JPH0696466B2 - Method for manufacturing β "alumina sintered body - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a β ″ -alumina sintered body. More specifically, it is suitable for sodium-sulfur batteries, etc. The present invention relates to a production method capable of inexpensively producing a dense and high-strength β ″ alumina sintered body having a conductive resistivity.

(Prior Art and Its Problems) A sodium-sulfur battery is a high temperature type secondary battery that operates at 300 to 350 ° C. Molten sodium is used as a negative electrode active material, and molten sulfur or sodium polysulfide is used as a positive electrode active material. As an electrolyte, β having sodium ion conductivity is used.
-Alumina (Na ₂ O ・ 11Al ₂ O ₃ ), β ″ -Alumina (Na ₂ O ・ 5
A sintered body made of a solid electrolyte such as Al ₂ O ₃ ) is used.
Since the sodium-sulfur battery requires particularly good performance and long service life, the solid electrolyte sintered body for sodium-sulfur battery needs the following characteristics.

(1) High density and no air permeability (2) High mechanical strength (3) Good ionic conductivity (4) Beta-alumina crystal grain size is uniform and there are no coarse particles of 100 μm or more Various manufacturing methods have been proposed for manufacturing a β ″ alumina sintered body having such characteristics.

For example, as a method for producing a Li ₂ O-stabilized β ″ alumina sintered body, a mixture of Na ₂ CO ₃ and α-Al ₂ O ₃ is previously calcined to form a calcined product containing β and β ″ alumina. In addition, separately prepared Li ₂ O ・ 5Al ₂ O ₃ (zetalithium aluminate) was mixed and calcined, and these two calcined products were further crushed and mixed in an organic solvent and sprayed again. The zeta process used as a raw material is known from Japanese Patent Publication No. 57-15063. In the zeta process, the dispersibility of Li ₂ O, which is a stabilizer, is improved, so that the formation of coarse particles is suppressed.

However, because two types of calcined products are created during raw material preparation,
Since the calcination process is required twice and it is necessary to use an organic solvent in order to suppress the decomposition of β and β ″ alumina calcination products with water when pulverizing and mixing two types of calcination products, However, the firing schedule is, for example, 200 ° C / min or more
An extremely advanced firing technique is required because it is a complex one that is held at a temperature of around 600 ° C for 5 to 10 minutes and then annealed at a temperature of 1500 ° C or less for several hours.

On the other hand, the aqueous solution spray-drying method is currently receiving attention for the zeta process. The aqueous solution spray drying method is a method of directly dissolving and mixing a water-soluble alkaline raw material species in a water solvent, and spray-drying a slurry-prepared product to obtain a granular raw material for molding. SSSD process (Slurry Solution Spray Drying)
It is considered to be effective in simplifying the raw material processing process and reducing costs for mass production of β ″ alumina bag tubes in the future. However, the β ″ alumina sintered body produced by the SSSD process has a β ′ conversion rate in the past. However, the ionic conductivity is high, and improvement of these characteristics has been a problem.

(Problems to be Solved by the Invention) An object of the present invention is to provide a β ″ alumina sintered body having a high β ″ conversion rate, a low ionic conduction resistivity, and sufficient strength, and a method for producing the same. .

(Means for Solving the Problems) The present invention is a β ″ -alumina calcination in which a water-soluble alkali raw material species and an alumina raw material are mixed and pulverized with water, spray-dried and granulated, and then molded and fired. In the method for producing a bound body, after the mixing and pulverization, the main component of the crystal phase is β ″ alumina phase and the average particle diameter is β ″ of 1 μm or more and 9 μm or less.
The present invention relates to a method for producing a β ″ alumina sintered body, which comprises adding and mixing 2% by weight or more and 45% by weight or less of alumina seed crystals.

(Operation) In the method for producing a β ″ alumina sintered body according to the present invention, a water-soluble alkali raw material seed and an alumina raw material are mixed and pulverized with water, and then the β ″ alumina pulverized product is added as a seed crystal. , Is important for improving the β ″ conversion of the crystal and reducing the ionic conductivity resistivity.

At this time, when the average grain size of the seed crystal is set to 1 to 9 μm and the addition amount is set to 2 to 45% by weight, the crystallites in the sintered body grow uniformly, so that the strength does not deteriorate. Average particle size is 1
If it is less than μm, the number of seed crystals that are absorbed and disappeared in the matrix increases, and the seed crystals do not function as nuclei for grain growth. Therefore, the effect of addition becomes insufficient, and partial abnormal grain growth occurs and the strength increases. Deteriorates. If the average grain size exceeds 9 μm, the number of seed crystals is insufficient, abnormal grain growth occurs, and strength deterioration occurs. The maximum diameter of the seed crystal is preferably 44 μm or less, and if it exceeds 44 μm, abnormal grain growth occurs around the coarse seed crystal, or the sinterability of the seed crystal is insufficient, resulting in strength deterioration. If the amount of seed crystals added is less than 2% by weight, the β ″ conversion is insufficient and the ionic conductivity is high.
In addition, abnormal grain growth occurs and the strength also decreases. Further, if the amount of seed crystals added exceeds 45% by weight, the sinterability of the molded body is lowered and the strength of the sintered body is lowered.

As described above, in the present invention, the manufacturing cost is low, β ″.
Β ″ in order to achieve both high ionic conductivity and high strength in the aqueous solution spray-drying manufacturing process of the alumina sintered body.
By adding an appropriate amount of crushed alumina sintered body, the β ″ conversion rate is improved and the microstructure is made uniform.

The average particle size of the seed crystals is more preferably 2 to 5 μm, and the addition amount of the seed crystals is more preferably 5 to 30% by weight.

(Example) FIG. 1 is a flow chart of the present invention in which a pulverized product of a β ″ alumina sintered body is added as a seed crystal to a water spray drying manufacturing process of a β ″ alumina sintered body.

In this example, Al ₂ O ₃ source as _{α-Al 2 O 3, Na} 2 O source as the water-soluble NaOH, MgO stabilizer, as the Li ₂ O source Mg (NO _{3) 2} · 6H
₂ O, MgCO ₃ and LiOH, and for comparison, α-Al ₂ O ₃ , N as a source for Al ₂ O ₃ as a raw material for the zeta process.
Na ₂ CO ₃ was used as the a ₂ O source, MgO as a NaOH stabilizer, and MgCO ₃ , Li ₂ CO ₃ and LiOH were used as the Li ₂ O sources.

As the β ″ alumina seed crystal, a crystal is prepared from powder that is not made in advance with the addition of seed crystals, and after coarse crushing, crushing is performed in an alumina ball mill for 5 to 100 hours in acetone to obtain seeds with different average particle sizes. Crystals B, C, D and E were obtained.
The 0.8 μm seed crystal A was prepared by crushing the same coarsely crushed product in acetone with a vibration mill for 24 hours. The β ″ ratio of the seed crystal is
It was 80%. The β "conversion rate is determined from the (110) peak intensity I _β of the β phase and the (01,11) peak intensity I _{β" of the} β "phase. It calculated using the formula of.

It calculated using the formula of.

Hereinafter, more specific experimental examples will be described.

First, Na ₂ O8.9% by weight for the systems containing MgO as a stabilizer, MgO2.1 wt%, such that the Al ₂ O ₃ 89.0 wt%,
The Na ₂ O9.0% by weight for the systems containing Li ₂ O as a stabilizer, Li ₂ O0.8 wt%, Al ₂ O ₃ so as to be 90.2 wt%,
α-alumina, sodium hydroxide, sodium carbonate, magnesium nitrate, magnesium carbonate, lithium hydroxide,
Raw materials such as lithium carbonate were prepared at the mixing ratio shown in Table 1.

Next, 200g-100g of this mixed powder and cobblestone (φ15Al ₂ O ₃ ) 1
Add distilled water to kg so that the water content is 60% of the mixed powder.
Mix and pulverize in a 3l pot for 20.5 hours, add a predetermined amount of β ″ alumina seed crystals of various particle sizes, and mix for an additional 0.5 hour. Seed crystals react with water and decompose, so the mixing time is 2 hours or less. Then, it is sifted through a 350-mesh sieve to remove coarse particles of 44 μm or more,
It was dried with a spray dryer, granulated, and sieved with a 100 mesh screen. Next, mold press (200 kg / cm ² ) into a 50 × 50 × 6 shape, then press rubber (1000 kg / cm ² ).
Then, it was fired under the predetermined firing conditions shown in Table 1. Na during firing
_In order to prevent the evaporation of the ₂ O component, the same compound powder as that of each molded body was filled in an alumina crucible and subjected to burial.

Also, the firing schedule is 1500 at 400 ° C for 1 hour.
Raise the temperature up to ~ 1600 ℃, hold at the maximum temperature for 10 ~ 60 minutes, and
The temperature was lowered to 1400 ° C. at a rate of 500 ° C., held at 1400 ° C. for 5 hours, and then lowered to a rate of 300 ° C. for 1 hour.

Preparation of a fired body by the zeta process as a comparative example was performed in the following procedure. That is, `` Am.Ceram.Soc.Bull., 5
6 , (2) .206, (1977) ”,“ Sintering Processes an
d Heat Treatment Schedules for Conductive Lithia S
As described in tabilized β ″ -Al ₂ O ₃ ″, two kinds of mixture are prepared and calcined separately to prepare two compounds, which are then crushed. Is crushed in a pot mill for 21 hours, and the resulting slurry is PVB
Was added as a binder and then granulated with a spray dryer. The composition of the first mixture was Li ₂ O.5.5Al ₂ O _3, and lithium carbonate and alumina were used as raw materials. The composition of the second mixture was Al ₂ O ₃ .5Al ₂ O ₃ , and sodium carbonate and alumina were used as raw materials. The calcination temperature of each mixture is
The calcination time at 1260 ° C. was 2 hours.

The subsequent molding and firing conditions were the same as those of the aqueous solution spray drying method described above. FIG. 2 shows a manufacturing flow chart in the zeta process.

For each of the above examples, the β ″ conversion rate, firing shrinkage rate, density (sintered body apparent density), four-point bending strength, and ionic conduction resistivity were measured. The results are shown in Table 1.

FIG. 3 shows the relationship between the amount of seed crystals added and the four-point bending strength. By adding the seed crystal in an amount of 2% (wt%, the same applies hereinafter) or more and 45% or less, a sintered body having a strength of 250 MPa or more can be obtained, and the strength is equivalent to that of the zeta process sintered body. If the amount added is less than 2%, the amount of seed crystals will be insufficient, and abnormal grain growth will occur.
It is considered that the strength has deteriorated. Since the amount of seed crystals is appropriate when the amount of seed crystals added is 2% or more and 45% or less,
Uniform grain growth occurs throughout, resulting in a homogeneous microstructure and a high-strength sintered body. If the amount of seed crystals added exceeds 45%, the sinterability will be reduced, and a low-density sintered body will be obtained, and the strength will be low.

Fig. 4 shows the relationship between the amount of seed crystals added and the β "conversion rate. The important point when manufacturing a β" alumina sintered body is that a β "phase with a low Na ion conductivity resistivity is sufficiently formed. Yes, the fourth
As can be seen from the figure, it is effective to add a seed crystal having a β ″ phase as the main component of the crystal phase, and it is effective to improve the β ″ conversion rate. β ″
It is possible to produce a β ″ alumina crystal having a high conversion rate.

Fig. 5 shows the relationship between the seed crystal addition amount and the Na ion conduction resistivity at 300 ° C. From this figure, it can be seen that by adding seed crystals of 2% or more, the Na ion conductivity resistivity at 300 ° C becomes 5 Ω · cm or less, and a β ″ alumina sintered body suitable for sodium-sulfur batteries can be obtained. It

Fig. 6 shows the relationship between the β "conversion rate and the Na ion conduction resistivity at 300 ° C. From this figure, it is clear that the improvement of the β" conversion rate is extremely effective in reducing the Na ion conduction resistivity. From a comparison between Examples 1 to 8 and Comparative Examples 6 and 7, it was found that the production method of the present invention is effective for improving the β ″ conversion rate when water-soluble sodium hydroxide is used as the sodium source. Further, when a water-soluble raw material is not used, the sinterability is lowered and only a low-strength sintered body can be obtained.

Further, FIG. 7 shows the relationship between the firing temperature and the sintered body density. In the manufacturing method of the present invention for the zeta process, from 1540 ° C
It is possible to obtain a sintered body with stable characteristics over a wide range of 1600 ° C. In addition, regarding the keep time, Example 1
As can be seen from FIG. 8, even if the time is not as short as 10 minutes or less as in the Zeta process, deterioration of properties due to grain growth is not recognized even if the keeping time is extended. For this reason, it is not necessary to control the sintering conditions very strictly as in the Zeta process, which is a great advantage in manufacturing.

Further, FIG. 8 shows the dependence of the four-point bending strength on the average grain size of the seed crystal when the seed crystal addition amount is 10% by weight. If the average particle size of the seed crystal is less than 1 μm, the size of the seed crystal is too small, so that the seed crystal is absorbed by the surrounding tissue and does not work effectively.
Coarse grains grow and the strength deteriorates to 250 MPa or less. Further, if the average grain size of the seed crystal exceeds 9 μm, the sinterability of the seed crystal deteriorates, so that the strength of the obtained sintered body deteriorates to 250 MPa or less.

Further, even if the seed crystal contains a coarse seed crystal of 44 μm or more, it will cause the strength deterioration of the sintered body. Therefore, after adding the seed crystal, it is sieved with a sieve of 325 mesh or more. It is desirable to remove coarse seed crystals.

(Effects of the Invention) As described above, the present invention has the following effects.

In the zeta process known as a method for producing a β ″ alumina sintered body, it is necessary to additionally calcine and pulverize two kinds of mixture when preparing raw materials, and to use an organic solvent when pulverizing the final formulation. On the other hand, the aqueous solution spray-drying method is a low-cost manufacturing method in which water can be used during mixing and crushing, and the mixing and crushing step can be clarified in a single operation, but the β ″ conversion rate is low, so the ionic conductivity is high, There was a problem of low strength.

According to the production method of the present invention, by adding a predetermined amount of a seed crystal having a β ″ phase as a main component of a crystal phase having a determined average particle size to the aqueous solution spray drying method, the β ″ conversion rate is improved. Since the conductivity resistivity is lowered and uniform grain growth occurs, the crystal strength is improved and a β ″ -alumina sintered body suitable for sodium-sulfur batteries etc. can be obtained. Since it is wider than that of the Zeta process, there is no need for rapid temperature rise in the zeta process and for a short time at high temperature, which is a great advantage in manufacturing.

[Brief description of drawings]

FIG. 1 is a flow chart showing the manufacturing method of the present invention, FIG. 2 is a flow chart showing a conventional zeta process, FIG. 3 is a graph showing the relationship between four-point bending strength and the amount of β ″ alumina seed crystal added, The figure is a graph showing the relationship between the β "conversion rate and the addition amount of β" alumina seed crystals, and Fig. 5 is the sodium ion conductivity resistivity at 300 ° C and β ".
FIG. 6 is a graph showing the relationship between the amount of seed crystals added, FIG. 6 is a graph showing the relationship between the Na ion conductive resistivity at 300 ° C. and the β ″ conversion rate, and FIG. 7 is an example of the present invention and the zeta process. Fig. 8 is a graph showing the relationship between the sintered body density and the sintering temperature in Fig. 8, and Fig. 8 is a graph showing the relationship between the four-point bending strength and the seed crystal average grain size.

Claims (1)

[Claims] 1. A method for producing a β ″ alumina sintered body, which comprises mixing a water-soluble alkali raw material species and an alumina raw material with water, pulverizing, spray-drying, granulating, and then molding and firing. After mixing and pulverizing, β ″ alumina seed crystals having a β ″ alumina phase as a main component of a crystal phase and having an average particle size of 1 μm or more and 9 μm or less are added and mixed in an amount of 2% by weight or more and 45% by weight or less. ″ Method for manufacturing alumina sintered body.