and/or by welding. 
         [0021]    The process of producing the housing parts of the first fuel cell unit is made particularly simple if the housing part is formed from a metal sheet. 
         [0022]    Preferably, provision is made for the housing part of the first fuel cell unit to be formed from a highly corrosion resistant steel. In consequence, adequate corrosion resistance of the housing part is obtained even at the high operating temperature of an SOFC (Solid Oxide Fuel Cell) fuel cell unit. 
         [0023]    It is particularly expedient, if the corrosion resistant steel which is commercially available under the trade name “Aluchrom Y” or else “FeCrAlY” is used as the material for the housing part. 
         [0024]    In principle, the ceramic coating can be formed from any ceramic material which has a sufficiently high specific electrical resistance at the operating temperature of the fuel cell stack. 
         [0025]    Ceramic coatings such as those comprising aluminium oxide and/or titanium dioxide and/or zirconium dioxide and/or magnesium oxide are particularly suitable. 
         [0026]    The ceramic coating can be produced for example by a thermal spraying process, in particular by an atmospheric plasma spraying process, by a vacuum plasma spraying process or by a flame spraying process. 
         [0027]    In a special embodiment of the seal arrangement in accordance with the invention, provision is made for the housing part of the first fuel cell unit to be formed from a metallic alloy which contains an oxidizable constituent. 
         [0028]    In particular, provision may be made for the metallic alloy to contain aluminium and/or zirconium as the oxidizable constituent. 
         [0029]    When an oxidizable constituent is present in the metallic alloy from which the housing part is formed, the ceramic coating can be produced by oxidation of the oxidizable constituent, for example, aluminium and/or zirconium of the metallic alloy. 
         [0030]    Preferably, the ceramic coating has a thickness of approximately 20 μm to approximately 1000 μm. 
         [0031]    A silver based braze in particular can be used for brazing the ceramic coating of the housing part of the first fuel cell unit to the housing part of the second fuel cell unit. 
         [0032]    Such a silver based braze can be used with or without an additive of copper. 
         [0033]    If the silver based braze without a copper additive is used, then it is expedient for the silver based braze to contain an additive of copper oxide since the silver based braze will better wet ceramic surfaces due to the additive of copper oxide. 
         [0034]    Furthermore, the silver based braze can comprise a titanium additive so as to improve the wetting properties. 
         [0035]    The braze used for brazing the ceramic coating of the housing part of the first fuel cell unit to the housing part of the second fuel cell unit is made from an intimate mixture of the components from which the braze alloy will only form in situ when heated up to the brazing temperature. 
         [0036]    Furthermore, an active braze can also be used for brazing the ceramic coating on the housing part of the first fuel cell unit to the housing part of the second fuel cell unit. 
         [0037]    Active brazes are metallic alloys which contain boundary-surface active elements (e.g. titanium, zirconium, hafnium, niobium and/or tantalum) in small quantities and are thus able to lower the boundary surface energy between a ceramic material and the braze melt to such an extent that wetting of the ceramic material by the braze can take place. 
         [0038]    The active brazing technique using active brazes enables ceramic-ceramic/metal compounds to be produced in the course of a single-step bonding process without a preceding step of metallizing the ceramic jointing surfaces. The wetting of the ceramic jointing surfaces by the braze is thus ensured by virtue of using the active braze. 
         [0039]    A suitable active braze is sold under the name “Copper ABA” by the company Wesgo Metals, 610 Quarry Road, San Carlos, Calif. 94070, USA for example. 
         [0040]    This active braze has the following composition: 2 percentage weight Al; 92.7 percentage weight Cu; 3 percentage weight Si; 2.3 percentage weight Ti. 
         [0041]    The at least one position of the housing part of the second fuel cell unit at which the housing part of the second fuel cell unit is brazed to the housing part of the first fuel cell unit can likewise be provided with an electrically insulating ceramic coating. However, due to the fact that the electrical insulation between the housings of the mutually successive fuel cell units is already ensured by the ceramic coating on the housing part of the first fuel cell unit, such a ceramic coating on the housing part of the second fuel cell unit is not of overwhelming necessity. 
         [0042]    In a preferred embodiment of the invention, provision is made for the housing part of the first fuel cell unit that is provided with the ceramic coating to be fixed to a second housing part of the first fuel cell unit, i.e. the selfsame fuel cell unit. 
         [0043]    In particular, provision may be made for the housing part of the first fuel cell unit that is provided with the ceramic coating to be welded and/or brazed to the second housing part of the first fuel cell unit. A particularly durable and rapidly and simply producible connection between the housing parts of the first fuel cell unit can be obtained in this way. 
         [0044]    The number of constructional elements necessary for the production of the fuel cell stack is reduced in an advantageous manner if provision is made for the second housing part of the first fuel cell unit to have substantially the same shape as the housing part of the second fuel cell unit to which the housing part of the first fuel cell unit that is provided with the ceramic coating is brazed. 
         [0045]    The second housing part of the first fuel cell unit can, in particular, comprise at least one fuel gas passage opening. 
         [0046]    As an alternative or in addition thereto, provision may be made for the second housing part of the first fuel cell unit to comprise at least one oxidizing agent passage opening. 
         [0047]    Furthermore, the second housing part of the first fuel cell unit may comprise at least one contact element for the purposes of making electrical contact with a neighbouring cathode electrolyte anode unit. 
         [0048]    As an alternative or in addition thereto, provision may also be made for the second housing part of the first fuel cell unit to comprise at least one passage opening through which, in the assembled condition of the fuel cell stack, a cathode electrolyte anode unit of the first fuel cell unit is accessible for enabling electrical contact to be made by another fuel cell unit of the fuel cell stack. 
         [0049]    Furthermore, provision may be made for a cathode electrolyte anode unit of the fuel cell unit to be fixed to the second housing part of the first fuel cell unit either directly or via a substrate of the cathode electrolyte anode unit, for example, by brazing and/or by welding. 
         [0050]    In a preferred embodiment of the invention, provision is made for the first housing part of the first fuel cell unit that is provided with the ceramic coating together with the second housing part of the first fuel cell unit that is connected to this housing part to form a complete two-piece housing of the first fuel cell unit without the need for further and especially metallic housing parts. 
         [0051]    This housing can enclose, in particular, a cathode electrolyte anode unit of the first fuel cell unit between the two housing parts. 
         [0052]    The further object of the present invention is to provide a method for manufacturing a fuel cell stack which comprises a plurality of fuel cell units that succeed one another in a stack direction wherein each of the fuel cell units comprises a housing incorporating at least one housing part consisting of a metallic material, said method enabling the housings of the fuel cell units to be connectable to one another in such a manner that an adequate electrical insulating effect, adequate gas-tightness and adequate mechanical rigidity are ensured even at a high operating temperature. 
         [0053]    In accordance with the invention, this object is achieved by a method which comprises the following process steps:
       preparing a housing part of a first fuel cell unit from a metallic material which is provided with a coating of a ceramic material;   brazing the housing part of the first fuel cell unit to a housing part of a second fuel cell unit by means of a metallic braze at least one position that is provided with the ceramic coating.       
 
         [0056]    Particular embodiments of the method in accordance with the invention form the subject matter of Claims  25  to  28 , the advantages thereof having already been explained in connection with the special embodiments of the seal arrangement in accordance with the invention. 
         [0057]    Further features and advantages of the invention form the subject matter of the following description and the graphic illustration of an exemplary embodiment. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0058]      FIG. 1  shows a schematic exploded illustration of the elements of a fuel cell unit; 
           [0059]      FIG. 2  a schematic exploded illustration of the fuel cell unit of  FIG. 1 , after a CEA unit of the fuel cell unit has been brazed to an upper housing part of the fuel cell unit; 
           [0060]      FIG. 3  a schematic exploded illustration of the fuel cell unit of  FIG. 2 , after the upper housing part and a lower housing part have been welded together, and a further second fuel cell unit of similar construction which is arranged above this first fuel cell unit in the stack direction of a fuel cell stack; 
           [0061]      FIG. 4  a schematic perspective illustration of the two fuel cell units of  FIG. 3 , after the upper housing part of the first fuel cell unit has been brazed to the lower housing part of the second fuel cell unit; 
           [0062]      FIG. 5  a schematic plan view of a fuel cell stack from above; 
           [0063]      FIG. 6  a partially sectional detailed perspective view of the fuel cell stack in the region of an oxidizing agent channel; 
           [0064]      FIG. 7  a schematic vertical section through the fuel cell stack in the region of the oxidizing agent channel, along the line  7 - 7  in  FIG. 5 ; 
           [0065]      FIG. 8  an enlarged exploded illustration of the region I in  FIG. 7 ; and 
           [0066]      FIG. 9  a schematic vertical section through the fuel cell stack in the region of a fuel gas channel, along the line  9 - 9  in  FIG. 5 . 
       
    
    
       [0067]    Similar or functionally equivalent elements are designated by the same reference symbols in each of the Figures. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0068]    A fuel cell stack bearing the general reference  100  that is illustrated in  FIGS. 4 to 9  comprises a plurality of fuel cell units  102  which are each of identical construction and are stacked one on top of the other along a vertical stack direction  104 . 
         [0069]    Each of the fuel cell units  102  comprises the components illustrated individually in  FIG. 1 , namely, an upper housing part  106 , a cathode electrolyte anode unit (CEA unit)  108 , a contact material  110 , a lower housing part  112  and spacer rings  190 . 
         [0070]    Furthermore, a first braze layer  116  for brazing the CEA unit  108  to the upper housing part  106  and a second braze layer  118  for brazing the upper housing part  106  to the lower housing part  112  of a second fuel cell unit  102  located thereabove is illustrated in  FIG. 1 . 
         [0071]    The upper housing part  106  is in the form of a shaped metal sheet and comprises a substantially rectangular and substantially flat metal plate  105  which is provided with a substantially rectangular central passage opening  120  through which, in the fully assembled state of the fuel cell unit, the CEA unit  108  of the fuel cell unit  102  is accessible for contact-making purposes by the lower housing part  112  of the fuel cell unit  102  located thereabove in the stack direction  104 . 
         [0072]    On the one side of the passage opening  120 , the upper housing part  106  is provided with a plurality of, three for example, fuel gas supply openings  122  which are arranged to alternate with a plurality of, four for example, oxidizing agent supply openings  124 . 
         [0073]    On the opposite side of the passage opening  120 , the upper housing part  106  is provided with a plurality of, four for example, fuel gas removal openings  126  which are arranged to alternate with a plurality of, three for example, oxidizing agent removal openings  128 . 
         [0074]    At the outer edge thereof, the metal plate  105  merges into an edge flange  107  which is aligned substantially parallel to the stack direction  104 . 
         [0075]    The oxidizing agent supply openings  124  and the oxidizing agent removal openings of the upper housing part  106  are surrounded in each case by a ring flange  135  which surrounds the opening concerned in ring-like manner and is aligned substantially parallel to the stack direction  104  (see  FIGS. 6 and 7 ). 
         [0076]    The upper housing part  106  is preferably made of a highly corrosion resistant steel, for example, from the alloy Crofer 22. 
         [0077]    The material Crofer 22 has the following composition: 
         [0078]    22 percentage weight chromium, 0.6 percentage weight aluminium, 0.3 percentage weight silicon, 0.45 percentage weight manganese, 0.08 percentage weight titanium, 0.08 percentage weight lanthanum, the remainder iron. 
         [0079]    This material is sold by the company ThyssenKrupp VDM GmbH, Plettenberger Straβe 2, 58791 Werdohl, Germany. 
         [0080]    As can be seen from  FIG. 8 , the upper housing part  106  is provided on the upper surface thereof facing the lower housing part  112  of a neighbouring fuel cell unit  102  with a ceramic coating  150  consisting of a ceramic material which has an electrically insulating effect at the operating temperature of the fuel cell unit  102 . 
         [0081]    The ceramic coating  150  on the upper housing part  106  may extend over the entire upper surface of the upper housing part  106  or merely over only those positions at which the upper housing part  106  is brazed to the lower housing part  112  of the fuel cell unit  102  located thereabove. 
         [0082]    This electrically insulating ceramic coating  150  is applied by means of a thermal spraying process to produce a layer thickness of, for example, approximately 30 μm up to, for example, approximately 500 μm. 
         [0083]    Processes that are suitable for this purpose are, for example, the atmospheric plasma spraying process, the vacuum plasma spraying process or the flame spraying process. 
         [0084]    The following insulating materials which can be applied by a thermal spraying process are suitable as the material for the ceramic coating  150  for example:
       99.5% aluminium oxide;   a mixture consisting of 97 percentage weight aluminium oxide and 3 percentage weight titanium dioxide;   yttrium-stabilized zirconium dioxide 5YSZ or 8YSZ;   a mixture of 70 percentage weight aluminium oxide and 30 percentage weight magnesium oxide;   an aluminium magnesium spinel.       
 
         [0090]    As an alternative to an upper housing part  106  having a ceramic insulating layer that was applied by a thermal spraying process, use can also be made of an upper housing part  106  consisting of a highly corrosion resistant steel containing aluminium that has been provided with a ceramic coating  150  of aluminium oxide by pre-oxidation of the aluminium-containing metallic material. 
         [0091]    In particular, such an upper housing part  106  can be formed from the steel alloy which is known by the name of “FeCrAlY” or else “Aluchrom Y”. 
         [0092]    The composition of the FeCrAlY alloy is as follows: 30 percentage weight chromium, 5 percentage weight aluminium, 0.5 percentage weight yttrium, the remainder iron. 
         [0093]    The upper housing part  106  which is stamped out from a metal sheet of this steel alloy and subjected to shaping processes is placed in an oxygen-containing atmosphere (in air for example) and held at a temperature of approximately 1100° C. for a period of time of two hours for example. As a result of this temperature treatment in an oxygen-containing atmosphere, the ceramic coating  150  consisting of aluminium oxide is produced on the upper surface of the upper housing part  106 . 
         [0094]    The CEA unit  108  comprises an anode  113 , an electrolyte  109  which is arranged above the anode  113  and a cathode  111  which is arranged above the electrolyte  109 . 
         [0095]    The anode  113  is formed from a ceramic material, from ZrO 2  or from a NiZrO 2 -Cermet (ceramic metal mixture) for example, which is electrically conductive at the operating temperature of the fuel cell unit (from approximately 800° C. to approximately 900° C.) and is porous in order to enable a fuel gas to pass through the anode  113  to the electrolyte  109  adjoining the anode  113 . 
         [0096]    A hydrocarbon-containing gas mixture or pure hydrogen can be used as the fuel gas for example. 
         [0097]    The electrolyte  109  is preferably in the form of a solid electrolyte, in particular, a solid oxide electrolyte, and consists of yttrium-stabilized zirconium dioxide for example. The electrolyte  109  is electronically non-conductive at normal temperatures and also at the operating temperature. By contrast however, the ionic conductivity thereof rises with increasing temperature. 
         [0098]    The cathode  111  is formed from a ceramic material which is electrically conductive at the operating temperature of the fuel cell unit, for example, from (La 0.8 Sr 0.2 ) 0.98 MnO 3 , and it is porous in order to enable an oxidizing agent, air or pure oxygen for example, to pass to the electrolyte  109  from an oxidizing agent chamber  130  adjoining the cathode  111 . 
         [0099]    The gastight electrolyte  109  of the CEA unit  108  extends up to the edge of the gas-permeable anode  113 , whereby the cathode surface is smaller than the surface of the anode so that the boundary region of the electrolyte  109  can be brazed to the upper housing part  106 . 
         [0100]    The contact material  110  that is arranged between the CEA unit  108  and the lower housing part  112  can be in the form of a net, a woven material or a fleece made of nickel wire for example. 
         [0101]    The lower housing part  112  is in the form of a sheet-metal shaped part and comprises a substantially rectangular plate  132  which is aligned perpendicularly to the stack direction  104  and merges at the edges thereof into an edge flange  136  which is aligned substantially in parallel with the stack direction  104 . 
         [0102]    The plate  132  comprises a substantially rectangular central contact field  138  which is provided with contact elements for enabling contact to be made with the contact material  110  on the one hand and the cathode  111  of a CEA unit  108  of a neighbouring fuel cell unit  102  on the other, said elements being of corrugated or pimpled shape for example. 
         [0103]    On the one side of the contact field  138 , the plate  132  is provided with a plurality of, three for example, fuel gas supply openings  140  which are arranged to alternate with a plurality of, four for example, oxidizing agent supply openings  142 . 
         [0104]    The fuel gas supply openings  140  and the oxidizing agent supply openings  142  of the lower housing part  112  are aligned with the respective fuel gas supply openings  122  and the oxidizing agent supply openings  124  of the upper housing part  106 . 
         [0105]    On the other side of the contact field  138 , the plate  132  is provided with a plurality of, four for example, fuel gas removal openings  144  which are arranged to alternate with a plurality of, three for example, oxidizing agent removal openings  146 . 
         [0106]    The fuel gas removal openings  144  and the oxidizing agent removal openings  146  of the lower housing part  112  are aligned with the respective fuel gas removal openings  126  and with the oxidizing agent removal openings  128  of the upper housing part  106 . 
         [0107]    The oxidizing agent removal openings  146  are preferably located opposite the fuel gas supply openings  140 , and the fuel gas removal openings  144  are preferably located opposite the oxidizing agent supply openings  142 . 
         [0108]    As can best be seen from  FIGS. 6 and 7 , the oxidizing agent supply openings  142  (in like manner to the oxidizing agent removal openings  146 ) of the lower housing part  112  are surrounded by a respective ring flange  148  which surrounds the opening concerned in ring-like manner and is aligned substantially parallel to the stack direction  104 . 
         [0109]    The lower housing part  112  is preferably made of a highly corrosion resistant steel, for example, from the previously mentioned alloy Crofer 22. 
         [0110]    Furthermore, for the purposes of mechanical stabilization of the fuel cell unit  102 , there are provided spacer rings  190  which are arranged in the region of the respective fuel gas supply openings  122  and  140  and in the region of the respective fuel gas removal openings  126  and  144  between the upper housing part  106  and the lower housing part  112  of the fuel cell unit  102  in order to maintain a spacing between the upper housing part  106  and the lower housing part  112  in this region. 
         [0111]    Each of the spacer rings  190  consists of a plurality of superimposed metal layers  192 , whereby fuel gas passage channels  194  that enable the passage of fuel gas through the spacer rings  190  are formed in the metal layers  192  by means of recesses. 
         [0112]    In order to produce the fuel cell units  102  illustrated in  FIG. 4  from the previously described individual components, one proceeds as follows: 
         [0113]    Firstly, the lower housing part  112  is provided with the ceramic coating  150  in the previously described manner. 
         [0114]    Subsequently, the electrolyte  109  of the CEA unit  108  is brazed along the edge of its upper surface to the upper housing part  106 , namely, to the lower surface of the region of the upper housing part  106  surrounding the passage opening  120  in the upper housing part  106 . 
         [0115]    As illustrated in  FIG. 1 , the brazing material needed for this purpose can be inserted between the electrolyte  109  and the upper housing part  106  in the form of a suitably cut brazing foil  116  or else it could be deposited on the upper surface of the electrolyte  109  and/or on the lower surface of the upper housing part  106  in the form of a bead of brazing material by means of a dispenser. Furthermore, it is also possible for the brazing material to be applied to the upper surface of the electrolyte  109  and/or to the lower surface of the upper housing part  106  by means of a pattern printing process, for example, a silk-screen printing process. 
         [0116]    A silver based braze incorporating a copper additive, for example a silver based braze with the composition (in mol %): Ag4Cu or Ag8Cu can be used as the brazing material. 
         [0117]    The brazing process takes place in an air atmosphere. The brazing temperature amounts to 1050° C. for example, the duration of the brazing process is approximately 5 minutes for example. When the brazing process is effected in air, copper oxide forms in situ. 
         [0118]    As an alternative thereto, a silver based braze without a copper additive could also be used as the brazing material. Such a copper-free braze offers the advantage of a higher solidus temperature (this amounts to approximately 960° C. without a copper additive, to approximately 780° C. with a copper additive). Since pure silver does not wet ceramic surfaces, copper(II) oxide is added to those silver based brazes without a copper additive for the purposes of reducing the edge angle. The brazing process utilising silver based brazes without a copper additive takes place in an air atmosphere or in an inert gas atmosphere, for example, under argon. 
         [0119]    In this case too, the brazing temperature preferably amounts to approximately 1050° C., the duration of the brazing process to approximately 5 minutes for example. 
         [0120]    As an alternative to brazing the CEA unit  108  into the upper housing part  106 , provision could also be made for a substrate upon which the CEA unit  108  has not yet been produced to be welded to the upper housing part  106  and, following the welding process, the electro-chemically active layers of the CEA unit  108 , i.e. the anode, electrolyte and cathode thereof, are produced successively on the substrate that has already been welded to the upper housing part  106  using a vacuum plasma spraying process. 
         [0121]    After the connection of the CEA unit  108  to the upper housing part  106 , the state illustrated in  FIG. 2  is reached. 
         [0122]    Subsequently, the contact material  110  and the spacer rings  190  are inserted between the lower housing part  112  and the upper housing part  106  and, if necessary, brazed and/or welded to the lower housing part  112  and/or to the upper housing part  106 , and then the lower housing part  112  and the upper housing part  106  are welded together in gas-tight manner along a welding seam  164  which extends around the outer edge of the edge flange  136  of the lower housing part  112  and the edge flange  107  of the upper housing part  106  and along welding seams  166  which extend around the inner edges of the ring flanges  148  of the lower housing part  112  and the ring flanges  135  of the oxidizing agent supply openings  124  and the oxidizing agent removal openings  128  of the upper housing part  106 . 
         [0123]    After this process step, the state illustrated in  FIG. 3  is reached. 
         [0124]    Now the side of the upper housing part  106  of the lower fuel cell unit  102   b  that is provided with the ceramic coating  150  and faces the lower housing part  112  of the upper fuel cell unit  102   a  is brazed directly by means of a brazing material to the side of the lower housing part  112  facing the upper housing part  106  whilst being weighted down. 
         [0125]    Here, the selfsame brazing materials can be used as were described previously in connection with the process of brazing the CEA unit  108  and the upper housing part  106 , and the brazing procedure can take place under the same conditions. 
         [0126]    Thus in particular and as illustrated in  FIG. 3 , the brazing material needed for this purpose can be inserted in the form of a suitably cut brazing foil  118  between the upper housing part  106  of the lower fuel cell unit  102   b  and the lower housing part  112  of the upper fuel cell unit  102   a , or else it could be applied in the form of a bead of brazing material to the upper surface of the upper housing part  106  and/or to the lower surface of the lower housing part  112  by means of a dispenser. Furthermore, it is also possible for the brazing material to be applied to the upper surface of the upper housing part  106  and/or to the lower surface of the lower housing part  112  by means of a pattern printing process, a silk-screen printing process for example. 
         [0127]    A silver based braze with a copper additive can be used as the brazing material, for example, a silver based braze having the composition (in mol percent): Ag-4Cu or Ag-8Cu. 
         [0128]    The brazing process takes place in an air atmosphere. The brazing temperature amounts to 1050° C. for example, the duration of the brazing process is approximately 5 minutes for example. Copper oxide forms in situ during the process of brazing in air. 
         [0129]    As an alternative thereto, a silver based braze without a copper additive could also be used as the brazing material. Such a copper-free braze offers the advantage of a higher solidus temperature (this amounts to approximately 960° C. without a copper additive, to approximately 780° C. with a copper additive). Since pure silver does not wet ceramic surfaces, copper(II) oxide is added to those silver based brazes without a copper additive for the purposes of reducing the edge angle. The brazing process utilising silver based brazes without a copper additive takes place in an air atmosphere or in an inert gas atmosphere, for example, under argon. 
         [0130]    Suitable silver based brazes without an additive of elementary copper have the composition (in mol percent): Ag-4CuO or Ag-8CuO for example. 
         [0131]    An additive of titanium can serve for the further improvement of the wetting process (reduction of the edge angle). An intimate mixture of the appropriate components in powder form is used for the production of the brazes. The braze alloy is formed in situ from this mixture. The titanium is added to this mixture in the form of titanium hydride. A metallic titanium is formed from the hydride at approximately 400° C. 
         [0132]    Suitable silver based brazes without an additive of elementary copper, but with an additive of titanium have the composition (in mol percent): Ag-4CuO-0.5Ti or Ag-8CuO-0.5Ti for example. 
         [0133]    In this case too, the brazing temperature preferably amounts to approximately 1050° C., the duration of the brazing process to approximately 5 minutes for example. 
         [0134]    Furthermore, active brazes can also be used as the brazing material for brazing the upper housing part  106  to the lower housing part  112 . 
         [0135]    Active brazes are metallic alloys which contain boundary-surface active elements (e.g. titanium, zirconium, hafnium, niobium and/or tantalum) in small quantities and are thus able to lower the boundary surface energy between a ceramic material and the braze melt to such an extent that wetting of the ceramic material by the braze can take place. 
         [0136]    The active brazing technique using active brazes enables ceramic-ceramic/metal compounds to be produced in the course of a single-step bonding process without a preceding step of metallizing the ceramic jointing surfaces. The wetting of the ceramic jointing surfaces by the braze is thus ensured due to the use of an active braze. 
         [0137]    A suitable active braze is sold under the name “Copper ABA” by the company Wesgo Metals, 610 Quarry Road, San Carlos, Calif. 94070, USA for example. 
         [0138]    This active braze has the following composition: 2 percentage weight Al; 92.7 percentage weight Cu; 3 percentage weight Si; 2.3 percentage weight Ti. 
         [0139]    The brazing process can be carried out, in particular, in accordance with the following temperature program:
       Insofar as the braze material is applied in the form of a braze paste, the braze paste is dried for a period of approximately 10 minutes at a temperature of approximately 150° C.   Subsequently, brazing takes place in three steps, whereby in a first step, those components that are to be brazed together are heated up for one hour from room temperature to a temperature of approximately 300° C., in a following second stage, the components that are to be brazed are heated up within three hours from a temperature of approximately 300° C. to a temperature of approximately 550° C. and in a third step, the components that are to be brazed together are heated up within three hours from a temperature of approximately 550° C. to a final temperature of approximately 1,050° C., whereby the final temperature is maintained for a time period of approximately 5 minutes for example.   After brazing has been effected, the components that have been brazed together are cooled to room temperature over a long period of time, for example, over night.       
 
         [0143]    In order to prevent an unwanted flow of the braze material beyond the region that is to be brazed, a braze stop material can be applied to those areas of the upper housing part  106  and the lower housing part  112  which should remain free of the braze material. 
         [0144]    Suitable braze stop materials are sold under the designations “Stopyt Liquid” or “Stopyt Liquid # 62A” by the company Wesgo Metals, 610 Quarry Road, San Carlos, Calif. 94070, USA. 
         [0145]    If the brazing process is effected in a vacuum or in an inert gas atmosphere, then care should be taken that the oxygen partial pressure does not drop below a certain lower limit since the cathode  111  of the CEA unit  108  will otherwise be destroyed. 
         [0146]    In the case of a cathode consisting of lanthanum strontium manganate (LSM), the lower limit for the oxygen partial pressure amounts to approximately 1 ppm (10 −4  bar); in the case of a cathode consisting of lanthanum strontium cobalt ferrite (LSCF) the lower limit for the oxygen partial pressure amounts to approximately 10 ppm (10 −3  bar). 
         [0147]    After the upper housing part  106  of the lower fuel cell unit  102   b  has been brazed to the lower housing part  112  of the upper fuel cell unit  102   a , the state illustrated in  FIG. 4  is reached. 
         [0148]    After two fuel cell units  102  have been connected together in this way, the fuel cell stack  100  can be gradually built up by successively brazing further fuel cell units  102  to the upper housing part  106  of the upper fuel cell unit  102   a  or to the lower housing part  112  of the lower fuel cell unit  102   b  in the stack direction  104  until the desired number of fuel cell units  102  is attained. 
         [0149]    In the finished fuel cell stack  100 , the respective mutually aligned fuel gas supply openings  122  and  140  of the upper housing parts  106  and the lower housing parts  112  form a respective fuel gas supply channel  172  which, in each fuel cell unit  102 , opens between the upper surface of the lower housing part  112  and the lower surface of the upper housing part  106  into a fuel gas chamber  174  which is formed between the upper surface of the contact field  138  of the lower housing part  112  on the one hand and the lower surface of the CEA unit  108  on the other. 
         [0150]    The respective mutually aligned fuel gas removal openings  126  and  144  of the upper housing parts  106  and the lower housing parts  112  form a respective fuel gas removal channel  176  which is open to the fuel gas chamber  174  in the region between the upper surface of the lower housing part  112  and the lower surface of the upper housing part  106  on the side of each fuel cell unit  102  that is located opposite the fuel gas supply channels  172 . 
         [0151]    The respective mutually aligned oxidizing agent supply openings  124  and  142  of the upper housing parts  106  and the lower housing parts  112  together form a respective oxidizing agent supply channel  178  which is open to the oxidizing agent chamber  130  of the fuel cell unit  102  in the region of each fuel cell unit  102  between the upper surface of the upper housing part  106  and the lower surface of the lower housing part  112  of the fuel cell unit  102  located thereabove in the stack direction  104 . 
         [0152]    In like manner, the respective mutually aligned oxidizing agent removal openings  128  and  146  of the upper housing parts  106  and the lower housing parts  112  form a respective oxidizing agent removal channel  180  which is arranged on the side of the fuel cell units  102  located opposite to the oxidizing agent supply channels  178  and likewise opens into the oxidizing agent chamber  130  of the fuel cell unit  102  in the region of each fuel cell unit  102  between the upper surface of the upper housing part  106  and the lower surface of the lower housing part  112  of the fuel cell unit  102  located thereabove in the stack direction  104 . 
         [0153]    In operation of the fuel cell stack  100 , a fuel gas is supplied to the fuel gas chamber  174  of each fuel cell unit  102  by way of the fuel gas supply channels  172  and the exhaust gas produced by oxidation at the anode  113  of the CEA unit  108  as well as any unused fuel gas is removed from the fuel gas chamber  174  through the fuel gas removal channels  176 . 
         [0154]    In like manner, an oxidizing agent, air for example, is supplied to the oxidizing agent chamber  130  of each fuel cell unit  102  through the oxidizing agent supply channels  178  and unused oxidizing agent is removed from the oxidizing agent chamber  130  through the oxidizing agent removal channels  180 . 
         [0155]    In operation of the fuel cell stack  100 , the CEA units  108  are, for example, at a temperature of 850° C. at which the electrolyte of each CEA unit  108  is conductive for oxygen ions. The oxidizing agent from the oxidizing agent chamber  130  picks up electrons at the cathode  111  and delivers doubly negatively charged oxygen ions to the electrolyte  109 , said ions then migrating through the electrolyte  109  to the anode  113 . At the anode  113 , the fuel gas from the fuel gas chamber  174  is oxidized by the oxygen ions from the electrolyte  109  and thereby donates electrons to the anode  113 . 
         [0156]    The electrons freed by the reaction at the anode  113  are supplied from the anode  113  by way of the contact material  110  and the lower housing part  112  to the cathode  111  of a neighbouring fuel cell unit  102  resting on the lower surface of the contact field  138  of the lower housing part  112  and thus make the cathode reaction possible. 
         [0157]    The lower housing part  112  and the upper housing part  106  of each fuel cell unit  102  are connected together in electrically conductive manner by the welding seams  164 ,  166 . 
         [0158]    However, the housings  182  of the fuel cell units  102  which succeed one another in the stack direction  104  that are formed in each case by an upper housing part  106  and a lower housing part  112  are electrically insulated from one another by the ceramic coatings  150  on the upper surface of the upper housing parts  106 . Hereby, due to the brazing of the upper housing parts  106  to the lower housing parts  112  of neighbouring fuel cell units  102 , a gastight connection between these components is ensured at the same time so that the oxidizing agent chambers  130  and the fuel gas chambers  174  of the fuel cell units  102  are separated in gas-tight manner from each other and from the environment of the fuel cell stack  100 . 
         [0159]    In a (not illustrated) variant of the previously described embodiment of a fuel cell stack  100 , provision is made for the electrically insulating ceramic coating not to be arranged on the upper surface of the upper housing part  106 , but instead, on the lower surface of the lower housing part  112 . 
         [0160]    In a further variant, provision is made for a respective electrically insulating ceramic coating to be provided on both the upper surface of the upper housing part  106  and the lower surface of the lower housing part  112 .