JP2012028504A - CaMnO3 TYPE THERMOELECTRIC CONVERSION MATERIAL AND METHOD FOR PRODUCING THE SAME - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a CaMnOtype thermoelectric conversion material having excellent performance as a thermoelectric conversion material, and a method for producing the CaMnOtype thermoelectric conversion material.SOLUTION: There are provided: a CaMnOtype thermoelectric conversion material having a pore structure therein and satisfying the following (1), (2) and (3) conditions; and a method for producing the CaMnOtype thermoelectric conversion material. (1) A crystallite diameter is in a range of 300 to 550 Å. (2) A relative density is in a range of 60 to 80%. (3) The number of pores existing per unit area (100 μm) is 40-60 in a photograph of a cross-section of the CaMnOtype thermoelectric conversion material. Among the pores, pores having the longest diameter (L) in a range of 300 to 1,600 nm account for 80-100%, pores having the longest diameter (L) of less than 300 nm account for less than 8%, and pores having the longest diameter (L) of more than 1,600 nm account for less than 12%.

Description

The present invention relates to a CaMnO ₃ type thermoelectric conversion material having excellent performance as a thermoelectric conversion material and a method for producing the same.

In conventional power generation systems, heat energy has been converted into kinetic energy, and the kinetic energy has been converted into electrical energy. Of these, there is a thermodynamic limit when converting heat into power, the available energy is reduced, and in order to increase its efficiency, it must be converted at a high temperature of at least 400 ° C. .
Thus, power generation by thermoelectric conversion that directly converts thermal energy into electrical energy or conversely, electrical energy into thermal energy is expected using the Peltier effect or Seebeck effect. Electric power generation by thermoelectric conversion is attracting attention because it enables effective use of exhaust heat and geothermal heat.

This thermoelectric conversion material is roughly classified into two types, n-type and p-type. A thermoelectric conversion element can be obtained by alternately arranging and connecting n-type thermoelectric conversion materials and p-type thermoelectric conversion materials so as to be electrically in series and thermally parallel. If a temperature difference is given between both surfaces of the thermoelectric conversion element, power generation can be performed. The performance of the thermoelectric conversion material can generally be evaluated by a dimensionless figure of merit (ZT). Here, the dimensionless figure of merit (ZT) is expressed by the formula: ZT = (S ² × σ × T) from Seebeck coefficient (S), electrical conductivity (σ), thermal conductivity (κ), and absolute temperature (T). ) / Κ.

  As thermoelectric conversion materials, for example, materials such as Pb—Te, Bi—Te, Si—Ge, and oxide ceramics are known. Among these, Pb-Te-based and Bi-Te-based compound semiconductors have excellent thermoelectric properties near room temperature and in the intermediate temperature range of 300 to 500 ° C, respectively, but are used in high temperature ranges exceeding 500 ° C. Not suitable for. Further, there is a problem that the material may contain an expensive and harmful element (for example, Te, Sb, Se, etc.). Si-Ge compound semiconductors have excellent thermoelectric properties at high temperatures and are characterized by not containing environmentally hazardous substances in the material, but have a problem of low heat resistance.

  On the other hand, oxide ceramic-based thermoelectric conversion materials do not necessarily contain expensive and harmful elements. Moreover, it is excellent also in heat resistance durability. Therefore, oxide ceramic-based thermoelectric conversion materials are attracting attention as materials that can replace compound semiconductors, and various proposals have been made for new materials having high thermoelectric properties and methods for producing the same.

  For example, Patent Document 1 discloses an electronic cooling element using, as an n-type thermoelectric conversion material, an oxide semiconductor containing a composite oxide containing strontium and titanium as a main component and having a predetermined amount of oxygen vacancies. .

  In Patent Document 2, the composition ratio and the constituent crystal phase (perovskite crystal structure) of the composite oxide composed of strontium oxide and titanium oxide or strontium oxide, barium oxide and titanium oxide are set in a specific range. A thermoelectric conversion material characterized by this is disclosed.

Patent Document 3 has a perovskite (ABO ₃ ) structure, the composition of which is represented by the general formula: (AE _1-y M1 _y ) (Mn _1-z M2 _z ) O _3-δ (where AE is 2 More than one kind of alkaline earth metal element, M1 is a rare earth element and one or more elements of Bi, Sn, Sb, In and Pb, M2 is an element of Ru, Nb, Mo, W and Ta 1 or two or more elements, 0 ≦ y ≦ 0.5, 0 ≦ z ≦ 0.5, y + z> 0, −0.5 ≦ δ ≦ + 0.5) In addition, a thermoelectric conversion material having a tolerance factor of 0.7 to 1.0 is disclosed.

Patent Document 4 discloses a perovskite-type calcium manganese oxide represented by composition formula (1): Ca _v M ¹ _w Mn _x M ² _y O _z , or composition formula (2): Ca _a M ³ _b. A layered perovskite type calcium manganese oxide represented by Mn _c M ⁴ _d O _e and selected from the group consisting of Ru, Rh, Pd, Ag, Re, Os, Ir, Pt and Au dispersed in the oxide An oxide composite containing at least one kind of metal fine particles, wherein the metal fine particles have an average particle size of 0.1 to 10 μm, and the content of the metal fine particles is 1 on the basis of the whole oxide composite. An oxide composite characterized in that it is ˜50% by weight and the density of the oxide composite is 80% or more of the theoretical density is disclosed. The oxide composite is a material having a high density and low electrical resistivity, a negative Seebeck coefficient having a large absolute value, and a large power factor that is an index representing the electrical characteristics of the thermoelectric material. It is a material with excellent thermoelectric conversion performance. Moreover, since it is a material having high strength and excellent workability, it is said that it can be processed with a high yield and is not easily damaged during use.

Japanese Patent Laid-Open No. 5-198847 JP-A-8-231223 JP 2003-142742 A JP 2009-117449 A

However, conventional oxide ceramic thermoelectric conversion materials generally replace heavy metals such as Bi, V, and In in the crystal lattice in order to obtain high performance. As a result, expensive and rare elements are used. This is not preferable.
Accordingly, the present invention provides a thermoelectric conversion material composed of calcium manganese oxide (hereinafter, also referred to as CaMnO ₃ type thermoelectric conversion material), without causing the substituted into the crystal lattice of the expensive and rare element, CaMnO ₃ type thermoelectric conversion It is an object of the present invention to provide a CaMnO ₃ type thermoelectric conversion material having improved material performance, for example, electrical conductivity, Seebeck coefficient, and thermal conductivity, and a method for producing the same.

As a result of intensive studies to solve the above-mentioned problems, the present inventors have found that a CaMnO ₃ type thermoelectric conversion material having specific conditions can achieve the above object, and have completed the present invention.

That is, the subject of this invention is achieved by following (1)-(4).
(1) A CaMnO ₃ type thermoelectric conversion material having a pore structure inside and satisfying the following conditions 1), 2) and 3).
1) The crystallite diameter is in the range of 300 to 550 mm.
2) The relative density is in the range of 60 to 80%.
3) In the cross-sectional photograph of the CaMnO ₃ type thermoelectric conversion material, the number of pores existing per unit area (100 μm ² ) is 40-60, and the longest diameter (L) is in the range of 300-1600 nm. The pores are 80 to 100%, the longest diameter (L) is less than 300 nm, less than 8%, and the longest diameter (L) is more than 1600 nm.
(2) CaMnO according to (1) above, wherein the average value of the aspect ratio of the pores having the longest diameter (L) in the range of 300 to 1600 nm is in the range of 2.0 to 4.0. Type ₃ thermoelectric conversion material.
(3) The above-mentioned (1) or (1), wherein the average value of the distance between the hole having the longest diameter (L) in the range of 300 to 1600 nm and the closest hole is in the range of 300 to 500 nm CaMnO ₃ type thermoelectric conversion material as described in 2).
(4) The method for producing a CaMnO ₃ type thermoelectric conversion material according to any one of (1) to (3), comprising the following steps (I) to (VI):
(I) A step of preparing a slurry having an average particle size of 0.3 μm or less by wet-grinding a calcium compound and a manganese compound (II) A step of drying the slurry obtained in the step (I) to prepare a powder (III) Step of pre-baking the powder obtained in the step (II) at 500 to 1000 ° C. to prepare the pre-fired powder (IV) Wet the pre-fired powder obtained in the step (III) A step of pulverizing to prepare a slurry having an average particle size of 0.1 to 3.0 μm (V) A step of drying the slurry obtained in the step (IV) to obtain CaMnO ₃ particles having an average particle size of 5 to 30 μm ( VI) A step of forming the CaMnO ₃ particles obtained in the step (V) and firing at 950 to 1200 ° C. to obtain a CaMnO ₃ sintered body.

According to the present invention, without thereby replacing the expensive and rare element in the crystal lattice, the performance of CaMnO ₃ type thermoelectric conversion material, for example, electrical conductivity, Seebeck coefficient, CaMnO ₃ type thermoelectric thermal conductivity is improved A conversion material and a method for producing the same can be provided.

2 is an FE-SEM photograph of the CaMnO ₃ type thermoelectric conversion material of Example 1. FIG. _{3 is} an FE-SEM photograph of a CaMnO ₃ type thermoelectric conversion material of Comparative Example 1. 4 is an FE-SEM photograph of a CaMnO ₃ type thermoelectric conversion material of Comparative Example 2.

(CaMnO ₃ type thermoelectric conversion material)
The CaMnO ₃ type thermoelectric conversion material according to the present invention is made of a CaMnO ₃ sintered body, has a pore structure inside, and satisfies the following conditions 1), 2) and 3). .
1) The crystallite diameter is in the range of 300 to 550 mm.
2) The relative density is in the range of 60 to 80%.
3) In the cross-sectional photograph of the CaMnO ₃ type thermoelectric conversion material, the number of pores existing per unit area (100 μm ² ) is 40-60, and the longest diameter (L) is in the range of 300-1600 nm. The pores are 80 to 100%, the longest diameter (L) is less than 300 nm, less than 8%, and the longest diameter (L) is more than 1600 nm.
Here, the longest diameter (L) of the hole is a line segment connecting two points on the outer edge of the cross-sectional structure of the hole existing on the cross-sectional photograph (note that the line segment is only inside the outer edge of the hole. Means the one with the longest length.

The CaMnO ₃ sintered body is formed of a metal composite oxide containing CaMnO ₃ as a main component and is represented by a general formula CaMnO _3-x .

In the present invention, the CaMnO ₃ sintered body has a crystallite diameter of 300 to 550 mm, preferably 350 to 500 mm, and most preferably 460 to 500 mm. When the crystallite diameter is 300 mm or more, high electrical conductivity and Seebeck coefficient can be maintained, and when it is 550 mm or less, the mechanical strength of the sintered body can be increased.

The relative density of CaMnO ₃ sintered body is 60-80%, preferably in the range of 65% to 80%, and most preferably in the range of 70-80%. When the relative density is in the above range, the denseness is increased, so that the electrical conductivity is improved and the increase in the thermal conductivity can be suppressed.

In the present invention, in the cross-sectional photograph of the CaMnO ₃ type thermoelectric conversion material, the number of holes present per unit area (100 μm ² ) is 40 to 60, preferably 46 to 57. Among the pores, the number ratio of holes having the longest diameter (L) in the range of 300 to 1600 nm is 80 to 100%, the number ratio of holes having the longest diameter (L) of less than 300 nm is less than 8%, The ratio of the number of holes whose major axis (L) exceeds 1600 nm is less than 12%. The ratio of the number of holes having the longest diameter (L) in the range of 300 to 1600 nm is preferably 90 to 95%, and the ratio of the number of holes having the longest diameter (L) of less than 300 nm is less than 6%. It is preferable that the number ratio of the holes having the longest diameter (L) exceeding 1600 nm is less than 8%. By setting it as the said range, the raise of heat conductivity can be suppressed, without reducing electrical conductivity remarkably.

With the above configuration, a CaMnO ₃ type thermoelectric conversion material with improved electrical conductivity, Seebeck coefficient, and thermal conductivity can be obtained without replacing expensive and rare elements in the crystal lattice.

In the CaMnO ₃ type thermoelectric conversion material, the average value of the aspect ratio of the pores having the longest diameter (L) in the range of 300 to 1600 nm is preferably in the range of 2.0 to 4.0. When the average value of the aspect ratio is within this range, most of the hole structures in the cross-sectional photograph have a non-circular structure including an ellipse. Although the mechanism of action is not clear, an increase in thermal conductivity can be suppressed without significantly reducing electrical conductivity in the case of a non-circular structure compared to a case where the hole cross section has a circular structure.

In the CaMnO ₃ type thermoelectric conversion material, it is preferable that the average value of the distance between the hole having the longest diameter (L) in the range of 300 to 1600 nm and the closest hole is in the range of 300 to 500 nm. When the average distance is less than 300 nm, it is difficult to suppress an increase in thermal conductivity. Moreover, when this average distance exceeds 500 nm, although heat conductivity can be suppressed, the tendency for electrical conductivity to fall becomes strong.

(Method for producing CaMnO ₃ type thermoelectric conversion material)
Next, a method for producing a CaMnO ₃ type thermoelectric conversion material will be described.
The method for producing a CaMnO ₃ type thermoelectric conversion material according to the present invention comprises (I) a step of wet-pulverizing a calcium compound and a manganese compound to prepare a slurry having an average particle size of 0.3 μm or less, (II) the step (I ) Drying the slurry obtained in step (ii) to prepare powder, (III) step of pre-baking the powder obtained in step (II) at 500 to 1000 ° C. to prepare a pre-baked powder, (IV) A step of preparing a slurry having an average particle size of 0.1 to 3.0 μm by wet-grinding the temporarily fired powder obtained in the step (III), and (V) obtained in the step (IV). The slurry is dried to obtain CaMnO ₃ particles having an average particle diameter of 5 to 30 μm. (VI) The CaMnO ₃ particles obtained in the step (V) are formed and fired at 950 to 1200 ° C. to be sintered with CaMnO _3. Obtaining a body.

<Process (I)>
In the method for producing a CaMnO ₃ type thermoelectric conversion material of the present invention, first, a calcium compound and a manganese compound are dispersed in a solvent and wet pulverized to prepare a slurry having an average particle size of 0.3 μm or less as a precursor.
The calcium compound used in the present invention is not particularly limited. For example, calcium carbonate (CaCO ₃ ), calcium nitrate (Ca (NO ₃ ) ₂ ), calcium oxalate (CaC ₂ O ₄ ), calcium acetate ( Ca (CH ₃ COO) ₂ ), calcium hydroxide (Ca (OH) ₂ ), calcium oxide (CaO), metal alkoxide, single metal, etc., may be used alone or in combination of two or more May be used.
The manganese compound used in the present invention is not particularly limited. For example, manganese carbonate (MnCO ₃ ), manganese nitrate (Mn (NO ₃ ) ₂ ), manganese oxalate (MnC ₂ O ₄ ), manganese acetate ( Mn (CH ₃ COO) ₂ ), manganese oxide (MnO, MnO ₂ , Mn ₂ O ₃ , Mn ₃ O ₄ ), metal alkoxide, metal carrier and the like may be mentioned, and one kind may be used alone or two or more kinds May be used in combination.

  The compounding ratio of the calcium compound and the manganese compound is preferably 0.98 to 1.02 in terms of the molar ratio of calcium to manganese (Ca / Mn), and more preferably 0.99 to 1.01. It is preferable that the amount of the calcium compound and the manganese compound is within the above range because by-products can be suppressed.

As the solvent for dispersing the calcium compound and the manganese compound, an aqueous solvent can be suitably used. Examples of the aqueous solvent include ion-exchanged water, distilled water, purified water, ultrapure water, and tap water. Among them, ion-exchanged water, distilled water, and ultrapure water are used because they do not contain impurity elements. It is preferred to use.
Moreover, when the total amount of the calcium compound and the manganese compound is 100 parts by mass, the amount of the solvent is preferably 233 to 1900 parts by mass, more preferably 300 to 900 parts by mass, and 567 to 900 parts by mass. Part is particularly preferred. A blending amount of the solvent within the above range is preferable because it can be pulverized and dried with good yield.

As the wet pulverization, for example, a wet bead mill, a ball mill, an attritor, a vibration mill or the like is preferably used, and a wet bead mill is preferably used from the viewpoint of pulverization efficiency.
In the step (I), the average particle size (precursor average particle size) of the calcium compound and the manganese compound in the slurry obtained by wet pulverization is 0.3 μm or less. When the average particle diameter of the precursor in the slurry is 0.3 μm or less, the raw material powder is uniformly pulverized and mixed.
The method for controlling the average particle size of the precursor during pulverization by wet pulverization includes, for example, a method of adjusting the pulverization time, a method of changing the particle size of a pulverizing medium such as beads, a method of adjusting pulverization energy, A method of diluting the partial concentration, a method of combining these, or the like can be employed. The time required for the wet pulverization is not generally described because of the influence of the production scale, but usually a range of 1 to 10 hours is recommended. More preferably, a range of 3 to 7 hours is recommended.

<Process (II)>
Next, the slurry obtained in the step (I) is dried to prepare a powder.
Although there is no restriction | limiting in particular as a drying method of a slurry, For example, the method of putting a slurry in a container and heating and drying, the method of drying with a spray dryer, the method of drying with a slurry dryer, etc. are mentioned.
Although there is no restriction | limiting in particular as drying temperature, 100-250 degreeC is preferable and what is necessary is just to continue a drying process until it will be in a state whose water concentration is 5% or less.

<Process (III)>
Next, the powder obtained in the step (II) is temporarily fired to prepare a temporarily fired powder.
This calcination is performed for the purpose of thermally decomposing the raw material to obtain CaMnO ₃ . The temperature of temporary baking is 500-1000 degreeC, 600-900 degreeC is preferable and 750-850 degreeC is more preferable. When the calcination temperature is 500 ° C. or higher, CaMnO ₃ particle growth is likely to occur due to decomposition of the raw material, and when the calcination temperature is 1000 ° C. or less, CaMnO ₃ particle growth can be moderately suppressed.
The calcination time is preferably 2 to 8 hours, more preferably 3 to 7 hours, and particularly preferably 4 to 6 hours. When the pre-baking time is 2 hours or longer, the decomposition of the raw material proceeds sufficiently, and when it is 8 hours or shorter, the CaMnO ₃ particle growth can be moderately suppressed.

<Process (IV)>
Next, a solvent is added to the calcined powder obtained in the step (III) and wet pulverized to prepare a slurry having an average particle size of 0.1 to 3.0 μm.
As the solvent used in step (IV), an aqueous solvent can be suitably used. Examples of the aqueous solvent include ion-exchanged water, distilled water, purified water, ultrapure water, and tap water. Among them, ion-exchanged water, distilled water, and ultrapure water are used because they do not contain impurity elements. It is preferred to use.
The blending amount of the solvent is preferably 200 to 1900 parts by weight, more preferably 300 to 900 parts by weight, and more preferably 500 to 900 parts by weight with respect to 100 parts by weight of the temporarily fired powder obtained in the step (III). Part is particularly preferred. A blending amount of the solvent within the above range is preferable because it can be pulverized and dried with good yield.

In step (IV), it is preferable to add a binder in order to improve moldability. Examples of such a binder include polyvinyl alcohol, acrylic resin, methyl cellulose, and the like. Among them, it is preferable to use polyvinyl alcohol because of its high adhesiveness to inorganic substances.
The binder is preferably 1 to 10 parts by mass, more preferably 2 to 8 parts by mass, and particularly preferably 3 to 5 parts by mass with respect to 100 parts by mass of the calcined powder. It is preferable for the amount of binder added to be in the above range since the moldability is improved.

As the wet pulverization method, for example, a wet bead mill, a ball mill, an attritor, a vibration mill or the like is preferably used, and a wet bead mill is preferably used from the viewpoint of pulverization efficiency.
In the step (IV), the average particle size of the temporarily fired powder in the slurry obtained by wet pulverization is 0.1 to 3.0 μm, preferably 0.5 to 2.5 μm, 0.8 More preferably, it is -1.5 micrometers. When the average particle size of the temporarily fired powder in the slurry is within the above range, the mechanical strength of the molded body after sintering can be increased.
The method for controlling the average particle size during pulverization by wet pulverization includes, for example, a method for adjusting the pulverization time, a method for changing the particle size of a pulverizing medium such as beads, a method for adjusting the pulverization energy, and a solid content concentration. It is possible to adopt a method of thinning, a method of combining these, and the like.

<Process (V)>
Next, the slurry obtained in the step (IV) is dried to obtain CaMnO ₃ particles.
As a method for drying the slurry, a known drying method can be appropriately selected and employed, and examples thereof include spray drying, vacuum drying, and drum drying. Of these, spray drying is preferred because the particles are spherical.
In the case of spray drying, the slurry obtained in the step (IV) is introduced into a spray dryer. As conditions, the spray dryer inlet temperature is 200 to 250 ° C, preferably 210 to 240 ° C, particularly preferably 220 to 230 ° C, and the outlet temperature is 100 to 150 ° C, preferably 110 to 140 ° C, particularly preferably. Is in the range of 110-120 ° C. By setting the inlet temperature and the outlet temperature in the above ranges, drying can be performed with good yield.

Thus, the average particle diameter of the CaMnO ₃ particles prepared in the step (V) is 5 to 30 μm, preferably 10 to 20 μm, particularly preferably 12 to 18 μm.

<Process (VI)>
Finally, the CaMnO ₃ particles obtained in the step (V) are formed and fired to obtain a CaMnO ₃ sintered body.
Although it does not restrict | limit especially as a method of shape | molding a molded object, For example, the method of uniaxial pressure molding, the method of isotropic pressure molding, the method of injection molding, the method of sheet forming, etc. are mentioned.

The firing temperature is 950 to 1200 ° C, preferably 1000 to 1150 ° C, and more preferably 1050 to 1100 ° C. When the firing temperature is 950 ° C. or higher, CaMnO ₃ particles can be sufficiently sintered, and when the firing temperature is 1200 ° C. or lower, the internal stress of the CaMnO ₃ sintered body is relaxed and the mechanical strength is increased. be able to.

By the above steps (I) to (VI), CaMnO ₃ particles having a controlled size are appropriately sintered, whereby non-spherical pores are formed inside the CaMnO ₃ sintered body. And the CaMnO ₃ sintered body 1) The crystallite diameter of the sintered body is 300 to 550 mm, 2) The relative density of the sintered body is 60 to 80%, and 3) The cross section of the CaMnO ₃ type thermoelectric conversion material In the photograph, the number of pores existing per unit area (100 μm ² ) is 40-60, of which the longest diameter (L) is in the range of 300-1600 nm, 80-100%, and the longest diameter (L ) Is less than 8% of vacancies less than 300 nm, and less than 12% of vacancies having a longest diameter (L) exceeding 1600 nm.
Further, the average value of the aspect ratio of the pores having the longest diameter (L) in the range of 300 to 1600 nm is in the range of 2.0 to 4.0, and the longest diameter (L) is in the range of 300 to 1600 nm. It becomes CaMnO ₃ type thermoelectric conversion material in which the average value of the distance between a certain vacancy and the nearest vacancy is in the range of 300 to 500 nm, without replacing expensive and rare elements in the crystal lattice, electrical conductivity, The Seebeck coefficient and thermal conductivity are improved.

  EXAMPLES Hereinafter, although an Example and a comparative example further demonstrate this invention, this invention is not restrict | limited to the following example.

The measurement methods and analysis methods employed in the examples and comparative examples are as follows.
[Average particle size]
Ion exchange water was added to a laser diffraction / scattering particle size distribution analyzer (Horiba, Ltd .: LA-700), and the sample was added so that the transmittance was 75 to 95%. Thereafter, about 20 ml of a 2 wt% sodium hexametaphosphate aqueous solution was added and subjected to ultrasonic dispersion for 1 minute, and the average particle size was measured.

[Crystallite diameter]
Using a high-temperature X-ray diffractometer (manufactured by Rigaku Corporation), the crystallite diameter is determined from the diffraction peak attributed to the (112) plane of the CaMnO ₃ X-ray diffraction pattern obtained by measurement under conditions of room temperature and atmosphere. Was calculated.

[Relative density]
Sintered density (measured density: .rho.1) of CaMnO ₃ sintered body, first, cut with precision cutting device CaMnO ₃ sintered body (KK maltose), the CaMnO ₃ sintered to 120 ° C., It dried with the dryer for 16 hours. The dimensions of the CaMnO ₃ sintered body were measured with a micrometer, the weight was weighed with an electronic balance, and the weight / dimension was calculated. The theoretical density (ρ2) J. et al. J. et al. Phys. chem. The calculated density of CaMnO ₃ described in solids 67 (2006) 1595, 4.577 g / cm ^3, was adopted and converted into a ratio (%) of (ρ1) / (ρ2) × 100 to obtain the relative density.

[Seebeck coefficient and electrical conductivity]
Using a thermoelectric property measuring apparatus (Ozawa Science Co., Ltd.), the Seebeck coefficient (μV / K) and electrical conductivity (S / cm) at 800 ° C. in the air atmosphere were determined.

[Thermal conductivity]
The thermal conductivity was calculated by the laser flash method. The thermal conductivity (W / mK) at 800 ° C. in a vacuum atmosphere was measured with a thermal conductivity measuring device (manufactured by ULVAC-RIKO).

[Dimensionless figure of merit]
The dimensionless figure of merit (ZT) was determined by the following formula from the Seebeck coefficient (S), electrical conductivity (σ), thermal conductivity (κ), and absolute temperature (T).
ZT = (S ² × σ × T) / κ

[Number of holes]
The number of pores per unit area (100 μm ² ) is obtained by ion-polishing a CaMnO ₃ sintered body with a cross section polisher (manufactured by JEOL Ltd.) and then by a field emission electron micrograph (magnification 10,000 times). In the photographic projections obtained by photographing, the number of vacancies existing in a region of 10 μm length × 10 μm width was measured. The measurement of the number of holes was performed for 10 arbitrary regions, and the average value was defined as the number of holes per unit area (100 μm ² ).

[Longest diameter of holes (L) and number ratio of holes (%)]
When measuring the number of holes, the longest diameter (L) of each hole is measured, and the number of holes, the number of holes having a longest diameter in the range of 300 to 1600 nm, and the number of holes having a longest diameter of less than 300 nm are measured. In addition, the number of holes exceeding the longest diameter of 1600 nm was determined. The measurement of the longest diameter of the holes is performed at 10 arbitrary regions in the same manner as in the case of the number of holes, and the number of holes corresponding to each longest diameter range is totaled to obtain the void corresponding to each longest diameter range. The ratio of the number of holes was calculated.

[Average aspect ratio]
In the present application, the aspect ratio was determined as follows.
1) In the above-mentioned 10 photographic projection views, a hole having a longest diameter in the range of 300 to 1600 nm is selected, a long axis (A) corresponding to the longest diameter is obtained, and the long axis (A) and the hole Two intersection points (intersection point x, intersection point y) with the outer edge are determined, and a distance (S) between xy is measured.
2) In the long axis (A), an intermediate point (z) for dividing the distance between xy into two equal parts is determined.
3) A straight line (B) perpendicular to the major axis (A) is obtained at the intermediate point (z), two intersection points (intersection u, intersection w) between the straight line (B) and the outer edge of the hole are determined, and u− Measure the distance (T) between w.
4) Obtain the value of (S) / (T).
5) For all the pores having the longest diameter (L) in the range of 300 to 1600 nm, the method of 1) to 4) is applied to obtain the value of (S) / (T), and the average value is calculated as the aspect ratio. And

[Distance to adjacent holes (R)]
In each of the ten photo projections, select a hole having the longest diameter in the range of 300 to 1600 nm, measure the distance to the hole that is present at the closest position, find the average value, It was set as the distance (R) with the vacant hole.
Note that the distance between the holes means the shortest distance between the holes. In addition, regarding the holes confirmed to exist at the shortest distance from other holes, the distance (R) is not measured again.

[Example 1]
<Process (I)>
A slurry was prepared by adding 174.97 g of calcium carbonate (CaCO ₃ ) and 217.28 g of manganese carbonate (MnCO ₃ · nH ₂ O) to 2222.7 g of ion-exchanged water, followed by stirring. This slurry was pulverized with a circulation type wet pulverizer (manufactured by Ashizawa Finetech Co., Ltd., trade name: LABSTAR) filled with 430 ml of ZrO ₂ beads having a bead diameter of 0.5 mm, with a peripheral speed of 2480 rpm and a circulation rate of 1 L / min. It grind | pulverized on the conditions for 5 hours. The average particle size of the solid content in the slurry after pulverization was 0.26 μm.
<Process (II)>
The slurry obtained from the circulation type wet pulverizer is collected, and the inlet temperature is 230 ° C., the outlet temperature is 110 ° C., and the atomizer rotational speed is obtained with a spray granulation dryer (trade name: LB-8 type, manufactured by Okawara Kako Co., Ltd.). It dried on condition of 30000 rpm and obtained powder.
<Process (III)>
The powder obtained in the step (II) was calcined at 800 ° C. for 5 hours in an air atmosphere using an electric furnace (trade name: KBF521N1 manufactured by Koyo Thermo Systems Co., Ltd.). Prepared.
<Process (IV)>
135 g of the pre-fired powder obtained in step (III) and 4.05 g of polyvinyl alcohol as a binder are added to 1125 g of ion-exchanged water, and the peripheral speed is 2480 rpm in the circulation type wet pulverizer used in step (I). Grinding was performed for 30 minutes under conditions of a circulation rate of 1 L / min. The average particle size of the solid content in the pulverized slurry was 1.061 μm.
<Process (V)>
The slurry obtained in the step (IV) is recovered from the circulation type wet pulverizer, and the slurry is spray granulated and dried with a tube pump while stirring with a stirrer set at a rotation speed of 200 rpm so that solids in the slurry do not settle. Machine (trade name: LB-8 type, manufactured by Ogawara Koki Co., Ltd.), spray dried at an inlet temperature of 230 ° C., an outlet temperature of 110 ° C. and an atomizer speed of 30000 rpm, and CaMnO having an average particle size of 15 to 20 μm ₃ spherical particles were obtained.
<Process (VI)>
The CaMnO ₃ spherical particles obtained in the step (V) were uniaxially pressed under a molding pressure of 1.5 ton / cm ² , and an electric furnace (trade name: KBF521N1 manufactured by Koyo Thermo Systems Co., Ltd.) was used. And fired at 1050 ° C. for 5 hours. The temperature increase rate from 400 ° C. to 1050 ° C. was 43.3 ° C./Hr, and after the completion of firing, the temperature was cooled to room temperature at a cooling rate of 24.8 ° C./Hr to obtain a CaMnO ₃ type thermoelectric conversion material. .

About the obtained CaMnO ₃ type thermoelectric conversion material, the crystallite diameter, relative density, the number of holes per unit area (100 μm ² ), the ratio of the number of holes having a longest diameter of 300 to 1600 nm, and the number of holes having a longest diameter of less than 300 nm Number ratio, number ratio of holes exceeding the longest diameter of 1600 nm, average value of aspect ratios of holes having a longest diameter of 300 to 1600 nm, and holes closest to a hole having a longest diameter of 300 to 1600 nm The average distance, Seebeck coefficient, electrical conductivity and thermal conductivity were measured. The results are shown in Table 1.
When the pore structure was measured with a field emission electron microscope (measurement magnification: 5000 times), it was confirmed that the pore structure was uneven and non-spherical. The results are shown in FIG.

[Example 2]
A CaMnO ₃ type thermoelectric conversion material was obtained in the same manner as in Example 1 except that the wet pulverization time in Step (I) in Example 1 was changed to 4 hours.
About the obtained CaMnO ₃ type thermoelectric conversion material, the crystallite diameter, relative density, the number of holes per unit area (100 μm ² ), the ratio of the number of holes having a longest diameter of 300 to 1600 nm, and the number of holes having a longest diameter of less than 300 nm Number ratio, number ratio of holes exceeding the longest diameter of 1600 nm, average value of aspect ratios of holes having a longest diameter of 300 to 1600 nm, and holes closest to a hole having a longest diameter of 300 to 1600 nm The average distance, Seebeck coefficient, electrical conductivity and thermal conductivity were measured. The results are shown in Table 1.
When the pore structure was measured with a field emission electron microscope (measurement magnification: 5000 times), it was confirmed that the pore structure was uneven and non-spherical.

[Example 3]
A CaMnO ₃ type thermoelectric conversion material was obtained in the same manner as in Example 1 except that the wet pulverization time in Step (I) in Example 1 was changed to 7 hours.
About the obtained CaMnO ₃ type thermoelectric conversion material, the crystallite diameter, relative density, the number of holes per unit area (100 μm ² ), the ratio of the number of holes having a longest diameter of 300 to 1600 nm, and the number of holes having a longest diameter of less than 300 nm Number ratio, number ratio of holes exceeding the longest diameter of 1600 nm, average value of aspect ratios of holes having a longest diameter of 300 to 1600 nm, and holes closest to a hole having a longest diameter of 300 to 1600 nm The average distance, Seebeck coefficient, electrical conductivity and thermal conductivity were measured. The results are shown in Table 1.
When the pore structure was measured with a field emission electron microscope (measurement magnification: 5000 times), it was confirmed that the pore structure was uneven and non-spherical.

[Comparative Example 1]
A CaMnO ₃ type thermoelectric conversion material was obtained in the same manner as in Example 1 except that the wet pulverization time in Step (I) in Example 1 was changed to 5 minutes.
About the obtained CaMnO ₃ type thermoelectric conversion material, the crystallite diameter, relative density, the number of holes per unit area (100 μm ² ), the ratio of the number of holes having a longest diameter of 300 to 1600 nm, and the number of holes having a longest diameter of less than 300 nm Number ratio, number ratio of holes exceeding the longest diameter of 1600 nm, average value of aspect ratios of holes having a longest diameter of 300 to 1600 nm, and holes closest to a hole having a longest diameter of 300 to 1600 nm The average distance, Seebeck coefficient, electrical conductivity and thermal conductivity were measured. The results are shown in Table 1.
When the pore structure was measured with a field emission electron microscope (measurement magnification: 5000 times), it was confirmed that the pore structure was uneven and non-spherical. The results are shown in FIG.

[Comparative Example 2]
A CaMnO ₃ type thermoelectric conversion material was obtained in the same manner as in Example 1 except that the wet pulverization time in step (I) in Example 1 was changed to 12 hours.
About the obtained CaMnO ₃ type thermoelectric conversion material, the crystallite diameter, relative density, the number of holes per unit area (100 μm ² ), the ratio of the number of holes having a longest diameter of 300 to 1600 nm, and the number of holes having a longest diameter of less than 300 nm Number ratio, number ratio of holes exceeding the longest diameter of 1600 nm, average value of aspect ratios of holes having a longest diameter of 300 to 1600 nm, and holes closest to a hole having a longest diameter of 300 to 1600 nm The average distance, Seebeck coefficient, electrical conductivity and thermal conductivity were measured. The results are shown in Table 1.
When the pore structure was measured with a field emission electron microscope (measurement magnification: 5000 times), it was confirmed that the pore structure was uneven and non-spherical. The results are shown in FIG.

From the results of Table 1, the CaMnO ₃ type thermoelectric conversion materials of Examples 1 to ₃ have improved electrical conductivity, Seebeck coefficient, and thermal conductivity compared to Comparative Examples 1 and 2, resulting in a high dimensionless figure of merit ( ZT) was found to be obtained.

The CaMnO ₃ type thermoelectric conversion material of the present invention can be effectively used as a thermoelectric conversion material because the electrical conductivity, Seebeck coefficient, and thermal conductivity can be improved without replacing expensive and rare elements in the crystal lattice.

Claims (4)

A CaMnO ₃ type thermoelectric conversion material having a pore structure inside and satisfying the following conditions 1), 2) and 3):
1) The crystallite diameter is in the range of 300 to 550 mm.
2) The relative density is in the range of 60 to 80%.
3) In the cross-sectional photograph of the CaMnO ₃ type thermoelectric conversion material, the number of pores existing per unit area (100 μm ² ) is 40-60, and the longest diameter (L) is in the range of 300-1600 nm. The pores are 80 to 100%, the longest diameter (L) is less than 300 nm, less than 8%, and the longest diameter (L) is more than 1600 nm. 2. The CaMnO ₃ type thermoelectric conversion according to claim 1, wherein an average value of aspect ratios of pores having the longest diameter (L) in a range of 300 to 1600 nm is in a range of 2.0 to 4.0. material. The average value of the distance between the hole having the longest diameter (L) in the range of 300 to 1600 nm and the closest hole is in the range of 300 to 500 nm. CaMnO ₃ type thermoelectric conversion material. The method for producing a CaMnO ₃ type thermoelectric conversion material according to any one of claims 1 to 3, comprising the following steps (I) to (VI).
(I) A step of preparing a slurry having an average particle size of 0.3 μm or less by wet-grinding a calcium compound and a manganese compound (II) A step of drying the slurry obtained in the step (I) to prepare a powder (III) Step of pre-baking the powder obtained in the step (II) at 500 to 1000 ° C. to prepare the pre-fired powder (IV) Wet the pre-fired powder obtained in the step (III) A step of pulverizing to prepare a slurry having an average particle size of 0.1 to 3.0 μm (V) A step of drying the slurry obtained in the step (IV) to obtain CaMnO ₃ particles having an average particle size of 5 to 30 μm ( VI) A step of forming the CaMnO ₃ particles obtained in the step (V) and firing at 950 to 1200 ° C. to obtain a CaMnO ₃ sintered body.