JP2013065585A - Thermoelectric conversion element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a thermoelectric conversion element capable of achieving both thermoelectric performance and reliability/durability.SOLUTION: A columnar thermoelectric conversion element for converting thermal energy into electrical energy or electrical energy into thermal energy by utilizing a temperature difference between one end surface and other end surface in the axial direction has a constitution including a fist thermoelectric conversion section forming a central layer and a second thermoelectric conversion section forming a coating layer formed with a predetermined thickness on a peripheral surface of the first thermoelectric conversion section. The first thermoelectric conversion section comprises NaCo-based oxide, and the second thermoelectric conversion section comprises CaCo-based oxide.

Description

  The present invention relates to a thermoelectric conversion element used in a thermoelectric conversion module.

Conventionally, a thermoelectric conversion module that directly converts thermal energy into electrical energy or electrical energy into thermal energy using the Seebeck effect or Peltier effect of a thermoelectric conversion element is known.
The structure of a general thermoelectric conversion module is shown in FIG. As shown in FIG. 1, the thermoelectric conversion module 1 includes a thermoelectric element pair 10 in which a thermoelectric conversion element 11 made of a p-type semiconductor and a thermoelectric conversion element 12 made of an n-type semiconductor are connected to a “π” type by a metal electrode 13. A large number are assembled and electrically connected in series, and sandwiched between two insulating substrates (for example, ceramic substrates) 14 and 15.

  When the flat thermoelectric conversion module 1 is arranged so that one surface (for example, the insulating substrate 14) is on the high temperature side and the other surface (for example, the insulating substrate 15) is on the low temperature side, a temperature difference is given between both surfaces. An electromotive force is generated. This electric power is taken out through current leads 16 and 17 connected to the thermoelectric conversion module 1. Conversely, when a current is passed through the thermoelectric conversion module 1 via the current leads 16 and 17, heat is generated on one surface (for example, the insulating substrate 14) and heat is generated on the other surface (for example, the insulating substrate 15).

  The thermoelectric conversion elements 11 and 12 used in the thermoelectric conversion module 1 are used for a long time in a state where one end is at a high temperature (about 900 ° C.) and the other end is at a normal temperature, so that the thermoelectric performance is high. High reliability and durability are required.

  The dimensionless figure of merit ZT represented by the following formula (1) is used for the performance evaluation of the thermoelectric conversion material. The higher the dimensionless figure of merit ZT, the better the thermoelectric conversion material with excellent thermoelectric performance. From equation (1), in order to increase the dimensionless figure of merit ZT, the Seebeck constant S is large (the Seebeck effect is large), the resistivity ρ is small (the Joule heat loss is small), and the thermal conductivity κ is It is desirable to be small (a temperature difference is maintained).

ZT = (S ² / (ρ · κ)) · T (1)
ZT: dimensionless figure of merit S: Seebeck constant (V / K)
ρ: resistivity (Ω · m)
κ: Thermal conductivity (W / m · K)
T: Absolute temperature (K)

  Thermoelectric conversion materials are roughly classified into metal-based materials and oxide-based materials. Examples of metal materials include bismuth / tellurium materials and silicide materials. These metal-based materials have a dimensionless figure of merit ZT compared to oxide-based materials, but they contain rare and toxic elements, and are prone to device degradation due to oxidation in high-temperature air at 600 ° C or higher. There are several disadvantages such as limited environment.

  On the other hand, oxide-based materials have a low dimensionless figure of merit ZT compared to metal-based materials, but many materials are stable at high temperatures of 873 K (600 ° C.) or higher. Among these, a single-crystal layered cobalt oxide is promising as a thermoelectric conversion material that can be used in a high-temperature atmosphere because the dimensionless figure of merit ZT exceeds 1 at 700 to 1000 K (427 to 727 ° C.).

However, since the layered cobalt oxide has anisotropy, the physical property value depends on the orientation of the crystallite. That is, even in the case of a layered cobalt oxide, the dimensionless figure of merit ZT is reduced to a fraction of that of a polycrystal having crystallites randomly oriented.
Therefore, the present inventors have made extensive studies focusing on the use of layered cobalt oxide as a thermoelectric conversion material, and using extrusion molding to improve the orientation of the crystallites of the layered cobalt oxide. It has been proposed to produce a conversion element (for example, Patent Document 1).

JP 2010-245089 A

By the way, examples of the layered cobalt oxide include NaCo-based oxides (for example, NaCo ₂ O ₄ ) and CaCo-based oxides (for example, Ca ₃ Co ₄ O ₉ ). In any case, the single crystal has a dimensionless figure of merit ZT of 1 or more at 955 K (682 ° C.), and has excellent thermoelectric performance.

In particular, NaCo-based oxides are polycrystalline and have a dimensionless figure of merit ZT of 0.5 to 0.7 at 955 K (982 ° C.), and are excellent in thermoelectric performance. However, since it contains Na, it is vulnerable to moisture, and if left in the atmosphere, the surface deteriorates and the electrode peels off or cracks in the material itself, resulting in poor stability.
On the other hand, CaCo-based oxides are more stable in the atmosphere than NaCo-based oxides, but because of their high anisotropy, the dimensionless figure of merit ZT for polycrystals is 973-1123 K (700-850 ° C.
) At about 0.1 to 0.3. Even when the technique described in Patent Document 1 is used, the dimensionless figure of merit ZT is about 0.1 to 0.2 at 973 to 1123 K (700 to 850 ° C.), which is lower than the NaCo-based oxide.
Thus, in a thermoelectric conversion element made of a single layered cobalt oxide, it is difficult to achieve both thermoelectric performance and reliability / durability.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a thermoelectric conversion element that can achieve both thermoelectric performance and reliability / durability.

A thermoelectric conversion element according to the present invention is a columnar thermoelectric conversion element for converting thermal energy into electrical energy or electrical energy into thermal energy using a temperature difference between one end face and the other end face in the axial direction. ,
A first thermoelectric conversion part serving as a core material, and a second thermoelectric conversion part serving as a coating layer formed with a predetermined thickness on the peripheral surface of the first thermoelectric conversion part,
The first thermoelectric converter is composed of a NaCo-based oxide;
The second thermoelectric conversion part is made of a CaCo-based oxide.

In the present invention, the first thermoelectric conversion portion serving as the core material is unstable in the atmosphere, but is covered with the second thermoelectric conversion portion that is stable in the atmosphere, and thus is not directly exposed to the atmosphere. Therefore, deterioration of the thermoelectric conversion element is effectively suppressed.
In addition, the core material is made of NaCo-based oxide having high thermoelectric performance, and the second thermoelectric conversion portion serving as the coating layer also contributes to thermoelectric conversion, so that high thermoelectric performance is ensured.
That is, according to this invention, the thermoelectric conversion element which can make thermoelectric performance and reliability and durability compatible is implement | achieved.

It is a figure which shows the structure of a thermoelectric conversion module. It is a figure which shows the structure of the thermoelectric conversion element which concerns on embodiment. It is a figure shown about the preparation methods of the thermoelectric conversion element using an extrusion method.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
The thermoelectric conversion element according to the embodiment is used as the p-type thermoelectric conversion element 11 in the thermoelectric conversion module 1 that converts thermal energy into electric energy using, for example, a temperature difference between one end face and the other end face in the axial direction. (See FIG. 1).

  FIG. 2 is a diagram illustrating a configuration of the thermoelectric conversion element according to the present embodiment. As shown in FIG. 2, the thermoelectric conversion element 11 is a columnar member, and is formed with a predetermined thickness on the first thermoelectric conversion portion 11 </ b> A serving as a core material and the peripheral surface of the first thermoelectric conversion portion 11 </ b> A. And a second thermoelectric conversion portion 11B that serves as a coating layer. That is, the thermoelectric conversion element 11 has a two-layer structure in which different thermoelectric conversion materials are formed concentrically.

11A of 1st thermoelectric conversion parts are unstable in air | atmosphere, but have high thermoelectric performance (The dimensionless figure of merit ZT in a polycrystal is 955K (682 degreeC) 0.5-0.7). Consists of oxides. The NaCo-based oxide is a substance containing Na among the layered cobalt oxide, and is represented by Na _x Co ₂ O ₄ (1.0 ≦ x ≦ 2.0). Among the NaCo-based oxides, Na _x Co ₂ O ₄ (1.0 ≦ x ≦ 1.4) is suitable as the first thermoelectric conversion unit 11A. This is because it has a small thermal conductivity due to random defects of Na atoms. Of these, NaCo ₂ O ₄ is more preferred because it has the lowest thermal conductivity.

The second thermoelectric conversion unit 11B is composed of a CaCo-based oxide that is stable in the atmosphere but has a lower thermoelectric performance than the NaCo-based oxide. The CaCo-based oxide is a material containing Ca among the layered cobalt oxides, and Ca _3−y A _y Co ₄ O ₉ (A: Bi, Sr, Mg, Gd, Y, K, Na) One or two elements, 0 ≦ y ≦ 0.6). Among the CaCo-based oxides, Ca _2.6 Bi _0.4 Co ₄ O ₉ , Ca _2.7 Sr _0.3 Co ₄ O ₉ , and Ca ₃ Co ₄ O ₉ are suitable as the second thermoelectric conversion unit 11B. This is because these materials have a high density when sintered and a low resistance value ρ, that is, a high thermoelectric performance (dimensionless figure of merit ZT). In particular, Ca _2.6 Bi _0.4 Co ₄ O ₉ is optimal because the dimensionless figure of merit ZT is highest at 0.2 to 0.3 at 973 to 1123 K (700 to 850 ° C.).

  The diameter of the first thermoelectric conversion unit 11 </ b> A and the thickness of the second thermoelectric conversion unit 11 </ b> B are designed to satisfy the thermoelectric performance required for the thermoelectric conversion element 11. When the outer diameter of the thermoelectric conversion element 11 is constant, the thermoelectric performance is improved as the diameter of the first thermoelectric conversion unit 11A is larger (the thickness of the second thermoelectric conversion unit 11B is thinner). However, if the diameter of the first thermoelectric conversion part 11A is too large, the thickness of the second thermoelectric conversion part 11B becomes too thin, and the stability in the atmosphere by the second thermoelectric conversion part 11B may be impaired. is there.

  Therefore, it is desirable that the thickness of the second thermoelectric conversion unit 11B is 150 to 1100 μm. By setting the thickness of the second thermoelectric conversion part 11B to 150 μm or more, the stability of the thermoelectric conversion element 11 in the atmosphere is ensured. On the other hand, if the thickness of the second thermoelectric conversion part 11B becomes too thick, the stability in the atmosphere becomes excessive and the thermoelectric performance is relatively lowered. The thickness is desirably 1100 μm or less.

The thermoelectric conversion element 11 is produced using, for example, an extrusion method. In the extrusion molding method, since the crystallites are oriented in the extrusion direction, the resistivity ρ in the axial direction (extrusion direction) of the thermoelectric conversion element 11 becomes small. That is, the thermoelectric conversion element 11 having high thermoelectric performance (dimensionalless figure of merit ZT) can be produced by using the extrusion method.
In addition, the extrusion molding method is preferable from the viewpoint of mass productivity because an extrusion molded body can be continuously produced. Furthermore, the finished product of the thermoelectric conversion element 11 can be obtained simply by cutting the extruded product in a direction perpendicular to the extrusion direction, which is also preferable from the processing surface.

  Specifically, as shown in FIG. 3, from the first die 101 for forming the first thermoelectric conversion unit 11A and the second die 102 for forming the second thermoelectric conversion unit 11B. The thermoelectric conversion element 11 can be produced by an extruder equipped with the double die 100. The through hole 101a formed in the first die 101 serves as a raw material flow path of the first thermoelectric conversion unit 11A, and the space 102a sandwiched between the first die 101 and the second die 102 is the second thermoelectric conversion. It becomes the flow path of the raw material of the part 11B.

  When producing the thermoelectric conversion element 11, first, as a raw material of the first thermoelectric conversion part 11A, a NaCo-based raw material powder, a binder (for example, an organic binder), and a plasticizer (for example, water) are kneaded to form a NaCo-based. An admixture (compound) is produced. Further, as a raw material of the second thermoelectric conversion unit 11B, a CaCo-based raw material powder, a binder (for example, an organic binder), and a plasticizer (for example, water) are kneaded to generate a CaCo-based mixture (compound).

When a NaCo-based mixture is introduced from the upstream side in the extrusion direction of the first die 101 and extruded at a predetermined extrusion pressure, the first thermoelectric conversion portion 11A is extrusion molded. The cross-sectional shape of the first thermoelectric conversion unit 11A is substantially the same as the shape (for example, a circle) of the extrusion port 101b of the first die 101.
Similarly, a CaCo-based mixture is introduced from the upstream side of the second die 102 in the extrusion direction and extruded at a predetermined extrusion pressure. Then, the CaCo-based mixture is pushed out from the clearance between the first thermoelectric conversion unit 11A and the extrusion port 102b of the second die 102, and the second thermoelectric conversion unit 11B is formed on the peripheral surface of the first thermoelectric conversion unit 11A. Extruded with a predetermined thickness.
Since both the NaCo-based admixture and the CaCo-based admixture are in the form of clay, the adhesion between them in the extruded body of the thermoelectric conversion element 11 is extremely good.

And after evaporating the water which is a plasticizer from an extrusion molding (drying process), pyrolyzing and vaporizing an organic binder (degreasing process), it heat-processes at high temperature and sinters (sintering process, for example, 950) ° C x 20 hours). Since both the first thermoelectric conversion part 11A and the second thermoelectric conversion part 11B are composed of a layered cobalt oxide, no distortion or the like occurs at the interface between them in the above process (especially the sintering process). That is, in the sintered compact of the thermoelectric conversion element 11, the first thermoelectric conversion part 11A and the second thermoelectric conversion part 11B are well bonded and are in an extremely stable state.
The produced sintered compact of the thermoelectric conversion element 11 is cut into a predetermined length (for example, 5 mm) and used for the thermoelectric conversion module 1.

  As described above, the thermoelectric conversion element 11 according to the embodiment uses the temperature difference between one end surface and the other end surface in the axial direction to convert thermal energy into electrical energy or columnar thermoelectric for converting electrical energy into thermal energy. 11A of 1st thermoelectric conversion parts which are conversion elements and become a core material, and 2nd thermoelectric conversion part 11B used as the coating layer formed in the surrounding surface of 11 A of 1st thermoelectric conversion parts by predetermined thickness Have. And the 1st thermoelectric conversion part 11A is comprised with the NaCo type oxide, and the 2nd thermoelectric conversion part 11B is comprised with the CaCo type oxide.

In the thermoelectric conversion element 11, the first thermoelectric conversion part 11A serving as the core material is unstable in the atmosphere, but is covered with the second thermoelectric conversion part 11B that is stable in the atmosphere, so that it is directly exposed to the atmosphere. Not. For this reason, the first thermoelectric converter 11A is not required to be stable in the atmosphere. Therefore, deterioration of the thermoelectric conversion element 11 is effectively suppressed.
In addition, the core material is made of NaCo-based oxide having high thermoelectric performance, and the second thermoelectric conversion portion 11B serving as the coating layer also contributes to thermoelectric conversion, so that high thermoelectric performance is ensured.
That is, in the thermoelectric conversion element 11, thermoelectric performance and reliability / durability are compatible.

[Example]
In Examples 1 to 3, NaCo ₂ O ₄ having a particle size of 1 to 3 μm was used as the raw material powder of the first thermoelectric conversion unit 11A. Further, Ca ₃ Co ₄ O ₉ having a particle size of 5 to 10 μm was used as the raw material powder for the second thermoelectric conversion part 11B.
And the thermoelectric conversion element 11 was produced using the extrusion forming method mentioned above. At this time, the outer diameter of the thermoelectric conversion element 11 is constant at 5.0 mm, and the thickness of the second thermoelectric conversion portion 11B is 200 μm (Example 1), 500 μm (Example 2), and 1000 μm (Example 3). Changed.
In Examples 4 to 6, the same raw material powder as in Example 1 was used as the first thermoelectric conversion part 11A, and Ca _2.6 Bi _0.4 Co having a particle diameter of 5 to 10 μm was used as the raw material powder of the second thermoelectric conversion part 11B. Thermoelectric conversion element 11 was fabricated using ₄ O ₉ .
In Examples 7 to 9, the same raw material powder as in Example 1 was used as the first thermoelectric conversion part 11A, and the Ca _2.7 Sr _0.3 Co having a particle diameter of 5 to 10 μm was used as the raw material powder of the second thermoelectric conversion part 11B. Thermoelectric conversion element 11 was fabricated using ₄ O ₉ .

In Comparative Example 1, a thermoelectric conversion element 11 made of a single NaCo-based oxide was produced by the same method as in the example using NaCo ₂ O ₄ having a particle size of 1 to 3 μm. In Comparative Examples 2 to 4, the particle diameters of Ca ₃ Co ₄ O ₉ (Comparative Example 2), Ca _2.6 Bi _0.4 Co ₄ O ₉ (Comparative Example 3), and Ca _2.7 Sr _0.3 Co ₄ O ₉ are 5 to 10 μm. Using (Comparative Example 4), a thermoelectric conversion element 11 made of a single CaCo-based oxide was produced by the same method.

  The thermoelectric conversion module 1 shown in FIG. 1 was produced using the thermoelectric conversion element 11 of Examples 1-9 and Comparative Examples 1-4, and each thermoelectric performance was evaluated. Specifically, 700 K (427 ° C.) when the insulating substrate 14 side is continuously raised from room temperature to 900 K (627 ° C.) while the insulating substrate 15 side of the thermoelectric conversion module 1 is cooled to water and maintained at room temperature. ) And 900K (627 ° C.), the Seebeck constant S, the resistivity ρ, and the thermal conductivity κ were measured, and the dimensionless figure of merit ZT (the initial value (A) of ZT in Tables 1 and 2) was calculated. Thereafter, the thermoelectric conversion module 1 was left in the room temperature atmosphere and cooled until the insulating substrate 14 side reached room temperature.

After the measurement, the insulating substrate 15 side was further cooled from water to maintain the room temperature, while the insulating substrate 14 side was continuously raised from room temperature to 950 K (677 ° C.) and left in this state for about 5 hours. Then, by leaving the thermoelectric conversion module 1 in the room temperature atmosphere and cooling the insulating substrate 14 side to room temperature, a temperature history from room temperature to 950 K (677 ° C.) was added, and this was repeated 10 times.
Thereafter, Seebeck constant S, resistivity ρ, heat conduction at 700 K (427 ° C.) and 900 K (627 ° C.) when continuously raised from room temperature to 900 K (627 ° C.) in the same manner as described above. The rate κ was measured again, the dimensionless figure of merit ZT (value after thermal history of ZT in Tables 1 and 2 (B)) was calculated, and the influence of thermal history was examined.

  The results are shown in Tables 1 and 2. The dimensionless figure of merit ZT (A, B) is preferably 0.20 or more at 700K (427 ° C.) and 0.25 or more at 900K (627 ° C.). The value of C = (B−A) / A · 100 [%]) is desirably −20% or more.

As shown in Table 1, the thermoelectric conversion element 11 has a two-layer structure of a first thermoelectric conversion unit 11A (inner layer) and a second thermoelectric conversion unit 11B (outer layer), and a second thermoelectric conversion unit 11B (outer layer). With the thickness of 150 to 1100 μm, good results are obtained for the dimensionless figure of merit ZT (A, B) at 700K (427 ° C.) and 900 K (627 ° C.) and the amount of change (C) in ZT. (Examples 1-9).
In particular, when the thickness of the second thermoelectric converter 11B (outer layer) is set to 300 to 700 μm, the dimensionless figure of merit ZT (B) after the thermal history hardly changes compared to the initial value (A) ( −3 ≦ C ≦ 3), the thermoelectric conversion element 11 having extremely small deterioration in thermoelectric performance was obtained (Examples 2, 5, and 8).

  On the other hand, as shown in Table 2, when the thermoelectric conversion element 11 has a single-layer structure made only of NaCo-based oxide, the initial value of the dimensionless figure of merit ZT is a good value, but the change amount C of ZT It was confirmed that the thermoelectric performance was significantly deteriorated by adding heat history (Comparative Example 1). Further, when the thermoelectric conversion element 11 has a single-layer structure made only of CaCo-based oxides, the dimensionless figure of merit ZT is small both in the initial value (A) and after the thermal history (B), and there is a problem in practical use. It was confirmed (Comparative Examples 2 to 4).

As mentioned above, the invention made by the present inventor has been specifically described based on the embodiment. However, the present invention is not limited to the above embodiment, and can be changed without departing from the gist thereof.
For example, the thermoelectric conversion element 11 may have a polygonal column shape instead of a cylindrical shape. Further, one of the first thermoelectric conversion unit 11A and the second thermoelectric conversion unit 11B may have a cylindrical shape, and the other may have a polygonal column shape.
When using extrusion molding, it is desirable to make the thermoelectric conversion element 11 cylindrical in order to prevent stress concentration in the manufacturing process. On the other hand, if the thermoelectric conversion element 11 has a polygonal column shape (particularly a quadrangular prism), the space factor of the thermoelectric conversion element 11 in the thermoelectric conversion module 1 can be increased.
Moreover, as a manufacturing method of the thermoelectric conversion element 11, a hot press method can also be utilized besides the extrusion molding method.

  The embodiment disclosed this time should be considered as illustrative in all points and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

DESCRIPTION OF SYMBOLS 1 Thermoelectric conversion module 10 Thermoelectric element pair 11, 12 Thermoelectric conversion element 11A 1st thermoelectric conversion part 11B 2nd thermoelectric conversion part 13 Metal electrodes 14, 15 Insulating substrate 16, 17 Current lead

Claims (9)

A columnar thermoelectric conversion element for converting thermal energy into electrical energy or electrical energy into thermal energy using a temperature difference between one end face and the other end face in the axial direction,
A first thermoelectric conversion part serving as a core material, and a second thermoelectric conversion part serving as a coating layer formed with a predetermined thickness on the peripheral surface of the first thermoelectric conversion part,
The first thermoelectric converter is composed of a NaCo-based oxide;
The thermoelectric conversion element, wherein the second thermoelectric conversion part is made of a CaCo-based oxide.   The thermoelectric conversion element according to claim 1, wherein the first thermoelectric conversion unit and the second thermoelectric conversion unit are formed by extrusion molding. The NaCo-based oxide is Na _x Co ₂ O ₄ (1.0 ≦ x ≦ 2.0),
The CaCo-based oxide is Ca _{3-y Ay} Co ₄ O ₉ (0 ≦ y ≦ 0.6, A: one or two elements of Bi, Sr, Mg, Gd, Y, K, Na) The thermoelectric conversion element according to claim 1 or 2, wherein The thermoelectric conversion element according to claim 3, wherein the NaCo-based oxide is Na _x Co ₂ O ₄ (1.0 ≦ x ≦ 1.4). The NaCo-based oxide is NaCo ₂ O ₄ ;
The thermoelectric according to claim 3 or 4, wherein the CaCo-based oxide is one of Ca _2.6 Bi _0.4 Co ₄ O ₉ , Ca _2.7 Sr _0.3 Co ₄ O ₉ , and Ca ₃ Co ₄ O _9. Conversion element.   6. The thermoelectric conversion element according to claim 1, wherein a thickness of the second thermoelectric conversion unit is 150 to 1100 μm.   The thermoelectric conversion element according to claim 6, wherein a thickness of the second thermoelectric conversion portion is 300 to 700 μm.   The thermoelectric conversion element according to any one of claims 1 to 7, wherein a cross-sectional shape perpendicular to the axial direction is substantially circular.   The thermoelectric conversion element according to any one of claims 1 to 7, wherein a cross-sectional shape perpendicular to the axial direction is substantially square or substantially rectangular.

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