Electrolyte matrices for MCFCs are used for the purpose of supporting a mixed molten carbonate salt of lithium carbonate (Li2CO3), potassium carbonate (K2CO3), sodium carbonate (Na2CO3) and the like in a high-temperature region around 650° C. Hence, the electrolyte matrices require a high supporting property for the molten carbonate salts, as well as properties such us particle shape stability, alkali resistance and thermal stability. As a constituent material meeting such required properties of the electrolyte matrices, lithium aluminate has been used; particularly, a fine γ-lithium aluminate has been suitably used which is excellent in the electrolyte supporting capacity and relatively large in the specific surface area.
Further, it is known that a fine α-lithium aluminate also is useful as electrolyte matrices for MCFCs. Patent Literature 1 discloses a method for producing a high-crystalline α-lithium aluminate. Further, Patent Literature 2 discloses a method of subjecting an alumina powder having a specific surface area of 100 m2/g or larger and a mixture of carbonate salts including lithium carbonate to heat treatment in a molten carbonate salt at 700 to 800° C. Further, Patent Literature 3 discloses a method of subjecting an aluminum hydroxide powder having a specific surface area of 100 m2/g or larger and a mixture of carbonate salts including lithium carbonate to heat treatment in a molten carbonate salt at 700 to 800° C.
In any of the above methods, alumina or lithium aluminate as a raw material is heat-treated in a molten carbonate salt. Such methods not only need to take as long a reaction time as 50 hours to 100 hours, but due to the character of the production methods, further need to inevitably comprise a step of cleaning and drying products in order to remove the carbonate salts, which cannot avoid the complexity and the high cost of the steps.
Further, Patent Literature 4 proposes to produce α-lithium aluminate by dry mixing and firing a porous γ-alumina and a lithium compound in an approximately stoichiometric molar ratio of Li/Al. When a fine α-lithium aluminate having a BET specific surface area of 10 m2/g or larger is produced by this production method, however, it can hardly have any thermal stability and chemical stability necessary for electrolyte matrices for MCFCs.
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