JP2003095741A - Metal oxide polycrystal, thermoelectric material, thermoelectric element and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a metal oxide polycrystal material having low electric resistance in which the c-axis of the crystal grains is oriented. SOLUTION: In the metal oxide polycrystal material, the c-axis of the metal oxide crystal grains constituting the polycrystal is oriented. The orientation degree of the c-axis by the Lotgering's method ranges from 0.7 to 1. The electric resistivity of the polycrystal at 500 deg.C is <=9 mΩcm.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a metal oxide polycrystal, a thermoelectric material containing the metal oxide polycrystal, a thermoelectric element, and a method for producing the metal oxide polycrystal.

[0002]

2. Description of the Related Art Nearly 70% of all primary energy supplied in Japan is discarded as waste heat. Effective use of this enormous amount of waste heat energy is one of the important issues in the 21st century.

There is thermoelectric power generation as a method capable of directly converting waste heat into electric energy without discharging carbon dioxide, radioactive substances and the like and without using moving parts such as a turbine. Since thermoelectric power generation directly contributes to the reduction of primary energy supply, it is a technology that is highly expected to be realized. One of the greatest advantages of thermoelectric power generation is that the scale of the waste heat source does not matter. Therefore, in thermoelectric power generation, it is expected to use various forms of heat sources such as waste heat from waste incineration plants, factories, automobiles, diesel engines, fuel cells; solar heat; catalytic heat of gas combustion.

The most important technical development task for realizing waste heat recovery by thermoelectric power generation is to develop a high-performance thermoelectric material (thermoelectric conversion material) having excellent heat resistance. Up to now, the practical application of a thermoelectric power generation system using a chalcogenide compound or an intermetallic compound has been studied. Since these materials have problems such as low heat resistance, low conversion efficiency, toxicity, and the use of rare elements, they have not been widely applied. In order to popularize thermoelectric power generation, have high conversion efficiency in a wide temperature range,
There is a need for a thermoelectric element that can be operated stably.

At present, as thermoelectric materials, oxide materials excellent in thermal and chemical stability are drawing attention. Generally, as a measure of thermoelectric conversion efficiency, the figure of merit (Z
T) is used. ZT = TS ² / ρκ [where T is absolute temperature, S is thermoelectromotive force, ρ is electrical resistivity,
κ indicates thermal conductivity] An excellent thermoelectric material is a substance having a large thermoelectromotive force (Seebeck coefficient) and a small electrical resistivity and thermal conductivity. Practical use of thermoelectric power generation requires thermoelectric materials having a ZT of more than 1.

The conventional wisdom has been that "metal oxides have low electrical conductivity, so their thermoelectric properties are low." recent years,
Pioneering research results that overturn this have been published by domestic research groups. According to this announcement, NaCo ₂ O ₄
And the single crystals of (Ca, Sr, Bi) ₂ Co ₂ O ₅ have a ZT greater than 1. In particular, the (Ca, Sr, Bi) ₂ Co ₂ O ₅ single crystal has been reported to have a ZT of more than 1 in the air at 600 ° C. or higher, and is expected for practical use of thermoelectric power generation.

The figure of merit of the above-mentioned oxide single crystal has cleared the target value for practical use, but the size of the single crystal is as small as about 1 mm or less, and the single crystal itself cannot be used as an electric heating element. .

In order to obtain an electric heating element, it is necessary to obtain a polycrystalline body having a high figure of merit. A conventional polycrystalline body is manufactured by sintering a metal oxide powder by an atmospheric pressure sintering method, a discharge plasma sintering method, or the like. However,
A polycrystalline body having a low resistance value and a high figure of merit has not been obtained. For example, regarding the polycrystal sintered body obtained by molding the powder of (Ca, Sr, Bi) ₂ Co ₂ O ₅ by die molding and sintering under normal pressure, the thermoelectric properties have been measured. Is about an order of magnitude lower than that of a single crystal.

[0009]

SUMMARY OF THE INVENTION The present invention has been made in view of the problems of the prior art, and is mainly a polycrystalline body in which the c-axis of the crystal grains is oriented and has a low electric resistance. Intended to provide the body.

[0010]

As a result of earnest research, the present inventor has found that a metal oxide polycrystal having a specific orientation can achieve the above object, and has completed the present invention.

That is, the present invention relates to the following metal oxide polycrystal, thermoelectric material, thermoelectric element, and manufacturing method. 1. The c-axis of the metal oxide crystal grains forming the polycrystal is an oriented metal oxide polycrystal, and the c-axis orientation degree by Lotgering's method is 0.7 to 1, and the polycrystal at 500 ° C. A metal oxide polycrystalline body having an electrical resistivity of 9 mΩcm or less. 2. 2. The metal oxide polycrystal as described in 1 above, wherein the metal oxide is at least one selected from the group consisting of the following compounds 1 to 4. Compound 1: Ca _xa A _ya Co ₄ O ₉ [wherein A represents Bi, Gd or Y, and 2.4 ≦ xa ≦ 3, 0 ≦ ya
≦ 0.6 and xa + ya = 3], Compound 2: Ca _xb Bi _yb B _zb Co ₄ O [wherein B represents Sr, Mg, Gd, Y, K or Na, and 2 ≦ xb
≦ 3, 0 ≦ yb ≦ 0.6, 0 <zb ≦ 1, and xb + yb + zb = 3], Compound 3: Ca _xc C _yc Co ₂ O ₅ [wherein C is Bi, Sr, Mg] , Gd, Y, K or Na, 1.
5 ≦ xc ≦ 2, 0 ≦ yc ≦ 0.5 and xc + yc = 2], and Compound 4: Ca _xd Bi _yd D _zd Co ₂ O ₅ [wherein D is Sr, Gd, Y, K or Indicates Na, 1.4 ≦ xd ≦
2, 0 ≦ yd ≦ 0.3, 0 <zd ≦ 0.3 and xd + yd + zd = 2] 3. 3. The metal oxide polycrystal as described in 1 or 2 above, wherein the figure of merit (ZT) represented by the following formula is 0.21 or more at 700 ° C. ZT = TS ² / ρκ [where T is absolute temperature (K), S is thermoelectromotive force (VK ^-1 ), ρ is electrical resistivity (Ωm), κ is thermal conductivity (Wm ^-1 K ^-1) )] 4. 1 to 3 above, in which the relative density of the polycrystal is 95% or more.
6. The metal oxide polycrystal as described in any one of 1. 5. 5. The metal oxide polycrystalline body as described in any one of 1 to 4, which has a thermoelectromotive force of 150 μVK ⁻¹ or more at 500 ° C. 6. A thermoelectric material containing the oxide polycrystal according to any one of 1 to 5 above. 7. A thermoelectric element containing the oxide polycrystal according to any one of 1 to 5 above. 8. A method for producing a metal oxide polycrystalline body, which comprises subjecting a metal oxide powder to spark plasma sintering and hot press sintering. 9. Spark plasma sintering, pressure is 20 ~ 60MPa, temperature rising rate is 10 ~ 200 ℃ / min, holding temperature is 750 ~ 1000 ℃, current is 400 ~ 800A, processing time is 1 ~ 30 minutes. Yes, and
8. The above-mentioned 8 characterized by being performed in an oxidizing atmosphere
7. The method for producing a metal oxide polycrystalline body according to. 10. The hot press sintering has a pressure of 5 to 15 MPa, a temperature rising rate of 2 to 5 ° C / min, a holding temperature of 850 to 1000 ° C, a treatment time of 30 minutes to 10 hours, and an oxidizing atmosphere. The method for producing a metal oxide polycrystal as described in 8 or 9 above, which is carried out below.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention relates to a metal oxide polycrystal in which the c-axes of the crystal grains constituting the polycrystal are oriented.

C according to the Lotgering's method of the polycrystalline body of the present invention
The degree of axial orientation (F value) is usually about 0.7 to 1. The C-axis orientation degree is defined by the following formula.

[0014]

[Formula 1]

The electrical resistivity of the polycrystalline body of the present invention is not particularly limited, but is usually about 9 mΩcm or less, preferably about 8.5 mΩcm or less, more preferably 2 to 8.5 mΩcm at 500 ° C.
It is a degree. The electrical resistivity of the polycrystalline body of the present invention is usually 1/2 to 1/4, preferably 1/3, as compared with the polycrystalline body obtained by sintering the metal oxide powder at atmospheric pressure. ~ 1/4
It will be about.

Examples of the metal oxide in the present invention include compound metal oxides such as compounds 1 to 4 shown below.

Compound 1: Ca _xa A _ya Co ₄ O ₉ [wherein A represents Bi, Gd or Y, and 2.4≤xa≤3, 0≤ya
≦ 0.6 and xa + ya = 3] In compound 1, A represents Bi, Gd or Y. As A, Bi or Gd is preferable, and Bi is more preferable. xa
Is usually about 2.4 to 3, preferably about 2.5 to 2.9, and more preferably about 2.6 to 2.8. ya is usually about 0 to 0.6,
It is preferably about 0.1 to 0.5, more preferably about 0.2 to 0.4.

Compound 2: Ca _xb Bi _yb B _zb Co ₄ O ₉ [wherein B represents Sr, Mg, Gd, Y, K or Na, and 2 ≦ xb
≦ 3, 0 ≦ yb ≦ 0.6, 0 <zb ≦ 1 and xb + yb + zb = 3] In Compound 2, B represents Sr, Mg, Gd, Y, K or Na. B is preferably Sr, K or Na, and Sr or
Na is more preferred. xb is usually about 2-3, preferably 2.
It is about 2 to 2.8, and more preferably about 2.3 to 2.7. yb
Is usually about 0 to 0.6, preferably about 0.1 to 0.5, and more preferably about 0.15 to 0.4. zb is usually about 0 to 1 (however, zb ≠ 0), preferably about 0.1 to 0.5, and more preferably
It is about 0.15 to 0.35.

Compound 3: Ca _xc C _yc Co ₂ O ₅ [wherein C represents Bi, Sr, Mg, Gd, Y, K or Na, and 1.
5 ≦ xc ≦ 2, 0 ≦ yc ≦ 0.5 and xc + yc = 2] In Compound 3, C is Bi, Sr, Mg, Gd, Y, K or N.
Indicates a. As C, Bi, Gd or Y is preferable, and Bi or Gd is more preferable. xc is usually about 1.5 to 2, preferably about 1.55 to 1.9, and more preferably about 1.55 to 1.8. yc is usually about 0 to 0.5, preferably about 0.1 to 0.45, and more preferably about 0.2 to 0.45.

Compound 4: Ca _xd Bi _yd D _zd Co ₂ O ₅ [wherein D represents Sr, Gd, Y, K or Na, and 1.4 ≦ xd ≦
2, 0 ≦ yd ≦ 0.3, 0 <zd ≦ 0.3 and xd + yd + zd = 2] In Compound 4, D represents Sr, Gd, Y, K or Na. D
Is preferably Sr, K or Na, and more preferably Sr or Na. xd is usually about 1.4 to 2, preferably 1.4 to
It is about 1.8, more preferably about 1.5 to 1.6. yd is
It is usually about 0 to 0.3, preferably about 0.2 to 0.3. zd
Is usually about 0 to 0.3 (however, zd ≠ 0), preferably 0.2 to 0.
It is about 3.

Among the complex oxides represented by the compounds 1 to 4, the complex oxide represented by the compound 1 is preferable. Among Compound 1, Ca _xa Bi _ya Co ₄ O ₉ [in the formula, 2.4 ≦ xa ≦ 3, 0
≦ ya ≦ 0.6 and xa + ya = 3] are preferable, Ca _xa Bi _ya Co ₄ O ₉ [wherein 2.5 ≦ xa ≦ 2.7, 0.3 ≦ ya ≦ 0.5 and xa + ya = 3] are more preferable, and Ca _2.6 Bi _0.4 Co ₄ O ₉ is particularly preferred.

The size of the metal oxide crystal grains constituting the polycrystalline body of the present invention is not particularly limited, but it is usually about 1 to 200 μm, preferably 3 as a measured value by electron microscope observation.
It is about 0 to 100 μm.

The relative density of the polycrystalline body of the present invention is not particularly limited, but is usually about 95% or more, preferably about 98% or more. The density of the polycrystal of the present invention is usually about 15 to 30%, preferably about 20 to 30% higher than that of the polycrystal obtained by sintering the metal oxide powder under normal pressure.
Relative density is the ratio of the measured density of the sample to the ideal density.
Means (%).

The thermoelectromotive force of the polycrystalline body of the present invention is not particularly limited, but is usually about 150 μVK ⁻¹ or more, preferably about 170 μVK ⁻¹ or more, more preferably 170 to 300 μV at 500 ° C.
It is about K ^-1 .

The figure of merit (ZT) of the polycrystalline body of the present invention is 700
At 0 ° C., it is usually about 0.21 or more, preferably about 0.21 to 0.3, more preferably about 0.24 to 0.3. The performance index is represented by the following formula.

ZT = TS²/ ρκ [Where T is absolute temperature (K), S is thermoelectromotive force (VK^-1), Ρ is electricity
Resistivity (Ωm), κ is thermal conductivity (Wm^-1K^-1)] As the crystal structure of the metal oxide crystal grains constituting the polycrystalline body
Is preferably a layered structure. For example, Ca₂Co₂O_FiveSingle crystal
Is CoO₆CoO shared by the octahedral unit₂Layer and 3 layer rock salt
Ca with type structure₂CoO₃Has a structure in which layers are stacked alternately
It (Ca, Sr, Bi)₂Co ₂O_FiveSingle crystal is Ca₂Co₂O_FiveSimilar to
In the crystal structure, a part of Ca is replaced by Sr and Bi
Have a structure.

The oxide polycrystal of the present invention is obtained by, for example, spark plasma sintering of metal oxide powder.
g) and hot press sintering and the like. The order of subjecting the spark plasma sintering to the hot press sintering is not particularly limited.

The particle diameter of the metal oxide powder as a raw material is not particularly limited, but as a measurement value by observation with an electron microscope,
It is usually about 0.5 to 10 μm, preferably about 1 to 5 μm.

The method for preparing the metal oxide powder as a raw material is
It is not particularly limited and can be obtained by a known method such as a solid phase method. The metal oxide powder can be prepared, for example, by firing a raw material capable of forming an oxide by firing. Examples of the raw material for the metal oxide powder include simple metals, metal compounds (carbonates, nitrates, hydroxides, etc.) and the like. The metal oxide powder may contain two or more kinds of metal elements. The metal oxide powder containing two or more kinds of metal elements can be prepared, for example, by firing two or more kinds of metal simple substances, metal compounds (carbonate, nitrate, hydroxide, etc.), and metal oxides. .

The firing conditions such as the firing temperature and the firing time for preparing the metal oxide powder can be appropriately set depending on the type of raw material used, the composition ratio and the like. The firing temperature is usually about 800 to 950 ° C, preferably about 850 to 900 ° C. The firing time is about 5 to 20 hours, preferably 10
~ 15 hours. The firing atmosphere is not particularly limited,
Examples thereof include the atmosphere and an oxidizing atmosphere such as an oxygen atmosphere. The firing means is not particularly limited, and any means such as an electric heating furnace, a gas heating furnace, and a light heating furnace can be adopted. In order to complete the reaction, the above calcined product may be pulverized and further calcined under the same conditions, if necessary. The metal oxide powder can be obtained by, for example, a method of pulverizing the fired product thus obtained.

In the present invention, the metal oxide powder or the sintered body obtained by subjecting the metal oxide powder to hot press sintering is subjected to spark plasma sintering. For example, the metal oxide powder as a raw material or the above-mentioned sintered body is put into a mold (for example, made of carbon, etc.) and heated and sintered by applying a pulsed DC voltage under uniaxial pressure.

The processing conditions in the spark plasma sintering can be appropriately set depending on the size of the mold used, the composition of the metal oxide powder, and the like. The pressure is usually about 20 to 60 MPa, preferably about 40 to 60 MPa. The rate of temperature increase is usually about 10 to 200 ° C / min, preferably about 50 to 150 ° C / min. The holding temperature is usually about 700 to 1100 ° C, preferably 750
It is about 1000 ℃. The current is usually about 350 to 800A, preferably about 400 to 600A. Treatment time (time to maintain a predetermined temperature) is usually about 1 to 30 minutes, preferably 2 to 1
It takes about 5 minutes. When a pulse current is applied, the peak current is usually about 400 to 800A, preferably about 500 to 600A. The pulse width is usually about 2-3 milliseconds, preferably 2.
It is about 4 to 2.6 milliseconds. The firing atmosphere is not particularly limited, and examples thereof include an oxidizing atmosphere such as the air and a vacuum atmosphere. When carbon or the like derived from a carbon mold is attached to the surface of the obtained sintered body, the surface of the sintered body may be polished and removed as necessary.

In the present invention, the metal oxide powder or the sintered body obtained by subjecting the metal oxide powder to the spark plasma sintering is hot-press sintered. For example, a metal oxide powder as a raw material or a sintered body obtained by spark plasma sintering is put in a mold (for example, made of ceramic such as alumina, made of metal, made of carbon) and heated and baked under uniaxial pressure.

The processing conditions in hot press sintering can be appropriately set according to the composition of the metal oxide powder as a raw material. The pressure is usually about 5 to 15 MPa, preferably about 8 to 12 MPa. The rate of temperature increase is usually about 2 to 10 ° C / min, preferably about 3 to 5 ° C / min. The holding temperature is usually about 850 to 1100 ° C, preferably about 900 to 1000 ° C. The treatment time (the time to maintain the predetermined temperature) is usually about 30 minutes to 10 hours, preferably about 1 to 8 hours. The firing atmosphere is not particularly limited, and examples thereof include the atmosphere and an oxidizing atmosphere such as an oxygen atmosphere.

According to the present invention, the same metal oxide powder is used for hot press sintering only, compared with a polycrystal obtained by about 30 to 50%, preferably 40 to 50% under the preferable conditions.
It is possible to provide a polycrystalline body having a high figure of merit of about%.

Since the polycrystalline body of the present invention has a high figure of merit, it is suitable as a thermoelectric material. The polycrystalline body of the present invention can be suitably used as a thermoelectric element by cutting out it into a desired shape and size as necessary, attaching electrodes, and the like.

[0037]

According to the present invention, a metal polycrystal having a high figure of merit can be obtained.

According to the method of the present invention, it is possible to manufacture an oxide thermoelectric material in which the c-axis orientation of the crystal grains forming the polycrystalline body is 0.7 or more.

The metal oxide obtained by the present invention is a polycrystal and can be obtained in a desired size, so that it can be suitably used as an electrothermal element (electrothermal power generation element).

[0040]

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to Examples of the present invention. The present invention is not limited to the examples below.

The atom sources used in the examples and comparative examples are as follows. * Bi source of bismuth oxide (Bi ₂ O ₃₎ * Sr source of strontium carbonate (SrCO ₃₎ * Ca source of calcium carbonate (CaCO ₃₎ * Co source of cobalt oxide (Co ₃ O ₄₎ * Y source of yttrium oxide (Y ₂ O ₃ ) * Gd source Gadolinium oxide (Gd ₂ O ₃ ) * Mg source Magnesium oxide (MgO) * Na source Sodium carbonate (Na ₂ CO ₃ ) * K source Potassium carbonate (K ₂ CO ₃ )

Example 1 First, a metal oxide powder was synthesized by the solid phase method. That is,
Desired metal ratio (Ca_2.6Bi _0.4Co_FourO₉) So that each atomic source
Was weighed and baked in air at 900 ° C for 12 hours
did. The obtained fired product was crushed to obtain powder. Grilled
By repeating the formation and crushing twice, Ca_2.6Bi_0.4Co_FourO
₉A powder was obtained. The particle size of the raw material powder can be determined by observing with an electron microscope.
The measured value was 2 μm. Scanning electron microscope observation
As a result, the powder shape was plate-like, reflecting the crystal structure.

The obtained Ca _2.6 Bi _0.4 Co ₄ O ₉ powder (6 g) was placed in a carbon mold (inner diameter 15 mm, outer diameter 35 mm, height 50 mm), and 5
Under uniaxial pressure of 0 MPa, a pulsed DC voltage (current: 500 A, peak current: 500 A, pulse width: 2.5 ms) was applied to perform spark plasma sintering. The temperature rising rate was 100 ° C./min, the holding temperature was 800 ° C., and the processing time (holding time) was about 5 minutes. Since carbon was attached to the surface of the obtained sintered body, the surface of the sintered body was polished to remove it.

Next, the spark plasma sintered body was further subjected to hot pressing. The spark plasma sintered body was placed in an alumina mold (inner diameter 20 mm, outer diameter 50 mm, height 50 mm), and this was sintered in the atmosphere under uniaxial pressure of 10 MPa. Temperature rising rate in hot press sintering is 4 ℃ / min, holding temperature is 950 ℃,
The processing time (holding time) was 2 hours.

The thermoelectric figure of merit at 700 ° C. of the obtained sample was ZT = 0.29, and the sample only subjected to spark plasma sintering described later (Comparative Example 1) or the sample only subjected to hot press treatment (Comparative Example 2). The figure of merit was improved by about 40%.

Examples 2 to 6 and Examples 8 to 9 Each atomic source was weighed so as to have the composition shown in Table 1, and the processing conditions of spark plasma sintering and hot press sintering are shown in Table 1. A polycrystalline sintered body having a composition ratio as shown in Table 1 was manufactured according to the method of Example 1 except that the above conditions were used. Table 1 shows the characteristics such as the figure of merit of each obtained polycrystal.

[0047]

[Table 1]

Comparative Example 1 A polycrystal was produced in the same manner as in Example 1 except that hot press sintering was not performed. That is, a polycrystal was obtained by subjecting the metal oxide powder obtained in the same manner as in Example 1 to spark plasma sintering only under the same conditions as in Example 1. Table 1 shows properties such as a figure of merit of the obtained polycrystal.

Comparative Example 2 A polycrystalline body was produced in the same manner as in Example 1 except that spark plasma sintering was not performed. That is, the metal oxide powder obtained in the same manner as in Example 1 was subjected only to hot press sintering under the same conditions as in Example 1 to obtain a polycrystalline body. Table 1 shows properties such as a figure of merit of the obtained polycrystal.

In order to clarify the influence of the above manufacturing process on the thermoelectric properties, as an example, Ca _2.6 Bi _0.4 Co ₄ O ₉
Thermoelectric properties of a sample (SPS-HP: Example 1) hot-pressed after spark-plasma sintering the powder (SPS: Comparative Example 1) only a sample subjected to spark plasma sintering, and a sample only hot-pressed (SPS-HP: Example 1). HP: It was compared with the thermoelectric characteristics of Comparative Example 2). That is, the SPS sample and the HP sample are the sintered body samples prepared by using the same metal oxide powder as the SPS-HP sample and only by the spark plasma sintering or the hot press treatment. The temperature dependence of the electrical resistivity of these samples is shown in Figure 1, the temperature dependence of the thermoelectromotive force in Figure 2, the temperature dependence of the thermal conductivity in Figure 3, and the temperature dependence of the figure of merit (ZT value). As shown in FIG.
The thermoelectromotive force and the thermal conductivity do not differ greatly among the above three samples. However, the electrical resistivity of the SPS-HP sample is the smallest, so the figure of merit is improved (Fig. 4).

C of each sample evaluated by Lotgering's method
The degree of axial orientation (F value) is F = 0.71 for SPS-HP samples and F = 0.5 for HP samples.
8, F = 0 in SPS sample.

Example 7 Spark plasma sintering was carried out after hot pressing.
A polycrystalline body was produced in the same manner as in Example 1 except that the treatment time by hot pressing was 4 hours.

Example 1 was also performed on a sample obtained by simply reversing the order of hot press sintering and spark plasma sintering.
It was confirmed that the sample showed the same thermoelectric properties as the sample obtained in. It was found that the order of spark plasma sintering and hot pressing does not significantly affect the thermoelectric properties.

[Brief description of drawings]

FIG. 1 is the temperature dependence of the electrical resistivity of the polycrystalline bodies obtained in Example 1 and Comparative Examples 1 and 2.

FIG. 2 is a temperature dependence of thermoelectromotive force of the polycrystalline bodies obtained in Example 1 and Comparative Examples 1 and 2.

FIG. 3 is the temperature dependence of the thermal conductivity of the polycrystalline bodies obtained in Example 1 and Comparative Examples 1 and 2.

FIG. 4 is temperature dependence of thermoelectric figure of merit (ZT) of the polycrystalline bodies obtained in Example 1 and Comparative Examples 1 and 2.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) H02N 11/00 C04B 35/00 JF term (reference) 4G030 AA03 AA04 AA07 AA08 AA09 AA11 AA12 AA28 AA43 BA01 BA21 CA02 GA23 GA29 4G075 AA23 AA27 BA02 BB10 CA02 CA15 CA47 CA62 DA18 FB04 FC11

Claims (10)

[Claims] 1. The c of metal oxide crystal grains constituting a polycrystalline body
A metal oxide polycrystal having an oriented axis, the Lotgerin
c-axis orientation degree by g's method is 0.7-1 and
A metal oxide polycrystal having an electrical resistivity at 0 ° C. of 9 mΩcm or less. 2. The metal oxide polycrystal according to claim 1, wherein the metal oxide is at least one selected from the group consisting of the following compounds 1 to 4. Compound 1: Ca _xa A _ya Co ₄ O ₉ [wherein A represents Bi, Gd or Y, and 2.4 ≦ xa ≦ 3, 0 ≦ ya
≦ 0.6 and xa + ya = 3], Compound 2: Ca _xb Bi _yb B _zb Co ₄ O ₉ [wherein B represents Sr, Mg, Gd, Y, K or Na, and 2 ≦ xb
≦ 3, 0 ≦ yb ≦ 0.6, 0 <zb ≦ 1 and xb + yb + zb = 3], Compound 3: Ca _xc C _yc Co ₂ O ₅ [wherein C is Bi, Sr, Mg, Indicates Gd, Y, K or Na, 1.
5 ≦ xc ≦ 2, 0 ≦ yc ≦ 0.5 and xc + yc = 2], and Compound 4: Ca _xd Bi _yd D _zd Co ₂ O ₅ [wherein D is Sr, Gd, Y, K or Indicates Na, 1.4 ≦ xd ≦
2, 0 ≦ yd ≦ 0.3, 0 <zd ≦ 0.3 and xd + yd + zd = 2] 3. A figure of merit (ZT) represented by the following formula is 700
The metal oxide polycrystal according to claim 1 or 2, which has a temperature of 0.21 or more at ° C. ZT = TS ² / ρκ [where T is absolute temperature (K), S is thermoelectromotive force (VK ^-1 ), ρ is electrical resistivity (Ωm), κ is thermal conductivity (Wm ^-1 K ^-1) )] 4. The metal oxide polycrystal as claimed in claim 1, wherein the polycrystal has a relative density of 95% or more. 5. The metal oxide polycrystalline body according to claim 1, which has a thermoelectromotive force of 150 μVK ⁻¹ or more at 500 ° C. 6. A thermoelectric material containing the oxide polycrystal according to any one of claims 1 to 5. 7. A thermoelectric device containing the oxide polycrystal according to claim 1. 8. A method for producing a metal oxide polycrystal, wherein the metal oxide powder is subjected to spark plasma sintering and hot press sintering. 9. Spark plasma sintering, the pressure is 20 ~ 60MP
a, temperature rising rate is 10 ~ 200 ℃ / min, holding temperature is 750 ~ 1000
The method for producing a metal oxide polycrystal according to claim 8, wherein the method is carried out in an oxidizing atmosphere at a temperature of 400C for 800 to 800A for a treatment time of 1 to 30 minutes. 10. Hot press sintering is performed at a pressure of 5 to 15
MPa, temperature rising rate is 2 to 5 ° C / min, holding temperature is 850 to 1000
The method for producing a metal oxide polycrystal according to claim 8 or 9, wherein the treatment is carried out in an oxidizing atmosphere for 30 minutes to 10 hours at 30 ° C.