JP4810087B2 - Oriented thermoelectric material and method for producing the same - Google Patents

Description

  The present invention relates to a thermoelectric material and a method for producing the same, and more particularly to an oriented thermoelectric material having crystal orientation and a method for producing the same.

Currently, much of the world's energy depends on the combustion energy of fossil fuels, but in the case of power generation systems that use thermal cycles, most of that energy is discarded as waste heat. Currently.
On the other hand, global environmental conservation has been debated on a global scale, and development of effective utilization technology for unused energy has been energetically promoted.
Among these, power generation using thermoelectric conversion can directly convert even relatively low-quality heat into electricity. As the environmental problems become more serious, expectations for thermoelectric conversion are increasing.
This thermoelectric conversion is the direct application of thermal energy using the Seebeck effect in which thermoelectromotive force is generated at both ends when a temperature difference is given to two different types of metals, thermoelectric conversion materials such as p-type semiconductors and n-type semiconductors. It is a technology that converts power into electric power and has excellent features such as no moving parts such as motors and turbines, and no waste.
Here, the figure of merit Z used for the performance evaluation of thermoelectric characteristics is expressed by the following formula (1).
Z = α ² / (κ · ρ) (1)
α: Seebeck coefficient
κ: Thermal conductivity
ρ: Specific resistance That is, it is necessary that the Seebeck coefficient is large and the thermal conductivity and the specific resistance are small.

Here, the Seebeck coefficient is a physical property value, so it depends on the material, but the thermal conductivity and specific resistance can be changed greatly depending on the microstructure and orientation of the material. A crystal structure control method for reducing the specific resistance has been studied.
That is, by improving the orientation of the crystal structure, the thermal conductivity and specific resistance can be reduced in a certain direction, and the thermoelectric characteristics in that direction can be improved.
For example, AxB ₂ Oy (A: Na, Li, K, Ca, Sr, Ba, Bi, Y, La, B: Mn, Fe, Co, Ni, Cu, 1 ≦ x ≦ 2, 2 ≦ y ≦ 4) type A thermoelectric element material having a structure, in particular, a NaCo ₂ O ₄ -based thermoelectric element material, is obtained by mixing cobalt hydroxide or cobalt oxide plate-like particles with a sodium metal salt, and the cobalt hydroxide or cobalt oxide particles are unidirectional. A thermoelectric element material and a method for manufacturing the thermoelectric element material have been proposed (for example, a sintered body in which the C-axis direction is oriented is formed by firing the compacted body and densifying the formed body to be densified (for example, , See Patent Document 1).

  Further, a conductive oxide having crystal anisotropy is formed by a reaction between ZnO having a shape anisotropy, which is a template for a crystal orientation material, or a precursor powder material thereof, and this ZnO or a precursor powder material thereof. The substance to be mixed is mixed, the mixed material is molded at room temperature so that the anisotropically shaped powder is oriented in one direction, the molded product is synthesized by heat treatment, and then sintered. A method for producing a crystal-oriented bulk ZnO-based sintered material and a thermoelectric conversion device produced thereby have been proposed (see, for example, Patent Document 2).

Furthermore, a thermoelectric material mainly composed of a combination of one or more elements selected from Group V elements and Group VI elements, or a thermoelectric material mainly composed of a combination of a metal and a metalloid material, or oxides thereof. In addition, when the thermoelectric material added with carbide, nitride, or a mixture thereof is sintered by direct current energization and pressurization, it is energized in a variable current range of 100 to 15000 A and a magnetic flux density of 0.1 T ≦ H ≦ 2.0 T (T: There has been proposed a method for producing a thermoelectric material characterized in that sintering is performed while applying a magnetic field in the range of (Tesla) to obtain the electrical orientation of the sintered body structure (see, for example, Patent Document 3).
JP 2000-219711 A JP 2002-16297 A JP 2001-223396 A

However, according to the methods proposed in Patent Document 1 and Patent Document 2 described above, it is possible to provide a sample that is certainly oriented to some extent. However, both have limitations on the degree of orientation. When the formed product is sintered or fired to be densified, the degree of orientation is lowered, so that the orientation is not sufficient.
Further, according to the method proposed in Patent Document 3, only the electric orientation is obtained by sintering in a magnetic field, and the crystal itself cannot be oriented because the magnetic field strength is small. On the contrary, the crystal structure to reduce the thermal conductivity and specific resistance in a certain direction by decreasing or eliminating the anisotropy of physical properties such as electrical resistance and thermal conductivity and increasing the degree of orientation of the crystal structure At present, it cannot be used for the purpose of a control method.

For this reason, a method capable of increasing the crystal orientation degree of the thermoelectric material and producing the thermoelectric material without reducing the orientation degree and a thermoelectric material having a sufficient degree of orientation have been desired.
Accordingly, an object of the present invention is to provide an oriented thermoelectric material having a large degree of crystal orientation and excellent thermoelectric properties, and a method for producing the same.

In order to solve the above problems, the invention according to claim 1 is directed to a thermoelectric material composed of an aggregate of fine particles, a thermoelectric material in which fine particles are densified by heat treatment, or the fine particles according to anisotropy of magnetic susceptibility, respectively. An oriented thermoelectric material in which the fine particles have shape anisotropy, and the fine particles have shape anisotropy and magnetic susceptibility anisotropy and have a thickness Ca ₃ Co ₄ Ox (8 .5 ≦ x ≦ 10). and, in the case where the major axis of the particulate X1, a minor Y1, that the thickness and d1, X1 / d1 or Y1 / d1 is I 5-10 der, the fine particles susceptibility It is characterized by being uniaxially oriented along the anisotropy .

According to a second aspect of the invention, at the invention of claim 1 Symbol mounting, and wherein the particle size of the fine particles is substantially uniform.

A third aspect of the invention is characterized in that, in the first or second aspect of the invention, the magnetic susceptibility in the thickness direction of the fine particles is larger than the magnetic susceptibility in a direction other than the thickness direction.

According to a fourth aspect of the present invention, there is provided a method for producing an oriented thermoelectric material by including a step of applying a magnetic field during the step, wherein the fine particles of the thermoelectric material according to any one of the first to third aspects are dispersed in a solvent. And a step of forming an oriented molded body by inserting the dispersion obtained in the dispersion step into a magnetic field.

According to a fifth aspect of the present invention, in the method for producing an oriented thermoelectric material by providing a step of applying a magnetic field during the process, the fine particles of the thermoelectric material according to any one of the first to third aspects are dispersed in a solvent. And a step of forming an oriented molded body by drying the dispersion obtained in the dispersion step in a magnetic field.

The invention according to claim 6 is the invention according to claim 4 or 5 , wherein the dispersion step of dispersing the fine particles of the thermoelectric material in a solvent, and the dispersion liquid obtained by the dispersion step or the fine particles obtained by drying the dispersion are subjected to a magnetic field. And a step of forming an oriented molded body by performing pressure molding in the interior.

The invention according to claim 7 is characterized by comprising a step of densifying the oriented molded body obtained by the method for producing an oriented thermoelectric material according to any one of claims 4 to 6 by heat treatment.

The invention according to claim 8 is the method for producing an oriented thermoelectric material according to claim 7 , wherein the step of densifying the oriented formed body by heat treatment is performed in a magnetic field.

  By utilizing the anisotropy of magnetic susceptibility, the degree of crystal orientation is increased by forming a thermoelectric material or a thermoelectric material molded body in a magnetic field and, if necessary, further densifying by heat treatment in the magnetic field. An oriented thermoelectric material having excellent thermoelectric properties can be obtained.

The alignment thermoelectric material of the present embodiment is a thermoelectric material composed of an aggregate of fine particles, a thermoelectric material in which fine particles are densified by heat treatment, or a thermoelectric material in which the fine particles are aligned along the anisotropy of magnetic susceptibility, A thermoelectric material in which fine particles have shape anisotropy, and the fine particles are circular or elliptical and have a thickness, and the major axis of the circle or ellipse is X1, the minor axis is Y1, and the thickness is d1. In this case, X1 / d1> 1 and Y1 / d1> 1.
In addition, the oriented thermoelectric material of the present embodiment is a thermoelectric material composed of an aggregate of fine particles, a thermoelectric material in which fine particles are densified by heat treatment, or a thermoelectric material in which the fine particles are oriented along the anisotropy of magnetic susceptibility. When the fine particle is a thermoelectric material having shape anisotropy, and the fine particle is a polygon and a fine particle having a thickness, the major axis of the polygon is X2, the minor axis is Y2, and the thickness is d2. , X2 / d2> 1 and Y2 / d2> 1.
In addition, the method for producing an oriented thermoelectric material of the present embodiment includes a thermoelectric material composed of an aggregate of fine particles, a thermoelectric material in which fine particles are densified by heat treatment, or the fine particles are oriented along the anisotropy of magnetic susceptibility, respectively. A method for producing a thermoelectric material in which fine particles have shape anisotropy, wherein the fine particles are hexagonal and thick fine particles, the major axis of the hexagon is X3, the minor axis is Y3, When the thickness is d3, X3 / d3> 1 and Y3 / d3> 1.

In the manufacturing method of the oriented thermoelectric material of the present embodiment, in addition to the above configuration, the particle diameter of the fine particles is preferably substantially uniform, and the magnetic susceptibility in the thickness direction of the fine particles is higher than the magnetic susceptibility in the direction other than the thickness direction. Is also preferably large.
The method for producing an oriented thermoelectric material of the present embodiment is a method for producing an oriented thermoelectric material by including a step of applying a magnetic field during the process, and a dispersion step of dispersing the fine particles of the thermoelectric material in a solvent; And a step of forming an oriented molded body by inserting the dispersion obtained in the dispersion step into a magnetic field.
The method for producing an oriented thermoelectric material of the present embodiment is a method for producing an oriented thermoelectric material by including a step of applying a magnetic field in the process, wherein the thermoelectric material fine particles are dispersed in a solvent, and And a step of forming an oriented molded body by drying the dispersion obtained in the dispersion step in a magnetic field.
In addition to the above-described configuration, the method for producing an oriented thermoelectric material of the present embodiment includes a dispersion step in which fine particles of the thermoelectric material are dispersed in a solvent, and a dispersion liquid obtained by the dispersion step or fine particles obtained by drying the dispersion liquid are added in a magnetic field. And a step of forming an oriented molded body by pressure forming.

The method for producing an oriented thermoelectric material of the present embodiment is characterized by comprising a step of densifying the oriented molded body obtained by the method for producing an oriented thermoelectric material by heat treatment.
The method for producing an oriented thermoelectric material according to this embodiment is characterized in that, in the method for producing an oriented thermoelectric material, the step of densifying the oriented compact by heat treatment is performed in a magnetic field.
Hereinafter, the manufacturing method of the oriented thermoelectric material of this embodiment will be described.
FIG. 1 shows an example of the manufacturing process of the present invention.
First, thermoelectric material fine particles are synthesized. The fine particles preferably have shape anisotropy in order to produce an oriented thermoelectric material. In particular, in a circular or elliptical fine particle having a circular or elliptical major axis of X1, a minor axis of Y1, and a thickness of d1, the major axis or minor axis divided by the thickness, that is, X1 / d1. Alternatively, Y1 / d1 is preferably larger than 1 for producing an oriented thermoelectric material. Similarly, in the case of polygonal and thick microparticles, when the polygonal major axis is X2, the minor axis is Y2, and the thickness is d2, the value obtained by dividing the major axis or minor axis by the thickness, that is, X2 / d2. Alternatively, Y2 / d2 is preferably larger than 1 for producing an oriented thermoelectric material. When this polygon is a hexagon, alignment becomes very easy, and depending on the alignment conditions, it can be aligned in a planar shape, which is particularly preferable for producing an alignment thermoelectric material.

  Here, since the anisotropy of the magnetic susceptibility changes depending on the size of the fine particles, in order to produce an oriented thermoelectric material using a magnetic field, it is more uniform that the fine particles have a uniform particle size. It is preferable from the viewpoint of improving productivity such as setting. The fine particles are required to have magnetic susceptibility anisotropy. That is, in order to produce an oriented thermoelectric material, the susceptibility: χ is small in any direction, the susceptibility is large in any other direction, and the difference in susceptibility in both directions: Δχ is as large as possible. In particular, it is preferable to synthesize the fine particles so that the magnetic susceptibility increases in the thickness direction of the fine particles because orientation by a magnetic field is easy. Further, when the crystal structure originally has anisotropy, it can also have anisotropy of magnetic susceptibility. Also from this meaning, the oxide layered compound is preferable as a raw material for producing an oriented thermoelectric material. Since this compound is layered, the magnetic susceptibility is greatly different between the stacking direction of the layers and the direction perpendicular thereto, and it is possible to improve thermoelectric properties by orienting in a magnetic field (step S1).

The next step is a step of preparing a dispersion in which the fine particles synthesized as described above are dispersed in a solvent.
As the dispersion solvent, any of water, organic solvent, and inorganic solvent can be used without any problem. In any case, it is necessary that the fine particles are dispersed in the solvent without agglomeration. Therefore, there is no problem even if ultrasonic dispersion is performed as necessary, or a surfactant or the like is added (step S2).

Subsequently, the next step is a step of inserting the above-mentioned dispersion of thermoelectric fine particles into a magnetic field to obtain a molded body. Fine particles having anisotropy of magnetic susceptibility can be oriented by a magnetic field. Depending on the state where the fine particles are placed, if the magnetic field strength is H,
H ² >> ² kT / Δχ
k: Boltzmann constant
T: Absolute temperature
Δχ: When the magnetic field intensity satisfies the anisotropy relationship of magnetic susceptibility, it becomes possible to orient the direction in which the magnetic susceptibility is large in the magnetic field application direction.

  However, since thermoelectric materials are generally not ferromagnetic, they require a very high magnetic field strength for their orientation. The magnetic field strength is preferably greater than 2T in order to orient the thermoelectric material, more preferably a magnetic field of 5T or more, and even more preferably a magnetic field of 10T or more. In order to obtain a molded body, the dispersion is dried in a magnetic field. Drying of the solvent may be natural drying, and there is no problem even if heat is applied as necessary. Moreover, when forming this molded object, a molded object with larger intensity | strength can be obtained by performing pressure molding. By this step, a molded body is formed in which fine particles having anisotropy of magnetic susceptibility are oriented in the direction of a magnetic field applied with a direction having a high magnetic susceptibility (step S3).

The subsequent process is a heat treatment process in a magnetic field in which the compact is densified by heat treatment to form a bulk body with high strength. This densification step by heat treatment includes a sintering step for bulking the fine particle raw material. In the conventional method for producing an oriented thermoelectric material, the degree of orientation is reduced in the step of densification by this heat treatment after forming an oriented shaped body. This is because some regions of particles or the like that have been orientated are oriented in a random direction by heat treatment.
On the other hand, when densification is performed by performing heat treatment (including sintering) in a magnetic field as in the present invention, the degree of orientation can be maintained even in this step. It is possible to form a bulk body of a thermoelectric material with good properties. However, in this case, since the temperature becomes high, the magnetic susceptibility may decrease, thereby reducing the anisotropy of the magnetic susceptibility. In that case, it is necessary to adjust the magnetic field strength accordingly. become.
The above describes the case where the thermoelectric material is made into an oriented molded body and then densified by heat treatment, but it is also possible to synthesize the thermoelectric material by heat treatment by using the thermoelectric material precursor in the same way as the oriented molded body. is there.

  A thermoelectric material (polycrystal) that does not have crystal orientation has a different magnetic anisotropy as shown in the conceptual diagram of FIG. 2 (dotted lines in the figure). The direction corresponds to the direction in which the magnetic susceptibility is large).

  In contrast, in the case of the present invention, when a dispersion containing thermoelectric fine particles is molded in a magnetic field, the thermoelectric fine particles follow the magnetic anisotropy as shown in the conceptual diagram of FIG. To obtain an oriented state. An oriented thermoelectric material (polycrystal) produced by sintering this in a magnetic field is a bulk in which the whole is oriented along the anisotropy of magnetic susceptibility as shown in the conceptual diagram of its cross section in FIG. It is a body.

  That is, the direction in which the magnetic susceptibility is large is aligned throughout the sample. For example, the oxide layered compound in which the anisotropy of electrical characteristics such as specific resistance or thermal conductivity corresponds to the anisotropy of the magnetic susceptibility. The thermoelectric material or the like composed of can improve the thermoelectric characteristics in a specific direction. For example, if the specific resistance can be reduced in a specific direction, the figure of merit Z in that direction increases even if other physical constants are combined, and excellent thermoelectric characteristics can be obtained in that direction. In addition, by using a direction with low thermal conductivity, by setting one end to a high temperature and the other end to a low temperature, it is possible to increase the temperature difference at both ends, thereby improving the power that can be extracted. That is why.

  As described above, when the present invention is used, a material having shape anisotropy and further having magnetic susceptibility anisotropy is dispersed in a solvent to form an oriented molded body in a magnetic field. A molded body oriented along the direction can be obtained. Further, heat treatment is performed in a magnetic field, and the oriented compact is densified to form a bulk material with high strength that remains oriented along the anisotropy of magnetic susceptibility (without reducing orientation). It becomes possible.

  As described above, it becomes possible to produce a thermoelectric material having a very good orientation that has been difficult to produce by a very simple method, and the oriented thermoelectric material produced in the present invention is , Which can have very high thermoelectric properties in a specific direction.

[Example 1]
As raw materials, cobalt oxide (Co ₃ O ₄ ) powder and calcium carbonate (CaCO ₃ ) powder are weighed so that the molar ratio is Ca: Co = 3: 4, and then inserted into a mortar to be uniform. Until well mixed. This mixture was inserted into an alumina boat and pre-baked at 800 ° C. for 10 hours in an oxygen atmosphere using an electric furnace.
The calcined product was pulverized, pressure-molded, inserted into an alumina boat, and further fired at 920 ° C. for 20 hours in an oxygen atmosphere using an electric furnace to prepare a sample.
After firing, the sample was pulverized to synthesize Ca ₃ Co ₄ O _x (8.5 ≦ x ≦ 10) fine particles. The fine particles were added to pure water and dispersed using ultrasonic waves.
When the fine particle dispersion was dried without applying a magnetic field, the fine particles had crystal axes oriented in random directions (Sample 1).
When this sample 1 was dried while applying a magnetic field of 10 T (Tesla) using a superconducting magnet, it was oriented in a uniaxial direction (sample 2).

[Example 2]
Using the same method as in Example 1, Ca ₃ Co ₄ O _x (8.5 ≦ x ≦ 10) fine particles were synthesized, and a dispersion was prepared in the same manner. This fine particle dispersion was inserted into a mold and subjected to pressure molding. When a magnetic field was not applied during pressure molding, the fine particles had their crystal axes oriented in random directions (Sample 3).
On the other hand, when pressure molding was performed using a superconducting magnet while applying a magnetic field of 10T, the sample was oriented in a uniaxial direction (Sample 4). Sample 4 was stronger than the sample 2 in the strength of the molded body.

Example 3
Using the same method as in Example 1, Ca ₃ Co ₄ O _x (8.5 ≦ x ≦ 10) fine particles were synthesized. When this fine particle is observed from above, the major axis is X1, the minor axis is Y1, and the thickness of the fine particle is d, X1 / d or Y1 / d is near 1 (microparticle A), and X1 / Fine particles (fine particles B) having d or Y1 / d of 5 to 10 were collected separately.
These two kinds of fine particles were added to pure water and dispersed using ultrasonic waves. These fine particle dispersions were inserted into a mold and subjected to pressure molding while applying a magnetic field of 10T.
Both molded bodies had uniaxial orientation, but the orientation was better when fine particles having shape anisotropy of the fine particles B were used.

Example 4
Using the same method as in the case of using the fine particles B of Example 3, an oriented molded body was formed by an aggregate of Ca ₃ Co ₄ O _x (8.5 ≦ x ≦ 10) fine particles (the applied magnetic field was also 10T). .
The molded body was sintered in an oxygen atmosphere at 920 ° C. for 20 hours. Sintering is performed by two methods, one is performed without applying a magnetic field using an electric furnace (Sample 5), and the other is applied with a 15T magnetic field using a high-temperature high magnetic field heat treatment apparatus. Sintering was carried out (Sample 6). In addition, the application direction of the magnetic field was matched with the direction in which the magnetic susceptibility of the oriented compact was large.
When the microstructures of both samples after sintering were observed using a scanning electron microscope, the sample (sample 5) sintered without applying a magnetic field had crystal grain orientation in a partial region of the sample. Disturbance was observed, but the sample (sample 6) that was sintered while applying a magnetic field had very good orientation.

〔The invention's effect〕
Claim 1
Since it has shape anisotropy, orientation by a magnetic field is improved, and thermoelectric characteristics can be improved in a specific direction.

Claims 2 and 3
The orientation by the magnetic field is improved, and the thermoelectric characteristics can be improved in a specific direction.

Claims 4 to 6
It has become possible to produce an oriented compact of thermoelectric fine particles using a simple method.

Claims 7 and 8
It has become possible to produce thermoelectric materials with very good orientation using a simple method.

An example of the manufacturing process of this embodiment is shown. It is a conceptual diagram of the thermoelectric material (polycrystal) which does not have the crystal orientation. It is the conceptual diagram which showed the mode of the orientation molded object with which the microparticles were uniaxially oriented in the direction with a large magnetic susceptibility. It is a conceptual diagram of an oriented thermoelectric material (polycrystal) in which fine particles are uniaxially oriented in the direction of high magnetic susceptibility.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Crystal grain of thermoelectric material 2 The direction of a dotted line is the direction where a magnetic susceptibility is large 3 The anisotropy of the magnetic susceptibility of a crystal grain is random

Claims (8)

A thermoelectric material composed of an aggregate of fine particles, a thermoelectric material in which fine particles are densified by heat treatment, or a thermoelectric material in which the fine particles are oriented along the anisotropy of magnetic susceptibility, and the fine particles have shape anisotropy. In the thermoelectric material
The fine particles have shape anisotropy and magnetic susceptibility anisotropy and a fine particle Ca ₃ Co ₄ Ox (8.5 ≦ x ≦ 10) having a major axis of X1, a minor axis of Y1, If the thickness was d1, X1 / d1 or Y1 / d1 5-10 der, oriented thermoelectric material the fine particles are characterized in that uniaxially oriented along the anisotropy of the magnetic susceptibility. Oriented thermoelectric material of claim 1 Symbol placement, wherein the particle size of the fine particles is substantially uniform. The oriented thermoelectric material according to claim 1 or 2, wherein a magnetic susceptibility in a thickness direction of the fine particles is larger than a magnetic susceptibility in a direction other than the thickness direction. In the method for producing an oriented thermoelectric material by providing a step of applying a magnetic field during the step,
A dispersion step of dispersing the fine particles of the thermoelectric material according to any one of claims 1 to 3 in a solvent;
A method for producing an oriented thermoelectric material, comprising the step of forming an oriented molded body by inserting the dispersion obtained in the dispersing step into a magnetic field. In the method for producing an oriented thermoelectric material by providing a step of applying a magnetic field during the step,
A dispersion step of dispersing the fine particles of the thermoelectric material according to any one of claims 1 to 3 in a solvent;
A method for producing an oriented thermoelectric material, comprising the step of drying the dispersion obtained in the dispersing step in a magnetic field to form an oriented molded body. A dispersion step of dispersing fine particles of the thermoelectric material in a solvent;
6. An alignment thermoelectric device according to claim 4 or 5 , further comprising a step of forming an alignment molded body by press-molding the dispersion obtained in the dispersion step or fine particles obtained by drying the dispersion in a magnetic field. Material manufacturing method. A method for producing an oriented thermoelectric material, comprising the step of densifying an oriented molded body obtained by the method for producing an oriented thermoelectric material according to any one of claims 4 to 6 by heat treatment. 8. The method for producing an oriented thermoelectric material according to claim 7 , wherein the step of densifying the oriented compact by heat treatment is performed in a magnetic field.

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