Patent Publication Number: US-2022223894-A1

Title: Electrolyte sheet for solid oxide fuel cells, method for producing electrolyte sheet for solid oxide fuel cells, and single cell for solid oxide fuel cells

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of International application No. PCT/JP2020/031092, filed Aug. 18, 2020, which claims priority to Japanese Patent Application No. 2019-189559, filed Oct. 16, 2019, the entire contents of each of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to an electrolyte sheet for solid oxide fuel cells, a method of producing an electrolyte sheet for solid oxide fuel cells, and a unit cell for solid oxide fuel cells. 
     BACKGROUND OF THE INVENTION 
     A solid oxide fuel cell (SOFC) is a device that produces electric energy through reactions of H 2 +O 2 →H 2 O+2e −  at the fuel electrode and (½)O 2 +2e − →O 2−  at the air electrode. A solid oxide fuel cell is a stack of unit cells each including an electrolyte sheet for solid oxide fuel cells made of a ceramic plate body and a fuel electrode and an air electrode that are formed on the electrolyte sheet. 
     For example, Patent Literature 1 discloses a method of producing a ceramic plate body including forming an unsintered laminate through hole by drilling in an unsintered laminate obtained by alternately stacking unsintered plate bodies and resin sheets or resin layers and compression-bonding the stack.
     Patent Literature 1: JP 2018-199256 A   

     SUMMARY OF THE INVENTION 
     As in the production method disclosed in Patent Literature 1, when the stack alternately including the unsintered plate bodies and the resin sheets or resin layers is compression-bonded, the outer edge of each unsintered plate body tends to spread out in directions perpendicular to the stacking direction, i.e., in the surface directions, as compared with the center of the unsintered plate body. The density of the unsintered plate body in the resulting unsintered laminate is therefore not uniform, which may result in variation of heat shrinkage degree inside the unsintered plate body during sintering of the unsintered plate body. Thus, when a ceramic plate body obtained by sintering an unsintered plate body provided with a through hole in the vicinity of the outer edge thereof is used to produce a unit cell, breakages such as cracking or chipping may occur in the vicinity of the through hole. In addition, gas leakage may occur during operations of a fuel cell with such a unit cell. In this respect, the production method disclosed in Patent Literature 1 can still be improved. 
     The present invention was made to solve the above issues, and aims to provide an electrolyte sheet for solid oxide fuel cells which is capable of preventing breakages such as cracking or chipping during production of a unit cell for solid oxide fuel cells as well as gas leakage during operations of a solid oxide fuel cell with such a unit cell. The present invention also aims to provide a method of producing the electrolyte sheet for solid oxide fuel cells. Furthermore, the present invention aims to provide a unit cell for solid oxide fuel cells which includes the electrolyte sheet for solid oxide fuel cells. 
     In a first embodiment, the electrolyte sheet for solid oxide fuel cells of the present invention includes: a ceramic plate body having a through hole penetrating therethrough in a thickness direction thereof, wherein a shortest distance between a first point on an outer edge of the ceramic plate body and a second point on a peripheral edge of the through hole is not shorter than 1 mm and not longer than 5 mm, and a warpage height in an area between the first point and the second point is not more than 150 μm. 
     In a second embodiment, the electrolyte sheet for solid oxide fuel cells of the present invention includes: a ceramic plate body having a through hole penetrating therethrough in a thickness direction thereof, wherein a shortest distance between a first point on an outer edge of the ceramic plate body and a second point on a peripheral edge of the through hole is not less than 0.5% and not more than 10.0% of a distance between the first point and a third point other than the first point among intersection points between the outer edge of the ceramic plate body and a virtual straight line connecting the first point and the second point, and a warpage height in an area between the first point and the second point is not more than 150 μm. 
     The method of producing an electrolyte sheet for solid oxide fuel cells of the present invention includes: pressing an unpressed body comprising an unsintered plate body containing a ceramic material powder to produce an unsintered body, the unpressed body having a first surface and a second surface opposing each other in a thickness direction of the unpressed body and side surfaces parallel to the thickness direction, the unpressed body being sandwiched between a first metal plate on the first surface and a second metal plate on the second surface and surrounded by a plate frame around the side surfaces thereof such that an elongation of a length of the unsintered body relative to a length of the unpressed body in a direction perpendicular to the thickness direction is within ±1.0%; forming an unsintered body through hole penetrating the unsintered body in the thickness direction; and firing the unsintered body to sinter the unsintered plate body into a ceramic plate body having a through hole penetrating therethrough in the thickness direction. 
     The unit cell for solid oxide fuel cells of the present invention includes: a fuel electrode; an air electrode; and the electrolyte sheet for solid oxide fuel cells of the present invention between the fuel electrode and the air electrode. 
     The present invention can provide an electrolyte sheet for solid oxide fuel cells which is capable of preventing breakages such as cracking and chipping during production of a unit cell for solid oxide fuel cells as well as gas leakage during operations of a solid oxide fuel cell with such a unit cell. The present invention can also provide a method of producing the electrolyte sheet for solid oxide fuel cells. Furthermore, the present invention can provide a unit cell for solid oxide fuel cells which includes the electrolyte sheet for solid oxide fuel cells. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic plan view of an example of an electrolyte sheet for solid oxide fuel cells of the present invention. 
         FIG. 2  is a schematic cross-sectional view of a portion taken along line A 1 -A 2  in  FIG. 1 . 
         FIG. 3  is a schematic plan view of an example of producing ceramic green sheets. 
         FIG. 4  is another schematic plan view of the example of producing ceramic green sheets. 
         FIG. 5  is yet another schematic plan view of the example of producing ceramic green sheets. 
         FIG. 6  is a schematic cross-sectional view of an example of producing an unsintered plate body. 
         FIG. 7  is a schematic plan view of an example of producing resin layers. 
         FIG. 8  is another schematic plan view of the example of producing resin layers. 
         FIG. 9  is yet another schematic plan view of the example of producing resin layers. 
         FIG. 10  is a schematic cross-sectional view of an example of producing an unpressed body. 
         FIG. 11  is a schematic perspective view of the unpressed body in  FIG. 10 . 
         FIG. 12  is a schematic cross-sectional view of an example of producing an unsintered body. 
         FIG. 13  is a schematic plan view of the assembly in  FIG. 12 . 
         FIG. 14  is another schematic cross-sectional view of the example of producing an unsintered body. 
         FIG. 15  is yet another schematic cross-sectional view of the example of producing an unsintered body. 
         FIG. 16  is a schematic cross-sectional view of an example of forming an unsintered body through hole. 
         FIG. 17  is another schematic cross-sectional view of the example of forming an unsintered body through hole. 
         FIG. 18  is a schematic cross-sectional view of an example of producing a ceramic plate body. 
         FIG. 19  is a schematic cross-sectional view of an example of a unit cell for solid oxide fuel cells of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The electrolyte sheet for solid oxide fuel cells (hereinafter, also referred to as the electrolyte sheet) of the present invention, the method of producing an electrolyte sheet for solid oxide fuel cells (hereinafter, also referred to as the method of producing an electrolyte sheet) of the present invention, and the unit cell for solid oxide fuel cells (hereinafter, also referred to as the unit cell) of the present invention are described below. The present invention is not limited to the following preferred embodiments, and may be suitably modified without departing from the gist of the present invention. Combinations of two or more preferred features described in the following preferred features are also within the scope of the present invention. 
     The drawings are schematic drawings, and the dimensions, the aspect ratio, the scale, and other parameters may differ from those of the actual products. 
     Electrolyte Sheet for Solid Oxide Fuel Cells 
     An example of the electrolyte sheet for solid oxide fuel cells of the present invention is described below.  FIG. 1  is a schematic plan view of an example of an electrolyte sheet for solid oxide fuel cells of the present invention.  FIG. 2  is a schematic cross-sectional view of a portion taken along line A 1 -A 2  in  FIG. 1 . 
     An electrolyte sheet  10  for solid oxide fuel cells shown in  FIG. 1  and  FIG. 2  is made of a ceramic plate body. 
     The ceramic plate body contains a sintered body of a solid electrolyte such as scandia-stabilized zirconia or yttria-stabilized zirconia. In particular, the electrolyte sheet  10  is preferably made of a ceramic plate body containing sintered scandia-stabilized zirconia. The electrolyte sheet  10 , which is made of a ceramic plate body containing sintered scandia-stabilized zirconia, has a higher conductivity. Thus, a solid oxide fuel cell with the electrolyte sheet  10  has a higher power generation efficiency. 
     In a plan view from a thickness direction of the electrolyte sheet  10  (vertical direction in  FIG. 2 ), the electrolyte sheet  10  has a square shape as shown in  FIG. 1 , for example. 
     Although not shown, in a plan view from the thickness direction, the electrolyte sheet  10  preferably has a substantially rectangular shape with rounded corners, more preferably a substantially square shape with rounded corners. In this case, all the corners may be rounded or some of the corners may be rounded. 
     The electrolyte sheet  10  is provided with through holes  10   h  penetrating the electrolyte sheet  10  in the thickness direction. The through holes  10   h  function as gas flow paths in a solid oxide fuel cell. 
     One through hole  10   h  may be provided or two or more through holes  10   h  may be provided. For example, four through holes  10   h  opposing the centers of the respective four sides of the electrolyte sheet  10  as shown in  FIG. 1  may be provided. 
     In a plan view from the thickness direction, the through holes  10   h  may have a circular shape as shown in  FIG. 1  or any shape other than a circular shape. 
     Each through hole  10   h  preferably has a hole diameter of not more than 20 mm. Also, the through hole  10   h  preferably has a hole diameter of not less than 5 mm. When the through hole  10   h  has a shape other than a circular shape in a plan view from the thickness direction, the diameter of a circle having an area equivalent to that of the shape above is taken as the hole diameter of the through hole  10   h.    
     In a plan view as in  FIG. 1 , the size of the electrolyte sheet  10  is, for example, 50 mm×50 mm, 100 mm×100 mm, 110 mm×110 mm, 120 mm×120 mm, or 200 mm×200 mm, for example. 
     The electrolyte sheet  10  has a thickness of preferably not more than 200 μm, more preferably not more than 130 μm. Also, the electrolyte sheet  10  has a thickness of preferably not less than 30 μm, more preferably not less than 50 μm. 
     The thickness of the electrolyte sheet  10  is determined as follows. First, the thickness is measured at randomly selected nine sites within a region excluding the portions 5 mm inside the outer edge of the electrolyte sheet  10  with a U-shape Frame Sheet Metal Micrometer (available from Mitutoyo Corporation, PMU-MX). The average of the thicknesses measured at the nine sites is calculated. The average is taken as the thickness of the electrolyte sheet  10 . 
     Preferably, recesses are scattered on a first main surface and a second main surface of the electrolyte sheet  10 , although not shown. With the recesses scattered on the first main surface and the second main surface of the electrolyte sheet  10 , the area of contact between the electrodes and gas is large in a solid oxide fuel cell with the electrolyte sheet  10 . This ultimately increases the power generation efficiency of the solid oxide fuel cell. The recesses may be scattered only on one of the first main surface and the second main surface of the electrolyte sheet  10 . 
     In a first embodiment of the electrolyte sheet  10 , with the shortest distance between an outer edge of the electrolyte sheet  10  and the peripheral edge of the through hole  10   h  being defined as the distance between a first point P 1  on the outer edge of the electrolyte sheet  10  and a second point P 2  on the peripheral edge of the through hole  10   h , the shortest distance is not shorter than 1 mm and not longer than 5 mm, and the warpage height in the area between the first point P 1  and the second point P 2  is not more than 150 μm. The present embodiment enables an electrolyte sheet  10  with no or less warpage in the vicinity of the outer edge thereof even when the through hole  10   h  is provided in the vicinity of the outer edge. The electrolyte sheet  10  is therefore capable of preventing breakages such as cracking or chipping during production of a unit cell for solid oxide fuel cells including the electrolyte sheet  10 . The electrolyte sheet  10  is also capable of preventing gas leakage during operations of a solid oxide fuel cell including the electrolyte sheet  10 . Thus, the solid oxide fuel cell can exhibit a higher power generation efficiency. 
     In the first embodiment of the electrolyte sheet  10 , the warpage height in the area between the first point P 1  and the second point P 2  is preferably not more than 100 μm. The warpage height may be 0 μm. 
     In the first embodiment of the electrolyte sheet  10 , when a plurality of through holes  10   h  is provided, at least one through hole  10   h  should have the features in the first embodiment, and preferably all the through holes  10   h  have the features in the first embodiment. There may be a through hole  10   h  failing to have the features in the first embodiment as long as at least one through hole  10   h  has the features in the first embodiment. 
     In the second embodiment of the electrolyte sheet  10 , with the shortest distance between the outer edge of the electrolyte sheet  10  and the peripheral edge of the through hole  10   h  being defined as the distance between a first point P 1  on the outer edge of the electrolyte sheet  10  and a second point P 2  on the peripheral edge of the through hole  10   h , and with a third point P 3  being defined as any point other than the first point P 1  among intersection points between the outer edge of the ceramic plate body and a virtual straight line connecting the first point P 1  and the second point P 2 , the shortest distance is not less than 0.5% and not more than 10.0% of the distance between the first point P 1  and the third point P 3 , and the warpage height in the area between the first point P 1  and the second point P 2  is not more than 150 μm. The present embodiment enables an electrolyte sheet  10  with no or less warpage in the vicinity of the outer edge thereof even when the through hole  10   h  is provided in the vicinity of the outer edge. The electrolyte sheet  10  is therefore capable of preventing breakages such as cracking or chipping during production of a unit cell for solid oxide fuel cells including the electrolyte sheet  10 . The electrolyte sheet  10  is also capable of preventing gas leakage during operations of a solid oxide fuel cell including the electrolyte sheet  10 . Thus, the solid oxide fuel cell can exhibit a higher power generation efficiency. 
     In the second embodiment of the electrolyte sheet  10 , when the electrolyte sheet  10  has a square shape as shown in  FIG. 1  in a plan view from the thickness direction, the distance between the first point P 1  and the third point P 3  corresponds to the length of one side of the square shape. 
     In the second embodiment of the electrolyte sheet  10 , the shortest distance is preferably not shorter than 1 mm and not longer than 5 mm. 
     In the second embodiment of the electrolyte sheet  10 , the warpage height in the area between the first point P 1  and the second point P 2  is preferably not more than 100 μm. The warpage height may be 0 μm. 
     In the second embodiment of the electrolyte sheet  10 , when a plurality of through holes  10   h  is provided, at least one through hole  10   h  should have the features in the second embodiment, and preferably all the through holes  10   h  have the features in the second embodiment. There may be a through hole  10   h  failing to have the features in the second embodiment as long as at least one through hole  10   h  has the features in the second embodiment. 
     In the electrolyte sheet  10 , the first point P 1  and the second point P 2  are determined as follows. First, the distance between the outer edge of the electrolyte sheet  10  and the peripheral edge of the through hole  10   h  is measured with, for example, a video measuring system “NEXIV VMZ-R6555” (available from Nikon Instech Co., Ltd.) to find the shortest distance. Of the two points defining the shortest distance, a point on the outer edge of the electrolyte sheet  10  is defined as the first point P 1  while the other point on the peripheral edge of the through hole  10   h  is defined as the second point P 2 . 
     In the electrolyte sheet  10 , the distance between the first point P 1  and the third point P 3  is measured with, for example, a video measuring system “NEXIV VMZ-R6555” (available from Nikon Instech Co., Ltd.). 
     On the electrolyte sheet  10 , the warpage height in the area between the first point P 1  and the second point P 2  is determined as follows. First, the electrolyte sheet  10  with the warpage as the measurement target is placed to be convex downward. With a wide-area 3D measurement system “VR-5000” available from Keyence Corporation, for example, the difference between the maximum height and the minimum height in the area between the first point P 1  and the second point P 2  is measured. The obtained measurement value is defined as the warpage height. 
     Method of Producing Electrolyte Sheet for Solid Oxide Fuel Cells 
     An example of the method of producing an electrolyte sheet for solid oxide fuel cells of the present invention is described below. 
     Producing Ceramic Green Sheets 
       FIG. 3  is a schematic plan view of an example of producing ceramic green sheets.  FIG. 4  is another schematic plan view of the example of producing ceramic green sheets.  FIG. 5  is yet another schematic plan view of the example of producing ceramic green sheets. 
     A ceramic material powder, a binder, a dispersant, an organic solvent, and the like are mixed to prepare a ceramic slurry. A first main surface of a carrier film is coated with the obtained ceramic slurry to produce ceramic green tape  1   t  as shown in  FIG. 3 . 
     The ceramic green tape  1   t  is preferably produced by tape casting, particularly preferably doctor blading or calendaring. In  FIG. 3 , the casting directions for the tape casting are indicated by X and the directions perpendicular to the casting directions are indicated by Y. 
     The ceramic material powder may be a solid electrolyte powder such as scandia-stabilized zirconia powder or yttria-stabilized zirconia powder. In particular, the ceramic material powder preferably contains scandia-stabilized zirconia powder. 
     The resulting ceramic green tape  1   t  is punched to obtain pieces having a predetermined size by a known technique as shown in  FIG. 4 , and the carrier film is removed from the pieces. Thus, ceramic green sheets  1   g  as shown in  FIG. 5  are produced. Punching of the ceramic green tape  1   t  and removal of the carrier film may be performed in any order. 
     Producing Unsintered Plate Body 
       FIG. 6  is a schematic cross-sectional view of an example of producing an unsintered plate body. 
     As shown in  FIG. 6 , an unsintered plate body  1   s  is produced by stacking three ceramic green sheets  1   g  and compression-bonding the stack. 
     The number of ceramic green sheets  1   g  used to produce the unsintered plate body  1   s  may be three as shown in  FIG. 6 , or two or four or more. The ceramic green sheets  1   g  may be compression-bonded, or may simply be stacked on one another without being compression-molded. When the unsintered plate body  1   s  includes a plurality of ceramic green sheets  1   g , the thickness of a ceramic plate body to be obtained can be controlled as appropriate in a simple manner. 
     The unsintered plate body  1   s  may be produced using one ceramic green sheet  1   g . In this case, the step shown in  FIG. 6  is omitted. 
     Producing Resin Layer 
       FIG. 7  is a schematic plan view of an example of producing resin layers.  FIG. 8  is another schematic plan view of the example of producing resin layers.  FIG. 9  is yet another schematic plan view of the example of producing resin layers. 
     First, a resin powder, a binder, a dispersant, an organic solvent, and the like are mixed to prepare a resin slurry. A first main surface of a carrier film is coated with the obtained resin slurry to produce resin tape  2   t  as shown in  FIG. 7 . 
     The resin tape  2   t  is preferably produced by tape casting, particularly preferably doctor blading or calendaring. In  FIG. 7 , the casting directions for the tape casting are indicated by X and the directions perpendicular to the casting directions are indicated by Y. 
     The resin powder is preferably made of a resin material that is poorly soluble in an organic solvent used in production of a resin slurry. The expression “poorly soluble in an organic solvent” herein means that when 0.1 g of a resin powder and 100 g of an organic solvent are mixed at room temperature (25° C.) for 24 hours, there remains a visually observable residue. The organic solvent used to prepare a resin slurry is, for example, at least one solvent (alone or in a mixture) selected from toluene, ethanol, isopropanol, butyl acetate, ethyl acetate, terpineol, and water. In this case, the resin powder is made of a crosslinked acrylic resin, for example. 
     Preferably, the resin powder has a spherical shape. When the resin powder has a spherical shape, its median size D 50  is, for example, not less than 0.3 μm and not more than 10 μm. 
     When the resin powder has a spherical shape, the median size D 50  of the resin powder is defined as the particle size at 50% in a cumulative particle size distribution curve of the resin powder expressed as cumulative percentage against particle size scale. The particle size distribution of the resin powder is measured with, for example, a laser diffraction particle size distribution measuring device. The median size D 50  used here is the equivalent spherical diameter because the resin powder may have a shape distorted through the production processes. 
     When the resin powder has a spherical shape, the resin powder has a smaller surface area per weight, so that the amount of binder required for preparation of a highly fluid resin slurry is reduced. This makes it possible to produce a resin layer having a high resin powder content, so that many recesses can be formed on a first main surface and a second main surface of a ceramic plate body to be obtained. 
     Next, the resin tape  2   t  is punched to obtain pieces having a predetermined size by a known technique as shown in  FIG. 8 , and the carrier film is removed from the pieces. Thus, resin sheets as resin layers  2   e  as shown in  FIG. 9  are produced. Punching of the resin tape  2   t  and removal of the carrier film may be performed in any order. 
     When producing the resin layer  2   e , a resin slurry may be applied to one or both of a first main surface and a second main surface of the unsintered plate body  1   s  instead of producing a resin sheet. 
     The resin layer  2   e  has a thickness after drying of not less than 10 μm and not more than 18 μm, for example. 
     Producing Unpressed Body 
       FIG. 10  is a schematic cross-sectional view of an example of producing an unpressed body.  FIG. 11  is a schematic perspective view of the unpressed body in  FIG. 10 . 
     As shown in  FIG. 10 , the unsintered plate body  1   s  containing a ceramic material powder and the resin layers  2   e  containing a resin powder  2   b  are stacked in the thickness direction (stacking direction) to produce an unpressed body  10   b . More specifically, the resin layer  2   e  is stacked on each of a first main surface (a top surface in  FIG. 10 ) and a second main surface (a bottom surface in  FIG. 10 ) of the unsintered plate body  1   s  to produce the unpressed body  10   b . In this step, the resin layer  2   e  may be stacked only on one of the first main surface and the second main surface of the unsintered plate body  1   s.    
     As shown in  FIG. 11 , the unpressed body  10   b  has a first surface  10   b A and a second surface  10   b B opposing each other in the thickness direction and side surfaces  10   b C parallel to the thickness direction. 
     The unpressed body  10   b  has a thickness of not less than 2 mm and not more than 8 mm, for example. 
     Producing Unsintered Body 
       FIG. 12  is a schematic cross-sectional view of an example of producing an unsintered body.  FIG. 13  is a schematic plan view of the assembly in  FIG. 12 .  FIG. 14  is another schematic cross-sectional view of the example of producing an unsintered body.  FIG. 15  is yet another schematic cross-sectional view of the example of producing an unsintered body. 
     First, as shown in  FIG. 12  and  FIG. 13 , the unpressed body  10   b  is sandwiched between a first metal plate  21  on the first surface  10   b A and a second metal plate  22  on the second surface  10   b B and is surrounded by a plate frame  23  around the side surfaces  10   b C, so that an assembly  30  is produced. 
     In a plan view as shown in  FIG. 13 , the shape of the first metal plate  21  and the shape of the unpressed body  10   b  are preferably similar to each other, and the shape of the second metal plate  22  and the shape of the unpressed body  10   b  are preferably similar to each other. 
     In a plan view as shown in  FIG. 13 , the shape of the area surrounded by the inner edge of the plate frame  23  and the shape of the unpressed body  10   b  are preferably similar to each other. 
     The first metal plate  21  and the second metal plate  22  may each be made of, for example, a metal such as stainless steel (SUS). 
     The first metal plate  21  and the second metal plate  22  may be made of the same material or different materials. 
     The first metal plate  21  and the second metal plate  22  each preferably have a thickness of not more than 3 mm. Also, the first metal plate  21  and the second metal plate  22  each preferably have a thickness of not less than 1 mm. 
     The first metal plate  21  and the second metal plate  22  may have the same thickness or different thicknesses. 
     The plate frame  23  may be made of, for example, a plastic material such as a metal or an elastic material such as rubber. 
     In production of the assembly  30 , a resin film made of a resin such as polyethylene terephthalate (PET), for example, is preferably disposed between the unpressed body  10   b  and the first metal plate  21  and between the unpressed body  10   b  and the second metal plate  22 . The unpressed body  10   b  easily adheres to the first metal plate  21  and the second metal plate  22 . Thus, with a resin film in between them, the assembly  30  is easily disassembled in a subsequent step. 
     Next, as shown in  FIG. 14 , the assembly  30  vacuum sealed in the bag  40  is sunk in water  60  in a pressure vessel  50 . Subsequently, the water  60  is pressurized by a pump  70 . Thus, a predetermined hydrostatic pressure is applied to the unpressed body  10   b  to press together the unsintered plate body  1   s  and the resin layers  2   e  by the hydrostatic pressure. 
     The bag  40  is made of a material such as a resin, for example. 
     As described above, an unsintered body  10   g  is produced in which the unsintered plate body  1   s  and the resin layers  2   e  are pressed together as shown in  FIG. 15 . The assembly  30  is then disassembled to take out the unsintered body  10   g.    
     When the unsintered plate body  1   s  and the resin layers  2   e  are pressed together by hydrostatic pressure, the resin layers  2   e  are pressed onto the first main surface and the second main surface of the unsintered plate body  1   s . This forms scattered recesses having a shape derived from the shape of the resin powder  2   b  on the first main surface and the second main surface of the unsintered plate body  1   s.    
     In production of the unsintered body  10   g , the unsintered plate body  1   s  and the resin layers  2   e  are pressed together such that an elongation of a length of the unsintered body  10   g  relative to a length of the unpressed body  10   b  in a direction perpendicular to the thickness direction, i.e., in a surface direction, is within ±1.0%. This makes the outer edge of the unsintered plate body  1   s  less apt to spread out. As a result, the variation of density of the unsintered plate body  1   s  in the unsintered body  10   g  is prevented or reduced, so that the variation of heat shrinkage degree inside the unsintered plate body  1   s  is prevented or reduced during the later-described sintering of the unsintered plate body  1   s . Thus, when the unsintered plate body  1   s  is sintered into a ceramic plate body  10   p  as described later, warpage of the ceramic plate body  10   p  in the vicinity of the outer edge thereof is prevented or reduced. Breakages such as cracking or chipping can therefore be prevented during production of a unit cell for solid oxide fuel cells including an electrolyte sheet made of such a ceramic plate body  10   p . Also, gas leakage can be prevented during operations of a solid oxide fuel cell including the electrolyte sheet made of the ceramic plate body  10   p . Thus, the solid oxide fuel cell can exhibit a higher power generation efficiency. 
     The expression “an elongation of a length of the unsintered body  10   g  relative to a length of the unpressed body  10   b  in a direction perpendicular to the thickness direction is within ±1.0%” means that the elongation of a length of the unsintered body  10   g  relative to a length of the unpressed body  10   b  in at least one direction perpendicular to the thickness direction is within ±1.0%. 
     The elongation (unit: %) of a length of the unsintered body  10   g  relative to a length of the unpressed body  10   b  is defined by the following formula (M). 
       Elongation=100×(“length of unsintered body 10 g ”−“length of unpressed body 10 b ”)/“length of unpressed body 10 b”   (M)
 
     The length of the unpressed body  10   b  and the length of the unsintered body  10   g  may each be measured with, for example, a video measuring system “NEXIV VMZ-R6555” (available from Nikon Instech Co., Ltd. 
     A positive elongation means that the unsintered body  10   g  became longer than the unpressed body  10   b  in a direction perpendicular to the thickness direction. A negative elongation means that the unsintered body  10   g  became shorter than the unpressed body  10   b  in a direction perpendicular to the thickness direction. 
     In order to control the elongation of the unsintered body  10   g  to be within ±1.0%, the unpressed body  10   b  is surrounded by the plate frame  23  around the side surfaces  10   b C, so that the plate frame  23  functions as a holding member for the unpressed body  10   b , more specifically for the unsintered plate body  1   s . Then, the conditions including the material(s) used to produce the plate frame  23 , the width of the plate frame  23  in a plan view, and the similarity ratio of the area surrounded by the inner edge of the plate frame  23  to the unpressed body  10   b  in a plan view are adjusted. The preferred conditions are described below. 
     The plate frame  23  is preferably made of a metal such as stainless steel. 
     When made of a metal, the plate frame  23  preferably has a width W of not less than 1 mm in a plan view as shown in  FIG. 13 . When the plate frame  2  is made of a metal and has a width W of less than 1 mm, the elongation of the unsintered body  10   g  described above may not be within ±1.0% because the plate frame  23  tends to be significantly deformed during production of the unsintered body  10   g.    
     When made of rubber, the plate frame  23  preferably has a width W of not less than 2 mm in a plan view as shown in  FIG. 13 . When the plate frame  23  is made of rubber and has a width W of less than 2 mm, the elongation of the unsintered body  10   g  described above may not be within ±1.0% because the plate frame  23  tends to be significantly deformed during production of the unsintered body  10   g.    
     The width W of the plate frame means the shortest distance between the inner edge and the outer edge of the plate frame. 
     When the plate frame  23  is made of a metal, the similarity ratio of the area surrounded by the inner edge of the plate frame  23  to the unpressed body  10   b  in a plan view as shown in  FIG. 13  is preferably not less than 1 and not more than 1.02, particularly preferably 1. In  FIG. 13 , the similarity ratio of the area surrounded by the inner edge of the plate frame  23  to the unpressed body  10   b  is 1, i.e., there is no gap between the unpressed body  10   b  and the plate frame  23 . When the similarity ratio of the area surrounded by the inner edge of the plate frame  23  to the unpressed body  10   b  is more than 1, there is a gap between the unpressed body  10   b  and the plate frame  23 . When the plate frame  23  is made of a metal and the similarity ratio of the area surrounded by the inner edge of the plate frame  23  to the unpressed body  10   b  is more than 1.02, there is a large gap between the unpressed body  10   b  and the plate frame  23 , and thus the elongation of the unsintered body  10   g  may not be within ±1.0%. 
     When the plate frame  23  is made of rubber, the similarity ratio of the area surrounded by the inner edge of the plate frame  23  to the unpressed body  10   b  in a plan view as shown in  FIG. 13  is preferably not less than 1 and not more than 1.02, particularly preferably 1. When the plate frame  23  is made of rubber and the similarity ratio of the area surrounded by the inner edge of the plate frame  23  to the unpressed body  10   b  is more than 1.02, there is a large gap between the unpressed body  10   b  and the plate frame  23 , and thus the elongation of the unsintered body  10   g  may not be within ±1.0%. 
     Specifically, as for the similarity ratio, the ratio of the inside dimension F of the plate frame  23  to the length E of the unpressed body  10   b  as shown in  FIG. 13  is preferably not less than 100% and not more than 102%, particularly preferably 100%.  FIG. 13  shows the case where the length E of the unpressed body  10   b  and the inside dimension F of the plate frame  23  are the same as each other. 
     The first metal plate  21  and the second metal plate  22  preferably have the same size as the area surrounded by the inner edge of the plate frame  23  in a plan view. In other words, in a plan view as shown in  FIG. 13 , the similarity ratio of the first metal plate  21  to the unpressed body  10   b  and the similarity ratio of the second metal plate  22  to the unpressed body  10   b  are each preferably not less than 1 and not more than 1.02, particularly preferably 1. 
     In the present step, hydrostatic pressure is used as the pressurization method, which applies uniform pressure to the unpressed body  10   b , more specifically to the unsintered plate body  1   s . Thus, use of hydrostatic pressure as the pressurization method contributes to prevention or reduction of variation of density of the unsintered plate body  1   s . In this viewpoint, hydrostatic pressure is preferably used as the pressurization method. Although the present step employs hydrostatic pressure as the pressurization method, pressurization may be performed by any other method. 
     The unsintered body  10   g  may be cut into a shape back-calculated from a possible heat shrinkage rate of the unsintered plate body  1   s  during firing of the unsintered body  10   g  (described later) and the desired shape of a ceramic plate body  10   p  to be obtained. 
     Forming Unsintered Body Through Hole 
       FIG. 16  is a schematic cross-sectional view of an example of forming an unsintered body through hole.  FIG. 17  is another schematic cross-sectional view of the example of forming an unsintered body through hole. 
     As shown in  FIG. 16  and  FIG. 17 , unsintered body through holes  10   gh  penetrating the unsintered body  10   g  in a thickness direction thereof are formed. 
     The unsintered body through holes  10   gh  are preferably formed with at least one drill DR. In this case, the unsintered body  10   g  is drilled with the drill DR from its first main surface to its second main surface or vice versa such that the unsintered body through holes  10   gh  penetrating the unsintered body  10   g  in the thickness direction are formed. The drilling with the drill DR may be performed under any conditions. 
     Only one unsintered body through hole  10   gh  may be formed or two or more unsintered body through holes  10   gh  may be formed. 
     Producing Ceramic Plate Body 
       FIG. 18  is a schematic cross-sectional view of an example of producing a ceramic plate body. 
     The unsintered body  10   g  is fired to burn off the resin layers  2   e  and sinter the unsintered plate body  1   s  into a ceramic plate body  10   p  provided with through holes  10   h  penetrating the ceramic plate body  10   p  in the thickness direction as shown in  FIG. 18 . 
     As described above, the variation of density of the unsintered plate body  1   s  is prevented or reduced during firing the unsintered body  10   g , so that the variation of heat shrinkage degree inside the unsintered plate body  1   s  is prevented or reduced. Thus, warpage of a ceramic plate body  10   p  to be obtained is prevented or reduced in the vicinity of the outer edge thereof. 
     Preferably, the firing the unsintered body  10   g  includes degreasing and sintering. 
     Thus, the ceramic plate body  10   p  is produced which is provided with the through holes  10   h  penetrating the ceramic plate body  10   p  in the thickness direction. 
     In the producing of the unsintered body in the method of producing an electrolyte sheet, the unsintered plate body  1   s  and the resin layers  2   e  are pressed together in a state where the unpressed body  10   b  is sandwiched between the first metal plate  21  on the first surface  10   b A and the second metal plate  22  on the second surface  10   b B and is surrounded by the plate frame  23  around the side surfaces  10   b C, such that an elongation of a length of the unsintered body  10   g  relative to a length of the unpressed body  10   b  in a direction perpendicular to the thickness direction is within ±1.0%. 
     Thus, in the ceramic plate body  10   p  provided with the through holes  10   h , as described above with reference to  FIG. 1 , the distance between the first point P 1  and the second point P 2  is not shorter than 1 mm and not longer than 5 mm, and the warpage height in the area between the first point P 1  and the second point P 2  is not more than 150 μm. In other words, the method of producing an electrolyte sheet described above enables the first embodiment of the electrolyte sheet for solid oxide fuel cells of the present invention which is made of the ceramic plate body  10   p  (for example, the electrolyte sheet  10  in  FIG. 1  and  FIG. 2 ). 
     Also, in the ceramic plate body  10   p  provided with the through holes  10   h , as described above with reference to  FIG. 1 , the distance between the first point P 1  and the second point P 2  is not less than 0.5% and not more than 10.0% of the distance between the first point P 1  and the third point P 3 , and the warpage height in the area between the first point P 1  and the second point P 2  is not more than 150 μm. In other words, the method of producing an electrolyte sheet described above enables the second embodiment of the electrolyte sheet for solid oxide fuel cells of the present invention which is made of the ceramic plate body  10   p  (for example, the electrolyte sheet  10  shown in  FIG. 1  and  FIG. 2 ). 
     In the method of producing an electrolyte sheet above, the ceramic plate body  10   p  is produced which is provided with scattered recesses on the first main surface and the second main surface. In the method of producing an electrolyte sheet above, the resin layers  2   e  were used to form such recesses. Yet, recesses may be formed on the first main surface and the second main surface of the unsintered plate body with a stamper in advance before sintering of the unsintered plate body. In this case, an unpressed body is produced using the unsintered plate body provided with recesses in the producing an unpressed body, an unsintered body is produced by pressing the unsintered plate body through pressing of the unpressed body, preferably by hydrostatic press, in the producing an unsintered body, and a ceramic plate body is produced through firing the unsintered body to sinter the unsintered plate body in the producing a ceramic plate body. 
     Unit Cell for Solid Oxide Fuel Cells 
     The following describes an example of the unit cell for solid oxide fuel cells of the present invention.  FIG. 19  is a schematic cross-sectional view of an example of the unit cell for solid oxide fuel cells of the present invention. 
     As shown in  FIG. 19 , a unit cell  100  for solid oxide fuel cells includes a fuel electrode  110 , an air electrode  120 , and an electrolyte sheet  130 . The electrolyte sheet  130  is disposed between the fuel electrode  110  and the air electrode  120 . 
     The fuel electrode  110  may be a known fuel electrode for solid oxide fuel cells. 
     The air electrode  120  may be a known air electrode for solid oxide fuel cells. 
     The electrolyte sheet  130  is the electrolyte sheet for solid oxide fuel cells of the present invention (for example, the electrolyte sheet  10  in  FIG. 1  and  FIG. 2 ). Thus, the unit cell  100  is capable of increasing the power generation efficiency of a solid oxide fuel cell with the unit cell. 
     Method of Producing Unit Cell for Solid Oxide Fuel Cells 
     The unit cell for solid oxide fuel cells of the present invention is produced by the following method, for example. 
     First, a slurry for a fuel electrode and a slurry for an air electrode are prepared. The slurry for a fuel electrode is prepared by mixing a powder of a material of a fuel electrode with a binder, a dispersant, a solvent, and the like as appropriate. The slurry for an air electrode is prepared by mixing a powder of a material of an air electrode with a binder, a dispersant, a solvent, and the like as appropriate. 
     The material of a fuel electrode may be a known material of a fuel electrode for solid oxide fuel cells. 
     The material of an air electrode may be a known material of an air electrode for solid oxide fuel cells. 
     The binder, dispersant, solvent, and other additives in a slurry for a fuel electrode may be those known in a method of forming a fuel electrode for solid oxide fuel cells. The binder, dispersant, solvent, and other additives in a slurry for an air electrode may be those known in a method of forming an air electrode for solid oxide fuel cells. 
     Then, a first main surface of the electrolyte sheet is coated with the slurry for a fuel electrode to a predetermined thickness and a second main surface of the electrolyte sheet is coated with the slurry for an air electrode to a predetermined thickness. These coating films are dried to form a green layer for a fuel electrode and a green layer for an air electrode. 
     The green layer for a fuel electrode and the green layer for an air electrode are then fired to form a fuel electrode and an air electrode. The firing conditions such as the firing temperature may be determined as appropriate depending on the material and the like of the fuel electrode or the air electrode. 
     EXAMPLES 
     Examples that more specifically disclose the electrolyte sheet for solid oxide fuel cells of the present invention are described below. The present invention is not limited to these examples. 
     Example 1 
     An electrolyte sheet of Example 1 was produced by the following method. 
     Producing Ceramic Green Sheets 
     Scandia-stabilized zirconia powder, a binder, a dispersant, and an organic solvent were compounded at a predetermined ratio. The organic solvent used was a 7:3 mixture by weight of toluene and ethanol. The compounded product was stirred with a medium made of partially stabilized zirconia at 1000 rpm for three hours to form a ceramic slurry. 
     Then, the ceramic slurry was formed into a sheet on a first main surface of a carrier film made of polyethylene terephthalate by a known technique of tape casting to give ceramic green tape. 
     The ceramic green tape was then punched by a known technique into pieces having a 100-mm-square shape in a plan view thereof after the firing, and the carrier film was removed from the pieces. Thus, ceramic green sheets were produced. 
     Producing Unsintered Plate Body 
     Three ceramic green sheets were stacked and compression-bonded to produce an unsintered plate body. 
     Producing Resin Layer 
     First, a resin powder, a binder, a dispersant, and an organic solvent were compounded at a predetermined ratio. The resin powder used was a spherical resin powder made of a crosslinked acrylic resin and having a median size D 50  of 0.3 μm. The organic solvent used was a 7:3 mixture by weight of toluene and ethanol. The compounded product was stirred with a medium made of partially stabilized zirconia at 1000 rpm for three hours to prepare a resin slurry. 
     Then, the resin slurry was formed into a sheet on a first main surface of a carrier film made of polyethylene terephthalate by a known technique of tape casting to give resin tape. 
     The resin tape was then punched by a known technique into pieces having the same size as the ceramic green sheets in a plan view, and the carrier film was removed from the pieces. Thus, resin sheets as resin layers were produced. Each resin sheet had a thickness after drying of not less than 10 μm and not more than 18 μm. 
     Producing Unpressed Body 
     The unsintered plate body and the resin sheets were stacked in the thickness direction (stacking direction) to produce an unpressed body. Here, the unpressed body had a first surface and a second surface opposing each other in the thickness direction and side surfaces parallel to the thickness direction. More specifically, the resin sheet was stacked on each of the first main surface and the second main surface of the unsintered plate body to produce an unpressed body. Thus, the resin sheet was disposed on each of the first surface of the unpressed body and the second surface of the unpressed body. In this step, 100 such unpressed bodies were produced. 
     Producing Unsintered Body 
     First, each of the 100 unpressed bodies was sandwiched between a first metal plate on the first surface and a second metal plate on the second surface and was surrounded by a plate frame around the side surfaces such that an assembly was produced. In a plan view thereof, the area surrounded by the inner edge of the plate frame and the unpressed body were both square-shaped and similar to each other. In production of the assembly, a polyethylene terephthalate film was disposed between the unpressed body and the first metal plate and between the unpressed body and the second metal plate. 
     The first metal plate and the second metal plate were each made of a stainless steel plate. The first metal plate and the second metal plate each had a thickness of 2 mm. The first metal plate and the second metal plate had the same size as the unpressed body in a plan view. In other words, in a plan view, the similarity ratio of the first metal plate to the unpressed body was 1, and the similarity ratio of the second metal plate to the unpressed body was 1. 
     The plate frame used was a stainless steel plate frame. In a plan view, the plate frame had a width of 2 mm. Th area surrounded by the inner edge of the plate frame had the same size as the unpressed body in a plan view, i.e., the same size as the first metal plate and the second metal plate in a plan view. In other words, in a plan view, the similarity ratio of the area surrounded by the inner edge of the plate frame to the unpressed body was 1. 
     Next, the assembly vacuum sealed in a plastic bag was sunk in water at 60° C. in a pressure vessel. Subsequently, the water was pressurized by a pump to apply a hydrostatic pressure of 1500 kgf/cm 2  (150 MPa) to the unpressed body, whereby the unsintered plate body and the resin layers were pressed together by hydrostatic pressure. The pressurization temperature was 60° C. 
     As described above, an unsintered body was produced in which the unsintered plate body and the resin layers were pressed together. Then, the assembly was disassembled to take out the unsintered body. 
     In the present step, the above procedure was performed on each of the 100 unpressed bodies, so that 100 unsintered bodies were produced. In other words, in the present step, the cycle of producing one unsintered body from one unpressed body was repeated 100 times. 
     Forming Unsintered Body Through Hole 
     In each of the 100 unsintered bodies, four unsintered body through holes penetrating the unsintered body in the thickness direction thereof were formed by drilling. The drilling was performed at a feed rate of 0.04 mm/revolution and a spindle speed of 2000 revolutions/minute. The unsintered body through holes were formed such that the through holes would oppose the centers of the respective four sides of the unsintered plate body and, in the ceramic plate body to be obtained by firing the unsintered body, the shortest distance between the outer edge of the ceramic plate body and the peripheral edge of each through hole would be 3 mm. Each unsintered body through hole had a hole diameter after the firing of 15 mm. 
     Producing Ceramic Plate Body 
     Each of the 100 unsintered bodies was fired in a furnace as follows. First, the unsintered body was degreased by holding the unsintered body at 400° C. for a predetermined time. The degreased unsintered body was sintered by holding the unsintered body at 1400° C. for five hours. As described above, the unsintered body was fired to burn off the resin layers and sinter the unsintered plate body into a ceramic plate body provided with four through holes penetrating the ceramic plate body in the thickness direction. In this step, each of the 100 unsintered bodies was fired, so that 100 ceramic plate bodies were produced. 
     Each ceramic plate body had a 100-mm-square shape in a plan view thereof and a thickness of 90 μm. The four through holes in each ceramic plate body were each formed at a position where the through hole opposes the center of the corresponding side among the four sides of the ceramic plate body. In each ceramic plate body, the shortest distance between the outer edge of the ceramic plate body and the peripheral edge of each of the four through holes was 3 mm. In other words, the shortest distance was 3% of the length of one side of the ceramic plate body. Each through hole had a hole diameter of 15 mm. 
     Thus, 100 electrolyte sheets (ceramic plate bodies) of Example 1 were produced. 
     Example 2 
     One hundred electrolyte sheets of Example 2 were produced as in Example 1, except that the conditions of the producing an unsintered body were changed to the following conditions. 
     In a plan view, the similarity ratio of the first metal plate to the unpressed body was 1.01. 
     In a plan view, the similarity ratio of the second metal plate to the unpressed body was 1.01. 
     In a plan view, the similarity ratio of the area surrounded by the inner edge of the plate frame to the unpressed body was 1.01. 
     Example 3 
     One hundred electrolyte sheets of Example 3 were produced as in Example 1, except that the conditions of the producing an unsintered body were changed to the following conditions. 
     The plate frame used was one made of natural rubber. 
     Example 4 
     One hundred electrolyte sheets of Example 4 were produced as in Example 1, except that the conditions of the producing an unsintered body were changed to the following conditions. 
     The plate frame used was one made of natural rubber. 
     In a plan view, the plate frame had a width of 3 mm. 
     Comparative Example 1 
     One hundred electrolyte sheets of Comparative Example 1 were produced as in Example 1, except that the conditions of the producing an unsintered body were changed to the following conditions. 
     In a plan view, the similarity ratio of the first metal plate to the unpressed body was 1.03. 
     In a plan view, the similarity ratio of the second metal plate to the unpressed body was 1.03. 
     In a plan view, the similarity ratio of the area surrounded by the inner edge of the plate frame to the unpressed body was 1.03. 
     Comparative Example 2 
     One hundred electrolyte sheets of Comparative Example 2 were produced as in Example 1, except that the conditions of the producing an unsintered body were changed to the following conditions. 
     The plate frame used was one made of natural rubber. 
     In a plan view, the plate frame had a width of 1 mm. 
     Evaluations 
     The electrolyte sheets of Examples 1 to 4 and Comparative Examples 1 and 2 were subjected to the following evaluations. 
     Elongation of Unsintered Body 
     In the producing an unsintered body, in each of the 100 cycles above, the length of the unpressed body and the length of the unsintered body in a direction perpendicular to the thickness direction were measured with a video measuring system “NEXIV VMZ-R6555” (available from Nikon Instech Co., Ltd.). Here, for the length of the unpressed body and the length of the unsintered body, the length of a specific side of a square shape in a plan view before pressurization and the length of the specific side of the square shape in the plan view after the pressurization were selected. The elongation of the unsintered body relative to the unpressed body in each of the 100 cycles was calculated from the formula (M) above. Table 1 shows the calculated elongations and the average of the elongations in each cycle. 
     Warpage Height 
     On each of the 100 electrolyte sheets, the warpage height in the area between the first point P 1  and the second point P 2  was measured for each of the four through holes by the method described above with reference to  FIG. 1 , with a wide-area 3D measurement system “VR-5000” available from Keyence Corporation. Table 1 shows the measured warpage heights at the respective sites and the average of the warpage heights. 
     Cracking and Chipping of Electrolyte Sheet in Unit Cell 
     First, a fuel electrode slurry containing nickel oxide powder and scandia-stabilized zirconia powder was prepared. Next, a first main surface of each electrolyte sheet was coated with the fuel electrode slurry by screen printing. The coating film of the fuel electrode slurry was dried to form a fuel electrode green layer. The fuel electrode green layer was then fired at 1300° C., so that a fuel electrode was formed. The fuel electrode had a thickness of 30 μm. 
     An air electrode slurry containing lanthanum strontium cobalt ferrite (LSCF) powder was prepared. Next, a gadolinia-doped ceria (GDC) layer as a barrier layer was formed on a second main surface of the electrolyte sheet, followed by coating of the barrier layer with the air electrode slurry by screen printing. The barrier layer had a thickness of 10 μm. The coating film of the air electrode slurry was dried to form an air electrode green layer. The air electrode green layer was then fired at 1000° C., so that an air electrode was formed. The air electrode had a thickness of 30 μm. 
     As described above, 100 unit cells were produced from the 100 electrolyte sheets. All the unit cells were visually checked for cracking and/or chipping of the electrolyte sheet in the vicinity of the outer edge thereof, and the number of cracked and/or chipped electrolyte sheets was counted. Table 1 shows the results. 
     Gas Leakage from Cell Stack 
     A separator made of a ferrite-based alloy material and provided with a channel was coated with three pastes as follows. The three pastes were: a glass-based sealing paste; a first conductive paste that originally mainly contains nickel oxide and mainly contains nickel after the reduction reaction; and a second conductive paste that contains a conductive oxide such as lanthanum strontium manganite (LSM). The peripheral edge of each main surface of the separator was coated with the glass-based sealing paste. The central area of the main surface of the separator in contact with the fuel electrode was coated with the first conductive paste. The central area of the main surface of the separator in contact with the air electrode was coated with the second conductive paste. Thus, 120 separators coated with the three pasts were produced. 
     Next, one cell stack was produced by alternately stacking six separators out of the 120 separators and five unit cells out of the 100 unit cells, so that 20 cell stacks were produced. In production of each cell stack, the separators and the unit cells were bonded to each other by applying a weight of about 5 kg and heat-processing the stack at 900° C. 
     Then, air was supplied to the 20 cell stacks at a pressure of 0.1 kgf/cm 2  (0.01 MPa) to evaluate air leakage. Specifically, an air supply amount of more than 5 cc/min used to maintain the air supply pressure (0.1 kgf/cm 2 ) was regarded as indicating significant air leakage, and the number of cell stacks with such an air supply amount was counted. Table 1 shows the results. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                   
                   
                   
                 Comparative 
                 Comparative 
               
               
                   
                 Example 1 
                 Example 2 
                 Example 3 
                 Example 4 
                 Example 1 
                 Example 2 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Elongation of 
                 Each cycle 
                 Within ±1.0 
                 Within ±1.0 
                 Within ±1.0 
                 Within ±1.0 
                 Over ±1.0 
                 Over ±1.0 
               
               
                 unsintered body (%) 
                 Average 
                 0.0 
                 1.0 
                 −1.0 
                 −0.5 
                 1.8 
                 −2.0 
               
               
                 Warpage height (μm) 
                 Each site 
                 ≤150 
                 ≤150 
                 ≤150 
                 ≤150 
                 &gt;150 
                 &gt;150 
               
               
                   
                 Average 
                 10 
                 70 
                 25 
                 95 
                 180 
                 360 
               
               
                 Cracking/chipping 
                 Chipping 
                 0 
                 0 
                 0 
                 0 
                 2 
                 3 
               
               
                 of electrolyte sheet 
                 Cracking 
                 0 
                 0 
                 0 
                 0 
                 4 
                 5 
               
               
                 in unit cell (number 
               
               
                 of electrolyte sheets) 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Gas leakage from cell stack 
                 0 
                 0 
                 0 
                 0 
                 2 
                 4 
               
               
                 (number of cell stacks) 
               
               
                   
               
            
           
         
       
     
     As shown in Table 1, the electrolyte sheets of Examples 1 to 4 had an elongation of the unsintered body of within ±1.0% in the producing an unsintered body, and thus had no or less warpage in the vicinity of the outer edge thereof. The electrolyte sheets of Examples 1 to 4 therefore caused no cracking or chipping when incorporated in a unit cell, and caused no gas leakage when incorporated in a cell stack. 
     As shown in Table 1, the electrolyte sheets of Comparative Examples 1 and 2 had an elongation of the unsintered body over ±1.0% in the producing an unsintered body, and thus caused warpage in the vicinity of the outer edge thereof. The electrolyte sheets of Comparative Examples 1 and 2 therefore caused cracking and/or chipping when incorporated in a unit cell and caused gas leakage when incorporated in a cell stack. 
     REFERENCE SIGNS LIST 
     
         
         
           
               1   g  ceramic green sheet 
               1   s  unsintered plate body 
               1   t  ceramic green tape 
               2   b  resin powder 
               2   e  resin layer 
               2   t  resin tape 
               10 ,  130  electrolyte sheet for solid oxide fuel cells (electrolyte sheet) 
               10   b  unpressed body 
               10   b A first surface of unpressed body 
               10   b B second surface of unpressed body 
               10   b C side surface of unpressed body 
               10   g  unsintered body 
               10   gh  unsintered body through hole 
               10   h  through hole 
               10   p  ceramic plate body 
               21  first metal plate 
               22  second metal plate 
               23  plate frame 
               30  assembly 
               40  bag 
               50  pressure vessel 
               60  water 
               70  pump 
               100  unit cell for solid oxide fuel cells (unit cell) 
               110  fuel electrode 
               120  air electrode 
             DR drill 
             E length of unpressed body 
             F inside dimension of plate frame 
             L virtual straight line 
             P 1  first point 
             P 2  second point 
             P 3  third point 
             W width of plate frame 
             X casting directions 
             Y directions perpendicular to casting directions