JPH1179836A - Electroconductive ceramics - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain electroconductive ceramics capable of enhancing the sinterability and carrying out the densification at a low temperature without deteriorating excellent electroconductivity possessed by LaCrO3 . SOLUTION: The electroconductive ceramics comprise La, Cr and Mg as metallic elements and contain a perovskite type compound oxide as a main crystal phase and Ca and at least one of Y, Yb, Sc, Sm, Nd, Dy and Pr in respective amounts of 0.01-2.0 pts.wt., based on 100 pts.wt. perovskite type compound oxide and expressed in terms of oxides.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to conductive ceramics, and more particularly to conductive ceramics having improved sinterability. The present invention relates to a conductive ceramic suitable for an electrode or the like.

[0002]

2. Description of the Related Art Conventionally, lanthanum chromite-based oxides (LaCrO ₃ ) have excellent chemical stability at high temperatures and high electron conductivity, and therefore have a high performance in a separator, a gas diffuser and an interface of a solid oxide fuel cell. Used as a connector.

FIG. 1 shows a solid oxide fuel cell having a flat plate shape. In this flat fuel cell, for example, Y ₂
La on the upper surface of the solid electrolyte 1 made of O ₃ stabilized ZrO ₂
A MnO ₃ -based air electrode 2 and a fuel electrode 3 such as Ni-zirconia are provided on the lower surface, and the connection between the cells is made by a LaCrO ₃ -based separator 4.

In a fuel cell unit, a gas containing oxygen, for example, air is flowed to the air electrode side, and fuel or fuel is flown to the fuel electrode side.
For example, power is generated at a temperature of 1000 to 1050 ° C. while flowing hydrogen gas. As the above separator material, Ca
Alternatively, a LaCrO ₃ material in which Sr is dissolved is used.

The cylindrical fuel cell uses the same material as the flat fuel cell, and a solid electrolyte, a fuel electrode, and a current collector called an interconnector are substantially concentric on a support tube made of an air electrode material. It is formed in a shape. In a cylindrical fuel cell, for example, a current collector of one cell and a fuel electrode of an adjacent cell are connected to each other via Ni felt or the like.

[0006]

The LaCrO ₃ -based material has a slow diffusion rate of cations, and a Cr component volatilizes from the material during the sintering process, so that the Cr ₂ O ₃ material is formed at the contact portion (neck portion) of the particles. It condenses and deposits as O ₃ and inhibits sintering. For this reason, it is necessary to perform sintering at a high temperature of 2000 ° C. or higher in the air or sintering in a reducing atmosphere while suppressing the evaporation and condensation of Cr. is necessary. The production of a material by such high-temperature sintering makes mass production of fuel cells extremely difficult from an economic viewpoint, and is a factor that increases the cost.

On the other hand, as a method for obtaining a LaCrO ₃ -based material at a low temperature, an electrochemical vapor deposition (EVD) method has been applied. However, although this method produces a LaCrO ₃ -based material at a relatively low temperature of 1400 ° C.,
Since the growth rate of LaCrO ₃ is low, mass productivity is lacking. In addition, this method has an economical problem because it requires the use of extremely expensive metal chloride as a starting material.

[0008]

The inventor of the present invention has studied a method for improving the sinterability at a low temperature, and as a result,
a, Cr, Y, Yb, Sc, Sm, Nd, Dy, Pr for LaCrO ₃ -based material containing a, Cr and Mg
By simultaneously adding at least one of
The present inventors have found that sinterability can be improved and densification can be performed at a low temperature without impairing the excellent electrical conductivity of LaCrO ₃ , and the present invention has been accomplished.

That is, the conductive ceramic of the present invention contains La, Cr and Mg as metal elements, has a perovskite-type composite oxide as a main crystal phase, and contains Ca and 100 parts by weight of the perovskite-type composite oxide. , Y, Y
At least one of b, Sc, Sm, Nd, Dy and Pr is contained in an amount of 0.01 to 2.0 parts by weight in terms of oxide.

[0010]

In the LaCrO ₃ -based material, in addition to the slow cation diffusion rate in the crystal, the Cr component is liable to evaporate preferentially. The so-called LaCrO ₃ -based material, which is deposited as Cr ₂ O ₃ and inhibits the diffusion of cations to deteriorate sinterability, is dominated by an evaporative condensation mechanism.

In contrast, the mechanism by which the material of the present invention sinters at a low temperature is not clear at present, but is interpreted as follows. Ca and Y in LaMgCrO ₃ material
When rare earth elements such as Ca coexist, a rare earth element such as Ca and Y forms a compound, and further generates a liquid phase at a temperature of about 1400 ° C. or higher.

Therefore, in the present material, the diffusion rate of cations in the grain boundary phase is increased, and at the same time, the evaporation of Cr is suppressed and the sinterability is greatly improved. At this time, if the content of rare earth elements such as Ca and Y is relatively large, after sintering, this liquid phase becomes C
It precipitates on the surface as a compound of a and a rare earth element.

When the conductive ceramic of the present invention is used as an electrode material for a fuel cell or the like, high electric conductivity is required. When La is replaced with Mg in LaCrO ₃ , holes are generated according to the following formula 1.

[0014]

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According to the above equation, the electric conductivity is proportional to the concentration of Mg ions substituted for Cr. Therefore, the conductive ceramic of the present invention has high conductivity and high stability even in a reducing atmosphere, and is thus useful as an electrode material for a current collecting member of a fuel cell.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION In the conductive ceramics according to the present invention, Ca, Y, and Y are used in comparison with a conventionally known LaCrO ₃ -based composite oxide containing La, Cr and Mg.
It is a great feature that a rare earth element such as Yb is simultaneously contained. LaCrO ₃ has an ABO ₃ type perovskite type composite oxide as a main crystal phase, and La has an A site, Mg, Cr
Is an element constituting the B site, and the ABO ₃ type perovskite composite oxide ideally has an A site element / B site element (atomic ratio) = 1.

On the other hand, in the present invention, Ca and 100 parts by weight of the perovskite-type composite oxide are
At least one of Y, Yb, Sc, Sm, Nd, Dy, and Pr is 0.01 to 2.0 in terms of oxide.
It contains parts by weight.

The reason why the combination of Ca and rare earth elements such as Y and Yb is included is that Ca, Y and Yb
This is for generating a liquid phase by coexisting with rare earth elements such as.

The reason that the amount is 0.01 to 2.0 parts by weight with respect to 100 parts by weight of the perovskite-type composite oxide is that Ca and rare earth elements such as Y and Yb are converted into oxides respectively. If the amount is less than 0.01 part by weight, the sinterability is deteriorated. If the amount is more than 2.0 parts by weight, the stability of the material in a reducing atmosphere is deteriorated and the material is decomposed. . Ca, Y, Yb, Sc, Sm,
At least one of Nd, Dy and Pr is desirably in the range of 0.1 to 1.0 part by weight in terms of oxide based on 100 parts by weight of the perovskite-type composite oxide. In particular, C
A combination of a and Y is desirable.

In the conductive ceramics of the present invention, the substitution ratio of Mg to Cr is 5 to 35 atoms in terms of atomic ratio from the viewpoint of imparting large electric conductivity and reducing the expansion of the material in a reducing atmosphere. %, Especially 10 to 2
Up to 0 atomic% is excellent. Further, according to the present invention, a part of Cr can be replaced by Mn, Ni, Co, Fe, or the like as long as the effect of the present invention is not affected, but the specific replacement ratio is Cr. On the other hand, it is preferably at most 30 atomic%, particularly preferably at most 10 atomic%.

The conductive ceramic of the present invention has a crystal structure of at least La, Cr and Mg.
The main crystal phase is a perovskite-type composite oxide having an element represented by the formula:
a and rare earth elements such as Y are present. Rare earth elements such as Ca and Y may be slightly dissolved in the perovskite-type composite oxide.

In addition to the main crystal phase, the substitution ratio of Mg is 3
If it exceeds 0 atomic%, MgO may precipitate, and if the sintering temperature exceeds about 1500 ° C., Ca and
In some cases, compounds of rare earth elements such as Y and Yb are precipitated, but within the scope of the present invention, the electric conductivity has a sufficiently large value.

The conductive ceramic of the present invention is, for example,
After mixing and pulverizing oxides or carbonates of La, Cr, and Mg, the mixture is calcined at 1000 to 1400 ° C., and ABO is applied.
_{A 3-} type perovskite-type composite oxide was prepared, and this ABO
_The perovskite-type composite oxide is obtained by adding a Ca oxide or a carbonate and at least one oxide or carbonate among Y, Yb, Sc, Sm, Nd, Dy, and Pr to the 3-type perovskite-type composite oxide. To 100 parts by weight, 0.01 to 2.0 parts by weight in terms of oxide were added, and after mixing and pulverization, the mixture was molded into a predetermined shape. 1700 ° C
For about 2 to 5 hours. According to such baking, a main crystal phase having an average particle size of 1 to 20 μm is obtained. From the viewpoint of promoting sintering, the oxygen partial pressure in the oxidizing atmosphere is preferably set to 10 ^-3 atm or more.

Incidentally, for example, La, Cr, Mg, Ca, Y
After mixing and grinding oxides or carbonates of
After calcination at 00 to 1400 ° C., an ABO ₃ type perovskite-type composite oxide is prepared, and crushed into particles having a size of 0.1 to 5 μm. It may be fired in an oxidizing atmosphere.

The conductive ceramic according to the present invention is suitably used as a current collector such as a separator or an interconnector in a fuel cell. Therefore, FIG.
Fig. 1 shows a typical structure of a flat fuel cell. According to FIG. 1, (La, Sr) MnO ₃ or (L) is formed on the upper surface of a plate-like solid electrolyte 1 made of Y ₂ O ₃ stabilized ZrO ₂ or the like.
a, Ca) An air electrode 2 made of MnO ₃ and the like, and a fuel electrode 3 made of Ni—ZrO ₂ (containing Y ₂ O ₃ ) cermet and the like are formed on the lower surface, and these are used as single cells to connect the cells. A current collecting member 4 (separator) is disposed as a member.

In such a cell, the air electrode 2 is in contact with an oxygen-containing gas such as air, the fuel electrode 3 is in contact with hydrogen gas or the like, and both the air electrode 2 and the fuel electrode 3 are made of a porous material. However, the current collecting member 4 is required to have high density and high electrical conductivity because one side thereof is in contact with the oxygen-containing gas and one side is in contact with the hydrogen gas to completely separate them. Is done. The conductive ceramic of the present invention is most preferably used as the current collecting member 4.

Further, also in the cylindrical fuel cell, the conductive ceramic of the present invention can be used as a current collecting member (interconnector) material for connecting the cells.

As described above, the above-mentioned conductive ceramic is a high-density body and has an electric conductivity of 1 particularly when the fuel cell is operated (about 1000 ° C.).
Since it is as high as 5 s / cm or more, it sufficiently satisfies the characteristics required of the current collecting member. In addition, this conductive ceramic is also excellent in durability against hydrogen, and is therefore excellent in long-term stability, which is one reason that it is suitable as a current collecting member.

[0029]

【Example】

Example 1 Commercially available La ₂ O ₃ , MgCO ₃ , Cr having a purity of 99.9%
_{Using 2} O ₃ powder, these were mixed at the ratios shown in Table 1, then mixed for 12 hours in a ball mill using zirconia balls, and calcined at 1200 ° C. for 5 hours to perform a solid phase reaction. A perovskite-type composite oxide powder was produced.
Thereafter, the perovskite-type composite oxide powder 1
100 parts by weight, commercially available 99.9% pure CaCO 3
₃ powder, Y ₂ O ₃ , Yb ₂ O ₃ , Sm ₂ O ₃ , Sc ₂
O ₃ , Nd ₂ O ₃ , and PrO powders were added in the amounts shown in Table 1 and mixed and pulverized again using zirconia balls for 10 hours. This was formed into a square prism having a side of 5 mm × 5 mm and a length of 45 mm, and fired in the atmosphere (oxygen partial pressure: 0.2 atm) under the conditions shown in Table 1.

With respect to the obtained sintered body, the open porosity of the sample was measured by the Archimedes method, and the sinterability was determined. In addition, a sample piece having a size of 3 mm × 3 mm and a length of 20 mm was prepared as described above, and one piece in air was prepared by a four-terminal method.
The electrical conductivity was measured at 000 ° C. In order to examine the stability of these samples in a hydrogen atmosphere, the samples were kept in a hydrogen atmosphere at 1000 ° C. for 24 hours, and then, when no change was observed on the surfaces of the samples, ×
Is attached. The results are shown in Table 1.

For comparison, a commercially available LaMg _0.10 Cr
_{The 0.90} powder was calcined in Ar at 2000 ° C. for 3 hours, and the open porosity and electric conductivity were measured according to the above-mentioned methods. The result is shown in Sample No. 1 of Table 1.

[0032]

[Table 1]

As can be seen from Table 1, Sample No. 2 in which the contents of Ca and rare earth elements such as Y and Yb are less than 0.01 part by weight has a large open porosity and poor sinterability. On the other hand, if the content exceeds 2.0% by weight as in sample No. 7, decomposition starts in a hydrogen atmosphere and cannot be used as a fuel cell separator or the like. It can be seen that all of the present invention have a high electric conductivity of 16 s / cm or more, and the stability of the material in a reducing atmosphere is sufficiently excellent.

Example 2 Using the samples of Nos. 1, 2, 4, 5, 8, and 16 in the above Example 1, the structure shown in FIG.
m, a separator having a thickness of 3 mm was produced. The average crystal grain size of this separator was 4 to 10 μm. 8 mol% of commercially available 99.9% purity Y ₂ O ₃ -92 mol%
Using ZrO ₂ powder, a dense solid electrolyte plate having a theoretical density ratio of 99.3% and a thickness of 0.25 mm was produced. A mixed powder of 70 wt% NiO-30 wt% zirconia (containing 8 mol% Y ₂ O ₃ ) having a thickness of 30 μm was applied to the lower surface, and baked at 1400 ° C. for 2 hours to obtain a fuel electrode.

Thereafter, La _0.9 S was applied on the upper surface of the solid electrolyte plate.
r _0.1 MnO ₃ powder is applied to a thickness of 30 μm,
Baking was performed at 0 ° C. for 2 hours to obtain an air electrode. This was sandwiched between the separators described above, oxygen gas was supplied to the air electrode side, and hydrogen gas was supplied to the fuel electrode side to generate power continuously at 1000 ° C. for 400 hours, and the output density at the time of power generation was measured.

[0036]

[Table 2]

From Table 2, it can be seen that Sample No. 19 as a comparative example was used.
Output was extremely small. In the case of sample No. 23, the cell was broken halfway. In contrast, the sample N of the present invention
o. 20, 21, and 22 showed stable power generation characteristics.

[0038]

As described in detail above, according to the present invention, L
The sinterability of the aCrO ₃ -based composition is improved, and a highly dense body having high electric conductivity and a low temperature of 1600 ° C. or less can be produced. Moreover, it is excellent in stability in a high-temperature hydrogen atmosphere, and is preferably used as a current collecting member such as an interconnector, a separator, and a gas diffuser that comes into contact with hydrogen such as a fuel cell, so that it is inexpensive and has a low fuel consumption. An electrode material that can cope with long-term stability as a battery can be provided.

[Brief description of the drawings]

FIG. 1 is a schematic view of a flat fuel cell.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Solid electrolyte 2 ... Air electrode 3 ... Fuel electrode 4 ... Current collecting member

Claims (1)

[Claims] 1. A metal oxide containing La, Cr and Mg as metal elements, a perovskite-type composite oxide as a main crystal phase, and C per 100 parts by weight of said perovskite-type composite oxide.
a, and at least one of Y, Yb, Sc, Sm, Nd, Dy, and Pr is 0.01 to 0.01% in terms of oxide.
A conductive ceramic, comprising 2.0 parts by weight.