JP2002252005A - Oxide ion electric conductor and its manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an oxide ion electric conductor, of which conductivity of the oxide ion electric conductor in a low temperature domain to a middle temperature domain of 250 to 600 deg.C is remarkably excellent, and by this, operation temperature of such as a fuel cell or the like can be lowered, and to provide a method for manufacturing the oxide ion electric conductor. SOLUTION: After mixing and forming lanthanum oxide (La2 O3 ) powder and germanium oxide (GeO2 ) powder with a rate that an X value is 8 or more and less than 10 in LaXGe6 O1.5 X+12 , which can finally be obtained, it is sintered and made to be an oxide ion electric conductor. The structure of the crystal of this LaXGe6 O1.5 X+12 (8<=X<10) belongs to an apatite type structure. When O<2-> 14 which occupy 2a site in the apatite type structure moves along a c axis direction, electric conduction of oxide ion (O<2-> ) takes place.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an oxide ion conductor and a method for producing the same, and more particularly, to an oxide ion conductor comprising a composite oxide of lanthanum and germanium and suitable as a solid electrolyte or the like for a fuel cell. The present invention relates to a conductor and a method for manufacturing the conductor.

[0002]

2. Description of the Related Art Solids capable of moving ions are widely known as ionic conductors. Ion as is known has a positive or negative charge, and therefore, the ionic conductor, a current flows along with the ions move.

[0003] Movable ions differ depending on the type of ionic conductor. For example, β-Al ₂ O ₃ having a composition of Na ₂ O · 11Al ₂ O ₃ is a good ion conductor of Na ⁺ , that is, a sodium ion conductor, and is used as a solid electrolyte of a sodium / sulfur battery. I have. AgI has long been known as a good silver ion conductor.

[0004] In recent years, attempts have been made to employ an oxide ion (O ²⁻ ) conductor as an electrolyte of a fuel cell which has been receiving attention as a low-pollution electricity supply source due to an increase in interest in environmental protection in recent years. In this case, since the oxide ion conductor is solid, the fuel cell can be entirely made of a solid material, and thus can have a simple structure. Moreover, since there is no liquid leakage, the frequency of maintenance work can be significantly reduced.

As the oxide ion conductor, one having a fluorite (CaF ₂ ) type crystal structure, for example, Y ₂ O ₃
Of stabilized ZrO ₂ , Mg doped with about 0.08 mol
Stabilized ZrO ₂ , Y ₂ doped with about 0.15 mol of O
O ₃ is Bi ₂ O _3, which is 0.25 moles doped, Gd ₂ O
CeO ₂ doped with about 0.25 mol of ₃ is typical. Particularly, the above two kinds of stabilized ZrO ₂ measure the oxygen concentration in a solid electrolyte of a fuel cell, a gas or a molten metal. Has been put to practical use as a partition of an oxygen sensor.

[0006] Examples of other oxide ion conductors include:
The crystal structure is perovskite (CaTiO_Three) Type structure
Things. That is, La_0.9Sr_0.1Ga
_0.8Mg _0.2O_ThreeAnd BaTh_0.9Gd_0.1O_ThreeEtc. and this
Are also used as fixed electrolytes for oxygen sensors and fuel cells.
Applications or applications to thermistors are being considered.

The reason why the oxide ion conductivity of the fluorite-type oxide ion conductor described above is good is that the oxide ion conductivity is 8%.
This is a high temperature range of about 00 to 1000 ° C. For this reason, when operating a fuel cell using this type of oxide ion conductor as a solid electrolyte, the temperature of the power generation cells constituting the fuel cell or the stacked stack in which a plurality of power generation cells are electrically connected is controlled. It is necessary to raise the temperature to about 800 to 1000 ° C. When the temperature is lower than this temperature range, the oxide ion conductivity of the solid electrolyte (oxide ion conductor) is low, so that the power generation efficiency is significantly reduced.

However, when the fuel cell is operated in such a high temperature range, a large amount of energy (electric power or the like) is inevitably required to heat the power generation cells or the stack. Moreover, in this case, an inexpensive metal material such as stainless steel cannot be used as a member constituting the fuel cell. This is because such a metal material has low mechanical strength and corrosion resistance in a high temperature range. Therefore, operation in a high-temperature range causes a problem of increasing the operation cost of the fuel cell.

In addition, the oxide ion conductor having a perovskite structure has a problem that the oxide ion conductivity is not always sufficient to employ the oxide ion conductor as a solid electrolyte of a fuel cell. I have.

On the other hand, JP-A-8-208333 and JP-A-11-71169 propose an oxygen ion conductor comprising a composite oxide of rare earth and Si, and a method for producing the same. Both publications state that 200-600 ° C
It is described that this composite oxide exhibits an excellent oxide ion conductivity as compared with the above two types of oxide ion conductors in a low temperature range to a medium temperature range.

Further, in the academic journal “Solid State Ioni”
cs ", 136-137 (2000 edition), pp. 31-37, discusses La ₁₀ Si ₆ O ₂₇ , La ₁₀ Ge ₆ O ₂₇ , and those in which a part of La in both is replaced by Sr. It has been done. It is also reported that such a composite oxide exhibits excellent oxide ion conductivity at 200 to 600 ° C. as compared with the above two types of oxide ion conductors.

[0012]

By the way, rare earth and S
When producing an oxide ion conductor composed of a composite oxide with i, it is customary to mutually sinter the rare earth oxide powder and the SiO ₂ powder at a temperature exceeding 1700 ° C. The reason is that the melting point of SiO ₂ is 1710 ° C., so that sintering of SiO ₂ does not sufficiently proceed at a temperature of 1700 ° C. or less. That is, it is difficult to obtain a sintered body (oxide ion conductor) having a strength that can withstand practical use.

However, the heating elements, heat insulating materials, reaction tubes and the like constituting the reaction furnace for sintering, when heated to a temperature exceeding 1700 ° C., have a drastically reduced durability. That is, the life of the reactor is significantly shortened, and therefore, the capital investment for producing an oxide ion conductor composed of a composite oxide of rare earth and Si increases. Eventually, in this case, there is a problem that the manufacturing cost of the oxide ion conductor rises.

On the other hand, in the above-mentioned academic journal, La ₁₀ G
e ₆ O ₂₇ and La _10-X S in which La is partially substituted with Sr
r _X Ge ₆ O ₂₇ is, after an equal molding a mixed powder of GeO _2, SiO ₂ and SrCO ₃ with 275 MPa, 1,600-16
It is described that it is obtained by sintering in a temperature range of 50 ° C.

However, the L thus obtained is
a ₁₀ Ge ₆ O ₂₇ and La _10-X Sr _X Ge ₆ O ₂₇ have L
It contains a lot of impurities such as a ₂ GeO ₅ and La ₂ Ge ₂ O ₇ . Those containing such impurities have remarkably low oxide ion conductivity in a medium to low temperature range as compared with pure ones. That is, in the composite oxide of lanthanum and germanium having this composition, a problem has been evident that the oxide ion conductivity cannot be improved due to the presence of impurities.

The present invention has been made in order to solve the above-mentioned problems, and has a remarkably excellent oxide ion conductivity in a low temperature range to a medium temperature range of 250 to 600 ° C., so that the operating temperature of a fuel cell or the like is low. It is an object of the present invention to provide an oxide ion conductor capable of lowering the temperature and a method for producing the same.

[0017]

In order to achieve the above-mentioned object, the present invention relates to a method for producing a crystal, wherein the crystal structure belongs to an apatite type structure, and the chemical formula is La _X Ge ₆ O _{1.5X + 12} (where 8 ≦
X <10), characterized by being composed of a composite oxide of lanthanum and germanium.

La _X Ge ₆ O belonging to the apatite type structure
_{In the 1.5X + 12} crystal, O ²⁻ occupying the 2a site can move relatively freely along the c-axis direction. For this reason, large oxide ion conductivity is exhibited even in a low to medium temperature range of 250 to 600 ° C. Therefore, this oxide ion conductor is suitable as a solid electrolyte for a fuel cell operated at a low temperature.

The crystal system of the crystal belongs to a hexagonal system,
And it is particularly preferable that the P6 ₃ / m to represent the space group of the crystal by Herman Morgan symbols. This is because when such a crystal system is employed, the oxide ion conductivity becomes the highest.

Further, according to the present invention, the material containing germanium as a constituent element and the material containing lanthanum as a constituent element are La _X Ge ₆ O _{1.5X + 12} (where 8 ≦ X <1
0), a mixing step of obtaining a mixed powder mixed in a ratio to form, a forming step of forming the mixed powder to form a compact, and sintering the compact to form an apatite-type crystal. And the chemical formula is La _X Ge ₆ O
And a sintering step of forming an oxide ion conductor composed of a composite oxide of lanthanum and germanium represented by _{1.5X + 12} (where 8 ≦ X <10).

As described above, the powder of the lanthanum-containing substance and the powder of the germanium-containing substance are mixed at a predetermined ratio, molded and then sintered, whereby the crystal has an apatite type structure, that is, , La _X Ge ₆ O _{1.5X + 12} (8 ≦ X <
10) can be easily obtained.

Further, according to the present invention, the substance containing germanium as a constituent element and the substance containing lanthanum as a constituent element are La _X Ge ₆ O _{1.5X + 12} (where 8 ≦ X <
A mixing step of obtaining a mixed powder mixed at a ratio of 10) and a heat treatment of the mixed powder, whereby the crystal structure belongs to the apatite type structure and the chemical formula is La _X Ge ₆
O _{1.5X + 12} (where 8 ≦ X <10), a granulating step of forming composite oxide particles of lanthanum and germanium, and pulverizing the particles to obtain a composite oxide powder And a molding step of molding the composite oxide powder to form a compact, and a sintering step of sintering the compact to form an oxide ion conductor composed of the composite oxide. It is characterized by the following. The term “granules” as used herein refers to aggregates of powder that are aggregated or bonded to such an extent that they can be pulverized in the pulverization step.

Also in this case, as in the above, the crystal
Is La with apatite type structure_XGe₆O _{1.5X + 12}(8 ≦ X <
10) can be easily obtained. And in this case,
Mutual solid solution of lanthanum oxide and germanium oxide and powder
And the grain growth is performed individually,
La with a G-shaped structure_XGe₆O_{1.5X + 12}(8 ≦ X <10) is raw
Obtaining a uniform and dense oxide ion conductor
Can be.

The heat treatment temperature in the granulation step may be a temperature at which the powders can be agglomerated or combined so that they can be pulverized, and more specifically, 700 to 1200 ° C. is sufficient.

In any case, the sintering temperature in the sintering step is preferably 1400 to 1700 ° C. Thus, according to the present invention, a sintering temperature of 170
Since the temperature can be set to 0 ° C. or lower, the life of the reaction furnace can be prolonged, and eventually, the production cost of the oxide ion conductor can be reduced. It should be noted, is less than 1400 ℃ not proceed grain growth efficiently.

As a specific means for obtaining the mixed powder, the lanthanum oxide powder and the germanium oxide are mixed in a ratio of 3.6 to less than 4.5 parts by weight of the lanthanum oxide powder to 1 part by weight of the germanium oxide powder. The powder may be mixed with the powder to obtain the mixed powder.

Alternatively, a mixed powder containing lanthanum, germanium, and oxygen at the above ratio may be directly obtained by a sol-gel method, a chemical vapor deposition (CVD) method, a spray pyrolysis method, or the like. Of course, a powder other than the oxide, for example, a carbonate powder may be prepared by the above-described method to obtain a mixed powder.

[0028]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of an oxide ion conductor and a method for manufacturing the same according to the present invention will be described below in detail with reference to the accompanying drawings.

The oxide ion conductor according to the present embodiment is a sintered body made of a composite oxide of lanthanum and germanium. And the composition in this composite oxide is
It is represented by La _X Ge ₆ O _{1.5X + 12} , and X is in the range of 8 or more and less than 10.

FIG. 1 shows the structure of a unit cell of La _X Ge ₆ O _{1.5X + 12} (8 ≦ X <10) with the viewpoint taken along the c-axis.
The unit cell 10 includes six GeO ₄ tetrahedrons 12 and 2
O ^2-14 occupying a site, 4f site or 6h
It is an apatite structure including La ³⁺ 16a and 16b occupying sites. In addition, GeO ₄ tetrahedron 12
Ge ⁴⁺ and O ²⁻ are not shown.

The crystal system of the unit cell 10 belongs to a hexagonal system. That is, in FIG. 1, the angle α at which the side AB in the a-axis direction and the side BF in the c-axis direction of the unit cell 10 intersect, the angle β at which the side BC and the side BF in the b-axis direction intersect with each other, and the side AB and the side The angles γ at which BC intersects are 9
0 °, 90 °, 120 °. And the side AB and the side B
C has the same length as each other, and the lengths of these sides AB and BC are different from the side BF.

A hexagonal crystal including the unit cell 10
The child (not shown) is a simple lattice. And this hexagonal
The grid is centered on a virtual spiral axis (not shown).
Operate 1/3 turn, and along the spiral axis, the side BF
When the translation is performed by half the length of
Position of each ion in the sample coincides. And this spiral
The mirror plane of the hexagonal lattice is orthogonal to the axis. That is, L
a_XGe₆O_{1.5X + 12}The space group of the crystal of (8 ≦ X <10)
When represented by the symbol of Herman Morgan, P6 _Three/ M and
Become.

The apatite-type structure La _X
Ge ₆ O _{1.5X + 12} why (8 ≦ X <10) is an oxide ion conductor, O ^2-14 is GeO occupying the 2a site
_This is considered to be because it is not involved in binding to tetrahedron 12 or La ³⁺ 16a. That is, the force acting on O ^2-14 is not strong, and thus O ^2-14 is 2a
This is because they can relatively freely move along the c-axis direction without being restricted by the site.

As will be understood from this, even in the case of a composite oxide of lanthanum and germanium, the oxide ion conductivity is low if the crystal structure is not an apatite type structure. Such unit cell in the composite oxide takes a structure that is different from the above-mentioned unit cell 10, Therefore, in the unit lattice, O ² can be freely moved as O ^2-14 in the unit cell 10 ^Because- can no longer exist.

In the present embodiment, _X in La _X Ge ₆ O _{1.5X + 12} is set to 8 or more and less than 10 so that the main structure of the complex oxide crystal becomes the apatite type structure shown in FIG. You. When X is less than 8, the crystal structure does not become an apatite type structure, and therefore, the oxide ion conductivity decreases. If the number is 10 or more, the composite oxide having an apatite structure contains a composite oxide having another structure, such as La ₂ Ge ₂ O _7, as an impurity. Eventually, also in this case, the oxide ion conductivity decreases. More preferably X ranges are 8 ≦ X ≦ 9.33. In this case, the majority of the crystals have an apatite structure (FIG. 1).
), And impurity phases of other structures are hardly observed. That is, the oxide ion conductivity is highest.

The oxide ion conductor has a thickness of 250 to 60.
It shows excellent oxide ion conductivity in a low to medium temperature range of 0 ° C. Specifically, the chemical formula is La _9.33 Ge ₆ O ₂₆
, 4.0 × 1 around 441 ° C.
It is about 0 ^-3 S / cm ² .

As described above, the oxide ion conductor according to the present embodiment includes stabilized ZrO ₂ doped with about 0.08 mol of Y ₂ O ₃ , BaTh _0.9 Gd _0.1 O ₃ , La ₁₀ Si ₆
Compared to oxide ion conductor according to the prior art, such as O ₂₇ shows excellent oxide ion conductivity at low temperature range to medium temperature range. Therefore, for example, in a fuel cell including the oxide ion conductor according to the present embodiment as a solid electrolyte, the same power generation characteristics can be obtained even when operating at a lower temperature than the fuel cell according to the related art. . For this reason,
The operating cost of the fuel cell can be reduced.

Next, a method for producing the oxide ion conductor will be described.

FIG. 2 shows a flowchart of a method for manufacturing an oxide ion conductor according to the first embodiment (hereinafter, referred to as a first manufacturing method). The first manufacturing method includes a mixing step S1 of mixing a lanthanum oxide powder and a germanium oxide powder to form a mixed powder, a forming step S2 of forming the mixed powder into a formed body, and sintering the formed body. And a sintering step S3 for forming an oxide ion conductor (sintered body).

First, in a mixing step S1, lanthanum oxide (La ₂ O ₃ ) powder and germanium oxide (GeO ₂ ) powder are mixed.

Here, the finally obtained La _X Ge ₆ O
The composition at _{1.5X + 12} , that is, the value of X changes depending on the ratio of La ₂ O ₃ powder and GeO ₂ powder in the mixed powder. For example, L for 1 part by weight of GeO ₂ powder
When the proportion of the a ₂ O ₃ powder is less than 3.6 parts by weight, X is less than 8. On the other hand, when the proportion of the La ₂ O ₃ powder exceeds 4.5 parts by weight, X exceeds 10. In short, in any case, a composite oxide of lanthanum and germanium having an apatite structure cannot be obtained, so that the oxide ion conductivity decreases.

Therefore, the ratio of La ₂ O ₃ powder is Ge
It is set to 3.6 parts by weight or more and less than 4.5 parts by weight with respect to 1 part by weight of O ₂ powder. In this case, X can be controlled within the range of 8 or more and less than 10.

Next, in a molding step S2, the mixed powder is molded. The molding method at this time is not limited to a specific method, and a known molding method such as a press molding method, a slurry casting method, and an extrusion molding method can be adopted. The shape of the molded body may be a shape according to the usage form.

Next, in the sintering step S3, the molding
By sintering the body, La_TwoO_ThreePowder and GeO_Twopowder
The powder is grown. That is, particles that are in contact with each other
To grow the joint and finally merge the particles
Large particles. Also, La _TwoO_ThreeAnd GeO_TwoAnd each other
Is dissolved, La_XGe₆O_{1.5X + 12}Complex oxide represented by
And Thereby, a sintered body, that is, an oxide ion
A conductor is obtained.

The sintering temperature is preferably 1400 to 1700 ° C. At a temperature lower than 1400 ° C., the grain growth does not proceed efficiently, and at a temperature higher than 1700 ° C., the durability of a heating element, a heat insulating material, a reaction tube and the like constituting a reaction furnace used for sintering rapidly decreases. It is because. A more preferred sintering temperature is 1450-1600 ° C.

As described above, in the first manufacturing method, La
_The sintering temperature at the time of obtaining _X Ge ₆ O _{1.5X + 12} can be lower than the sintering temperature at the time of obtaining La ₁₀ Si ₆ O ₂₇ which is the oxide ion conductor according to the prior art. . For this reason,
The life of the reactor can be extended, and the production cost can be reduced.

Since the ratio between the La ₂ O ₃ powder and the GeO ₂ powder is set as described above, the obtained L
The value of X in a _X Ge ₆ O _{1.5X + 12} will be in the range of less than 8 or more 10. That is, has a crystal of apatite type structure belongs to the hexagonal system and space group is represented by P6 ₃ / m,
For this reason, a composite oxide having excellent oxide ion conductivity is obtained.

By the way, La ₂ O ₃ or GeO ₂ and La _X
The crystal structure is different from Ge ₆ O _{1.5X + 12} . For example, the crystal structure of La ₂ O ₃ is a rutile structure, which is significantly different from an apatite structure. For this reason, when the compact is directly sintered as in the first production method, La ₂ O ₃ and Ge
Since the crystal structure change accompanying the solid solution with O ₂ and the grain growth proceed simultaneously, if the driving force in sintering is small, either the crystal structure change or the grain growth may not proceed sufficiently. is there. Therefore, it is desirable that the step of solid-solving La ₂ O ₃ and GeO ₂ with each other be a step different from the step of grain growth, as described below.

FIG. 3 shows a flowchart of a method for manufacturing an oxide ion conductor according to the second embodiment (hereinafter, referred to as a second manufacturing method). Steps for performing operations similar to those in the first manufacturing method are given the same names, and detailed descriptions thereof are omitted.

The second manufacturing method comprises a mixing step S10 of mixing La ₂ O ₃ powder and GeO ₂ powder to form a mixed powder, and a heat treatment of the mixed powder to form a composite oxide particle of lanthanum and germanium. Granulating step S20 to form a body, and pulverizing step S30 to pulverize the granule to form a composite oxide powder
And a molding step S40 of molding the composite oxide powder to form a compact, and a sintering step S50 of sintering the compact to form an oxide ion conductor.

First, in the mixing step S10, the first
The La ₂ O ₃ powder and the GeO ₂ powder are mixed in accordance with the mixing step S1 in the production method described above. Of course, also in this case, both powders are obtained from the finally obtained La _X Ge ₆ O _{1.5X + 12}
Are mixed in such a ratio that the value of X in the range of 8 to less than 10. That is, the ratio of La ₂ O ₃ powder is GeO
₍₂₎ It is set to 3.6 parts by weight or more and less than 4.5 parts by weight with respect to 1 part by weight of powder.

Next, in the granulation step S20, the mixed powders are fused to each other to a degree that can be pulverized by heat treatment. That is, the powders are aggregated or combined so that they can be pulverized into granules. If a dense sintered body is formed at this point, it becomes extremely difficult to pulverize.

In other words, the heat treatment temperature in the granulating step S20 is set to such an extent that remarkable grain growth of the mixed powder does not occur. In this case, since it is a La ₂ O ₃ powder and a GeO ₂ powder, a temperature of 700 to 1200 ° C. is sufficient.

At this time, La ₂ O ₃ and GeO _{2 form a} solid solution with each other. That is, in the second production method, La _X Ge ₆ O _{1.5X + 12} is generated at this point. The heat treatment in the granulation step S20 may be performed until the generation of La _X Ge ₆ O _{1.5X + 12} is completed, specifically, about 2 hours. Here, _X in La _X Ge ₆ O _{1.5X + 12}
Is not less than 8 and less than 10.

When the heat treatment of the mixed powder is performed at the temperature and time as described above, the obtained La _X Ge ₆ O
The cohesive force or binding force of the _{1.5X + 12} granules is such that even a mortar can be easily crushed.

Next, in the grinding step S30, La _X
The particles of Ge ₆ O _{1.5X + 12} are pulverized to powder. The pulverizing method is not particularly limited, and may be performed in a mortar, but is preferably a method such as a ball mill that can make the particle diameter of the powder substantially uniform. This makes it difficult for pores to remain in the sintered body, so that an oxide ion conductor having excellent strength and toughness can be obtained.

Next, in a molding step S40, a molded body is produced according to the molding step S2 of the first manufacturing method.

Finally, in a sintering step S50, the compact is sintered according to the sintering step S3 to obtain an oxide ion conductor. Also in the second production method, the sintering temperature is preferably 1400 to 1700 ° C, more preferably 1450 to 1600 ° C, as in the sintering step S3.

In this case, the step of solid-solving La ₂ O ₃ and GeO ₂ with each other (granulation step S20) and the step of growing grains (sintering step S50) are separately performed. For this reason, the driving force required for the structural change of the crystal and the driving force required for the grain growth are reduced. Therefore, it is possible to obtain a homogeneous oxide ion conductor in which La _x Ge ₆ O _{1.5X + 12} (8 ≦ X <10) having an apatite structure is entirely formed.
That is, in this case, La _X which is not an apatite type structure
There is no concern that Ge ₆ O _{1.5X + 12} is generated and the oxide ion conductivity is reduced.

Further, in the pulverizing step S30, the particles are pulverized so that the particle diameters become substantially uniform, whereby La ₂ O ₃
Even when the particle diameters of the powder and the GeO ₂ powder are significantly different from each other, the oxide ion conductor can be obtained as a dense sintered body. Such a dense sintered body has sufficient strength and toughness to withstand practical use.

As described above, according to the first and second manufacturing methods,
If La_TwoO_ThreePowder and GeO_TwoMix with powder at specified ratio
And then sintering, the structure of the crystal
It has an apatite-type structure,
La that indicates conductivity_XGe₆O _{1.5X + 12}(8 ≦ X <10)
Can be easily obtained. Moreover, the sintering temperature is 170
0 ° C. or lower, so that oxide ion conductor
Can be reduced in manufacturing cost.

In the first and second production methods described above, the forming steps S2 and S40 and the sintering steps S3 and S50
However, the forming and the sintering may be performed simultaneously by employing a hot pressing method or a hot isostatic pressing (HIP) method.

In the first and second production methods, the lanthanum oxide powder and the germanium oxide powder are mixed to obtain a mixed powder. For example, a lanthanum carbonate and a germanium carbonate are mixed. For example, a mixed powder may be obtained by using a powder of a substance other than the oxide.
Of course, also in this case, the ratio of each powder is set so that La _X Ge ₆ O _{1.5X + 12 in} which X is 8 or more and less than 10 is obtained.

Further, a mixed powder containing lanthanum, germanium and oxygen may be obtained by performing a sol-gel method, a CVD method or a spray pyrolysis method. In this case, La _X Ge ₆ O _{1.5X + 12} (8 ≦ X <10)
The various conditions may be controlled so as to obtain a mixed powder. Of course, also in this case, a powder made of a substance other than the oxide may be produced.

[0065]

EXAMPLE 33.82 g of La ₂ O ₃ powder and 9.34 g of GeO ₂ powder were mixed for 16 hours in a wet ball mill using 100 g of ethyl alcohol as a solvent. Note that 500 g of ZrO ₂ balls were used. afterwards,
The solvent was volatilized and removed by a rotary evaporator to obtain a mixed powder.

Next, the Al ₂ O ₃ containing this mixed powder was
Crucible is placed in the reactor and placed at 1200 ° C. for 2 hours.
Heat treatment was performed in an air atmosphere to obtain granules. Then, the granules were pulverized by a wet ball mill under the same conditions as above to obtain powder. X-ray diffraction measurement of this powder revealed that it was La ₈ Ge ₆ O ₂₄ having an apatite structure.

Further, this powder was formed into a disk having a diameter of 12 mm and a thickness of 3 mm by a die press and an isostatic pressing method. Thereafter, the disk is placed in a reaction furnace with the disk mounted on a jig made of a stabilized ZrO ₂ sintered body, and sintered at 1500 ° C. for 2 hours in an air atmosphere to obtain La ₈ Ge ₆ O. _An oxide ion conductor made of ₂₄ sintered bodies was used. This is referred to as Example 1.

Also, oxide ion conductors of various compositions were obtained in accordance with Example 1 except that the La ₂ O ₃ powder and the GeO ₂ powder were mixed at the weights shown in FIG. These are referred to as Examples 2 to 4, respectively. The composition of the oxide ion conductor of Example 2-4 are shown together in Figure 4.

For comparison, La ₂ O ₃ powder 36.1 was used.
After mixing 6 g with 7.99 g of GeO ₂ powder, an oxide ion conductor composed of a La ₁₀ Ge ₆ O ₂₇ sintered body was finally obtained according to Example 1. This is referred to as Comparative Example 1.

Further, stabilized ZrO doped with 0.08 mol of Y ₂ O ₃ which is a general oxide ion conductor is used.
₂ and an oxide ion conductor made of a La ₁₀ Si ₆ O ₂₇ sintered body were prepared. These are referred to as Comparative Examples 2 and 3, respectively. Of course, the dimensions of the oxide ion conductors of Comparative Examples 2 and 3 were the same as the dimensions of the oxide ion conductors of Examples 1 to 4.

First, the X-ray diffraction pattern of the powder obtained by pulverizing the granules in Example 3 is shown in FIG. In FIG. 5, the black circles indicate peaks attributed to the apatite structure. From FIG. 5, it can be seen that La _9.33 Ge ₆
It is understood that the crystal structure of O ₂₆ is an apatite structure, and no other structure is observed.

FIG. 5 shows La ₉ Ge ₆ O _25.5 (Example 2), La ₈ Ge ₆ O ₂₄ (Example 1) and La ₁₀ Ge ₆
The X-ray diffraction pattern of O ₂₇ (Comparative Example 1) is also shown. By comparing the pattern of Example 3 with these patterns, it was found that the peak attributable to the apatite structure was broad at an X value of 9.33, in other words, the peak of the apatite structure was It is clear that there is a tendency to warp.

Separately, the oxide ion conductivity of each of the oxide ion conductors of Examples 1 to 4 and Comparative Examples 1 to 3 was measured. That is, after grinding the sintered body to a thickness of 2 mm, a Pt paste was applied to both end surfaces so that the diameter became 6 mm. Then, a Pt lead wire having a diameter of 0.1 mm was arranged on the Pt paste. Then 120 ° C
After drying the Pt paste by holding for 1 hour at, the baking was performed by holding at 700 ° C. for 2 hours.

Further, the Pt lead wire was connected to an impedance analyzer 4192A manufactured by Hewlett-Packard Company, and the AC impedance was measured at a frequency of 5 Hz to 1.3 MHz, and the oxide ion conductivity was calculated from the measurement results. The results are shown in FIG. 6 as a function of temperature. FIG. 6 shows that the oxide ion conductors of Examples 1 to 4
It is clear that the oxide ion conductivity in the temperature range of 7 ° C. or less is higher than that of the oxide ion conductor of Comparative Example 1. The reason for this is shown in FIG.
This is presumably because La ₁₀ Ge ₆ O ₂₇ cannot maintain the apatite structure. Also,
It is appreciated that La _9.33 Ge ₆ O ₂₆ (Example 3) exhibits the best oxide ion conductivity.

FIG. 6 further shows that the oxide ion conductors of Examples 1 to 4 exhibited excellent oxide ion conductivity in comparison with Comparative Example 2 in the temperature range of 250 to 600 ° C.
It can be seen that in the temperature range of 350 ° C. or more, the oxide ion conductivity superior to Comparative Example 3 is exhibited. This means that L
By adopting an oxide ion conductor composed of a _X Ge ₆ O _{1.5X + 12} (8 ≦ X <10) as a solid electrolyte,
This suggests that a fuel cell capable of operating in a low to medium temperature range of 50 to 600 ° C. can be configured.

[0076]

As described above, according to the oxide ion conductor of the present invention, the value of X in La _X Ge ₆ O _{1.5X + 12} is controlled to be 8 or more and less than 10. Since the crystal structure of such a complex oxide belongs to the apatite structure, the complex oxide has a low temperature range of 250 to 600 ° C.
It shows excellent oxide ion conductivity even in the middle temperature range. Such a complex oxide (oxide ion conductor)
For example, it can be adopted as a suitable solid electrolyte in a fuel cell.

Further, according to the method for manufacturing an oxide ion conductor according to the present invention, a lanthanum oxide powder and a germanium oxide powder are mixed at a predetermined ratio, molded, and then sintered, whereby the crystal structure is reduced. Apatite type structure,
For this reason, La _X Ge ₆ exhibiting excellent oxide ion conductivity.
O _{1.5X + 12} (8 ≦ X <10) can be easily obtained. Moreover, since the sintering temperature can be set to 1700 ° C. or lower, the effect that the production cost of the oxide ion conductor can be reduced can be achieved.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of a unit cell of La _X Ge ₆ O _{1.5X + 12} (8 ≦ X <10) constituting an oxide ion conductor according to the present embodiment.

FIG. 2 is a flowchart of a method for manufacturing an oxide ion conductor according to the first embodiment (first manufacturing method).

FIG. 3 is a flowchart of a method for manufacturing an oxide ion conductor according to a second embodiment (second manufacturing method).

FIG. 4 shows La in Examples 1 to 4 and Comparative Examples 1 to 3.
₅ is a table showing the mixing ratio of ₂ O ₃ powder and GeO ₂ powder and the composition of the oxide ion conductor.

FIG. 5 is an X-ray diffraction pattern of a composite oxide powder obtained in Examples 1, 2, 3 and Comparative Example 1.

FIG. 6 is a graph showing the relationship between the oxide ion conductivity and the temperature in the oxide ion conductors of Examples 1 to 4 and Comparative Examples 1 to 3.

[Explanation of symbols]

10 Unit lattice 12 GeO ₄
Tetrahedron 14: oxide ion (O ²⁻ ) 16a, 16b: lanthanum ion (La ³⁺ )

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 5H026 AA06 BB01 BB06 BB08 EE11 HH00 HH08

Claims (8)

[Claims] (1) The crystal structure belongs to the apatite type structure, and the chemical formula is La _X Ge ₆ O _{1.5X + 12} (where 8 ≦ X <1).
An oxide ion conductor comprising a composite oxide of lanthanum and germanium represented by 0). 2. A oxide ion conductor according to claim 1, wherein, when the crystal system of the crystal belongs to the hexagonal system, and representing the space group of the crystal by Hermann Mauguin symbol P6 ₃
/ M. 3. A material containing germanium as a constituent element and a material containing lanthanum as a constituent element are La _X
A mixing step of obtaining a mixed powder mixed in such a ratio that Ge ₆ O _{1.5X + 12} (where 8 ≦ X <10) is formed; a forming step of forming the mixed powder to form a formed body; By sintering, the crystal structure belongs to the apatite structure and the chemical formula is La _X Ge ₆ O
_A sintering step of forming an oxide ion conductor comprising a composite oxide of lanthanum and germanium represented by _{1.5X + 12} (where 8 ≦ X <10); How to make the body. 4. A substance containing germanium as a constituent element and a substance containing lanthanum as a constituent element are La _X
A mixing step of obtaining a mixed powder mixed in such a ratio that Ge ₆ O _{1.5X + 12} (where 8 ≦ X <10) is generated, and a heat treatment of the mixed powder, whereby the crystal structure belongs to the apatite type structure. And the chemical formula is La _X Ge ₆ O
_{1.5X + 12} (where 8 ≦ X <10), a granulating step of forming composite oxide particles of lanthanum and germanium, and a pulverizing step of pulverizing the particles to form a composite oxide powder And a molding step of molding the composite oxide powder to form a molded body, and a sintering step of sintering the molded body to form an oxide ion conductor made of the composite oxide. A method for producing an oxide ion conductor, comprising: 5. The method for producing an oxide ion conductor according to claim 4, wherein the heat treatment temperature in said granulating step is set to 700 to 1200 ° C. 6. The method according to claim 3, wherein a sintering temperature in the sintering step is 1400 to 1400.
A method for producing an oxide ion conductor, wherein the temperature is 1700 ° C. 7. The method according to claim 3, wherein in the mixing step, the lanthanum oxide powder is at least 3.6 parts by weight and at least 4.5 parts by weight with respect to 1 part by weight of the germanium oxide powder. A method for producing an oxide ion conductor, comprising mixing a lanthanum oxide powder and a germanium oxide powder in a proportion of less than 10 parts by weight. 8. The method according to claim 3, wherein the mixed powder is obtained by performing a sol-gel method, a chemical vapor deposition method or a spray pyrolysis method in the mixing step. A method for producing an oxide ion conductor.