JP2006057124A - Clathrate compound and thermoelectric conversion element using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a clathrate compound having excellent thermoelectric conversion characteristics and a thermoelectric conversion element using the same. <P>SOLUTION: The clathrate compound represented by the following composition formulae (1) to (3) is provided: (1) Ba<SB>8</SB>M<SB>x</SB>Si<SB>46-x</SB>(wherein, 1≤x≤6 is satisfied; and M represents Cu, Ag or Au); (2) A<SB>y</SB>Ba<SB>8-y</SB>M<SB>x</SB>Si<SB>46-x</SB>(wherein, 1≤x≤6 and 1≤y<8 are satisfied; A represents a rare earth element; and M represents Cu, Ag or Au); (3) A<SB>y</SB>Ba<SB>8-y</SB>M<SB>z</SB>Ge<SB>46-z</SB>(wherein, 1≤y<8 and 1≤z≤6 are satisfied; A represents a rare earth element; and M represents Cu, Ag or Au). The thermoelectric conversion element using the compound is also provided. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a clathrate compound and a thermoelectric conversion element using the same.

  Thermoelectric conversion elements using the Seebeck effect can convert thermal energy into electrical energy. Utilizing this property, it is possible to convert exhaust heat exhausted from industrial / consumer processes and mobile objects into effective electric power, and thermoelectric conversion elements are attracting attention as energy-saving technologies in consideration of environmental problems.

Conventionally, bismuth / tellurium-based materials, silicon / germanium-based materials, lead / tellurium-based materials, and the like are known as thermoelectric conversion materials used for thermoelectric conversion elements. Furthermore, a thermoelectric conversion material formed by molding and baking zinc oxide powder doped with aluminum is known (see, for example, Patent Document 1).
JP 2002-118296 A

  An object of this invention is to provide the novel clathrate compound suitable for a thermoelectric conversion element, and a thermoelectric conversion element using the same.

That is, the present invention
<1> A clathrate compound represented by the following composition formula (1).
Ba ₈ M _x Si _46-x (1 ≦ x ≦ 6) (1)
(In composition formula (1), M represents Cu, Ag, or Au.)

<2> A clathrate compound represented by the following composition formula (2).
A _y Ba _8-y M _x Si _46-x (1 ≦ x ≦ 6, 1 ≦ y <8) (2)
(In composition formula (2), A represents a rare earth element, and M represents Cu, Ag, or Au.)

<3> A clathrate compound represented by the following composition formula (3).
A _y Ba _8-y M _z Ge _46-z (1 ≦ y <8, 1 ≦ z ≦ 6) (3)
(In composition formula (3), A represents a rare earth element, and M represents Cu, Ag, or Au.)

<4> A thermoelectric conversion element which is a sintered body of a clathrate compound represented by the following composition formula (1).
Ba ₈ M _x Si _46-x (1 ≦ x ≦ 6) (1)
(In composition formula (1), M represents Cu, Ag, or Au.)

<5> A thermoelectric conversion element which is a sintered body of a clathrate compound represented by the following composition formula (2).
A _y Ba _8-y M _x Si _46-x (1 ≦ x ≦ 6, 1 ≦ y <8) (2)
(In composition formula (2), A represents a rare earth element, and M represents Cu, Ag, or Au.)

<6> A thermoelectric conversion element which is a sintered body of a clathrate compound represented by the following composition formula (3).
A _y Ba _8-y M _z Ge _46-z (1 ≦ y <8, 1 ≦ z ≦ 6) (3)
(In composition formula (3), A represents a rare earth element, and M represents Cu, Ag, or Au.)

  ADVANTAGE OF THE INVENTION According to this invention, the clathrate compound which has the outstanding thermoelectric conversion characteristic, and a thermoelectric conversion element using the same can be provided.

Hereinafter, the clathrate compound of the present invention and the thermoelectric conversion element using the same will be described in detail.
<Clathrate compound>
The clathrate compound 1 of the present invention is represented by the following composition formula (1).
Ba ₈ M _x Si _46-x (1 ≦ x ≦ 6) (1)
(In composition formula (1), M represents Cu, Ag, or Au.)

  When x in the composition formula (1) is less than 1, the clathrate compound 1 may not exhibit sufficient characteristics as a thermoelectric conversion material. When x is larger than 6, a clathrate compound having properties as a thermoelectric conversion material may not be obtained.

  The figure of merit ZT, which is one of the performance indexes of the thermoelectric conversion material, is represented by the following formula (A).

ZT = α ² σT / κ (A)

  Here, α, σ, κ, and T represent the Seebeck coefficient, electrical conductivity, thermal conductivity, and measurement temperature, respectively.

  In order to improve the performance of the thermoelectric conversion material, it is necessary to increase the figure of merit ZT.

  The Seebeck coefficient α, electrical conductivity σ, and thermal conductivity κ in the formula (A) are represented by the following formulas (B), (C), and (D), respectively.

α = k _B / (e (−log (n / n ₀ ) + δ)) (B)
σ = enμ (C)
κ = κ _p + κ _e (D)

Here, k _B represents a Boltzmann constant, e represents an electric charge, n represents a carrier concentration, n ₀ represents a temperature-dependent constant, and δ represents a dimensionless constant (motion term) that is weakly dependent on temperature. Μ represents mobility, κ _p represents thermal conductivity related to the lattice component, and κ _e represents thermal conductivity related to the carrier component. Further, κ _p is represented by the following formula (E).

κ _p = Cvl / 3 (E)

  Here, C represents specific heat, v represents the speed of sound, and l represents the mean free path of phonons.

  From the above equation, it can be seen that decreasing the carrier concentration improves the Seebeck coefficient but decreases the electrical conductivity. In addition, increasing the electrical conductivity increases the thermal conductivity. Therefore, it is difficult to consider improving the figure of merit ZT based on these relational expressions.

When the figure of merit ZT is rewritten based on the expressions α, σ, and κ, the following expression (F) is obtained. If the electron energy E can be written as E = P ² / 2m (where P is momentum),

ZT∝m ^3/2 μ / (κ _p + κ _e ) (F)

  Here, m represents an effective mass.

In the region where the carrier concentration is 10 ²⁵ to 10 ²⁶ / m ³ as in the thermoelectric conversion material, the thermal conductivity of κ _p is more dominant than κ _e , so how to reduce κ _p is a problem. . Therefore, Formula (F) can be considered as ZT∝m ^3/2 μ / κ _p . Here, m, μ, and κ _p can be considered as independent parameters. Therefore, in order to improve the figure of merit ZT, the effective mass m and the mobility μ are increased, and the heat related to the lattice component is increased. it can be seen that it is sufficient to lower the conductivity kappa _p.

  The clathrate compound 1 can improve the effective mass as compared with the clathrate compound using Ga as M in the composition formula (1). Therefore, the clathrate compound 1 can improve the figure of merit ZT.

  In the clathrate compound 1, the preferred range of x is 3 ≦ x ≦ 6, and more preferably 5 ≦ x ≦ 6.

The clathrate compound 2 of the present invention is represented by the following composition formula (2).
A _y Ba _8-y M _x Si _46-x (1 ≦ x ≦ 6, 1 ≦ y <8) (2)
(In composition formula (2), A represents a rare earth element, and M represents Cu, Ag, or Au.)

  In the composition formula (2), in the region where x <1, sufficient thermoelectric performance may not be obtained, and in the region where x> 6, unreacted substances and by-products are generated.

  A preferable range of x in the composition formula (2) is 3 ≦ x ≦ 6, and more preferably 5 ≦ x ≦ 6.

  In the composition formula (2), the crystallinity is lowered in the region of y <1 and the crystal structure is different in the region of y> 8, and sufficient thermoelectric characteristics cannot be obtained.

  A preferable range of y in the composition formula (2) is 1 ≦ y ≦ 3.

  In the clathrate compound 2, among the rare earth elements represented by A, La, Ce, Yb, and Lu are preferable, and La and Ce are particularly preferable.

  The clathrate compound 2 is obtained by substituting a part of Ba of the clathrate compound 1 with a rare earth element. Thereby, the effective mass and band gap of the clathrate compound 2 can be improved more than that of the clathrate compound 1.

  In general, the thermoelectric conversion material has a property that the temperature at which the Seebeck coefficient α peaks increases as the band gap increases. As the band gap increases, the T at which the figure of merit ZT is optimal increases, and the value of the figure of merit ZT also increases.

  The reason why the peak temperature of the Seebeck coefficient increases as the band gap increases is as follows. First, in the situation where many electrons are thermally excited from the valence band to the conduction band, the contribution from the valence band and the conduction band strongly cancel each other, resulting in a decrease in the Seebeck coefficient. Therefore, when the band gap is increased, the cancellation occurs at a higher temperature, so that the peak temperature is increased.

The clathrate compound 3 of the present invention is represented by the following composition formula (3).
A _y Ba _8-y M _z Ge _46-z (1 ≦ y <8, 1 ≦ z ≦ 6) (3)
(In composition formula (3), A represents a rare earth element, and M represents Cu, Ag, or Au.)

  In the composition formula (3), in the region of y <1, the crystallinity is lowered and sufficient thermoelectric performance may not be obtained. In the region of y> 8, a clathrate compound cannot be generated due to unreacted substances and by-products, and thus sufficient thermoelectric performance cannot be obtained.

  A preferable range of y in the composition formula (3) is 1 ≦ y ≦ 3.

  In the composition formula (3), the crystallinity is lowered in the region of z <1, and in the case of z> 6, the clathrate compound cannot be generated due to unreacted substances or by-products, so that sufficient thermoelectric performance cannot be obtained.

  A preferable range of z in the composition formula (3) is 3 ≦ z ≦ 6, and more preferably 5 ≦ z ≦ 6.

  In the clathrate compound 3, among the rare earth elements represented by A, La, Ce, Yb, and Lu are preferable, and La and Ce are particularly preferable.

  The clathrate compound 3 is obtained by changing the Si of the clathrate compound 2 to Ge. Even in a Ge-based clathrate compound, the band gap can be improved by adding rare earth elements.

<Method for producing clathrate compound>
The clathrate compound of the present invention can be produced through a melting step of melting the constituent elements constituting the clathrate compound, but is not limited to this method. As a melting temperature, 1000-1500 degreeC is preferable, 1000-1400 degreeC is more preferable, 1200-1400 degreeC is especially preferable. The melting time is preferably 10 to 100 minutes, more preferably 10 to 60 minutes, and particularly preferably 20 to 60 minutes. As a melting method, an arc melting method, a high-frequency heating method, or the like can be used.

  In the production of the clathrate compound of the present invention, an annealing treatment may be performed after the melting step. The treatment time for the annealing treatment is preferably 100 hours or more, and more preferably 150 to 200 hours. The annealing temperature is preferably 950 to 1350 ° C, and more preferably 1100 to 1300 ° C.

<Thermoelectric conversion element>
The thermoelectric conversion element of the present invention is a sintered body of the clathrate compound of the present invention. The thermoelectric conversion element of the present invention can be produced through a fine particle forming step of forming the clathrate compound of the present invention into fine particles and a sintering step of sintering the fine particles obtained in the fine particle forming step. The method is not limited.

  In the fine particle formation step, fine particles can be obtained by pulverizing the clathrate compound using a ball mill or a mortar. The particle diameter of the fine particles is preferably 150 μm or less, and more preferably 90 μm or less. Further, a so-called flowing gas evaporation method can be used in which a vapor of a clathrate compound is generated in a vacuum and the vapor is blown off with a high-pressure inert gas to obtain fine particles. Details of the flowing gas evaporation method are described in Japanese Patent Publication No. 5-9483. In the sintering step, the fine particles can be sintered using a discharge plasma sintering method, a hot press sintering method, a hot isostatic pressing method, or the like.

  As a sintering condition in the case of using the discharge plasma sintering method, the temperature is preferably 650 to 950 ° C, and more preferably 700 to 900 ° C. The sintering time is preferably 20 to 120 minutes, more preferably 30 to 90 minutes. The pressure is preferably 25 to 40 MPa, more preferably 30 to 40 MPa.

  The formation of the clathrate compound of the present invention can be confirmed by X-ray diffraction. Specifically, if the sample after firing shows only the clathrate phase by X-ray diffraction, it can be confirmed that the clathrate compound has been synthesized.

EXAMPLES Hereinafter, although this invention is demonstrated further in detail using an Example, this invention is not limited by the following Example.
[Example 1]
In clathrate compound 1, the band gap and effective mass when M = Cu, Ag, Au; x = 6 were calculated. As a comparison object, the band gap and effective mass were also calculated for Ba ₈ Ga ₁₆ Si ₃₀ . The WIEN2k code was used to calculate the electronic structure. In this code, the FLAPW method based on the APW + local orbital method is used. Muffin-tin radii were Ba: 1.59 mm, Cu: 1.26 mm, Ag, Au: 1.27 mm, Si: 1.11 mm and Ga: 1.22 mm. Further, the interaction between exchange phases used a formula by Perdew-Burke-Ernzerrof. For the FLAPW method, see Solid State Comm. 114, 15 (2000), and for the mathematical expression by Perdew-Burke-Ernzerrof, see Phys. Rev. B45, 3865 (1996).

  From the calculation of the electronic structure, a band structure in which the conduction electron band is minimized in the vicinity of the M point was obtained. The band near the point M was approximated by the following formula (G).

Ek = (p ² / 2m) + Eg (G)

  Here, Ek represents the total energy of electrons near the M point, Eg represents a band gap, m represents an effective mass, and p represents momentum.

  Using formula (G), the effective mass and the band gap were determined so that the results of the electronic structure calculation would match. The obtained calculation results are shown in Table 1.

  From Table 1, it can be seen that the clathrate compound 1 using Cu, Ag and Au has a larger effective mass than the clathrate compound using Ga.

[Example 2]
The effective mass of La ₂ Ba ₆ Au ₆ Si ₄₀ which is clathrate compound 2 was calculated in the same manner as in Example 1. In this case, the La Muffin-tin radius was 1.59 mm. The band gap of La ₂ Ba ₆ Au ₆ Si ₄₀ was also calculated. The band gap was calculated using the WIEN2k code as well as the effective mass. The results are shown in Table 2 together with the results of Ba ₈ Au ₆ Si ₄₀ which is clathrate compound 1.

  As is clear from Table 2, it is understood that the effective mass and the band gap can be improved by adding rare earth elements.

[Example 3]
The band gap of La ₂ Ba ₆ Au ₆ Ge ₄₀ which is clathrate compound 3 was calculated by the same method as in Example 2. As a comparison object, the band gap was also calculated for Ba ₈ Au ₆ Ge ₄₀ . In this case, the Ge Muffin-tin radius was 1.22 mm. The results are shown in Table 3.

  As can be seen from Table 3, the band gap can also be improved by adding rare earth elements in the Ge-based clathrate compounds.

Claims (6)

A clathrate compound represented by the following composition formula (1).
Ba ₈ M _x Si _46-x (1 ≦ x ≦ 6) (1)
(In composition formula (1), M represents Cu, Ag, or Au.) A clathrate compound represented by the following composition formula (2).
A _y Ba _8-y M _x Si _46-x (1 ≦ x ≦ 6, 1 ≦ y <8) (2)
(In composition formula (2), A represents a rare earth element, and M represents Cu, Ag, or Au.) A clathrate compound represented by the following composition formula (3).
A _y Ba _8-y M _z Ge _46-z (1 ≦ y <8, 1 ≦ z ≦ 6) (3)
(In composition formula (3), A represents a rare earth element, and M represents Cu, Ag, or Au.) A thermoelectric conversion element which is a sintered body of a clathrate compound represented by the following composition formula (1).
Ba ₈ M _x Si _46-x (1 ≦ x ≦ 6) (1)
(In composition formula (1), M represents Cu, Ag or Au.) A thermoelectric conversion element which is a sintered body of a clathrate compound represented by the following composition formula (2).
A _y Ba _8-y M _x Si _46-x (1 ≦ x ≦ 6, 1 ≦ y <8) (2)
(In composition formula (2), A represents a rare earth element, and M represents Cu, Ag, or Au.) A thermoelectric conversion element which is a sintered body of a clathrate compound represented by the following composition formula (3).
A _y Ba _8-y M _z Ge _46-z (1 ≦ y <8, 1 ≦ z ≦ 6) (3)
(In composition formula (3), A represents a rare earth element, and M represents Cu, Ag, or Au.)