JP4341440B2 - Thermoelectric conversion material - Google Patents

Description

  The present invention relates to a thermoelectric conversion material that directly converts heat into electricity. Thermoelectric modules manufactured using thermoelectric conversion materials efficiently convert unused energy such as exhaust heat from automobiles, various manufacturing plants, power plants, garbage incineration facilities, etc. into electricity efficiently. It contributes and has the effect of suppressing the emission of carbon dioxide, which has become a problem in recent years.

A thermoelectric conversion material is a material that has the property of converting heat energy directly into electrical energy (Seebeck effect) by creating a temperature difference at both ends of the material. The performance index ( Figure of Merit) Z is used, and a larger value means higher performance.
Z = α ² σ / κ (K ⁻¹ ) (1)
Here, α: Seebeck coefficient (V / K), σ: electrical conductivity (S / m), and κ: thermal conductivity (W / m · K).

Since Z is a dimension of the reciprocal of temperature, a dimensionless figure of merit ZT expressed as a dimensionless quantity is often used by multiplying this by an absolute temperature. In addition, the numerator α ² σ of the figure of merit is called an output factor and may be used as a measure of performance.

  FIG. 1 is a diagram showing a dimensionless figure of merit of a typical conventional thermoelectric conversion material. In the figure, a broken line and a solid line indicate a p-type and an n-type, respectively. The alternate long and short dash line indicates a curve having a dimensionless figure of merit of 1 as a measure of performance. As can be seen from this figure, the thermoelectric material needs to be properly used depending on the operating temperature range due to the temperature dependence of its characteristics. Of these thermoelectric conversion materials, the characteristics and characteristics of three typical materials that have been put to practical use will be briefly described.

As can be seen in FIG. 1, the BiTe-based material is a material that exhibits the highest performance index in the low temperature range, and is currently the most utilized thermoelectric material that has been put to practical use as a thermoelectric cooling material. As a practical material, a solid solution of Bi ₂ Te ₃ and Sb ₂ Te ₃ material is used, and the electrical characteristics are controlled by the ratio and addition of Bi ₂ Se ₃ or the like.

  The crystal structure of the BiTe-based material is a layered compound, and the physical properties are also strong anisotropy, making it possible to construct a high-performance element, but it has the property of being easily cleaved in a plane perpendicular to the c-axis The mechanical strength is difficult, and it is necessary to devise a sintering method.

  PbTe-based materials are materials used in the middle temperature range of 800K or less, and US SNAP (Systems for Nuclear Auxiliary Power), which develops a power generation system RTG (Radioactive Thermoelectric Generator) that uses the heat of the reactors used in space. It became the central material of the plan. The RTG produced by this project was installed in Apollo 12-17 and the exploration satellite Pioneer and Viking.

For use as a practical material, the characteristics are controlled using Ag ₂ Te or Na for the p-type and PbI ₂ or PbBr ₂ for the n-type as the dopant. However, since this material is easily oxidized in the atmosphere, it is necessary to devise measures such as enclosing it in a special container together with an inert gas in order to use it in the atmosphere.

As shown in FIG. 1, the material system having the highest performance index is a GeTe-AgSbTe ₂ system called TAGS, and the performance index Z at 400 ° C. (absolute temperature 673.15 K) is 2 × 10. It is a p-type material having a dimensionless figure of merit ZT which is as large as ⁻³ / K and whose dimensionless figure of merit ZT, which is the product of the figure of merit Z and the absolute temperature T, greatly exceeds 1. However, it is a material that is difficult to use, such as a structural phase transition occurring in the operating temperature range.

  The SiGe-based material is a material having excellent thermoelectric properties up to 1270 K. As a famous application example, there is an application to RTG mounted on a Voyage of a deep space exploration spacecraft.

  In order to use as a practical material, B is used as a dopant for p-type and P is used for n-type to control electrical characteristics. In recent years, it has been found that the addition of GaP can greatly reduce the thermal conductivity, and the figure of merit has been dramatically improved. The characteristics of the SiGe-based material shown in FIG. 1 indicate the characteristics of the material to which GaP is added.

  As described above, BiTe-based materials have the highest performance in the low temperature region, and are widely used practically as thermoelectric cooling materials. However, as a material used for thermoelectric power generation in the middle temperature range, which is typically an automobile exhaust gas, there is only a PbTe-based material such as TAGS that has high performance, but there are special examples such as space use. For a long time, there has been a demand for a practical high-performance thermoelectric conversion material in this intermediate temperature range.

  In addition, it has been said that there is no high-performance material exceeding ZT = 1 except for TAGS for a long time, and there is a “ZT = 1 wall”, but recently, a material exceeding ZT = 1. The system is now reported.

The ZnSb-based material described in the present invention is a material with a history, and a well-known example is the application example called “partisan rice”. It was used as. However, it has not received much attention because of its higher performance BiTe and PbTe materials. However, recently, there has been a report (see Non-Patent Document 1) that β-Zn ₄ Sb ₃ has achieved a figure of merit that exceeds TAGS (ZT is about 1.3 at 700K). It is a frightening material.

Proceedings of 15th International Conference on Thermoelectrics, p.150, 1996

  As described above, although ZnSb materials have been reported to have high performance, research into practical use has not progressed very easily. It is generally known among thermoelectric material researchers that the cause is a problem in the mechanical strength of the material.

Another problem is that the ZnSb powder is difficult to sinter. Therefore, there is a report that sintering requires high temperature and high pressure. However, β-Zn ₄ Sb ₃ has a β → γ transformation point in the vicinity of 490 ° C., and when sintering is attempted at a high temperature, the sintered body is cracked due to the phase transformation. In addition, sintering at a high pressure makes the apparatus large and the cost is greatly increased. Therefore, when sintering is performed by a normal uniaxial hot press, the density becomes low and the electrical conductivity is poor and the performance is low. Also, the strength is low due to the low density, which is a problem in practical use.

Accordingly, an object of the present invention is to provide a thermoelectric material having higher mechanical strength in a β-Zn ₄ Sb ₃ -based thermoelectric material having high thermoelectric performance. Thereby, the possibility of the efficient use of the unused energy in the middle temperature region represented by the exhaust gas of automobiles, which has not had a high performance thermoelectric conversion material that can withstand general use in the past, can be opened.

The present invention achieves the above object by providing the following thermoelectric conversion material.

That is, the present invention relates to a thermoelectric conversion material that is a sintered body of β-Zn ₄ Sb ₃ and contains at least one of Pb, Bi, and Sn as an additive component.

In the present invention, the additive component is Pb, and the Pb content is 0.5 to 1.2% in terms of a molar ratio to β-Zn ₄ Sb ₃ . About.

In particular, as a preferred form, the β-Zn ₄ Sb ₃ sintered body is a particle in which fine single crystal grains having a particle diameter of less than 20 μm are primary particles and irregular crystal grains having a particle diameter of less than 20 μm are closely packed. It is a sintered body having a fine structure filled with the primary particles between polycrystalline grains having a diameter of 10 μm to a diameter of 200 μm.

In the present invention, a sintered body of β-Zn ₄ Sb ₃ , which contains at least one of Pb, Bi, and Sn as an additive component, so that a sintered body of β-Zn ₄ Sb ₃ A thermoelectric conversion material with improved mechanical strength can be provided. Further, the sintered body of β-Zn ₄ Sb ₃ has a particle diameter of 10 μm, in which fine single crystal grains having a particle diameter of less than 20 μm are primary particles, and irregular crystal grains having a particle diameter of less than 20 μm are closely packed. A thermoelectric material having excellent thermoelectric properties and mechanical strength can be provided by having a fine structure filled with the primary particles between the 200 μm polycrystalline grains.

In the thermoelectric material, the additive component is Pb, and the Pb content is 0.5 to 1.2% in terms of a molar ratio with respect to β-Zn ₄ Sb ₃ . An excellent thermoelectric conversion material having a figure of merit ZT of up to 2.3 can be provided.

  Hereinafter, the thermoelectric conversion material of the present invention will be described first.

The basic crystal structure of the thermoelectric conversion material of the present invention is a sintered body of β-Zn ₄ Sb ₃ and includes at least one of Pb, Bi, and Sn as an additive component. Pb, Bi, and Sn is to improve the sinterability of the _β-Zn 4 Sb _3, to strengthen the bonds between _β-Zn 4 Sb ₃ particles, while maintaining a high thermoelectric properties, mechanical sintered body Strength can be increased.

The content of the additive components of Pb, Bi, and Sn in the sintered body is not particularly limited as long as preferable thermoelectric performance and mechanical strength can be obtained. Preferably, the content relative to β-Zn ₄ Sb ₃ The molar ratio is 10% or less, further preferably 0.2 to 8%, and more preferably 0.5 to 5%. When the content of the additive component is small, the effect of improving the strength is small. On the other hand, when the content of the additive component is too large, the precipitation of the element alone and the heterogeneous phase are generated more than necessary, which tends to cause deterioration of the thermoelectric performance. is there.

In addition to the above-described strength improvement effect, β-Zn ₄ Sb ₃ to which Pb is added has a particularly remarkable effect on thermoelectric performance. In particular, in a sintered body of β-Zn ₄ Sb ₃ having a Pb content of 0.5 to 1.2% in terms of a molar ratio to β-Zn ₄ Sb ₃ , the dimensionless figure of merit ZT is the highest. Excellent thermoelectric performance of 2.3. As described above, in the dimensionless figure of merit ZT indicating thermoelectric performance, materials exceeding ZT = 1 have not appeared until recently. In the β-Zn ₄ Sb ₃ of the present invention, as already described, the patent application filed by the present inventors for the invention that ZT = 1.6 was achieved at 400 ° C. even in the case of no addition. No. 2002-11454, which is described in detail.

  The present invention is the first in the development of a material exhibiting a particularly excellent performance that has not been achieved so far, in addition to the above-described strength improvement effect, the dimensionless figure of merit ZT exceeds 1.6 and the maximum is ZT = 2.3. It is a success.

In a preferred form of the sintered body of β-Zn ₄ Sb ₃ , fine single crystal grains are used as primary particles, and the primary particles are filled between polycrystalline grains in which fine amorphous crystals are closely packed. It consists of a sintered body of β-Zn ₄ Sb ₃ having the structure:

  The primary particles (single crystal grains) have a particle size of less than 20 μm, preferably 15 μm or less, more preferably 10 μm. The polycrystalline grains have a particle size of 10 to 200 μm, preferably a particle size of 15 to 150 μm, and more preferably a particle size of 20 to 100 μm. The inside of the polycrystalline grains has a structure in which amorphous crystal grains having a grain size of less than 20 μm, preferably 15 μm or less, more preferably 10 μm or less are densely and uniformly distributed.

  As described above, the sintered body is a fine particle composed of primary particles (single crystal grains) having a particle size of less than 20 μm and polycrystalline grains having an amorphous crystal particle having a particle size of less than 20 μm. It preferably has a structure.

  The primary particles (single crystal grains) and polycrystalline grains constituting the sintered body and the amorphous grains constituting the polycrystalline grains have the following particle size measurement method: It is measured by.

[Measuring method of particle size]
1. The sample (sintered body) is cut and the cross section is polished.
2. The sample polished surface is chemically etched so that the grain boundaries can be visually confirmed with a microscope.
3. Magnify the etched surface with an optical microscope. Under the same conditions, take a standard-scale photo.
4). Read a photo with a scanner and import it into a computer as an image. At that time, in order to know the enlargement ratio, the standard scale photograph taken under the same conditions is taken in the same way.
5. Trace the grain boundaries of the obtained image on the computer screen, leaving only the trace image.
6). From the obtained trace image, the particle size is measured by image processing software “NIH Image (Public domain software, by Wayne Rasband, National Institute of Health, USA)”. As a measurement principle, the number of pixels surrounded by the trace is counted as an area, and converted into a diameter of a circle having the same area as a particle diameter. At that time, since the enlargement ratio can be calculated by measuring the scale bar on the screen, it is divided by the enlargement ratio to obtain the actual particle size.

In the sintered body of β-Zn ₄ Sb ₃ having a dense microstructure as described above, the electric conductivity is large. For this reason, when the carrier density is considered constant, the electric conductivity is almost constant even if the Seebeck coefficient is almost constant. Since the contribution is large, the output factor (power factor) is greatly improved. Further, in such a fine structure, the thermal conductivity is lowered, and the figure of merit is greatly increased also from this aspect. The above-mentioned fine microstructure and the manufacturing method thereof are described in detail in Japanese Patent Application No. 2002-11454 filed by the present inventors.

  Next, a preferred embodiment of the method for producing a thermoelectric conversion material of the present invention will be described.

First, a predetermined amount of Zn powder, Sb powder, and an additive component composed of at least one of Pb, Bi, and Sn are weighed, and a molten material of β-Zn ₄ Sb _{3 that is} a sintering raw material by a melting method Manufacturing. Next, the molten material is pulverized and classified to produce β-Zn ₄ Sb ₃ having a particle size of less than 20 μm and β-Zn ₄ Sb ₃ molten material having a particle size larger than the particle size of the molten material and not more than 200 μm. To do. Next, these melted materials are mixed to obtain a sintered raw material. And _β-Zn 4 Sb ₃ ingot material of particle size less than 20 [mu] m, the mixing ratio of the large and 200μm following _β-Zn 4 Sb ₃ ingot material than the particle diameter of the particle size solution lumber, by weight, preferably The former: the latter = 1: 5 to 100, more preferably 1:10 to 50.

  Next, the sintered body raw material is filled in a mold, heated and pressurized to produce a sintered body.

  The heating / pressurizing conditions are preferably a temperature of 400 ° C. to 550 ° C. and a pressure of 30 to 200 MPa, more preferably a temperature of 450 ° C. to 490 ° C. and a pressure of 40 to 100 MPa. Examples of the heating / pressurizing means include a hot press method and a HIP method.

The thermoelectric conversion material of the present invention contains at least one additive component of Pb, Bi, and Sn, and thus has a much higher strength than a β-Zn ₄ Sb ₃ sintered body that does not contain an additive component. It becomes the material which has.

Hereinafter, the effects of the present invention will be specifically described with reference to Examples and Comparative Examples.

Example 1
Weighed 84.0396 g of Zn powder (Rare Metallic, purity 99.99%, particle size 10-30 mesh) and 115.604 g of Sb powder (Rare Metallic, purity 99.99%, particle size 10-30 mesh). Three powder samples were prepared in which Zn was 0.3 at% richer than the stoichiometric composition. For each powder sample, Pb was weighed to a molar ratio of 3% with respect to β-Zn ₄ Sb ₃ (eg, 3 mol of Pb with respect to 100 mol of β-Zn ₄ Sb ₃ ), Added to each powder sample and mixed. These were sealed by introducing an inert gas into an ampule tube. This ampule tube was set in a melting and stirring furnace, and a melted β-Zn ₄ Sb ₃ melted material was produced. Next, this melted material was dry-pulverized with a jet mill to prepare a raw material 1 having a particle size of less than 20 μm. Further, the melted material was dry-pulverized with an automatic mortar to prepare a raw material 2 having a particle size of 20 to 200 μm. These raw materials 1 and 2 were dry-mixed in a V blender for 24 hours at a weight ratio of raw material 1: raw material 2 = 1: 15 to obtain a sintered raw material. This sintered raw material was filled in a graphite mold and hot-pressed at 470 ° C. for 300 minutes under a pressure of 40 MPa to obtain a β-Zn ₄ Sb ₃ sintered body.

The density of the sintered body measured by the Archimedes method is 6.25 to 6.39 g / cm ³ , and the crystal structure (HW Mayer, I. Mikhail, and K. Schubert, J The theoretical density calculated from Less Common Metals 59, 43 (1978)) was significantly higher than 6.078 g / cm ³ .

The obtained sintered body was pulverized and subjected to powder X-ray diffraction measurement. As a result, as shown in FIG. 2, a pattern consistent with the β-Zn ₄ Sb ₃ single-phase data, which is a crystal structure that has been generally known as reliable data, was obtained. The relationship between the powder X-ray diffraction data and the density is unknown.

A test piece of 3w × 1.5t × 20L was cut out from the sintered body produced as described above, the Seebeck coefficient and the electrical conductivity were measured, and the output factor (power factor) was calculated. As a result, it has been clarified that all of them have the same characteristics as the β-Zn ₄ Sb ₃ sintered body produced in the same manner as described above without addition of elements. Further, the dimensionless figure of merit obtained by measuring the thermal conductivity has the same characteristics as the β-Zn ₄ Sb ₃ sintered body produced in the same manner as described above without addition of elements, and has been reported in the past. It was confirmed that the thermoelectric performance was superior to the dimensionless figure of merit 1.3 (at 700 K) of the —Zn ₄ Sb ₃ -based material. The results are shown in Table 1.

When a three-point bending test piece of 4 w × 3 t × 20 L was cut out from the sintered body and a three-point bending test was performed, a high value exceeding the β-Zn ₄ Sb ₃ sintered body without element addition shown in the following comparative example. Got. The results are shown in Table 1. This fracture surface is as shown in FIG. 5, and the fracture has a form of intragranular fracture, and it has been proved that the bond between the particles is also strong.

Example 2
Instead of Pb, which is the additive element of Example 1, Bi was added in a molar ratio of 3% with respect to β-Zn ₄ Sb _{3 in the same} manner as in Example 1, and β- A Zn ₄ Sb ₃ sintered body was obtained. The firing conditions, measurement conditions, and the like are all the same as in Example 1. FIG. 3 shows a powder X-ray diffraction pattern obtained by pulverizing and measuring the obtained sintered body. The characteristic measurement results are shown in Table 1, and the fracture surface of this sintered body is shown in FIG.

Example 3
Instead of Pb, which is the additive element of Example 1, Sn was added in a molar ratio of 3% with respect to β-Zn ₄ Sb _{3 in the same} manner as in Example 1, and β- A Zn ₄ Sb ₃ sintered body was obtained. The firing conditions, measurement conditions, and the like are all the same as in Example 1. FIG. 4 shows a powder X-ray diffraction diagram obtained by pulverizing and measuring the obtained sintered body. The characteristic measurement results are shown in Table 1, and the fracture surface of this sintered body is shown in FIG.

Example 4
Was added to 0.5% in the molar ratio with respect to Pb is _beta-Zn 4 Sb ₃ is a added element of Example 1, to obtain a _beta-Zn 4 Sb ₃ sintered body of three samples . The firing conditions, measurement conditions, and the like are all the same as in Example 1. The powder X-ray diffraction pattern obtained by crushing and measuring the obtained sintered body was confirmed to be the same pattern as that of FIG. The characteristic measurement results are shown in Table 1. It was also confirmed that the fracture surface of the sintered body showed the same aspect as that of FIG.

Example 5
Was added to a 1.0% by molar ratio relative to Pb is _beta-Zn 4 Sb ₃ is a added element of Example 1, to obtain a _beta-Zn 4 Sb ₃ sintered body of three samples . The firing conditions, measurement conditions, and the like are all the same as in Example 1. The powder X-ray diffraction pattern obtained by crushing and measuring the obtained sintered body was confirmed to be the same pattern as that of FIG. The characteristic measurement results are shown in Table 1. It was also confirmed that the fracture surface of the sintered body showed the same aspect as that of FIG.

Example 6
Was added to a 1.2% by molar ratio relative to Pb is _beta-Zn 4 Sb ₃ is a added element of Example 1, to obtain a _beta-Zn 4 Sb ₃ sintered body of three samples . The firing conditions, measurement conditions, and the like are all the same as in Example 1. The powder X-ray diffraction pattern obtained by crushing and measuring the obtained sintered body was confirmed to be the same pattern as that of FIG. The characteristic measurement results are shown in Table 1. It was also confirmed that the fracture surface of the sintered body showed the same aspect as that of FIG.

Comparative Example 1
A β-Zn ₄ Sb ₃ sintered body was obtained by the same process as in Example 1 except that the third element was not added and a predetermined amount of Zn and Sb was used.

The density measured by Archimedes method was 6.25 g / cm ³ , which was equivalent to the density of the sample of Example 1. Table 1 shows the results of processing the obtained sintered body into a sample piece similar to Example 1 and measuring the three-point bending strength.

It is a figure which shows the figure of merit of the conventional typical thermoelectric conversion material. FIG. 3 is a powder X-ray diffraction diagram obtained by pulverizing and measuring the β-Zn ₄ Sb ₃ sintered body to which 3% of Pb is added, obtained in Example 1. _{4 is} a powder X-ray diffraction diagram obtained by pulverizing and measuring a β-Zn ₄ Sb ₃ sintered body to which 3% of Bi is added obtained in Example 2. FIG. _{4 is} a powder X-ray diffraction pattern obtained by pulverizing and measuring a β-Zn ₄ Sb ₃ sintered body added with 3% of Sn obtained in Example 3. FIG. It is a SEM photograph of the bending fracture surface after the three-point bending test of the β-Zn ₄ Sb ₃ sintered body added with 3% Pb obtained in Example 1. It is a SEM photograph of the bending fracture surface after the three-point bending test of the β-Zn ₄ Sb ₃ sintered body added with 3% Bi obtained in Example 2. It is a SEM photograph of the bending fracture surface after the three-point bending test of the β-Zn ₄ Sb ₃ sintered body added with 3% of Sn obtained in Example 3.

Claims (3)

β-Zn ₄ Sb ₃ sintered body containing at least one of Pb, Bi, and Sn as an additive component, and the content of the additive component in the sintered body is β-Zn _{4. A} thermoelectric conversion material having a molar ratio with respect to ₄ Sb ₃ of 0.2 to 10% . 2. The thermoelectric conversion material according to claim 1, wherein the additive component is Pb, and the Pb content is 0.5 to 1.2% in terms of a molar ratio to β-Zn ₄ Sb ₃ . In the sintered body of β-Zn ₄ Sb ₃ , fine single crystal grains having a particle diameter of less than 20 μm are used as primary particles, and irregular crystal grains having a particle diameter of less than 20 μm are closely packed, and the particle diameter is from 10 μm to 200 μm. The thermoelectric conversion material according to claim 1, which is a sintered body having a fine structure filled with the primary particles between the polycrystalline grains.

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