JPH0987024A - Electrically conductive porous ceramic - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce an electrically conductive porous ceramic capable of being used as an electrode material of a solid electrolyte type fuel cell because of its high open pore, high bending strength and electrical conductivity. SOLUTION: This electrically conductive porous ceramic consisting of a lanthanum based multiple oxide expressed by the formula, ABO3 and having a perovskite type crystal structure is a porous material having >=20% open pore, >=50MPa bending strength and >=200S/m electrical conductivity.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a conductive porous ceramics, and more particularly to a cathode electrode material for a solid oxide fuel cell and a conductive porous ceramics as an electrode material using the same.

[0002]

2. Description of the Related Art As a cathode electrode material (air electrode) of a solid oxide fuel cell, a ceramic sintered body such as lanthanum-manganese (LaMnO ₃ ) which is a lanthanum-based composite oxide excellent in high temperature conductivity is used. Used (Japanese Patent Laid-Open No. 6-20
(See Japanese Patent No. 6781). The electrode material of such a sintered body is manufactured by a known technique of general ceramics and by melting, etc., but in any case, in order to smooth gas permeation of air or oxygen which is an oxidant. In addition, as a characteristic, an open porosity of 20% or more is required.

However, the porous sintered body having pores (including open / closed pores) has a problem that the strength decreases as the porosity increases, and recently, there is a demand for improving the strength to secure reliability against cracking. There is a strong demand. At present, whisker (fiber reinforced ceramics), which is known as a ceramics reinforcement material, is used to improve sinterability by finely pulverizing lanthanum and manganite powder.
Studies have been conducted to overcome brittleness by the addition of, and studies on pore-forming materials that create pores.

In the case of these pores, the fact that the pore diameter and the shape of the pores strongly influence the strength is that the following Griffith equation σf = K _1c / Yc ^1/2 σf: fracture strength, K _1c : fracture toughness, Y: It can be easily estimated from a constant depending on the shape of the defect in the material, c: the size of the defect. That is, from this equation, in order to improve the strength while ensuring a predetermined open porosity,
It is understood that the pore size (defect size) may be reduced and the pore shape (defect shape) may be U-shaped rather than V-shaped.

[0005]

However, the open porosity required for gas permeability of air or oxygen, which is an oxidizer of a solid oxide fuel cell, and strength and toughness (resistance to fracture) related to reliability. It is very difficult to satisfy the contradictory improvement of properties at the same time, and there are many problems to be solved. However, it is possible to improve the properties of open porosity and strength in a well-balanced manner, and it is considered that the combination of the ceramics that is the base material (matrix) and the pore-forming material that forms the pores is particularly important for that. The problems in combining the matrix and the pore-forming material at this time are the firing temperature and the atmosphere, bond strength and reactivity, and, as is clear from the Griffith equation, the powder characteristics of the pore-forming material. The size and shape of the pores in the base material of the sintered body, which bears a large amount of pressure, are mentioned.

[0006]

SUMMARY OF THE INVENTION The present invention relates to a conductive porous ceramics for achieving the above-mentioned improvement of both open porosity and strength, which is a perovskite type crystal represented by the general formula ABO _3. Conductive porous ceramics composed of structured lanthanum-based composite oxides with open porosity of 20% or more and bending strength of 50MP
It is characterized in that the conductivity is a or more and the conductivity is 200 S / m or more.

The present invention will be described in detail below. The present invention provides a conductive porous ceramics composed of a lanthanum-based composite oxide having a perovskite type crystal structure represented by the general formula ABO _3, which is used as a cathode electrode material of a solid oxide fuel cell, and has an open porosity, Bending strength and conductivity are specified, and if the open porosity is less than 20%, air or oxygen gas as an oxidant will not permeate smoothly in the above electrode material, so it should be 20% or more. If the bending strength is less than 50 MPa, the reliability against cracking is low and there is a practical problem.
It is necessary to make it above MPa, and the conductivity is 200
If it is less than S / m, a sufficient function and effect as a cathode electrode material cannot be obtained, so it is necessary to set it to 200 S / m or more.

The present inventor diligently studied a method for achieving the above-mentioned improvement in both open porosity and strength.
For this, lanthanum manganite in the finest possible powder is used, and as a pore-forming material, it is easy to form pores due to the progress of a uniform oxidation reaction at a low temperature, which is a fine powder and has a specific surface area (BET method). We have found that it is better to use large carbon powders.

Therefore, this is a lanthanum strontium manganite powder (LaSrMnO ₃ ) having a perovskite type crystal structure calcined at 1,000 to 1,300 ° C. and an average particle size of 1 to 5 μm, and a specific surface area of 1 to 10 μm. Is 2
Carbon powder of 00m ² / g or more is mixed with polyvinyl alcohol (PVA) which is a binder in a ball mill to form a slurry, and when granulated by a known technique, the carbon powder agglomerated in the ball mill is uniformly dispersed. As a result, a stable open porosity, a uniform pore size and a sharp pore size distribution with a small pore size can be obtained.

In the porous ceramics thus obtained, the strength is improved by making the carbon powder uniform, but the granulated powder obtained here has a uniform composition. By subjecting this to a primary molding with a die press and then a secondary molding with an isotropic and isobaric CIP (rubber press), a molded product having a more uniform composition can be obtained. The compact is then sintered in an oxidizing atmosphere to form a flat plate of conductive LaSrMnO ₃ porous ceramics. If the sintering temperature is less than 1,300 ° C, the porosity is good but the pore size is large. However, since the density of the sintered body is low, the strength decreases, and if it is higher than 1,500 ℃, the strength increases but the open porosity decreases, so it is necessary to set the temperature to 1,300 to 1,500 ℃. It The conductive porous ceramics thus obtained had an open porosity of 25%,
Pore diameter is 1μm, bending strength is 60MPa, conductivity is 200
It will have the desired physical properties of S / m (1,000 ° C).

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Next, embodiments of the present invention will be described with reference to Examples and Comparative Examples. Here, the open porosity was measured by the mercury penetration method, the bending strength was measured by the three-point bending test method, and the conductivity was measured by the direct current four-terminal method.

[0012]

【Example】

Example 1 La with an average particle size of 3.5 μm synthesized in a perovskite structure
75% by weight of SrMnO ₃ powder with an average particle size of 5.9
23% by weight of carbon powder with a specific surface area of 210 m ² / g in μm and 2% by weight of polyvinyl alcohol (PVA) as a binder were mixed, granulated and molded, and then 1,4 in an oxidizing atmosphere.
When heated to 50 ℃ and sintered, the diameter is 205mm and the thickness is 2mm.
The flat plate-shaped porous ceramic of was obtained. When the physical properties of the porous ceramics thus obtained were investigated,
It had an open porosity of 25%, a bending strength of 60 MPa and an electrical conductivity of 200 S / m.

Example 2 La having an average particle size of 2.0 μm synthesized in a perovskite structure
72% by weight of SrMnO ₃ powder, average particle size of 1.0μ as a pore former
20% by weight of carbon black having a specific surface area of 290 m ² / g in m,
And 8% by weight of methyl cellulose (MC) as a binder are mixed, granulated and molded, and then 1,3 in an oxidizing atmosphere.
When heated to 00 ℃ and sintered, the diameter is 205mm and the thickness is 2mm.
Since the flat plate-shaped porous ceramics were obtained, its physical properties were examined. As a result, the open porosity was 27%, the bending strength was 52 MPa, and the electrical conductivity was 200 S / m.

Comparative Example La synthesized with a perovskite structure and having an average particle size of 2.3 μm
75% by weight of SrMnO ₃ powder, 0.7μ average particle size as pore former
23% by weight of carbon black with a specific surface area of 100 m ² / g
And polyvinyl alcohol (PVA) as binder
After mixing 2% by weight, granulating and molding, heating at 1,400 ℃ in an oxidizing atmosphere and sintering, a diameter of 205 mm,
Since a flat type porous ceramics with a thickness of 2 mm was obtained, its physical properties were examined.
62 MPa, low open porosity of 17% and conductivity of 170
It was S / m.

[0015]

INDUSTRIAL APPLICABILITY The present invention relates to a conductive porous ceramics, which is produced by mixing a lanthanum-based composite oxide having a perovskite type crystal structure and an appropriate pore-forming material. It has an open porosity of 20% or more, a bending strength of 50 MPa or more, and an electric conductivity of 200 S / m or more, and is particularly useful as a cathode electrode material of a solid oxide fuel cell.

Claims (1)

[Claims] 1. A conductive porous ceramics composed of a lanthanum-based composite oxide having a perovskite type crystal structure represented by the general formula ABO ₃ has an open porosity of
Conductive porous ceramics having a porosity of 20% or more, a bending strength of 50 MPa or more, and an electrical conductivity of 200 S / m or more.