JP3091100B2 - Method for producing conductive ceramics - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a solid oxide fuel cell separator, the gas diffuser, and a current collector material or suitable MCrO ₃ as a ceramic heating element such as the interconnector (M: the periodic table group 3a elements )
The present invention relates to a method for producing a conductive ceramics.

[0002]

2. Description of the Related Art MCrO ₃ (M: Group 3a element of the periodic table)
The compound represented by is excellent in chemical stability at high temperature and has high electron conductivity, so that it can be used for current collector materials such as solid oxide fuel cell separators, gas diffusers and interconnectors, or ceramic heating elements. Applications are being considered. Recently, an MCrO ₃ -based material in which M or Cr is partially substituted with Ca, Sr, Ba, Mg or the like in order to enhance electric conductivity has been proposed.

In a solid oxide fuel cell, as shown in FIG. 1, LaMnO _{3 in} which porous La is replaced by Ca or Sr on one surface of an electrolyte 1 of Y ₂ O ₃ stabilized ZrO _2.
Is formed as an air electrode 2 and porous Ni-Z is formed on the other surface.
A fuel cell 3 made of rO ₂ (containing Y ₂ O ₃ ) is formed to constitute a single cell. This single cell is composed of the above-mentioned LaCrO
It is sandwiched between ₃ series separators 4.

On the other hand, as a ceramic heating element which operates at a high temperature, a resistor such as platinum is formed on the surface of alumina which is an insulating ceramic, or a resistor such as tungsten is incorporated inside. . An advantage of this type of heating element is that the operating temperature is as high as 700 ° C., but the voltage applied to the resistance is not uniform, resulting in an uneven heating temperature and a small heating area. There are disadvantages.

To overcome this problem, Japanese Patent Application No. 5-103
As described in No. 117, a LaCrO ₃ -based self-heating type ceramic heating element is also being studied.

[0006]

Problems to be Solved by the Invention As described above, MCrO ₃ (M = Group 3a element of the periodic table) material is suitable as a current collecting material for a solid oxide fuel cell or a ceramic heating element. However, in the MCrO ₃ -based material, in addition to the slow diffusion rate of cations, the Cr component volatilizes from the material during the sintering process, and condenses as Cr ₂ O _{3 at} the contact portion (neck portion) of the particles. Accumulates and inhibits sintering.
For this reason, it is necessary to perform sintering at a high temperature of 2000 ° C. or higher in the atmosphere, or to perform sintering in a reducing atmosphere while suppressing the evaporation and condensation of the Cr component. Even when sintering in an atmosphere that can suppress the evaporation and condensation of Cr, 1800
Firing at a high temperature of at least ℃ is required.

The production of a material at such a high temperature makes the mass production of fuel cells and the application to ceramic heating elements extremely difficult from an economic viewpoint, and is a major factor in increasing the product cost. Or JP-A-2-11
No. 1632, La _{1-x + y} Ca _x CrO
_Although there is a method in which Ca is excessively added and calcined at a low temperature as shown in _3, the material may undergo phase separation when used at around 1000 ° C., resulting in degraded characteristics. For this reason, there is a problem that this material is not necessarily a stable material depending on use conditions.

On the other hand, as a method for producing a LaCrO ₃ -based material at a low temperature, an electrochemical vapor deposition (EVD) method is known. However, although this method can produce a LaCrO ₃ material at a relatively low temperature of 1400 ° C.,
The growth rate of LaCrO ₃ is low, so that there is a disadvantage that mass productivity is lacking. In addition, this method has an economical problem because it requires the use of extremely expensive metal chlorides as starting materials.

[0009]

Means for Solving the Problems The present inventors have proposed MCr
As a result of studying an additive for enhancing the sintering property of an O ₃ (M = Group 3a element of the periodic table) material, the MCrO
_A second composite oxide containing an element belonging to Group 3a of the periodic table and at least one selected from the group consisting of Co, Mn, Ni, and Fe is added to the first composite oxide made of a ternary material.
By adding and mixing at a ratio of ５０50% by volume, 140
The present inventors have found that high sintering can be achieved at the same time as enhancing sinterability in an oxidizing atmosphere at 0 to 1700 ° C., leading to the present invention.

Further, according to the present invention, in the above structure, an oxide containing a Group 3a element of the periodic table is added to the first composite oxide at a ratio of 30 mol% or less, so that It has also been found that the sinterability can be improved.

Hereinafter, the present invention will be described in detail. MCr of the present invention
According to the method for producing an O ₃ -based (M; Group 3a element of the periodic table) conductive ceramic, as a main component in the starting material, a Group 3a element of the periodic table as a metal element, Cr,
A first composite oxide containing at least one selected from the group consisting of Ca, Sr, Ba and Mg is used. The composition according to the atomic ratio of this powder is

[0012]

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X + y + z = 2, 0.005
It is desirable to satisfy ≦ y / x + y + z ≦ 0.3.
Substitution with these alkaline earth elements (element A) is necessary to increase the conductivity of the ceramics,
> Y / x + y + z, the electrical conductivity decreases and y / x + y + z
When x + y + z> 0.3, material decomposition occurs in a hydrogen / water vapor atmosphere or corrosion of the material surface is not preferable. Particularly preferably, 0.01 ≦ y / x + y + z
≦ 0.1. Note that M1 is specifically La,
Y, Yb, Sc, Sm, Dy, Nd, Pr, Ce, Gd
And at least one selected from the group of Er.

Further, according to the first composite oxide, one obtained by substituting a part of Cr in the above formula 1 with Mn, Ni, Fe, Co at a ratio of 30 atomic% or less with respect to Cr is used. You may.

The first composite oxide has an ABO ₃ type perovskite crystal, for example, an oxide powder of a Group 3a element (M) of the periodic table, a Cr ₂ O ₃ powder and Ca, At least one oxide selected from the group consisting of Ba, Sr and Mg and, if desired, a mixture of powders of other metal oxides weighed and mixed at the above-mentioned predetermined ratio are mixed in an oxidizing atmosphere at 1000 to 1600 ° C. After heat treatment to form a solid solution, pulverization is performed by a well-known method such as a ball mill to obtain a composite oxide powder of 0.1 to 10 μm. In the synthesis of the composite oxide,
Instead of the above oxide powders, hydroxides, carbonates, nitrates, acetates, etc. of the respective constituent metals capable of forming an oxide by heat treatment can also be used.

According to the present invention, the first MCrO ₃ -based material is used.
Of the group 3a element of the periodic table and C
A second composite oxide containing at least one selected from the group consisting of o, Mn, Ni, and Fe is added and mixed at 1 to 50% by volume. Specifically, the second composite oxide has the following atomic ratio composition:

[0017]

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Wherein a and b in Chemical Formula 2 are a + b
= 2, 0.9 ≦ b / a + b ≦ 1.1. Note that, specifically, M1, La, Y, Yb, S
At least one selected from the group consisting of c, Sm, Dy, Nd, Pr, Ce, Gd, and Er.

This second composite oxide is also composed of an ABO ₃ type perovskite crystal, (M 2) is a constituent element of the A site and B element is a constituent element of the B site. Although the ratio is 1: 1, the same crystal is maintained even if the a / b ratio changes somewhat. Therefore, the a and b ratios were determined as described above.

[0020] The second composite oxide is a compound of the periodic table 3a
A powder obtained by weighing and mixing at least one oxide powder selected from the group consisting of oxide powders of group elements (M2), Co, Mn, Ni and Fe in the above-mentioned predetermined ratio,
After heat treatment in an oxidizing atmosphere at 0 to 1600 ° C. to form a solid solution, pulverization is performed by a known method such as a ball mill to obtain a composite oxide powder of 0.1 to 10 μm. In the synthesis of the composite oxide, a hydroxide, a carbonate, a nitrate of each constituent metal capable of forming an oxide by heat treatment instead of the above-described oxide powders,
Acetate and the like can also be used.

According to the present invention, the second composite oxide is added to the first composite oxide at a ratio of 1 to 50% by volume, particularly 5 to 20% by volume. The reason why the addition amount is set in the above range is that if less than 1% by volume, the effect of improving the sinterability is not recognized, and if it exceeds 50% by volume, the sinterability deteriorates or becomes unstable in hydrogen. This is because adverse effects such as the occurrence of the problem occur.

Further, according to the present invention, the first composite oxide can contain an oxide containing a Group 3a element of the periodic table in an amount of 30 mol% or less, particularly 10 mol% or less, in order to enhance sinterability. . When the content of the Group 3a element oxide in the periodic table exceeds 30 mol%, the sinterability deteriorates. This oxide containing a Group 3a element of the periodic table includes Y ₂ O ₃ , La ₂ O ₃ , and Yb ₂ O ₃
In addition to metal oxides such as YCrO ₃ , YbCrO ₃ ,
A composite oxide such as LaCrO ₃ may be used. In this case, the second composite oxide is mixed so as to be 1 to 50% by volume based on the total amount.

Next, two types of composite oxides mixed in the above ratio, or two types of composite oxides and an oxide containing Group 3a of the Periodic Table are molded by desired molding means, for example, a die pressing method.
It is formed into an arbitrary shape by a cold isostatic pressing method, an extrusion molding method, a doctor blade method or the like.

Then, in firing the molded body obtained as described above, 1 to 1 in an oxidizing atmosphere such as air.
Bake for about 0 hours. However, if the oxygen partial pressure during firing is low, the evaporation of CrO ₃ and the solid solution of Mg and Sr are promoted and the sintering is hindered. Therefore, the oxygen partial pressure may be 10 ^-3 atm or more. desirable. Further, according to the present invention, the use of the powder treated as described above
By firing at a low temperature of 00 to 1700 ° C., a high-density sintered body having a porosity of 1% or less can be produced.

The firing temperature is appropriately adjusted depending on the application. For example, when used as a current collecting member of a fuel cell, the open porosity must be reduced to 1% or less, particularly 0.5% or less. A firing temperature of 1400-1700 ° C. is optimal.

In the conductive ceramics obtained as described above, the complexation of the above-mentioned complex oxide of the formula 1 and the complex oxide of the formula 2 progresses, and the above-mentioned M1 and
Group 3a element of the periodic table of M2, Cr, Ca, Ba,
A ceramic having a main crystal phase containing a perovskite composite oxide containing at least one kind of alkaline earth element selected from the group consisting of Sr and Mg and at least one kind selected from the group consisting of Co, Mn, Ni and Fe. Of these metal elements, M1, M2, Ca, Ba, and Sr form crystals as A-site constituent elements, and Mg, Co, Mn, Ni, and Fe form crystals as B-site constituent elements.

When an oxide containing a Group 3a element in the periodic table is added, the oxide containing a Group 3a element in the periodic table may precipitate as a second crystal phase. Furthermore, in the first composite oxide shown in Chemical formula 1, when the compounding amount of Mg and Sr is high, a small amount of MgO and SrO may be precipitated depending on heat treatment conditions and sintering conditions during the synthesis of the composite oxide. However, if the proportion of the second crystal phase is 20% by volume or less based on the entire crystal phase, the main crystal phase forms a continuous phase even if the second crystal phase is dispersed in the main crystal phase. Therefore, it hardly hinders electric conductivity.

The average crystal grain size of the main crystal phase of the perovskite composite oxide in the sintered body is 0.5 to 30.
μm is desirable.

Further, with respect to the amount of metal impurities in the ceramics, from the viewpoint of improving the creep resistance at high temperatures, the amount of Al and Si is preferably 2 atomic% or less based on the total amount of metal elements in terms of metal. This is because if the amounts of Al and Si each exceed 2 atomic%, a glass phase is formed at the grain boundary, and the creep resistance at high temperatures tends to deteriorate. The preferred ranges are each 0.5 atomic% or less.

Next, a case where the conductive ceramic of the present invention is used as a current collecting member of a fuel cell will be described. In flat type fuel cell shown in FIG. 1, 3 to 15 mol% of Y ₂ O ₃ or stabilized Zr containing the Yb ₂ O ₃
O ₂ or 5 to 30 mol% of _{Y 2 O 3, Yb 2 O} 3, G
On one surface of the solid electrolyte 1 made of CeO ₂ containing one of d ₂ O ₃ , for example, La is added with 10 to 20 atomic% of S
An air electrode 2 made of a material such as porous LaMnO ₃ or Japanese Patent Application No. 5-66935 substituted with r and Ca, and a porous 60 volume% Ni-40 as a fuel electrode 3 on the other surface.
A volume% ZrO ₂ (containing Y ₂ O ₃ ) cermet is formed. A current collecting member 4 called an interconnector for connecting the cells as a single cell electrically connects the air electrode to the fuel electrode of an adjacent cell. The conductive ceramic 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 and hydrogen, CO and CO ₂ gas, etc. are supplied to the fuel electrode. For this reason,
One side of the current collecting member 4 comes into contact with the oxidizing gas and the other side comes into contact with the reducing gas, and the necessity of completely isolating the reducing gas requires high electrical conductivity and high density. The porosity is preferably 1% or less, particularly preferably 0.5% or less. In a cylindrical fuel cell, the conductive ceramic of the present invention can also be used as an interconnector for a cylindrical fuel cell.

Next, the case where the conductive ceramic of the present invention is used as a cylindrical heating element will be described. FIG.
The heating element shown in (1) comprises a resistor 5 made of a cylindrical sintered body and electrodes 6 and 7 formed on 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. In addition to the cylindrical shape shown in FIG. 2, the heating element can be made arbitrarily, such as a flat plate shape, a cylindrical spiral, or a honeycomb structure. 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 preferably 10% or less.

[0032]

An important characteristic of MCrO ₃ (M: element of Group 3a of the periodic table) is that it has electrical conductivity from an oxidizing atmosphere to a reducing atmosphere. For example, when La is replaced with Ca in LaCrO ₃ , holes are generated according to the following formula ₃ , which contributes to electric conduction. La to Sr or Ca
The same applies to the case of substituting with Mg or replacing Cr with Mg. Since the electrical conductivity of LaCrO ₃ is controlled by the substitution ratio of Ca as shown in the following chemical formula ₃ , it is a major feature that the LaCrO ₃ has stable product performance without being affected by the use atmosphere.

[0033]

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LaCrO ₃ or the like has a melting point of 220
Since it has a temperature of 0 ° C. or higher, it has excellent chemical stability, and can be used in a wide range from an oxidizing atmosphere such as air to a reducing atmosphere such as hydrogen.

However, the conventional LaCrO ₃ material could be sintered only at an extremely high temperature. The reason is that the Cr component is likely to evaporate preferentially from LaCrO ₃ at high temperature, and this deposits as Cr ₂ O _{3 at} the contact portion of the powder particles, the so-called neck portion, during sintering, impeding the diffusion of cations and burning This is considered to be due to progress in a so-called evaporation-condensation mechanism, which deteriorates the bondability. In addition, the third part of the periodic table other than La
Similarly, oxides of group a element and Cr also have poor sinterability due to evaporation of Cr.

As described above, when evaporation is involved in sintering, the surface energy, which is the driving force for sintering, is reduced. According to the present invention, the decrease in surface energy, which is the driving force for sintering, for the evaporation of Cr is reduced. The purpose of the present invention is to enhance the sinterability by promoting and compensating for the diffusion of components by causing a solid phase reaction to act during sintering. For example, another different metal oxide is added to the first composite oxide of LaCrO ₃ . At that time, if the crystal structure and lattice constant of the metal oxide to be added are different from those of the first composite oxide, the molar volumes of the original system and the production system are largely different, and generally, the production system has a smaller molar volume, so the product This results in leaving many pores inside. However, additives such as LaMnO
₃ , elements of Group 3a of the periodic table and Co, Mn, Ni
And the second composite oxide containing at least one member selected from the group consisting of Fe and Fe has similar crystal structures and lattice constants in the original system and the production system. As a result, the generation of pores due to the solid phase reaction with the first composite oxide is suppressed, so that a dense sintered body can be obtained.

As described above, according to the present invention, the surface energy, which is the driving force of sintering for the evaporation of Cr, can be reduced by applying a solid-phase reaction during sintering. It promotes and compensates, and can improve sinterability.

Further, the method for producing a conductive ceramic according to the present invention is suitable for producing a current collecting member of a fuel cell, and in particular, can be fired in an oxidizing atmosphere at a lower temperature than conventional ones. For example, La forming an air electrode
It can be fired simultaneously with MnO ₃ -based conductive ceramics, Y ₂ O ₃ -stabilized ZrO ₂ forming a solid electrolyte, and the like.

[0039]

【Example】

Example 1 A commercially available 99.9% pure group 3a element oxide of the periodic table,
SrCO ₃ , CaCO ₃ , BaCO ₃ , MgO, Cr ₂
Using O ₃ , these were mixed so as to have the first composite oxide composition shown in Table 1, mixed in a ball mill using zirconia balls for 12 hours, and then calcined at 1400 ° C. for 5 hours to perform solid phase reaction. To produce a first perovskite composite oxide powder. Also, 99.9% purity for some samples
Was added to and mixed with the first composite oxide at a ratio shown in Table 1.

On the other hand, commercially available 99.9% purity group 3a element oxide of the periodic table, Mn ₂ O ₃ , NiO, FeO, Co
Using each powder of O, these were mixed so as to have the second composite oxide composition shown in Table 1, mixed in a ball mill using zirconia balls for 12 hours, and calcined at 1400 ° C. for 5 hours to solidify. A phase reaction was performed to produce a second perovskite composite oxide powder.

The first composite oxide and the second composite oxide
Was mixed at a ratio shown in Table 1 and then pulverized for 10 hours with a rotary mill using zirconia balls to obtain 4 × 4 × 4
After forming into a 0 mm square prism, 1400-16
Baking was performed at a temperature of 00 ° C. for 3 to 5 hours.

The open porosity of the obtained sintered body was measured by the Archimedes method to determine the sinterability. Further, Pt is baked on this sintered body, and the electrode is used as an electrode, the distance between the voltage terminals is set to 20 mm, and 1000 μm is applied by a DC four-terminal method.
C., the electric conductivity in the atmosphere was measured. In addition, 1000
The sample was left standing in hydrogen at 24 ° C. for 24 hours to examine the durability in a hydrogen atmosphere.

For comparison, a commercially available purity of 99.8% was used.
La _0.9 Sr _0.1 CrO ₃ and La _0.9 Ca _0.1 CrO
_The same evaluation as above was performed on the sintered body of Sample No. ₃ which was fired in Ar at 2,000 ° C. for 1 hour.
2 is shown.

[0044]

[Table 1]

[0045]

[Table 2]

As is clear from the results in Tables 1 and 2,
In Samples Nos. 1 and 2 in which no composite oxide was added, the sinterability was poor and the open porosity was 28% or more. Y ₂
Add O ₃ , La ₂ O _3, etc. (Sample No. 78, 7
In 9), the sinterability is considerably improved, but the open porosity is 4%.
That is all.

On the other hand, when the second composite oxide is added, the sinterability is greatly improved, and an open porosity of 1% or less is achieved. Further, when an oxide containing a Group 3a element of the periodic table is added, the open porosity is improved to 0.5% or less. The electric conductivity was almost the same as that of the first composite oxide even when the second composite oxide was added.

However, in Sample No. 13 in which the amount of the second composite oxide added is less than 1% by volume and in Samples No. 49 to 53 exceeding 50% by volume, the open porosity exceeds 1%, and Also decreased in stability.

[0049]

As described above, the conductive ceramics of the present invention can produce a dense sintered body at a lower temperature than conventional materials, and when the composition is controlled, the electrical conductivity is equivalent to that of the conventional products. It has conductivity, which makes it easier to manufacture the fuel cell as a current collecting member or a heating element, and also reduces the manufacturing cost.

[Brief description of the drawings]

FIG. 1 is a view for explaining the structure of a flat fuel cell.

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

[Description of Signs] 1 Electrolyte 2 Air electrode 3 Fuel electrode 4 Current collecting member 5 Resistor 6, 7 Electrode

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-4-219364 (JP, A) JP-A-4-219366 (JP, A) JP-A-4-214069 (JP, A) JP-A-4-219 331764 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) C04B 35/00 C04B 35/495 C04B 35/42 C04B 35/50

Claims (2)

(57) [Claims] 1. A group 3a element of the periodic table as a metal element;
For a first composite oxide containing Cr and at least one selected from the group consisting of Ca, Sr, Ba and Mg, the first composite oxide is composed of a Group 3a element of the periodic table and Co, Mn, Ni and Fe. After adding and mixing 1 to 50% by volume of a second composite oxide containing at least one selected oxide, the mixed powder is molded and fired in an oxidizing atmosphere at 1400 to 1700 ° C. Manufacturing method of ceramics. 2. A group 3a element of the periodic table as a metal element;
A main component mixture comprising a first composite oxide containing Cr, at least one selected from the group consisting of Ca, Sr, Ba and Mg, and an oxide containing a Group 3a element of the Periodic Table of 30 mol% or less. On the other hand, elements of Group 3a of the periodic table, Co, M
After adding and mixing 1 to 50% by volume of a second composite oxide containing at least one selected from the group consisting of n, Ni and Fe, this mixed powder is molded and fired in an oxidizing atmosphere at 1400 to 1700 ° C. A method for producing a conductive ceramic.