JP3210803B2 - Method for producing conductive ceramics - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a conductive ceramic having a LaCrO ₃ composition.
More specifically, the present invention relates to a method for producing a conductive ceramic suitable for a current collecting member such as a separator, a gas diffuser, and an interconnector of a fuel cell, and a current collecting member for MHD power generation.

[0002]

2. Description of the Related Art Lanthanum chromite oxide (LaCr)
O ₃ ) is used as a current collecting member such as a separator, a gas diffuser, and an interconnector of a solid oxide fuel cell because of its excellent chemical stability at high temperature and high electron conductivity.

FIG. 1 shows a solid oxide fuel cell having a flat plate shape. In a flat fuel cell, for example, Y ₂ O ₃
One of the solid electrolytes 1 made of stabilized ZrO ₂ has (LaS
r) MnO ₃ or (lacA) MnO ₃ system of the air electrode 2, Ni over Y ₂ O ₃ fuel electrode 3, such as stabilized zirconia cermet is provided on the other, the connection between the cell consists of LaCrO ₃ based ceramic separator 4 is performed. The fuel cell generates power at a temperature of 1000 to 1050 ° C. while flowing a gas containing oxygen such as air on the side of the air electrode 2 and a fuel gas such as hydrogen gas on the side of the fuel electrode 3. As the above-mentioned ceramics for separators, C
A LaCrO ₃ -based material in which aO or SrO is dissolved is used.

[0004]

Problems to be Solved by the Invention In addition to the slow diffusion rate of cations, LaCrO ₃ -based ceramics volatilize Cr components from the material during the sintering process, and cause Cr contact at the contact portion (neck portion) of the particles. _It condenses and deposits as ₂ O ₃ and hinders sintering. Therefore, the Cr ₂ In either in the atmosphere is sintered at a high temperature of at least 2000 ° C., or a reducing atmosphere
It is necessary to perform sintering while suppressing the evaporation and condensation of O ₃ , but even in this case, a high temperature of 1800 ° C. or more is required. 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 LaCrO ₃ ceramics at a low temperature, electrochemical vapor phase synthesis (EV
D) Method is applied. However, this method is
Although LaCrO ₃ system material is made relatively low temperature and 400 ° C., lacks mass productivity for the growth rate of the LaCrO ₃ is low, also, it is necessary to use a very expensive metal chloride as the starting material in this method Therefore, there was an economic problem.

[0006]

Means for Solving the Problems The inventors of the present invention have solved the above-mentioned problems and have studied various methods for improving the sinterability at a low temperature. As a result, a perovskite type containing at least La and Cr has been obtained. After synthesizing the composite oxide powder, the sinterability can be dramatically improved by adding a composite oxide of a Group 3a element of the periodic table and MgO at a predetermined ratio to this powder. 1450-1600 ° C
It has been found that high densification by firing at a low temperature becomes possible, and the present invention has been achieved.

That is, in the method for producing a conductive ceramic of the present invention, a perovskite-type composite oxide powder containing at least La and Cr as metal elements is added with an oxide of a Group 3a element of the periodic table in an amount of 0.01 to 0.01%. A powder mixture containing 30% by weight and MgO at a ratio of 0.01 to 30% by weight is formed into a predetermined shape, and then fired in an oxidizing atmosphere at 1450 to 1600 ° C. to obtain a high-density body. It is assumed that.

Hereinafter, the present invention will be described in detail. According to the method for producing a conductive ceramic of the present invention, first, a perovskite-type composite oxide powder containing at least La and Cr is synthesized. In this composite oxide, part of La is C
Those substituted with an alkaline earth element such as a or Sr can also be employed.

[0009]

Embedded image

When x, y and z in the formula are 0.
01 ≦ x ≦ 0.5, 0.8 ≦ y ≦ 1.2, 0 ≦ z ≦ 0.
Those satisfying No. 3 are preferably adopted.

The composite oxide powder can be prepared by using an oxide of a metal element constituting the composite oxide or a powder of a nitrate, carbonate, acetate or the like capable of forming an oxide by heat treatment. After blending so as to satisfy the relationship shown in No. 1, by subjecting this to a heat treatment at a temperature of 1000 to 1500 ° C., a perovskite-type composite oxide can be synthesized.

Next, according to the present invention, a powder of a Group 3a element oxide of the periodic table and a powder of MgO are added to the perovskite-type composite oxide powder synthesized as described above. The oxide of the Group 3a element of the periodic table accounts for 0.01 to 30% by weight, particularly 1 to 10% by weight,
O is added at a rate of 0.01 to 30% by weight, particularly 1 to 10% by weight. The reason why the amounts of these additional components were limited to the above ranges was that the oxides of the Group 3a elements of the periodic table and MgO were used.
When either one or the above components are less than the above lower limit, excellent sinterability is not exhibited, and a sintered body having pores is formed, and conversely, when each component exceeds the above upper limit, the sintered body becomes Is inhibited by the added component, and the electric conductivity is reduced. When La ₂ O ₃ is selected as the Group 3a element oxide of the periodic table, if it exceeds 30% by weight, the stability in a hydrogen / steam atmosphere is reduced. Further, the total amount of the added components is desirably 50% by weight or less in order to suppress a decrease in electric conductivity.

The mixed oxide and the added components added and mixed in the above ratios are sufficiently mixed by a ball mill or the like, and then mixed with a desired forming means, for example, a die press, a cold isostatic press, an extrusion molding or the like. After forming into a sheet or a thin film by a doctor blade method or a slurry dipping method, firing is performed.

The firing is performed in an oxidizing atmosphere such as air.
It can be fired at a temperature of from 00 to 1700C, especially from 1450 to 1600C. The firing temperature at this time is 1300 ° C
If it is less than 30, sufficient sintering cannot be expected. However, if the firing temperature is too high, restrictions such as a firing furnace occur and the cost required for firing increases, so the upper limit was set to 1700 ° C.

According to the present invention, the conductive ceramic produced by the above production method is a high-density sintered body having an open porosity of 0.5% or less, and has a crystal structure of at least La, Cr, possibly Ca, Sr, Ba
And a perovskite-type crystal containing an alkaline earth element as the main crystal phase. Further, according to the present invention, as a phase other than the main crystal phase, a Group 3a element of the periodic table due to the additive component is used. Containing oxide and MgO or MgCr
_It has a structure in which a phase composed of ₂ O ₄ is dispersed as a grain boundary phase.
The phase other than the perovskite-type crystal phase, if present between the two main crystal grains, impedes the transfer of electrons across the grain boundary and causes a reduction in the electrical conductivity of the sintered body. It is desirable that the crystal grains be precipitated at a size of 50 nm to 3 μm, particularly 50 nm to 1 μm at an interface between three crystal grains, a so-called triple point. In order to make the grain boundary phase exist at the triple point, the components of the grain boundary phase are aggregated at the triple point by maintaining the firing at a temperature of 1450 to 1600 ° C. for 1 hour or more under an oxygen partial pressure of 10 ⁻³ atm or more. be able to. This sintered body is, in addition to being a high-density body, chemically stable in an oxidizing and reducing atmosphere and having an electric conductivity of 10 at the operating temperature (1000 ° C.) of the fuel cell.
It is as high as s / cm.

Therefore, the conductive ceramic according to the present invention is suitably used, for example, as an electrode material in a fuel cell. Specifically, in the flat fuel cell of FIG. 1, a separator (current collecting member) 4 used as a member for connecting between the cells has one surface in contact with an oxygen-containing gas and one with a hydrogen gas. It is required to have a role of completely separating them, and to have high density and high electrical conductivity.

As described above, the above-described conductive ceramic is a high-density body having an open porosity of 1% or less.
Since the electric conductivity, particularly when the fuel cell operates (about 1000 ° C.), is as high as 10 s / cm or more,
The characteristics required as the current collecting member are sufficiently satisfied. 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. Therefore, it is most suitably used as the separator 4 in the flat fuel cell of FIG. Furthermore, in a cylindrical fuel cell, it can be used as an interconnector for connecting between cells.

[0018]

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.

On the other hand, in the material of the present invention, by adding an oxide of a Group 3a element of the periodic table and MgO, the element of the Group 3a of the periodic table reacts with the evaporated Cr component to form a liquid phase. Which interacts with MgO to form a liquid phase with a low melting point.
It is considered that the evaporation of the components is suppressed and the diffusion rate of the cation in the grain boundary phase is increased, so that the sinterability is greatly improved.

By the addition of the Group 3a element oxide and MgO of the periodic table, sintering at a low temperature of 1450 to 1600 ° C. is realized, and a dense body having a porosity of 0.5% or less is produced. You can do it.

Therefore, it is not necessary to fire at a high temperature of 1800 ° C. or more as in the prior art, so that it is not necessary to use a special heat-resisting firing furnace or the like. Can be.

Further, since the conductive ceramic obtained by the present invention is highly dense and has high conductivity at a high temperature, it can be used as a current collecting member or a fuel electrode for connecting fuel cells. Can be used. In that case, the LaMnO ₃ -based electrode material or Y ₂
Solid electrolyte such as O ₃ -stabilized ZrO ₂ and 1500 to 17
Simultaneous firing in a temperature range of 00 ° C is also possible,
It is also possible to reduce the manufacturing cost of the fuel cell.

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 by Ca in LaCrO ₃ , holes are generated according to the following formula 2.

[0024]

Embedded image

According to the above equation, the electric conductivity is proportional to the concentration of Ca ions substituted for La. In LaCrO ₃ , if the substitution amount of Ca, Sr, Mg, etc. is small, the electric conductivity is small, and if the substitution amount is large, it is precipitated without substitution and lowers the electric conductivity. In the composition of the perovskite-type composite oxide represented by Chemical Formula 1 described above, the substitution ratio x of the alkaline earth element with respect to La was specified in the above-described range.

As a result, 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. It is.

[0027]

【Example】

Example 1 Commercially available La ₂ O ₃ , Cr ₂ O ₃ , Sr with a purity of 99.9%
Using CO ₃ , CaCO ₃ , and BaCO ₃ , they were mixed at the ratios shown in Table 1, mixed for 12 hours in a ball mill using zirconia balls, and calcined at 1400 ° C. for 5 hours to perform solid phase reaction. Was performed to produce a perovskite-type composite oxide powder. Further, the powder of the Group 3a element oxide of the periodic table and Mg
O powder was added and mixed and pulverized again using zirconia balls for 10 hours. One piece of this mixture is 5 mm x 5 mm,
It was formed into a square prism having a length of 45 mm and fired in the air under the firing conditions shown in Table 1.

With respect to the obtained sintered body, the open porosity of the sample was measured by the Archimedes method to determine the sinterability. 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. For comparison, a commercially available raw material having a composition of La _0.9 Sr _0.1 CrO _{3 was} added at 2100 ° C. for 1 hour.
Open porosity and electrical conductivity were measured using the material fired in the atmosphere for a period of time. In order to examine the stability of the sample in a hydrogen atmosphere, the sample was kept at 1000 ° C. in a hydrogen atmosphere containing 5% water vapor for 24 hours. A mark of X in which remarkable surface etching was observed was given. The results are shown in Table 1.

[0029]

[Table 1]

As is clear from the results shown in Table 1, Sample No. 3 containing no Group 3a element oxide or MgO at all in the periodic table.
In Sample No. 2 to which only MgO 1 and MgO were added, 170
Densification was not possible even at a temperature of 0 ° C. Also,
In sample No. 3 to which only La ₂ O ₃ was added, 1400 ° C.
Samples Nos. 10, 17, and 24 in which the densification was insufficient at an open porosity of 1.5% at the calcination temperature, and La ₂ O ₃ , Y ₂ O ₃ and MgO exceeded 30% by weight, The conductivity was low and the stability in a hydrogen / water vapor atmosphere was poor.

For these comparative examples, the periodic table 3a
According to the method of the present invention in which the group-group element oxide and MgO are added in combination, a highly dense sintered body having a porosity of 0.5% or less could be produced. In addition, it was confirmed that this sintered body had a high electric conductivity at 1000 ° C. of 10 s / cm or more, and was sufficiently usable as an electrode material. Further, it was excellent without decomposition in a high-temperature hydrogen atmosphere.

In addition, EPMA,
As a result of X-ray diffraction measurement, scanning electron micrograph, and TEM analysis, 50 nm to 1 μm of the Group 3a element oxide of the periodic table and MgO were formed at the triple point of the perovskite-type main crystal phase grain boundary.
A precipitate having a size of was detected.

[0033]

As described in detail above, according to the present invention, L
The sinterability of the aCrO ₃ -based composition is improved, high electrical conductivity is obtained, and a dense body can be produced at a low temperature of 1450 to 1600 ° C. Moreover, it has excellent stability in a high-temperature hydrogen atmosphere. For example, interconnectors, separators,
By being suitably used as a current collecting member such as a gas diffuser, it is possible to provide a current collecting material which is inexpensive and can cope with long-term stability as a fuel cell.

[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

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-4-108667 (JP, A) JP-A-7-187768 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C04B 35/42-35/50 CA (STN) REGISTRY (STN)

Claims (1)

(57) [Claims] 1. An oxide of a Group 3a element of the periodic table in an amount of 0.01 to 30% by weight based on a perovskite-type composite oxide powder containing at least La and Cr as metal elements.
Is formed into a predetermined shape by adding a powder mixture having a content of 0.01 to 30% by weight, and then fired in an oxidizing atmosphere at 1450 to 1600 ° C. to form a high-density ceramic. Manufacturing method.