JP2000272954A - Storage of hygroscopic ceramic - Google Patents

Abstract

PROBLEM TO BE SOLVED: To store a hygroscopic ceramic having the increasing rate of weight of a specific % or higher than it after stored in an atmosphere with a relative humidity of a specified % or higher than it for prescribed days without deteriorating characteristics for a long period of time by keeping the ceramic in a container having a relative humidity of a specific % or lower than it. SOLUTION: The relative humidity of a container in which a hygroscopic ceramic having <=0.01% increasing rate of weight after stored at >=15% relative humidity for 60 days is stored is <=10%. This method is preferable as a storage method for a ceramic having a strength of <=90% initial strength after stored in an atmosphere having >=42% relative humidity for >=60 days, a ceramic having ionic conductivity with a specific resistance value of 1.1 times as much as that before the storage and a ceramic having >=4 μm thickness of deterioration layer caused by moisture absorption occurring on the surface after stored in an atmosphere with >=15% relative humidity for 30 days. This method is preferable as a storage method for β-alumina ceramic. A β-alumina tube has the same characteristics as those immediately after baking even after long-term preservation and exhibits high airtightness.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for storing hygroscopic ceramics. More specifically, as in the case of sodium ion conductors such as beta-alumina ceramics and sodium ion conductive glass (NASICON), hydrophilic ions present in the ceramic sintered body move to the surface by contact with moisture. A hygroscopic ceramic that causes a crystal structure collapse due to non-uniform composition distribution and ion migration, or a reduction in surface activity and surface structure collapse due to the formation of a reactant with water on the surface, The present invention relates to a method for inexpensively and effectively storing the characteristics for a long period of time without deteriorating its characteristics.

[0002]

2. Description of the Related Art Among hygroscopic ceramics, beta alumina ceramics and sodium ion conductive glass (NAS)
Ceramics such as ICON) have alkali ion conductivity and are therefore used as solid electrolytes in batteries and various sensors. However, since alkali ions have high reactivity with water, a phenomenon occurs in which alkali ions move from the inside of the ion conductor to the surface of the ion conductor and near the surface, and react with water.

[0003] As a result, the internal composition of the sintered body becomes non-uniform, the crystal structure is collapsed, and the surface activity of the ion conductor is reduced due to the formation of a reaction product of water and alkali ions on and near the surface of the ionic conductor. And problems such as collapse of the surface structure. For example, beta-alumina ceramics are known to deteriorate due to moisture absorption, causing a decrease in mechanical strength and a decrease in sodium ion conductivity.

FIG. 1 shows a general structure of a sodium-sulfur battery. Operating temperature of sodium sulfur battery is 300
At about ° C., the sulfur (sodium polysulfide) 1 of the positive electrode and the metallic sodium 4 of the cathode are liquefied, and the metallic sodium is partially present as a vapor inside the battery. For this reason, the atmosphere inside the battery needs to be kept at an inert gas or vacuum, and a high airtightness is required at the joint of each battery member. The present inventors have conventionally stored the beta-alumina solid electrolyte tube 2 in a desiccator in which the relative humidity was maintained at about 30%. It has been confirmed that the beta-alumina solid electrolyte tube stored under the conventional conditions does not show any deterioration in mechanical strength and electrical characteristics until storage for 6 months. However, when assembled in a sodium-sulfur battery, there was a problem that high airtightness could not be obtained at the glass joint 7 shown in FIG.

[0005] As a result of investigating the cause of the lack of airtightness,
A deteriorated layer having a low density is formed in the thickness direction from the surface of the solid alumina solid electrolyte tube stored at a relative humidity of about 30%, and it is apparent that high airtightness cannot be obtained due to this. became. Further, since the thickness of the deteriorated layer increases as the storage period becomes longer, it is considered that the mechanical strength is deteriorated when the thickness exceeds a certain thickness. An example of a SEM observation photograph of this deteriorated layer is shown in FIG.
(Thickness of the deteriorated layer: about 35 μm).

In addition, since sodium polysulfide is used for the positive electrode and metallic sodium is used for the negative electrode, the moisture introduced into the battery reacts with the positive and negative electrode active materials and impairs the activity. In particular, it has a high reaction activity with metallic sodium which is a negative electrode active material, generates sodium oxide and sodium hydroxide, and inhibits ionization of sodium and exchange of electrons and sodium at the beta alumina ceramics / metallic sodium interface. Therefore, moisture adsorbed and brought into a member used for a battery such as beta alumina ceramics significantly lowers battery performance. For this reason, it is desired that the amount of water adsorbed on the beta alumina ceramics be reduced as much as possible.

[0007] A countermeasure common to the above problems is to lower the humidity under storage conditions. For example, JP-A-51-1
No. 48710 discloses a method for storing beta-alumina ceramics using a desiccant. In particular,
A desiccant such as silica gel or calcium chloride is applied to a sealable container such as a polyethylene or polyvinyl chloride bag.
A method of containing and sealing with alumina is shown. According to this method, it is described that weight increase during storage due to moisture adsorption and deterioration of mechanical strength of a sintered body can be suppressed. Japanese Patent Application Laid-Open No. 2-14872 discloses a method of storing beta-alumina ceramics which is sealed after vacuum sealing or sealing with an inert gas.

[0008]

However, the storage method using a desiccant is effective at the initial stage (approximately 2 to 3 months) to suppress weight change and deterioration of mechanical strength, During the storage period, the formation of the porous degraded layer generated on the surface of the highly hygroscopic ceramic and near the surface cannot be sufficiently suppressed. Further, the vacuum sealing or the storage method of sealing after sealing the inert gas uses a special container or an expensive inert gas that can withstand the decompression treatment, and thus has a problem that the storage cost is increased.

In the past, the humidity of the storage atmosphere of hygroscopic ceramics has not been investigated or studied in detail, so how much concrete control can be performed to completely suppress the deterioration and alteration of hygroscopic ceramics? Was not found. An object of the present invention is to solve the above-mentioned problems and to provide a storage method that enables long-term storage of highly hygroscopic ceramics.

[0010]

According to the first aspect of the present invention, there is provided a container for storing a hygroscopic ceramic having a weight increase rate of 0.01% or more after being stored for 60 days in an atmosphere having a relative humidity of 15% or more. The point is to make the relative humidity 10% or less. By storing the hygroscopic ceramic under such conditions, the weight increase rate after a long-term storage of 2 years or more can be reduced by 0.02%.
% Or less. This increase in weight is presumed to be caused by moisture absorption and / or a reaction product of the moisture absorption and the ionic component constituting the ceramic. Also,
More preferable results are obtained when the relative humidity in the storage container is 5% or less. By storing the hygroscopic ceramic under such conditions, the rate of weight increase after long-term storage for two years or more can be suppressed to substantially 0%.

According to a second aspect of the present invention, there is provided a container for storing a hygroscopic ceramic having a strength after storage in an atmosphere having a relative humidity of 42% or more for 60 days or less of 90% or less of an initial strength before the storage. It is essential that the humidity be 10% or less. By storing the hygroscopic ceramic under such conditions, the deterioration in strength after long-term storage for two years or more can be suppressed to substantially 0%.

According to a third aspect of the present invention, there is provided an ionic conductivity in which the specific resistance after storage for 60 days in an atmosphere having a relative humidity of 42% or more is 1.1 times or more the specific resistance before storage. The gist is that the relative humidity in the container storing the hygroscopic ceramics is set to 10% or less. By storing the hygroscopic ceramic under such conditions, the rate of change of the specific resistance value after long-term storage for two years or more can be suppressed to substantially 0%.

According to a fourth aspect of the present invention, there is provided a container for storing a hygroscopic ceramic in which the thickness of a deteriorated layer caused by moisture absorption generated on the surface is 4 μm or more after storage in an atmosphere having a relative humidity of 15% or more for 30 days. The point is to make the relative humidity 10% or less. By storing the hygroscopic ceramic under such conditions, the thickness of the deteriorated layer generated on the surface after long-term storage of two years or more can be suppressed to substantially 0 μm.

The invention according to claim 5 is the invention according to claims 1 to 4.
The gist is that the hygroscopic ceramic described in any of the above is beta alumina ceramic. It is suitable as a storage method for beta-alumina ceramics which has high hygroscopicity and in which the mechanical strength and electrical characteristics are significantly deteriorated due to the moisture absorption.

[0015]

EXAMPLES The evaluation material of the present invention was made of beta-alumina ceramic, which is best known as a hygroscopic ceramic, in the form of a bottomed cylinder having a length of 400 mm, an outer diameter of 45 mm and a wall thickness of 2.5 mm ( Hereinafter, it is referred to as a beta alumina tube.)
Was used. The initial characteristic values before storage of the beta alumina tube used in the test were as follows: relative density is 99.0% or more, radial crushing strength immediately after firing was 350 MPa, and specific resistance value was 3.2 Ω-cm.
Met.

The test was performed by changing the relative humidity in the storage container of the beta alumina tube. Specifically, the relative humidity was set to 4 ± 1%, 9 ± 1%, 15 ± 1%, 20 ± 1%, 42 ± 1%.
The conditions were 1% and 86 ± 1%. Storage temperature is 20
± 2 ° C. The humidity was adjusted by using a desiccator with a dehumidifier in the case of the relative humidities of 4 ± 1% (Example 1) and 9 ± 1% (Example 2). Further, a relative humidity of 15 ± 1% which is a comparative example.
(Comparative Example 1), 20 ± 1% (Comparative Example 2), 42 ± 1%
(Comparative Example 3), 86 ± 1% (Comparative Example 4), “International Critical Tables”, Vol. 1, p. 67, Mc
Adjustment was performed using a solution that gave a constant humidity as described in Graw-Hill (1926). Specifically, in Comparative Example 1, lithium chloride monohydrate was used, and in Comparative Example 2, potassium acetate saturated aqueous solution was used.
In Comparative Example 3, a saturated aqueous solution of zinc nitrate hexahydrate was used.
Used a saturated aqueous solution of potassium hydrogen sulfate. The evaluation period is 7, 14, 28, 56, 120, 210
390 days, 480 days, 570 days and 750 days. For these samples, the following items (1) to (5) were evaluated.

[0017] Measurement of weight increase rate Weight increase rate ΔW of beta-alumina tube every elapse of predetermined period
(%) Was calculated according to the following formula 1. However, W ₀ is the initial weight (weight before storage), W ₁ is the weight after storage a predetermined number of days. Four samples were measured for each condition, and the average value was calculated. The results are shown in Table 1 and FIG.

[0018]

１W = 100 × (W ₁ −W ₀ ) / W ₀

[0019] Measurement of radial crushing strength ratio The strength deterioration of the beta-alumina tube every predetermined period was evaluated using radial crushing strength (σ _r ), which is the breaking strength when a ring-shaped test piece obtained by cutting was compressed in the radial direction. . Here, the “ring _radius σ _r ” is calculated by the following Equation 2. However, P is the maximum load at break (MP
a) and D are the outer diameter (mm) of the test piece, d is the thickness (mm) of the test piece, and L is the width of the test side. Measurement method is JIS
It was measured according to Z2507. The test piece was produced by cutting out 10 rings of 3 mm width from a beta alumina tube. Here, the “ring compression strength ratio” refers to the ratio (unit:%) of the crushing intensity after storage for a predetermined period to the initial crushing intensity.
The results are shown in Table 2 and FIG.

[0020]

Σ _r = P × (D−d) / (L × d ² )

[0021] Measurement of change rate of specific resistance value The specific resistance value of the beta-alumina tube at every elapse of a predetermined period is determined by contacting metallic sodium inside and outside the cylinder of the beta-alumina tube in a glove box at 350 ° C under an argon atmosphere. Then, the resistance value of the sintered body was measured by a four-terminal method. Three beta-alumina tubes were measured for each condition, and the “rate of change in specific resistance value (ΔR)” was calculated using the following equation (3). However, R ₁ is the specific resistance value after a predetermined period storage, R ₀ is a specific resistance value before the storage. Therefore, the “rate of change in specific resistance value” here does not indicate a change with time in the resistance value when the resistance value of the beta alumina tube is measured “continuously”. When the “rate of change in specific resistance value (ΔR)” is positive, it indicates that the specific resistance value has increased. The results are shown in Table 3 and FIG.

[0022]

ΔR = 100 × (R ₁ −R ₀ ) / R ₀

[0023] Measurement of Degraded Layer Thickness The thickness of the degraded layer formed on the surface of the beta-alumina tube every predetermined period was measured as follows. That is, a ring-shaped test piece was cut out from a beta-alumina tube, the cross section was smoothed by mirror polishing, observed by SEM, and the thickness of the deteriorated layer formed on the surface was measured and determined. Since the thickness of the deteriorated layer varies slightly depending on each sample and each part, for each condition, five test pieces are taken from four beta alumina tubes, and the measurement results of a total of 20 points are averaged. I asked. The results are shown in Table 4 and FIG.

[0024]

[Table 1]

[0025]

[Table 2]

[0026]

[Table 3]

[0027]

[Table 4]

Table 1 and FIG. 3 show the results of the rate of weight increase at each relative humidity. These results show that the beta alumina tube stored in accordance with the present invention had substantially no change in weight even after 750 days had elapsed, indicating that moisture absorption had not occurred. In particular, the sample stored in the atmosphere of the relative humidity of 5 ± 1% shown in Example 1 did not change in weight at all. On the other hand, in each comparative example, it can be seen that the rate of weight increase tends to increase as the relative humidity increases. In particular, in Comparative Examples 3 and 4, it can be seen that surface layer peeling due to remarkable deterioration occurred and a large amount of moisture was absorbed.

Table 2 and FIG. 4 show the results of the radial crushing strength ratio at each relative humidity. From these results, it can be understood that the beta alumina tube stored according to the present invention did not cause any deterioration in strength even after storage for 750 days, and maintained the strength at the start of storage. On the other hand, in Comparative Example 3 and Comparative Example 4, it can be seen that the deterioration of the strength is rapidly progressing. In particular, in Comparative Examples 3 and 4, surface layer peeling due to remarkable deterioration occurred. In Comparative Examples 1 and 2, no surface peeling was observed as in Comparative Examples 3 and 4, but the strength gradually decreased, and after about 750 days, 70% in Comparative Example 1 and Comparative Example 1 In No. 2, it deteriorated to about 60%.

Table 3 and FIG. 5 are results showing the rate of change of the specific resistance value at each relative humidity. From these results, it can be seen that in the beta-alumina tube stored according to the present invention, the specific resistance value did not deteriorate (increase in resistance) at all. on the other hand,
In Comparative Examples 3 and 4, the specific resistance was sharply increased, and it is considered that the surface of the beta-alumina tube was altered due to the adsorption of moisture, and the sodium ion conductivity was significantly impaired. In particular, in Comparative Examples 3 and 4, surface layer peeling due to remarkable deterioration occurred. In Comparative Example 2, the resistivity did not change until after 120 days, but the resistance gradually increased thereafter. In Comparative Example 1, no change over time was observed until about one year, but thereafter, the resistance gradually increased.

Table 4 and FIG. 6 show the results showing the thickness of the deteriorated layer at each relative humidity. From these results, it can be seen that no deteriorated layer was formed in the beta alumina tube stored according to the present invention. On the other hand, Comparative Examples 1 to 4
In, a deteriorated layer is generated from the early stage of storage. Further, as can be seen from the results of Comparative Example 2, the deteriorated layer is formed even during the storage period of about 120 days, which does not affect the change rate of the radial crushing strength rate and the specific resistance value. It has been confirmed that even when a sodium-sulfur battery cell is manufactured using such a beta-alumina tube, airtightness is not maintained. Further, from the results of Comparative Examples 3 and 4, it is considered that the surface is peeled off when the thickness of the deteriorated layer is about 200 μm or more.

[0032]

According to the storage method of the present invention, a hygroscopic ceramic can be stored for a long period of time without deteriorating its characteristics. Specifically, the composition distribution of the hygroscopic ceramic generated during long-term storage becomes non-uniform, the crystal structure collapses due to the movement of ions, or the surface activity decreases due to the formation of a reactant with water on the surface, The collapse of the structure can be prevented. In particular, beta alumina tubes stored using the storage method of the present invention have no deterioration in mechanical strength and electrical characteristics due to moisture absorption, and do not have a deteriorated layer formed on the surface from the initial stage of deterioration. Therefore, even after storage for a long period of time, it is possible to exhibit not only the same properties as immediately after firing but also high airtightness.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing a general structure of a sodium-sulfur battery.

FIG. 2 is an SEM image of a cross section of a deteriorated layer generated on the surface of a beta alumina tube.

FIG. 3 is a graph showing a weight increase rate with respect to storage days.

FIG. 4 is a graph showing a radial crushing strength ratio with respect to storage days.

FIG. 5 is a graph showing a change rate of a specific resistance value with respect to storage days.

FIG. 6 is a graph showing a change in thickness of a deteriorated layer with respect to the number of storage days.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Sulfur or sodium polysulfide 2 Beta-alumina solid electrolyte tube 3 Alpha-alumina insulating ring 4 Metal sodium 5 Cathode current collector 6 Anode container 7 Glass joint

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 3E067 AA11 AB99 BA01A CA07 EE25 4G030 AA03 AA36 BA03 CA07 GA22

Claims (5)

[Claims] 1. A method for storing hygroscopic ceramics, wherein the weight gain after storage for 60 days in an atmosphere having a relative humidity of 15% or more is 0.01% or more, wherein a container for storing the hygroscopic ceramics is provided. A method for storing hygroscopic ceramics, wherein the relative humidity of the ceramic is 10% or less. 2. The strength after storage for 60 days in an atmosphere having a relative humidity of 42% or more is 9% of the initial strength before the storage.
A method for storing hygroscopic ceramics, which is 0% or less, wherein the relative humidity in a container for storing the hygroscopic ceramics is 10% or less. 3. Storage of hygroscopic ceramics having ionic conductivity whose specific resistance after storage for 60 days in an atmosphere having a relative humidity of 42% or more is 1.1 times or more the specific resistance before storage. A method for storing hygroscopic ceramics, wherein the relative humidity in a container for storing the hygroscopic ceramics is 10% or less. 4. A method for storing hygroscopic ceramics, wherein after storage in an atmosphere having a relative humidity of 15% or more for 30 days, the thickness of a degraded layer due to moisture absorption generated on the surface is 4 μm or more. A method for storing hygroscopic ceramics, wherein the relative humidity in the container for storing ceramics is 10% or less. 5. The storage method for a hygroscopic ceramic according to claim 1, wherein said hygroscopic ceramic is beta-alumina ceramic.