Method for producing beta-alumina sintered compact

Abstract

Provided is a method for producing a beta-alumina sintered compact that includes: a step for preparing a bottomed cylindrical first molded body containing a raw material composition of beta-alumina; a step for placing the first molded body on a first setter placed at a predetermined location in a firing furnace and having a horizontal mounting surface, with the first molded body inverted and the open end facing downward; a step for firing the first molded body to produce a bottomed cylindrical first beta-alumina sintered compact; a step for measuring the straightness of the first beta-alumina sintered compact and identifying the bend direction; a step for preparing a bottomed cylindrical second molded body containing a raw material composition of beta-alumina; a step for placing the second molded body on a second setter placed at the same location as the first setter in the firing furnace and having a mounting surface inclined downward toward the side opposite to the bend direction, with the second molded body inverted and the open end facing downward; and a step for firing the second molded body and producing a bottomed cylindrical second beta-alumina sintered compact having better straightness than the first beta-alumina sintered compact.

Description

Manufacturing method of β-alumina sintered body

[0001] The present invention relates to a method for producing a cylindrical β-alumina sintered body with a bottom, and particularly to a method for producing a cylindrical β-alumina sintered body with a bottom that can be used as a solid electrolyte in a sodium-sulfur battery (hereinafter also referred to as a "NAS battery").

[0002] A sodium-sulfur battery (hereinafter also referred to as a "NAS battery") has a configuration in which, for example, a bottomed tubular part made of a solid electrolyte such as β-alumina is placed inside a metal container on the positive electrode side, which serves as a storage case, and sodium is housed inside the bottomed tubular part as a negative electrode active material, and sulfur is housed outside the bottomed tubular part as a positive electrode active material. During discharge, sodium-sulfur batteries generate electricity as ionized sodium permeates the solid electrolyte and reacts with sulfur to produce sodium polysulfide, while charging is achieved by the reverse reaction, which produces sodium and sulfur.

[0003] β-alumina has selective permeability to sodium ions and extremely high ionic conductivity for sodium ions (i.e., low electrical resistance), and therefore plays an important role as a solid electrolyte, such as a partition wall separating the positive and negative electrodes of sodium-sulfur batteries.

[0004] A bottomed cylindrical β-alumina sintered body (hereinafter also referred to as a "β-alumina tube") is produced by pulverizing and mixing raw materials such as an aluminum source, a magnesium source, and a sodium source in water to obtain a slurry, granulating the slurry, forming it into a bottomed cylindrical compact, and then firing the compact.

[0005] High dimensional accuracy is required for β-alumina tubes used in NAS batteries. For example, it is necessary to stably produce β-alumina tubes with excellent straightness by suppressing bending during firing. To address this issue, several methods have been proposed, including a method in which the bottom of a cylindrical beta-alumina tube with a bottom is covered with a specific platinum cap and then placed in a firing vessel for firing (Patent Document 1), a method in which a gas release port is provided at the bottom of the firing vessel when the beta-alumina tube is placed in a firing vessel for firing (Patent Document 2), and a method in which the tube bottom is placed upright so as to be in contact with a setter (a firing table) for firing (Patent Document 3).

[0006] Japanese Patent Application Laid-Open No. 2001-247378 Japanese Patent Application Laid-Open No. 2001-253780 Japanese Patent Application Laid-Open No. 2001-261451

[0007] However, when attempting to industrially manufacture β-alumina tubes, it is necessary to simultaneously fire a large number of compacts in a firing furnace. In this case, the thermal history of the numerous compacts arranged in the firing furnace varies depending on the positions of the compacts in the firing furnace and the positions of the compacts in the firing vessel, as these conditions, such as the air flow in the firing furnace and the distance from the burner, change. For this reason, it is not easy to maintain a high level of straightness for all β-alumina tubes. While it is possible to improve straightness by utilizing the prior art techniques described above, there are limitations.

[0008] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide, in one embodiment, a method for manufacturing a β-alumina sintered body that enables the manufacture of a bottomed cylindrical β-alumina sintered body with high straightness.

[0009] The present inventors have conducted extensive research to solve the above problems and have created the present invention, which is exemplified below.

[0010] preparing a first cylindrical shaped body with a bottom that contains a raw material composition of β-alumina; placing the first cylindrical shaped body in an inverted state with an open end facing downward on a first setter that is placed at a predetermined position in a firing furnace and has a horizontal mounting surface; firing the first cylindrical shaped body to produce a first cylindrical β-alumina sintered body with a bottom; measuring the straightness of the first β-alumina sintered body and identifying the direction of bending; preparing a second cylindrical shaped body with a raw material composition of β-alumina; placing the second cylindrical shaped body in an inverted state with an open end facing downward on a second setter that is placed at the same position as the first setter in the firing furnace and has a mounting surface that is inclined downward toward the opposite side to the direction of bending; and firing the second cylindrical shaped body to produce a second cylindrical β-alumina sintered body with improved straightness than the first β-alumina sintered body. [Aspect 2] The method for producing a β-alumina sintered body according to Aspect 1, wherein the steps of preparing the first and second compacts each include a step of granulating a slurry obtained by dispersing the β-alumina raw material composition in water using a spray dryer to prepare first granules, a step of secondary drying the first granules to prepare second granules, and a step of molding the second granules into a bottomed cylindrical shape using a dry isostatic pressing method. [Aspect 3] The method for producing a β-alumina sintered body according to Aspect 1 or 2, wherein the β-alumina raw material composition contains aluminum oxide, magnesium hydroxide, sodium carbonate, sodium bicarbonate, tantalum oxide, and zirconium dioxide. [Aspect 4] The method for producing a β-alumina sintered body according to Aspect 3, wherein the β-alumina raw material composition further contains citric acid monohydrate. [Aspect 5] The method for producing a β-alumina sintered body according to Aspect 3 or 4, wherein the β-alumina raw material composition further contains an acrylic acid-based binder. [Aspect 6] The method for producing a β-alumina sintered body according to any one of Aspects 3 to 5, wherein the β-alumina raw material composition further contains triacetin. [Aspect 7] The method for producing a β-alumina sintered body according to any one of Aspects 3 to 6, wherein the β-alumina raw material composition further contains a polyoxyalkylene glycol-based lubricant.[Aspect 8] The method for producing a β-alumina sintered body according to any one of Aspects 3 to 7, wherein the β-alumina raw material composition further contains octanol. [Aspect 9] The method for producing a β-alumina sintered body according to any one of Aspects 1 to 8, wherein the β-alumina raw material composition contains 75 to 80 parts by mass of aluminum oxide, 5 to 6 parts by mass of magnesium hydroxide, 12 to 13 parts by mass of sodium carbonate, 3 to 4 parts by mass of sodium bicarbonate, 0.5 parts by mass or less of tantalum oxide, and 0.5 to 1.5 parts by mass of zirconium dioxide, relative to 100 parts by mass of the total of aluminum oxide, magnesium hydroxide, sodium carbonate, and sodium bicarbonate. [Aspect 10] The method for producing a β-alumina sintered body according to Aspect 9, wherein the β-alumina raw material composition further contains 0.5 parts by mass or less of citric acid monohydrate, relative to 100 parts by mass of the total of aluminum oxide, magnesium hydroxide, sodium carbonate, and sodium bicarbonate. [Aspect 11] The method for producing a β-alumina sintered body according to Aspect 9 or Aspect 10, wherein the β-alumina raw material composition further contains an acrylic acid-based binder in an amount of 0.5 to 1 part by mass, calculated as solids, when the total amount of aluminum oxide, magnesium hydroxide, sodium carbonate, and sodium bicarbonate is 100 parts by mass. [Aspect 12] The method for producing a β-alumina sintered body according to any one of Aspects 9 to 11, wherein the β-alumina raw material composition further contains triacetin in an amount of 0.1 to 0.5 parts by mass when the total amount of aluminum oxide, magnesium hydroxide, sodium carbonate, and sodium bicarbonate is 100 parts by mass. [Aspect 13] The method for producing a β-alumina sintered body according to any one of Aspects 9 to 12, wherein the β-alumina raw material composition further contains a polyoxyalkylene glycol-based lubricant in an amount of 0.1 to 0.5 parts by mass when the total amount of aluminum oxide, magnesium hydroxide, sodium carbonate, and sodium bicarbonate is 100 parts by mass. [Embodiment 14] The method for producing a β-alumina sintered body according to any one of embodiments 9 to 13, wherein the β-alumina raw material composition further contains 0.1 parts by mass or less of octanol when the total amount of aluminum oxide, magnesium hydroxide, sodium carbonate, and sodium bicarbonate is 100 parts by mass.[Aspect 15] The method for producing a β-alumina sintered body according to any one of Aspects 9 to 14, wherein the β-alumina raw material composition contains sodium carbonate and sodium bicarbonate in a molar ratio of sodium carbonate:sodium bicarbonate = 7:3 to 8:2. [Aspect 16] The method for producing a β-alumina sintered body according to Aspect 2 or any one of Aspects 3 to 15 dependent on Aspect 2, wherein the slurry of the β-alumina raw material composition has a volume-based median diameter (D50) of 0.5 to 0.8 μm as measured with a laser diffraction particle size analyzer. [Aspect 17] The method for producing a β-alumina sintered body according to Aspect 2 or any one of Aspects 3 to 16 dependent on Aspect 2, wherein the slurry of the β-alumina raw material composition has a viscosity of 200 cP or less at 25°C as measured with a single cylindrical rotational viscometer specified in JIS Z8803:2011. [Aspect 18] The method for producing a β-alumina sintered body according to Aspect 2 or any of Aspects 3 to 17 dependent on Aspect 2, wherein the second granules subjected to the molding step have a SiO2 concentration of 320 ppm by mass or less, an Fe2O3 concentration of 200 ppm by mass or less, a TiO2 concentration of 1000 ppm by mass or less, a CaO concentration of 100 ppm by mass or less, and a KO concentration of 50 ppm by mass or less. [Aspect 19] The method for producing a β-alumina sintered body according to Aspect 2 or any of Aspects 3 to 18 dependent on Aspect 2, wherein the second granules subjected to the molding step are free of fibrous foreign matter having an average fiber diameter of 10 μm or more and a length of 300 μm or more. [Aspect 20] The method for producing a β-alumina sintered body according to Aspect 2 or any of Aspects 3 to 19 dependent on Aspect 2, wherein the moisture content of the second granules used in the molding step is 1.3 to 1.9 mass %. [Aspect 21] The method for producing a β-alumina sintered body according to Aspect 2 or any of Aspects 3 to 20 dependent on Aspect 2, wherein the molding step includes a step of pressurizing the second granules filled in a molding space formed by the inner peripheral surface of a molding rubber mold, the outer peripheral surface of a mandrel, the lower surface of a top rubber stopper, and the upper surface of a bottom punch in a pressure vessel by compressing the molding space from the outer peripheral side of the rubber mold with a pressure medium.Aspect 22: The method for producing a β-alumina sintered body according to Aspect 21, wherein the pressurizing step comprises increasing the pressure from 0 MPa (gauge pressure) to 190-230 MPa (gauge pressure), maintaining the pressure at 190-230 MPa (gauge pressure) for 1-2 seconds, and reducing the pressure to 0 MPa (gauge pressure). Aspect 23: The method for producing a β-alumina sintered body according to Aspect 22, wherein the pressurizing step comprises increasing the pressure from 20 MPa (gauge pressure) to 190-230 MPa (gauge pressure) at an average pressure increasing rate of 10-15 MPa / sec, and the reducing step comprises reducing the pressure from 190-230 MPa (gauge pressure) to 10 MPa (gauge pressure) at an average pressure reducing rate of 35-45 MPa / sec. [Aspect 24] The method for producing a β-alumina sintered body according to any one of Aspects 21 to 23, wherein the rubber mold has an inner layer portion having a durometer hardness of 85 to 95° as measured in accordance with JIS K6253-3:2012, and a portion other than the inner layer having a durometer hardness of 45 to 55° as measured in accordance with JIS K6253-3:2012. [Aspect 25] The process for producing the first β-alumina sintered body and the process for producing the second β-alumina sintered body each include a first heating step of heating from room temperature to 1050 to 1100°C at an average heating rate of 100 to 200°C / hr, a first holding step of holding the temperature in the temperature range of 1050 to 1100°C for 30 to 90 minutes following the first heating step, a second heating step of heating to 1390 to 1460°C at an average heating rate of 100 to 150°C / hr following the first holding step, a second holding step of holding the temperature in the temperature range of 1390 to 1460°C for 90 to 150 minutes following the second heating step, and a second holding step of holding the temperature in the temperature range of 50 to 100°C / hr following the second holding step. A method for producing a β-alumina sintered body according to any one of Aspects 2 to 24, which are dependent on Aspect 2, including: a third heating step of heating the material to 1570 to 1630°C at a heating rate; a third holding step of holding the material at a temperature range of 1570 to 1630°C for 30 to 90 minutes following the third heating step; a first heating step of lowering the material to 1460 to 1530°C at an average heating rate of 350 to 450°C / hr following the third holding step; a fourth holding step of holding the material at a temperature range of 1460 to 1530°C for 30 to 90 minutes following the first heating step; and a second heating step of lowering the material to room temperature at an average heating rate of 100 to 500°C / hr following the fourth holding step.[Aspect 26] The method for manufacturing a β-alumina sintered body according to any one of Aspects 1 to 25, wherein the Na ion conduction resistivity of the second β-alumina sintered body at 320 °C is 3.2 Ω·cm or less. [Aspect 27] The method for manufacturing a β-alumina sintered body according to any one of Aspects 1 to 26, wherein the apparent density of the second β-alumina sintered body is 3.22 g / cc or more. [Aspect 28] The method for manufacturing a β-alumina sintered body according to any one of Aspects 1 to 27, wherein the maximum particle diameter of the ceramic particles constituting the second β-alumina sintered body is 200 μm or less. [Aspect 29] The method for manufacturing a β-alumina sintered body according to any one of Aspects 1 to 28, wherein the internal hydraulic pressure fracture strength of the second β-alumina sintered body is 173 MPa or more. [Aspect 30] The method for manufacturing a β-alumina sintered body according to any one of Aspects 1 to 29, wherein the straightness of the second β-alumina sintered body is 0.75 mm or less.

[0011] According to the method for manufacturing a β-alumina sintered body according to an embodiment of the present invention, it is possible to manufacture a bottomed cylindrical β-alumina sintered body with high straightness. This manufacturing method is particularly effective in improving the yield when industrially producing a bottomed cylindrical β-alumina sintered body.

[0012] It is a schematic longitudinal sectional view of a NAS battery according to an embodiment of the present invention. It is a flowchart showing each process performed in the granulation process. It is an example of a schematic longitudinal sectional view showing a bottomed cylindrical molding space formed by a molding rubber mold and a mandrel. It is a heat curve schematically showing the relationship between the firing temperature and the elapsed time. It is an exemplary schematic diagram for explaining the structure of the firing container. It is a schematic enlarged view near the gas discharge outlet of FIG. 5-1. It is a schematic plan view of the bottom of the firing container as viewed from above. It schematically shows a state where a plurality of firing containers containing a molded body are placed on a cart and loaded into a firing furnace to fire the molded body. It is a schematic diagram conceptually showing that when a bend occurs in a β-alumina tube obtained by using a horizontal setter, the straightness of the β-alumina tube is improved by using an inclined setter thereafter. It is a schematic diagram for explaining a method for measuring the Na ion conduction resistivity of a β-alumina tube.

[0013] Next, an embodiment of the present invention will be described in detail with reference to the drawings, taking as an example a β-alumina tube used as a solid electrolyte in a sodium-sulfur battery (NAS battery). However, it should be understood that the present invention is not limited to the following embodiment, and that appropriate design changes, improvements, etc. may be made based on the ordinary knowledge of those skilled in the art within the scope of the present invention.

[0014] (1. Overall Structure of NAS Battery) FIG. 1 shows a schematic longitudinal cross-sectional view illustrating an example of the overall structure of a NAS battery 10. In one embodiment, the NAS battery 10 includes a positive electrode side body portion 21, a positive electrode side bottom cover 24 joined to the lower end of the positive electrode side body portion 21, a positive electrode ring fitting 22 joined to the upper end of the positive electrode side body portion 21, a positive electrode terminal 23 joined to the upper end of the positive electrode ring fitting 22, a negative electrode ring fitting 31 positioned more inward than the positive electrode ring fitting 22 and electrically insulated from the positive electrode ring fitting 22 by an insulating ring 40, and a negative electrode side upper cover 32 joined to the upper end of the negative electrode ring fitting 31. A negative electrode terminal 39 is provided on the upper surface of the negative electrode side upper cover 32. The negative electrode side upper cover 32 and the negative electrode terminal 39 can be provided, for example, as an integrally molded product.

[0015] The NAS battery (single cell) is housed in a sleeve tube 80. A heat-insulating and / or insulating covering 82 is wrapped and fixed around the outer periphery of the sleeve tube 80. Examples of the covering 82 include mica sheets and fire-resistant carbon sheets. Mica sheets have heat-insulating and insulating properties and can be used for a variety of purposes. For example, flexible mica of MT66 as specified in JIS C2255:1992 can be suitably used. For example, the covering 82 can be formed by wrapping a heat-insulating mica sheet, a fire-resistant carbon sheet, and an insulating mica sheet around the sleeve tube 80 in this order from the inner periphery to the outer periphery. In addition, a heat-insulating and / or insulating plate material 84 is adhesively fixed to the bottom surface of the sleeve tube 80. Examples of the plate material 84 include an insulating mica plate.

[0016] Metals such as aluminum or aluminum alloys can be used as materials for the positive electrode side body portion 21, the positive electrode side bottom cover 24, the positive electrode ring fitting 22, the positive electrode terminal 23, the negative electrode ring fitting 31, and the negative electrode side top cover 32. For example, an aluminum alloy A3003 as defined in JIS H4000:2014 can be suitably used. Furthermore, welding, particularly electron beam welding, can be suitably employed for joining metal parts, such as the joining between the positive electrode side body portion 21 and the positive electrode side bottom cover 24, the joining between the positive electrode side body portion 21 and the positive electrode ring fitting 22, the joining between the positive electrode ring fitting 22 and the positive electrode terminal 23, and the joining between the negative electrode ring fitting 31 and the negative electrode side top cover 32.

[0017] Aluminum oxide such as α-alumina is preferably used as the material for the insulating ring 40. The insulating ring 40 is joined to the outer periphery of the upper end of a bottomed tubular (typically cylindrical) solid electrolyte 50. The insulating ring 40 and the solid electrolyte 50 can be joined with glass. The solid electrolyte 50 is made of β-alumina.

[0018] A bottomed cylindrical anode container 36 (typically cylindrical) is provided inside the bottomed cylindrical solid electrolyte 50, containing sodium 3 as the anode active material. A small hole 36a is provided at the bottom of the anode container 36. Materials that can be used for the anode container 36 include high-chromium steel, stainless steel (e.g., SUS304), aluminum alloys, and SPCC (cold-rolled steel plate). A bottomed cylindrical safety tube 70 is provided outside the anode container 36 and inside the solid electrolyte 50. Aluminum or an aluminum alloy can be used as the material for the safety tube 70. For example, an aluminum alloy A3003 as specified in JIS H4000:2014 can be suitably used. The gap between the solid electrolyte 50 and the safety tube 70 (the gap in the radial direction (horizontal direction in the figure)) is preferably 30 to 100 μm, more preferably 50 to 80 μm. A thickness of 30 μm or more is preferable because sodium can be moved without increasing pressure loss, and a thickness of 100 μm or less is preferable from the standpoint of safety.

[0019] Additionally, sulfur 2 as a positive electrode active material is contained within a space (positive electrode space) 28 surrounded by the positive electrode side body 21 and the positive electrode side bottom lid 24 outside the bottomed tubular solid electrolyte 50. Because sulfur 2 is an insulator, a positive electrode current collector 27 is generally provided to ensure electrical continuity between the positive electrode and the negative electrode and reduce the internal resistance of the battery. The positive electrode current collector 27 may be a member made of a felt material made of conductive carbon fiber and / or graphite fiber. By impregnating the positive electrode active material sulfur 2 and arranging the positive electrode current collector 27 so as to abut both the inner circumferential surface of the positive electrode side body 21 and the outer circumferential surface of the bottomed tubular solid electrolyte 50, electrical continuity between the positive electrode and the negative electrode is ensured and the internal resistance of the battery is also reduced.

[0020] An inert gas such as argon, helium, or neon is sealed in the space 37 inside the negative electrode container 36 as a pressure (negative electrode side pressure) generating source, and nitrogen gas is sealed in the positive electrode space 28 as a pressure (positive electrode side pressure) generating source.

[0021] During discharge, sodium 3 molten in the anode container 36 is supplied into the safety tube 70 through the small hole 36a due to the pressure of the inert gas, filling the safety tube 70. Furthermore, overflowing sodium 3 is supplied to the space (anode space) 38 between the solid electrolyte 50 and the safety tube 70. Some of the sodium 3 supplied between the solid electrolyte 50 and the safety tube 70 has released electrons to the external circuit through the anode terminal 39 in the anode space 38, becoming sodium ions. These sodium ions then pass through the solid electrolyte 50 and enter the cathode space 28, where they react with sulfur 2 and electrons supplied from the external circuit through the cathode terminal 23 to produce sodium polysulfide. This allows a voltage of, for example, about 1.8 to 2.3 V to be generated.

[0022] During charging, when a voltage is applied from an external circuit via the positive electrode terminal 23 and the negative electrode terminal 39, the sodium polysulfide releases electrons to the external circuit via the positive electrode terminal 23 to generate sulfur and sodium ions, and the generated sodium ions permeate (pass) through the solid electrolyte 50 and move to the negative electrode space 38. The sodium ions that have moved to the negative electrode space 38 move inward, over the upper end of the safety tube 70, and then move through the small hole 36a into the negative electrode container 36. They then react with electrons supplied from the external circuit via the negative electrode terminal 39 to become electrically neutral (become sodium 3), thereby converting electrical energy into chemical energy.

[0023] The following method can be used to seal the inert gas in the anode can 36. First, the anode can 36 is inverted so that the small hole 36a is at the top, and heated and melted sodium 3 is poured into the anode can 36 through the small hole 36a. The sodium 3 is cooled and solidified. Next, the anode can 36 is turned upside down and placed in the solid electrolyte 50. Next, the opening of the solid electrolyte 50 is sealed under an inert gas atmosphere, and the sodium 3 is melted by heating. At this time, the molten sodium moves to the bottom of the anode can 36, and the inert gas moves to the top instead. In this way, the inert gas is sealed in the upper part of the anode can 36. Using an inert gas as the gas to be sealed in the anode can 36 prevents the material constituting the anode can 36 from reacting with the sealed gas and consuming it, thereby preventing a decrease in the pressure of the inert gas.

[0024] A preferred method for sealing nitrogen gas in the positive electrode space 28 is to place solid sodium azide, such as in pellet form, in the positive electrode space 28 during battery assembly, and then generate nitrogen gas by thermal decomposition after the positive electrode space 28 is sealed. This is because electron beam welding, which has high joining reliability, can be performed in a vacuum during battery assembly.

[0025] The positive electrode side body portion 21 may have a constriction 21 a. The constriction 21 a provides the positive electrode side body portion 21 with a spring effect, which can mitigate expansion and contraction of the positive electrode side body portion 21 due to thermal changes. After the positive electrode solidifies, the portion where the positive electrode current collector 27 is present is fixed with sulfur or sodium polysulfide and becomes immobile, so the constriction 21 a is preferably located above the position where the positive electrode current collector 27 is housed.

[0026] (2. Manufacturing Method of β-Alumina Sintered Body) The composition of β-alumina is a non-stoichiometric compound of Na2O and Al2O3, as shown by Na2O·(5-11)Al2O3. Typical crystal structures include β-Al2O3 (Na2O·(9-11)Al2O3) and β"-Al2O3 (Na2O·(5-7)Al2O3). β"-Al2O3, which has a relatively high sodium ratio, is preferred for achieving higher sodium ion conductivity. However, the β" phase lacks crystalline stability. Therefore, it is desirable to add MgO to β-alumina as a crystalline stabilizer, substituting a portion of the aluminum element to stabilize the β"-Al2O3 crystal. While Li2O can also be used as a crystalline stabilizer, MgO is preferred because it reacts easily with sodium.

[0027] In a preferred embodiment, the β-alumina sintered body contains β"-AlO as a main component, and for example, the β" ratio is preferably 80% or more, and more preferably 90% or more. The β" ratio can be determined by Rietveld analysis from the identification and peak intensity of the crystalline phase by X-ray diffraction. The crystalline phase can be identified using powder XRD analysis software (JADE) based on the database (JPCPDS).

[0028] (2-1. Granulation Step) The granulation step includes a step of granulating a slurry in which a raw material composition of β-alumina is dispersed in water using a spray dryer to prepare first granules, and a step of secondary drying the first granules to prepare second granules. Figure 2 is a flowchart showing each treatment performed in the granulation step.

[0029] The raw material composition of β-alumina contains aluminum oxide as an aluminum source, magnesium hydroxide as a magnesium source, sodium carbonate and sodium bicarbonate as sodium sources, and preferably contains tantalum oxide and zirconium dioxide as sintering aids.

[0030] Aluminum oxide (α-Al2O3) is used as the aluminum source. The particle size of the aluminum oxide used as the raw material is preferably small; specifically, the volume-based median diameter (D50) measured with a laser diffraction particle size analyzer is preferably 60 μm or less, and particularly preferably 40 μm or less. However, taking into consideration handling during weighing and raw material charging, aluminum oxide with a diameter of, for example, 20 to 60 μm can be suitably used. When the particle size of the aluminum oxide is within the above range, coarse crystals are unlikely to form in the resulting β-alumina sintered body, making it easier to obtain a dense, low-electrical resistance body.

[0031] The use of magnesium hydroxide (Mg(OH)2) as the magnesium source makes it possible to reduce raw material costs. The particle size of the magnesium hydroxide used as the raw material is preferably such that the volume-based median diameter (D50) measured with a laser diffraction particle size distribution analyzer is 5 μm or less, and particularly preferably 1 μm or less; for example, magnesium hydroxide having a particle size of 0.5 to 1.0 μm can be suitably used. When the particle size of the magnesium hydroxide is within the above range, the distribution of the magnesium source in the resulting slurry and the granules obtained in the granulation step becomes uniform, and a β-alumina sintered body with better properties can be obtained.

[0032] The combined use of sodium carbonate (Na2CO3) and sodium bicarbonate (NaHCO3) as sodium sources increases the bicarbonation rate in the slurry, enabling the production of granules with reduced Na segregation. Suppressing Na segregation is preferable because Na segregation can lead to the formation of pores during the sintering process, reducing the density of the β-alumina sintered body. Furthermore, NaHCO3 has a lower solubility in solvents such as water than Na2CO3, resulting in a faster precipitation rate during drying during granulation, thereby achieving uniform dispersion within the granules. Furthermore, due to its buffering effect, when the raw materials are mixed to form a slurry, the pH of the slurry can be controlled low (around 10), enabling the use of organic binders that would be unusable with Na2CO3 alone due to their high pH. The particle size of the sodium carbonate (NaCO) and sodium bicarbonate (NaHCO) used as raw materials is preferably a volume-based median diameter (D50) of 300 μm or less, particularly preferably 250 μm or less, as measured with a laser diffraction particle size distribution analyzer. However, in consideration of ease of dissolution and ease of handling during weighing and raw material charging, those with a size of, for example, 150 to 200 μm can be suitably used.

[0033] The raw material composition of β-alumina preferably contains sodium carbonate and sodium hydrogen carbonate in a molar ratio of sodium carbonate:sodium hydrogen carbonate of 6:4 to 9:1, more preferably a molar ratio of sodium carbonate:sodium hydrogen carbonate of 7:3 to 8:2, so that the raw material composition of β-alumina is dissolved in a slurry state while readily precipitating Na immediately during granulation using a spray dryer.

[0034] Tantalum oxide (Ta2O5) and zirconium dioxide (ZrO2) are sintering aids. Tantalum oxide (Ta2O5) is particularly effective in increasing the ionic conductivity of sodium ions, while zirconium dioxide (ZrO2) is highly effective in increasing the strength of sintered bodies. Furthermore, using both in combination offers the advantage of broadening the optimal firing temperature range, resulting in superior quality stability. The particle size of the tantalum oxide used as a raw material is preferably 3 μm or less, with a volume-based median diameter (D50) of 1 μm or less, as measured by a laser diffraction particle size analyzer, and is particularly preferably 0.5 to 1.0 μm. The particle size of the zirconium dioxide used as a raw material is preferably 3 μm or less, with a volume-based median diameter (D50) of 1 μm or less, as measured by a laser diffraction particle size analyzer, and is particularly preferably 0.5 to 1.0 μm. However, as will be described later, when zirconium dioxide is used as the grinding medium, the worn-out portion of the grinding medium can be used as a raw material, so there is no need to add zirconium dioxide in addition to the grinding medium.

[0035] In a preferred embodiment, the β-alumina raw material composition contains 70 to 85 parts by mass of aluminum oxide, 3 to 8 parts by mass of magnesium hydroxide, 10 to 15 parts by mass of sodium carbonate, 2 to 5 parts by mass of sodium bicarbonate, 0.1 to 1.0 part by mass of tantalum oxide, and 0.9 to 1.5 parts by mass of zirconium dioxide, relative to 100 parts by mass of the total of aluminum oxide, magnesium hydroxide, sodium carbonate, and sodium bicarbonate. In a more preferred embodiment, the β-alumina raw material composition contains 75 to 80 parts by mass of aluminum oxide, 5 to 6 parts by mass of magnesium hydroxide, 12 to 13 parts by mass of sodium carbonate, 3 to 4 parts by mass of sodium bicarbonate, 0.1 part by mass or more and 0.5 part by mass or less of tantalum oxide, and 0.5 to 1.5 parts by mass of zirconium dioxide, relative to 100 parts by mass of the total of aluminum oxide, magnesium hydroxide, sodium carbonate, and sodium bicarbonate.

[0036] To obtain a β-alumina sintered body with good properties, it is preferable to use raw materials of as high purity as possible. Furthermore, to prevent segregation of the raw materials in the granules obtained during the granulation process, it is important to mix the raw materials as uniformly as possible during slurry preparation. However, magnesium hydroxide is difficult to disperse in the slurry, and the viscosity of the slurry increases, making it difficult to control the viscosity during storage and the granulation process. Therefore, it is preferable that the β-alumina raw material composition further contains citric acid monohydrate. Citric acid monohydrate can function as a dispersant, so adding citric acid monohydrate improves the dispersibility of magnesium hydroxide, enabling uniform mixing of the raw materials and stabilizing the viscosity of the slurry during storage and the granulation process.

[0037] The amount of citric acid monohydrate added is preferably 0.5 parts by mass or less, and preferably 0.1 to 0.5 parts by mass, when the total of aluminum oxide, magnesium hydroxide, sodium carbonate, and sodium bicarbonate is 100 parts by mass. The timing of adding citric acid monohydrate is preferably before the slurry is pulverized and mixed. This is because adding citric acid monohydrate after pulverization and mixing does not fully exhibit its dispersibility effect. The particle size of the citric acid monohydrate used as a raw material is preferably 300 μm or less, and particularly preferably 250 μm or less, in terms of volume-based median diameter (D50) measured with a laser diffraction particle size distribution analyzer. For example, citric acid monohydrate having a D50 of 150 to 200 μm can be suitably used.

[0038] As described above, due to the combined use of sodium carbonate (Na2CO3) and sodium bicarbonate (NaHCO3) as sodium sources, adjusting the pH of the raw material slurry to a weakly alkaline range of 8 to 11 enhances the effectiveness of the organic binder and broadens the range of binder options. Therefore, from the perspective of the strength and rigidity of the compact, it is preferable for the raw material composition of β-alumina to further contain an organic binder. Examples of organic binders include acrylic acid-based binders and polyethylene-based binders, with acrylic acid-based binders being preferred. Furthermore, the organic binder is preferably provided in the form of an aqueous solution. The amount of acrylic acid-based binder added is preferably 0.1 to 1.5 parts by mass, and more preferably 0.5 to 1 part by mass, in terms of solid content, based on 100 parts by mass of the total of aluminum oxide, magnesium hydroxide, sodium carbonate, and sodium bicarbonate, in order to ensure the strength of the compact while suppressing volatile content and improving sinterability, and further to omit or simplify the degreasing process.

[0039] The β-alumina raw material composition preferably further contains a plasticizer to improve the flexibility of the granules. Examples of plasticizers include glycerin-based plasticizers and phthalic acid-based plasticizers, and among these, it is preferable to contain a glycerin-based plasticizer, particularly triacetin. The amount of triacetin added is preferably 0.1 to 1.0 parts by mass, and more preferably 0.1 to 0.5 parts by mass, relative to 100 parts by mass of the total of aluminum oxide, magnesium hydroxide, sodium carbonate, and sodium bicarbonate.

[0040] The raw material composition of β-alumina preferably further contains a lubricant for the purpose of improving the sliding of primary particles and intergranular sliding. Examples of lubricants include stearic acid-based lubricants and polyether-based lubricants, and among these, it is preferable to contain a polyoxyalkylene glycol-based lubricant. The amount of polyoxyalkylene glycol-based lubricant added is preferably 0.1 to 1.0 parts by mass, and more preferably 0.1 to 0.5 parts by mass, relative to 100 parts by mass of the total of aluminum oxide, magnesium hydroxide, sodium carbonate, and sodium bicarbonate.

[0041] The raw material composition of β-alumina preferably further contains an antifoaming agent for the purpose of removing bubbles in the slurry. The antifoaming agent preferably contains octanol. The amount of octanol added is preferably 0.1 parts by mass or less, and preferably 0.01 to 0.05 parts by mass, relative to 100 parts by mass of the total of aluminum oxide, magnesium hydroxide, sodium carbonate, and sodium bicarbonate.

[0042] A slurry in which the raw material composition of β-alumina is dispersed in water can be prepared by placing the above-mentioned raw materials in a desired mass ratio together with water in an agitator mill, and grinding and mixing them. Conditions such as the amount of water added to the agitator mill, grinding time, and rotation speed can be appropriately set so that the resulting slurry has a predetermined pH, particle size, and viscosity. The pH of the slurry is preferably 8 to 11, and more preferably 9.5 to 10.5.

[0043] It is preferable to vary the grinding time depending on the type of raw material. The grinding time for aluminum oxide using an agitator grinder can be 6 to 16 hours, the grinding time for magnesium hydroxide can be 1 to 2 hours, and the grinding time for organic binders can be 1 to 2 hours. This is due to the need for fine grinding and uniform mixing. For this reason, it is preferable to feed the raw materials into the agitator grinder at different times.

[0044] A grinding medium can be placed in the agitator grinder. Zirconium dioxide is preferred as the grinding medium because it can be used as a raw material. While the particle size of zirconium dioxide added as a sintering aid is small, the particle size of zirconium dioxide added as a grinding medium is preferably large. Specifically, the particle size of the zirconium dioxide added as a grinding medium is preferably 200 to 1000 μm, more preferably 300 to 800 μm, in terms of volume-based median diameter (D50) measured using a laser diffraction particle size distribution analyzer. The rotation speed of the rotor blades of the agitator grinder can be, for example, 800 to 1400 rev / min. When zirconium dioxide is used as the grinding medium, the content of zirconium dioxide in the raw material composition for β-alumina can be calculated from the weight loss due to wear of the added grinding medium. Furthermore, the wear rate (the rate at which the diameter of the grinding medium decreases) can be predicted from the grinding time.

[0045] The slurry prepared in this manner is preferably stored in a tank equipped with an agitator until it is supplied to the spray dryer, in order to maintain the particle dispersion state. The prepared slurry also contains various foreign matter. Examples of foreign matter include iron-based foreign matter, coarse foreign matter, and fibrous foreign matter. These foreign matter may be contained in the raw materials, or may be mixed into the slurry during processing, storage, or transportation in the granulation process. If these foreign matter get mixed into the β-alumina tube, pinholes or cracks may form, adversely affecting the durability of the β-alumina tube. Therefore, it is desirable to remove them as much as possible.

[0046] The foreign matter in the raw material mainly includes iron-based foreign matter and coarse foreign matter. For this reason, it is desirable to first remove iron-based foreign matter and coarse foreign matter from the prepared slurry. A method for removing iron-based foreign matter includes a method using a magnetic separator. A method for removing coarse foreign matter includes sieving. For example, a multistage vibrating sieve specified in JIS Z8801-1:2019, which uses a filter with a mesh size of 30 to 50 μm (e.g., 45 μm) and a filter with a mesh size of 15 to 25 μm (e.g., 20 μm) in sequence, can be suitably used.

[0047] Foreign matter that may be mixed in during the process primarily includes iron-based foreign matter and fibrous foreign matter. Iron-based foreign matter, in particular, can be mixed in simply by transporting the slurry through piping using a pump, so it is desirable to perform spray drying immediately before the process. Examples of fibrous foreign matter include lint and hair. A suitable method for removing fibrous foreign matter is a high-performance filter capable of removing microfibers, such as a multi-stage vibrating sieve with a multilayer structure consisting of a twill weave filter (mesh size of 20 to 70 μm, e.g., 20 μm, as specified in JIS Z8801-1:2019) → a twill weave filter (mesh size of 20 to 70 μm, e.g., 32 μm, as specified in JIS Z8801-1:2019) → a twill weave filter (mesh size of 20 to 70 μm, e.g., 32 μm, as specified in JIS Z8801-1:2019).

[0048] The particle size of the particles of the β-alumina raw material composition in the slurry supplied to the spray dryer is preferably 0.5 to 0.8 μm, more preferably 0.6 to 0.75 μm, in terms of volume-based median diameter (D50) measured with a laser diffraction particle size distribution analyzer. Furthermore, the slurry supplied to the spray dryer preferably has a viscosity at 25° C. measured with a single cylindrical rotational viscometer specified in JIS Z8803:2011 of 200 cP or less, more preferably 50 to 100 cP.

[0049] In this way, various foreign matters are removed, and the resulting β-alumina raw material composition slurry having the predetermined pH, particle size, and viscosity is granulated using a spray dryer to prepare first granules. The drying temperature during granulation affects the crystal structure of the Na compound that constitutes the precipitated first granules. If the outlet temperature of the spray dryer is too low, the precipitated Na2CO3 is likely to contain water of crystallization. This deteriorates the disintegrability of the granules, making it difficult to increase the density of the compact. On the other hand, if the outlet temperature of the spray dryer is too high, NaHCO3 begins to partially decompose. As a result, even if the moisture content in the granules is reduced, the fluidity is poor and they are prone to agglomeration during storage. Therefore, the outlet temperature of the spray dryer is preferably 101 to 109°C, and more preferably 103 to 107°C.

[0050] Granulation using a spray dryer homogenizes the raw material composition of β-alumina and improves its moldability in the subsequent molding process. It is preferable to use a spray dryer that employs a rotary atomizer spray method and a two-point collection method. In the case of the rotary atomizer method, particle size can be easily controlled by the disk rotation speed. Furthermore, in the case of the two-point collection method, the spray dryer has a drying chamber and a cyclone, and spherically granulated particles with good flowability are obtained below the drying chamber, while only fine particles are obtained below the cyclone, making particle size control even easier. The resulting first granules may be classified as needed.

[0051] The first granules obtained by the spray dryer are then subjected to secondary drying to prepare second granules. The first granules are in a high-temperature state, and when they are cooled under high humidity, the Na present on the granule surface combines with the surrounding water, easily precipitating crystals of a Na compound (Na2CO3·H2O). Precipitation of Na compound crystals on the granule surface tends to cause the granules to bond and agglomerate, reducing the fluidity of the granules. Therefore, during the process of cooling the first granules to room temperature, it is desirable to keep the moisture content around the granules as low as possible. A continuous fluidized bed dryer can be suitably used for secondary drying. It is preferable to gradually cool the first granules during secondary drying; for example, the temperature of the second granules after secondary drying is preferably 20-30°C. After secondary drying, it is preferable to remove iron-based foreign matter that may be mixed in during the spray dryer and secondary drying. A magnetic separator can be used as a method for removing iron-based foreign matter.

[0052] (2-2. Molding Step) The second granules prepared by the above-described procedure are subjected to the molding step. The second granules can be directly subjected to the molding step by pneumatic transport. However, if there is a waiting time before the molding step, they can be stored in a flash blender via pneumatic transport and then pneumatically transported from the flash blender to a molding device. However, when storing the second granules in the flash blender, it is preferable to periodically agitate the second granules using an air blower or the like to prevent the moisture in the second granules from combining with Na2CO3 and precipitating sodium needle crystals. Furthermore, it is desirable to remove iron-based foreign matter that may be mixed in due to agitation in the flash blender or friction with the piping during pneumatic transport just before the molding step. A method for removing iron-based foreign matter includes the use of a magnetic separator. Furthermore, to remove second granules that have become coarse due to agglomeration during storage in the flash blender, the second granules may be classified using a vibrating sieve such as a rotary sifter during pneumatic transport. Removing the coarse secondary granules is advantageous in terms of ensuring the strength of the β-alumina tube.

[0053] The moisture content of the second granules of the β-alumina raw material composition to be subjected to the molding step is preferably 1.0 to 2.0% by mass, more preferably 1.3 to 1.9% by mass, in order to ensure homogeneity of the molded body by having good fluidity and disintegrability. Here, the moisture content is measured by the loss on drying method specified in JIS K0068:2001.

[0054] From the viewpoint of ensuring good flowability when filling a molding space, the second granules of the raw material composition of β-alumina to be subjected to the molding step preferably have a volume-based median diameter (D50) measured with a sonic sieving measuring device (e.g., Sonic Sifter GILSON GA-6 manufactured by Seishin Enterprise Co., Ltd.) of 30 to 100 μm, more preferably 40 to 70 μm.

[0055] The second granules of the β-alumina raw material composition to be subjected to the molding step desirably contain few impurities. Specifically, the second granules to be subjected to the molding step preferably have a SiO concentration of 320 mass ppm or less, an FeO concentration of 200 mass ppm or less, a TiO concentration of 1000 mass ppm or less, a CaO concentration of 100 mass ppm or less, and a KO concentration of 50 mass ppm or less. The second granules of the β-alumina raw material composition to be subjected to the molding step more preferably have a SiO concentration of 145 mass ppm or less, an FeO concentration of 170 mass ppm or less, a TiO concentration of 95 mass ppm or less, a CaO concentration of 85 mass ppm or less, and a KO concentration of 45 mass ppm or less. The impurity concentrations are measured by X-ray fluorescence analysis using the glass bead method.

[0056] The second granules of the β-alumina raw material composition to be subjected to the molding step are preferably free of fibrous foreign matter. Here, fibrous foreign matter is defined as foreign matter having an average fiber diameter of 10 μm or more and a length of 300 μm or more. The average fiber diameter refers to the average value of fiber diameters (lengths in the direction perpendicular to the direction of fiber extension) measured at five locations at 50 μm intervals near the center of the fiber length (extension direction) using a microscope.

[0057] The presence or absence of fibrous foreign matter in the second granules is determined by the following procedure. First, 20 L of water and 0.5 L of octanol are placed in a container and stirred at 650 rpm, while 10 kg of the second granules are gradually added over 40 minutes. Next, after the entire amount of the second granules is added, stirring is continued at 650 rpm for 0.5 hours. After stirring is stopped, the mixture is allowed to settle for at least 1 hour. Next, the supernatant octanol after settling is collected by suction, and the octanol is passed through a twill-woven stainless steel sieve (manufactured by Manabe Kogyo Co., Ltd., mesh (number of mesh holes on one side between the center of the metal and the center of 25.4 mm (1 inch)): length x width = 270 x 2000, wire diameter (mm): length x width = 0.035 x 0.028), and the surface remaining on the sieve is observed under a microscope. If no fibrous foreign matter is confirmed by microscopic observation, it is determined that no fibrous foreign matter is present in the second granules.

[0058] In the molding process, the second granules of the β-alumina raw material composition are molded into a bottomed cylindrical shape using a dry isostatic pressing (CIP) process. CIP molding consists of the steps of granule filling, pressurization, decompression, and compact removal. For example, the second granules can be suitably molded using a CIP molding apparatus equipped with a molding rubber mold, a mandrel disposed in the hollow portion of the molding rubber mold, a pressure vessel having a pressing means for pressing the molding rubber mold from the outer periphery toward the central axis, and the molding rubber mold disposed within the pressure vessel. Examples of the pressing means include a method in which an elastic body such as rubber is disposed on the outer periphery of the molding rubber mold, a space is provided between the outer periphery of the elastic body and the inner periphery of the pressure vessel through which a pressure medium such as oil can flow, and oil is poured into the space to press the elastic body against the molding rubber mold.

[0059] FIG. 3 illustrates a schematic longitudinal cross-sectional view of a molding space 300 formed by the inner peripheral surface of a molding rubber mold 301 installed in a CIP molding apparatus, the outer peripheral surface of a mandrel 302, the lower surface of a top rubber stopper 303, and the upper surface of a bottom punch 304. During molding, first, with the top rubber stopper 303 removed, the mandrel 302 with the bottom punch 304 attached is inserted into the hollow portion of the molding rubber mold 301 from below and fixed in place, and second granules are filled into the molding space 300 from above. Once the filling of the second granules is complete, the molding space 300 is closed with the top rubber stopper 303. Next, the second granules filled in the molding space 300 are pressurized by compressing the molding space 300 from the outer peripheral side of the molding rubber mold 301 toward the central axis O using a pressure medium. No special heating is required during pressurization. In this manner, the second granules filled in the molding space 300 are compression-molded into a bottomed cylindrical shape. After the compression molding is completed, the pressure medium is released to restore the molding rubber mold 301, thereby detaching the molded body from the molding rubber mold 301, and then the top rubber stopper 303 is raised and the bottom punch 304 is lowered, thereby removing the bottomed cylindrical molded body.

[0060] In order to stably and efficiently obtain a bottomed cylindrical compact having the desired strength, the pressurizing step preferably includes the steps of increasing the pressure from 0 MPa (gauge pressure) to 180 to 260 MPa (gauge pressure), maintaining the pressure of 180 to 260 MPa (gauge pressure) for 0.1 to 5 seconds, and reducing the pressure to 0 MPa (gauge pressure). More preferably, the pressurizing step includes the steps of increasing the pressure from 0 MPa (gauge pressure) to 190 to 230 MPa (gauge pressure), maintaining the pressure of 190 to 230 MPa (gauge pressure) for 1 to 2 seconds, and reducing the pressure to 0 MPa (gauge pressure).

[0061] In the pressure increasing step, the pressure is preferably increased from 20 MPa (gauge pressure) to 180 to 260 MPa (gauge pressure) at an average pressure increase rate of 8 to 17 MPa / sec, and more preferably increased from 20 MPa (gauge pressure) to 190 to 230 MPa (gauge pressure) at an average pressure increase rate of 10 to 15 MPa / sec.

[0062] In the depressurizing step, the pressure is preferably reduced from 180 to 260 MPa (gauge pressure) to 10 MPa (gauge pressure) at an average depressurization rate of 30 to 50 MPa / second, and more preferably from 190 to 230 MPa (gauge pressure) to 10 MPa (gauge pressure) at an average depressurization rate of 35 to 45 MPa / second.

[0063] The molding rubber mold 301 is preferably composed of a relatively hard inner layer portion 301a and a relatively soft portion other than the inner layer 301b. The inner layer portion 301a has the surface that comes into contact with the second granules, i.e., the inner peripheral surface of the molding rubber mold 301 described above. Because the inner layer portion 301a is relatively hard, the second granules are less likely to adhere to the inner peripheral surface of the molding rubber mold 301, which reduces the likelihood of the outer surface of the molded body adhering to the inner peripheral surface of the molding rubber mold 301 and causing partial peeling or cracking of the outer surface of the molded body. Furthermore, because the portion other than the inner layer 301b is relatively soft, the recovery speed is slower, which reduces the load on the molded body when depressurized, and reduces the likelihood of cracking of the molded body.

[0064] Specifically, the inner layer portion 301a preferably has a durometer hardness of 80 to 100°, more preferably 85 to 95°, as measured in accordance with JIS K6253-3:2012. The portion 301b other than the inner layer preferably has a durometer hardness of 40 to 60°, more preferably 45 to 55°, as measured in accordance with JIS K6253-3:2012.

[0065] The thickness of the inner layer portion 301a that comes into contact with the second granules is preferably 1 to 5 mm, and more preferably 1 to 3 mm.

[0066] The mandrel 302 preferably has a base material made of an iron-based material and a top surface layer made of diamond-like carbon (DLC) formed on the top surface of the base material. The diamond-like carbon (DLC) has a maximum height R max is preferably 1.0 μm or less, and more preferably 0.3 μm or less. Furthermore, the diamond-like carbon (DLC) preferably has a contact angle with water of 75° or more. When mandrel 302 has such an outermost surface layer, second granules are less likely to adhere to the outer peripheral surface of mandrel 302, which has the effect of reducing the likelihood of problems such as partial peeling or cracks occurring in the inner surface of the molded body due to the inner surface of the molded body adhering to the outer peripheral surface of the mandrel.

[0067] Here, the maximum height R max is measured in accordance with JIS B0601-1982. In this specification, the contact angle with water is measured using a contact angle meter DM100 manufactured by Kyowa Interface Science Co., Ltd. or an equivalent measuring instrument.

[0068] After molding, the shape and dimensions can be adjusted by grinding as needed. A visual inspection may also be performed to check for cracks, dirt, scratches, foreign matter, and remaining shavings. While the visual inspection may be performed manually, it is preferable to use an inspection device that can acquire inspection images using a camera and automatically determine whether or not a predetermined standard is met based on the inspection images.

[0069] (2-3. Firing Process) The bottomed cylindrical compact thus obtained is fired to produce a β-alumina tube. Firing is usually carried out using a firing furnace. Examples of firing furnaces include batch or continuous firing furnaces. Firing generally comprises a temperature-raising process, a temperature-holding process, and a temperature-lowering process. When used as a solid electrolyte for NAS batteries, the conditions for each process are set so that the resulting β-alumina sintered body has high sodium ion conductivity, high apparent density, no coarse particles, and high mechanical strength. Since firing can be carried out at high temperatures of 1600°C or higher in the firing furnace, it is preferable to use an air atmosphere.

[0070] 4 shows a heat curve that schematically illustrates the relationship between firing temperature and elapsed time. Optimizing the heat curve makes it possible to produce a β-alumina sintered body without calcining, thereby significantly reducing the production cost of the β-alumina sintered body. In one embodiment, the firing process includes a first heating step in which the temperature is raised from room temperature to 950-1200°C at an average heating rate of 50-250°C / hr, a first holding step in which the temperature is held in the temperature range of 950-1200°C for 15-120 minutes following the first heating step, a second heating step in which the temperature is raised to 1350-1550°C at an average heating rate of 50-200°C / hr following the first holding step, a second holding step in which the temperature is held in the temperature range of 1350-1550°C for 60-180 minutes following the second heating step, and a second holding step in which the temperature is held in the temperature range of 1350-1550°C for 30-150°C / hr following the second holding step. The firing temperature conditions include a third heating step in which the temperature is increased to 1550-1650°C at an average heating rate of 100°C / hr, a third holding step in which the temperature is maintained at 1550-1650°C for 15-120 minutes following the third heating step, a first temperature decreasing step in which the temperature is decreased to 1400-1550°C at an average temperature decreasing rate of 200-500°C / hr following the third holding step, a fourth holding step in which the temperature is maintained at 1400-1550°C for 15-120 minutes following the first temperature decreasing step, and a second temperature decreasing step in which the temperature is decreased to room temperature at an average temperature decreasing rate of 100-500°C / hr following the fourth holding step. Note that the above firing temperature conditions refer to the ambient temperature inside the firing furnace. Note that the second temperature decreasing step may be performed to a degree that does not damage the kiln materials due to thermal shock, for example, by natural cooling, and strict temperature control is not required.

[0071] In a preferred embodiment, the firing step includes a first heating step of heating from room temperature to 1050-1100°C at an average heating rate of 100-200°C / hr, a first holding step of holding the temperature in the temperature range of 1050-1100°C for 30-90 minutes following the first heating step, a second heating step of heating to 1390-1460°C at an average heating rate of 100-150°C / hr following the first holding step, a second holding step of holding the temperature in the temperature range of 1390-1460°C for 90-150 minutes following the second heating step, and a second holding step of heating the material at an average heating rate of 50-100°C / hr following the second holding step. The method includes a third heating step in which the temperature is increased to 1570 to 1630 ° C. at a heating rate, a third holding step in which the temperature is maintained in the temperature range of 1570 to 1630 ° C. for 30 to 90 minutes following the third heating step, a first temperature decreasing step in which the temperature is decreased to 1460 to 1530 ° C. at an average temperature decreasing rate of 350 to 450 ° C. / hr following the third holding step, a fourth holding step in which the temperature is maintained in the temperature range of 1460 to 1530 ° C. for 30 to 90 minutes following the first temperature decreasing step, and a second temperature decreasing step in which the temperature is decreased to room temperature at an average temperature decreasing rate of 100 to 500 (e.g., 200 to 350) ° C. / hr following the fourth holding step. Note that the above temperature conditions refer to the atmospheric temperature in the firing furnace.

[0072] The temperature increase rate can be controlled by, for example, the flow rate of the gas in the furnace, the burner output, the burner position, the number of burners, etc. The temperature decrease rate can be controlled by, for example, the flow rate of the cooling air.

[0073] When firing the cylindrical compact with a bottom, it is preferable to place the compact in a firing vessel to prevent evaporation and scattering of sodium, which is the main component. The firing vessel is preferably made of magnesium oxide (MgO) of high purity (e.g., 99% by mass or more) because MgO is less likely to react with sodium.

[0074] A gas outlet is preferably provided at the bottom of the firing container. This allows gases other than sodium vapor to be discharged outside the firing container by utilizing the physical properties of sodium vapor, which has a lighter specific gravity than air. During firing, moisture, water of crystallization, binder decomposition gas, raw material decomposition gas, etc. are generated from the compact. If these gases fill the firing container, they can adversely affect the sintering of the compact, inhibiting shrinkage and preventing the desired dimensions, or reducing the strength and density of the sintered compact. Therefore, these generated gases, which have a heavier specific gravity than air, are discharged outside the firing container through a gas outlet provided at the bottom of the firing container. In other words, generated gases that adversely affect the sintering of the compact do not fill the firing container, and at the same time, sodium vapor is prevented from escaping. This makes it possible to obtain β-alumina sintered bodies with excellent dimensional properties and sintered body properties without changing the composition of β-alumina.

[0075] FIG. 5-1 shows an exemplary schematic diagram of a firing container 501, FIG. 5-2 shows a schematic enlarged view of the vicinity of the gas release outlet in FIG. 5-1, along with a state in which a compact 514 is placed on a setter 511, and FIG. 5-3 shows a schematic plan view of the bottom of the firing container viewed from above. The firing container 501 has an alumina bottom plate 508, an MgO divided bottom plate 506 placed on the bottom plate 508 and divided into four, and an MgO bottom plate 505 placed on the MgO divided bottom plate 506. The alumina bottom plate 508 may be divided (e.g., into four or nine parts). Gas release ports 502 are provided in a total of four locations on the peripheral wall of the bottom of the firing container 501. A setter 511 is provided on the MgO base plate 505, and the compact 514 is placed upside down on the setter 511 with the open end facing downward, and then placed in the firing container 501 for firing.

[0076] During firing, the compact 514 expands as it heats up and contracts as it cools. When placing the compact 514, it is preferable to place a straightening ring 515 with an inner circumferential surface along the outer circumferential surface near the open end of the compact 514. The material of the straightening ring 515 is preferably spinel (molar ratio MgO:Al2O3 = 1.25:1 (MgO 1.25 or more is required)), because it has the strength to withstand the force applied when the compact expands and is less likely to react with sodium gas. The distance between the outer circumferential surface of the compact 514 and the inner circumferential surface of the straightening ring 515 before heating can be, for example, 1 to 3 mm, expressed as the distance normal to the inner circumferential surface of the straightening ring 515. The inner circumferential shape of the straightening ring is adjusted to match the required cross-sectional outer circumferential shape (e.g., circular) of the β-alumina sintered compact. As a result, for example, if the required cross-sectional outer peripheral shape of the β-alumina sintered body is circular, the outer peripheral side surface of the molded body 514 will come into contact with the inner peripheral side surface of the correction ring 515 during expansion, thereby undergoing shape correction, so that contraction can begin in a state of high circularity, and it is possible to obtain a β-alumina sintered body with improved circularity.

[0077] To facilitate the release of gas trapped in the internal space on the inner peripheral side of the compact 514, the porosity of the setter is preferably 5 to 20%. The material of the setter 511 is preferably spinel (MgO:Al2O3 = 1.25:1 in molar ratio (MgO 1.25 or more is required)) because it is easy to adjust the porosity of the setter and is less likely to react with Na gas. The firing container 501 can be placed on beams arranged at a predetermined interval within the firing furnace. Pads may be interposed between the firing container 501 and the beams to prevent deformation of the alumina bottom plate 508 and the MgO dividing base plate 506. To improve straightness, it is preferable that the pads do not deform and remain flat at high temperatures of approximately 1600°C, so high-strength, high-density pads are desirable. For example, a support 512 may be erected on a base plate 513, and a recrystallized SiC beam (lower stage) 510 and a recrystallized SiC beam (upper stage) 509 may be arranged on the support 512 so as to be perpendicular to each other, and the firing container 501 may then be placed on the recrystallized SiC beam (upper stage) 509. In this case, one or more recrystallized SiC pads 507 may be sandwiched between the firing container 501 and the recrystallized SiC beam (upper stage) 509.

[0078] By increasing the size of the firing vessel, multiple molded bodies (e.g., 5 to 15 bodies) can be accommodated in one firing vessel and fired simultaneously. In other words, compared to a method in which molded bodies are individually placed in a firing vessel and fired, the manufacturing process is simplified and the area occupied by the setter per unit number of molded bodies is reduced, which reduces fuel costs and ultimately enables a significant reduction in manufacturing costs.

[0079] When multiple compacts are simultaneously fired in a firing furnace, regardless of whether the compacts are housed in firing containers, the thermal history of the multiple compacts arranged in the firing furnace varies depending on the position of the compacts in the furnace, as conditions such as the airflow within the firing furnace, the distance from the burner, and the direct flame from the burner (elliptical or linear flame) change. Figure 6 shows a schematic diagram of multiple firing containers 501 containing the compacts placed on a cart 605 and loaded into a firing furnace 602 (a shuttle kiln or a tunnel kiln), and the interior of the firing furnace is heated using a flame from a burner 603 to fire the compacts. The multiple firing containers 501 are arranged in the direction of travel of the cart 605 (vertical direction) and in the direction perpendicular to that (horizontal direction) (e.g., vertical x horizontal = 3 to 6: 6 to 10).

[0080] For this reason, when multiple compacts are fired simultaneously in a firing furnace, the straightness of the resulting β-alumina sintered bodies is not consistent, and some do not meet the quality control standards. In this case, it is extremely difficult to change the firing conditions (adjusting the positions of the multiple compacts placed in the firing furnace, the position, number and output of the burners, the flow rate of the gas in the furnace, etc.) each time so that all β-alumina sintered bodies meet the quality control standards. The present inventors have investigated a simple method for adjusting the straightness of the β-alumina sintered bodies and have found that adjusting the inclination of the mounting surface of the setter on which the compacts are placed is effective.

[0081] Specifically, we confirmed the bending direction and position in the firing furnace (the position through which the β-sintered body passes in the firing furnace in the case of a continuous furnace) of a β-sintered body that was placed on a setter with a horizontal mounting surface (horizontal setter) and fired. We then found that the straightness of a molded body placed in the same position in the firing furnace could be improved by placing it on a setter with a mounting surface that slopes downward toward the opposite side of the bending direction (inclined setter) and firing it under the same other conditions as before.

[0082] A schematic diagram conceptually illustrating this is shown in Figure 7. The bottomed, cylindrical β-alumina sintered body 710a on the left was obtained by firing a bottomed, cylindrical compact in an inverted position with its open end 711a facing downward on a setter 720a with a horizontal mounting surface. Measurement of the straightness of the β-alumina sintered body 710a revealed that it was bent at a 50° angle relative to the south. Therefore, for the next firing, the compact to be placed in the same position as the β-alumina sintered body 710a in the firing furnace was placed in an inverted position with its open end 711b facing downward on a setter 720b with a mounting surface tilted downward at 230°, the opposite direction of the bending. If the compact is fired under the same firing conditions as the previous firing (meaning that the firing conditions are not intentionally changed, excluding unavoidable fluctuations in firing conditions), the straightness of the β-alumina sintered body 710b on the right obtained by the next firing will be improved.

[0083] In this specification, the straightness of a bottomed cylindrical β-alumina sintered body (β-alumina tube) refers to a value measured by the following method. The β-alumina tube is placed upright on a horizontal surface with its open end facing downward. Next, a laser scanning micrometer is used to determine the virtual center position (Ct) of the β-alumina tube near the bottom end (height Hb) and the virtual center position (Cb) of the β-alumina tube near the top end (height Ht). The absolute value |C| of the difference in horizontal distance from the vertical knife edge to Ct and Cb is calculated. With the knife edge position fixed, the β-alumina tube is rotated 10° in increments, and the absolute value |C| is similarly calculated each time the β-alumina tube is rotated once. Of all the absolute values ​​|C| obtained during one rotation, the maximum value is calculated as the straightness of the β-alumina tube: |C| × (height of the β-alumina tube) ÷ (Ht - Hb).

[0084] The inclination of the inclined setter can be adjusted depending on the straightness of the beta-alumina sintered body with bending so as to improve the straightness of the next sintered compact. For example, if the bending is significant, the inclination of the inclined setter can be increased. For example, if the straightness of a beta-alumina sintered body obtained by sintering a bottomed cylindrical molded body having a height of 450 to 650 mm, an inner diameter of 50 to 70 mm, and a wall thickness of 1.5 to 2.0 mm exceeds 0.75 mm, the straightness can be improved by setting the gradient (vertical distance / horizontal distance) of the inclined setter in the range of 0.3 / 100 to 0.7 / 100, preferably 0.4 / 100 to 0.6 / 100. Experience has shown that beta-alumina sintered bodies with extremely large bending are not manufactured, so it is sufficient to prepare a single level of inclined setter with a gradient of, for example, 0.5 / 100; there is no need to prepare multiple levels of inclined setters.

[0085] Therefore, according to one embodiment of the present invention, there is provided a method for producing a β-alumina sintered body, the method comprising the steps of: preparing a first cylindrical shaped body with a bottom that contains a β-alumina raw material composition; placing the first cylindrical shaped body in an inverted state with its open end facing downward on a first setter that is placed at a predetermined position in a firing furnace and has a horizontal mounting surface; firing the first cylindrical shaped body to produce a first cylindrical β-alumina sintered body with a bottom; measuring the straightness of the first β-alumina sintered body and identifying the direction of bending; preparing a second cylindrical shaped body with a β-alumina raw material composition; placing the second cylindrical shaped body in an inverted state with its open end facing downward on a second setter that is placed at the same position as the first setter in the firing furnace and has a mounting surface that is inclined downward toward the opposite direction to the bending direction; and firing the second cylindrical shaped body to produce a second cylindrical β-alumina sintered body with improved straightness than the first β-alumina sintered body.

[0086] The criterion for determining whether or not a tilted setter should be used in the next firing may be appropriately set depending on the straightness required for the bottomed cylindrical β-alumina sintered body. For example, if the straightness of the first β-alumina sintered body exceeds 0.75 mm, a tilted setter may be used in the next firing to produce a second β-alumina sintered body with improved straightness, for example, a straightness of 0.75 mm or less.

[0087] As described above, it is desirable that the bottomed cylindrical β-alumina sintered body has high sodium ion conductivity, high apparent density, no coarse particles, and high mechanical strength.

[0088] Specifically, the Na ion conductive resistivity of the first and second β-alumina sintered bodies at 320°C is preferably 3.5 Ω cm or less, and more preferably 3.2 Ω cm or less. The lower the Na ion conductive resistivity, the better, but there is a limit. Therefore, from the viewpoint of ease of manufacture, the Na ion conductive resistivity of the first and second β-alumina sintered bodies at 320°C is preferably 2.5 Ω cm or more, and more preferably 2.7 Ω cm or more. Therefore, the Na ion conductive resistivity of the first and second β-alumina sintered bodies at 320°C is preferably, for example, 2.5 to 3.5 Ω cm, and even more preferably 2.7 to 3.2 Ω cm.

[0089] In this specification, the Na ion conduction resistivity of a bottomed cylindrical β-alumina sintered body (β-alumina tube) is measured by the following method. As shown in Figure 8, the β-alumina tube is placed in a stainless steel cylindrical container and heated to 140°C. The inside and outside of the β-alumina tube are filled with molten metallic Na up to about one-third of the height of the β-alumina tube. The temperature of the β-alumina tube is then raised to 400°C, and the voltage is measured while a constant current is applied. While continuing to measure the voltage, the temperature of the β-alumina tube is gradually lowered from 380°C to 300°C. The temperature of the β-alumina tube is lowered to 140°C, and the filled metallic Na is recovered. The tube is then returned to room temperature. 24 hours after the voltage measurement, the β-alumina tube is treated with alcohol, and the outer diameter and wall thickness of the β-alumina tube are then measured. Then, from the measured values ​​of the filling height of the metallic Na solution, the outer diameter and the wall thickness of the β-alumina tube, the conductive resistivity of Na ions at each temperature (= resistance value × (pi × outer diameter × filling height) / wall thickness) is calculated, and an approximate straight line is found by the least squares method to determine the conductive resistivity of Na ions at 320°C.

[0090] The apparent density of the first and second β-alumina sintered bodies is preferably 3.22 g / cc or more, and more preferably 3.23 g / cc or more. The higher the apparent density, the better, but there is a limit. Therefore, from the viewpoint of ease of production, the apparent density of the first and second β-alumina sintered bodies is preferably 3.25 g / cc or less, and more preferably 3.24 g / cc or less. Therefore, the apparent density of the first and second β-alumina sintered bodies is preferably, for example, 3.22 to 3.25 g / cc, and more preferably 3.23 to 3.24 g / cc.

[0091] In this specification, the apparent density of the β-alumina sintered body is measured by the Archimedes method.

[0092] The maximum particle size of the ceramic particles constituting the first and second β-alumina sintered bodies is preferably 200 μm or less, and more preferably 100 μm or less. The smaller the maximum particle size, the better, but there is a limit. Therefore, from the viewpoint of ease of production, the maximum particle size of the ceramic particles constituting the first and second β-alumina sintered bodies is preferably 5 μm or more, and more preferably 50 μm or more. Therefore, the maximum particle size of the first and second β-alumina sintered bodies is preferably, for example, 5 to 200 μm, and more preferably 50 to 100 μm.

[0093] In this specification, the maximum particle size of the ceramic particles constituting the bottomed cylindrical β-alumina sintered body is measured by the following procedure. The β-alumina sintered body is sliced ​​near the center in the longitudinal direction to obtain a tube (e.g., a cylinder) with a length of 10 mm in the central axis direction. Next, the tube is cut along the central axis to obtain a sample that is 1 / 4 the size of the tube (in the case of a cylinder, a cross-sectional arc shape with a central angle of 90°). Next, an arbitrary point on the cut surface of the obtained sample is roughly finished with a diamond grinding wheel having a grain size of #280 as specified in JIS R6001-2:2017, and then intermediately finished with a diamond grinding wheel having a grain size of #800 as specified in JIS R6001-2:2017 (Electrical Resistance Testing Method), and the cumulative height fraction (d s50 The ceramic particles are mirror-polished with a diamond paste having a diameter of approximately 1.0 μm. The particles are then etched with phosphoric acid, and the particle diameter is observed under a microscope (magnification × 20), and the maximum particle diameter (circle-equivalent diameter) of the ceramic particles contained within a field of view (2 mm × 45 mm) is measured. The same polishing and measurement are performed on any one point on the cut surface when cut along the central axis, and the maximum particle diameter is measured. The maximum value of both is then taken as the measured value.

[0094] The internal hydrostatic burst strength of the first and second β-alumina sintered bodies is preferably 173 MPa or more, and more preferably 220 MPa or more. The higher the internal hydrostatic burst strength, the better, but there is a limit. Therefore, from the viewpoint of ease of manufacture, the internal hydrostatic burst strength of the first and second β-alumina sintered bodies is preferably 300 MPa or less, and more preferably 250 MPa or less. Therefore, the internal hydrostatic burst strength of the first and second β-alumina sintered bodies is preferably, for example, 173 to 300 MPa, and more preferably 220 to 250 MPa.

[0095] In this specification, the internal water pressure burst strength of a bottomed cylindrical β-alumina sintered body is measured by the following method. A bottomed cylindrical β-alumina sintered body is placed upright with its bottom facing downwards, and a bag-shaped rubber tube is inserted into the inside of the β-alumina sintered body. Water is supplied to the rubber tube to inflate it, thereby applying pressure to the entire inner surface of the β-alumina sintered body. The pressure is increased until the β-alumina sintered body bursts, and the pressure at the time of bursting is taken as the measured value.

[0096] Additionally, the obtained cylindrical β-alumina sintered body with a bottom may be subjected to a visual inspection to check for the presence or absence of abnormalities such as cracks, peeling, stains, pores, adhesion of foreign matter, inclusion of foreign matter, etc. The visual inspection may be performed manually, but it is preferable to use an inspection device that can acquire an inspection image using a camera and automatically determine whether or not the inspection image satisfies a predetermined standard based on the inspection image.

[0097] Furthermore, the obtained β-alumina sintered body may be subjected to finishing processes such as cutting and grinding the open end.

[0098] (1. Granulation Step) The following raw materials were prepared. Each raw material was used with the highest possible purity. Aluminum oxide: powder (D50 = 50 μm) Magnesium hydroxide: powder (D50 = 1 μm) Sodium carbonate: powder (D50 = 200 μm) Sodium bicarbonate: powder (D50 = 200 μm) Tantalum oxide (sintering aid): powder (D50 = 1 μm) Zirconium dioxide (grinding medium): powder (D50 = 500 μm) Citric acid monohydrate (dispersant): powder (D50 = 150 μm) Acrylic acid binder: aqueous solution, Aron AS1100 (Toagosei Co., Ltd.) Triacetin (plasticizer): liquid Polyoxyalkylene glycol lubricant: liquid, Newpol 50HB2000 (Sanyo Chemical Industries, Ltd.) Octanol (antifoaming agent): liquid

[0099] The above raw materials were placed in a stirring mill containing 1,130 L of water and pulverized and mixed at a rotation speed of 1,250 rev / min to prepare a slurry of a raw material composition of β-alumina. The blending amounts of each raw material are shown below. The timing of adding the raw materials to the stirring mill was changed for each raw material so that the stirring time would be the value shown below. The shorter the stirring time, the later the timing of adding the raw material.・Aluminum oxide: amount 1,130 kg, stirring time 15 hours ・Magnesium hydroxide: amount 81.56 kg, stirring time 2 hours ・Sodium carbonate: amount 182.56 kg, stirring time 2 hours ・Sodium hydrogen carbonate: amount 48.30 kg, stirring time 2 hours ・Tantalum oxide (sintering aid): amount 1.31 kg, stirring time 19 hours ・Zirconium dioxide (grinding media): amount 18 kg, stirring time 19 hours (the entire amount was worn away and included in the raw material composition of β-alumina.) ・Citric acid monohydrate (dispersant): amount 1.13 kg, stirring time 19 hours ・Acrylic acid binder: amount 36.09 kg (solids concentration 30% by mass), stirring time 2 hours ・Triacetin (plasticizer): amount 1.44 kg, stirring time 2 hours Polyoxyalkylene glycol lubricant: amount 2.89 kg, mixing time 2 hours Octanol (antifoaming agent): amount 0.14 kg, mixing time 2 hours

[0100] The pH of the prepared slurry was 10.0. The prepared slurry was discharged from the agitator mill, transported through a pipe, and supplied to a spray dryer. During the slurry transportation, the following foreign matter removal treatments were carried out in order. A tank equipped with an agitator was installed in the slurry transportation route, and the slurry was temporarily stored in the tank while being appropriately stirred. - Removal of iron-based foreign matter from raw materials using a magnetic separator - Removal of coarse foreign matter from raw materials using a multi-stage vibrating sieve (a multi-stage vibrating sieve using a filter with a mesh size of 45 μm and a filter with a mesh size of 20 μm, as specified in JIS Z8801-1:2019, in that order) - Removal of iron-based foreign matter that gets mixed in during the process using a magnetic separator - Removal of fibrous foreign matter that gets mixed in during the process using a multi-stage vibrating sieve with a multi-layer filter (the multi-layer filter has a multi-layer structure of a twill weave filter (mesh size of 20 μm as specified in JIS Z8801-1:2019) → a twill duvet weave filter (mesh size of 32 μm as specified in JIS Z8801-1:2019) → a twill weave filter (mesh size of 32 μm as specified in JIS Z8801-1:2019)).・A magnetic separator is used to remove iron-based foreign matter that gets mixed in during the process (performed just before supplying the slurry to the spray dryer).

[0101] The D50 and viscosity at 25° C. of the particles of the raw material composition of β-alumina in the slurry supplied to the spray dryer were measured by the above-mentioned measuring method, and the D50 was 0.65 μm and the viscosity was 80 cP.

[0102] Next, the slurry that had been subjected to the foreign matter removal treatment was supplied to a spray dryer and granulated to prepare first granules, and a process of secondary drying the first granules to prepare second granules was carried out continuously. As the spray dryer, a rotary atomizer type spray dryer with a two-point collection system was used, and the rotation speed was set to 10,000 rev / min, the inlet temperature to 230°C, and the outlet temperature to 105°C. Secondary drying was carried out using a continuous fluidized bed dryer, and the interior was gradually cooled so that the outlet temperature was about 25°C. In addition, a magnetic separator was installed downstream of the continuous fluidized bed dryer to remove iron-based foreign matter.

[0103] (2. Molding Process) The second granules of the β-alumina raw material composition obtained in the above granulation process were pneumatically transported to a flash blender, stored in the flash blender for a predetermined time, and then supplied to a CIP molding device by pneumatic transport. A magnetic separator was installed immediately before the CIP molding device to remove iron-based foreign matter that may have been mixed in due to friction with the piping during pneumatic transport. (Moisture Content) The moisture content of the second granules used in the molding process was measured using the method described above and was found to be 1.6% by mass. (D50) The D50 of the second granules used in the molding process was measured using the method described above and was found to be 55 μm. (Impurity Concentration) The impurity concentrations of the second granules to be subjected to the molding step were measured using the method described above, and were found to be 60 ppm by mass for SiO, 110 ppm by mass for Fe, 35 ppm by mass for Ti, 20 ppm by mass for CaO, and 10 ppm by mass for K. (Fibrous Foreign Matter) The second granules to be subjected to the molding step were inspected using the method described above to determine whether or not they contained fibrous foreign matter, and no fibrous foreign matter was found.

[0104] As shown in Figure 3, the CIP molding apparatus had a molding space 300 formed by the inner peripheral surface of a molding rubber die 301, the outer peripheral surface of a mandrel 302, the lower surface of a top rubber stopper 303, and the upper surface of a bottom punch 304. The molding rubber die 301 was composed of a relatively hard inner layer portion 301a (durometer hardness of 90° measured in accordance with JIS K6253-3:2012) and a relatively soft portion 301b other than the inner layer (durometer hardness of 50° measured in accordance with JIS K6253-3:2012). The thickness of the inner layer portion 301a that comes into contact with the second granules was set within a range of 1.5 to 2.5 mm. The mandrel 302 was composed of a base material made of an iron-based material and an outermost surface layer (maximum height R max The surface roughness was 1 μm and the contact angle with water was 75°.

[0105] A CIP molding machine was used to carry out the steps of granule filling, pressurization, decompression, and compact removal to form a cylindrical compact (height 600 mm, inner diameter 50-70 mm, wall thickness 1.5-2.0 mm) with a bottom at room temperature. The pressurization step involved the following steps in order: - A step of increasing the pressure from 0 MPa (gauge pressure) to 20 MPa (gauge pressure) at an average rate of 10 MPa / sec; - A step of increasing the pressure from 20 MPa (gauge pressure) to 216 MPa (gauge pressure) at an average rate of 13 MPa / sec; - A step of maintaining a pressure of 190-216 MPa (gauge pressure) for 2 seconds; - A step of reducing the pressure from 190 MPa (gauge pressure) to 10 MPa (gauge pressure) at an average rate of 40 MPa / sec; - A step of reducing the pressure from 10 MPa (gauge pressure) to 0 MPa (gauge pressure) at an average rate of 3 MPa / sec.

[0106] The appearance of the resulting cylindrical molded body with a bottom was inspected using a camera, and no abnormalities such as cracks or stains were found.

[0107] (3. Firing Process) A large number of bottomed cylindrical compacts obtained in the above-mentioned forming process were fired in a continuous firing furnace (tunnel kiln). For firing, an MgO firing container 501 with a gas outlet at the bottom, as shown in Figures 5-1 to 5-3, was prepared, and each compact was placed inverted with its open end facing downward on nine horizontal setters laid at the bottom of the firing container 501. Also, as shown in Figure 6, a plurality of firing containers 501 were placed on one cart 605 and loaded into a firing furnace 602. A total of 32 firing containers 501 were placed on one cart 605, with four rows in the direction of travel of the cart 605 and eight rows in a direction perpendicular thereto.

[0108] The atmosphere in the firing furnace was air, and the firing process was carried out by carrying out the following steps in order. a first heating step in which the temperature is increased from room temperature to 1080°C at an average heating rate of 150°C / hr; a first holding step in which the temperature is maintained at 1080°C for 60 minutes following the first heating step; a second heating step in which the temperature is increased to 1430°C at an average heating rate of 150°C / hr following the first holding step; a second holding step in which the temperature is maintained at 1430°C for 110 minutes following the second heating step; a third heating step in which the temperature is increased to 1600°C at an average heating rate of 60°C / hr following the second holding step; a third holding step in which the temperature is maintained at 1600°C for 45 minutes following the third holding step; a first cooling step in which the temperature is reduced to 1495°C at an average cooling rate of 400°C / hr following the first cooling step; a fourth holding step in which the temperature is maintained at 1495°C for 45 minutes following the fourth holding step; a second cooling step in which the temperature is reduced to room temperature at an average cooling rate of 300°C / hr following the fourth holding step. The above temperature conditions refer to the atmospheric temperature in the firing furnace.

[0109] The straightness of each of the β-alumina tubes thus obtained was measured, and although most of them had a straightness of 1.0 mm or less, some bent β-alumina tubes with a straightness of about 1 to 2 mm were also confirmed. Therefore, the setter on which such bent β-alumina tubes were placed was identified and replaced with a setter having a mounting surface (gradient = 0.5 / 100) that was inclined downward toward the opposite side to the bending direction of the β-alumina tube.

[0110] Then, a large number of molded bodies formed by the same manufacturing method as described above were fired under the same firing conditions as above except for replacing the setter, to produce β-alumina tubes.The straightness of all of the β-alumina tubes for which the setter was replaced was improved, reaching within 0.75 mm.

[0111] (4. Characteristics of β-alumina tubes) One of the β-alumina tubes with improved straightness in this way was selected from the right end of the last row, and the β" ratio, sodium ion conductive resistivity, and apparent density were measured using the methods described above. This is because, from experience, the temperature of the β-alumina tubes at both ends of the last row tends to be low, and therefore the sodium ion conductive resistivity is likely to be high and the apparent density is likely to be low. In addition, one of the β-alumina tubes with improved straightness in this way was selected from the third row from the front and the fourth row from the right in the direction of travel, and the maximum particle diameter and internal hydrostatic crushing strength of the ceramic particles constituting the β-alumina tube were measured using the methods described above. This is because, from experience, the temperature of the β-alumina tubes near the center tends to be high, and therefore grain growth is likely to be promoted, and therefore strength is likely to decrease. The results are shown below. β" ratio: 93% Na ion conductive resistivity: 3.0 Ω cm Apparent density: 3.24 g / cc・Maximum particle diameter of ceramic particles that make up the β-alumina tube: 80 μm ・Internal water pressure breaking strength: 230 MPa

[0112] Furthermore, one of the β-alumina tubes with improved straightness was randomly selected and subjected to a visual inspection using a camera, and no abnormalities such as cracks or stains were found.

[0113] 2: Sulfur 3: Sodium 10: NAS battery 21: Positive electrode side body 22: Positive electrode ring fitting 23: Positive electrode terminal 24: Positive electrode side bottom lid 27: Positive electrode current collector 28: Positive electrode space 31: Negative electrode ring fitting 32: Negative electrode side upper lid 36: Negative electrode container 36a: Small hole 37: Space 38: Negative electrode space 39: Negative electrode terminal 40: Insulating ring 50: Solid electrolyte 70: Safety tube 80: Sleeve tube 82: Cover 84: Plate material 300: Molding space 301: Molding rubber mold 301a: Inner layer portion 301b: Part 302: Mandrel 303: Top rubber stopper 304: Bottom punch 501: Sintering container 502: Gas release port 505: Base plate 506: Divided base plate 507: Pad 508: Bottom plate 509: Beam (upper stage) 510: Beam (lower stage) 511: Setter 512: Support 513: Base plate 514: Molded body 515: Correction ring 602: Sintering furnace 603: Burner 605: Cart 710a: β-alumina sintered body 710b: β-alumina sintered body 711a: Open end 711b: Open end 720a: Setter 720b: Setter

Claims

1. A method for producing a β-alumina sintered body, comprising: a step of preparing a first cylindrical, bottomed compact containing a β-alumina raw material composition; a step of placing the first cylindrical, bottomed compact in an inverted state with its open end facing downward on a first setter that is placed at a predetermined position in a firing furnace and has a horizontal mounting surface; a step of firing the first cylindrical, bottomed compact; a step of measuring the straightness of the first β-alumina sintered body and identifying the direction of bending; a step of preparing a second cylindrical, bottomed compact containing a β-alumina raw material composition; a step of placing the second cylindrical, bottomed compact in an inverted state with its open end facing downward on a second setter that is placed at the same position as the first setter in the firing furnace and has a mounting surface that is inclined downward toward the opposite side to the bending direction; and a step of firing the second cylindrical, bottomed compact.

2. A method for producing a beta-alumina sintered body as described in claim 1, wherein the steps of preparing the first and second compacts each include a step of granulating a slurry in which the beta-alumina raw material composition is dispersed in water using a spray dryer to prepare first granules, a step of secondary drying the first granules to prepare second granules, and a step of molding the second granules into a bottomed cylindrical shape using a dry isostatic pressing method.

3. The method for producing a β-alumina sintered body according to claim 1, wherein the raw material composition of β-alumina contains aluminum oxide, magnesium hydroxide, sodium carbonate, sodium bicarbonate, tantalum oxide, and zirconium dioxide.

4. The method for producing a β-alumina sintered body according to claim 3, wherein the raw material composition of β-alumina further contains citric acid monohydrate.

5. The method for producing a β-alumina sintered body according to claim 3, wherein the raw material composition of β-alumina further contains an acrylic acid-based binder.

6. The method for producing a β-alumina sintered body according to claim 3, wherein the raw material composition of β-alumina further contains triacetin.

7. The method for producing a β-alumina sintered body according to claim 3, wherein the raw material composition of β-alumina further contains a polyoxyalkylene glycol-based lubricant.

8. The method for producing a β-alumina sintered body according to claim 3, wherein the raw material composition of β-alumina further contains octanol.

9. The method for producing a β-alumina sintered body according to claim 1, wherein the raw material composition of β-alumina contains 75 to 80 parts by mass of aluminum oxide, 5 to 6 parts by mass of magnesium hydroxide, 12 to 13 parts by mass of sodium carbonate, 3 to 4 parts by mass of sodium bicarbonate, 0.5 part by mass or less of tantalum oxide, and 0.5 to 1.5 parts by mass of zirconium dioxide, when the total of aluminum oxide, magnesium hydroxide, sodium carbonate, and sodium bicarbonate is 100 parts by mass.

10. A method for producing a β-alumina sintered body as described in claim 9, wherein the β-alumina raw material composition further contains 0.5 parts by mass or less of citric acid monohydrate when the total of aluminum oxide, magnesium hydroxide, sodium carbonate, and sodium bicarbonate is 100 parts by mass.

11. A method for producing a β-alumina sintered body according to claim 9, wherein the β-alumina raw material composition further contains 0.5 to 1 part by mass, calculated as solid content, of an acrylic acid-based binder when the total of aluminum oxide, magnesium hydroxide, sodium carbonate, and sodium hydrogencarbonate is 100 parts by mass.

12. A method for producing a β-alumina sintered body according to claim 9, wherein the β-alumina raw material composition further contains 0.1 to 0.5 parts by mass of triacetin when the total of aluminum oxide, magnesium hydroxide, sodium carbonate, and sodium hydrogencarbonate is 100 parts by mass.

13. A method for producing a β-alumina sintered body according to claim 9, wherein the β-alumina raw material composition further contains 0.1 to 0.5 parts by mass of a polyoxyalkylene glycol-based lubricant when the total amount of aluminum oxide, magnesium hydroxide, sodium carbonate, and sodium hydrogencarbonate is 100 parts by mass.

14. A method for producing a β-alumina sintered body as described in claim 9, wherein the β-alumina raw material composition further contains 0.1 parts by mass or less of octanol when the total of aluminum oxide, magnesium hydroxide, sodium carbonate, and sodium bicarbonate is 100 parts by mass.

15. A method for producing a β-alumina sintered body according to claim 9, wherein the raw material composition of β-alumina contains sodium carbonate and sodium bicarbonate in a molar ratio of sodium carbonate:sodium bicarbonate = 7:3 to 8:

2.

16. A method for producing a β-alumina sintered body according to claim 2, wherein the slurry of the β-alumina raw material composition has a volume-based median diameter (D50) of 0.5 to 0.8 μm as measured by a laser diffraction particle size distribution analyzer.

17. A method for producing a beta-alumina sintered body as described in claim 2, wherein the slurry of the beta-alumina raw material composition has a viscosity of 200 cP or less at 25°C as measured using a single cylindrical rotational viscometer specified in JIS Z8803:2011.

18. A method for producing a β-alumina sintered body as described in claim 2, wherein the second granules subjected to the molding step have a SiO2 concentration of 320 mass ppm or less, an Fe2O3 concentration of 200 mass ppm or less, a TiO2 concentration of 1000 mass ppm or less, a CaO concentration of 100 mass ppm or less, and a K2O concentration of 50 mass ppm or less.

19. A method for producing a β-alumina sintered body as described in claim 2, wherein the second granules subjected to the molding step are free of fibrous foreign matter having an average fiber diameter of 10 μm or more and a length of 300 μm or more.

20. A method for producing a β-alumina sintered body according to claim 2, wherein the moisture content of the second granules subjected to the molding step is 1.3 to 1.9 mass %.

21. A method for producing a beta-alumina sintered body as described in claim 2, wherein the molding step includes a step of pressurizing the second granules filled in a molding space formed in a pressure vessel by the inner peripheral surface of the molding rubber mold, the outer peripheral surface of the mandrel, the lower surface of the top rubber stopper, and the upper surface of the bottom punch, by compressing the molding space from the outer peripheral side of the rubber mold with a pressure medium.

22. A method for producing a β-alumina sintered body as set forth in claim 21, wherein the pressurizing step includes the steps of increasing the pressure from 0 MPa (gauge pressure) to 190 to 230 MPa (gauge pressure), maintaining the pressure at 190 to 230 MPa (gauge pressure) for 1 to 2 seconds, and reducing the pressure to 0 MPa (gauge pressure).

23. A method for producing a β-alumina sintered body as set forth in claim 22, wherein the pressurizing step includes increasing the pressure from 20 MPa (gauge pressure) to 190-230 MPa (gauge pressure) at an average pressure increasing rate of 10-15 MPa / sec, and the depressurizing step includes decreasing the pressure from 190-230 MPa (gauge pressure) to 10 MPa (gauge pressure) at an average depressurizing rate of 35-45 MPa / sec.

24. A method for producing a β-alumina sintered body as described in claim 21, wherein the rubber mold has an inner layer portion having a durometer hardness of 85 to 95° measured in accordance with JIS K6253-3:2012, and a portion other than the inner layer having a durometer hardness of 45 to 55° measured in accordance with JIS K6253-3:2012.

25. The process for producing a first β-alumina sintered body and the process for producing a second β-alumina sintered body each include a first heating step of heating from room temperature to 1050-1100°C at an average heating rate of 100-200°C / hr, a first holding step of holding the temperature in the temperature range of 1050-1100°C for 30-90 minutes following the first heating step, a second heating step of heating to 1390-1460°C at an average heating rate of 100-150°C / hr following the first holding step, a second holding step of holding the temperature in the temperature range of 1390-1460°C for 90-150 minutes following the second heating step, and a second holding step of holding the temperature in the temperature range of 1390-1460°C for 50-180 minutes following the second holding step.

3. The method for producing a β-alumina sintered body according to claim 2, further comprising: a third heating step in which the temperature is increased to 1570-1630°C at an average heating rate of 00°C / hr; a third holding step in which the temperature is maintained in the temperature range of 1570-1630°C for 30-90 minutes following the third heating step; a first temperature decreasing step in which the temperature is decreased to 1460-1530°C at an average temperature decreasing rate of 350-450°C / hr following the third holding step; a fourth holding step in which the temperature is maintained in the temperature range of 1460-1530°C for 30-90 minutes following the first temperature decreasing step; and a second temperature decreasing step in which the temperature is decreased to room temperature at an average temperature decreasing rate of 100-500°C / hr following the fourth holding step.

26. A method for producing a β-alumina sintered body according to claim 1, wherein the second β-alumina sintered body has a sodium ion conduction resistivity of 3.2 Ω·cm or less at 320°C.

27. A method for producing a β-alumina sintered body according to claim 1, wherein the apparent density of the second β-alumina sintered body is 3.22 g / cc or more.

28. A method for producing a β-alumina sintered body according to claim 1, wherein the maximum particle size of the ceramic particles constituting the second β-alumina sintered body is 200 μm or less.

29. A method for producing a β-alumina sintered body according to claim 1, wherein the second β-alumina sintered body has an internal water pressure fracture strength of 173 MPa or more.

30. A method for producing a β-alumina sintered body according to claim 1, wherein the straightness of the second β-alumina sintered body is 0.75 mm or less.

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