JPH08175868A - Production of electroconductive ceramic - Google Patents

Abstract

PURPOSE: To obtain an electroconductive ceramic suitable for current collectors of dense fuel cells and exothermic elements by baking at a low temperature. CONSTITUTION: This method for producing an electroconductive ceramic is to add and mix 1-50 volume % of a second compound oxide containing 3a group elements of the periodic table and at least one oxide selected from a group comprising Co, Mn, Ni and Fe to a first compound oxide containing 3a group elements of the periodic table, Cr and at least one oxide selected from a group comprising Ca, Sr, Ba and Mg as metal elements, mold the mixture powder, bake the molded product in an oxidizing atmosphere at 1400-1700 deg.C and obtain the electroconductive ceramic. In the above mentioned constitution, <=30 mole % of oxides containing 3a group elements of the periodic table can be added to the first compound oxide in order to improve the degree of sintering.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to MCrO ₃ (M: Group 3a element of the periodic table) suitable as a current collector or ceramic heating element for separators, gas diffusers and interconnectors of solid oxide fuel cells. )
TECHNICAL FIELD The present invention relates to a method for producing a conductive ceramic.

[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 a large electron conductivity, so that it can be used as a collector material for a solid electrolyte fuel cell, a gas diffuser, and a current collector material such as an interconnector or a ceramic heating element. Applications are under consideration. Recently, a MCrO ₃ -based material in which a part of M or Cr is replaced by Ca, Sr, Ba, Mg or the like in order to enhance electric conductivity has also been 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.

On the other hand, in a high temperature operating ceramic heating element, 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. . The advantage of this type of heating element is that the operating temperature is as high as 700 ° C. However, the voltage applied to the resistance is non-uniform, resulting in non-uniform heat generation, and a small heating area. There is a drawback of.

To overcome this problem, Japanese Patent Application No. 5-103
As described in No. 117, a self-heating type ceramic heating element of LaCrO ₃ system is also under study.

[0006]

As described above, the MCrO ₃ (M = group 3a element of the periodic table) -based material is a material suitable as a current collecting material for a solid oxide fuel cell and various ceramic heating elements. However, in addition to the slow diffusion rate of cations, the MCrO ₃ -based material volatilizes Cr components from the material during the sintering process and condenses as Cr ₂ O _{3 in} the particle contact part (neck part). Deposit and inhibit sintering.
Therefore, it is necessary to sinter at a high temperature of 2000 ° C. or higher in the air, or to sinter while suppressing evaporation and condensation of the Cr component in a reducing atmosphere. 1800 even when sintering in an atmosphere that can suppress the evaporation and condensation of Cr
Firing at a high temperature of ℃ or more is required.

The production of the material at such a high temperature makes the mass production of fuel cells and the application to the ceramic heating element extremely difficult from an economical point of view, and is a major factor of increasing the product cost. Alternatively, Japanese Patent Laid-Open No. 2-11
La _{1-x + y} Ca _x CrO as described in 1632.
There is also a method of adding Ca in excess and firing at a low temperature as in ₃ , but when this material is used at around 1000 ° C., phase separation may occur and the characteristics may deteriorate. Therefore, there is a problem that this material is not always a stable material depending on the usage conditions.

On the other hand, an electrochemical vapor phase synthesis (EVD) method is known as a method for producing a LaCrO ₃ material at a low temperature. However, although this method can produce a LaCrO ₃ based material at a relatively low temperature of 1400 ° C.,
Since the growth rate of LaCrO ₃ is slow, there is a drawback that mass productivity is lacking. In addition to this, this method is economically problematic because it requires the use of an extremely expensive metal chloride as a starting material.

[0009]

The present inventors have found that MCr
As a result of investigating an additive for enhancing the sinterability of O ₃ (M = group 3a element of the periodic table) -based material, the above-mentioned MCrO
_For the first composite oxide composed of a ₃ system material, a second composite oxide containing an element of Group 3a of the periodic table and at least one selected from the group consisting of Co, Mn, Ni and Fe is used.
By adding and mixing at a ratio of ˜50% by volume, 140
The present invention has been accomplished by discovering that sinterability can be enhanced in an oxidizing atmosphere of 0 to 1700 ° C. and, at the same time, sintering can be performed with high density.

Further, according to the present invention, in the above-mentioned constitution, the oxide containing a group 3a group 3a element of the periodic table is added to the first composite oxide in a proportion of 30 mol% or less. It has also been found that its sinterability can be improved.

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

[0012]

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When expressed as follows, x + y + z = 2, 0.005
It is desirable to satisfy ≦ y / x + y + z ≦ 0.3.
Substitution with these alkaline earth elements (A elements) is necessary in order to increase the conductivity of ceramics, and is 0.005
> Y / x + y + z, the electric conductivity becomes small, and y /
When x + y + z> 0.3, material decomposition occurs in a hydrogen / steam atmosphere, or corrosion of the material surface is remarkable, which 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.

According to the first composite oxide, a part of Cr in the above chemical formula 1 is replaced with Mn, Ni, Fe, and Co at a ratio of 30 atomic% or less with respect to Cr. May be.

The first composite oxide has an ABO ₃ type perovskite type crystal, and is, for example, an oxide powder of a Group 3a element (M) of the Periodic Table, Cr ₂ O ₃ powder and Ca, At least one oxide selected from the group consisting of Ba, Sr, and Mg, and optionally, other metal oxide powders, which are weighed and mixed so as to have the above-mentioned predetermined ratio, in an oxidizing atmosphere at 1000 to 1600 ° C. After heat treatment to form a solid solution, it is pulverized 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 above composite oxide,
Instead of the above oxide powders, it is also possible to use hydroxides, carbonates, nitrates, acetates and the like of the respective constituent metals capable of forming oxides by heat treatment.

According to the present invention, the first of the above-mentioned MCrO ₃ system is used.
With respect to the complex oxide of
1 to 50% by volume of a second composite oxide containing at least one selected from the group consisting of o, Mn, Ni and Fe is added and mixed. Specifically, this second composite oxide has an atomic ratio composition shown below.

[0017]

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

This second composite oxide is also composed of an ABO ₃ type perovskite type crystal. (M 2) is the A site, the B element is the B site constituent element, and in the chemical formula 1, usually a: b is Although it is 1: 1, a similar crystal is maintained even if the a / b ratio is changed to some extent. Therefore, the a and b ratios are set as described above.

The above-mentioned second composite oxide is the periodic table 3a.
An oxide powder of a group element (M2) and at least one oxide powder selected from the group consisting of Co, Mn, Ni and Fe were weighed and mixed to give the above-mentioned predetermined ratio.
After heat treatment in an oxidizing atmosphere of 0 to 1600 ° C. to form a solid solution, it is pulverized by a well-known method such as a ball mill to obtain a composite oxide powder of 0.1 to 10 μm. In synthesizing the composite oxide, instead of the oxide powders described above, hydroxides, carbonates, and nitrates of constituent metals capable of forming oxides by heat treatment,
An acetate salt or the like can also be used.

According to the present invention, the second composite oxide is added to the first composite oxide in an amount of 1 to 50% by volume, particularly 5 to 20% by volume. The amount of addition is set in the above range because if it is less than 1% by volume, the effect of improving the sinterability is not observed, and if it exceeds 50% by volume, the sinterability is deteriorated or unstable in hydrogen. This is because a harmful effect such as is caused.

Further, according to the present invention, the first composite oxide may contain 30 mol% or less, particularly 10 mol% or less, of an oxide containing a Group 3a element of the periodic table in order to improve sinterability. . If the oxide of the Group 3a element of the Periodic Table exceeds 30 mol%, the sinterability will decrease. This Group 3a element-containing oxide of the periodic table is Y ₂ O ₃ , La ₂ O ₃ , Yb ₂ O ₃
In addition to metal oxides such as YCrO ₃ , YbCrO ₃ ,
It may be a complex oxide such as LaCrO ₃ . In this case, the second composite oxide is mixed so as to be 1 to 50% by volume in the total amount.

Next, the two kinds of composite oxides mixed in the above ratio, or the two kinds of composite oxides and the Group 3a-containing oxide of the periodic table are formed by a desired molding means, for example, a die pressing method,
It is molded 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 is applied in an oxidizing atmosphere such as air.
Bake for about 0 hours. However, when the oxygen partial pressure during firing is low, the evaporation of CrO ₃ and the solid solution of Mg and Sr are promoted to inhibit the sintering, so that the oxygen partial pressure is 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 its use. For example, when it is used as a current collecting member of a fuel cell, it is necessary to reduce the open porosity to 1% or less, particularly 0.5% or less. A firing temperature of 1400 to 1700 ° C is optimal.

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

When an oxide containing a group 3a element of the periodic table is added, the oxide containing a group 3a element of the periodic table may be precipitated as a second crystal phase. Further, in the first composite oxide represented by Chemical Formula 1, when the blending amount of Mg and Sr is high, a small amount of MgO and SrO may be precipitated depending on the heat treatment conditions and the sintering conditions during the composite oxide synthesis. However, if the proportion of the second crystal phase is 20% by volume or less with respect to all the crystal phases, the main crystal phase forms a continuous phase even if the second crystal phase is dispersed in the main crystal phase. Therefore, the electric conductivity is hardly hindered.

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

Further, regarding the amount of metal impurities in the ceramics, from the viewpoint of improving the creep resistance at high temperatures, the amounts of Al and Si are preferably 2 atomic% or less in terms of metal, based on the total amount of metal elements. This is because when each of the amounts of Al and Si exceeds 2 atomic%, a glass phase is formed at the grain boundaries, and the creep resistance at high temperature tends to be deteriorated. The preferable range is 0.5 atom% or less.

Next, the case where the conductive ceramics of the present invention is used as a current collecting member of a fuel cell will be described. The stabilized Zr containing ₃ to 15 mol% of Y ₂ O ₃ or Yb ₂ O ₃ in the flat plate fuel cell shown in FIG.
O ₂ or 5-30 mol% 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 containing 10 to 20 atomic% of S is added.
An air electrode 2 made of a material such as porous LaMnO ₃ substituted with r or Ca or Japanese Patent Application No. 5-66935, and a porous 60 vol% Ni-40 as a fuel electrode 3 on the other surface.
Volume% ZrO ₂ (containing Y ₂ O ₃ ) A cermet is formed. A current collecting member 4 called an interconnector 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.

[0032]

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

[0033]

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LaCrO _{3 and the} like have a melting point of 220.
Since the temperature is 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 can be sintered only at an extremely high temperature. The reason for this is that at high temperatures, the Cr component in LaCrO ₃ is likely to evaporate preferentially, and during sintering this deposits as Cr ₂ O _{3 on} the contact portion of powder particles, the so-called neck portion, which impedes the diffusion of cations and burns. It is considered that this is because the so-called evaporation-condensation mechanism proceeds, which deteriorates the binding property. In addition, the third periodic table other than La
Similarly, the oxides of Group a elements and Cr also have poor sinterability due to evaporation of Cr.

When evaporation is involved in the sintering as described above, the surface energy which is the driving force for sintering is reduced, but according to the present invention, the surface energy which is the driving force for sintering with respect to the evaporation of Cr is reduced. It is intended to enhance the sinterability by accelerating the diffusion of the components by compensating for the solid phase reaction during the 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 the 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. This results in many pores being left inside. However, as an additive, for example, LaMnO
_{As shown in 3,} the elements of Group 3a of the periodic table and Co, Mn, Ni
The second composite oxide containing at least one selected from the group consisting of Fe and Fe has similar crystal structures and lattice constants in the original system and the production system, and thus the molar volumes of the original system and the production system are about the same. As a result, the generation of pores due to the solid-phase reaction with the first composite oxide is suppressed, and as a result, a dense sintered body can be obtained.

As described above, according to the present invention, the surface energy, which is the driving force for sintering with respect to the evaporation of Cr, can be reduced by causing the solid-phase reaction to act during the sintering, resulting in the diffusion of the components. It can be accelerated and compensated, and the sinterability can be improved.

The method for producing a conductive ceramics according to the present invention is suitable for producing a current collecting member for a fuel cell, and in particular, it can be fired in an oxidizing atmosphere at a lower temperature than before, For example, La forming an air electrode
It can be fired at the same time as MnO ₃ based conductive ceramics, Y ₂ O ₃ stabilized ZrO ₂ forming a solid electrolyte, and the like.

[0039]

【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
Using O ₃ , these were blended so as to have the first composite oxide composition shown in Table 1, mixed for 12 hours in a ball mill using zirconia balls, and then calcined at 1400 ° C. for 5 hours to perform a solid phase reaction. Then, the first perovskite composite oxide powder was produced. In addition, the purity of some samples is 99,9%
Using the Group 3a element oxide of the Periodic Table, the components were added and mixed in the ratio shown in Table 1 with respect to the first composite oxide.

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

Then, these first complex oxide and second
After mixing the composite oxides in the ratio shown in Table 1, it was pulverized in a rotary mill for 10 hours using zirconia balls, and then 4 × 4 × 4.
After being formed into a 0 mm square pole, 1400 to 16 in air
Firing 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 was baked on this sintered body, and this was used as an electrode, and the distance between the voltage terminals was set to 20 mm.
The electrical conductivity in the atmosphere was measured at ℃. In addition, 1000
The sample was left 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%
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 No. ₃ sintered at 2000 ° C. for 1 hour in Ar, and the results are shown in Table 1.
Shown in 2.

[0044]

[Table 1]

[0045]

[Table 2]

As is clear from the results of Tables 1 and 2, the second
In Samples Nos. 1 and 2 in which the complex oxide of No. 2 was not added at all, the sinterability was poor and the open porosity was 28% or more. Y ₂
Add O ₃ or La ₂ O ₃ (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 the open porosity is 1% or less. 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. Moreover, the electrical conductivity was about the same as that of the first composite oxide even when the second composite oxide was added.

However, in Samples No. 13 and 49 to 53 in which the amount of the second composite oxide added was less than 1% by volume and in Samples No. 49 to 53 exceeding 50% by volume, the open porosity exceeded 1%, and in the hydrogen. Stability was also reduced.

[0049]

As described above, the conductive ceramics of the present invention can produce a dense sintered body at a low temperature as compared with the conventional materials, and when the composition is controlled, the electrical conductivity is the same as that of the conventional products. Since it has conductivity, it can be more easily manufactured as a current collecting member or a heating element of a fuel battery cell, and the manufacturing cost thereof 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 Codes] 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 number Internal reference number FI technical display location // H01M 8/02 Y H05B 3/14 B 0380-3K (72) Inventor Shoji Yamashita Kagoshima Prefecture Kokubun 1-4 Yamashita-cho, Ichi, Kyocera Stock Company Research Institute

Claims (2)

[Claims] 1. A group 3a element of the periodic table as a metal element,
For the first composite oxide containing Cr and at least one selected from the group of Ca, Sr, Ba and Mg, from the group 3a element of the periodic table and the group of Co, Mn, Ni and Fe 1 to 50% by volume of a second composite oxide containing at least one selected from the above, and then mixed and molded, and the mixed powder is molded and fired in an oxidizing atmosphere at 1400 to 1700 ° C. Method of manufacturing ceramics. 2. A group 3a element of the periodic table as a metal element,
A first composite oxide containing Cr and at least one selected from the group consisting of Ca, Sr, Ba and Mg, and a main component mixture comprising 30 mol% or less of an oxide containing a Group 3a element of the periodic table of 30 mol% or less. In contrast, 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 ceramics, comprising: