Sintering aid for lanthanum chromite refractories

• US 4,749,632 A
• Filed: 10/23/1986
• Issued: 06/07/1988
• Est. Priority Date: 10/23/1986
• Status: Expired due to Fees

• Current Assignee: Argonne National Laboratory, ARCH Development Corporation (University of Chicago)
• Original Assignee: United States Department of Energy
• Inventors: Flandermeyer, Brian K., Dusek, Joseph T., Anderson, Harlan U., Poeppel, Roger B.
• Primary Examiner(s): Hearn, Brian E.
• Assistant Examiner(s): Kalafut, Stephen J.
• Application Number:
  US06/922,429
• Time in Patent Office: 593 Days
• Field of Search: 429/12, 429/33, 429/32, 501/87, 501/88, 501/152
• US Class Current: 429/468
• CPC Class Codes: C25B 9/65 Means for supplying current; Electrode connections; Electric inter-cell connections
• CPC Class Codes: H01M 2008/1293 Fuel cells with solid oxide electrolytes
• CPC Class Codes: H01M 2300/0074 Ion conductive at high temperature
• CPC Class Codes: H01M 8/0215 Glass; Ceramic materials
• CPC Class Codes: H01M 8/2435 with monolithic core structure, e.g. honeycombs
• CPC Class Codes: Y02E 60/50 Fuel cells
• CPC Class Codes: Y10T 29/49112 including laminating of indefinite length material

Abstract

An electronically conductive interconnect layer for use in a fuel cell or other electrolytic device is formed with sintering additives to permit densification in a monolithic structure with the electrode materials. Additions including an oxide of boron and a eutectic forming composition of Group 2A metal fluorides with Group 3B metal fluorides and Group 2A metal oxides with Group 6B metal oxides lower the required firing temperature of lanthanum chromite to permit densification to in excess of 94% of theoretical density without degradation of electrode material lamina. The monolithic structure is formed by tape casting thin layers of electrode, interconnect and electrolyte materials and sintering the green lamina together under common densification conditions.

Claims

1. A method of forming an electronically conductive, integral layer including lanthanum chromite comprising:

depositing a layer including particulate lanthanum chromite containing a minor fraction of a sintering aid which includes a eutectic affording composition of at least two components providing a minor portion of a liquid phase in said layer during the sintering step, said eutectic affording composition having a melting point substantially below the melting point of said lanthanum chromite;

sintering said layer at a sufficiently high temperature of not more than 1800°

K. to form and densify an integral layer including lanthanum chromite.

2. The method of claim 1 wherein said sintering aid includes an oxide of boron.

3. The method of claim 1 wherein said eutectic affording composition is selected from the class consisting of metals from Groups 2A, 3B, and 6B combined with anions selected from the oxides and fluorides.

4. The method of claim 1 wherein said eutectic affording composition is selected from the class consisting of YF₃ -CaF₂, YF₃ -MgF₂, LaF₃ -CaF₂, LaF₃ -MgF₂, and CaO-Cr₂ O₃.

5. The method of claim 2 wherein said oxide of boron is selected from the group consisting of lanthanum borates, yttrium borates, boron oxide and boric acid and wherein said eutectic affording composition is selected from the class consisting of metals from Groups 2A, 3B, and 6B combined with anions selected from the oxides and fluorides.

6. The method of claim 2 wherein said oxide of boron is selected from the class consisting of lanthanum borates, yttrium borates, boron oxide and boric acid and wherein said eutectic affording composition is selected from the class consisting of YF₃ -CaF₂, YF₃ -MgF₂, LaF₃ -CaF₂, LaF₃ -MgF₂, and CaO-Cr₂ O₃.

7. The method of claim 6 wherein said layer of lanthanum chromite and sintering aid is deposited on a substrate of cathodic material including lanthanum manganate followed by depositing a superstrate of anodic material including transition metal, zirconia cermet to form a green composite lamina and said lamina is sintered at not more than 1700°

K. to form an integral three layered structure with said layer of lanthanum chromite of more than 94% theoretical density.

8. The method of claim 1 wherein said sintering aid is present at a level of at least 2 weight percent in respect to the lanthanum chromite.

9. A method of forming an electrolytic composite suitable for use as an interconnection layer between series connected solid oxide cells comprising:

providing a layer of first electrode material including an electrochemical catalyst,

depositing on said layer of first electrode material, a layer including particulate lanthanum chromite with a sintering aid including an oxide of boron and a eutectic affording composition, said eutectic affording composition selected from the class of eutectic affording systems consisting of metal compounds capable of providing a liquid phase at temperatures below 1600°

K., said class of metal compounds consisting of metals from Groups 2A, 3B, and 6B combined with anions selected from the oxides and fluorides, said eutectic affording composition including at least two components, and providing a minor portion of a liquid phrase in said layer including particulate lanthanum chromite during sintering;

depositing a layer of second electrode material on said lanthanum chromite layer to form a three-layered structure;

sintering said structure at a temperature of 1400°

-1700°

K. to bond said layers into an integral lamination having an electronically conductive, lanthanum chormite layer interconnecting said layer of first electrode material to said layer of second electrode material.

10. The method of claim 9 wherein said first electrode material includes an anodic material including a cermet of a transition metal with stabilized zirconia and wherein said second electrode material includes strontium-doped lanthanum manganate as cathode material.

11. The method of claim 9 wherein said oxide of boron is selected from the group consisting of lanthanum borates, yttrium borates, boric acid and boron oxide.

12. The method of claim 9 wherein said eutectic affording composition is selected from the class consisting of YF₃ -CaF₂, YF₃ -MgF₂, LaF₃ -CaF₂, LaF₃ MgF₂, and CaO-Cr₂ O₃.

13. The method of claim 9 wherein said oxide of boron and said eutectic affording composition each are included in said lanthanum chromite containing layer at a concentration of at least 2% by weight.

14. The method of claim 9 wherein each of said layers is deposited by tape casting a slip of particulate material, solvent and polymeric binder, wherein said solvent and polymeric binder is driven off as vapor in said sintering step.

15. A lamination suitable for sintering at a temperature below 1700°

K. to form an integral three-layered wall for use as an interconnection between series connected solid oxide fuel cells, said lamination comprising;

a first layer including lanthanum manganite;

a second layer containing lanthanum chromite and a sintering aid, said sintering aid including an oxide of boron combined with a eutectic affording composition selected from the fluorides and the oxides of the Group 2A, Group 3B and Group 6B metals, said eutectic affording composition including at least two components, and providing a minor portion of a liquid phase in said second layer during sintering; and

a third layer containing a cermet of a transition metal and stabilized zirconia.