JP2519374B2 - Method of firing beta-alumina solid electrolyte - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for firing a beta-alumina solid electrolyte capable of obtaining a beta-alumina solid electrolyte having both high mechanical strength and low electric resistance characteristics.

[0002]

2. Description of the Related Art A beta-alumina solid electrolyte used as a solid electrolyte for a sodium-sulfur battery, for example, is produced by firing a beta-alumina molded body obtained by pressure-molding beta-alumina particles. Further, this beta-alumina solid electrolyte is required to have high mechanical strength in order to exhibit stable durability for a long period of time, and low electrical resistance characteristics in order to exert excellent battery performance. .

However, in order to obtain high mechanical strength, it is necessary to suppress abnormal growth of crystal grains, which causes a decrease in strength, to form a homogeneous crystal. Therefore, it is desirable to lower the firing temperature. In order to obtain low electric resistance characteristics, it is necessary to make the crystal grains as large as possible, so it is desirable to raise the firing temperature. Therefore, conventionally, it has been difficult to set the firing conditions of the beta-alumina solid electrolyte which is a homogeneous and large crystal grain that satisfies both characteristics at the same time.

As one solution to overcome this difficulty, Japanese Patent Publication No. Sho 61-12869 discloses a beta-alumina solid electrolyte in which beta-alumina particles are densified by repeating heating and cooling a plurality of times. Has been proposed. However, when attempting to actually implement this method, the process becomes complicated due to the need for multiple temperature raising and lowering firing processes, and when considering a continuous firing furnace for mass production, the middle of the furnace Since a cooling zone has to be provided, the loss is large in terms of equipment and energy, and there is a problem of lack of practicality.

[0005]

DISCLOSURE OF THE INVENTION The present invention solves the above-mentioned problems of the prior art and monotonically increases the temperature of the beta-alumina molded body without lowering the temperature of the beta-alumina molded body and holds it in the maximum temperature range for a predetermined time. Later, on the premise of the firing method of the beta-alumina solid electrolyte that monotonically decreases the temperature without raising the temperature in the middle, and yet provides a firing method of the beta-alumina solid electrolyte that can simultaneously satisfy high mechanical strength and low electrical resistance characteristics. It was made to do.

[0006]

The method for calcination of a beta-alumina solid electrolyte of the present invention is to raise the temperature of the beta-alumina molded body without lowering the temperature during the calcination, hold it in the maximum temperature region for a predetermined time, and then raise the temperature during the calcination. In the method of firing beta-alumina solid electrolyte that lowers the temperature without heating, the temperature is stopped at a temperature at which beta-alumina in the temperature raising process reaches 65 to 80% of the theoretical density, and the temperature is raised again after holding for a predetermined time. It is characterized by temporarily lowering the temperature raising rate, stopping the temperature lowering in the uniform growth temperature region of the crystal grains in the middle of the temperature lowering step, holding it for a predetermined time and then lowering the temperature again or temporarily lowering the temperature lowering rate. To do.

FIG. 1 shows a basic form of the above-mentioned firing temperature curve of the present invention, in which the temperature increases monotonically as A, B and C up to the maximum temperature region shown as D, and E and F. ，
G has a firing temperature curve that monotonically decreases. For this reason, the temperature is lowered in the middle of firing and then raised again.
-12869 is basically different from the method described in the publication.

In the present invention, during such a temperature raising process,
As indicated by B, the temperature rise is stopped, the temperature is maintained for a predetermined time, and then the temperature is raised again, or an abnormal grain growth suppressing portion in which the temperature rise rate is temporarily reduced is provided. This abnormal grain growth suppressing portion B has a density (relative density) of beta-alumina of 65 to the theoretical density.
The temperature reaches 80%. As shown in Fig. 2, the density of beta-alumina gradually approaches the theoretical density as the firing temperature is raised, but the firing temperature for densification varies depending on the beta-alumina composition. The temperature at which the density reaches 65-80% cannot be uniquely determined for beta alumina.

For example, in the case of the beta-alumina composition shown in FIG. 2, this temperature corresponds to 1450 ° C. to 1500 ° C., and the temperature rise is stopped in this temperature range and held for about 1 hour, or in this temperature range. Temporarily reduce the heating rate to 50 ° C / hr or less. As a result, a large number of beta alumina crystal particles having a uniform fine particle diameter are generated in the beta alumina molded body,
Generation of abnormal grains is suppressed. In addition, the abnormal grain growth suppressing portion B
Even if the relative density of beta alumina is less than 65%, the above effect does not occur. This is because the generation of crystal particles is insufficient at such a low temperature. On the contrary, even if the relative density of beta-alumina exceeds 80%, the above effect does not occur. This is because abnormal grains have already been generated at such a high temperature.

As described above, according to the present invention, the abnormal grain growth suppressing portion B is provided to generate matrix grains having a uniform grain size, and then the temperature is raised to the maximum temperature range D to quench the structure. The maximum temperature varies depending on the beta-alumina composition, but in the case of the beta-alumina composition shown in FIG. 2, the maximum temperature is 1550 to 1600 ° C., and this temperature is maintained for 0.5 hour, for example. As a result, beta-alumina
It is possible to obtain a beta-alumina solid electrolyte having high mechanical strength, which is compacted at a high density of 98% or more.

Next, the temperature lowering step is started, but if the temperature is lowered at a normal speed, the crystal grains of uniform fine grain size generated in the abnormal grain growth suppressing portion B remain as it is, so that a low electric resistance characteristic is obtained. I can't. In view of this, in the present invention, the temperature is stopped in the homogeneous growth temperature range of the crystal grains during the temperature lowering step, the temperature is held again for a predetermined time, and then the temperature is lowered again, or the homogeneous growth portion F in which the temperature lowering rate is temporarily reduced is provided. This temperature also varies depending on the composition of beta-alumina, but practically, it may be set to a temperature range lower by 50 to 200 ° C. from the maximum temperature. The homogeneous growth portion F is also kept at a constant temperature for, for example, 2 hours as shown in FIG. 1, or in this temperature range, the temperature rising rate is temporarily reduced to 30 ° C./hr or less. As a result, the crystal grains grow uniformly and have a large grain size, and a beta-alumina solid electrolyte having low electrical resistance characteristics can be obtained.

[0012]

Examples of the present invention will be described below. External diameter 68 mm, inner diameter 60 mm, total length 510 mm, which was pressure molded from beta-alumina particles
The beta-alumina molded body of was baked under various firing conditions, and the density (unit:
g / cm ³ ) and mechanical strength (unit MPa) and electric resistance characteristics (unit Ωcm) were measured. Table 1 shows an example in which the temperature condition of the temperature raising step is changed, Table 2 shows an example in which the temperature condition of the cooling step is changed, and Table 3
Shows an example in which the maximum temperature condition is changed.

[0013]

[Table 1]

[0014]

[Table 2]

[0015]

[Table 3]

[0016]

As is clear from the data of the above examples, according to the method for calcination of the beta-alumina solid electrolyte of the present invention, the beta-alumina in the middle of the temperature raising step has a theoretical density of
Stop raising the temperature when it reaches 65-80%, hold it for a certain period of time, then raise the temperature again, or temporarily lower the rate of temperature increase.
In addition, by stopping the temperature reduction in the homogeneous growth temperature range of the crystal grains during the temperature lowering process, holding it for a predetermined time and then lowering the temperature again or temporarily lowering the temperature lowering rate, it is possible to prevent the generation of abnormal grains and make each grain uniform. Can be grown to obtain a beta-alumina solid electrolyte having both high mechanical strength and low electric resistance characteristics, and characteristics equal to or higher than those of the conventional firing method in which heating and cooling are repeated can be obtained. Further, according to the method for firing a beta-alumina solid electrolyte of the present invention, it is possible to perform firing without loss in terms of equipment and energy without the need to interpose a temperature lowering process in the middle of firing as in the conventional case. Therefore, the present invention has an extremely great contribution to the industrial development as a method for firing a beta-alumina solid electrolyte that solves the conventional problems.

[Brief description of drawings]

FIG. 1 is a graph showing a typical firing temperature curve of the present invention.

FIG. 2 is a graph showing the relationship between the firing temperature and the relative density of the beta-alumina molded body having the composition of the example.

Claims (1)

(57) [Claims] 1. A method for calcination of a beta-alumina solid electrolyte in which the temperature of the beta-alumina molded body is raised without lowering the temperature during the firing, the temperature is held in the maximum temperature region for a predetermined time, and then the temperature is lowered without raising the temperature during the firing. Beta-alumina in the middle of the warming process stops heating at a temperature of 65 to 80% of the theoretical density, holds it for a certain period of time and then either raises the temperature again or temporarily lowers the temperature rising rate. In the method for calcination of beta-alumina solid electrolyte, the temperature is stopped in the uniform growth temperature region of the crystal particles, the temperature is maintained for a predetermined time, and then the temperature is decreased again or the temperature decrease rate is temporarily decreased.