Electrolyte sheet for solid oxide fuel cell, unit cell for solid oxide fuel cell and solid oxide fuel cell equipped with same, method for testing electrolyte sheet for solid oxide fuel cell, and method for manufacturing electrolyte sheet for solid oxide fuel cell

1. An electrolyte sheet for a solid oxide fuel cell, wherein

both sides of the electrolyte sheet have 30 or less cracks including those with a depth of less than 20 μm and a length of less than 100 μm per 30 mm×30 mm wide section as detected by a fluorescent flaw-detecting penetration test, the each section being obtained by dividing the electrolyte sheet into sections;

wherein each of the 30 or less cracks detected by the fluorescent flaw-detecting penetration test are a fluorescent spot formed by a fluorescent paint that penetrates in the cracks during the test,

the electrolyte sheet includes a zirconia-based oxide,

the zirconia-based oxide is a zirconia stabilized with an oxide of at least one element selected from the group consisting of scandium, yttrium, cerium and ytterbium, and

the electrolyte sheet is formed from a green sheet including a binder whose acid value ratio is 0.01 to 0.1.

2. The electrolyte sheet for a solid oxide fuel cell according to claim 1 , wherein the each section has 7 or less cracks as detected based on an image obtained with a CCD camera.

3. The electrolyte sheet for a solid oxide fuel cell according to claim 1 , wherein the zirconia-based oxide is a zirconia stabilized with an oxide of scandium.

4. The electrolyte sheet for a solid oxide fuel cell according to claim 1 , wherein a thickness of the electrolyte sheet is 50 μm to 400 μm and a plane area thereof is 50 cm 2 to 900 cm 2 .

5. The electrolyte sheet for a solid oxide fuel cell according to claim 4 , wherein a thickness of the electrolyte sheet is 80 μm to 300 μm and a plane area thereof is 50 cm 2 to 900 cm 2 .

6. The electrolyte sheet for a solid oxide fuel cell according to claim 1 , wherein a surface roughness Ra is 0.02 μm to 1.0 μm.

7. A unit cell for a solid oxide fuel cell comprising:

a fuel electrode;

an air electrode; and

an electrolyte sheet for a solid oxide fuel cell according to claim 1 , the electrolyte sheet intervening between the fuel electrode and the air electrode,

wherein each of the 30 or less cracks detected by the fluorescent flaw-detecting penetration test are a fluorescent spot formed by a fluorescent paint that penetrates in the cracks during the test detected by the fluorescent flaw-detecting penetration test conducted by a capillary action of a fluorescent penetrant on the surface of the electrolyte sheet,

the electrolyte sheet includes a zirconia-based oxide,

the zirconia-based oxide is a zirconia stabilized with an oxide of at least one element selected from the group consisting of scandium, yttrium, cerium and ytterbium, and

the electrolyte sheet is formed from a green sheet including a binder whose acid value ratio is 0.01 to 0.1.

8. A solid oxide fuel cell comprising the unit cell for a solid oxide fuel cell according to claim 7 .

9. The electrolyte sheet for a solid oxide fuel cell according to claim 1 , wherein at least one of the both sides of the electrolyte sheet has a flat-shaped surface.

10. The unit cell for a solid oxide fuel cell according to claim 7 , wherein at least one of the both sides of the electrolyte sheet has a flat-shaped surface.

11. The electrolyte sheet for a solid oxide fuel cell according to claim 1 , wherein said 30 or less cracks with a depth of less than 20 urn and a length of less than 100 urn per 30 mm×30 mm wide section are linear, chain streak, indentation, punctate, circular holes, or dents.

12. The electrolyte sheet for a solid oxide fuel cell according to claim 1 , wherein said 30 or less cracks with a depth of less than 20 μm and a length of less than 100 μm per 30 mm×30 mm wide section are linear.

13. The electrolyte sheet for a solid oxide fuel cell according to claim 1 , wherein said 30 or less cracks with a depth of less than 20 μm and a length of less than 100 μm per 30 mm×30 mm wide section are linear streaks and/or linear indentations.

14. The electrolyte sheet for a solid oxide fuel cell according to claim 1 , wherein said 30 or less cracks with a depth of less than 20 μm and a length of less than 100 μm per 30 mm×30 mm wide section are chain streaks and/or chain indentations.

15. The electrolyte sheet for a solid oxide fuel cell according to claim 1 , wherein at least one side of the electrolyte sheet has 6 or 7 cracks as detected based on an image obtained with a CCD camera.

16. The electrolyte sheet for a solid oxide fuel cell according to claim 1 , wherein the electrolyte sheet is manufactured by being fired next to a plurality of convexes of an alumina-based spacer sheet.

17. A method of manufacturing an electrolyte sheet for a solid oxide fuel cell, the method comprising:

preparing an alumina-based spacers each having convex-shaped surfaces;

preparing green sheets for the electrolyte sheet, each of the green sheet including a zirconia-based oxide and a binder;

stacking the alumina-based spacers and the green sheets alternatively; and

firing the green sheets after the stacking,

wherein an acid value ratio of the binder is 0.01 to 0.1.

18. The method according to claim 17 , wherein degree of penetration of the green sheets is 0.01 to 0.3 mm/min.