JP4914588B2 - Method for producing zirconia sheet - Google Patents

Description

  The present invention relates to a zirconia sheet used as an electrolyte for a fuel cell, a method for producing the zirconia sheet, and a method for measuring residual carbon in the zirconia sheet.

  Ceramic sheets mainly composed of zirconia have excellent mechanical strength, toughness, wear resistance, chemical resistance, corrosion resistance, and the like, and thus are used in various structural materials, blades, firing setters, and the like. In addition, since it has high oxygen ion conductivity, it has recently been utilized as a solid electrolyte for fuel cells.

  This zirconia sheet is generally produced by the following method. First, a slurry obtained by adding a solvent to a mixture containing zirconia particles and a binder is molded by a doctor blade method or an extrusion molding method, and then the solvent is evaporated and scattered to obtain a zirconia green sheet. Next, the organic compound such as a binder is decomposed and removed by heating usually at about 200 to 500 ° C. Then, a dense zirconia sheet is obtained by further sintering at 1300 to 1500 ° C.

  In industrially producing zirconia sheets, it is conceivable to increase the zirconia green sheets per unit volume of the firing furnace and the unit residence time in the firing furnace in the heating step in order to increase productivity. That is, heating is performed by laminating a plurality of zirconia green sheets to increase the number occupied in the effective volume of the firing furnace and to increase the rate of temperature rise. However, under such manufacturing conditions, the larger the firing furnace, the more uneven the heat is in each part of the green sheet. As a result, warping and undulation may occur in the finally obtained sheet.

  Therefore, the present inventors have developed a technique for sandwiching a green sheet between porous sheets in order to suppress warpage and the like when the green sheets are laminated and sintered (Patent Document 1). In addition, when firing ceramic green sheets after stacking multiple shelves, a technology that suppresses variations in the dimensions of the ceramic sheets by specifying the shelves, the volume occupied by the space, and the space between the shelves. (Patent Document 2). Furthermore, as a method for minimizing the occurrence of warpage during firing, Patent Document 3 discloses a technique for controlling an average temperature increase rate during firing using an organic component or the like contained in a green sheet as an index.

  However, the present inventors have found that another problem may arise when using a mass-produced zirconia sheet as an electrolyte for a fuel cell.

  That is, when a large amount of zirconia green sheet is industrially fired, a large amount of decomposition gas or the like generated by decomposition or disappearance of an organic component such as a binder, or a large amount of oxygen is consumed for this purpose, Since the oxygen concentration is greatly reduced, the organic component may remain without being completely decomposed and removed. This organic component remains as carbon between the zirconia particles in the sintering process. And since this residual carbon has electrical conductivity, the transport number is lowered by giving electrical conductivity to the electrolyte as a result. Furthermore, there arises a problem that a solid-phase reaction with zirconia is generated to generate zirconia carbide, thereby reducing the oxygen ion conductivity of the electrolyte.

In recent years, dense ceramic sheets made from submicron ceramic fine powders have been used favorably. However, when such high surface area fine powders are used as raw materials, many binders are needed for green sheet molding. In particular, the organic components are hardly decomposed and removed. Accordingly, a means for solving the deterioration of characteristics due to residual carbon is desired.
JP-A-8-151275 (Claims, FIG. 3) JP 2001-206777 A (Claims) JP 2001-247373 A (Claims)

  As described above, a technique for suppressing warpage and dimensional variation has been known in the industrial production of zirconia sheets by firing a large amount of zirconia green sheets. However, the mass-produced zirconia sheet has electrical conductivity, so that a short-circuit current in the opposite direction to oxygen ions is generated in the zirconia sheet, reducing the electromotive force generated on both sides of the sheet, and reducing the oxygen ion conductivity. In some cases, the characteristics of the fuel cell electrolyte deteriorate. In addition, as an evaluation of the characteristics required for such an electrolyte, it is inefficient to measure electric conductivity, residual carbon, etc. one by one.

  Therefore, the problem to be solved by the present invention is a method for producing a zirconia sheet that does not give up its properties as an electrolyte, even when mass-producing a zirconia sheet used as an electrolyte for a fuel cell, and a mass produced product. In spite of the fact, there is a need to provide a zirconia sheet having excellent characteristics as an electrolyte. Another object of the present invention is to provide a method for easily measuring the properties of a zirconia sheet as an electrolyte.

  In order to solve the above-mentioned problems, the present inventor has conducted extensive research on the mass production conditions of zirconia sheets. As a result, when zirconia sheets are mass-produced, the characteristics of the electrolyte are deteriorated due to the fact that a large amount of oxygen is consumed at one time and a large amount of cracked gas is generated in the process of decomposing and removing organic components by heating. As a result, it was found that the organic component was not sufficiently decomposed. Therefore, it has been found that in mass production of zirconia sheets, it is important to supply oxygen sufficiently in the firing process so as not to cause oxygen shortage around the green sheets.

  Further, the present invention was completed by finding that the characteristics of the zirconia sheet as a fuel cell electrolyte can be judged very simply and easily by measuring the whiteness.

  That is, the method for producing a zirconia sheet according to the present invention is characterized in that the degreasing step of the zirconia green sheet that is a precursor of the zirconia sheet is performed in an atmosphere having an oxygen concentration of 13% or more.

  In the above method, the oxygen concentration is preferably 15% or more. This is because the decomposition of the organic component is more surely promoted and the residual carbon is suppressed.

  The zirconia sheet of the present invention was measured in accordance with JIS 8148 (2001) for an electrolyte sheet laminate in which a zirconia sheet produced under the same conditions was laminated under the zirconia sheet to a thickness of 2 mm or more. The whiteness value is 80% or more. The zirconia sheet having a whiteness in the above range has residual carbon remarkably suppressed and is useful as a fuel cell electrolyte.

  As the zirconia sheet, those made of cubic zirconia and having a whiteness value of 82% or more, and those made of tetragonal zirconia and having a whiteness value of 84% or more are suitable. This is because these zirconia sheets are particularly excellent as a fuel cell electrolyte.

  The method for measuring the residual carbon of a zirconia sheet according to the present invention is a JIS 8148 (2001) in which a zirconia sheet manufactured under the same conditions is laminated under a zirconia sheet to be measured to form an electrolyte sheet laminate having a thickness of 2 mm or more. ) Is used as an index.

  In the present invention, residual carbon of the zirconia sheet is indirectly measured by whiteness. This method is much easier and simpler than directly measuring electrical conductivity, ionic conductivity, and the like. Further, according to the method for producing a zirconia sheet according to the present invention, residual carbon is suppressed, mass production of zirconia sheets having an excellent characteristic as a fuel cell electrolyte that is an electrical insulator and has high ionic conductivity. Can do. Therefore, the present invention is extremely useful industrially as a technique for mass production of zirconia sheets suitable as an electrolyte membrane for fuel electrons.

  The manufacturing method of the zirconia sheet which concerns on this invention is characterized by performing the degreasing | defatting process of a zirconia green sheet in the atmosphere whose oxygen concentration is 13% or more.

  In the production method of the present invention, first, a zirconia green sheet is produced. The production method is not particularly limited, and a conventional method may be applied. For example, zirconia raw material particles are coated with a slurry containing an organic binder and a solvent, and if necessary, a dispersant or a plasticizer, on a smooth sheet, such as a polyester sheet, by a doctor blade method and dried to evaporate the solvent. There is a way to remove it. In addition, you may manufacture by the extrusion method from the kneaded material of the fuse | melted binder and zirconia raw material particle | grains.

  The zirconia particles used as a raw material may be produced by a conventional method, or commercially available ones may be used. However, it is preferable to use a finer average particle diameter. If zirconia particles with a small average particle size are used as raw materials, a dense zirconia sheet with good sinterability can be obtained, but carbon tends to remain in the conventional method, so use relatively large zirconia particles. I had to do it. However, since the present invention solves such a problem, according to the present invention, it is possible to utilize the advantages of zirconia particles having a small average particle diameter.

  Moreover, as zirconia particle | grains, the thing with a uniform particle diameter is suitable. Specifically, zirconia particles having an average particle size of 0.01 to 0.5 μm and 90% by volume or more of particles having a particle size of 1.5 μm or less are preferable. More preferably, the average particle diameter is 0.05 to 0.3 μm, and 90% by volume or more of the particles have a particle diameter of 1 μm or less, and more preferably the average particle diameter is 0.08 to 0.2 μm, 90 volume. % Or more of particles having a particle size of 0.3 μm or more and 0.8 μm or less is preferable. The average particle diameter may be measured by a conventional method, for example, using a laser diffraction particle size distribution measuring apparatus LA-920 manufactured by Horiba, Ltd., and measuring with 0.2 mass% sodium metaphosphate aqueous solution as a dispersion medium. do it.

Higher thermal, mechanical, electrical, and chemical properties are required for zirconia sheets used for solid electrolyte membranes in fuel cells. Examples of the electrolyte material that satisfies these required characteristics include zirconia, alkaline earth metal oxides such as MgO, CaO, SrO, and BaO; Y ₂ O ₃ , La ₂ O ₃ , CeO ₂ , Pr ₂ O ₃ , and Nd ₂ O. _{3, Sm 2 O 3, Eu} 2 O 3, Gd 2 O 3, Tb 2 O 3, Dy 2 O 3, Ho 2 O 3, Er 2 O 3, Yb rare earth metal oxides such as ₂ O _3; Sc ₂ Examples include zirconia-based ceramics to which other metal oxides such as O ₃ , Bi ₂ O ₃ , In ₂ O _{3 and the} like are added, and one or more of them are selected and used alone or as a mixture Can do. Furthermore, it is recommended as a preferable one that 0.01 to 5% by mass of at least one selected from the group consisting of SiO ₂ , Al ₂ O ₃ and TiO ₂ is blended.

Among them, zirconia stabilized with ₃ to 10 mol% Y ₂ O ₃ , or 4 to 12 mol% Sc ₂ O ₃ or 4 to 15 mol% Yb ₂ O ₃ is particularly preferable as the electrolyte. Further, it is zirconia to which about 0.01 to ₂ % by mass of SiO ₂ , Al ₂ O ₃ , TiO ₂ , CeO _{2 and the} like are added.

  There is no particular limitation on the type of binder used for the production of the green sheet, and conventionally known organic binders can be appropriately selected and used. Examples of organic binders include ethylene copolymers, styrene copolymers, acrylate and methacrylate copolymers, vinyl acetate copolymers, maleic acid copolymers, vinyl butyral resins, and vinyl acetal resins. And vinyl formal resins, vinyl alcohol resins, waxes, celluloses such as ethyl cellulose, and the like.

  Among these, it is necessary to have excellent debinding properties in the heating process at 200 to 500 ° C., and also suppress variation in green sheet moldability, punching workability, strength, and shrinkage rate during degreasing and sintering. In view of the above, a (meth) acrylate copolymer polymer having a number average molecular weight of 20,000 to 250,000, more preferably 50,000 to 200,000 is recommended.

  The use ratio of the zirconia particles to the binder is preferably 5 to 30 parts by mass, more preferably 10 to 20 parts by mass with respect to the former 100 parts by mass. When the amount of the binder used is insufficient, the green sheet moldability is lowered, and the strength and flexibility are insufficient. On the other hand, if the amount is too large, not only will it be difficult to adjust the viscosity of the slurry, but the decomposition and release of the binder component during degreasing and sintering will increase and become severe, resulting in large variations in shrinkage ratio, resulting in a sheet with small dimensional variations. In addition, the binder tends to remain as residual carbon.

  When producing a green sheet from a slurry by a doctor blade method, the solvent used is water; alcohols such as methanol, ethanol, 2-propanol, 1-butanol, 1-hexanol; acetone, 2-butanone, etc. Ketones; aliphatic hydrocarbons such as pentane, hexane and heptane; aromatic hydrocarbons such as benzene, toluene, xylene and ethylbenzene; and acetates such as methyl acetate, ethyl acetate and butyl acetate. Is used by appropriately selecting from. These solvents can also be used alone, or two or more of them can be appropriately mixed and used. The amount of these solvents used should be appropriately adjusted in consideration of the viscosity of the slurry at the time of forming the green sheet, and preferably the slurry viscosity is in the range of 1 to 50 Pa · s, more preferably 2 to 20 Pa · s. It is better to make adjustments.

  In the preparation of the slurry, in order to promote peptization and dispersion of the ceramic raw material powder, polyelectrolytes such as polyacrylic acid and ammonium polyacrylate; organic acids such as citric acid and tartaric acid; isobutylene or styrene and maleic anhydride Copolymers and ammonium salts or amine salts thereof; Dispersants comprising copolymers of butadiene and maleic anhydride and ammonium salts thereof; Dibutyl phthalate and dioctyl phthalate for imparting flexibility to green sheets Phthalates such as: plasticizers composed of glycols such as propylene glycol and glycol ethers; and surfactants and antifoaming agents can be added as necessary.

The slurry obtained by mixing the above components is formed into a green sheet by the doctor blade method after adjusting the viscosity. Of course, a kneaded material obtained by adding zirconia raw material particles to a molten binder may be extruded. The obtained green sheet may be punched or cut into an appropriate size by any method. The green sheet may have any shape such as a circle, an ellipse, a square, or a square with R (R), and a similar circle, ellipse, square, or square with an R in the sheet. One or two or more may be included. Furthermore, the area of the sheet is 100 cm ² or more, preferably 150 cm ² or more. In addition, this area means the area of the outer periphery including the area of this hole, when there exists a hole in a sheet | seat. In addition, the thickness of the green sheet is not particularly limited, but the thickness of the finally obtained zirconia sheet is preferably 0.02 to 1 mm, and may be about 0.03 to 1.2 mm.

Next, one or more zirconia green sheets are sandwiched between porous plates, placed in an electric heater-type degreasing furnace, batch-type gas combustion furnace, or tunnel-type continuous electric firing furnace, and heated with an appropriate temperature program. And degrease. At this time, mass production is aimed at in the present invention. In the case of a degreasing furnace, 500 to 50,000 sheets are degreased at a total area of 5 to 500 m ² at a time. More preferably, 1000 to 30,000 sheets are degreased at a time. The internal volume of the degreasing furnace to be used can be 0.5 to 2.5 m ^3, and more preferably 0.8 to ² m ³ . Moreover, in the case of a batch-type baking furnace, 1000-200,000 sheets and a total area of 10-2000 m < ² > are degreased at once. More preferably, 3000 to 100,000 sheets are degreased at a time. As an internal volume of the baking furnace to be used, it can be set as 1-10 m < ³ >, for example, More preferably, it is 1-5 m < ³ >.

  In the prior art, oxygen concentration in the degreasing process has not been sufficiently considered apart from non-oxide green sheets such as AlN. However, when the temperature gradually rises in the degreasing process and the decomposition and removal of organic components such as the binder proceeds, the present inventors consume a large amount of oxygen and cause a temporary oxygen shortage, which causes carbon to remain. I found out. This tendency is particularly strong in the case of mass production where the interval between zirconia green sheets becomes narrow, and the characteristics of the zirconia sheet as a fuel cell electrolyte are deteriorated. Therefore, in the present invention, the oxygen concentration in each part of the surface of each zirconia green sheet is always kept at 13% or more (preferably 15% or more) through a degreasing process in mass production of zirconia sheets. This is because organic components in the zirconia green sheet are decomposed and removed by supplying sufficient oxygen in the degreasing step.

  The conditions for the degreasing step of the zirconia green sheet can be those of a conventional method. In general, the temperature is set to 200 to 500 ° C., gradually heated to a predetermined temperature over about 15 to 30 hours, and maintained at the predetermined temperature for about 1 to 5 hours.

  In the degreasing step, it is preferable to measure the oxygen concentration constantly or at regular intervals. The method for measuring the oxygen concentration is not particularly limited. For example, a zirconia oxygen concentration meter can be used. The oxygen concentration should be measured at a position where the interval between zirconia green sheets is narrow. This is because the oxygen concentration tends to decrease particularly in such a place, and if the oxygen concentration is sufficient here, it is considered that the oxygen concentration in other places is also sufficient.

  The zirconia green sheet subjected to the degreasing step is further sintered. This sintering process narrows the space between the zirconia particles, and a dense zirconia sheet is obtained. In the present invention, even when fine zirconia particles are used, the organic components are sufficiently decomposed and removed in the green sheet degreasing process, so that the carbon is not trapped between the particles even in the sintering process. The amount is significantly suppressed.

  There are no particular limitations on the specific conditions of the sintering step, and a conventional method may be used. For example, the temperature may be gradually raised to a predetermined temperature of 1300 to 1500 ° C. over 15 to 30 hours, maintained at the predetermined temperature for 1 to 5 hours, and then allowed to cool.

  The zirconia sheet produced by the method of the present invention is intended for use as an electrolyte for fuel cells. Accordingly, the thinner the thickness, the better, but considering the strength, it is preferably 0.02 to 1 mm, more preferably 0.05 to 0.8 mm.

  The zirconia sheet produced by the method of the present invention is excellent as an electrolyte for a fuel cell because residual carbon is remarkably suppressed and it is electrically insulating and has high ionic conductivity. Such characteristics can be measured by measuring insulation resistance and ionic conductivity, but it is difficult to perform such measurement for each lot, for example, in the field of mass production. Therefore, in the present invention, by using whiteness as an index of residual carbon, it is possible to easily and simply measure residual carbon and electrolyte characteristics.

  This whiteness is an index representing the degree of whiteness of pulp or paper, and is indicated by the ratio of the amount of reflected light where the light reflection amount of the magnesium oxide standard white plate is 100 and the true black is 0. The measuring method is based on JIS 8148 (2001) “Paper, paperboard, and pulp-ISO whiteness (diffuse blue light reflectance) measuring method”. Here, the ISO whiteness is a filter having a specification stipulated in ISO 2469, having an effective wavelength of 457 nm and a half-value width of 44 nm, or a function corresponding thereto, and the amount of ultraviolet light contained in the light irradiated to the test piece is CIE illuminant. It is an intrinsic reflectance when measured with a reflectance meter adjusted to correspond to C.

  In this whiteness measurement, the specimen is diffusely illuminated by a prescribed device. Then, the light reflected in the normal direction with respect to one side of the test piece is measured using a photovoltaic cell or a diode through a predetermined glass filter, or a diode array in which a plurality of diodes that sense different effective wavelengths are arranged. taking measurement. The whiteness is determined directly from the output of the photovoltaic cell or by calculation using an appropriate weighting function from the output of the diode.

  The apparatus used in the whiteness measurement has a geometric, spectroscopic and measurement specification defined in ISO 2469, calibrated in accordance with the provision of ISO 2469, and a reflectometer having a blue light reflectance measurement function. It is.

  With respect to the test piece to be measured stipulated in JIS 8148, in the case of paper and paperboard, “the number of test pieces needs to be a number that does not change the reflectance even if it is doubled”. This is because the reflectance increases as the number of sheets to be overlapped generally increases, so a sufficient number of sheets are stacked until the reflectance does not change, that is, the change in reflectance is within ± 1%. It is supposed to be measured after being combined.

  Such a situation also applies to a zirconia sheet having a thickness of 0.02 to 1 mm. The reflectance is low because the transmittance is high when the number is small, and the reflectance increases as the number of sheets to be superimposed increases. However, according to the knowledge by the present inventors, when the thickness is set to 2 mm or more, more preferably 3 mm or more by laminating zirconia sheets, the change in reflectance is within ± 1%. Therefore, in the present invention, the whiteness is measured for a sheet laminate in which a zirconia sheet manufactured under the same conditions is laminated under a zirconia sheet whose whiteness is to be measured to have a thickness of 2 mm or more. As described above, if the zirconia sheets are laminated to 2 mm or more, the change in whiteness is almost eliminated, but even if they are unnecessarily overlapped, they are wasted. When used as an electrolyte for a fuel cell, the thickness of one zirconia sheet generally does not exceed 3 mm.

  The whiteness of the zirconia sheet of the present invention is 80% or more. This is because if it is less than 80%, more carbon than the allowable amount remains, and the characteristics as a fuel cell electrolyte are poor. Moreover, since the amount of residual carbon is so small that the said whiteness value is high, the said value is preferable 82% or more, More preferably, it is 84% or more. Further, in the case of a zirconia sheet made of cubic zirconia, the whiteness value is preferably 82% or more, but in the case of a zirconia sheet made of tetragonal zirconia, the whiteness value is preferably 84% or more.

  When the carbon remaining on the zirconia sheet is directly measured, a part of the sheet must be destroyed, and the measurement is complicated and the accuracy is not sufficient. On the other hand, according to the method for measuring residual carbon according to the present invention, it is possible to easily and simply measure the amount of residual carbon that serves as an index of characteristics as a fuel cell electrolyte, and it is not necessary to break the sheet. Therefore, the measuring method of the present invention is an excellent method suitable for quality control at the time of mass production.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. It is also possible to implement, and they are all included in the technical scope of the present invention.

Example 1
3 parts by weight of yttria-stabilized zirconia powder (manufactured by Sumitomo Osaka Cement Co., Ltd., trade name “OZC3Y”, specific surface area: 8 m ² / g, average particle diameter: 0.6 μm) and alumina powder (manufactured by Showa Denko KK A mixture with 0.5 parts by mass of a trade name “AL-160SG”) was pulverized with a bead mill to prepare a mixed powder having an average particle size of 0.1 μm and a 90% by volume of 0.44 μm. Methacrylic acid ester copolymer (2-ethylhexyl methacrylate: 95%, dimethylaminoethyl methacrylate: 4%, hydroxypropyl acrylate: 1% copolymer, average molecular weight: 180,000, glass) 14 parts by mass of a binder comprising a transition point: −9 ° C. in terms of solid content, 2 parts by mass of dibutyl phthalate as a plasticizer, and 50 parts by mass of a mixed solvent containing toluene / isopropyl alcohol (mass ratio: 3/2) as a solvent. Was put into a nylon resin ball mill with an internal volume of 100 L charged with zirconia balls having a diameter of 10 mm, and milled at about 45 rpm for 40 hours to prepare a raw slurry. The slurry had a viscosity of 0.8 Pa · s at room temperature (25 ° C.).

  The raw material slurry was transferred to a jacketed round bottom cylindrical vacuum degassing vessel with an internal volume of 50 L equipped with a bowl-shaped stirrer, and the pressure was reduced at a jacket temperature of 40 ° C. while rotating the stirrer at a speed of 30 rpm (about 4 to 21 kPa), and the viscosity was adjusted to 3 Pa · s to obtain a slurry for coating. This coating slurry is transferred to a slurry dam of a coating device, coated on a polyethylene terephthalate (PET) film by the doctor blade method, and a dryer (50 zones at 80 ° C, 80 ° C and 110 ° C) following the coating section. ) At a speed of 0.2 m / min and dried to obtain a green sheet having a width of 95 cm and a thickness of about 180 μm. This green sheet was punched into a circle having an outer diameter of about 145 mm to obtain a firing green sheet.

This green sheet for firing is a hot air circulation type degreasing furnace (manufactured by Koyo Lindberg Co., Ltd.) having an internal volume of 1 m ³ in which a sensor unit connected to a zirconia oxygen concentration meter (LC-800 manufactured by Toray Industries, Inc.) is arranged in the furnace. Degreasing was performed by an inert gas oven, furnace dimensions: 1000 × 1000 × 1000 mm. In a degreasing furnace, 15 sheets of alumina green and silica, 15 sheets of green sheet for firing, 15 sheets of firing green sheet, outer diameter of 150 mmφ, thickness of 0.2 mm, porosity of 28%, 16 porous alumina spacers 4 laminates were alternately stacked. Three shelves on which the laminate was placed were arranged in the lateral direction and two in the depth direction. Furthermore, a 25 mm high getter was placed at each of the four corners of each shelf board, and 15 stages of shelf boards were stacked. Therefore, 5400 green sheets for firing are loaded in the degreasing furnace. This was degreased by setting the temperature program and the amount of introduced air in Table 1. Table 1 also describes the results of measuring the oxygen concentration in the furnace in each temperature region.

After degreasing, these shelves are transferred to a gas-fired firing furnace with a zirconia oxygen concentration meter (manufactured by Mino Ceramics Co., Ltd., trade name “high temperature superior kiln”, effective internal volume: 1 m ³ (1100 × 1100 × 830 mm), A 3 mol% yttria-stabilized zirconia sheet having an outer diameter of 120 mmφ and a thickness of 150 μm was obtained by firing for 3 hours at 1450 ° C. The temperature profile was as shown in Table 1. Surface of the obtained zirconia sheet Was visually observed to be flat and white without warping or undulation.

Example 2
100 parts by mass of 10 mol% scandia-1 mol% ceria stabilized zirconia-based powder (manufactured by Daiichi Rare Elemental Chemical Co., Ltd., trade name “10Sc1CeSZ”, specific surface area: 9 m ² / g, average particle size: 0.35 μm), A raw material in the same manner as in Example 1 except that a mixed powder of 1 part by mass of alumina powder (trade name “Daimicron-TM-DAR”, manufactured by Daimei Chemical Co., Ltd., 90 volume% diameter: 0.92 μm) was used. A slurry was prepared. This raw material slurry was transferred to a jacketed round bottom cylindrical vacuum degassing vessel equipped with a bowl-shaped stirrer and having a volume of 50 L. While the stirrer was rotated at a speed of 30 rpm, the pressure was reduced at a jacket temperature of about 45 ° C. 4-21 kPa) under reduced pressure, the viscosity was adjusted to 6.8 Pa · s, and a coating slurry was obtained.

  Using the obtained coating slurry, a green sheet having a width of 95 cm and a thickness of about 335 μm was obtained in the same manner as in Example 1. This green sheet was punched into a circle having an outer diameter of about 145 mm to obtain a firing green sheet.

  In Example 1 above, firing was performed in the same manner except that 11 porous alumina spacers and 10 firing green sheets were alternately stacked. Therefore, 3600 green sheets for firing are inserted in the firing furnace. This was degreased by setting the temperature program and the amount of introduced air in Table 1.

  After the degreasing, a 10 mol% scandia 1 mol% ceria stabilized zirconia sheet having an outer diameter of 120 mmφ and a thickness of 300 μm was obtained in the same manner as in Example 1 above. Further, when the surface of the sheet was visually observed, it was flat and white without warping or undulation.

Example 3
The green sheet for firing obtained in the same manner as in Example 1 was degreased and fired in the gas combustion firing furnace with an oxygen concentration sensor used in the firing step of Example 1. In the firing furnace, four laminates in which 20 green sheets for firing and 21 porous alumina spacers similar to those in Example 1 were alternately stacked were placed on the same shelf as in Example 1. Three shelves on which the laminated body is placed are arranged on a movable firing rack in the horizontal direction and three in the depth direction, and further, getters with a height of 20 mm are arranged at the four corners of each shelf board. 22 layers were stacked. Accordingly, 15840 green sheets for firing are loaded in the firing furnace. This was set to the temperature program of Table 1 and the amount of introduced air, and degreased and fired. The amount of introduced air is 4200 L / min. Table 1 also describes the results of measuring the oxygen concentration in the furnace in each temperature region.

  The obtained 3 mol% yttria-stabilized zirconia sheet had an outer diameter of 120 mmφ and a thickness of 150 µm. Further, when the surface of the obtained zirconia sheet was visually observed, it was flat and white without warping or undulation.

Comparative Example 1
In Example 1 above, degreasing was performed by setting the temperature program and the amount of introduced air in Table 2. After degreasing, the same temperature combustion program of 500 ° C. or higher was baked in the same manner as in Example 1 using the same gas combustion type baking furnace as in the example. The obtained 3 mol% yttria-stabilized zirconia sheet had an outer diameter of 120 mmφ and a thickness of 150 µm. Moreover, when the sheet | seat surface was observed visually, curvature and a wave | undulation were recognized by the sheet | seat on the shelf arrange | positioned at the upper part in a furnace. Further, the peripheral edge portion of the sheet was colored a little grayish white.

Comparative Example 2
In Example 1 described above, degreasing was performed by setting the temperature program, the amount of introduced air, and the amount of introduced nitrogen in Table 2. After degreasing, firing was performed in the same manner as in Comparative Example 1 above. The obtained 3 mol% yttria-stabilized zirconia sheet had an outer diameter of 120 mmφ and a thickness of 300 µm. When the surface of the sheet was visually observed, it was flat but the entire surface was colored grayish white.

  As described above, it has been clarified that the surface of the zirconia sheet is colored when the oxygen concentration in the baking furnace is less than 13% in the degreasing step. On the other hand, it was demonstrated that when the oxygen concentration in the degreasing step is always 13% or more, such coloring can be suppressed and a white zirconia sheet can be obtained.

Test example 1
In the state where 15 sheets of the zirconia sheets produced in Example 1 and Comparative Example 1 were stacked, the whiteness was measured with an ISO whiteness-compatible high-speed spectrophotometer (CMS-355PX, manufactured by Murakami Color Research Laboratory). Moreover, in the zirconia sheet manufactured in Example 2 and Comparative Example 2, seven sheets were stacked and the whiteness was measured in the same manner. Furthermore, about these zirconia sheets, the electrical resistance was measured using the insulation resistance meter. The results are shown in Table 3.

  As the result, the higher the whiteness, that is, the lower the coloring, the higher the electrical resistance of the zirconia sheet. This is considered to be based on the fact that residual carbon that causes coloring is also responsible for electron conduction. Therefore, it was proved that the whiteness of the zirconia sheet is an indicator of the amount of residual carbon, and consequently an indicator of electron conductivity.

  Further, as the method of the present invention, the zirconia sheet produced with an oxygen concentration of 13% or more in the degreasing step has a high whiteness and a low electronic conductivity, so that it is very excellent as an electrolyte for a fuel cell.

Claims (7)

A method for producing a zirconia sheet used as a solid electrolyte for a fuel cell ,
Preparing a slurry containing at least zirconia particles, an organic binder and a solvent;
Forming the slurry to obtain a zirconia green sheet; and degreasing the zirconia green sheet at 200 to 500 ° C .;
In the degreasing process of zirconia green sheets, when using a degreasing furnace, a total area of 5 to 500 m ² of 500 to 50,000 zirconia green sheets is degreased at once, and when using a batch-type firing furnace, the total area 10 to 2000 m ² of 3000 to 100,000 zirconia green sheets are degreased at once, and oxygen is measured to keep the oxygen concentration measured between the zirconia green sheets at 13% or more. Method.   The method for producing a zirconia sheet according to claim 1, wherein the oxygen concentration is 15% or more.   The manufacturing method of the zirconia sheet | seat of Claim 1 or 2 heated over 15 to 30 hours to the predetermined temperature of 200-500 degreeC.   The manufacturing method of the zirconia sheet | seat in any one of Claims 1-3 maintained at the predetermined temperature of 200-500 degreeC for 1 to 5 hours. The manufacturing method of the zirconia sheet in any one of Claims 1-4 except the case where oxygen concentration is not adjusted in a degreasing process. The method for producing a zirconia sheet according to claim 1 , wherein a degreasing furnace having an internal volume of 0.5 to 2.5 m ³ is used. Zirconia sheet manufacturing method according to claim 1, internal volume used batch type firing furnace 1 to 10 m ^3.