Abstract:
An improved ceramic interconnect component for a solid oxide fuel cell having good electrical conductivity thermodynamic stability in the presence of fuel and a coefficient of thermal expansion matching closely that of zirconia electrolytes is disclosed. The interconnect is a lanthanum strontium chromate material containing minor quantities of calcia, and iron and, optionally, very minor quantities of cobalt, as dopants.

Description:
This application is a continuation of application Ser. No. 08/080,924, filed Jun. 21, 1993, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field: 
     The instant invention relates to solid oxide fuel cells and particularly to ceramic interconnect materials having good electrical properties. 
     2. State of the Art: 
     Solid oxide fuel cells (SOFC&#39;s) are structured to convert the energy of combustion directly to electrical energy. Low molecular weight, residue-free gases, especially natural gas, carbon monoxide, hydrogen and other clean-burning gases, are employed as fuels. A solid electrolyte, e.g. ZrO 2 , which rapidly transports oxygen ions is an essential component in SOFC&#39;s. 
     Typical SOFC&#39;s are illustrated in the following U.S. patents: 
     U.S. Pat. No. 4,476,198 Ackerman, et al. 
     U.S. Pat. No. 4,816,036 Kotchick 
     U.S. Pat. No. 4,476,196 Poeppel, et al. 
     The fuel cell operation is shown schematically in FIG. A, wherein oxygen is introduced at the cathode, dissociates to form oxygen ions by picking up electrons from the external circuit. The oxygen ions flow through the electrolyte (which is at an elevated temperature ˜700° C. or more) to combine with hydrogen, for example, in a combustion reaction (exothermic). The electrochemical heat of reaction and the internal resistance maintains the fuel cell at an efficient operating temperature, i.e., one at which the ceramic electrolyte, typically ZrO 2 , is an efficient transporter of oxygen ions. The combustion reaction (half cell reaction at the anode) is as follows: 
     
         O.sup.= +H.sub.2 →H.sub.2 O+2e.sup.- 
    
     The electrons freed by this reaction are available as electrical energy to perform useful work. The circuit must be complete so that the electrons are available at the cathode-electrolyte interface to participate in the dissociation of oxygen molecules into oxygen ions, to wit: 
     
         O.sub.2 +4e.sup.- →2O.sup.= 
    
     Ceramic interconnect devices interconnect one cell to another electrically and act as channels for both the gaseous fuel and oxygen, as illustrated in FIG. B. While FIG. B shows only two cells connected by a single interconnect, it is typical that a plurality of interconnects are used to form a &#34;stack&#34; of cells, thus serially connecting one cell to another from an electrical standpoint. 
     The interconnect must be a good conductor of electricity, have a coefficient of thermal expansion (CTE) which closely matches the electrolyte, e.g. zirconia, and be thermodynamically stable simultaneously at high oxygen partial pressures in oxygen or air and low oxygen partial pressures in the fuel gas at cell operating temperatures. Many materials may satisfy one or two of these requirements, but the lack of effective, long lasting interconnects has thus far retarded the development of a commercially usable fuel cell, such as those made of lanthanum strontium chromite (LSC). 
     SUMMARY OF THE INVENTION 
     An effective, durable interconnect for SOFC&#39;s has been invented. The interconnect is a lanthanum strontium calcium chromite, identified herein as LS2C, which preferably contains minor quantities of cobalt and/or iron. The ceramic composition has the following formula: 
     
         La.sub.0.99-(w+x) Sr.sub.w Ca.sub.x Cr.sub.1-(y+z) Co.sub.y Fe.sub.z O.sub.3 
    
     wherein 
     W (Sr)=0.08 to 0.14 
     X (Ca)=0.02 to 0.08 
     Y (Co)=0.00 to 0.05 
     Z (Fe)=0.00 to 0.05 
     when W=0.08, X=0.02 and Y and Z=0, then the formula is 
     
         La.sub.0.89 Sr.sub.0.08 Ca.sub.0.02 Cr.sub.1.0 O.sub.3 
    
     which is the interconnect material in its simplest compositional form. 
     The properties of the composition are generally improved for purposes as an interconnect by including a minor amount of cobalt and/or iron with cobalt being a preferred dopant. The composition of this invention provides an interconnect with excellent electrical conductivity in the presence of a fuel gas and a coefficient of thermal expansion which closely matches zirconia, the currently preferred electrolyte material. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an elevational view of an interconnect device; 
     FIG. 2 is an elevational view of the interconnect of FIG. 1 rotated 90° about a vertical axis; and 
     FIG. 3 is a plan view of the interconnect of FIG. 1; 
     FIG. 4 is a schematic of a fuel cell; 
     FIG. 5 is a schematic of a two-cell fuel cell with an interconnect. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An improved ceramic interconnect has been invented. The interconnect is composed of a ceramic material which meets the exacting criteria as set forth in the following table: 
     
                       TABLE 1______________________________________Criterion     Requirement   Failure Mode______________________________________Gas Impermeability         &gt;˜95% Density                       Direct                       Combustion                       Lower                       Performance                       Local Hot SpotsStructural    No Mechanical or                       CrackedIntegrity     Microstructural                       Electrolytes         Degradation   Low Stack                       PerformanceElectrical    &gt;2Ω.sup.-1 cm.sup.-1                       Low StackConductivity                PerformanceCompatible Thermal         10.5 ± 0.5 × 10.sup.-                       InadequateExpansion (˜Zr0.sub.2)         .sup.6 /°C.                       Bonding                       Cracked                       ElectrolytesThermodynamic Minimal Change in                       DecreasedStability     Conductivity over                       Performance         50,000 hrs    Eventual Stack                       Failure______________________________________ 
    
     The lanthanum strontium calcium chromite (LS2C) composition of the instant invention meets the criteria set forth in the above table. 
     While various lanthanum strontium chromite materials have been used for various purposes, the lanthanum strontium calcium chromite compositions as set forth herein have exceptional properties as an interconnect for SOFC&#39;s. 
     The properties of various chromite materials such as LSC and YSC compositions are set forth in Table 2: 
     
                                           TABLE 2__________________________________________________________________________PROPERTIES FOR SELECTED INTERCONNECT MATERIALS                    Air  Fuel                             Sinter-    Sinter-    CTE  Cond.                         Cond.                             ingComposition    ing   Density               10.sup.-                    (1/  (1/ Yield(Acronym)    Temp. (% TD)               .sup.6 /°C.)                    Ωcm)                         Ωcm)                             (%)__________________________________________________________________________Minimum  &lt;1700°        C.          94   ˜10.5                    &gt;10  &gt;2  &gt;80requirementLa.sub..83 Sr.sub..16 Cr.sub..98    1680°        C.          94   9.8-11.0                    1.5-3                         0.3 90Fe.sub..02 O.sub.3(LSFeC)La.sub.xs Sr.sub..16 Cr.sub..98    1680°        C.          96   9.6  2.5-4                         0.3 90Fe.sub..02 O.sub.3xs = 0.85 - 0.87(L.sub.xs FeC)La.sub..83 Ca.sub..16 Cr.sub..90    1500°        C.          98   10-12.8                    23-30                         1.7 70Co.sub.10 O.sub.3 (L3C)Y.sub..83 Ca.sub..16 Cr.sub..84    1450°        C.          98   10-11.7                    23-30                         1.7 60Co.sub..16 O.sub.3 (Y3C)La.sub..83 Sr.sub..13 Ca.sub..03    1650°        C.          94   9.6-10.6                    13-23                         3-6 90CrO.sub.3 (LS2C)__________________________________________________________________________ 
    
     The composition identified as LS2C (lanthanum strontium calcium chromite) has the best properties of the various compositions tested. 
     The LS2C composition has excellent gas impermeability, which is critical so that there is no &#34;leaking&#34; of fuel gas in molecular form through the electrolyte, especially to the oxygen side since combustion at the cathode side not only decreases electrical efficiency (reduced oxygen ion migration) but it also tends to destroy the cathode materials. The LS2C compositions can be pressureless sintered in air to a density of &gt;94% of theoretical. 
     Not only does the coefficient of thermal expansion (CTE) of LS2C match closely that of zirconia, it has a very uniform CTE over a wide temperature range. 
     While the electrical conductivity of LS2C in air is not particularly advantageous when compared to compositions L3C and Y3C, and had the material been tested in air only it might have been discarded as an interconnect candidate, the conductivity and stability thereof in fuel is especially good, often an order of magnitude better than any of the other LSC materials tested in fuel. Such good conductivity in fuel was not predictable, especially since its conductivity in air was poorer than either L3C or Y3C. Since there is air on one side of the interconnect and gas on the other side, it is necessary that an interconnect material have good conductivity in both air and gas. 
     LS2C was the only material tested which met minimum conductivity criteria for both air and fuel. 
     The unique interconnect compositions of the instant invention may be formed in various ways, as set forth in Table 3. 
     
                       TABLE 3______________________________________POWDER SYNTHESIS TECHNIQUESTechnique  Advantages      Disadvantages______________________________________Gel Process      Establish       Expensive      Technology      Precursors      Moderately      NO.sub.x Evolu-      Scaleable       tion/Pollution      Provides Active      PowderSolid State      Industrial Scale                      Poor Phase      Production      Homogeneity      Inexpensive Raw Low Activity      Mtrls/Processing                      PowderCo-precipitation      Provides Active Large Volume      Powder          Reduction During      High Production Calcining      Capacity      Good Stoichiometric      ControlGlycine-Nitrate      Highly Reactive Very Low      Powder          Production      Good Homogeneity                      Expensive                      Precursors______________________________________ 
    
     For the purposes of the instant invention, a gel process method of the type generally employed in the preparation of ceramic powders, which provides a homogeneous reactive powder, has generally been preferred, although any other process involving liquid precursors which insure chemical homogeneity on the molecular level are desirable as a method of powder preparation. The initial powder, as contained from the gel process, however, is frequently unacceptable for part fabrication and must be optimized by proper calcining and milling procedures. 
     Extending calcining at high temperature allows for particle coarsening and eliminates the ultra-fine &lt;0.1 μm particulates that are a result of the gel process. The powder is then reactivated by ball milling and optimally has a final particle size of 0.9 to 0.7 μm and a surface area of 6.0 to 6.5 m 2  /g. This combination of size and surface area creates a highly active powder than can be easily consolidated by uniaxial pressures of 5 to 15 ksi with a green density in excess of 55% TD. Sintering these parts at 1600° to 1680° C. for 2 hours leads to a final density of &gt;95%. 
     Interconnects have been fabricated by the following methods as set forth in Table 4: 
     
                       TABLE 4______________________________________Fabrication Approaches for Interconnect Materials                  Key                  FabricationApproach  Examples     Technology                            Results______________________________________Sintering Aids     LSC + MgF.sub.2                  Uniform   94% TD     LSC + CoCl.sub.3                  Mixing    Precipitation     LSC + CaCrO.sub.4                  Sinter at of Liquid                  1500° C.                            Phase at                            Grain                            BoundaryEutectic  La(Ca,Co)CrO.sub.3                  Minimal   97% TDLiquid Phase     Y(Ca,Co)CrO.sub.3                  Calcining Reactive with                  Reduced   Setter                  Milling   Materials                  Sinter at Warpage                  1450° C.Hot Pressing     Any LaCrO.sub.3                  Minimal   Very Low                  Powder    Yield                  Preparation                            Extensive                  Sinter at Side                  1450° C.                            ReactionsCalcining and     LSC          Moderate  95% TDMilling   LSFeC        Calcining Easy Handling     LS2C         Vigorous  Flat     L.sub.xs FeC Milling   High Yields                  Sinter at                  1650° C.______________________________________ 
    
     While sintering aids promote liquid phases in the sintering process, these may result in adverse conditions at the grain boundaries or in adverse high temperature instability. 
     Thus, preferred powders useful in forming invention interconnects of LS2C are preferably made by a gel process, following by calcining and milling as described herein. Such powders preferably have a particle size of about 0.6 to 1.0 μm, essentially no fine particles, i.e. &lt;0.1 μm and preferably a surface area of about 5.5 to 7.0 m 2  /g. Also, such powder preferably has no low melting ingredients. 
     Although it is preferred to make the LS2C powders via a gel process and proper calcining and milling, other techniques may be utilized as long as the powder is active, i.e. having the particle size and surface area described above. 
     Interconnects formed via the powder and processing techniques described herein can be formed in various shapes by uniaxial pressing and conventional sintering. It is advantageous that densities &gt;˜95% TD can be obtained without hot pressing, which tends to limit the types of shapes formed.