JP2018019014A - Thermoelectric conversion material, and thermoelectric conversion module arranged by use thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide: a thermoelectric conversion material which enables the achievement of a low environmental load and a low cost and which can impart a high thermoelectric conversion property; and a thermoelectric conversion module.SOLUTION: A thermoelectric conversion material comprises a compound represented by the following general formula (1): ReTT'XX'Y(1). (In the formula (1), 0≤α≤2, 0≤β≤8, -1≤γ≤1, and -0.3≤μ≤0.3; and Re represents at least one element selected from a group consisting of an alkali metal element, an alkali earth metal and a rare earth element, T represents at least one element selected from a group consisting of Co, Rh and Ir, T' represents at least one element selected from a group consisting of Fe, Mn, Ni, Cr, V, Ti, Zr, Nb, Mo, Ru, Os, Re, W, Ta and Hf, X represents at least one element selected from a group consisting of Sn, Ge and Si, X' represents at least one element selected from a group consisting of Al, Ga, In, P, Sb and Bi, and Y is S.)SELECTED DRAWING: Figure 6

Description

  The present invention relates to a thermoelectric conversion material and a thermoelectric conversion module using the same.

Thermoelectric conversion elements have been known for a long time as one of the methods for using exhaust heat, and Bi ₂ Te ₃ has been put into practical use as a thermoelectric conversion material that is relatively efficient in a temperature range of 200 ° C. or lower. Yes. Bi-Te materials have a high figure of merit ZT> 1 and high conversion efficiency, but both Bi and Te are expensive, and Te is extremely toxic. For this reason, a conventional thermoelectric conversion element such as a Bi-Te system cannot be supplied to the market stably in large quantities at low cost, and is generally unlikely to be widely spread. For this reason, what can be mass-produced, cost-reduced, and environmental load reduction is calculated | required as a thermoelectric conversion element.

As a material system having a low environmental load, Patent Document 1 discloses a thermoelectric conversion material based on a full Heusler alloy Fe ₂ VAl. This is composed of elements with low environmental load and relatively low cost, such as Fe, V, and Al, and is known to exhibit excellent thermoelectric conversion efficiency mainly in a relatively low temperature range of 200 ° C. or lower. ing. Full-Heusler alloy is a material system that is valuable for industrial applications because it does not use toxic rare metals like Bi-Te-based materials.

  The temperature range of exhaust heat differs depending on the type, and the exhaust heat exhausted at subways and substations is relatively low, 200 ° C or less, while exhaust heat exhausted from garbage incineration plants and automobile exhaust. The heat is in a relatively high temperature range of approximately 300 to 600 ° C. For this reason, establishment of an energy recovery technique suitable for each temperature range is required.

  A skutterudite compound is known as a thermoelectric conversion material that can be used in a temperature range of 300 to 600 ° C. The skutterudite compound can realize a low thermal conductivity by utilizing the rattling effect. For example, Non-Patent Document 1 describes a material having a high thermoelectric conversion characteristic of about ZT = 1.9 as a thermoelectric conversion material using a skutterudite compound.

Japanese Patent Laid-Open No. 2004-253618

G. Rogletal. / ActaMateria vol. 63 (2014) pp30-43

  Although the thermoelectric conversion material of patent document 1 can obtain high thermoelectric conversion efficiency in a temperature range of 200 ° C. or lower, for example, the thermoelectric conversion efficiency in a temperature range exceeding 300 ° C. has not been considered.

  On the other hand, although the thermoelectric conversion material of Non-Patent Document 1 can obtain high thermoelectric conversion efficiency in the temperature range of 300 to 600 ° C., it contains a highly toxic element such as Sb or As, and therefore it is difficult to realize a low environmental load. is there. Accordingly, there is a need for a thermoelectric conversion material that is low in environmental load and low in cost, and that can obtain high thermoelectric conversion efficiency in a temperature range including a temperature range exceeding 300 ° C.

  SUMMARY OF THE INVENTION An object of the present invention is to obtain a thermoelectric conversion material and a thermoelectric conversion module that can achieve low environmental load and low cost and can obtain high thermoelectric conversion characteristics.

A preferred embodiment of the thermoelectric conversion material according to the present invention is characterized by including a compound represented by the following general formula (1).
Re _α T _8-β T ′ _β X _12-μ X ′ _μ Y _{12 + γ} (1)
(In the formula (1), 0 ≦ α ≦ 2, 0 ≦ β ≦ 8, −1 ≦ γ ≦ 1, −0.3 ≦ μ ≦ 0.3, and Re is Li, Na, K, Rb, Selected from the group consisting of Cs, Mg, Ca, Sr, Ba, Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu At least one element, T is at least one element selected from the group consisting of Co, Rh and Ir, and T ′ is Fe, Mn, Ni, Cr, V, Ti, Zr, Nb , Mo, Ru, Os, Re, W, Ta and Hf, and X is at least one element selected from the group consisting of Sn, Ge and Si; X ′ is at least one element selected from the group consisting of Al, Ga, In, P, Sb and Bi. Yes, Y is S.)
As a preferred embodiment of the thermoelectric conversion module according to the present invention, a first thermoelectric conversion material layer and a first thermoelectric conversion material layer are provided adjacent to the first thermoelectric conversion material layer, and the first thermoelectric conversion material layer is a conductive type. Different thermoelectric conversion material layers, upper electrodes provided in contact with the first thermoelectric conversion material layer and the second thermoelectric conversion material layer, the first thermoelectric conversion material layer and the first thermoelectric conversion material layer A lower electrode provided in contact with the surface opposite to the surface on which the upper electrode is installed, and the first thermoelectric conversion material layer or the second thermoelectric conversion material. At least one of the layers is a layer formed of a thermoelectric conversion material containing a compound represented by the following general formula (1).
Re _α T _8-β T ′ _β X _12-μ X ′ _μ Y _{12 + γ} (1)
(In the formula (1), 0 ≦ α ≦ 2, 0 ≦ β ≦ 8, −1 ≦ γ ≦ 1, −0.3 ≦ μ ≦ 0.3, and Re is Li, Na, K, Rb, Selected from the group consisting of Cs, Mg, Ca, Sr, Ba, Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu At least one element, T is at least one element selected from the group consisting of Co, Rh and Ir, and T ′ is Fe, Mn, Ni, Cr, V, Ti, Zr, Nb , Mo, Ru, Os, Re, W, Ta and Hf, and X is at least one element selected from the group consisting of Sn, Ge and Si; X ′ is at least one element selected from the group consisting of Al, Ga, In, P, Sb and Bi. Yes, Y is S.)

  According to the present invention, it is possible to realize a thermoelectric conversion material and a thermoelectric conversion module that can be reduced in environmental load and cost and can obtain high thermoelectric conversion characteristics.

It is a figure which shows the crystal structure of a skutterudite compound. It is a figure which shows the band structure of a skutterudite compound. It is a figure which shows the band structure of a skutterudite compound. It is a figure which shows the band structure of a skutterudite compound. It is a figure which shows the band structure of a skutterudite compound. It is a figure which shows the relationship between (DELTA) VEC and a Seebeck coefficient. It is a figure which shows the relationship between (DELTA) VEC and a power factor. It is a figure which shows the relationship between (DELTA) VEC and ZT. It is a schematic sectional drawing which shows an example of the thermoelectric conversion element which concerns on embodiment. It is a schematic sectional drawing which shows an example of the thermoelectric conversion element which concerns on embodiment. It is a schematic whole perspective view which shows an example of the thermoelectric conversion module which concerns on embodiment. It is a figure which shows an example of the cascade type thermoelectric conversion module which concerns on embodiment. It is a figure which shows Table 1 of the valence electron number of each element.

The thermoelectric conversion material of the embodiment has a skutterudite-type crystal structure and contains a skutterudite compound represented by the following general formula (1).
Re _α T _8-β T ′ _β X _12-μ X ′ _μ Y _{12 + γ} (1)
(In the formula (1), 0 ≦ α ≦ 2, 0 ≦ β ≦ 8, −1 ≦ γ ≦ 1, −0.3 ≦ μ ≦ 0.3, and Re is Li, Na, K, Rb, Selected from the group consisting of Cs, Mg, Ca, Sr, Ba, Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu At least one element, T is at least one element selected from the group consisting of Co, Rh and Ir, and T ′ is Fe, Mn, Ni, Cr, V, Ti, Zr, Nb , Mo, Ru, Os, Re, W, Ta and Hf, and X is at least one element selected from the group consisting of Sn, Ge and Si; X ′ is at least one element selected from the group consisting of Al, Ga, In, P, Sb and Bi. Yes, Y is S.)
The skutterudite-type crystal structure is a crystal structure having symmetry of the space group Im-3 or the space group R-3. For example, CoAs ₃ and CoSb ₃ are known as conventional thermoelectric conversion materials having a skutterudite-type crystal structure. FIG. 1A shows the crystal structure of CoAs ₃ or CoSb ₃ , and FIGS. 1B and 1C show the crystal structure of the skutterudite compound represented by the general formula (1).

The crystal structure shown in FIG. 1A is composed of an element T that is a transition metal element and X _VB = As and Sb that are typical elements. Specifically, for example, CoAs ₃ or CoSb ₃ .

The crystal structure shown in FIG. 1B is a crystal structure in which the element X _VB in FIG. 1A is replaced with the typical element X and element Y, and is represented by the general formula T ₈ X ₁₂ Y _12. The That is, the crystal structure shown in FIG. 1B is a case where α = 0, β = 0, γ = 0, and μ = 0 in the general formula (1). Specifically, for example, a CoSn _1.5 S _1.5 crystal structure in which the As site of CoAs ₃ shown in FIG. 1A is replaced with the element X = Sn and the element Y = S can be mentioned.

  The element X = Sn, Ge, Si, and the element Y = S are harmless unlike Sb, As, etc., and can be suitably used as a thermoelectric conversion material as an inexpensive and non-toxic element. That is, by using a skutterudite compound in which Sb or As is substituted with Sn, Ge, Si, S, or the like, it is possible to produce a thermoelectric conversion material that is low in environmental load and inexpensive.

  The thermoelectric conversion characteristics of the thermoelectric conversion material depend on the electronic state of the substance. For example, the bottom of the conductor in the band structure has a flat band shape, and a crystal structure with a heavy effective mass is likely to obtain a high thermoelectromotive force, and has a high potential as a thermoelectric conversion material. Yes.

From the above viewpoint, Sb and As in a skutterudite compound such as CoSb ₃ and CoAs ₃ that have been conventionally used as thermoelectric conversion materials are replaced with non-toxic S and Sn or the like as shown in FIG. In the terdite type crystal structure, element substitution or element doping is further performed to control the electronic state of the compound (see FIG. 1C).

Specifically, for example, a part of Co = element T in CoSn _1.5 S _1.5 (see FIG. 1B) is replaced with an element T ′ which is a transition metal element such as Fe, By adding the element Re, which is an alkali metal element, alkaline earth metal element, or rare earth element, to the cage-like site of the skutterudite crystal structure, the electronic state of the skutterudite compound is modulated, and the thermoelectric conversion characteristics are improved. It is possible to control.

In the crystal structure shown in FIG. 1C, part of the element T in the crystal structure shown in FIG. 1B is replaced with the element T ′, and the element Re is added to the crystal structure shown in FIG. Crystal structure, 8 transition metal element sites (T site and T ′ site), 24 typical element sites (X site and Y site), rare earth element, alkaline earth metal element or alkali metal It has two element sites (Re sites). Accordingly, the crystal structure shown in FIG. 1C is represented by the general formula Re _α T _8−β T ′ _β X ₁₂ Y ₁₂ (0 ≦ α ≦ 2, 0 ≦ β ≦ 8). That is, the crystal structure shown in FIG. 1C is a case where γ = 0 and μ = 0 in the general formula (1). Specifically, for example, a crystal structure in which an element Re such as a rare earth element is added to a space of a crystal structure of CoSn _1.5 S _1.5 .

  In the general formula (1), the number of elements Y is basically the same as the total number 12 of the elements X and X ′. However, according to the type of the element selected as the element X or the element X ′, the number of elements Y is appropriately adjusted within the range of −1 ≦ γ ≦ 1, and the number of valence electrons of the entire compound is adjusted. Also good.

FIG. 2A shows the band structure of CoSb ₃ having a skutterudite-type crystal structure obtained by the first principle calculation. As shown in FIG. 2A, the band structure of CoSb ₃ reflects a highly localized electronic state derived from the 3d orbital of Co, and the vicinity of the bottom of the conduction band has a flat band shape. Yes.

FIG. 2B shows a band structure of CoSn _1.5 S _1.5 having a skutterudite type crystal structure obtained by the first principle calculation. CoSn _1.5 S _1.5 also shows a semiconductor band structure like CoSb _3, and the bottom of the conduction band has a band shape close to a flat with relatively little dispersion relation. That is, CoSn _1.5 S _1.5 , like CoSb ₃ , may have a high thermoelectromotive force as a thermoelectric conversion material.

3A to 3C show band structures obtained by first-principles calculations for various skutterudite compounds obtained by element substitution or element doping from a predetermined crystal structure. For example, by changing La _0.5 Fe ₂ Co ₂ Sn ₆ S ₆ La in FIG. 3A (i) to Y and Sc, which are lighter elements of the same family, the vicinity of the bottom of the conduction band in the band structure is flat. While maintaining the band shape, it can be confirmed that the band is displaced toward the low energy side (FIGS. 3A (ii) (iii)).

Further, for example, by changing Ba _0.5 Fe ₂ Co ₂ Sn ₆ S ₆ Ba in FIG. 3A (iv) to Sr, the vicinity of the bottom of the conduction band in the band structure is compared with the case where the rare earth element is changed. Then, although the fluctuation range is slightly small, it can be confirmed that the band is slightly displaced toward the high energy side while maintaining the flat band shape (FIG. 3A (v)).

  Therefore, it can be confirmed that the N-type thermoelectric conversion characteristics can be adjusted to a suitable state by changing the type of the additive element Re.

Further, for example, in La _0.5 Fe ₂ Co ₂ Sn ₆ S ₆ in FIG. 3A (i), the ratio of La as the element Re and Fe as the element T ′ is changed to change LaFe ₂ Co ₂ Sn ₆ S. ₆ (see FIG. 3B (i)), it can be confirmed that the vicinity of the bottom of the conduction band in the band structure is displaced toward the low energy side while maintaining a flat band shape.

For example, the ratio of Co and Fe in La _0.5 Fe ₂ Co ₂ Sn ₆ S ₆ in FIG. 3A (i) is changed from Co: Fe = 1: 1 to Co: Fe = 1: 3, By using La _0.5 CoFe ₃ Sn ₆ S ₆ (see FIG. 3C (i)), it can be confirmed that the band structure has changed significantly.

  As described above with reference to FIGS. 3A to 3C, for example, the element Re is added to the space of the skutterudite crystal structure, or a part of the element T is replaced with the element T ′ to control the electronic state. By doing so, it is possible to adjust the thermoelectric conversion characteristics within a suitable range.

  Adjustment of the thermoelectric conversion characteristics requires, for example, control of the carrier density by element substitution or element doping as described above. As an evaluation index, the amount of change in valence number density (VEC) ΔVEC (hereinafter referred to as valence electron number change ΔVEC). Can be used. The valence number change ΔVEC is a change amount of VEC from the valence number density (VEC) of the composition before the composition control.

For example 'in _beta X ₁₂ Y _12, a part of the site of the element T element T' formula Re _alpha T _8-beta T when substituting the valence number change ΔVEC from _Re α T ₈ X ₁₂ Y ₁₂ It is the valence number _{n T} of the element T, when 'the number of valence electrons of n _T' element T and is determined by _{ΔVEC = (n T '-n T} ) × β. For example, when the element T = Co and the element T ′ = Fe and the element Re is not added (α = 0), the valence electron change ΔVEC when the element T is replaced by the element T ′ is n _{T ′} = 8, n _{Since T} = 9, ΔVEC = (n _{T ′} −n _T ) × β = (8−9) × β = −β. That is, ΔVEC depends on the amount of Co substituted by Fe.

Further, for example, in the general formula _{Re α Co 8 Sn 12 S 12} , when the addition of Re _alpha, valence number change ΔVEC from Co _{8 Sn 12 S} 12, when the number of valence electrons of the element Re and _{n Re,} ΔVEC = α × n _Re . Specifically, for example La, Y, valence number _n Re = 3 of Sc, valence electron number _n Re = 2 for alkaline earth elements, or the number of valence electrons _{n Re} = 1 for alkali metal elements, appropriately alpha × It can be obtained by substituting for n _Re . Therefore, the valence electron change ΔVEC in this case depends on the valence electron number of the element added to the Re site.

  In order to investigate the possibility of controlling the thermoelectric conversion characteristics by controlling the Fermi level, the following evaluation was performed. In addition, the performance of the thermoelectric conversion material is evaluated by a dimensionless figure of merit (ZT) of the following formula (2).

上記式（２）において、σは電気伝導率、Ｓはゼーベック係数、κは熱伝導率、Ｓ^２σはパワーファクターである。一般に、ＺＴが高いものほど性能が良いため、式（２）より、ゼーベック係数及び電気伝導率が高く、熱伝導率の低い材料が熱電変換材料として望ましい。

In the above formula (2), σ is the electrical conductivity, S is the Seebeck coefficient, κ is the thermal conductivity, and S ² σ is the power factor. In general, the higher the ZT, the better the performance. Therefore, from Equation (2), a material having a high Seebeck coefficient and electrical conductivity and a low thermal conductivity is desirable as the thermoelectric conversion material.

_{LaCoFe 3 Sn 6 S 6, YCoFe} 3 Sn 6 S 6, ScCoFe 3 Sn 6 S 6, BaFe 2 Co 2 Sn 6 S 6, SrFe 2 Co 2 Sn 6 S 6, CaFe 2 Co 2 Sn 6 S 6, Ba 0 For the skutterudite compounds of _.5 FeCo ₃ Sn ₆ S ₆ and CoSn _1.5 S _1.5 , the change in Seebeck coefficient when the number of valence electrons was changed by first-principles calculation was examined. The evaluation results are shown in FIG.

  FIG. 4 (a) is a diagram showing the relationship between the valence electron number change ΔVEC and Seebeck coefficient of each skutterudite compound at 500K, and FIG. 4 (b) is the valence of each skutterudite compound at 800K. It is a figure which shows the relationship between electron number change (DELTA) VEC and a Seebeck coefficient.

  In FIGS. 4 (a) and 4 (b), the valence electron number change ΔVEC indicates the valence electron number change per unit cell shown in FIG. 1 (b) or 1 (c). The values of ΔVEC when the carrier density is changed from each skutterudite compound by element substitution or element doping are calculated by the first principle calculation. This also applies to FIGS. 5 and 6 described later.

In FIG. 4, ΔVEC can be adjusted as follows. Table 1 in FIG. 11 shows the number of valence electrons of each element. For example, in the case of _CoSn 1.5 _{S 1.5,} a part of the Co as _{Co 1-x Fe x Sn 1.5} S 1.5 by replacing Fe, it is possible to change the number of valence electrons . As shown in FIG. 11, since Fe has 8 valence electrons and Co has 9 valence electrons, the total valence electron number of the system is reduced by replacing part of Co with Fe. For example, in the case of formula _{Co 8-x Fe x Sn 12} S 12 _is a valence number change ΔVEC = -x from _CoSn 1.5 _{S 1.5. Therefore,} the _{Co 8-x Fe x Sn 12} S 12, by varying the substitution amount x of Co by Fe, and adjust the total number of valence electrons of the system, the DerutaVEC, be adjusted in the range of ΔVEC ≦ 0 Is possible.

_Further, the _CoSn 1.5 _{S 1.5,} the _{Co 8-x Ni x Sn 12} S 12 was replaced with more number of valence electrons than Co Ni (valence number _10), the _CoSn 1.5 _{S 1.5} The change in the number of valence electrons ΔVEC = x. For this reason, in Co _8-x Ni _x Sn ₁₂ S ₁₂ , the total valence electron number of the system is adjusted by changing the amount x of substitution of Co by Ni, and ΔVEC is adjusted in the range of ΔVEC ≧ 0. Is possible.

Further, in the case of LaCoFe ₃ Sn ₆ S ₆ , the total valence electrons of the system can be obtained by substituting a part of La with, for example, La _2-x Ba _x Co ₂ Fe ₆ Sn ₁₂ S _12. Can be adjusted. As shown in FIG. 11, since La has 3 valence electrons and Ba has 2 valence electrons, replacing La with Ba reduces the total number of valence electrons in the system. For example, in the case of the composition formula La _2−x Ba _x Co ₂ Fe ₆ Sn ₁₂ S ₁₂ , the valence number change ΔVEC = −x from LaCoFe ₃ Sn ₆ S ₆ is obtained. For this reason, in La _2−x Ba _x Co ₂ Fe ₆ Sn ₁₂ S ₁₂ , the substitution amount x of Ba by La is changed to adjust the total valence electron number of the system, and ΔVEC is in a range of ΔVEC ≦ 0. It is possible to adjust with.

The adjustment of the valence number of LaCoFe ₃ Sn ₆ S ₆ can also be performed by increasing the amount of Co, such as La ₂ Co _{2 + x} Fe _6-x Sn ₁₂ S ₁₂ . In this case, the change in the number of valence electrons from LaCoFe ₃ Sn ₆ S _{6 is} ΔVEC = x. For this reason, in La ₂ Co _{2 + x} Fe _6-x Sn ₁₂ S ₁₂ , the total valence electron number of the system is adjusted by changing the substitution amount x of Fe by Co, and the change in valence number ΔVEC becomes ΔVEC ≧ 0 It is possible to adjust within the range.

As shown in FIG. 4A and FIG. 4B, when ΔVEC <0, that is, when P-type thermoelectric conversion characteristics are shown, the Seebeck coefficient shows a positive value, and both have high absolute values. Indicated. Among them, CaCo ₂ Fe ₂ Sn ₆ S ₆ , SrCo ₂ Fe ₂ Sn ₆ S ₆ , BaCo ₂ Fe ₂ Sn ₆ S ₆ , and LaCoFe ₃ Sn ₆ S ₆ are in the range of −1.0 ≦ ΔVEC <0. By adjusting the number, a Seebeck coefficient exceeding 300 V / K at the maximum at 500K was obtained, and excellent thermoelectric conversion characteristics are expected.

Further, when ΔVEC> 0, that is, when an N-type thermoelectric characteristic was exhibited, the Seebeck coefficient showed a negative value, and both showed high absolute values. Among them, CoSn _1.5 S _1.5 has obtained a high absolute value for the Seebeck coefficient in both 500K and 800K by adjusting the number of valence electrons in the range of 0 <ΔVEC ≦ 1.0. High thermoelectric conversion characteristics are expected.

_{LaCoFe 3 Sn 6 S 6, YCoFe} 3 Sn 6 S 6, ScCoFe 3 Sn 6 S 6, BaFe 2 Co 2 Sn 6 S 6, SrFe 2 Co 2 Sn 6 S 6, CaFe 2 Co 2 Sn 6 S 6, Ba 0 For the skutterudite compounds of _.5 FeCo ₃ Sn ₆ S ₆ and CoSn _1.5 S _1.5 , the ΔVEC dependence of the power factor when the number of valence electrons was changed by first-principles calculation was examined. The evaluation results are shown in FIG.

  FIG. 5 (a) is a diagram showing the relationship between the valence electron number change ΔVEC and power factor of each skutterudite compound at 500K, and FIG. 5 (b) shows the valence of each skutterudite compound at 800K. It is a figure which shows the relationship between electron number change (DELTA) VEC and a power factor.

As shown in FIGS. 5A and 5B, when ΔVEC <0, that is, when P-type thermoelectric conversion characteristics are exhibited, all of the skutterudite compounds showed a high power factor. Among them, LaCoFe ₃ Sn ₆ S ₆ and YCoFe ₃ Sn ₆ S ₆ are high at ΔVEC <0 in both 500K and 800K by adjusting the number of valence electrons in the range of −1.0 ≦ ΔVEC <0. A power factor has been obtained, and high thermoelectric conversion performance can be expected as a P-type thermoelectric conversion material.

On the other hand, as shown in FIG. 5, when ΔVEC> 0, that is, when N-type thermoelectric conversion characteristics are exhibited, all showed a high power factor. Among them, CoSn _1.5 S _1.5 and Ba _0.5 FeCo ₃ Sn ₆ S ₆ have a high power factor by adjusting the number of valence electrons within the range of 0 <ΔVEC ≦ 1.0. As an N-type thermoelectric conversion material, high thermoelectric conversion performance can be expected.

As shown in FIG. 4 and FIG. 5, as a P-type thermoelectric conversion material, for example, LaCoFe ₃ Sn ₆ S ₆ , when β ≧ 4 in the general formula (1), that is, more than Co as a transition metal element When a large amount of Fe is contained, high thermoelectric conversion performance is easily obtained by controlling the number of valence electrons. On the other hand, as an N-type thermoelectric conversion material, for example, CoSn _1.5 S _1.5 or Ba _0.5 FeCo ₃ Sn ₆ S ₆ , when β ≦ 4 in the general formula (1), that is, a transition metal When the element contains more Co than Fe, high thermoelectric conversion performance is easily obtained by controlling the number of valence electrons.

_{LaCoFe 3 Sn 6 S 6, YCoFe} 3 Sn 6 S 6, ScCoFe 3 Sn 6 S 6, BaFe 2 Co 2 Sn 6 S 6, SrFe 2 Co 2 Sn 6 S 6, CaFe 2 Co 2 Sn 6 S 6, Ba 0 _.5 FeCO ₃ Sn ₆ S ₆ , CoSn _1.5 S _1.5 for skutterudite compounds, the ΔVEC dependence of ZT at 800 K when the number of valence electrons was changed by first-principles calculation did. The evaluation results are shown in FIG. As shown in FIG. 6, all of the skutterudite compounds show high values of ZT when the value of ΔVEC is within a range of ± 0.3.

As described above, the adjustment of the number of valence electrons in each skutterudite compound Re _α T _8-β T ′ _β X ₁₂ Y _{12 + γ} is such that the values of α and β are 0 ≦ α ≦ 2 and 0 ≦ β ≦ 8, respectively. Adjustment can be made within the range, or the type of element Re, element T, element T ′, element X, and element Y can be appropriately selected from the elements listed above and changed.

The adjustment of the number of valence electrons of the general formula (1) as shown in _{(Re α T 8-β T} 'β X 12-μ X' μ Y 12 + γ), the general formula _{Re α T 8-β T '} β Alternatively, a part of the element X in X ₁₂ Y _{12 + γ} may be replaced with another element X ′. As described above, in the general formula (1), the element X ′ is at least one element selected from the group consisting of Al, Ga, In, P, Sb, and Bi, and μ is −0.3. ≦ μ ≦ 0.3.

  The skutterudite compound described above can be obtained by mixing the raw material powder containing the constituent elements of each compound by, for example, mechanical alloying and then heat-treating it as necessary. Moreover, the skutterudite compound demonstrated above can also be obtained as a thin film by sputtering using the target material containing the structural element of each compound, for example.

  As the thermoelectric conversion material, the above-mentioned skutterudite compound nanoparticles are used as the main phase, and the grain boundary phase having a composition different from that of the main phase skutterudite compound is formed at the grain boundaries of the nanoparticles. Also good. The grain boundary phase preferably contains at least one element selected from the group consisting of P, Sb, Mg, Al, Cu, Ga, Na, Li, K, Sn, and Bi.

The thermoelectric conversion material having such a grain boundary phase is, for example, a powder material containing a constituent element of a grain boundary phase, such as MgAl ₂ and MgAlCu, and a raw material powder of a skutterudite compound mixed by a mechanical alloying method. Then, it can obtain by baking.

  Accordingly, at least one element selected from the group consisting of P, Sb, Mg, Al, Cu, Ga, Na, Li, K, Sn, and Bi is included in the grain boundaries of the skutterudite compound nanoparticles. A thermoelectric conversion material in which a grain boundary phase is formed can be obtained. A thermoelectric conversion material having such a grain boundary phase has a higher electric conductivity and a thermoelectric conversion material having a higher ZT than a thermoelectric conversion material having no grain boundary phase.

  The particle size of the nanoparticles of the skutterudite compound is preferably 1 to 500 nm. By setting the particle diameter of the nanoparticles to 500 nm or less, a high surface area can be obtained as the skutterudite compound particles, and sufficient thermoelectric conversion efficiency can be obtained. Moreover, complication of a manufacturing process can be prevented by making the particle diameter of the nanoparticles of a skutterudite compound 1 nm or more.

  The thermoelectric conversion material according to the above-described embodiment can be suitably used as a thermoelectric conversion element to be described later in a temperature range of 573 K or higher. For example, the engine exhaust heat of a car is relatively high at 573 to 873K. The thermoelectric conversion material according to the embodiment can efficiently recover and utilize such a relatively high temperature exhaust heat.

  The crystal structure of the thermoelectric conversion material can be easily confirmed by X-ray diffraction (XRD). Further, a lattice image can be observed with an electron microscope such as TEM (Transmission Electron Microscope), and a single crystal or polycrystal structure can be confirmed from a spot-like pattern or a ring-like pattern in an electron beam diffraction image. The composition distribution is EPMA (Electron Probe Micro Analyzer) such as EDX (Energy Dispersive X-ray spectroscopy), SIMS (Secondary Ionization Mass Spectro), P Information on the state density of the material can be confirmed by ultraviolet photoelectron spectroscopy or X-ray photoelectron spectroscopy. The electrical conductivity and carrier density can be confirmed by electrical measurement and Hall effect measurement using a 4-terminal method. The Seebeck coefficient can be confirmed by giving a temperature difference to both ends of the sample and measuring the voltage difference between both ends. The thermal conductivity can be confirmed by a laser flash method.

  Hereinafter, an example of sample preparation of the thermoelectric conversion material according to the embodiment will be shown. Here, the manufacturing example is an example, and it is needless to say that the manufacturing condition is not limited thereto.

<Experimental example>
(Sample preparation example 1)
Co powder having a purity of 99.9% and SnS powder were mixed at a ratio of a composition ratio of 2: 3, and an alloy powder having a composition of CoSn _1.5 S _1.5 was produced by a mechanical alloying method. CoSn _1.5 S _1.5 has a crystal structure in which the As site of CoAs ₃ is replaced with Sn and S, and the change in valence number ΔVEC from CoAs ₃ is about zero.

(Sample preparation example 2)
La powder of 99.9% purity, Fe powder, Co powder, and SnS powder were mixed at a ratio of 1: 3: 1: 6, and LaCoFe ₃ Sn ₆ S ₆ was mixed by mechanical alloying. After producing an alloy powder having a composition, the obtained alloy powder was heat-treated under high pressure. As a result of structural analysis of the obtained powder sample by X-ray diffraction, a crystal structure peak derived from a skutterudite structure was observed. LaCoFe ₃ Sn ₆ S ₆ is a crystal structure in which a part of the Co site of CoSn _1.5 S _1.5 is substituted with Fe and La is further added, and valence electrons from CoSn _1.5 S _1.5 The number change ΔVEC is approximately zero.

(Sample preparation example 3)
99.9% purity La powder, Ba powder, Fe powder, Co powder and SnS powder were mixed at a ratio of 0.95: 0.05: 3: 1: 6, and then used with a ball mill. The sample was pulverized to produce an alloy powder having a composition of La _0.95 Ba _0.05 CoFe ₃ Sn ₆ S ₆ . Thereafter, the obtained alloy powder was heat-treated under high pressure. As a result of structural analysis of the obtained powder sample by X-ray diffraction, a crystal structure peak derived from a skutterudite structure was observed. La _0.95 Ba _0.05 CoFe ₃ Sn ₆ S ₆ has a crystal structure in which part of the La site of LaCoFe ₃ Sn ₆ S ₆ is replaced with Ba, and the valence number change from LaCoFe ₃ Sn ₆ S ₆ ΔVEC is about −0.1.

(Sample preparation example 4)
Co _0.99 Ni _0.01 Sn _{1 in} which a part of Co in CoSn _1.5 S _1.5 prepared in Sample Preparation Example 1 was replaced with Ni, Mn, Cr, V, Zr, Hf, Ta, and W. _{.5 S 1.5, Co 0.99 Mn 0.01} Sn 1.5 S 1.5, Co 0.99 Cr 0.01 Sn 1.5 S 1.5, Co 0.99 V 0.01 Sn _1.5 S _1.5 , Co _0.99 Zr _0.01 Sn _1.5 S _1.5 , Co _0.99 Hf _0.01 Sn _1.5 S _1.5 , Co _0.99 Ta _0.01 An alloy powder having a composition of Sn _1.5 S _1.5 and Co _0.99 W _0.01 Sn _1.5 S _1.5 was prepared.

For example, in the case of Co _0.99 Ni _0.01 Sn _1.5 S _1.5 , the ratio of Co powder, Ni powder, and SnS powder at a composition ratio of 0.99: 0.01: 1.5 Then, an alloy powder was prepared by the same method as in Sample Preparation Example 1. Alloy powders having other compositions were produced in the same manner.

(Sample preparation example 5)
Sputtering is performed on a Si substrate having a thermal oxide film using a mixed target having a composition ratio of La, Fe, Co, Sn, and S of 1: 3: 1: 6: 6 to form LaCoFe ₃ Sn ₆ S ₆ . A thin film having a thickness of about 300 nm having a composition was prepared, and heat treatment was performed in a nitrogen atmosphere at 800 ° C. for 1 hour. As a result of structural analysis by X-ray diffraction of the thin film, a peak of the skutterudite structure was observed.

(Sample measurement example 1)
For the samples prepared in Sample Preparation Example 1 and Sample Preparation Example 2, the voltage at both ends was measured with one end at room temperature and the other end at a temperature 20 ° C. higher than room temperature with a temperature difference of 20 ° C. at both ends. The Seebeck coefficient was measured by measuring the difference. As a result, the sample obtained in Sample Preparation Example 1 had a high Seebeck coefficient of 350 V / K, and the sample obtained in Sample Preparation Example 2 had a high Seebeck coefficient of 100 V / K. The thermal conductivity was 10 mW / Kcm for the sample obtained in Sample Preparation Example 1 and 9 mW / Kcm for the sample obtained in Sample Preparation Example 2 at room temperature.

  Therefore, it was confirmed that a thermoelectric conversion material having a high thermoelectromotive force can be obtained as a p-type thermoelectric conversion material by controlling the Seebeck coefficient by changing the VEC by adjusting the element doping amount or element substitution.

  As the sample preparation method, various known methods other than the sample preparation examples described above can be used. For example, a melting method, a rapid solidification method, or a hot press method may be used. A sintering method may be used, a vacuum deposition method such as molecular beam epitaxy which is a thin film manufacturing method may be used, and chemical vapor deposition using a transition metal complex or the like may be used in combination. Further, for example, sulfur is heated to be vaporized, and an inert gas such as argon is used as a carrier gas, and sent to a reaction chamber charged with a La—Co—Fe—Sn alloy, and the sulfur vapor is introduced into La—Co—Fe—. You may use the method made to react with Sn alloy.

  According to the embodiment described above, there is little fear of depletion, and it is compared with a conventional thermoelectric conversion material using a skutterudite compound containing As or Sb in the main skeleton by combining materials that are inexpensive and non-toxic. Thus, it is possible to produce a thermoelectric conversion material with low environmental load and low cost. In addition, by adding, for example, an element having a different valence number to a skutterudite compound having a predetermined composition, or by substituting an element having a different valence number, the number of valence electrons of the entire compound is controlled, It becomes possible to produce a thermoelectric conversion material having a high thermoelectromotive force.

(Example of module production)
Next, a thermoelectric conversion element using the above-described thermoelectric conversion material and a thermoelectric conversion module using the same will be described with reference to FIGS. FIG. 7 is a schematic cross-sectional view illustrating an example of the thermoelectric conversion element according to the embodiment. A thermoelectric conversion element 100 shown in FIG. 7 is a thermoelectric conversion element using the n-type thermoelectric conversion material according to the above-described embodiment as the n-type thermoelectric conversion material layer 104, and is on the upper surface side of the n-type thermoelectric conversion material layer 104. An electrode 102a (first upper electrode) and an electrode 102b (first lower electrode) are provided on the lower surface side, and the temperature difference is such that the electrode 102a side of the n-type thermoelectric conversion material layer 104 is at a high temperature and the electrode 102b side is at a low temperature. , Current flows in the direction 120 from the low temperature side to the high temperature side. In FIG. 7, when the p-type thermoelectric conversion material layer 103 is provided instead of the n-type thermoelectric conversion material layer 104, the current flows in a direction 130 from the high temperature side toward the low temperature side.

  FIG. 8 is a schematic cross-sectional view illustrating an example of the thermoelectric conversion element according to the embodiment. A thermoelectric conversion element 200 shown in FIG. 8 is a thermoelectric conversion element using the n-type thermoelectric conversion material according to the above-described embodiment as the n-type thermoelectric conversion material layer 104, and is adjacent to the n-type thermoelectric conversion material layer 104. Thus, a p-type thermoelectric conversion material layer 103 is provided. In the following, a case where the n-type thermoelectric conversion material layer 104 is installed as the first thermoelectric conversion material layer and the p-type thermoelectric conversion material layer 103 is installed as the second thermoelectric conversion material layer will be described.

  The thermoelectric conversion element 200 has a π-type structure in which an n-type thermoelectric conversion material layer 104 and a p-type thermoelectric conversion material layer 103 are connected to each other on the upper side surface in FIG. Have. The electrode 102c is an electrode as a second upper electrode provided on the p-type thermoelectric conversion material layer 103 and an electrode 102a as a first upper electrode provided on the n-type thermoelectric conversion material layer 104 (FIG. 7). Are electrically connected and integrally formed. On the other hand, an electrode 102b (first lower electrode) and an electrode 102d (second lower electrode) are separately provided on the lower side surfaces in FIG. 9 of the n-type thermoelectric conversion material layer 104 and the p-type thermoelectric conversion material layer 103, respectively. ing.

  In the π-type thermoelectric conversion element 200 shown in FIG. 8, when the electrode 102c side is at a high temperature and the electrodes 102b and 102d are at a low temperature, the p-type thermoelectric conversion material layer 104 is moved from the low temperature side to the high temperature side. Inside the thermoelectric conversion material layer 103, a current flows in a direction 140 from the high temperature side to the low temperature side.

  According to the thermoelectric conversion element 100 illustrated in FIG. 7 and the thermoelectric conversion element 200 illustrated in FIG. 8, high conversion efficiency can be obtained even in a temperature range of 300 to 600 ° C., for example.

  Next, a thermoelectric conversion module using the above-described n-type thermoelectric conversion material will be described with reference to FIGS. FIG. 9 is a schematic overall perspective view illustrating an example of the thermoelectric conversion module according to the embodiment.

  The thermoelectric conversion module 300 shown in FIG. 9 is configured by arranging a large number of π-type thermoelectric conversion elements 200 shown in FIG. 8 on the same surface. The thermoelectric conversion module 400 shown in FIG. Is a cascade type thermoelectric conversion module using the π-type thermoelectric conversion element 200 shown in FIG. 8, and any of the π-type thermoelectric conversion elements 200 shown in FIG. Desired voltage and current can be obtained. In particular, the cascade-type thermoelectric conversion module 400 shown in FIG. 10 can be applied to a case where the temperature difference between the high temperature side and the low temperature side is large or the temperature distribution is different.

  In FIG. 9, FIG. 9B is a perspective view in a state where the electrode 102c in FIG. 9A is removed. In operation, the thermoelectric conversion module 300 shown in FIG. 9 is given a temperature difference so that a temperature gradient is formed in a direction perpendicular to the device surface, as indicated by an arrow 150 in FIG. 9A.

  In FIG. 9, an electrode 102 b (first lower electrode) and an electrode 102 d (second lower electrode) are provided on the lower side surfaces of the n-type thermoelectric conversion material layer 104 and the p-type thermoelectric conversion material layer 103 in FIG. 9. An electrode 102e that is electrically connected and integrally formed is provided. The electrode 102e is provided on the lower side in FIG. 9 of the n-type thermoelectric conversion material layer 104 and the p-type thermoelectric conversion material layer 103 so as to be in contact with both.

  FIG. 10 is a diagram illustrating an example of a cascade type thermoelectric conversion module according to the embodiment. 10A is a schematic overall perspective view of a cascade type thermoelectric conversion module, and FIG. 10B is a cross-sectional view of a part of the thermoelectric conversion module shown in FIG. 10A. .

  10B, in the cascade type thermoelectric conversion module 400, the first-stage n-type thermoelectric conversion material layer 104 and the p-type thermoelectric conversion material layer 103, and the second-stage n-type thermoelectric conversion material. An insulating substrate 101 is provided between the layer 104 and the p-type thermoelectric conversion material layer 103. An insulating substrate 101 is provided on the upper surface side of the first-stage n-type thermoelectric conversion material layer 104 and p-type thermoelectric conversion material layer 103 with the electrode 102c interposed therebetween. An insulating substrate 101 is provided on the lower surface side of the second-stage n-type thermoelectric conversion material layer 104 and p-type thermoelectric conversion material layer 103 with the electrode 102e interposed therebetween.

  According to the thermoelectric conversion module 300 shown in FIG. 9 and the thermoelectric conversion module 400 shown in FIG. 10, a thermoelectric conversion module with high conversion efficiency can be provided even in a temperature range of about 300 to 600 ° C. Therefore, the thermoelectric conversion module 300 and the thermoelectric conversion module 400 described above can be suitably used for in-vehicle use, for example.

DESCRIPTION OF SYMBOLS 100, 200 ... Thermoelectric conversion element, 300, 400 ... Thermoelectric conversion module, 101 ... Insulating substrate, 102a, 102b, 102c, 102e ... Electrode, 103 ... P-type thermoelectric conversion material layer, 104 ... N-type thermoelectric conversion material layer, 120 ... direction of current flow when n-type thermoelectric conversion material is used, 130 ... direction of current flow when p-type thermoelectric conversion material is used, 140 ... direction of current flow, 150 ... arrow

Claims (9)

A thermoelectric conversion material comprising a compound represented by the following general formula (1):
Re _α T _8-β T ′ _β X _12-μ X ′ _μ Y _{12 + γ} (1)
(In the formula (1), 0 ≦ α ≦ 2, 0 ≦ β ≦ 8, −1 ≦ γ ≦ 1, −0.3 ≦ μ ≦ 0.3, and Re is Li, Na, K, Rb, Selected from the group consisting of Cs, Mg, Ca, Sr, Ba, Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu At least one element, T is at least one element selected from the group consisting of Co, Rh and Ir, and T ′ is Fe, Mn, Ni, Cr, V, Ti, Zr, Nb , Mo, Ru, Os, Re, W, Ta and Hf, and X is at least one element selected from the group consisting of Sn, Ge and Si; X ′ is at least one element selected from the group consisting of Al, Ga, In, P, Sb and Bi. Yes, Y is S.)   The thermoelectric conversion material according to claim 1, wherein the thermoelectric conversion material is a P-type thermoelectric conversion material, and β ≧ 4 in the general formula (1).   The thermoelectric conversion material according to claim 1, wherein the thermoelectric conversion material is an N-type thermoelectric conversion material, and β ≦ 4 in the general formula (1).   Selected from the group consisting of P, Sb, Mg, Al, Cu, Ga, Na, Li, K, Sn, and Bi formed in the main phase formed by the nanoparticles of the compound and the grain boundaries of the nanoparticles. 2. The thermoelectric conversion material according to claim 1, further comprising: a grain boundary phase that includes at least one kind of element having a composition different from that of the main phase.   The thermoelectric conversion material according to claim 4, wherein the nanoparticles have a particle size of 1 to 500 nm. The thermoelectric conversion material includes a compound in which the number of valence electrons of LaCoFe ₃ Sn ₆ S ₆ or CoSn _1.5 S _1.5 is adjusted in a range of −0.3 ≦ ΔVEC ≦ 0.3. The thermoelectric conversion material according to claim 1. A first thermoelectric conversion material layer;
A second thermoelectric conversion material layer provided adjacent to the first thermoelectric conversion material layer and having a conductivity type different from that of the first thermoelectric conversion material layer;
An upper electrode provided in contact with the first thermoelectric conversion material layer and the second thermoelectric conversion material layer;
A lower electrode provided on the first thermoelectric conversion material layer and the second thermoelectric conversion material layer in contact with the surface opposite to the installation surface of the upper electrode;
At least one of the first thermoelectric conversion material layer or the second thermoelectric conversion material layer is a layer formed of a thermoelectric conversion material containing a compound represented by the following general formula (1). Thermoelectric conversion module.
Re _α T _8-β T ′ _β X _12-μ X ′ _μ Y _{12 + γ} (1)
(In the formula (1), 0 ≦ α ≦ 2, 0 ≦ β ≦ 8, −1 ≦ γ ≦ 1, −0.3 ≦ μ ≦ 0.3, and Re is Li, Na, K, Rb, Selected from the group consisting of Cs, Mg, Ca, Sr, Ba, Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu At least one element, T is at least one element selected from the group consisting of Co, Rh and Ir, and T ′ is Fe, Mn, Ni, Cr, V, Ti, Zr, Nb , Mo, Ru, Os, Re, W, Ta and Hf, and X is at least one element selected from the group consisting of Sn, Ge and Si; X ′ is at least one element selected from the group consisting of Al, Ga, In, P, Sb and Bi. Yes, Y is S.)   The thermoelectric conversion module according to claim 7, wherein the thermoelectric conversion module is used in a temperature range of 300 to 600 ° C.   The thermoelectric conversion module according to claim 8, wherein the thermoelectric conversion module is for vehicle use.

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