JP4423122B2 - Method for producing oriented thermoelectric material - Google Patents

Description

The present invention relates to a method of manufacturing a thermoelectric material, and more particularly, to a method for manufacturing a thermoelectric material having a crystal orientation.

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 discussed on a global scale, and the 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 use of the Seebeck effect in which thermoelectromotive force is generated at both ends when a temperature difference is given to two different metals or thermoelectric conversion materials such as a p-type semiconductor and an n-type semiconductor. 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)
In the formula (1), α is the Seebeck coefficient, κ is the thermal conductivity, and ρ represents the specific resistance.

  That is, when the Seebeck coefficient is large and the thermal conductivity and specific resistance are small, the figure of merit is large, in other words, an element having a large figure of merit can be obtained. Here, Seebeck coefficient is a physical property value determined by the material, but thermal conductivity and specific resistance can be changed greatly depending on the microstructure (fine structure and orientation) of the material. In order to reduce the specific resistance, a method of controlling the crystal structure has been studied. That is, by controlling and improving the orientation of the crystal structure, it is possible to reduce the thermal conductivity and specific resistance in a certain direction, and it is possible to improve the thermoelectric characteristics in such a development direction. is there.

For example, in Japanese Patent Application Laid-Open No. 2000-211971 of Patent Document 1, AxB ₂ Oy (A is Na, Li, K, Ca, Sr, Ba, Bi, Y, La, and B is Mn, Fe, Co, Ni, Cu, where x is in the range of 1 ≦ x ≦ 2 and y is in the range of 2 ≦ y ≦ 4). In particular, the NaCo ₂ O ₄ -based thermoelectric element material is a mixture of cobalt hydroxide or cobalt oxide plate-like particles and a sodium metal salt, and molded so that the cobalt hydroxide or cobalt oxide particles are oriented in one direction. It is disclosed that a sintered body oriented in the C-axis direction is produced by firing and densifying the compact.

  Japanese Patent Application Laid-Open No. 2002-16297 of Patent Document 2 discloses ZnO having a shape anisotropy as a template for a crystal orientation material or a precursor powder material thereof, and ZnO or a precursor powder material thereof. Is mixed with a substance that generates a conductive oxide having crystal anisotropy by the reaction of, and the mixed material is molded at room temperature so that the anisotropically shaped powder is oriented in one direction. And a method for producing a crystallographic bulk ZnO-based sintered material and a thermoelectric conversion device produced thereby have been proposed.

Furthermore, Japanese Patent Application Laid-Open No. 2001-223396 of Patent Document 3 discloses a thermoelectric material or a metal and a semimetal mainly composed of a combination of one or more elements selected from Group 5 elements and Group 6 elements of the Periodic Table. When a thermoelectric material mainly composed of a combination of system materials or a thermoelectric material added with an oxide, carbide, nitride, or a mixture thereof is sintered by direct current energization and pressurization, a current is applied in a variable current range of 100 to 15000 A. And a method for producing a thermoelectric material, characterized by sintering while applying a magnetic field in a magnetic flux density range of 0.1T ≦ H ≦ 2.0T (T: Tesla) to obtain an electrical orientation of the sintered body structure. Has been proposed.
JP 2000-219711 A JP 2002-16297 A JP 2001-223396 A

  However, according to the method proposed by Japanese Patent Application Laid-Open Nos. 2000-211971 and 2002-16297, it is possible to provide a sample that is certainly oriented to some extent. However, there is a problem that the orientation is not yet sufficient.

In addition, according to the method proposed in Japanese Patent Application Laid-Open No. 2001-223396, only the electrical orientation is obtained by sintering in a magnetic field, and the crystal itself cannot be oriented. Crystal structure control to reduce or eliminate the anisotropy of physical properties such as electrical resistance and thermal conductivity, increase the degree of orientation of the crystal structure, and reduce the thermal conductivity and specific resistance in a certain direction At present, it cannot be used for the purpose of a method.
Accordingly, a method capable of producing a thermoelectric material having a sufficiently large crystal orientation degree and a thermoelectric material having a sufficient degree of orientation have been desired.

Therefore, an object of the present invention is to cool the thermoelectric material fine particles and facilitate orientation by a magnetic field when forming a thermoelectric material or a thermoelectric material molded body in a magnetic field by utilizing anisotropy of magnetic susceptibility. Accordingly, an object of the present invention is to provide a method for producing a thermoelectric material having a large degree of crystal orientation and excellent thermoelectric properties.

A first feature of the present invention is a method for producing an oriented thermoelectric material by including a step of applying a magnetic field in the process, using thermoelectric material fine particles having anisotropy of magnetic susceptibility, and using the thermoelectric material fine particles as a solvent. And a step of producing an alignment molded body by drying the dispersion in a magnetic field, and the step of forming the alignment molded body includes at least a step of cooling the thermoelectric material fine particles. It is in having.

According to a second aspect of the present invention, in the method for producing an oriented thermoelectric material by providing a magnetic field application step, thermoelectric material fine particles having magnetic anisotropy are used, and the thermoelectric material fine particles are used as a solvent. And a step of forming a dispersion by dispersing the dispersion or the fine particles obtained by drying the dispersion in a magnetic field to form an alignment molded body, The step of performing is provided with a step of cooling at least the thermoelectric material fine particles .

The third feature of the present invention is that, in the method for producing an oriented thermoelectric material, the temperature at which the thermoelectric fine particles are cooled in the cooling step is higher than the freezing point of the solvent used in the dispersion.

A fourth feature of the present invention is that, in the method for producing an oriented thermoelectric material, fine particles having magnetic anisotropy are used as the thermoelectric material fine particles.

A fifth feature of the present invention resides in that fine particles having shape anisotropy are used as thermoelectric material fine particles in the method for producing an oriented thermoelectric material.

According to the present invention, by cooling the thermoelectric fine particles, the orientation due to the magnetic field is improved, and thereby it is possible to produce a thermoelectric material having a good orientation.

In addition , by cooling the thermoelectric fine particles, the orientation by the magnetic field is improved, and thereby it is possible to produce an oriented molded body of thermoelectric fine particles having a good orientation.

In addition , since the thermoelectric fine particles have anisotropy, the orientation due to the magnetic field during cooling of the thermoelectric fine particles is improved, thereby making it possible to produce an oriented molded body of thermoelectric fine particles with good orientation. became.

Hereinafter, the present invention will be described with reference to embodiments.
FIG. 1 shows an example of the manufacturing process of the present invention.
First, thermoelectric material fine particles are synthesized. These fine particles are required to have magnetic susceptibility anisotropy. That is, the magnetic susceptibility χ in an arbitrary direction is small, the magnetic susceptibility is large in a specific direction, and an orientation thermoelectric material is produced when the difference Δχ (magnetic susceptibility anisotropy) between the two directions is as large as possible. Is preferred. In addition, it is preferable that the thermoelectric material fine particles have shape anisotropy for producing an oriented thermoelectric material. Since the anisotropy of the magnetic susceptibility changes depending on the size of the thermoelectric material fine particles, the magnetic field to be applied is more uniform when the particle diameters of the fine particles are uniform in order to produce an oriented thermoelectric material using a magnetic field. It is preferable from the viewpoint of setting and improving productivity.

Further, when anisotropy originally exists in the crystal structure of the fine particles used, it is possible to have anisotropy of magnetic susceptibility by using such raw materials. From this point of view, the oxide layered compound is preferable as a raw material for producing the oriented thermoelectric material of the present invention.
Since such a compound has a layered structure, the magnetic susceptibility differs greatly between the stacking direction of the layers and the direction perpendicular to the direction, and thermoelectric properties can be improved by orienting in a magnetic field. That is why.

Next, a process for preparing a dispersion in which the fine particles synthesized as described above are dispersed in a solvent will be described.
As the dispersion medium used in this step, any of water, an organic dispersion medium, and an inorganic dispersion medium may be used, and there is no particular limitation. Whichever dispersion medium is used, it is necessary that the fine particles are dispersed in the dispersion without agglomeration. Therefore, there is no problem if the dispersion is stabilized by applying ultrasonic waves as necessary, or by adding a surfactant, a binder or the like to stabilize the dispersion.

Next, the process of cooling the thermoelectric material fine particles will be described.
Fine particles having anisotropy of magnetic susceptibility can be oriented by a magnetic field. Although it depends on the state where fine particles are placed, if the magnetic field strength is H, the following formula (2)
H ² >> ² kT / Δχ (2)
In this formula (2), k is a Boltzmann constant, T is an absolute temperature, and Δχ is anisotropy of magnetic susceptibility.

In the case of the magnetic field intensity satisfying the relationship of the formula (2), it becomes possible to orient the direction with a large magnetic susceptibility in the magnetic field application direction.
Here, since the absolute temperature T can be reduced by cooling the thermoelectric fine particles, the thermoelectric material fine particles are more easily oriented even when the same magnetic field strength is used.
One idea of the present invention is to apply a magnetic field while cooling. That is, as a general rule, the cooling temperature should be as low as possible. However, when the thermoelectric material fine particles are dispersed in the dispersion liquid, the cooling temperature needs to be higher than the solidification temperature of the dispersion medium used in the dispersion liquid.

Next, the process of inserting the thermoelectric fine particle dispersion into a magnetic field to form a molded body will be described.
Since thermoelectric materials are generally not ferromagnetic, they require a very high magnetic field strength for their orientation. As the magnetic field strength, for example, it is preferable to apply the thermoelectric material by applying greater than 2T (Tesla), more preferably 5T or more, and even more preferably 10T or more in the present invention.
In order to obtain a molded body, the dispersion is dried in a magnetic field. Moreover, when forming this molded object, a molded object with larger intensity | strength can be obtained by performing pressure molding. By this drying or pressure molding, 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. Even during this step, a molded article with higher orientation can be formed by cooling the thermoelectric material fine particles.

Next, a densification process by heat treatment in a magnetic field for forming a bulk body having a high strength by densifying the compact by heat treatment will be described.
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. If it does in this way, the part etc. of the particle | grains etc. which were orientated with much heat will face the random direction by heat processing.

  On the other hand, in the present invention, when molding is performed in a magnetic field, the thermoelectric fine particles are cooled to form a molded body having a very high orientation, and further heat treatment (including sintering) in a magnetic field. By carrying out the above, it becomes possible to maintain the degree of orientation by densification, so that it is possible to form a bulk body of thermoelectric material with extremely good orientation. However, in this case, it is necessary to set the magnetic field strength accordingly so that the magnetic susceptibility does not decrease and the anisotropy of the magnetic susceptibility does not decrease because the temperature becomes high. 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 make the thermoelectric material precursor into an oriented molded body by the same method and synthesize the thermoelectric material by heat treatment. is there.
A thermoelectric material (polycrystal) that does not have crystal orientation has a magnetic anisotropy that is in a different direction as shown in the conceptual diagram of FIG. (The direction of the dotted line in the figure shows the direction corresponding to the direction of high magnetic susceptibility) On the other hand, the oriented thermoelectric material (polycrystal) produced according to the present invention is shown in the conceptual diagram in FIG. In addition, the entire sample is oriented along the anisotropy of the magnetic susceptibility, that is, the direction in which the magnetic susceptibility is large is aligned throughout the sample. In a thermoelectric material or the like composed of an oxide layered compound corresponding to the anisotropy of electrical characteristics such as a rate, the thermoelectric characteristics can be improved 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, thermoelectric fine particles having shape anisotropy and magnetic anisotropy are dispersed in a solvent, and after cooling the thermoelectric fine particles, an oriented compact is formed in a magnetic field. By forming, a molded body having a high orientation along the magnetic anisotropy can be obtained. Since the orientation of this molded body is very high, heat treatment is further performed in a magnetic field, and the orientation molded body is densified, so that it remains oriented along the anisotropy of magnetic susceptibility (does not degrade orientation) It becomes possible to form a bulk body with high strength.

  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)
EXAMPLES Hereinafter, although an Example demonstrates this invention further, it cannot be overemphasized that this invention is restrained by these Examples and is not interpreted.

Co ₃ O ₄ powder and Na ₂ CO ₃ powder were sufficiently mixed and then baked at 880 ° C. for 20 hours using an electric muffle furnace.
After firing, this sample was pulverized to synthesize fine particles of Na _x Co ₂ O _y (where x is in the range of 1 ≦ x ≦ 2 and y is in the range of 2 ≦ y ≦ 4). The fine particles were added to toluene and dispersed using ultrasonic waves.
The fine particle dispersion was cooled to −80 ° C. in advance, and then the fine particle dispersion was dried while applying a 10 T magnetic field using a superconducting magnet to prepare Sample 1.

(Comparative Example 1)
Sample 2 was prepared in the same manner as in Example 1, except that the fine particle dispersion of Example 1 was dried while applying a 10 T (Tesla) magnetic field using a superconducting magnet without cooling.

  When the cross sections of Sample 1 (Example 1) and Sample 2 (Comparative Example 1) were observed using an electron microscope, the microstructure of Sample 1 was more oriented.

In the same manner as in Example 1, Na _x Co ₂ O _y (1 ≦ x ≦ 2, 2 ≦ y ≦ 4) fine particles were synthesized, and the fine particles were added to ethyl alcohol and dispersed using ultrasonic waves. .
The fine particle dispersion was cooled to −100 ° C. in advance, and then the fine particle dispersion was dried while applying a magnetic field of 10 T using a superconducting magnet.

(Comparative Example 2)
Sample 4 was prepared in the same manner as in Example 2 except that the fine particle dispersion was dried while applying a magnetic field of 10 T (Tesla) using a superconducting magnet without cooling. .

  When the cross sections of Sample 3 (Example 2) and Sample 4 (Comparative Example 2) were observed using an electron microscope, the microstructure of Sample 3 was more oriented.

In the same manner as in Example 2, Na _x Co ₂ O _y (x is 1 ≦ x ≦ 2 and y is 2 ≦ y ≦ 4) fine particles were synthesized, and similarly, a dispersion in which the solvent was ethyl alcohol Was made. This fine particle dispersion was inserted into a mold, cooled to −100 ° C., and then subjected to pressure molding while applying a 10 T magnetic field using a superconducting magnet.

(Comparative Example 3)
As in Example 3, after inserting the fine particle dispersion into the mold and then performing pressure molding while applying a magnetic field of 10 T (Tesla) using a superconducting magnet without cooling, the same as in Example 3 Thus, Sample 6 was produced.

  When the cross sections of Sample 5 and Sample 6 were observed using an electron microscope, the microstructure of Sample 5 was more oriented.

An example of the manufacturing process of this invention 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.

Explanation of symbols

1 Thermoelectric material crystal grains 2 Dotted line direction with large magnetic susceptibility 3 Crystal grain magnetic anisotropy is random 4 Thermoelectric material fine particles

Claims (5)

In the method for producing an oriented thermoelectric material having a step of applying a magnetic field, a step of using a thermoelectric material fine particle having anisotropy of magnetic susceptibility and dispersing the thermoelectric material fine particle in a solvent to generate a dispersion, and the dispersion A method of producing an oriented molded body by drying the material in a magnetic field, wherein the step of producing the oriented molded body includes a step of cooling at least the thermoelectric material fine particles. . In the method for producing an oriented thermoelectric material having a step of applying a magnetic field, a step of using a thermoelectric material fine particle having anisotropy of magnetic susceptibility and dispersing the thermoelectric material fine particle in a solvent to generate a dispersion, and the dispersion Alternatively, the step of forming an oriented molded body by press-molding fine particles obtained by drying the dispersion in a magnetic field, and the step of forming the oriented molded body cools at least the thermoelectric material fine particles. method for producing oriented thermoelectric material characterized by having a step. 3. The method for producing an oriented thermoelectric material according to claim 1, wherein a cooling temperature in the cooling step is higher than a freezing point of the dispersion. In the production method of the oriented thermoelectric material according to claim 1-3, method for producing oriented thermoelectric material characterized by using fine particles having an anisotropic magnetic susceptibility as the thermoelectric material particles. In the production method of the oriented thermoelectric material according to claim 1-4, method for producing oriented thermoelectric material characterized by using fine particles having a shape anisotropy as the thermoelectric material particles.

Images