JP4400861B2 - 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 crystal orientation, that is, an oriented thermoelectric material in which crystals are oriented in a certain direction, 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 metals or thermoelectric conversion materials such as p-type 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 performance evaluation of thermoelectric characteristics is expressed by the following equation.
Z = α ² / (κ · ρ)
α: Seebeck coefficient
κ: Thermal conductivity
ρ: specific resistance

  That is, in order to obtain a large figure of merit Z, it is necessary that the Seebeck coefficient α is large and the thermal conductivity κ and the specific resistance ρ are small.

  Here, Seebeck coefficient α is a physical property value, so it depends on the material. However, thermal conductivity κ and specific resistance ρ can be changed greatly depending on the microstructure and orientation of the material. Various crystal structure control methods for reducing κ and specific resistance ρ have been studied.

  That is, by improving the orientation of the crystal structure, it becomes possible to reduce the thermal conductivity κ and the specific resistance ρ in a certain direction. As a result, the thermoelectric characteristics in that direction are improved, that is, the figure of merit Z is increased. Will be able to.

  For example, Japanese Unexamined Patent Application Publication No. 2000-211971 (Patent Document 1) describes AxB2Oy (A: Na, Li, K, Ca, Sr, Ba, Bi, Y, La, B: Mn, Fe, Co, Ni, Cu). 1 ≦ x ≦ 2, 2 ≦ y ≦ 4) thermoelectric element material having a structure. In particular, the NaCo 2 O 4 -based thermoelectric element material is a mixture of cobalt hydroxide or cobalt oxide plate-like particles and sodium metal salt, which is molded so that the cobalt hydroxide or cobalt oxide particles are oriented in one direction. A thermoelectric element material and a manufacturing method thereof have been proposed in which a sintered body with the C-axis direction oriented is produced by firing and densifying the body.

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

  Furthermore, Japanese Patent Application Laid-Open No. 2001-223396 (Patent Document 3) discloses a thermoelectric material or a combination of a metal and a semi-metal material mainly composed of a combination of one or more elements selected from Group V elements and Group VI elements. In the sintering by direct current energization pressurization of a thermoelectric material having a main component, or oxides, carbides, nitrides or mixtures thereof added thereto, current is applied in a variable current range of 100 to 15000 A, and magnetic flux density A method for producing a thermoelectric material characterized in that sintering is performed while applying a magnetic field in a range of 0.1 T ≦ H ≦ 2.0 T (T: Tesla) to obtain an electrical orientation of the sintered body structure. Yes.

JP 2000-219711 A JP 2002-016297 A JP 2001-223396 A

  However, according to the methods proposed in Patent Document 1 and Patent Document 2, it is possible to provide a sample that is certainly oriented to some extent, but there is a limit in 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.

  In addition, according to the method proposed in Patent Document 3, electrical orientation is only obtained by sintering in a magnetic field, and the crystal itself cannot be oriented because the magnetic field strength is small. On the other hand, crystals that 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 tissue control method.

  Accordingly, there has been a strong demand for a method capable of increasing the crystal orientation degree of the thermoelectric material and producing the thermoelectric material without reducing the degree of orientation, and a thermoelectric material having a sufficient degree of orientation.

  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, and a thermoelectric material formed by the production method.

The first feature of the present invention is that in an oriented thermoelectric material in which crystals are oriented in a certain direction, the oriented thermoelectric material contains at least a ferrimagnetic component and a diamagnetic component .

According to a second aspect of the present invention, in the method for producing an oriented thermoelectric material by forming the thermoelectric material in a magnetic field, the method includes the step of adding a ferrimagnetic component to the thermoelectric material. is there.

According to the first aspect of the present invention, by including ferrimagnetism, orientation by a magnetic field is possible, thereby improving thermoelectric characteristics in a specific direction.

According to invention of Claim 2 , it became possible to manufacture the thermoelectric material with favorable orientation using a simple method.

  In the present invention, a thermoelectric material or a thermoelectric material molded body is formed in a magnetic field by adding a magnetic component such as a paramagnetic component, a ferromagnetic component, or a ferrimagnetic component to the thermoelectric material. A method of manufacturing a thermoelectric material having a large crystal orientation and excellent thermoelectric characteristics by performing densification by heat treatment, and a thermoelectric material formed by the manufacturing method are obtained.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 is a diagram showing an example of the manufacturing process of the present invention.
This example includes a thermoelectric material fine particle synthesis step (step S10), a magnetic component addition step (step S20), a dispersion preparation step (step S30), a molding step in a magnetic field (step S40), and a heat treatment in a magnetic field. (Step S50). Note that the step of creating the dispersion (step S30) is not necessary when the dispersion is not particularly used.

Hereinafter, each step will be described in detail.
(A) The first step is a thermoelectric material fine particle synthesis step.
The fine particles in this case are required to have magnetic susceptibility anisotropy. That is, the magnetic susceptibility χ is small in any direction, the magnetic susceptibility χ is large in any other direction, and the difference Δχ between the two directions is as large as possible to produce an oriented thermoelectric material using a magnetic field. Is preferred.

  Here, an example of fine particles is described, but the fine particles are not necessarily required. Further, when the crystal structure originally has anisotropy, it can also have anisotropy of magnetic susceptibility.

  Also in this sense, for example, an oxide layered compound is preferable as a raw material for producing an oriented thermoelectric material. Since this compound is in the form of a layer, the magnetic susceptibility χ is greatly different between the stacking direction of the layers and the direction perpendicular thereto, and the thermoelectric characteristics can be improved by orientation with a magnetic field.

(B) The next step is a magnetic component addition step (step S20).
In consideration of the formation of an oriented thermoelectric material by a magnetic field, paramagnetic components, ferromagnetic components, and ferrimagnetic components are suitable as magnetic components to be added.

  It is more preferable that these magnetic components have magnetic susceptibility anisotropy. Moreover, there is no problem even if the added magnetic component is subjected to a uniform distribution process as necessary so that it is uniformly distributed throughout.

  Since the amount to be added varies depending on the magnetic characteristics and the shape of the magnetic component to be added, an optimal addition amount may be appropriately selected depending on the magnetic component to be added.

  In consideration of the formation of the alignment thermoelectric material by a magnetic field and the thermoelectric properties of the alignment thermoelectric material, the thermoelectric material preferably contains at least a diamagnetic component. In this case, it is more preferable that the anisotropy of the diamagnetic susceptibility is large.

(C) The next step is a step of producing a dispersion (step S30) in which the thermoelectric fine particles to which the magnetic component is added as described above are dispersed in a solvent. As described above, this step can be omitted when the dispersion is not particularly used.

  There is no particular problem even if any of water, an organic solvent, and an inorganic solvent is used as the dispersion. 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.

  Here, there is no problem even if the magnetic component added in the above step is not added in the above step (without passing through the above step) and added in this step. In that case, there is no problem even if the magnetic component is added as it is, and there is no problem if it is added after the magnetic component is made into a dispersion. What is necessary is just to select suitably.

(D) The next step is a molding step in a magnetic field (step S40), that is, the above-described dispersion of thermoelectric fine particles (an aggregate when no dispersion is used) is inserted into the magnetic field to form a molded body. It is a process.

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. By this step, a molded body is formed in which fine particles having anisotropy of magnetic susceptibility are oriented in the direction of the applied magnetic field in the direction in which the magnetic susceptibility is large.

  Here, as in the present invention, when a magnetic component such as a paramagnetic component, a ferromagnetic component, and a ferrimagnetic component is added to the thermoelectric material, it is possible to produce a thermoelectric material with better orientation. Is possible.

  Furthermore, depending on the magnetic characteristics and shape of the magnetic component to be added, an oriented thermoelectric material can be produced even with a smaller magnetic field strength, which can contribute to the improvement of productivity.

(E) The subsequent process is a process of heat treatment in a magnetic field (step S50) 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 heat treatment (including sintering) in a magnetic field, the degree of orientation can be maintained even in this step. It is possible to form a bulk body. 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.

  Even in this step, when a magnetic component such as a paramagnetic component, a ferromagnetic component, and a ferrimagnetic component is added to the thermoelectric material as in the present invention, a thermoelectric material with better orientation is obtained. It is possible to manufacture.

  Furthermore, depending on the magnetic characteristics and shape of the magnetic component to be added, an oriented thermoelectric material can be produced even with a smaller magnetic field strength, which can contribute to the improvement of productivity.

  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.

  FIG. 2 is a conceptual diagram of a thermoelectric material (polycrystal) that does not have crystal orientation, and each of the crystal grains a of the thermoelectric material that is a constituent element varies in magnetic anisotropy (randomly). (In FIG. 2, the direction of the dotted line in each crystal grain a corresponds to the direction of high magnetic susceptibility).

  On the other hand, the oriented thermoelectric material (polycrystal) produced according to the present invention makes it easier to make the whole magnetic anisotropy easier by utilizing the property that the added magnetic component is oriented in the applied magnetic field direction. That is, the direction in which the magnetic susceptibility is large can be aligned in the entire sample.

  FIG. 3 is a conceptual diagram of an oriented thermoelectric material (polycrystal) produced according to the present invention, in which magnetic components (paramagnetism, ferromagnetism, ferrimagnetism) of added magnetic fine particles b are oriented in the applied magnetic field direction, A state in which the direction in which the magnetic susceptibility is large is uniform in the entire sample including the thermoelectric material fine particles c (in FIG. 3, the direction of the dotted line corresponds to the direction in which the magnetic susceptibility is large).

  For example, in a thermoelectric material composed of an oxide layered compound in which the anisotropy of electrical characteristics such as specific resistance or thermal conductivity corresponds to the anisotropy of magnetic susceptibility, the thermoelectric The characteristics can be improved.

  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. become.

  As described above, when the present invention is used, anisotropy of magnetic susceptibility is obtained by adding a magnetic component such as a paramagnetic component, a ferromagnetic component, and a ferrimagnetic component to a thermoelectric material and forming an oriented compact in a magnetic field. It is possible to obtain a molded body oriented along the.

  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.

Next, specific examples will be described.
Example 1
Co ₃ O ₄ powder and Na ₂ CO ₃ powder were sufficiently mixed and then baked at 880 ° C. for 20 hours in an electric muffle furnace.

After firing, this sample was pulverized to synthesize Na _x Co ₂ O _y (1 ≦ x ≦ 2, 2 ≦ y ≦ 4) fine particles.

  As a result of molding the fine particles while applying a magnetic field of 10 T (Tesla) using a superconducting magnet, a molded body oriented in a uniaxial direction could be formed (Sample 1).

On the other hand, composite fine particles obtained by adding 5 wt% of FeCr ₂ O ₄ fine particles as a paramagnetic component in pulverized Na _x Co ₂ O _y (1 ≦ x ≦ 2, 2 ≦ y ≦ 4) fine particles are similarly used. As a result of molding using a superconducting magnet while applying a magnetic field of 10 T (Tesla), a molded body oriented in the uniaxial direction was formed in this case as well (Sample 2).

  As a result of evaluating the orientation of Sample 1 and Sample 2 using a scanning electron microscope, it was found that the orientation of Sample 2 was better.

(Example 2)
In Example 1, as a magnetic component added to Na _x Co ₂ O _y (1 ≦ x ≦ 2, 2 ≦ y ≦ 4) fine particles, MnZn ferrite (38 mol% MnO-6.4 mol% ZnO-) as a ferrimagnetic component is used. As a result of forming composite fine particles to which 5 wt% of Fe ₂ O ₃ ) was similarly applied using a superconducting magnet while applying a magnetic field of 10 T, a molded body oriented in a uniaxial direction could be formed in this case as well (sample) 3).

  As a result of evaluating the orientation using a scanning electron microscope, the orientation of sample 3 was better than that of sample 1.

(Example 3)
When forming a compact using the same method as in Example 2, the magnetic field strength of the superconducting magnet was changed, and the orientation was similarly evaluated using a scanning electron microscope.

When only Na _x Co ₂ O _y (1 ≦ x ≦ 2, 2 ≦ y ≦ 4) fine particles were used, good orientation was confirmed at a magnetic field strength of 5 T or more, whereas Na _x Co ₂ When composite fine particles in which 5 wt% of MnZn ferrite (38 mol% MnO—6.4 mol% ZnO—Fe ₂ O ₃ ) is added to O _y (1 ≦ x ≦ 2, 2 ≦ y ≦ 4) fine particles are used, 2T Good orientation was confirmed at the above magnetic field strength.

  Thus, it has been found that when a magnetic component is added, a good alignment molded body can be formed even with a smaller magnetic field strength.

Example 4
The molded body (Sample 1) formed in Example 1 and the molded body (Sample 3) formed in Example 2 were sintered in air at 920 ° C. for 12 hours. Sintering was performed using a high temperature high magnetic field heat treatment apparatus while applying a 15 T magnetic field. 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 with a scanning electron microscope, the sample (sample 3) to which the ferrimagnetic component was added had better orientation. Thus, when the magnetic component was added, the orientation was good even in the sintered body.

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 in which the magnetic fine particle and the thermoelectric material fine particle were uniaxially oriented in the direction with a large magnetic susceptibility.

Explanation of symbols

a: Crystal grain of thermoelectric material b: Magnetic fine particle c: Thermoelectric material fine particle S10: Thermoelectric material fine particle synthesis step S20: Magnetic component addition step S30: Dispersion liquid preparation step S40: Molding step in magnetic field S50: In magnetic field Heat treatment process

Claims (2)

An oriented thermoelectric material in which crystals are oriented in a certain direction, wherein the oriented thermoelectric material contains at least a ferrimagnetic component and a diamagnetic component .   An alignment thermoelectric material manufacturing method for forming an alignment thermoelectric material by forming a thermoelectric material in a magnetic field, comprising the step of adding a ferrimagnetic component to the thermoelectric material. .

Images