JP2011210870A - Double phase thermoelectric conversion material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a double phase thermoelectric conversion material which independently controls the Seebeck coefficient S, the electric resistivity ρ, and the thermal conductivity κ of materials which decide the capacity ZT of the thermoelectric conversion material, in contrast to the conventional thermoelectric conversion material of a single substance phase, and further can be used in a moderate high temperature area of a relatively high temperature, for example, 200°C or higher with the excellent thermoelectric capacity.SOLUTION: In the thermoelectric conversion material having a half Heusler crystal structure consisting of at least one type or more of elements selected from a group consisting of Ti, Zr, Hf, V, Nb, Ta, Mn, Fe, Co, Ni, Cu, Si, Ge, Sn, Sb, and Bi, the double phase thermoelectric conversion material consists of a plurality of types of phases, and is formed that a volume ratio of a first phase to a second phase of the different type from the first phase is in a range of 80:20 to 20:80.

Description

  The present invention relates to a multiphase thermoelectric conversion material that can directly convert thermal energy into electricity or electricity into thermal energy.

  A thermoelectric conversion material is a material that can directly convert thermal energy into electricity, or that can directly convert electrical energy into thermal energy, that is, can be heated and cooled by applying electricity. One thermoelectric conversion module is formed by electrically connecting a number of pairs of p / n thermoelectric conversion materials obtained by combining p-type thermoelectric conversion materials and n-type thermoelectric conversion materials in series. If a thermoelectric conversion module is used, waste heat, which has not been used so far, can be converted into electricity to effectively use energy.

The performance of the thermoelectric conversion material is evaluated by the figure of merit Z. The figure of merit Z is represented by the following formula (1) using the Seebeck coefficient S, the thermal conductivity κ, and the electrical resistivity ρ.
Z = S ² / (κρ) Equation (1)

Further, the performance of the thermoelectric conversion material may be evaluated by the product of the figure of merit Z and the temperature T. In this case, it is expressed by the following equation (2) obtained by multiplying both sides of equation (1) by temperature T (where T is an absolute temperature). ZT shown in Formula (2) is called a dimensionless figure of merit and serves as an index indicating the performance of the thermoelectric conversion material. The thermoelectric conversion material has higher thermoelectric performance at the temperature T as the ZT value is larger.
ZT = S ² T / (κρ) Equation (2)

  From Equations (1) and (2), an excellent thermoelectric conversion material is a material that can increase the value of the figure of merit ZT, that is, a material that has a large Seebeck coefficient S, and a small thermal conductivity κ and electrical resistivity ρ. It can be said.

Furthermore, when evaluating the performance of the thermoelectric conversion material from an electrical viewpoint, the output factor P represented by the following formula (3) may be used.
P = S ² / ρ Formula (3)

Moreover, the maximum conversion efficiency η _max of the thermoelectric conversion material is expressed by the following formula (4).
η _max = {(T _h −T _c ) / T _h } {(M−1) / (M + (T _c / T _h ))} Equation (4)

M in the equation (4) is represented by the following equation (5). Here, _Th is the temperature at the high temperature end of the thermoelectric conversion material, and T _c is the temperature at the low temperature end.
M = {1 + Z (T _h + T _c ) / 2} ^−0.5 formula (5)

  From the above formulas (1) to (5), it can be seen that the thermoelectric conversion efficiency of the thermoelectric conversion material improves as the performance index and the temperature difference between the high temperature end and the low temperature end increase.

Typical thermoelectric conversion materials that have been studied so far include Bi ₂ Te ₃ system, PbTe system, AgSbTe ₂ -GeTe system, SiGe system, skutterudite system represented by CoSb ₃ , Zn ₄ Sb ₃ system, There are FeSi ₂ -based, B ₄ C-based, NaCo ₂ O ₄ -based oxide, Ca ₃ Co ₄ O ₉ -based oxide, and the like. However, only the Bi ₂ Te ₃ system is put into practical use among them. In the thermoelectric conversion module using the Bi ₂ Te ₃ series thermoelectric conversion material, the temperature range that can be used as a power generation application is limited to the range of about 250 ° C. that the Bi ₂ Te ₃ series material can withstand from near room temperature.

  Therefore, a thermoelectric conversion material that can be used in a wide temperature range from room temperature to 1000 ° C. has been demanded in that various waste heat is effectively used. Thermoelectric conversion materials that have been researched and developed so far consist of a single substance phase having almost the same crystal structure with as few impurity phases as possible in order to improve the performance of the thermoelectric conversion material. Therefore, the thermoelectric conversion performance is limited by a single material phase.

  On the other hand, in order to improve the performance of thermoelectric conversion materials, a superlattice structure of two kinds of material phases is artificially constructed, and the increase in Seebeck coefficient and the reduction in thermal conductivity by the superlattice structure are utilized, Thermoelectric conversion materials with good performance have been developed (Non-Patent Document 1).

R. Venkatasuburamanian et al., Nature, Vol. 413 (2001), p. 597-602

  However, since the Seebeck coefficient S, the electrical resistivity ρ, and the thermal conductivity κ are determined by a function of the carrier concentration, these three parameters cannot be controlled independently in the conventional single-material thermoelectric conversion material. . That is, as is clear from the above equation (2), the performance ZT of the thermoelectric conversion material is determined by the Seebeck coefficient S, the electrical resistivity ρ, and the thermal conductivity κ of the material, so that it is still improved in terms of obtaining higher thermoelectric performance. There was room for.

  In addition, an artificially constructed superlattice structure as described in Non-Patent Document 1 has a property of forming a compound thermodynamically or forming a solid solution with each other. It was thermodynamically unstable. Therefore, when used at a relatively high temperature, for example, at a temperature of 200 ° C. or higher, the superlattice structure collapses due to element diffusion, and there is a problem that it is difficult to use in a medium to high temperature region of 200 ° C. or higher. .

  As a result of intensive studies in order to improve the thermoelectric performance without artificially constructing a superlattice structure, the present inventors have obtained the following knowledge. That is, in the conventional thermoelectric conversion material, it is said that the thermoelectric performance can be improved by reducing the impurity phase to a single phase, but it is good by coexisting multiple phases instead of a single phase. We found new knowledge that a good thermoelectric performance can be obtained.

  According to the present invention, the volume ratio between the first phase and the second phase different from the first phase is in the range of 80:20 to 20:80. A composite thermoelectric conversion material is provided.

  That is, according to the present invention, the electrical conductivity performance and the thermal conductivity performance can be individually controlled by combining two or more kinds of phases. Therefore, the dimensionless figure of merit ZT can be reduced by reducing the electrical resistivity ρ and the thermal conductivity κ. Can be increased. Further, a composite thermoelectric conversion material composed of two or more kinds of thermodynamically stable phases can be used instead of the thermodynamically unstable superlattice structure.

  According to the present invention, a thermoelectric conversion material having excellent thermoelectric performance in a wide temperature range is provided.

1 is a structure diagram of a thermoelectric conversion material of Example 1. FIG. 4 is a structural diagram of a thermoelectric conversion material of Comparative Example 1. FIG. It is a figure showing the temperature dependence of the Seebeck coefficient of n-type thermoelectric conversion material. It is a figure showing the temperature dependence of the electrical resistivity of an n-type thermoelectric conversion material. It is a figure showing the temperature dependence of the output factor of an n-type thermoelectric conversion material. It is a figure showing the temperature dependence of the thermal conductivity of an n-type thermoelectric conversion material. It is a figure showing the temperature dependence of the dimensionless figure of merit of n type thermoelectric conversion material.

  The multiphase thermoelectric conversion material of the present invention is composed of a plurality of types of phases, and may be at least two types. The second phase of a type different from the first phase is a phase having a composition different from that of the first phase.

  When two types of phases were included in the conventional multiphase thermoelectric conversion material, the volume ratio of the phase with a small amount and the phase with a large amount was less than 20 and the volume ratio of the phase with a small amount was less than 20. On the other hand, in the multiphase thermoelectric conversion material of the present invention, the volume ratio of the first phase and the second phase is preferably in the range of 80:20 to 20:80, 70:30 to 30:70, Furthermore, the range of 65:35 to 35:65 is more preferable. Thereby, higher thermoelectric performance is obtained.

  In the multiphase thermoelectric conversion material of the present invention, the phase with the highest volume ratio is preferably 80% or less, and more preferably 70% or less with respect to the entire multiphase thermoelectric conversion material. Thereby, higher thermoelectric performance can be obtained. In addition, the phase having the second highest volume fraction is preferably 20% or more, and more preferably 30% or more with respect to the entire multiphase thermoelectric conversion material. Thereby, more stable thermoelectric performance can be obtained.

  In the multiphase thermoelectric conversion material of the present invention, the plurality of types of phases are composed of Ti, Zr, Hf, V, Nb, Ta, Mn, Fe, Co, Ni, Cu, Si, Ge, Sn, Sb, and Bi. It consists of at least one element selected from the group. Among these, Ti, Zr, Hf, Fe, Co, Ni, Si, Ge, Sn, Sb, and Bi are particularly preferable from the viewpoint that a half-Heusler structure can be formed.

  In the multiphase thermoelectric conversion material of the present invention, the first phase and / or the second phase has a structure represented by the following formula (6). For example, a (Ti, Zr, Hf, Ta) NiSn based multiphase thermoelectric conversion material is preferable.

T _t M _m X _x formula (6)
In formula (6), 0.5 ≦ t ≦ 1.5, 0.5 ≦ m ≦ 1.5, 0.5 ≦ x ≦ 1.5, T is Ti, Zr, Hf, V, Nb and Ta M is at least one element selected from the group consisting of Mn, Fe, Co, Ni and Cu, and X is Si, Ge, Sn, Sb. And at least one selected from the group consisting of Bi.

In the multiphase thermoelectric conversion material of the present invention, the first phase and / or the second phase have a half-Heusler crystal structure. Thereby, a large output factor can be obtained. Examples of the half-Heusler compound include (Ti, Zr, Hf) ₁ Ni ₁ Sn ₁ .

  Next, the manufacturing method of the double phase thermoelectric conversion material of this invention is demonstrated.

  The multiphase thermoelectric conversion material of the present invention comprises eutectic reaction, eutectoid reaction, peritectic reaction, peritectic reaction, segregation reaction, segregation reaction, decomposition of non-equilibrium material phase, decomposition of solid solution, and composite processes thereof. Can be formed. Therefore, for example, dissolution method, rapid solidification method (gas atomization, water atomization, single roll method, twin roll method), mechanical alloying method (ball mill method), hot press method, heat sintering method, discharge plasma molding method, or heat treatment It can be manufactured by appropriately combining methods.

  A more specific manufacturing method will be described below.

First, an example in which the melting method and the heat treatment method are combined will be described.
Pure metal raw material is put in an arc melting furnace at a predetermined ratio, and heated and melted in an Ar gas atmosphere to obtain a target ingot. The ingot is heated to 1000 to 1200 ° C. in a vacuum or an inert gas atmosphere, held for 100 hours, and then cooled to room temperature to obtain the desired double-phase thermoelectric conversion material.

Next, an example in which the rapid solidification method and the discharge plasma molding method are combined will be described.
Pure metal raw material is put in a crucible of a gas atomizer at a predetermined ratio, heated and dissolved in an inert gas atmosphere, and then gas atomized to obtain a raw material powder. The obtained powder is put into a carbon die and heated to a temperature of 600 ° C. to 1000 ° C. while applying a pulse current under a pressure of 40 MPa to 60 MPa in a vacuum or an inert gas atmosphere. After holding for 10 minutes to 60 minutes, a thermoelectric conversion material having high thermoelectric performance in a wide temperature range of interest can be obtained by cooling to room temperature.

  It was confirmed by powder X-ray diffraction that the obtained multiphase thermoelectric conversion material was composed of two or more kinds of substance phases in any of the above-described production methods.

  In the multiphase thermoelectric conversion material of the present invention, the structure of the multiphase coexistence is derived from the thermodynamic properties of the material, and the volume ratio of different types of phases can be adjusted by controlling the composition of the material.

  As described above, the multiphase thermoelectric conversion material of the present invention has been described in detail using the embodiments and examples. However, the multiphase thermoelectric conversion material of the present invention is not limited to the above contents, and the scope of the present invention is not limited thereto. All modifications and changes are possible without departing from the scope.

  Hereinafter, the multiphase thermoelectric conversion material of the present invention will be specifically described with reference to examples.

(Examples 1-4, Comparative Example 1)
Pure metals Ti, Zr, Hf, Ta, Ni and Sn were weighed so as to have a predetermined ratio as shown in Table 1, respectively, put in an arc melting furnace, and heated and melted in an Ar gas atmosphere. Each ingot was obtained. This ingot was heated to a temperature of 1100 ° C. in a vacuum, held for 100 hours, and then cooled to room temperature to obtain a target multiphase thermoelectric conversion material.

About the obtained multiphase thermoelectric conversion material, powder X-ray diffraction and scanning electron microscope observation were performed, and the volume ratio of multiple phases was computed. Moreover, the Seebeck coefficient of the thermoelectric conversion material in the temperature range of room temperature to 700 ° C. using a thermoelectric performance evaluation apparatus (Thermoelectric measurement apparatus ZEME-2 manufactured by ULVAC-RIKO, Inc. and laser flash method thermal constant measurement apparatus TC-7000H). S, electrical resistivity ρ, and thermal conductivity κ were measured, and dimensionless figure of merit ZT and output factor P (P = S ² / ρ) were calculated. These results are shown below (results) and FIGS.

(result)
Example 1
(First Phase) (Ti _0.33 Zr _0.33 Hf _0.33 Ta _0.01 ) Ni ₁ Sn ₁
(Second Phase) (Ti _0.76 Zr _0.12 Hf _0.12 ) Ni ₁ Sn ₁
Volume ratio (first phase): (second phase) = 60: 40
Example 2
(First Phase) (Ti _0.33 Zr _0.33 Hf _0.33 Ta _0.01 ) Ni ₁ Sn ₁
(Second Phase) (Ti _0.76 Zr _0.12 Hf _0.12 ) Ni ₁ Sn ₁
Volume ratio (first phase): (second phase) = 65: 35
Example 3
(First Phase) (Ti _0.33 Zr _0.33 Hf _0.33 Ta _0.01 ) Ni ₁ Sn ₁
(Second Phase) (Ti _0.76 Zr _0.12 Hf _0.12 ) Ni ₁ Sn ₁
Volume ratio (first phase): (second phase) = 55: 45
Example 4
(First Phase) (Ti _0.33 Zr _0.33 Hf _0.33 Ta _0.01 ) Ni ₁ Sn ₁
(Second Phase) (Ti _0.76 Zr _0.12 Hf _0.12 ) Ni ₁ Sn ₁
Volume ratio (first phase): (second phase) = 50: 50
Comparative Example 1
(First Phase) (Ti _0.33 Zr _0.33 Hf _0.33 Ta _0.01 ) Ni ₁ Sn ₁
(Second Phase) (Ti _0.76 Zr _0.12 Hf _0.12 ) Ni ₁ Sn ₁
Volume ratio (first phase): (second phase) = 5: 95

1 is a diagram showing a structural cross-sectional photograph of n-type Ti _0.4 Zr _0.3 Hf _0.3 Ta _0.01 Ni ₁ Sn ₁ of Example 1. FIG. From FIG. 1, it was confirmed that the multiphase thermoelectric conversion material of Example 1 had a two-phase structure of a white phase (first phase) and a gray phase (second phase). Both phases had a half-Heusler crystal structure.

FIG. 2 is a diagram showing a structural cross-sectional photograph of n-type Ti _0.8 Zr _0.1 Hf _0.1 Ta _0.01 Ni ₁ Sn ₁ of Comparative Example 1. From FIG. 2, it was confirmed that the thermoelectric conversion material of Comparative Example 1 has a two-phase structure of a white phase (first phase) and a gray phase (second phase). However, the white phase was slight.

3 is a diagram showing the temperature dependence of the Seebeck coefficient of the n-type thermoelectric conversion material, FIG. 4 is a diagram showing the temperature dependence of the electrical resistivity of the n-type thermoelectric conversion material, and FIG. Fig. 6 shows the temperature dependence of the output factor, Fig. 6 shows the temperature dependence of the thermal conductivity of the n-type thermoelectric conversion material, and Fig. 7 shows the temperature dependence of the dimensionless figure of merit of the n-type thermoelectric conversion material. FIG.
When comparing Example 1 to Example 4 and Comparative Example 1, the output factor of Comparative Example 1 is almost the same as that of Examples 1 to 4 as shown in FIG. Referring to the relationship between the thermal conductivity and the temperature in FIG. 6, it can be seen that the thermal conductivity of Comparative Example 1 is considerably larger than those of Examples 1 to 4. This is because the volume ratio of the white phase (first phase) to the gray phase (second phase) is 5:95, and there is no reduction in thermal conductivity due to the white phase (first phase). It is considered sufficient. Therefore, the dimensionless figure of merit ZT of Comparative Example 1 was small and was 0.6 at 500 ° C. From FIG. 7, it was found that the maximum value of the dimensionless figure of merit ZT of the n-type thermoelectric conversion materials of Examples 1 to 4 reached 0.9 to 1.1 at 500 ° C. Comparing Example 1, 3, 4 and Example 2 in FIG. 7, in the temperature region higher than 500 ° C., the dimensionless figure of merit ZT tends to increase as the ratio of the first phase increases. However, it has been found that if the ratio of the first phase is too high, it tends to be low.

Claims (4)

  A multiphase thermoelectric conversion material comprising a plurality of types of phases, wherein the volume ratio of the first phase and the second phase of a type different from the first phase is in the range of 80:20 to 20:80 . In the multiphase thermoelectric conversion material according to claim 1,
The plurality of types of phases are at least one selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Mn, Fe, Co, Ni, Cu, Si, Ge, Sn, Sb, and Bi. A multiphase thermoelectric conversion material consisting of elements. In the multiphase thermoelectric conversion material according to claim 1 or 2,
The multiphase thermoelectric conversion material in which the first phase and / or the second phase has a structure represented by the following formula (i).
T _t M _m X _x formula (i)
In formula (i), 0.5 ≦ t ≦ 1.5, 0.5 ≦ m ≦ 1.5, 0.5 ≦ x ≦ 1.5, T is Ti, Zr, Hf, V, Nb and Ta M is at least one element selected from the group consisting of Mn, Fe, Co, Ni and Cu, and X is Si, Ge, Sn, Sb. And at least one selected from the group consisting of Bi. In the multiphase thermoelectric conversion material according to any one of claims 1 to 3,
A multiphase thermoelectric conversion material in which the first phase and / or the second phase has a half-Heusler crystal structure.