JP2003306380A - Method for producing composite oxide sintered body - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a polycrystalline sintered body of a Co-based composite oxide having excellent thermoelectric conversion performance. <P>SOLUTION: The method for producing a composite oxide sintered body comprises using plate-like crystals of a composite oxide represented by the formula: Ca<SB>2.2-3.6</SB>Na<SB>0-0.8</SB>Sr<SB>0-0.8</SB>Bi<SB>0-0.8</SB>Co<SB>4</SB>O<SB>8.8-9.2</SB>as a raw material, molding the plate-like crystals with the crystal planes being arranged in the same direction, and then sintering the obtained moldings under uniaxial compression. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a complex 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, most of the heat generated by combustion in factories, refuse incinerators, etc. is discarded into the atmosphere without being converted into other energy. 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 thermoelectric power generation using such thermoelectric conversion, one end of the thermoelectric conversion material is placed in a high temperature part generated by waste heat, the other end is placed in the atmosphere (room temperature), and conductors are connected to both ends of each. Electricity can be obtained only by itself, and no moving device such as a motor or turbine required for general power generation is required. 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.

Therefore, thermoelectric power generation using waste heat has not yet been put to practical use. Therefore, it is expected to develop a material which is composed of elements having low toxicity and high abundance, is excellent in heat resistance, chemical durability, etc. and has high thermoelectric conversion efficiency.

In recent years, a Co-based composite oxide containing Ca, Sr, Bi, Na and the like has been reported as a material having excellent durability and high thermoelectric conversion efficiency, and its 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 actually applied as a thermoelectric conversion material, it is desirable to use a 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 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 it is an object of the present invention to provide a sintered body of a Co-based complex oxide having excellent thermoelectric conversion performance. It is the main purpose.

[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, according to the method of aligning the crystal planes of the Co-based complex oxide plate crystals that have grown well and then sintering by pressing in the direction perpendicular to the crystal planes, the orientation of the crystal grains is very good. It was found that a uniform high-density sintered body can be obtained, and the obtained sintered body has excellent thermoelectric conversion performance, and 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 Sr 0 to _0.8 Bi
_{0 to 0.8} Co ₄ O _8.8 to _9.2 is used as a raw material, and plate-shaped crystals of the complex oxide are used as a raw material. A method for producing a complex oxide sintered body, comprising: 2. The method for producing a complex oxide sintered body according to the above item 1, wherein the plate-like crystal of the complex oxide satisfies the following conditions (1) to (3): (1) Opposite grown two faces (2) The longest side of the grown surface has a length of 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. 3. Item 1 or 2 above, wherein the method of aligning the crystal plane directions of the plate-like crystals is any one of the following (1) to (3):
A method for producing the composite oxide sintered body according to: (1) a method for filtering a slurry containing plate crystals, (2)
A method of thinning a slurry containing plate crystals by a doctor blade method, and (3) a method of drying a slurry containing plate crystals in a magnetic field. 4. Item 4. The method according to any one of Items 1 to 3, wherein the method of sintering under uniaxial pressure is a method of sintering in a state in which pressure is applied in a direction perpendicular to the growth surface of the plate crystal. 5. General formula: Ca _{2.2 to 3.6} Na 0 to _0.8 Sr 0 to _0.8 Bi
_A sintered body of a composite oxide represented by ₀ to _0.8 Co ₄ O _8.8 to 9.2, and 100 μ at an absolute temperature of 300 to 973K.
A composite oxide sintered body having a Seebeck coefficient of V / K or more. 6. General formula: Ca _{2.2 to 3.6} Na 0 to _0.8 Sr 0 to _0.8 Bi
_0-0.8 Co ₄ O _8.8-9.2 Sintered body of complex oxide represented by _{10 mΩ} at an absolute temperature of 300-973K
A composite oxide sintered body having an electric resistivity of not more than cm. 7. General formula: Ca _{2.2 to 3.6} Na 0 to _0.8 Sr 0 to _0.8 Bi
_0-0.8 Co ₄ O _8.8-9.2 Sintered body of complex oxide represented by 3 W / m at an absolute temperature of 300-973K
A composite oxide sintered body having a thermal conductivity of K or less. 8. General formula: Ca _{2.2 to 3.6} Na 0 to _0.8 Sr 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 to 973 K: (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. 9. A p-type thermoelectric conversion material comprising the composite oxide sintered body according to any one of items 5 to 8 above. 10. A thermoelectric conversion module comprising the p-type thermoelectric conversion material according to item 9.

[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.
A plate crystal of a complex oxide represented by 0 to _0.8 Sr 0 to _0.8 Bi 0 to _0.8 Co ₄ O _8.8 to _9.2 is used.

The complex oxide represented by the above general formula is Co
A unit cell in which six oxygens are octahedrally coordinated around is a CoO ₂ layer spread in layers so that one side thereof is shared, and MO-CoO-MO (M is a salt having a salt) structure.
And at least one of Ca, Sr, Bi, and Na) are stacked in this order to have a structure in which they are alternately laminated in the c-axis direction. By using a plate-shaped crystal of a complex oxide having such a structure as a raw material and producing a sintered body by the method described below, a sintered body having excellent thermoelectric conversion performance in which the directions of crystal planes of crystal grains are aligned is obtained. You can get a unity.

The shape of the plate-like crystal of the complex oxide is not particularly limited, but it is preferable that the following conditions (1) to (3) are satisfied. The plate crystal of the composite oxide 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 shape of the plate-like crystal of the composite oxide, the average value measured for 100 crystals arbitrarily selected by microscopic observation may be within the above range.
Of the 100 crystals measured, 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.

A scanning electron micrograph showing an example of the crystal structure of a plate-like crystal of a complex oxide used as a raw material is shown in FIG. From this electron micrograph, it can be seen that the plate crystal has a well-grown surface, that is, an ab surface.

As the method for producing the plate-like crystal of the complex oxide, various methods can be adopted as long as it is a method capable of producing the plate-like crystal of the complex oxide satisfying the above-mentioned conditions. For example, solid-phase reaction method, sol-gel method, hydrothermal synthesis method and the like can be used, but particularly, single crystal production methods such as a flux method, a zone melt method, a pulling method, and a glass annealing method via a glass precursor are used. It can be used suitably.

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. Usually, the cooling rate is preferably about 500 ° C./sec or more, more preferably 10 ³ ° C./sec or more.

Next, the solidified product formed by quenching is heat-treated in an oxygen-containing atmosphere, so that the composite oxide 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 in air or in 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 material 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.

When the plate crystal is produced by the flux method, for example, various chlorides such as NaCl, CaCl ₂ and SrCl ₂ are used as the flux component, and the raw material is dissolved in the melted flux component. So that
Then, by gradually cooling, plate crystals of the target complex oxide can be grown in the molten salt.

Raw material used for producing plate crystals
May be appropriately selected according to the manufacturing method.
Use simple substances, oxides, various compounds (carbonates, etc.)
You can Specific examples of such a raw material include Ca
As a source, calcium oxide (CaO), calcium chloride
(CaCl₂), Calcium carbonate (CaCO₃), Nitric acid
Lucium (Ca (NO₃)₂), Calcium hydroxide (Ca
(OH)₂), Alkoxide compounds (dimethoxycalci)
Umm (Ca (OCH₃)₂), Diethoxy calcium (C
a (OC₂H_Five)₂), Dipropoxy calcium (Ca
(OC₃H₇)₂) Etc.) and the like, and Sr source
Strontium oxide (SrO), strontium peroxide
Tium (SrO₂), Strontium carbonate (SrC
O₃), Strontium nitrate (Sr (NO₃)₂), Hydroxy
Strontium oxide (Sr (OH)₂), Alkoxide
Compound (dimethoxystrontium (Sr (OC
H₃)₂), Diethoxystrontium (Sr (OC
₂H_Five)₂), Dipropoxystrontium (Sr (OC₃
H₇)₂) Etc.) and the like, and as the Bi source, acid
Bismuth chloride (Bi₂O ₃), Bismuth nitrate (Bi (N
O₃)₃), Bismuth chloride (BiCl₃), Bisma hydroxide
Su (Bi (OH)₃), An alkoxide compound (Bi (O
CH₃)₃, Bi (OC₂H_Five)₃, Bi (OC₃H₇)₃etc)
Etc., and as the Na source, sodium oxide
(Na₂O), sodium nitrate (NaNO₃), Nato chloride
Ium (NaCl), sodium hydroxide (NaOH),
Alkoxide compound (NaOCH₃, NaOC₂H_Five, N
aOC₃H₇Etc.) and the like, and as a Co source,
Cobalt oxide (CoO, Co₂O₃, Co₃O_Four), Chloride
Baltic (CoCl₂), Cobalt carbonate (CoCO₃), Glass
Cobalt acid (Co (NO₃)₂), Cobalt hydroxide (Co
(OH)₂), Alkoxide compounds (dipropoxycoba
Routt (Co (OC₃H₇)₂) Etc.) etc.
It In addition to the above compounds, the structure of the target complex oxide
You may use the compound which contains two or more types of element.

In the method of the present invention, the above-mentioned compound oxide is molded by aligning the crystal planes of the plate-like crystals of the above-mentioned compound oxide, and then sintering the obtained molded product under uniaxial pressure to obtain the desired compound oxide. A sintered compact is manufactured.

The method of aligning the crystal plane directions of the plate crystals is not particularly limited, but for example, (1) a method of filtering a slurry containing the plate crystals (filtration method), (2)
A method of thinning the slurry containing the plate crystals by a doctor blade method (doctor blade method), (3) a method of drying the slurry containing the plate crystals in a magnetic field (alignment method in a magnetic field), 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: After preparing a slurry containing plate crystals of the complex oxide described above, by filtering this slurry, the well-grown surface of the plate crystals can be oriented parallel to the filter paper or filter surface. it can. At this time, a suction filtration method or the like can be appropriately applied.

The type of solvent for forming the slurry is not particularly limited as long as it can uniformly disperse the plate-like crystal as a raw material, and for example,
Water and 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 plate crystals of the above complex oxide is poured onto a base material such as a carrier tape to form a doctor blade. This is a method of orienting a plate crystal by forming a thin film by passing through a gap called a knife blade, that is, between slits.

Known conditions may be appropriately applied to specific conditions of the doctor blade method. (3) Magnetic field orientation method: After preparing a slurry containing the above-mentioned composite oxide plate crystals, the slurry can be dried in a magnetic field to orient the crystal axes of the plate crystals in one direction. it can. 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 plate crystals are uniformly dispersed is formed as in the above-mentioned filtration method. Good.

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, the crystal planes of the plate-like crystals are formed in the same direction by the above-described method, and the obtained formed body is sintered under uniaxial pressure, whereby the crystal planes are aligned in a very good direction. A sintered body can be obtained. Moreover, since the obtained sintered body is obtained by sintering the plate crystals under pressure, it becomes an extremely 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 are appropriately selected depending on the size of the mold to be used, the composition of the composite oxide constituting the molded body, etc. so that a dense sintered body is formed. Just set it. 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 10 hours.

Since the composite oxide used as a raw material in the method of the present invention has a structure in which two different types of sublattices are alternately laminated in the c-axis direction, the ab plane grows well in a general manufacturing method. By arranging the crystal planes of these raw materials in the same direction and then sintering them under uniaxial pressure, it is possible to obtain a composite oxide sintered body in which the crystal planes are very well aligned.

Such a sintered body exhibits a low electric resistivity because the ab plane having a low electric resistance is aligned in one of all crystal grains and has a high density. . Therefore, according to the method of the present invention, at least 30 which is a practical temperature range as a thermoelectric conversion material.
It is possible to obtain a sintered body having a low electrical resistivity of 10 mΩcm or less in the temperature range of 0 to 973 K and 6 m.
It is also possible to obtain a sintered body having a very low electrical resistivity of Ωcm or less.

Further, according to the method of the present invention, at least 3
In the temperature range of 00 to 973K, a sintered body having a high Seebeck coefficient (S) of 100 μV / K or more can be obtained.

Further, the sintered body has a low value of thermal conductivity and can exhibit a thermal conductivity of 3 W / mK or less in a temperature range of at least 300 to 973K.

As described above, the sintered body obtained by the method of the present invention has a temperature range of at least 300 to 973K which is a practical temperature range as a thermoelectric conversion material.
It has a high Seebeck coefficient and a low electric resistivity and low thermal conductivity, and can exhibit excellent thermoelectric conversion performance even outside this temperature range.

The composite oxide sintered body obtained by the method of the present invention utilizes the above-mentioned characteristics, for example, as a thermoelectric conversion material used at high temperature in air, which was impossible with conventional intermetallic compound materials. It can be used effectively. Therefore, by incorporating the composite oxide sintered body 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 so far. Further, application to a thermoelectric module using the Peltier effect is also possible.

FIG. 2 shows a schematic view of an example of a thermoelectric conversion module using the thermoelectric conversion material comprising the composite oxide sintered body of the present invention as a p-type thermoelectric conversion element. The structure of the thermoelectric conversion module is the same as a known thermoelectric conversion module,
The composite oxide sintered body of the present invention is a p-type thermoelectric conversion module, which is a thermoelectric power generation module including a substrate for a high temperature portion, a substrate for a low temperature portion, a p-type thermoelectric conversion material, an n-type thermoelectric conversion material, an electrode, a lead wire, and the like. Used as a material.

[0045]

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).

[0048]

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

The raw materials used for producing the plate-like crystals of the composite oxide in each example are as follows. * Ca source: Calcium carbonate (CaCO ₃₎ * Sr source: strontium carbonate (SrCO 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 Preparation of Composite Oxide CaCO ₃ and Co ₃ O ₄ were mixed at an element ratio of Ca: Co = 1: 3 to prepare a raw material mixed powder for producing a plate crystal, and SrC was prepared.
l ₂ and CaCl ₂ were mixed so as to have an element ratio of Sr: Ca = 5: 1 to prepare a mixed powder for flux. After mixing these raw material mixed powders and the mixed powder for flux in a weight ratio of 1: 2 and heating to 900 ° C.,
The mixture was gradually cooled to 600 ° C. at a cooling rate of 1 ° C./hour, and then allowed to cool to room temperature. Then, the flux was removed by washing with water to obtain a plate-shaped crystal of a composite oxide. The obtained plate crystals had an average composition of Ca _2.9 Co _4.0 O _9.1 , and the lengths of the average longest side and the average shortest side were 1 mm and 500 μm, respectively.
The average thickness was 10 μm.

The size of the plate crystal is the average value of the measured values of 100 crystals measured by microscopic observation.

Production of Sintered Body 10 g of the plate crystal obtained by the above-mentioned method was added to ethanol 200
After mixing in ml to disperse uniformly, the resulting dispersion was suction filtered. After filtration, on the filter paper, the well-grown surface of plate crystals (ab surface) was oriented parallel to the filter paper surface.

The composite oxide having the crystal planes aligned in this way was hot-press sintered under uniaxial pressure. The pressing direction was perpendicular to the well-grown surface (ab surface) of the plate-like crystal, pressure 12 MPa, sintering temperature 820 ° C., sintering time 20.
It was time.

FIG. 3 shows an X-ray diffraction diagram of the surface of the obtained sintered body which is perpendicular to the pressure axis during sintering. In FIG. 3, (0
The diffraction peak that is indexed with 0l) appears strongly,
It is shown that the ab planes of the crystal grains of the sintered body are aligned perpendicular to the pressing axis.

FIG. 4 is a graph showing the temperature dependence of the electrical resistivity of the sintered body obtained in Example 1 at 300 to 973K. From this graph, Example 1
It can be seen that the sintered body obtained in 1. has a low electrical resistivity of about 1 mΩcm. In all the examples described below, the electric resistivity is 10 mΩc at 300 to 973K.
It was a value below m.

Further, FIG. 5 is a graph showing the temperature dependence of the Seebeck coefficient at 300 to 973K. From this graph, it can be seen that the sintered body obtained in Example 1 exhibits a high Seebeck coefficient exceeding 120 μV / K in the temperature range of 300 K or higher. In all Examples described below, the Seebeck coefficient was a value exceeding 100 μV / K at 300 to 973K.

FIG. 6 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 was
It can be seen that the thermal conductivity is as low as about 2.3 W / mK or less. It should be noted that even in all the examples described later, 300
The heat conductivity was as low as 3 W / mK or less at ˜973 K.

Examples 2 to 19 Composite oxide plates having the average composition (element ratio) shown in Table 1 or 2 below and the shape (longest side, shortest side and thickness) shown in Table 1 or Table 2 Using plate-like crystals, the plate-like crystals were made to have the same crystal plane by the filtration method as in Example 1, and then sintered under uniaxial pressure.

The hot pressing method described in the section of the sintering method of 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, and the discharge plasma 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.

For each sintered body obtained in each example, 9
The thermoelectric conversion index (ZT) at 73K is shown in the table. 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, in all the examples,
ZT was a value of more than 1.0 at 973K.

ZT = S ² Tσ / κ S: Seebeck coefficient, T: absolute temperature, σ: electric conductivity,
κ: Thermal conductivity

[0061]

[Table 1]

[0062]

[Table 2]

[Brief description of drawings]

FIG. 1 is a scanning electron micrograph showing the appearance of plate crystals of a complex oxide used as a raw material.

FIG. 2 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. 3 is an X-ray diffraction diagram of the complex oxide sintered body obtained in Example 1.

FIG. 4 is a graph showing the temperature dependence of the electrical resistivity of the composite oxide sintered body obtained in Example 1.

FIG. 5 is a graph showing the temperature dependence of the Seebeck coefficient of the composite oxide sintered body obtained in Example 1.

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

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Claims (10)

[Claims] 1. A general formula: Ca _{2.2 to 3.6} Na 0 to _0.8 Sr.
Using _0~0.8 Bi _0~0.8 Co ₄ O _{8 .8~9.2} plate crystals of the complex oxide represented by a raw material, after molding align the direction of crystal plane of the plate-like crystals, obtained A method for producing a composite oxide sintered body, comprising: sintering a molded body under uniaxial pressure. 2. A plate-like crystal of a composite oxide, which has the following (1) to
The method for producing a complex oxide sintered body according to claim 1, which satisfies the condition (3): (1) a plate-shaped crystal having opposite two grown faces, and (2) growth. Length of the longest side of the 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. 3. A method for aligning the directions of crystal planes of a plate crystal is
The method for producing a composite oxide sintered body according to claim 1 or 2, which is any one of the following (1) to (3): (1) A method for filtering a slurry containing plate crystals, (2)
A method of thinning a slurry containing plate crystals by a doctor blade method, and (3) a method of drying a slurry containing plate crystals in a magnetic field. 4. 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. 5. A general formula: Ca _{2.2 to 3.6} Na 0 to _0.8 Sr.
_0-0.8 a sintered body of Bi _{0 to 0.8} Co ₄ O ₈ complex oxide represented by _.8～9.2, characterized by having a Seebeck coefficient of more than 100 uV / K in absolute temperature 300~973K Composite oxide sintered body. 6. A general formula: Ca _{2.2 to 3.6} Na 0 to _0.8 Sr.
_0-0.8 a sintered body of Bi _{0 to 0.8} Co ₄ O ₈ complex oxide represented by _.8～9.2 composite characterized by having the following electrical resistivity 10mΩcm in absolute temperature 300~973K Oxide sintered body. 7. A general formula: Ca _{2.2 to 3.6} Na 0 to _0.8 Sr.
_0-0.8 a sintered body of Bi _{0 to 0.8} Co ₄ O ₈ complex oxide represented by _.8～9.2, and characterized by the following thermal conductivity of 3W / mK in the absolute temperature 300~973K A complex oxide sintered body. 8. A general formula: Ca _{2.2 to 3.6} Na 0 to _0.8 Sr.
_0-0.8 a sintered body of Bi _{0 to 0.8} Co ₄ O ₈ complex oxide represented by _.8～9.2, composite oxide sintered, characterized in that it has the following characteristics in absolute temperature 300~973K Body: (1) Seebeck coefficient of 100 μV / K or more, (2) Electric resistivity of 10 mΩcm or less, (3) Thermal conductivity of 3 W /
mK or less. 9. A p-type thermoelectric conversion material comprising the composite oxide sintered body according to claim 5. 10. A thermoelectric conversion module containing the p-type thermoelectric conversion material according to claim 9.