JPH0625032B2 - Manufacturing method of β-alumina ceramics - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention relates to a method for producing β-alumina-based ceramics used in a battery or a thermoelectric conversion device, and particularly to an alkaline component in a β-alumina-based compact sintering process. In order to suppress volatilization, the present invention relates to a molded body housed in a container.

[Prior Art and Problems] Beta-alumina-based ceramics are generally Na ₂ O.x.
This refers to a ceramic made of a non-stoichiometric compound represented by Al ₂ O ₃ (where x represents a number of 5 to 11), and is typical as a conductive ceramic. However, this β-
In the case of alumina-based ceramics, alkaline components (for example, Na ₂ O) easily volatilize during sintering,
As the composition fluctuates, it becomes difficult to densify it, which hinders its use. Therefore, various proposals have hitherto been made to suppress the volatilization of alkali components during the sintering. For example, a method of embedding or packing a β-alumina-based compact to be sintered (hereinafter, simply referred to as “molding”) in a β-alumina powder or sintering (US Pat. No. 3,468,719); Sintered by storing in a container (Japanese Patent Publication No. 57-15063); The β-alumina sintered tube is used as the inner wall and the α-alumina sintered tube is used as the outer wall. How to store and sinter (Japanese Patent Publication No. 57-3423)
6); A method in which a compact is housed in a firing furnace mainly composed of β-alumina and composed of a refractory material composed of a binder and a sodium salt, or a firing furnace in which the inner surface of concrete is covered with the refractory material and sintered. (U.S. Pat. No. 4,211,570); a method of accommodating and sintering the compact in a container obtained by treating the inner surface of the crucible composed of 50% or more α-alumina with an alkali (US Pat. No. 4,263,381); and the like.

An object of the present invention is to completely suppress volatilization of an alkaline component from a β-alumina-based compact during firing for a long period of time by a completely different means from the above-mentioned prior art to prevent compositional variation and densification, and thus to achieve ionization. It is to develop a method for producing β-alumina-based ceramics having excellent copper electric conductivity and mechanical strength.

[Solution] As a result of earnest research to solve such problems, the present inventor has found that
Using a predetermined α-Al ₂ O ₃ -containing material as a base material, a β-Al ₂ O ₃ -containing composition is subjected to a two-step treatment with a predetermined alkali component-containing composition and a predetermined β-Al ₂ O ₃ -containing composition, and β- When the alumina-based compact was sintered, it was found that β-alumina-based ceramics excellent in ionic conductivity and mechanical strength can be stably provided, and the present invention has been completed. That is, the present invention solves the above problems by the following means.

In a method for producing a β-alumina-based ceramics, including a step of housing a β-alumina-based compact in a container and sintering the compact, the container is subjected to the following steps: (a) The compact can be housed It has a shape and contains at least 50 wt% α-alumina, and finally has a relative theoretical density.
A step of preparing a base material which becomes a sintered body of 90% or more, (b) a step of first treating the inner surface of the base material with a composition containing at least 20 wt% of an alkali component in terms of oxide, and (c) the base A method for producing β-alumina-based ceramics, which comprises obtaining the inner surface of the material by a secondary treatment with a composition containing at least 80 wt% of β-alumina.

[Preferred Embodiment and Action] Hereinafter,% is% by weight unless otherwise specified.

The base material has a shape capable of accommodating the molded body and serves as a basis for the final container shape. For example, a bottomed cylindrical shape may be mentioned, but a square tube shape tube may be appropriately selected according to the outer shape of the molded body to be housed. This substrate contains at least 50% α-alumina
Must be included. This is because if it is less than 50%, the alkaline component cannot be sufficiently applied to the inner surface of the substrate in the next step. It is preferably 80% or more. As the balance composition,
Examples include refractories of magnesia and zirconia.
Further, unavoidable impurities such as SiO ₂ ,
It may contain CaO. The base material is preferably a sintered body, but may be a molded body (unsintered) or a pre-sintered body as long as it is a sintered body by the primary and secondary treatments described later. The relative theoretical density is 90% or more, more preferably 95% or more.

Next, the inner surface of the base material is primarily treated with a composition containing an alkaline component as an essential component. This is because the alkaline component is applied to the inner surface of the base material. The composition used in this primary treatment (primary composition) must contain at least 20% of the alkali component in terms of oxide. If it is less than 20%, the alkaline component cannot be sufficiently applied to the inner surface of the base material, and the bond strength with β-alumina to be applied in the subsequent secondary treatment is also insufficient. The amount of alkaline component is preferably 35
% Or more, more preferably 40% or more. As long as the primary composition contains an alkaline component, it is in the form of powder, solution (for example, aqueous solution), paste (using polyvinyl butyral, ethyl cellulose as a binder).
It can be used in various forms such as. Examples of the alkali component include alkali metal (Li, Na, etc.) salts (for example, carbonates, nitrates, acetates, sulfates) or oxides. In particular, Na is an essential component and Li is an optional small amount component.
The alkaline component may be used alone,
Practically, α-alumina, to reduce the effect on the furnace,
It is preferably used as a mixture with β-alumina or the like. When it is used in powder form, 10
It is recommended to heat at a temperature of 00 to 1500 ° C for 1 hour or more. It is preferably 1300 ° C or higher. When used in solution form,
After pouring this into the base material, the solvent may be evaporated. Also, when used in paste form, it is recommended to apply this on the inner surface of the substrate and then heat at a temperature of 1000 to 1500 ° C or higher.

Next, the inner surface of the base material subjected to the primary treatment is further subjected to a secondary treatment with a composition containing β-alumina as an essential component. β-
This is because alumina is applied to the inner surface of the base material to suppress volatilization of the alkaline component from the compact during sintering of the compact. The composition used in this secondary treatment (secondary composition) must contain at least 80% β-alumina. If it is less than 80%, it is impossible to stably maintain the volatilization suppression of the alkaline component from the compact during sintering of the compact. The amount of β-alumina is preferably 90% or more, more preferably 95% or more. β-alumina is the one represented by Na ₂ O.xAl ₂ O ₃ as well as the compact to be sintered. Na-β-alumina (Na ₂ O.9-11Al ₂
O ₃ ), Na-β ″ -alumina (Na ₂ O.5-7Al
₂ O ₃ ). For Na-β ″ alumina,
Li ₂ O and MgO may be added to stabilize the crystal phase. The balance of the secondary composition is preferably an alkali metal salt, which is an essential component of the primary composition. 1500 ~ 20 after filling this secondary composition into the substrate
It is recommended to heat at a temperature of 00 ° C for 30 minutes or longer. The preferred temperature is
1500 to 1700 ° C.

The heating may be performed simultaneously in the primary treatment and the secondary treatment. The thickness of the layer applied by the primary treatment and the secondary treatment usually reaches a few millimeters depending on the form of paste or powder, but it is effective even if it can be applied to a few hundred micrometers.

Next, the β-alumina-based compact to be sintered is placed in the container thus obtained and, for example, 1300 to 20 in an electric furnace.
Hold at 0.5 ° C. for 0.5 to 2 hours to sinter. Further, after sintering, it may be annealed at, for example, 1300 to 1500 ° C.

The β-alumina-based compact to which the present invention is applied is Na-β.
-Alumina and Na-β "-alumina are included.
Some of the Na ions are converted to other metals (eg Li, K, Ag, C
Substituted β-alumina obtained by substituting u, Pb) ions, or modified β-alumina obtained by adding another oxide such as an alkaline earth metal oxide or a rare earth element oxide may be used.

The β-alumina-based ceramics produced by applying the present invention can be used in batteries, especially sodium-sulfur secondary batteries; and thermoelectric converters, especially AMTEC (Alkali Metal Thermo).
It is suitable as a conductive ceramics used for electric converters, etc.

[Examples] Examples of the present invention will be described below. A comparative example will also be described.

Example 1 Preparation of base material: A one-end closed tubular crucible containing 92% of α-alumina and a relative theoretical density of 92% is prepared as a base material. The inner surface of this crucible was subjected to primary and secondary treatments by the following procedure.

Primary treatment: A crucible is filled with a mixed powder of sodium carbonate 40%, α-alumina 20% and β-alumina 40%, and heated in an electric furnace at 1500 ° C for 4 hours. After that, it is slowly cooled, the crucible is taken out from the electric furnace, and the residual powder in the crucible is removed.

Secondary treatment: The crucible was filled with a mixed powder consisting of 90% β-alumina and 10% sodium carbonate and placed in an electric furnace.
Heat at 1600 ° C for 30 minutes. After that, it is slowly cooled, the crucible is taken out from the electric furnace, and the residual powder in the crucible is removed.

Next, the β-alumina compact was placed in the container thus obtained, and the compact was sintered by holding it in an electric furnace at 1600 ° C for 30 minutes. The sintering was repeated under these sintering conditions. As a comparative example, one not subjected to the secondary treatment is shown (Comparative Example 1).

The physical properties of the obtained β-alumina sintered body are measured and calculated,
The results are shown in the table below. The “non-penetrability” was examined with a color check solution.

Examples 2 and 3 Base material: Same as Example 1 Primary treatment: Alkaline component (NaNO _{3 of} Example 2, CH ₃ COONa of Example 3), β-alumina powder composition Secondary treatment: Composition consisting only of β-alumina Sintering: same as in Example 1 (see table for specific conditions and results) As a comparative example, a composition containing a small amount of alkali component in the primary composition (Comparative Example 2), The one that was not treated at all (Comparative Example 3) is also shown in the display.

Example 4 ・ Substrate: α-alumina 80%, relative theoretical density 95% ・ Primary treatment: A saturated aqueous solution of NaNO ₃ was injected, and the temperature was 100 ° C.
Heat to evaporate water by heating. Secondary treatment: Composition consisting of β-alumina only. Sintering: Same as Example 1 (Refer to the table for specific conditions and results.) As a comparative example, the substrate was treated. Similarly, the one not subjected to (Comparative Example 4) is shown in the table.

As is clear from the above table, in each example, a β-alumina sintered body having excellent relative theoretical density, specific resistance value and impermeability as compared with the corresponding comparative example can be obtained. In addition, Example 1
Shows almost the same sintered body characteristics as the corresponding comparative example when the number of times of sintering is 1, but the difference becomes remarkable as the number of times of sintering increases (Table 2). Therefore, Example 1 is excellent. It is possible to stably manufacture a sintered body having the above characteristics. It is considered that this is because the alkali component is almost completely volatilized when it is used for one-time sintering because the coating state of the alkali component on the inner surface of the substrate is insufficient only by the primary treatment.

Further, as a primary treatment, the same treatment as in Example 4 was conducted except that a paste consisting of 100 parts of NaCO _{3 and} 10 parts of binder (ethyl cellulose) was applied to the inner surface of the base material, and the characteristics of the sintered body were examined. Showed excellent results.

[Effect] As described above, according to the present invention, volatilization of the alkaline component from the compact can be sufficiently suppressed during the sintering of the β-alumina compact, so that the composition variation can be prevented and the compact having a desired composition can be obtained. It is possible to manufacture a large β-alumina ceramics. Further, since the volatilization suppressing action of the alkali component is stably exhibited for a long period of time, it is most suitable for repeated firing, and the desired β-alumina ceramics can be mass-produced. It is considered that the stable volatilization suppressing action is due to the fact that the β-alumina surface layer of the container volatilizes sodium oxide (Na ₂ O) at the time of firing to increase the sodium partial pressure in the container.

Therefore, the β-alumina-based ceramics produced in this manner has a high ionic conductivity and a high mechanical strength, and is therefore extremely useful as various high-strength conductive materials.

Further, since the present invention relates to a ceramics container, there is no complexity of taking out a set of a molded body and a sintered body as in a method of powder filling, the life of the container is long, and an expensive precious metal is not required. Moreover, since the present invention relates to the α-alumina base material, it does not have the problem of the method using the β-alumina base material (tube). That is, in the method relating to the β-alumina base material (tube), it takes time and labor to manufacture the tube, and the life is insufficient, resulting in an increase in manufacturing cost, but the present invention does not have such a problem.

Thus, the industrial value of the present invention is very high.

Claims (1)

[Claims] 1. A method for producing a β-alumina ceramics, which comprises the step of accommodating a β-alumina-based compact in a container and sintering the compact, wherein the container is subjected to the following steps: (a) forming It has a shape capable of accommodating a body and contains at least 50 wt% α-alumina, and finally has a relative theoretical density of 90%.
The step of preparing a base material to be the above sintered body, (b) the step of primarily treating the inner surface of the base material with a composition containing at least 20 wt% of an alkali component in terms of oxide, and (c) the inner surface of the base material Is obtained by a secondary treatment with a composition containing at least 80 wt% of β-alumina.