JPH0214891A - Reduction of surface defect of beta-alumina ceramic - Google Patents

Abstract

PURPOSE:To remove the cracks and improve the service life against deterioration of the title ceramic by impregnating said cracks existing on the surface of a beta-alumina sintered form with a solution containing a component capable of giving beta-alumina followed by heat treatment. CONSTITUTION:The defects such as hairy cracks existing on the surface of a beta-alumina sintered form are impregnated, under reduced pressure and/or increased pressure action, with a liquid consisting of a solution or slurry containing a component capable of giving the same composition as said sintered form, followed by heat treatment to allow beta-alumina to be left at said defects, thus removing said cracks.

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field) The present invention relates to a method for reducing surface defects in β-alumina ceramic inxes, and particularly to a method for improving the deterioration life of β-alumina tubes used in sodium-sulfur storage batteries. It is.

(Background technology) Sodium-based batteries have traditionally been used as a type of power storage battery.
Sulfur batteries are known. This battery uses molten metal sodium as the anode (cathode) and molten sulfur as the cathode (anode), and separates them with a solid electrolyte that is selectively permeable to Na'' ions. As such a solid electrolyte, β-alumina ceramics is used for -C, and it is disclosed in Japanese Patent Application Laid-open No. 54-85332 and Japanese Patent Publication No. 62-439.
As shown in Japanese Patent No. 50, etc., it has a tube-like shape with one end closed.

By the way, the quality of β-alumina ceramics used as a solid electrolyte is said to affect the characteristics and life of the battery.First of all, in order to prevent penetration of the bipolar active material and short circuit, 3.20g/ In addition to the need for a high sintered density of cm3 or higher (theoretical density -3.26 g/cm'), it is said that having no defects on the surface is effective in avoiding deterioration. In particular, cracks such as elongated arc-shaped cracks that extend inward from the surface cause concentration of current (Na' ion flow) at their tips and extend in the thickness direction of β-alumina ceramics. In the end, the facts that led to its destruction became clear. In other words, as a result of the current concentration at the tip of the crack, a large amount of metallic Na is deposited at the tip, and when the precipitated metallic Na moves along the crack, the crankshaft Internal pressure acts due to friction with the wall surface, and a force (Poiseuille pressure) that tries to widen the crack acts, causing the crack to extend in the thickness direction.

Note that cracks such as arc-shaped cracks, which are defects existing on the surface of β-alumina ceramics, are caused by β-alumina ceramics.
It is generated from the surface of alumina ceramics during the middle to late stages of firing or after firing due to the action of some external force such as thermal shock or mechanical shock, and its length (1) and the maximum diameter of its cross section ( The aspect ratio (ff/D) expressed as the ratio to D) is about 10 to 100, and the above-mentioned current concentration is likely to occur at the tip of such an elongated crack.

Therefore, cracks such as hair-like cracks, which are surface defects, could not be sufficiently eliminated simply by increasing the sintering density of β-alumina ceramics (sintered body).
Furthermore, no proposals have been made to date on measures to eliminate defects such as cracks localized on the surface, avoid current concentration based on them, and improve the deterioration life. be.

(Top of Solution Section) The present invention was made against the background of the above, and the problem to be solved is hair-like cracks localized on the surface of β-alumina ceramics. The objective is to provide an effective method to effectively reduce the defects of such surface defects, and also to avoid current concentration due to the growth of such surface defects and improve its deterioration life.

(Solution Means) In order to solve this problem, the present invention applies a reduced pressure and/or pressurizing action to defects such as hair-like cracks existing on the surface of a β-alumina sintered body. After impregnating with a solution or slurry containing a component that provides β-alumina with the same composition as the body, heat treatment is performed to leave β-alumina in the hair-like crank and other defects, resulting in -. The gist of this is to try to eliminate kushitsuku.

In short, in the present invention, β-alumina ceramics (sintered body) is immersed in a solution or slurry containing components that give β-alumina of the same composition, and hair-like cracks etc. localized on the surface of the After the solution or slurry has entered the defective part, heat treatment is performed to cause β-alumina to remain from the solution or slurry that has entered the defective part, and to deposit it in the defective part such as a hair-like crank, thereby filling the space. By burying it, its disappearance will be achieved.

By the way, as is well known, the β-alumina ceramic whose surface defects are reduced according to the method of the present invention has a chemical composition of -fllQ as shown in the table below. , usually Ap, zo3. L
After 1zcO3 (MgO), Na, and CO3 are uniformly blended, it is calcined and pulverized into a desired shape such as a tube shape (bag tube shape) with one end closed.
2.5-7. It is molded using a cold isostatic pressure (CIP) method using a pressure of about Ot/cm2, and then
It is manufactured by firing such a molded body at a temperature of about 1500°C or higher.

The β-alumina ceramics subjected to IJ in this way are generally considered to be a high-density sintered body of 3.20 g 7 cm or more, but as mentioned above, the surface has hair-like structures. Since defective parts such as cranks are localized, the following measures are taken to eliminate such defective parts.

That is, in order to fill in the surface defects of the β-alumina ceramics, the β-alumina ceramics are mixed into a solution or slurry containing components that have substantially the same chemical composition as the β-alumina ceramics and remain after firing. They are forced to immerse themselves.

Note that such an immersion solution may contain, for example, β as a solute.
- Water containing inorganic salts that provide alumina components, alkoxides, and other components that remain as oxides after calcination;
Alcohol frequently, benzene.

There are solutions that use toluene or other liquids that volatilize during calcination as a solvent, and it is also possible to use solutions that do not use such solvents and are made only of the liquid of the solute itself, such as a liquid of an alkoxide. It is also possible to use a slurry made by using a pulverized product of β-alumina ceramic itself or a finely pulverized product of the solute component and finely dispersing it in the solvent. Such a pulverized material does not necessarily need to be dissolved in a solvent as long as it is sufficiently finely divided to be able to penetrate into surface defects.

By the way, considering that surface defects such as cranks are to be filled with β-alumina provided in the above-mentioned solution or slurry to eliminate the cranks, it is desirable that such a solution or slurry has as much residual component as possible after firing. is clear. Therefore, when using a solution, the higher the solute concentration is, the better, and when using a slurry, the larger the amount of pulverized material dispersed therein, the more desirable. In addition, volatile components or disappearing components contained in the solute molecules themselves, such as NO! -, CH3COO-, etc., the smaller the proportion, the more effective it is to fill in surface defects.

Then, the target β-alumina ceramic from which surface defects are to be removed is immersed in a solution or slurry containing such predetermined components. Since the solution or slurry cannot be sufficiently impregnated and filled when the solution or slurry enters, the present invention makes use of reduced pressure and/or pressurization to effectively impregnate the solution or slurry. This is what I tried to do. That is, gas components exist in the surface defects of β-alumina ceramics,
Since this hinders the penetration of the solution or slurry, in order to remove the gas components in such surface defects, it is recommended to use a depressurizing effect or pressurizing effect to push the solution or slurry. It is to make it. More specifically, (a) a method of reducing the pressure of β-alumina ceramics and solution/slurry to below atmospheric pressure and then returning it to atmospheric pressure, and (b) a method of reducing the pressure of β-alumina ceramics and solution/slurry to below atmospheric pressure. After that, a method is adopted in which the pressure is increased and then the pressure is returned to atmospheric pressure.

Next, the β-alumina ceramic impregnated with a solution or slurry containing predetermined components in this manner is subjected to heat treatment, whereby β-alumina is absorbed from the impregnating solution or slurry into surface defects such as hair-like cracks. These surface defects will be filled in by depositing on the surface. Note that such heat treatment is carried out at an appropriate temperature depending on the type of β-alumina imparting component contained in the solution or slurry, but is generally carried out at a temperature of 1000 to 1 in an oxidizing atmosphere.
It is carried out at a temperature of about 600°C.

Then, by heat treatment in this oxidizing atmosphere,
The β-alumina-providing components present in the impregnating solution or slurry are oxidized to form the desired β-alumina and fill in surface defects. However, such oxidation is not necessary when pulverized β-alumina ceramics itself is used as the β-alumina imparting component.
The pulverized material is directly sintered into the crack by heat treatment.

In this way, the surface defects of β-alumina ceramics are filled with the β-alumina component of the same material, and their disappearance is attempted. It goes without saying that if such surface defects cannot be sufficiently filled in by a single operation, such operations may be repeated as many times as necessary.

(Examples) Below, some examples of the present invention will be shown to clarify the present invention more specifically, but the present invention is not limited in any way by the description of such examples. It goes without saying that this is not acceptable.

In addition to the following examples and the above-mentioned specific description, the present invention includes various changes, modifications, and changes based on the knowledge of those skilled in the art, as long as they do not depart from the spirit of the present invention. It should be understood that improvements and the like may be made.

Example 1 The weight composition ratio as an oxide is LizO: 0.75°Na
zO: 8.85. ALO*: A β-alumina ceramic solid electrolyte bag tube with a value of 90.40 was used as raw materials, A120z, L1zcOa and Na2CO.

It was obtained by firing a molded body formed by a CrP molding method. In addition, such a bag tube has an outer diameter of 1
5 +nmφ, inner diameter is 13mmφ, length is 200111
111, and its high density is 3.22 g/
cm 3 and the theoretical density of 3.26 g 7 cm
” was 99%.

On the other hand, methoxylithium (LiOCHz) was used so that the weight ratio as an oxide after heat treatment was the same as that of the β-alumina ceramic bag tube.

Methoxysodium (N a OCH:l ) and triethoxyaluminum (AI!, (OC,H5)3:
1 of each solute was weighed and sufficiently dissolved in anhydrous isopropyl alcohol solvent contained in a stainless steel vacuum container with an internal volume of 5I until a precipitate of the solute was formed.

Next, the β-alumina ceramic bag tube was immersed in the solution obtained in this way, and then the inside of the stainless steel container was
By vacuum pump, vacuum degree: l Torr, time: 1
The β-alumina bag tube was sufficiently impregnated with the above solution by reducing the pressure for 0 minutes and then returning to normal pressure.

After this impregnation operation, the β-alumina bag tube was taken out from the stainless steel container, and with the solution still attached to the bag tube, it was left at room temperature for about 30 minutes to roughly dry, and then heated at 120°C. It was completely dried in a drying dish for 1 hour to slowly evaporate the solvent, anhydrous isopropyl alcohol. Thereafter, the β-alumina bag tube was heat treated at 1300° C. for 1 hour in an electric furnace in an air atmosphere. Then, the above operation was repeated twice.

When the defect density on the surface of the thus obtained calcined β-alumina bag tube was investigated by a conventional water-soluble fluorescent penetrant test (Zygro) test, it was found that the density of indications that fluoresce under ultraviolet rays at defective areas was approximately equal to the unit square. The average number was less than 1 piece per senna meter. On the other hand, the indications for the β-alumina bag tubes that were not subjected to the solution adhesion and heat treatment were 20 on average per square centimeter, and after adhering the prescribed solution according to the method of the present invention as described above, It has been found that the surface defects of β-alumina bag tubes can be significantly reduced by heat treatment.

Example 2 The weight composition ratio of the oxide after heat treatment is MgO
:2.27. NazO: 8.73. AfzOl :89
, OO, magnesium chloride (MgC12
), sodium hydroxide (Nail ()) and aluminum chloride (/lcf, ) were weighed, and placed in distilled water stored in a plastic vacuum container with an internal volume of 5 μm until a precipitate of these solutes was formed. In order to prevent the formation of precipitates as much as possible, hydrochloric acid was added to the solution to make it weakly acidic.

Next, a β-alumina ceramic solid electrolyte bag tube having the same oxide weight ffHJl composition ratio was immersed in the above solution, and the inside of the reduced pressure container was heated to a vacuum degree of 1 Torr for a time of 10 using a vacuum pump. The β-alumina bag tube was sufficiently impregnated with the solution by reducing the pressure for a few minutes and then returning to normal pressure.

After the impregnation treatment, the β-alumina bag tube is taken out from the plastic vacuum container, and with the solution still attached to the bag tube, it is completely dried in a dryer at 120°C for 1 hour. The solvent water was evaporated. Thereafter, the β-alumina bag tube was heat-treated at 1300° C. for 2 hours in an electric furnace in an atmospheric atmosphere. Then, the above operation was repeated twice.

When the defect density on the surface of the thus obtained calcined β-alumina bag tube was investigated by a normal Zygro test, it was found that the density of indicators that fluoresce under ultraviolet rays at defective areas was less than 2 on average per unit square centimeter. there were.

As a result, it was found that by repeating the solution impregnation treatment and heat treatment as described above, surface defects existing on the surface of the β-alumina bag tube could be significantly reduced.

Example 3 Weight composition ratio as oxide is Lizo: 0.75
°Na, O: 8.85. Af203: 90.40
The β-alumina ceramics was dry-pulverized to a size of 1 mm or less using a roll crusher. Next, the coarsely ground material was passed through a sieve with an opening of 149 μm, and 200 g of the passed ground material and 40 g of distilled water were put into an attritor pulverizer, and finely milled until the center particle size of the slurry became 1 μm or less. Shattered. Note that the particle size of the obtained slurry was measured using a Sedigraph.

Next, the slurry thus obtained was transferred to a stainless steel vacuum container, and a β-alumina ceramic solid electrolyte bag tube having the same oxide composition ratio was immersed in the slurry, and the inside of the stainless steel container was then moved using a vacuum pump. The β-alumina bag tube was sufficiently impregnated with the β-alumina fine particles in the slurry by reducing the pressure under the conditions of vacuum degree: 1 Torr and time: 30 minutes, and then returning to normal pressure.

After this impregnation treatment, the β-alumina bag tube was taken out from the stainless steel container, and left with the slurry attached to the bag tube at room temperature to approximately dry it, and then 1
Completely dried in an oven at 20'C for 1 hour. Thereafter, the β-alumina bag tube was heat-treated at 1300° C. for 1 hour in an electric furnace in an atmospheric atmosphere. Also,
The above operation was repeated three times.

As a result, when the defect density on the surface of β-alumina bag tubes subjected to such heat treatment was investigated by a normal Zygro test, it was found that the density of indicators that emit fluorescence due to ultraviolet rays at defective areas was on average 3 per unit square centimeter. It was less than one. Therefore, β-
It has been found that the surface defects of the β-alumina bag tube can be significantly reduced by sufficiently impregnating the β-alumina bag tube with a slurry of fine alumina particles and then subjecting the tube to heat treatment.

(Effect of the invention) As is clear from the above explanation, according to the present invention, β-
By immersing alumina ceramics in a solution or slurry containing components that provide such β-alumina ceramics, sufficiently impregnating it, and then heat-treating it, the substantial amount localized on the surface of such β-alumina ceramics is It can be advantageously used as a solid electrolyte in sodium-sulfur batteries, for example, because it eliminates surface defects and effectively eliminates hair-like cracks that tend to cause local current concentration.
Thereby, it was possible to avoid current concentration and thereby effectively improve the deterioration life of β-alumina ceramics, and this is where the great industrial significance of the present invention lies.

Claims (1)

[Claims] A solution or slurry containing a component that provides β-alumina with the same composition as the sintered body by applying reduced pressure and/or pressure to defects such as hair-like cracks existing on the surface of the β-alumina sintered body. After impregnating with β, heat treatment is performed to form β in defects such as hair-like cracks.
- A method for reducing surface defects in β-alumina ceramics, characterized in that alumina remains to eliminate hair-like cracks.