JP2003246678A - Method of producing compound oxide sintered compact - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a polycrystal sintered compact of a Co-based compound oxide which has excellent thermoelectric transducing performance. <P>SOLUTION: The fine powder of a compound oxide expressed by the general formula of Ca<SB>2.2-3.6</SB>Na<SB>0-0.8</SB>Bi<SB>0-0.8</SB>Co<SB>4</SB>O<SB>8.8-9.2</SB>and planar crystals are mixed, and the direction of the crystal faces of the planar crystals is uniformized. After that, sintering is performed under uniaxial pressurization, so that the compound oxide sintered compact is produced. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a composite oxide sintered body having excellent thermoelectric conversion performance and a method for producing the same.

[0002]

2. Description of the Related Art In Japan, the effective energy yield from the primary energy supply is only about 30%, which is about 70%.
Eventually, as much as% of the energy is wasted as heat into the atmosphere. In addition, heat generated by combustion in factories, refuse incinerators, etc. is not converted into other energy and is discarded into the atmosphere. In this way, we humans waste a great deal of heat energy in vain, and obtain only a small amount of energy from the act of burning fossil energy.

In order to improve the energy yield,
It is effective to make use of the thermal energy that is discarded in the atmosphere. To that end, thermoelectric conversion, which directly converts thermal energy into electric energy, is an effective means. This thermoelectric conversion utilizes the Seebeck effect, and is an energy conversion method that generates a potential difference by generating a temperature difference between both ends of the thermoelectric conversion material to generate electricity.
In this thermoelectric power generation, one end of the thermoelectric conversion material is placed in the high temperature part generated by waste heat, the other end is placed in the atmosphere (room temperature), and electricity is obtained simply by connecting a conductor to each end. No moving devices such as motors and turbines are needed for general power generation. Therefore, the cost is low, gas is not emitted due to combustion, etc., and power can be continuously generated until the thermoelectric conversion material deteriorates.

As described above, thermoelectric power generation is expected as a technology that plays a part in solving energy problems that are a concern in the future, but in order to realize thermoelectric power generation, it has high thermoelectric conversion efficiency, heat resistance, and heat resistance. It is necessary to supply a large amount of thermoelectric conversion material excellent in chemical durability and the like.

At present, intermetallic compounds are known as substances having high thermoelectric conversion efficiency. However, the thermoelectric conversion efficiency of the intermetallic compound is about 10% at maximum,
Moreover, it can only be used in air at temperatures below about 500K. Further, depending on the type of intermetallic compound, there are some that use toxic elements or rare elements as constituent elements.

For this reason, thermoelectric power generation using waste heat has not yet been put to practical use, is composed of elements with little toxicity and abundance, is excellent in heat resistance, chemical durability, etc., and has high thermoelectricity. Development of materials having conversion efficiency is expected.

In recent years, Co containing Ca, Bi, Na and the like has been used as a material having excellent durability and high thermoelectric conversion efficiency.
-Based complex oxides have been reported, and their practical application is considered promising. However, although these composite oxides show high performance as a single crystal, in a polycrystalline body such as a sintered body, the performance deteriorates to about 1/3 or less. It is considered that the main cause of the deterioration in the performance of such a polycrystalline body is that the electric resistance becomes higher than that of the single crystal.

When the above Co-based composite oxide is applied as a thermoelectric conversion material in practice, it is desirable to use a polycrystalline sintered body because a large-sized material having an arbitrary shape can be easily produced. Therefore, it is desired to improve the thermoelectric conversion performance in the polycrystalline sintered body of the Co-based composite oxide.

[0009]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems of the prior art, and provides a polycrystalline sintered body of Co-based complex oxide having excellent thermoelectric conversion performance. The main purpose is that.

[0010]

Means for Solving the Problems The present inventor has conducted extensive studies to achieve the above objects. As a result, they have found that it is effective to align the crystal axes of the crystals in order to reduce the electric resistance of the polycrystalline sintered body. Then, the plate crystals of the Co-based composite oxide are mixed with the fine powder of the Co-based composite oxide, the crystal planes of the plate-like crystals that have grown well are aligned, and then pressure is applied in the direction perpendicular to the crystal faces. According to the sintering method, it is possible to obtain a high-density sintered body in which the crystal grain orientations are very well aligned, and the obtained sintered body has excellent thermoelectric conversion performance. The present invention has been completed here.

That is, the present invention provides the following composite oxide sintered body, a method for producing the same, and a thermoelectric material using the sintered body. 1. General formula: Ca _{2.2 to 3.6} Na 0 to _0.8 Bi 0 to _0.8 Co
₄ O _8.8-9.2 The fine powder of the complex oxide and the plate crystal are mixed, the crystal planes of the plate crystal are aligned, and then the mixture is sintered under uniaxial pressure. Method for manufacturing sintered compact. 2. In the fine powder of a complex oxide and a plate crystal used as a raw material, the length of the longest side of the plate crystal is twice or more the length of the longest side of the fine powder. Body manufacturing method. 3. Item 3. The complex according to Item 1 or 2, wherein the mixing ratio of the fine powder of the complex oxide and the plate crystal is 5 to 95% by weight of the fine powder and 95 to 5% by weight of the plate crystal, based on the total amount of both. Method for manufacturing oxide sintered body. 4. Fine powder of complex oxide, longest side length is 50μm
The method for producing a complex oxide sintered body according to any one of Items 1 to 3, wherein the powder is a powder having a longest side length / a shortest side length of 5 or less. 5. The method for producing a complex oxide sintered body according to any one of the above items 1 to 4, wherein the plate-like crystal of the complex oxide satisfies the following conditions (1) to (3): (1) relative (2) The longest side length of the grown surface is 100.
μm or more, the length of the shortest side is 10 μm or more, the length of the longest side / the length of the shortest side is 100 or less, and (3) the thickness between two opposed grown surfaces is 50 μm or less. hand,
The length / thickness of the shortest side of the grown surface is 5 or more. 6. The method for producing a complex oxide sintered body according to any one of the above items 1 to 5, wherein the method of aligning the crystal plane directions of the plate crystals is any one of the following (1) to (3): 1) A method of filtering a slurry containing a mixture of fine powder of complex oxide and plate crystals, (2) A method of thinning a slurry containing a mixture of fine powder of complex oxide and plate crystals by a doctor blade method, (3) A method of drying a slurry containing a mixture of fine powder of complex oxide and plate crystals in a magnetic field. 7. 7. The method according to any one of the above items 1 to 6, wherein the method of sintering under uniaxial pressure is a method of sintering in a state of being pressed in a direction perpendicular to the growth surface of the plate crystal. 8. General formula: Ca _{2.2 to 3.6} Na 0 to _0.8 Bi 0 to _0.8 Co
A compound oxide sintered body represented by ₄ O _8.8 to _9.2 ,
A composite oxide sintered body having a Seebeck coefficient of 100 μV / K or more at an absolute temperature of 300 K or more. 9. General formula: Ca _{2.2 to 3.6} Na 0 to _0.8 Bi 0 to _0.8 Co
A compound oxide sintered body represented by ₄ O _8.8 to _9.2 ,
A composite oxide sintered body having an electric resistivity of 10 mΩcm or less at an absolute temperature of 300 K or more. 10. General formula: Ca _{2.2 to 3.6} Na 0 to _0.8 Bi 0 to _0.8 C
A composite oxide sintered body represented by o ₄ O _8.8 to _9.2 , which has a thermal conductivity of 3 W / mK or less at an absolute temperature of 300 K or more. 11. General formula: Ca _{2.2 to 3.6} Na 0 to _0.8 Bi 0 to _0.8 C
o ₄ O _8.8 to _9.2, which is a composite oxide sintered body, having the following characteristics at an absolute temperature of 300 K or higher: (1) Seebeck coefficient of 100 μV / K or more, (2) electrical resistivity of 10 mΩcm or less, (3) thermal conductivity of 3 W /
mK or less. 12. A p-type thermoelectric conversion material comprising the composite oxide sintered body according to any one of items 8 to 11. 13. A thermoelectric power generation module including the p thermoelectric conversion material according to item 12.

[0012]

BEST MODE _FOR CARRYING OUT THE INVENTION In the method for producing a complex oxide sintered body of the present invention, the raw material is represented by the general formula: Ca _{2.2 to 3.6} Na.
It is necessary to use a mixture of fine powder of the complex oxide represented by ₀ to _0.8 Bi 0 to _0.8 Co ₄ O _8.8 to _9.2 and plate crystals. The fine powder and the plate crystal may be those represented by the above general formula, and the fine powder and the plate crystal may have the same composition or different compositions.

The complex oxide represented by the above general formula is Co
A CoO ₂ layer in which a unit cell in which six oxygen atoms are octahedrally coordinated with each other shares one side thereof and a layer of CoO ₂ and rock salt
MO-CoO-MO having a (NaCl) structure (M is C
a, Bi and at least one of Na) and a layer stacked in this order have a structure in which they are alternately laminated in the c-axis direction. By using a mixture of fine powder of complex oxide and plate crystal having such a structure as a raw material and producing a sintered body by the method described later, excellent thermoelectric properties in which the crystal planes of crystal grains are aligned A sintered body having conversion performance can be obtained.

The fine powders and plate crystals used as raw materials are as follows.
It is preferable that the difference in particle size is large, and it is usually preferable that the length of the longest side of the plate-like crystal is twice or more the length of the longest side of the fine powder.

The mixing ratio of the fine powder of the complex oxide and the plate crystal is preferably about 5 to 95% by weight of the fine powder and about 95 to 5% by weight of the plate crystal, based on the total amount of both. .

The shape of the fine powder of the complex oxide is not particularly limited, but the length of the longest side is about 50 μm or less, and the length of the longest side / the length of the shortest side is about 5 or less. It is preferable.

Regarding the shape of the plate crystal of the complex oxide,
Although not particularly limited, it is preferable to satisfy the following conditions (1) to (3). Incidentally, the plate-like crystal of the composite oxide is
It is preferably a single crystal. (1) A plate-shaped crystal having two opposite grown faces, and (2) the longest side length of the grown face is 100.
μm or more, the length of the shortest side is 10 μm or more, the length of the longest side / the length of the shortest side is 100 or less, and (3) the thickness between two opposed grown surfaces is 50 μm or less. hand,
The length / thickness of the shortest side of the grown surface is 5 or more.

Regarding the respective shapes of the fine powder of the complex oxide and the plate-like crystals, the average value measured for each 100 crystals arbitrarily selected by microscopic observation may be within the above range. Of each of 100 crystals, 70% or more of the crystals are preferably in the above range, 90% or more of the crystals are more preferably in the above range, and all the crystals are in the above range. Is most preferred.

1 and 2 are scanning electron micrographs showing an example of the crystal structure of the fine powder of the complex oxide used as a raw material and the plate crystal. Among these, FIG. 1 is an electron micrograph of a fine powder crystal, and FIG. 2 is an electron micrograph of a plate crystal. From this electron micrograph, it can be seen that the fine powder has a nearly spherical shape, and the plate crystal has a well-grown surface, that is, an ab surface.

The method for producing the fine powder of the complex oxide is not particularly limited, and various methods can be adopted as long as the method can produce the fine powder of the complex oxide satisfying the above-mentioned conditions.

For example, a complex oxide having a predetermined composition is produced by a known method such as a solid phase reaction method, a sol-gel method and a hydrothermal synthesis method, and if necessary, a predetermined method is performed by a mechanical grinding method such as milling. By pulverizing until the size becomes, fine powder of the composite oxide as a raw material can be obtained.

Of these, the method for producing the complex oxide fine powder by the solid phase reaction method will be briefly described.
After mixing the raw materials so that the mixing ratio is the same as the metal ratio of the target composite oxide, the powder is calcined at 800 to 900 ° C. for about 10 hours in an oxygen-containing atmosphere such as air to give a calcined powder. And Then, the calcined powder is pressure-molded and fired in an oxygen-containing atmosphere to obtain a target composite oxide. As the oxygen-containing atmosphere, for example, an atmosphere such as the atmosphere or an oxygen stream having a flow rate up to about 300 ml / min can be adopted. The firing means is not particularly limited, and any means such as an electric heating furnace and a gas heating furnace can be adopted. The firing temperature and the firing time are not particularly limited as long as the target composite oxide can be obtained. For example, 920
The firing may be performed at about 1100 ° C. for about 20 to 40 hours. By finely pulverizing the thus obtained fired product, a complex oxide fine powder can be obtained.

The raw material can be used without particular limitation as long as it can form the target oxide by firing, and for example, simple metals, oxides, various compounds (carbonates, etc.) and the like can be used. For example, as a Ca source, calcium oxide (CaO), calcium chloride (CaCl ₂ ), calcium carbonate (CaCO ₃ ), calcium nitrate (Ca (N
O ₃ ) ₂ ), calcium hydroxide (Ca (OH) ₂ ), alkoxide compound (dimethoxy calcium (Ca (OCH 2
₃ ) ₂ ), diethoxy calcium (Ca (OC
₂ H ₅ ) ₂ ), dipropoxy calcium (Ca (OC
₃ H ₇ ) ₂ )) or the like can be used, and bismuth oxide (Bi ₂ O ₃ ) or bismuth nitrate (Bi (N
O ₃ ) ₃ ), bismuth chloride (BiCl ₃ ), bismuth hydroxide (Bi (OH) ₃ ), alkoxide compound (Bi (O
_{CH 3) 3, Bi (OC} 2 H 5) 3, Bi (OC 3 H 7) 3) or the like can be used, sodium oxide as a source of Na (Na ₂ O), sodium nitrate (NaNO _3), chloride Sodium (NaCl), sodium hydroxide (NaOH),
Alkoxide compounds (NaOCH ₃ , NaOC ₂ H ₅ , N
aOC ₃ H ₇ ) or the like can be used, and as a Co source, cobalt oxide (CoO, Co ₂ O ₃ , Co ₃ O ₄ ), cobalt chloride (CoCl ₂ ), cobalt carbonate (CoCO ₃ ), cobalt nitrate (Co (NO ₃ ) ₂ ), cobalt hydroxide (Co
(OH) ₂ ), alkoxide compounds (dipropoxycobalt (Co (OC ₃ H ₇ ) ₂ ), etc.) and the like can be used. In addition to the above compounds, compounds containing two or more kinds of constituent elements of the target composite oxide may be used.

As the method for producing the plate-shaped crystal of the composite oxide, various methods can be adopted as long as they can produce the plate-shaped crystal of the composite oxide satisfying the above-mentioned conditions. For example, a solid phase reaction method, a sol-gel method, a hydrothermal synthesis method, etc. can be applied, but in the case of producing a plate-shaped single crystal, for example, a flux method, a zone melt method, a pulling method, a glass precursor is used. A single crystal manufacturing method such as a glass annealing method can be preferably used.

The specific conditions of each of these methods may be appropriately determined so that a complex oxide having a desired composition is formed.

For example, a glass annealing method via a glass precursor will be briefly described. First, a raw material is melted and rapidly cooled to be solidified. The melting condition at this time may be any condition that can uniformly melt the raw material, but in order to prevent contamination from the melting container and evaporation of the raw material components, for example, when a crucible made of alumina is used, 1200 ~
It is preferable to heat it to about 1400 ° C. to melt it. The heating time is not particularly limited, and heating may be performed until the raw material substance is uniformly melted, and the heating time is usually about 30 minutes to 1 hour. The heating means is not particularly limited, and any means such as an electric heating furnace and a gas heating furnace can be adopted. The atmosphere at the time of melting may be an oxygen-containing atmosphere such as air or an oxygen stream of about 300 ml / min or less. However, when the raw material contains a sufficient amount of oxygen, it is melted in an inert atmosphere. Is also good.

The quenching conditions are not particularly limited, but the quenching may be carried out under the condition that at least the surface portion of the solidified material formed becomes a glassy amorphous layer. For example, the melt may be poured onto a metal plate and rapidly cooled by means such as compression from above. The cooling rate is usually about 500 ° C./sec or more, and preferably 10 ³ ° C./sec or more.

Next, the solidified product formed by rapid cooling is heat-treated in an oxygen-containing atmosphere, so that the composite oxide of the present invention grows as a single crystal from the surface of the solidified product.

The heat treatment temperature may be set to about 880 to 930 ° C., and the heat treatment may be performed in an oxygen-containing atmosphere such as air or an oxygen stream. When heating in an oxygen stream, for example, the heating may be performed in an oxygen stream having a flow rate of about 300 ml / min or less. The heat treatment time is not particularly limited,
The heating time may be determined according to the desired degree of growth of the single crystal, but the heating time is usually about 60 to 1000 hours.

The mixing ratio of the raw materials can be determined according to the composition of the target composite oxide. In particular,
When a complex oxide single crystal is formed from the amorphous layer portion on the surface of the solidified material, the composition of the melt of the amorphous portion is taken as the liquid phase composition, and the composition of the solid phase in phase equilibrium with this Since the oxide single crystal grows, the composition of the starting material can be determined by the relationship between the composition of the melt phase and the solid phase (single crystal) in equilibrium with each other.

In the case of manufacturing by the flux method,
For example, various chlorides such as NaCl, CaCl ₂ , SrCl _{2 and the} like are used as flux components, heated so that the raw material is dissolved in the melted flux components, and then slowly cooled to obtain the purpose in the molten salt. It is possible to grow a plate-shaped crystal of the complex oxide.

The raw material used in producing the plate crystal may be appropriately selected according to the production method. For example, the same metal simple substance as the raw material used in producing the above-mentioned fine powder of complex oxide, It can be selected from oxides, various compounds (carbonates, etc.), and the like.

In the method of the present invention, after the fine powder of the above complex oxide and the plate-like crystals are mixed, the crystal faces of the plate-like crystals in the obtained mixture are aligned, and then sintered. A composite oxide sintered body is manufactured.

The method for aligning the crystal plane directions of the plate-like crystals is not particularly limited, but for example, (1) a method of filtering a slurry containing the mixture (filtration method), (2) containing the mixture. A method of thinning the slurry by a doctor blade method (doctor blade method), (3) a method of drying a slurry containing the mixture in a magnetic field (magnetic field orientation method), and the like can be applied. According to these methods, it is possible to obtain a molded product in which the well-grown faces of the plate-like crystals are aligned substantially parallel to a certain direction.

Hereinafter, each of the above methods (1) to (3) will be described in more detail. (1) Filtration method: A slurry containing a mixture of the above-mentioned fine powder of complex oxide and plate crystals is prepared and then filtered to make the well-grown surface of the plate crystals parallel to the filter paper or filter surface. Can be oriented. On this occasion,
A suction filtration method or the like can be applied as appropriate.

The type of solvent for forming the slurry is not particularly limited as long as it can uniformly disperse the mixture of the fine powder as the raw material and the plate-like crystals, and for example, water or Various organic solvents can be used. The concentration of the composite oxide in the slurry is not particularly limited, and may be appropriately determined so that a uniform slurry can be formed and a slurry having an appropriate filtration rate is formed.

If necessary, a viscosity adjusting agent, a dispersant or the like may be added to the slurry. (2) Doctor blade method: The doctor blade method is a known method as a thin film forming method. For example, a slurry containing a mixture of the above-mentioned fine powder of complex oxide and plate crystals is placed on a substrate such as a carrier tape. This is a method of pouring and passing through a gap of a knife blade called a doctor blade, that is, between slits to form a thin film, and orienting plate crystals.

Known specific conditions may be appropriately applied to specific conditions of the doctor blade method. (3) Magnetic field orientation method: After preparing a slurry containing a mixture of the above-mentioned fine powder of complex oxide and a plate crystal, the slurry is dried in a magnetic field so that the crystal axis of the plate crystal is unidirectional. Can be oriented. This method utilizes the anisotropy of the magnetization of the crystal, and the plate crystal is oriented so that the well-grown surface of the plate crystal is perpendicular to the direction of the magnetic field.

The method for preparing the slurry and the type of solvent that can be used are not particularly limited, and may be appropriately determined so that a slurry in which the above mixture is uniformly dispersed is formed, as in the above-mentioned filtration method.

The strength of the magnetic field is not particularly limited, but it is usually about 1 to 5 T (tesla). By drying the slurry in such a magnetic field to remove the solvent, it is possible to obtain a molded product in which the crystal planes of plate-like crystals are arranged in a certain direction. In the method of the present invention, after aligning the crystal planes of the plate-like crystals by the method described above, by sintering the mixture of complex oxides under uniaxial pressure,
It is possible to obtain a sintered body in which the crystal plane directions are very well aligned. Moreover, since the obtained sintered body is obtained by mixing fine powder with plate crystals and sintering this under pressure, it becomes a very high density sintered body.

Regarding the pressing direction during sintering, the well-grown surface (ab) of the plate-like crystals oriented in a fixed direction is used.
The direction perpendicular to the (plane), that is, the direction parallel to the c axis of the plate crystal.

There is no particular limitation on the sintering method, and any method may be used as long as the compact obtained by orienting the plate-like crystals can be sintered under pressure to produce a dense compact. Examples of such a sintering method include a hot press sintering method and a discharge plasma sintering method under pressure (SPS method).

The specific sintering conditions are not particularly limited, and a dense sintered body is formed depending on the size of the mold used, the composition of the metal oxide powder constituting the molded body, and the like. It may be set appropriately. The firing atmosphere is not particularly limited, and examples thereof include an oxidizing atmosphere such as the air and a vacuum atmosphere.

As a specific example of the sintering conditions, in the hot press sintering method, for example, the pressure is about 10 to 20 MPa, the sintering temperature is about 700 to 850 ° C., and the sintering time is about 5 to 40 hours. do it. In the spark plasma sintering method, for example, the pressure is about 10 to 50 MPa, the sintering temperature is about 800 to 900 ° C., and the sintering time is 1
It may be about 0 minutes to 1 hour.

Since the composite oxide used as a raw material in the method of the present invention has a structure in which two different sublattices are alternately laminated in the c-axis direction, the ab plane grows well in a general manufacturing method. By sintering such a raw material under uniaxial pressure, grain growth of the fine powder crystal is restricted to the plane perpendicular to the pressure axis, and the plate crystal acts as a pressure medium. Two-dimensional grain growth is promoted. As a result, it is possible to obtain a complex oxide sintered body in which the crystal planes of crystal grains are extremely well aligned.

As described above, the complex oxide sintered body obtained by the method of the present invention is prepared by mixing the fine powder of the complex oxide and the plate crystal, and aligning the crystal planes of the plate crystal before sintering. It is obtained by uniaxial pressure sintering, and has a low electrical resistance.
The b-plane is almost aligned in one position in all the crystal grains, and is densified by pressure sintering.

Such a sintered body has a low electric resistivity because the ab plane having a low electric resistance is aligned in one of all the crystal grains and the density is increased. 10 mΩc at temperatures above 300K
The electrical resistivity may be m or less.

Further, the sintered body has a high Seebeck coefficient (S) of 100 μV / K or more at a temperature of 300 K or more.
Can be indicated.

Further, the sintered body has a low thermal conductivity, and is 3 W / mK at an absolute temperature of 300 K or more.
It may exhibit the following thermal conductivity:

As described above, the sintered body obtained by the method of the present invention has a high Seebeck coefficient, a low electric resistivity and a low thermal conductivity, and exhibits excellent thermoelectric conversion performance. it can.

The composite oxide single crystal obtained by the method of the present invention is effective as a thermoelectric conversion material used at high temperature in air, which was impossible with the conventional intermetallic compound materials, by utilizing the above-mentioned characteristics. Can be used for. Therefore, by incorporating the composite oxide single crystal into the system as a p-type thermoelectric conversion element of a thermoelectric power generation module, it becomes possible to effectively use the thermal energy that has been discarded in the atmosphere. Further, application to a thermoelectric module using the Peltier effect is also possible.

FIG. 3 shows a schematic view of an example of a thermoelectric power generation module using the thermoelectric conversion material comprising the composite oxide of the present invention as a p-type thermoelectric conversion element. The structure of the thermoelectric power generation module is similar to that of a known thermoelectric power generation module, and includes a high temperature part substrate, a low temperature part substrate, a p-type thermoelectric conversion material, an n-type thermoelectric conversion material, an electrode, a lead wire, and the like. It is a module and the composite oxide of the present invention is used as a p-type thermoelectric conversion material.

[0053]

According to the method for producing a complex oxide sintered body of the present invention, a complex oxide having a high Seebeck coefficient is used as a raw material, and high density polycrystalline sintering in which the crystal grains are arranged in the same direction. You can get the body.

The obtained composite oxide sintered body is a polycrystal of a metal oxide having a high figure of merit (ZT) and is highly useful as a high performance thermoelectric material.

In particular, since the composite oxide sintered body obtained by the method of the present invention is a polycrystalline body obtained by a sintering method, it can be easily produced in a desired size.
It can be used for various applications as a thermoelectric conversion material (thermoelectric conversion element).

[0056]

EXAMPLES The present invention will be described in more detail with reference to examples.

In each of the examples, the raw materials used for producing the fine powder of the composite oxide and the plate crystal were:
It is as follows. * Ca source: Calcium carbonate (CaCO ₃₎ * Bi source: bismuth oxide (Bi ₂ O ₃₎ * Na source: Sodium carbonate (Na ₂ CO ₃₎ * Co source: cobalt oxide (Co ₃ O ₄₎ Example 1 Composite Preparation of Oxides Powders of CaCO ₃ , Bi ₂ O ₃ , Na ₂ CO ₃ and Co ₃ O ₄ were mixed with Ca: Bi: Na: Co = 2.4: 0.3: 0.
The mixture was mixed so that the element ratio was 3: 4, and the mixture was baked at 600 ° C. in air for 10 hours. Then, it was crushed by a ball mill, and the powder was pressed into a disk shape. This disc was heated at 880 ° C. for 20 hours in an oxygen stream of 300 ml / min. The obtained sintered product was crushed by a ball mill to obtain a complex oxide fine powder. The obtained fine powder had an average composition of Ca _2.4 Na _0.3 Bi _0.3 Co ₄ O _9.1 , and the average longest side and the average shortest side had lengths of 1 μm and 0.5 μm, respectively.

On the other hand, CaCO ₃ and Co ₃ O ₄ are replaced with Ca: Co.
= 1: 3 is mixed in an element ratio to prepare a raw material mixed powder for producing a plate crystal, and SrCl ₂ and CaCl ₂ are added to Sr: Ca = 5:
The mixed powder for flux was prepared by mixing so as to have an element ratio of 1. The raw material mixed powder and the mixed powder for flux are mixed at a weight ratio of 1: 2, and the mixture is heated to 900 ° C.
After heating up to 750 ° C., it was gradually cooled at a cooling rate of 1 ° C./hour. Then remove the flux by washing with water,
Plate-like crystals of complex oxide were obtained. The obtained plate-like crystals had an average composition of Ca _2.9 Co _4.0 O _9.1 , lengths of the average longest side and the average shortest side of 1 mm and 500 μm, and an average thickness of 10 μm.

The sizes of the fine powder and the plate-like crystals mentioned above are the average values of the measured values for each 100 crystals measured by microscopic observation.

Production of Sintered Body The fine powder obtained by the above method and the plate-like crystals were mixed so that the ratio of the plate-like crystals would be 20% by weight based on the total amount of both.

10 g of this mixture was added to 200 ml of ethanol.
After mixing in and uniformly dispersing, the resulting dispersion was suction filtered. In the mixture of complex oxides remaining on the filter paper after filtration, the well-grown surface of plate crystals (ab surface) was oriented parallel to the filter paper surface.

This mixture was hot pressed and sintered under uniaxial pressure. The pressing direction is the well-grown surface (ab) of the plate crystal.
Plane), pressure 12 MPa, sintering temperature 820
C., and the sintering time was 20 hours.

FIG. 4 shows the X-ray diffraction pattern of the surface of the obtained sintered body, which is parallel to the X-ray diffraction pattern of the surface perpendicular to the pressure axis during sintering. In FIG. 4, a diffraction peak indexed by (001) appears strongly on a plane perpendicular to the pressure axis, and a diffraction intensity of peaks other than (001) increases on a plane parallel to the pressure axis. This result means that the ab planes of the crystal grains of the sintered body are aligned perpendicular to the pressing axis.

Further, with respect to the sintered body obtained in Example 1, a scanning electron microscope (SEM) photograph of a plane parallel to the pressing axis is shown in FIG. From FIG. 5, it can be seen that the plate-shaped crystal grains that have grown are stacked in the pressing axis direction, and the ab plane that has grown well extends from the front side to the back side of the photograph.

Furthermore, FIG. 6 is a graph showing the temperature dependence of the Seebeck coefficient at 300 to 973 K for the sintered body obtained in Example 1. This graph also shows, as a comparative example, the temperature dependence of the Seebeck coefficient for a sintered body obtained by sintering the same fine powder as that used in Example 1 without uniaxially pressing. From FIG. 6, it can be seen that the sintered body obtained in Example 1 exhibits a higher Seebeck coefficient than the comparative example in the entire temperature range. In all Examples described below, the Seebeck coefficient is 10 at a temperature of 300 K or higher.
The value was higher than 0 μV / K.

FIG. 7 is a graph showing the temperature dependence of the electrical resistivity of the sintered body obtained in Example 1 at 300 to 973K. As a comparative example, this graph also shows the temperature dependence of the electrical resistivity of a sintered body obtained by sintering the same fine powder as that used in Example 1 without uniaxially pressing. From this graph, it can be seen that the sintered body obtained in Example 1 has a low electrical resistivity of about 2 mΩcm, whereas the sintered body of the comparative example has a high electrical resistivity of 10 mΩcm or more. . In all the examples described below, the electrical resistivity is 10 m at a temperature of 300 K or higher.
It was a value below Ωcm.

FIG. 8 shows the sintered body of Example 1 in 3
The temperature dependence of the thermal conductivity at 73 to 973K is shown as a graph. From this graph, the sintered body of Example 1 has 3
It can be seen that the thermal conductivity is as low as W / mK or less.
The thermal conductivity is 3 W in all the examples described later.
It was a low value of / mK or less.

FIG. 9 shows 3 pieces of the sintered body of Example 1.
The temperature dependence of the thermoelectric figure of merit (ZT) at 73 to 973K is shown as a graph. This graph also shows, as a comparative example, the temperature dependence of ZT for a sintered body obtained by sintering the same fine powder as that used in Example 1 without uniaxially pressing. Here, ZT is a value defined by the following equation and represents the thermoelectric conversion efficiency of the material. The higher this value, the higher the conversion efficiency. In the present invention, ZT was a value of more than 1.0 at 973K in all the examples.

ZT = S ² T / ρκ S: Seebeck coefficient, T: absolute temperature, ρ: electrical resistivity,
κ: Thermal conductivity Examples 2 to 90 Fine powders of complex oxides and plate crystals shown in Tables 1 to 10 below were used, and the crystal planes of the plate crystals were aligned by the method shown in each table, followed by uniaxial addition. Sintered under pressure.

In each table, the filtration method described in the section of the orientation method is a method for orienting plate crystals by suction filtration in the same manner as in Example 1, and the slit method is the doctor blade described above. This is a method corresponding to the method, and is a method of orienting the plate-like crystals by stirring and mixing the fine powder of the composite oxide and the plate-like crystals in a Florinate and passing through a slit having a width of 1 to 3 mm.

Further, the hot pressing method described in the section of the sintering method in each table is a method of sintering in the same manner as in Example 1 at the pressure, temperature and sintering time described in the table.
The method is a method of sintering by the discharge plasma method at the pressure, temperature and sintering time described in the table. In any of these cases, the pressing direction was perpendicular to the well-grown surface (ab surface) of the plate crystal.

Regarding each sintered body obtained in each example, 9
The thermoelectric conversion index (ZT) at 73K is shown in the table.

[0073]

[Table 1]

[0074]

[Table 2]

[0075]

[Table 3]

[0076]

[Table 4]

[0077]

[Table 5]

[0078]

[Table 6]

[0079]

[Table 7]

[0080]

[Table 8]

[0081]

[Table 9]

[0082]

[Table 10]

[Brief description of drawings]

FIG. 1 is a scanning electron micrograph showing the crystal structure of a composite oxide fine powder used as a raw material.

FIG. 2 is a scanning electron micrograph showing a crystal structure of a plate crystal of a composite oxide used as a raw material.

FIG. 3 is a schematic diagram of a thermoelectric power generation module using the composite oxide sintered body of the present invention as a thermoelectric conversion material.

FIG. 4 is an X-ray diffraction diagram of the complex oxide sintered body obtained in Example 1.

5 is a scanning electron microscope (SEM) photograph showing the crystal structure of a plane parallel to the pressure axis of the sintered body obtained in Example 1. FIG.

FIG. 6 is a graph showing the temperature dependence of the Seebeck coefficient of the composite oxide sintered body obtained in Example 1.

7 is a graph showing the temperature dependence of the electrical resistivity of the composite oxide sintered body obtained in Example 1. FIG.

FIG. 8 is a graph showing the temperature dependence of the thermal conductivity of the composite oxide sintered body obtained in Example 1.

FIG. 9 is a graph showing the temperature dependence of the thermoelectric figure of merit of the complex oxide sintered body obtained in Example 1.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) H01L 35/34 C04B 35/00 E H02N 11/00 J F term (reference) 4G030 AA03 AA08 AA28 AA43 BA02 BA21 GA11 GA20 GA23 GA29 4G048 AA04 AA05 AB01 AB06 AC08 AD04 AD06 AE05

Claims (13)

[Claims] 1. General formula: Ca _{2.2 to 3.6} Na 0 to _0.8 Bi
_{0 to 0.8} Co ₄ O _8.8 to _9.2 Fine powder of the complex oxide and plate crystals are mixed, the crystal planes of the plate crystals are aligned, and then sintered under uniaxial pressure. And a method for producing a composite oxide sintered body. 2. The fine powder and the plate-like crystal of the composite oxide as a raw material, wherein the length of the longest side of the plate-like crystal is at least twice the length of the longest side of the fine powder. Manufacturing method of composite oxide sintered body. 3. The mixing ratio of the fine powder of the composite oxide and the plate crystal is 5 to 95% by weight of the fine powder, based on the total amount of both.
And 95 to 5% by weight of plate crystals, The method for producing a complex oxide sintered body according to claim 1 or 2. 4. The composite oxide fine powder has a longest side length of 5
0 μm or less and the length of the longest side / the length of the shortest side is 5
It is the following powders, The manufacturing method of the complex oxide sintered compact in any one of Claims 1-3. 5. A plate-shaped crystal of a composite oxide has the following (1) to
The method for producing a composite oxide sintered body according to any one of claims 1 to 4, which satisfies the condition (3): (1) A plate-shaped structure crystal having opposite two grown faces. (2) The length of the longest side of the grown surface is 100
μm or more, the length of the shortest side is 10 μm or more, the length of the longest side / the length of the shortest side is 100 or less, and (3) the thickness between two opposed grown surfaces is 50 μm or less. hand,
The length / thickness of the shortest side of the grown surface is 5 or more. 6. A method of aligning the directions of crystal planes of a plate crystal is
The method according to any one of the following (1) to (3):
5. A method for producing a complex oxide sintered body according to any one of 5: (1) a method of filtering a slurry containing a mixture of fine powder of complex oxide and plate crystals, (2) fine powder of complex oxide A method of thinning a slurry containing a mixture of plate crystals by a doctor blade method, and (3) a method of drying a slurry containing a mixture of fine powder of complex oxide and plate crystals in a magnetic field. 7. The method according to claim 1, wherein the method of sintering under uniaxial pressure is a method of sintering in a state of being pressed in a direction perpendicular to the growth surface of the plate crystal. 8. A general formula: Ca _{2.2 to 3.6} Na 0 to _0.8 Bi.
_0-0.8 Co ₄ O _8.8-9.2 Sintered body of complex oxide represented by 100 μV / at absolute temperature of 300K or higher
A composite oxide sintered body having a Seebeck coefficient of K or more. 9. General formula: Ca _{2.2 to 3.6} Na 0 to _0.8 Bi
_0-0.8 Co ₄ O _8.8-9.2 Sintered body of complex oxide represented by _{10 mΩcm} at an absolute temperature of 300 K or higher
A composite oxide sintered body having the following electrical resistivity. 10. General formula: Ca _{2.2 to 3.6} Na 0 to _0.8 Bi
A composite oxide sintered body represented by ₀ to _0.8 Co ₄ O _8.8 to _9.2 , which has a thermal conductivity of 3 W / mK or less at an absolute temperature of 300 K or more. . 11. A general formula: Ca _{2.2 to 3.6} Na 0 to _0.8 Bi.
A composite oxide sintered body represented by ₀ to _0.8 Co ₄ O _8.8 to _9.2 , which has the following characteristics at an absolute temperature of 300 K or higher: (1) Seebeck coefficient Is 100 μV / K or more, (2) electric resistivity is 10 mΩcm or less, (3) thermal conductivity is 3 W /
mK or less. 12. A p-type thermoelectric conversion material comprising the composite oxide sintered body according to claim 8. 13. A thermoelectric power generation module comprising the p thermoelectric conversion material according to claim 12.