JPH08253363A - Baceo3-based ionically conductive ceramics and its production - Google Patents

Abstract

PURPOSE: To improve a ceramics strength without substantially spoiling ionic conductivity by bringing the ceramics to develop a BaAl2 O4 spinel phase by adding Al2 O3 to the Ba-Ce-Gd-O-based ceramics. CONSTITUTION: This BaCeO3 -based ionically conductive ceramics has a constitution prepared by adding Al2 O3 to (1) powder of Ba1+x Ce1-y Gdy O3-a [0<=(x)<=0.2, 0<(y)<0.3; (a) is a defective number of oxygen per unit formula and 0.5>(a)>0] or BaAl2 O4 to (2) powdery BaCe1-y Gdy O3-a [0<(y)<0.3; (a) is a defective number of oxygen per unit formula and 0.5>(a)>0], then baking the mixture after mixing. This ionically conductive ceramics has a baked constitution in which BaAl2 O4 spinel phases are dispersed in the mother body of BaCe1-y Gdy O3-a [(y) and (a) have the same meaning as above].

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to BaCeO ₃ -based ion conductive ceramics that can be used in electrochemical devices such as fuel cells, hydrogen / water vapor sensors, and solid electrolytes for hydrogen separation. The present invention relates to BaCe _1-x Gd _x O _{3 -a} oxygen / proton mixed ion conductive ceramics having improved strength and _a method for producing the same.

[0002]

2. Description of the Related Art As an ionic conductor, there are a solution type, a molten salt type and a solid type, and an electrochemical device such as a battery uses a solution type and a molten salt type having a high ionic conductivity. There are problems such as liquid leakage and evaporation in the device, and there are great expectations for developing a solid-type ion-conducting electrochemical device. However, many complex oxide ion conductors have been reported, but their ionic conductivity is low and their chemical stability is poor.

Among these, a perovskite type oxide sintered body having BaCeO ₃ as a base material, particularly BaCe _1-X Gd _x O in which a part of Ce is replaced with a rare earth element, typically Gd
_3-a (0 <x <0.3, a is the number of oxygen vacancies per unit formula amount, 1.5>a> 0, hereinafter abbreviated as BCG) is chemically stable and relatively ionic. It is known that, in an atmosphere with high conductivity and a hydrogen source and an oxygen source, both oxygen ions and protons behave as a mixed ionic conductor involved in conduction. For example, in JP-A-5-28820, in view of the fact that an ionic conductor that is chemically stable and has high conductivity characteristics has not been synthesized until then, control of the powder particle size before final firing or before final firing is performed. BCG perovskite-type oxide in which x is 0.16 to 0.23, more preferably 0.2 in the above formula, through powder treatment with a non-aqueous solvent, performing dehydration vacuum drying, and selecting the firing temperature. The method of synthesizing a sintered body is described. Also,
Ceramics 27 (1992) No. 2, 112-11
On page 6, an introduction of some types of proton-conducting solids, entitled "High Temperature Proton-Conducting Ceramics and Their Applications",
As the conduction characteristics of the perovskite-type proton-conductive oxide, the measurement results of the electrical conductivity in hydrogen of various synthetic ceramics in which a part of Ce of SrCeO ₃ and BaCeO ₃ is replaced with a rare earth element are shown. As examples of its application, high-temperature hydrogen and water vapor sensors, fuel cells, etc. have been introduced.

[0004]

This BCG high temperature type proton conductor exhibits high ionic conductivity at a temperature of 1000 ° C. or less as compared with the currently used stabilized zirconia-based oxygen ion conductor. It has the advantages that it can be expected to be applied to electrochemical devices that can be used at lower temperatures than stabilized zirconia-based oxygen ion conductors, and that electrode reactions can be carried out smoothly by raising the temperature, and these advantages were utilized. Although it is expected to be applied to many electrochemical devices, the problem is that the strength is weak. According to the measurement by the present inventor, the sintered body density is 99.4%.
The three-point bending (flexural strength) strength of the sintered body of No. 3 is only 75 MPa. Whatever form is used, a three-point bending (flexural strength) strength of at least 100 MPa is required to obtain reliable mechanical strength.

An object of the present invention is to improve the strength of BCG sintered ceramics without substantially sacrificing ionic conductivity.

[0006]

As a result of research, the present inventor has found that it is effective to solve this problem by expressing a BaAl ₂ O ₄ spinel phase by adding 20 mol% or less of Al ₂ O _3. I found something. BaAl ₂ O ₄ can also be added. Based on this finding, the present invention provides Ba _{1 + X} Ce _1-y Gd _y O _3-a (0 ≦ x ≦ 0.2,
0 <y <0.3, a is the number of oxygen vacancies per unit formula amount, 1.5>a> 0) Al ₂ O _{3 in} powder or BaC
e _1-y Gd _y O _3-a (0 <y <0.3, a is the number of oxygen vacancies per unit formula amount, 1.5>a> 0) Powder with BaAl
Disclosed is a BaCeO ₃ -based ion conductive ceramic having a structure in which ₂ O ₄ is added and mixed and then sintered. This sintered structure is BaCe _1-y Gd _y O
_3-a (0 <y <0.3, a is the number of oxygen vacancies per unit formula amount, 1.5>a> 0) has a sintered structure in which a BaAl ₂ O ₄ spinel phase is dispersed in the matrix. It is characterized by
The present invention also relates to Ba _{1 + X} Ce _1-y Gd _y O _3-a (0 ≦ x ≦
0.2, 0 <y <0.3, a is the number of oxygen vacancies per unit formula amount, and 1.5>a> 0) powder to Al ₂ O ₃ powder or Al ₂ O ₃ whiskers or BaCe _{1 -y} Gd _y O
_3-a (0 <y <0.3, a is the number of oxygen vacancies per unit formula amount, 1.5>a> 0) BaAl ₂ O ₄ powder was added to the powder, mixed and molded, Also provided is a method for producing a BaCeO ₃ -based ion conductive ceramics, which is characterized by sintering thereafter.

[0007]

It is considered that the following reaction occurs by adding Al ₂ O ₃ powder to BCG powder and mixing it and then sintering the compact: Ba _{1 + x} Ce _1-y Gd _y O _3-a (0 ≦ x ≦ 0.2, 0 <
y <0.3, 1.5>a> 0) + xAl ₂ O ₃ → BaA
l ₂ O ₄ + BaCe _1-y Gd _y O _3-a The mechanism for improving the strength is BaAl ₂ O ₄ having a large strength.
It is considered to be a dispersion strengthening mechanism of the spinel phase.

The amount of Al ₂ O ₃ added is 20 mol% or less,
It is preferably 2 to 18 mol%, more preferably 4 to 11 mol%. If the amount of Al ₂ O ₃ added exceeds 20 mol%, the sinterability deteriorates and the sintered body density decreases. If the amount is 20 mol% or less, a corresponding addition effect is exhibited depending on the addition amount, but it is preferable to add at least 2 mol% in order to obtain a meaningful strength improving effect. B instead of Al ₂ O ₃
A suitable amount of aAl ₂ O ₄ may be added.

The BCG powder can be produced by a usual sintering method using a mixed powder of oxides, carbonates, acetates, etc. of respective elements in a predetermined ratio. For example, barium carbonate (BaCO ₃ ), cerium oxide (CeO ₂ ), and gadolinium oxide (Gd ₂ O ₃ ) powder can be used as the starting powder to synthesize by a solid-phase reaction method. BCG powder is obtained by sufficiently mixing these starting powders in a predetermined ratio, firing the molded product at, for example, 1300 ° C. for about 10 hours, and pulverizing the product.

For example, granular or whisker-like Al ₂ O ₃ was added to the BCG powder and thoroughly mixed with a ball mill, and then CI was added.
Molded with P, 5 to 15 at 1450 to 1650 ° C
Sintering is done for a time. Sintering is preferably carried out in an oxygen atmosphere.

In this way, BCG is Al ₂ O ₃ or B
A BaCeO ₃ based ion conductive ceramics having improved strength by having a sintered structure in which aAl ₂ O ₄ is added and reacted is obtained. This is due to the high strength of BaAl ₂
It is considered that the strength was improved by dispersing the O ₄ spinel phase in the matrix.

The addition of alumina has revealed a notable phenomenon. BCG shows a large change in the coefficient of thermal expansion in the range of 600 to 800 ° C. which is considered to be caused by the phase change from orthorhombic to cubic (as a reference, “Denki Ka”).
gaku 62 (4) (1994) 326-331). However, by adding alumina, it is possible to reduce or eliminate the large change in the coefficient of thermal expansion in the range of 600 to 800 ° C. Figure 1
₃ consisting of a BCG powder sintered body, a BCG + 5 mol% Al ₂ O ₃ powder sintered body and a BCG + 10 mol% Al ₂ O ₃ powder sintered body 3
Seed samples at 1000 ° C at a heating rate of 300 ° C / hr
3 shows curves 1, 2 and 3 in which the respective thermal expansion coefficients (%) when heated above are measured. The measurement was performed using the differential thermal analyzer of Rigaku TAS-200 system. Curve 1 shows a large change in the coefficient of thermal expansion in the 600-800 ° C. range, whereas curve 2 reduces the thermal expansion and curve 3 with 10% alumina has substantially eliminated it. There is. Therefore, the addition of Al ₂ O ₃ improves the dimensional stability during heating and reduces or avoids the thermal stress when incorporated into a structural material.

[0013]

【Example】

(Examples and Comparative Examples) BCG powder (specific surface area: 12 m ² /
g) granular Al ₂ O ₃ powder (average particle diameter: 0.05 μm, specific surface area: 80 m ² / g) or Al ₂ O ₃ whiskers (average diameter: 5 μm, length: 100 to 200 μm) in 5 mol% and 10 mol% (only for whiskers) was added, mixed by a ball mill, CIP molded, and then sintered at 1500 ° C. for 10 hours in an oxygen atmosphere. A sample without addition of Al ₂ O ₃ was also prepared as a comparative example. A platinum paste was baked on the sintered body, and the conductivity in hydrogen was measured.

[0014]

[Table 1]

From Table 1, it can be seen that the strength of the sintered ceramics is improved by about 1.7 times without substantially sacrificing the ionic conductivity, and the strength is improved by adding Al ₂ O ₃ . I understand.

[0016]

The strength of the BCG sintered ceramics is improved without substantially sacrificing the ion conductivity at high temperatures, and the dimensional stability during heating is improved.

[Brief description of drawings]

FIG. 1 BCG powder sintered body (curve 1), BCG + 5 mol%
Al ₂ O ₃ powder sintered body (curve 2) and BCG + 10 mol%
It is a graph which shows the measurement result of the coefficient of thermal expansion (%) at the time of heating of 3 types of samples which consist of Al ₂ O ₃ powder sintered body (curve 3).

Claims (4)

[Claims] 1. Ba _{1 + X} Ce _1-y Gd _y O _3-a (0 ≦ x
≦ 0.2, 0 <y <0.3, a is the number of oxygen vacancies per unit formula amount, 1.5>a> 0) Al ₂ O _{3 in} powder or BaCe _1-y Gd _y O _{3 -a} (0 <y <0.3, a is the number of oxygen vacancies per unit formula amount, 1.5>a> 0) A structure in which BaAl ₂ O ₄ was added to and mixed with the powder and then sintered. BaCeO ₃ based ion conductive ceramics characterized by having 2. BaCe _1-y Gd _y O _3-a (0 <y <
0.3, a is the number of oxygen vacancies per unit formula amount, 1.5
>A> 0) BaCeO ₃ based ion conductive ceramics having a sintered structure in which a BaAl ₂ O ₄ spinel phase is dispersed in a matrix. 3. Ba _{1 + X} Ce _1-y Gd _y O _3-a (0 ≦ x
≦ 0.2, 0 <y <0.3, a is the number of oxygen vacancies per unit formula amount, and 1.5>a> 0) is added with Al ₂ O ₃ powder or Al ₂ O ₃ whiskers. A method for producing a BaCeO ₃ -based ion conductive ceramics, which comprises mixing, shaping, and then sintering. 4. BaCe _1-y Gd _y O _3-a (0 <y <
0.3, a is the number of oxygen vacancies per unit formula amount, 1.5
>A> 0) BaAl ₂ O ₄ powder is added to the powder, mixed, shaped, and then sintered, which is a method for producing a BaCeO ₃ based ion conductive ceramics.