JPH08268750A - Electrically conductive ceramics - Google Patents

Abstract

PURPOSE: To suppress expansion and shrinkage due to a change of an oxidizing- reducing atmosphere. CONSTITUTION: This electrically conductive ceramics is made of a multiple oxide contg. at least La and Cr as metallic elements. When the entire compsn. of the constituent metallic elements of the multiple oxide by atomic ratio is represented by the formula Lax (M1) (M2)z Crp (where M1 is at least one kind of element selected from among Mg, Ca, Sr and Ba and M2 is Zr or a combination of Zr with at least one kind of element selected from among Hf and the group IIIa element of the Periodic Table except La), (x), (y), (z) and (p) in the formula satisfy x+y+z+p=2, x >0, z>0, p>0, 0.002<=y<=0.9 and 0.001<=y-z<=0.8.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a LaCrO ₃ -based conductive ceramic suitable as a current collecting material such as a separator, a gas diffuser, and an interconnector of a solid oxide fuel cell, or a ceramic heating element.

[0002]

2. Description of the Related Art A compound represented by LaCrO ₃ is excellent in chemical stability at high temperature and has high electron conductivity, and therefore, it is a current collecting material such as a separator, a gas diffuser, and an interconnector of a solid oxide fuel cell. Alternatively, application to a ceramic heating element is under consideration.
Recently, in order to improve electric conductivity, Ca, Sr,
A LaCrO ₃ -based material in which a part of La or Cr is replaced by Ba or Mg is also proposed.

In the solid oxide fuel cell, as shown in FIG. 1, one surface of the electrolyte 1 of Y ₂ O ₃ -stabilized ZrO ₂ is LaMnO _{3 in} which porous La is replaced with Ca and Sr.
Is formed as the air electrode 2, and porous Ni-Z is formed on the other surface.
A single cell is formed by forming a fuel electrode 3 made of rO ₂ (containing Y ₂ O ₃ ). This single cell is the above-mentioned LaCrO.
It is sandwiched between ₃ series separators 4. In addition, power generation is performed at a temperature of 1000 to 1050 ° C. by supplying air (oxygen) to the air electrode side of the cell and fuel (hydrogen) to the fuel electrode side.

On the other hand, as a ceramic heating element which operates at a high temperature, a resistance element called a PTC thermistor typified by barium titanate is known. This element is characterized in that it has a self-temperature control function because it has a high electric resistance at high temperatures, but it is used in a relatively low temperature range up to about 350 ° C. On the other hand, as a ceramic heating element whose operating temperature is 400 ° C. to 1200 ° C., as described in Japanese Patent Application No. 5-103117, LaCrO _{3 is used.}
A self-heating type ceramic heating element made of a system material has also been proposed.

[0005]

This LaCrO ₃ type material is suitable as a current collecting material for solid oxide fuel cell and various ceramic heating elements as described above, but it was produced in an oxidizing atmosphere. The LaCrO ₃ based material has a fatal defect that it expands in a reducing atmosphere. Therefore, when the atmosphere is repeatedly changed to the oxidized state / reduced state, expansion and contraction are repeated, and cracks are generated in the material, resulting in the problem that the fuel cell and the heating element are destroyed.

[0006]

The inventors of the present invention have repeatedly studied a method for suppressing expansion / contraction with respect to the above problems, and as a result, in a composite oxide containing at least La and Cr, La Alternatively, they have found that the problem of expansion and shrinkage can be suppressed and the problem can be solved by substituting Zr at the same time as substituting a part of Cr with alkaline earth elements such as Mg and Ca.

That is, the conductive ceramics of the present invention is a conductive ceramics composed of a composite oxide containing at least La and Cr as metal elements, and has an overall composition based on the atomic ratio of the metal elements constituting the composite oxide. The following formula 1

[0008]

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X, y, z and p in the chemical formula 1
However, x + y + z + p = 20 <x, 0 <z, 0 <p 0.002 ≦ y ≦ 0.9 0.001 ≦ yz ≦ 0.8 is satisfied.

The reason why the values of x, y, z and p in Chemical formula 1 are limited to the above values in the conductive ceramics of the present invention will be explained. First, the y value indicating the amount of at least one selected from the group consisting of Mg, Ca, Sr and Ba is 0.0
When it is less than 02, the electric conductivity becomes small, and the essential characteristics as a conductive ceramic cannot be obtained, so that it cannot be used as a separator of a fuel cell, an interconnector, or a heating element. Further, when the y value is larger than 0.9, the electric conductivity is reduced and the sinterability is deteriorated. Furthermore, the difference (yz) between the above y value and Zr, or the z value indicating the amount of Zr and the combination of Zr and at least one selected from the group consisting of elements of Group 3a of the periodic table excluding La and Hf (yz).
Is less than 0.001, the electric conductivity is small, and conversely, if it exceeds 0.8, the sinterability is deteriorated. Further, when the z value is 0, the effect of suppressing expansion / contraction cannot be obtained. According to the present invention, a more desirable range from the above viewpoint is x + y + z + p = 20 <x, 0 <p, 0.3 ≦ y ≦ 0.6 0.1 ≦ z ≦ 0.3 0.1 ≦ yz ≦ 0.4.

According to the conductive ceramics of the present invention, a part of Cr can be replaced with Mn, Fe or Co at a ratio of 30 atomic% or less with respect to Cr. in this case,
Mn, Fe, Co, etc. are calculated as p.

Regarding x, which represents the La amount in the above chemical formula 1, when the A element is Mg, 1.6 / (1.8 + x + y) ≤x≤3.2 / (1.8+
The range of x + y) is good. This means that x is 3.0 / (1.8 + x +
If it exceeds y), the amount of La ₂ O ₃ precipitated increases depending on the firing conditions, and the material may decompose by reacting with moisture in the air or carbon dioxide gas. Also, x is 1.6 / (1.8+
If it is smaller than (x + y), the sinterability is lowered, and a temperature of 1700 ° C. or higher is required to sinter the material, which is not economical.

On the other hand, when the A element is Ca, Ba or Sr, 1.6 / (1.7 + x + y) ≤x≤3.2 / (1.7+) for the same reason as in the case of Mg.
It is desirable to satisfy x + y).

In the conductive ceramics of the present invention, the main crystal phase is composed of a perovskite type crystal represented by the general formula ABO ₃ , and Ca, Sr and Ba in M 1 form an A site together with La and Mg. Constitutes the B site with Cr. Further, the Zr of M2 or the Group 3a element of the periodic table almost always constitutes the B site, although it depends on the added amount.

As for the microstructure, when M1 is Mg, the y value is about 0.2, and when M1 is Ca, Sr, or Ba, the y value exceeds about 0.3.
_At least MgO, Ca in the perovskite main crystal of ₃
Crystals of O, SrO, BaO, etc. are dispersed and precipitated. Also,
According to the conductive ceramics of the present invention, MgO or Caa
In addition to O, in some cases only a slight amount of La ₂ O ₃ , Y
There are cases where solid solution oxides of ₂ O ₃ , ZrO ₂ or La and elements of Group 3a of the Periodic Table are precipitated, but there is no particular problem because these materials do not affect the sinterability and electrical conductivity. .

Precipitation of MgO, CaO and BaO has the effect of controlling the coefficient of thermal expansion of ceramics. La
The thermal expansion coefficient of CrO ₃ solid solution is smaller than that of the solid electrolyte, and thermal stress occurs when a fuel cell is constructed,
The cell may be destroyed in some cases. According to the present invention, by precipitating crystals of MgO, CaO or the like, the coefficient of thermal expansion can be made to match the coefficient of thermal expansion of the solid electrolyte as a composite oxide of LaCrO ₃ solid solution and MgO or CaO.

From the standpoint of strength, the average crystal grain size of MgO and CaO dispersed and precipitated is preferably 50 μm or less, and more preferably 10 μm or less. The LaCrO ₃ solid solution phase, which is the main crystal phase, is also preferably 50 μm or less, particularly preferably 20 μm or less, from the viewpoint of strength. The average crystal grain size of these crystal phases is 50 μm
This is because the mechanical strength of the material decreases and the material may be destroyed by thermal shock if it exceeds.

The conductive ceramics of the present invention are required to have high creep resistance when used at high temperatures. The creep resistance is greatly affected by Al and Si in the ceramics, and it is desirable to control the total amount of Al and Si to 5% by weight or less, particularly 2% by weight or less in order to improve the creep resistance.

When the conductive ceramic of the present invention is used as an interconnector or a separator of a fuel cell, the thermal expansion coefficient of this ceramic is stabilized by Y ₂ O ₃ or Yb ₂ O ₃ which is a solid electrolyte. Zr
LaMnO _{3 formed} by adding O ₂ , CaO, SrO, etc.
By making it coincide with the system air electrode, generation of thermal stress can be suppressed and cell destruction can be prevented. Therefore, it is necessary to adjust the coefficient of thermal expansion of the conductive ceramics in the range of 10 to 11 × 10 ⁻⁶ / ° C. in the temperature range of room temperature to 1000 ° C. From such a point, a desirable range of Chemical Formula 1 is x + y + z + p = 20 <x, 0 <z, 0 <p, 0.3 ≦ y ≦ 0.6 0.1 ≦ yz−0.4.

In the conductive ceramic of the present invention, M
Specific examples of the Group 3a element of the periodic table constituting a part of 2 include Y, Yb, Sc, Sm, Dy, Nd, Pr and C.
At least one selected from the group consisting of e, Gd, and Er.

For producing the conductive ceramics of the present invention
Is La as the starting material₂O₃, Cr₂O₃Of powder
Others, oxides or heat of the metals that make up M1 and M2
Compounds such as carbonates and nitrates that form oxides by treatment
And the metal element ratios of them satisfy the above-mentioned chemical formula 1.
Ball mix using zirconia balls
After mixing with a vibrating mill, add this powder to the desired powder.
Pulverize to a powder particle size, and use this as the desired molding means,
For example, die press, cold isostatic press, extrusion molding, etc.
Molded into an arbitrary shape with a temperature of 1400 to 1700 ° C
2 to 7 o'clock in the air or in an inert atmosphere such as Ar
It is made by firing for a while. To improve product dimensional accuracy
2 to 5 at a temperature of 1300 to 1600 ° C once before molding
Using powder that has been partially calcined for a period of time to form a solid solution powder
It may be molded and fired. In addition, Mg, Ca, etc. are included in advance.
Having LaCrO₃After preparing the solid solution powder, MgO,
La₂O ₃, Y₂O₃, ZrO₂Add a specified amount of
May be manufactured.

Next, the case where the conductive ceramics of the present invention is used as a current collecting member such as an interconnector or a separator of a fuel cell will be described. In the flat type fuel cell shown in FIG. 1, _{3 to} 15 mol% of Y ₂ O ₃ , Y
Stabilized ZrO ₂ containing b ₂ O ₃ , Sc ₂ O ₃ or Er ₂ O ₃ or 5 to 30 mol% Y ₂ O ₃ , Yb ₂
On one surface of the solid electrolyte 1 made of CeO ₂ containing one of O ₃ and Gd ₂ O ₃ , for example, porous LaMnO ₃ in which La is replaced with 10 to 20 atomic% of Sr and Ca or Japanese Patent Application No. Air electrode 2 made of materials such as 5-66935
On the other side as the fuel electrode 3, which is porous 60% by volume N
i-40 volume% ZrO ₂ (containing Y ₂ O ₃ ) A cermet is formed. A current collecting member 4 called a separator that connects the cells with each other as a single cell electrically connects the air electrode and the fuel electrode of the adjacent cell. The conductive ceramics of the present invention is used as the current collecting member 4. In such a cell, air or oxygen gas is supplied to the air electrode 2, hydrogen, CO, CO ₂ gas, etc. are supplied to the fuel electrode. For this reason,
Since one surface of the current collecting member 4 is in contact with the oxidizing gas and the other surface is in contact with the reducing gas, and it is necessary to completely isolate these from each other, high electrical conductivity and high compactness are required. The porosity is preferably 1% or less, particularly preferably 0.5% or less. Further, in a cylindrical fuel cell, the conductive ceramics of the present invention can also be used as an interconnector of a cylindrical fuel cell.

Next, the case where the conductive ceramics of the present invention is used as a cylindrical heating element will be described. Figure 2
The heating element shown in (1) is composed of a resistor 5 made of a cylindrical sintered body and electrodes 6 and 7 formed at both ends. The conductive ceramic of the present invention is used as the resistor 5. This heating element can be operated at a temperature of 400 to 1200 ° C. by applying a voltage of 50 V or less to the electrodes 6 and 7. As the heating element, in addition to the cylindrical shape shown in FIG. 2, a flat plate shape, a cylindrical spiral, a honeycomb structure, or the like can be arbitrarily manufactured. The heating element is not necessarily required to be dense, but from the viewpoint of high temperature strength and creep resistance of the element, the open porosity is preferably 20% or less, particularly 10% or less.

[0024]

The important characteristic of LaCrO ₃ is that it has electrical conductivity in an oxidizing atmosphere to a reducing atmosphere. For example, in LaCrO ₃ , when a part of Cr is replaced with Mg, holes are generated according to the following chemical formula 2, and this causes electric conduction.

[0025]

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However, LaCrO ₃ is known to expand in a reducing atmosphere at high temperature. This expansion is

[0027]

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It is considered that oxygen is released from the crystal in the reducing atmosphere, the repulsive force between the cations is increased, and as a result, the crystal lattice is expanded, as shown in FIG. La
CrO ₃ will eventually be destroyed if an operation is performed such that expansion / contraction is repeatedly caused by changes in the atmosphere. To give a specific example, one of the separators or interconnectors of a fuel cell is exposed to an oxygen atmosphere and the other is exposed to a reducing atmosphere such as hydrogen. Tensile stress occurs on the
When the fuel cell is stopped and hydrogen is stopped and returned to the oxidizing atmosphere, the fuel cell contracts. Therefore, if such operation stop of the fuel cell is repeated, LaCrO ₃ is destroyed and, as a result, the cell itself is also destroyed.

In the present invention, as a result of various studies on methods for suppressing expansion / contraction associated with changes in the oxidizing / reducing atmosphere, Mg, Ca, Sr, Ba partially replaces La or Cr, and at the same time, at least Zr is reduced. It has been found that the substitution can reduce the expansion in the reducing atmosphere. The reason for this is not clear, but it is presumed that by substituting Zr, the interaction between cations such as La and Mg, Ca, etc. was reduced, or the generation of oxygen ion vacancies was suppressed. To be done.

[0030]

【Example】

Example 1 Commercially available 99.9% pure Group 3a element oxide of the periodic table,
SrCO ₃ , CaCO ₃ , BaCO ₃ , MgO, Cr ₂
O ₃ and ZrO ₂ were blended so as to have the first composite oxide composition shown in Tables 1 and 2, and the mixture was mixed for 24 hours in a ball mill using zirconia balls.
After molding to a length of × 45mm, it is heated at 1500 ° C in air for 5
Burned for hours.

The open porosity of the obtained sintered body was measured by the Archimedes method to determine the sinterability. Further, a Pt electrode was baked on this sintered body, and using this as an electrode, the distance between the voltage terminals was set to 20 mm, and the Pt electrode was set to 100 by the DC 4-terminal method.
The electrical conductivity in the atmosphere was measured at 0 ° C. In addition, 100
After allowing the sample to stand in hydrogen at 0 ° C. for 24 hours, the expansion coefficient of the sintered body was measured at room temperature. Also, for comparison, 2000
LaMg _0.2 Cr _0.8 O ₃ and La calcined in Ar at ℃
_0.8 Ca _0.2 CrO ₃ was also evaluated in the same manner as above. The results are shown in Tables 1 and 2.

[0032]

[Table 1]

[0033]

[Table 2]

As is clear from the results shown in Tables 1 and 2, Mg
Sample No. 4 having an atomic ratio y of 0.002 smaller than 0.002 has low electric conductivity. Sample No. 18, in which the atomic ratio y of Mg and Ca exceeds 0.9, has poor electrical conductivity and an open porosity of 15
%, The sinterability also deteriorates. Mg and Zr
Similarly, Samples No. 3 and No. 31 having an atomic ratio difference (yz) smaller than 0.001 also have low electric conductivity. (Y-
In Sample No. 17 in which z) is larger than 0.8, the sinterability is extremely poor and the open porosity exceeds 30%. Furthermore, the expansion coefficient is 0.2 in the conventional products of Sample No. 1 and 2 with z value of 0.
%, But the ceramics of which composition is controlled according to the present invention all have a coefficient of expansion of 0.1% or less,
In particular, it showed excellent characteristics of 0.08% or less. Among these, 0 <x, 0 <z, 0 <p, 0.3 ≦ y ≦ 0.6, 0.
10 to 11 × 10 for samples satisfying 1 ≦ yz ≦ 0.4
It had a coefficient of thermal expansion of ^-6 / ° C.

Example 2 In Table 1 of Example 1, samples No. 1, 2, 9, 12 and 2 were used.
5 composition powder was fired at 1500 ° C. for 5 hours, the thickness was 2 mm, the size was 50 mm, and the width was 1 mm at 1 mm intervals.
A separator having grooves with a depth of 1 mm formed in a grid pattern was produced. In addition, a commercially available purity of 99.9% and a thickness of 0.2 mm,
Stabilized Z containing 8 mol% Y ₂ O ₃ having a size of 50 mm
_On one surface of the rO ₂ sheet, La _0.8 having a powder particle size of about 2 μm
Sr _0.2 MnO ₃ cathode powder and 70 on the other side
Each of the volume% Ni-30 volume% ZrO ₂ (containing 8 mol% Y ₂ O ₃ ) fuel electrode powder was screen-printed to a thickness of 30 μm and baked at 1300 ° C. for 3 hours. This was sandwiched between the separators described above, oxygen gas was flown on the air electrode side, and hydrogen gas was flown on the fuel electrode side, and power was generated at 1000 ° C., which was 0.18 to 0.20 W / in conventional samples No. 1 and 2. cm
² shows the power generation performance, while the sample No.
In Nos. 9, 12, and 25, good power generation characteristics of 0.21 to 0.25 W / cm ² were exhibited.

After power was generated at 1000 ° C. for 100 hours, it was cooled to room temperature in an N ₂ atmosphere and further heated to 1000 ° C. in a hydrogen atmosphere to generate power. This one
As a result of repeating 0 times, the separator was destroyed 6 times for the conventional sample No. 1 and 5 times for the sample No. 2, but all 1 in the samples No. 9, 12, 25 of the present invention.
It showed excellent stability without breaking even after 0 times.

[0037]

As described above, the conductive ceramics of the present invention can suppress expansion in a reducing atmosphere as compared with conventional materials while maintaining high conductivity, and therefore, for example, fuel Even when it is used as a battery separator or a heating element, expansion and contraction are suppressed even during repeated operation, and stable characteristics are obtained. Further, manufacturing as a heating element can be facilitated and the manufacturing cost can be reduced.

[Brief description of drawings]

FIG. 1 is a diagram for explaining the structure of a flat plate type fuel cell unit.

FIG. 2 is a diagram for explaining the structure of a heating element.

[Explanation of symbols]

 1 Electrolyte 2 Air electrode 3 Fuel electrode 4 Current collecting member 5 Resistor 6, 7 Electrode

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI technical display location // H05B 3/14 0380-3K H05B 3/14 B

Claims (1)

[Claims] 1. A metal element containing at least La and Cr.
It is a conductive ceramics composed of a complex oxide containing, and the overall composition based on the atomic ratio of metal elements constituting the complex oxide is represented by the following chemical formula 1. In the formula 1, x, y, z and p are x + y + z + p = 2 0 <x, 0 <z, 0 <p, 0.002 ≦ y ≦ 0.9 0.001 ≦ y−z ≦ Conductive ceramics characterized by satisfying 0.8.