JP4900819B2 - Thermoelectric material and manufacturing method thereof - Google Patents

Description

  The present invention relates to a thermoelectric material and a manufacturing method thereof, and more particularly to a thermoelectric material mainly composed of a half-Heusler compound and a manufacturing method thereof.

Thermoelectric conversion refers to the direct conversion of electrical energy into thermal energy for cooling and heating, and conversely, thermal energy into electrical energy using the Seebeck effect and Peltier effect. Thermoelectric conversion
(1) Do not discharge excess waste during energy conversion,
(2) Effective use of exhaust heat is possible.
(3) It can continuously generate power until the material deteriorates.
(4) No moving devices such as motors and turbines are required and maintenance is not required.
Therefore, it has been attracting attention as a highly efficient energy utilization technology.

As an index for evaluating the characteristics of heat energy and electric energy, that is, the characteristics of the thermoelectric material, generally, the figure of merit Z (= S ² σ / κ, where S: Seebeck coefficient, σ: electric conduction Degree, κ: thermal conductivity), or a dimensionless figure of merit ZT expressed as a product of the figure of merit Z and the absolute temperature T indicating the value. The Seebeck coefficient represents the magnitude of electromotive force generated by a temperature difference of 1K. Thermoelectric materials each have their own Seebeck coefficient, and are broadly classified into those having a positive Seebeck coefficient (p-type) and those having a negative Seebeck coefficient (n-type).

The thermoelectric material is usually used in a state where a p-type thermoelectric material and an n-type thermoelectric material are joined. Such a junction pair is generally called a “thermoelectric element”. The performance index of the thermoelectric element depends on the performance index Z _{p of the p-} type thermoelectric material, the performance index Z _{n of} the n-type thermoelectric material, and the shape of the p-type and n-type thermoelectric materials, and the shape is optimized If you are the greater the Z _p and / or Z _n, the figure of merit of the thermoelectric element increases is known. Therefore, in order to obtain a high figure of merit thermoelectric device, the figure of merit Z _p, be used with high Z _n thermoelectric material is important.

As such a thermoelectric material,
(1) Bi-Te-based, Pb-Te-based, Si-Ge-based compound semiconductors,
(2) Co-based oxide ceramics such as Na _x CoO ₂ (0.3 ≦ x ≦ 0.8), (ZnO) _m In ₂ O ₃ (1 ≦ m ≦ 19), Ca ₃ Co ₄ O ₉ ,
(3) a skutterudite compound such as Zn—Sb, Co—Sb, or Fe—Sb,
(4) Half-Heusler compounds such as ZrNiSn are known.

  Among these, Bi-Te-based and Pb-Te-based compound semiconductors exhibit high ZT in the low temperature range, but cannot be used in the middle / high temperature range, and have a large environmental load such as Pb, Te, Sb, etc. There is a problem of containing a large amount. Further, Ge—Si-based compound semiconductors have low characteristics in the middle / low temperature range of 600 ° C. or lower (ZT <0.5). Further, Co-based oxide ceramics can be used even in a high temperature range, and there is a report that ZT> 1 in a single crystal, but ZT <0.5 in a bulk material.

  The skutterudite compound is a p-type thermoelectric material that exhibits relatively high thermoelectric properties in the middle / low temperature range. Further, it is known that certain skutterudite compounds have ZT> 1 at 527 ° C. (800 K). For example, since the exhaust gas temperature of an automobile is about 800 K, it is expected that a highly efficient waste heat recovery system can be obtained by using such a thermoelectric element using a skutterudite compound.

Furthermore, the half-Heusler compound is an n-type thermoelectric material that exhibits relatively high thermoelectric properties in the middle and low temperature regions. Here, the “half-Heusler compound” refers to a series of compounds having a structure in which half of the Cu site atoms of the Heusler alloy Cu ₂ AlMn are lost. Since the half-Heusler compound exhibits relatively high thermoelectric properties in the middle / low temperature range, it is considered as one of the candidate materials for the partner material to be used in combination with the skutterudite compound. Therefore, various proposals have conventionally been made regarding the composition of the half-Heusler compound.

For example, Patent Document 1 and Non-Patent Document 1 disclose thermoelectric materials made of a half-Heusler compound having a composition of Zr _0.5 Hf _0.5 Ni _1-x Pd _x Sn _0.99 Sb _0.01 (x = 0.2 or 0.5). Has been. In the same document,
(1) By substituting part of Ni with Pd, mass defects are introduced into the crystal lattice, thereby reducing the thermal conductivity of the lattice;
(2) The electrical resistivity is reduced by 1 at% Sb doping, and
(3) The dimensionless figure of merit ZT at 800 K of a half-Heusler compound having a composition of Hf _0.5 Zr _0.5 Ni _0.8 Pd _0.2 Sn _0.99 Sb _0.01 is 0.7 (maximum value),
Is described.

Non-Patent Document 2 discloses a thermoelectric material made of a half-Heusler compound having a composition of Ti _0.5 (Zr _0.5 Hf _0.5 ) _0.5 NiSn _0.998 Sb _0.002 . In the same document,
(1) The point that the (Zr, Hf) site is replaced with Ti greatly reduces the thermal conductivity and increases the Seebeck coefficient.
(2) Doping Sb at the Sn site increases the electrical conductivity, and
(3) The dimensionless figure of merit ZT becomes 1.5 at 700K,
Is described.

Non-Patent Document 3 discloses a half-Heusler compound having a composition of (Zr _x Hf _1-x ) _0.7 Ti _0.3 NiSn (x = _{0.3, 0.4, 0.5,} 0.6, _0.7 ). A thermoelectric material is disclosed. This document describes that in the (Zr _0.7 Hf _0.3 ) _0.7 Ti _0.3 NiSn composition, the dimensionless figure of merit ZT at 800 K is 0.32 (maximum value).
Further, Non-Patent Document 4, the general formula: thermoelectric consisting half-Heusler compound represented by _{Zr 1-xy Y x Nb y} NiSn (y-x = 0.02,0.02 ≦ y ≦ 0.05) A material is disclosed. This document describes that when x = 0 (Zr _0.98 Nb _0.02 NiSn), ZT reaches the maximum value (0.34) at 573K.
Furthermore, Patent Documents 2 to 4 describe methods for producing various thermoelectric materials using a rapid solidification method. Patent Document 2 describes that quartz glass is used as a nozzle used for rapid solidification.

US Patent Application Publication No. 2004/0112418 Q. Shen et al., Appl. Phys. Lett., 79, 4165 (2001) N. Shutoh et al., Proc. 22nd Int. Conf. Thermoelectrics (2003), P312 Ken Kurosaki et al., The 1st Annual Meeting of the Thermoelectric Society of Japan, p24 Shigeru Katsuyama et al., The 1st Annual Meeting of the Thermoelectric Society of Japan, p22 JP 2006-269870 A JP 2006-269931 A JP 2006-165125 A

In a thermoelectric element using a p-type thermoelectric material made of a skutterudite compound, in order to obtain high thermoelectric conversion efficiency, a counterpart material (n-type thermoelectric material) exhibiting a high ZT in the middle / low temperature range is required. A guideline for practical use of thermoelectric materials is ZT> 1.
However, with conventional half-Heusler compounds, it is difficult to achieve ZT> 1. Note that Non-Patent Document 2 describes a half-Heusler compound having a ZT of 1.5 at 700K. However, the reported example of Non-Patent Document 2 has poor reproducibility, and in current academic societies, the authenticity of data is described. Is currently being questioned.

  Furthermore, many of the skutterudite compounds exhibiting high thermoelectric properties in the middle / low temperature range have a problem of containing a large amount of elements having a large environmental load such as Sb. Therefore, there is a demand for a p-type thermoelectric material that has a relatively low content of elements with a large environmental load and that exhibits high thermoelectric properties in the middle / low temperature range.

  In general, when a rapid solidification method is used for a meltable material, it is possible to improve the uniformity of the phase, refine the grain size, and the like. Therefore, the rapid solidification method is used for the synthesis of alloy materials. It has been reported that the thermoelectric performance is improved by using the rapid solidification method in the process of synthesizing the thermoelectric material (see Patent Documents 2 to 4). In the rapid solidification method, a glass nozzle is generally used (see Patent Document 2). However, in the case of producing a thermoelectric material using the rapid solidification method, there has never been an example in which the causal relationship between the nozzle material and the thermoelectric characteristics has been studied.

The problem to be solved by the present invention is to provide a thermoelectric material exhibiting relatively high thermoelectric properties in the middle / low temperature range and a method for producing the same.
In addition, another problem to be solved by the present invention is to provide a thermoelectric material that exhibits relatively high thermoelectric properties in the middle and low temperature ranges and that has a relatively low content of elements with a large environmental load. is there.
Furthermore, another problem to be solved by the present invention is to provide a method for producing a thermoelectric material with little deterioration in thermoelectric properties due to the mixing of impurities, and a thermoelectric material obtained using the method.

In order to solve the above-described problems, the thermoelectric material according to the present invention includes a half-Heusler compound having the following configuration.
(1) The half-Heusler compound has an AgAsMg crystal structure.
(2) The half-Heusler compound has 6 valence electrons per atom.
(3) In the half-Heusler compound, at least two of the three sites of the AgAsMg type crystal structure each include two or more kinds of atoms having different valence electrons.
(4) The O concentration ([O]) and the Si concentration ([Si]) in the half-Heusler compound satisfy the following formula (a).
2.5 ≦ 3.305-5.10 [O] −0.540 [Si] (a)
(5) The O concentration in the half-Heusler compound is 0.10 at% or less, and the Si concentration is 0.10 at% or less.
(6) The thermoelectric material is obtained by rapidly cooling and solidifying the molten metal of the half-Heusler compound using a boron nitride nozzle heated at 600 ° C. or higher in an inert gas atmosphere.
Moreover, the manufacturing method of the thermoelectric material which concerns on this invention makes it a summary to have the following structures.
(1) The manufacturing method of the thermoelectric material is as follows:
A dissolution step of dissolving the raw materials formulated to be a half-Heusler compound according to the present invention;
A rapid cooling step for rapidly solidifying the molten metal obtained in the melting step;
With
The quenching step includes spraying or dripping the molten metal onto a cooling medium using a boron nitride nozzle.
(2) pre-SL made of boron nozzle nitride, than is also obtained by heating at 600 ° C. or higher in an inert gas atmosphere.

In a half-Heusler compound, when a part of atoms occupying at least two sites is replaced with an atom having a different valence electron number so that the number of valence electrons per atom is maintained at 6, the mobility of carriers does not decrease. The thermal conductivity κ can be reduced and the Seebeck coefficient S can be increased.
In addition, when a half-Heusler compound having a predetermined composition is manufactured using a rapid solidification method, when a conventionally used quartz nozzle is used, an element contained in the main component reacts with the quartz nozzle. As a result, a composition shift of the half-Heusler compound occurs. In addition, due to the reaction with the quartz tube, silicon and oxygen are mixed and the thermoelectric properties are deteriorated.
On the other hand, when a boron nitride nozzle is used as the rapid solidification nozzle, reaction between the constituent elements and the nozzle is suppressed. In particular, when a boron nitride nozzle heated in advance in an inert gas atmosphere at 600 ° C. or higher is used, it is possible to suppress unintended impurity contamination to a minimum. Therefore, a thermoelectric material reflecting the charged composition can be obtained. Moreover, since contamination with impurities can be suppressed to a minimum, thermoelectric characteristics and reproducibility are also improved.

Hereinafter, an embodiment of the present invention will be described in detail.
[1. Thermoelectric material (1)]
The thermoelectric material according to the first embodiment of the present invention is characterized by including a half-Heusler compound having the following configuration.
(1) The half-Heusler compound has an AgAsMg crystal structure.
(2) The half-Heusler compound has 6 valence electrons per atom.
(3) In the half-Heusler compound, at least two of the three sites of the AgAsMg type crystal structure each include two or more kinds of atoms having different valence electrons.
(4) The O concentration ([O]) and the Si concentration ([Si]) in the half-Heusler compound satisfy the following formula (a).
2.5 ≦ 3.305-5.10 [O] −0.540 [Si] (a)

[1.1. Crystal structure]
FIG. 1 shows a schematic diagram of a unit cell of an AgAsMg type crystal structure. The half-Heusler compound has an AgAsMg type crystal structure (space group F43m) and is represented by a general formula: XYZ. The X atom and the Z atom are respectively a 4a (0, 0, 0) site (hereinafter simply referred to as “4a site”) and a 4b (1/2, 1/2, 1/2) site (hereinafter simply referred to as “4a site”). It is located at “4b site”), and the X and Z atoms form a rock salt structure. Y atom is an octahedrally coordinated pocket (center of a cube composed of X atom and Z atom), that is, 4c (1/4, 1/4, 1/4) site (hereinafter simply referred to as “4c Site "). The other pockets, that is, 4d (3/4, 3/4, 3/4) sites (hereinafter simply referred to as “4d sites”) are empty.

[1.2. Number of valence electrons]
In the half-Heusler compound according to the present embodiment, the number of valence electrons per atom is 6. Half-Heusler compounds with 6 valence electrons per atom (or 18 total valence electrons) are known to exhibit semiconducting properties and have moderately large Seebeck coefficient S and electrical resistivity ρ. ing.
Since the half-Heusler compound XYZ is a compound of X: Y: Z = 1: 1: 1, the number of valence electrons #e per atom is represented by the following formula (1).
_{#E = (# e X + #} e Y + # e Z) / 3 ··· (1)
Here, #e _X , #e _Y, and #e _Z are the valence electron numbers of the X atom, the Y atom, and the Z atom, respectively. When each site is occupied by a plurality of types of atoms, #e _X , #e _Y, and #e _Z are the average number of valence electrons of the atoms occupying each site.

  In the present invention, the “valence electron number” means the number of electrons contributing to the chemical bond. Table 1 below shows the number of valence electrons of each atom.

[1.3. Constituent elements]
The half-Heusler compound according to the present invention is characterized in that at least two of the three sites described above contain two or more atoms having different valence electrons.
In particular,
(1) Either one of the 4a site or the 4b site includes one or more atoms having 4 valence electrons and one or more atoms having 5 valence electrons,
The other site contains one or more atoms having 4 valence electrons and one or more atoms having 3 valence electrons,
When the 4c site contains one or more atoms having 10 valence electrons,
(2) Either one of the 4a site or the 4b site includes one or more atoms having a valence number of 4 and one or more atoms having a valence number of 5;
The other site contains one or more atoms having 4 valence electrons,
When the 4c site contains one or more atoms having 10 valence electrons and one or more atoms having 9 valence electrons,
(3) One or two or more atoms having 4 valence electrons, 1 or 2 or more atoms having 5 valence electrons, and valence electrons on either the 4a site or the 4b site Including one or more atoms having a number of 3,
On the other site, one or more atoms having 4 valence electrons, one or more atoms having 3 valence electrons, and one or 2 atoms having 5 valence electrons Containing more than species of atoms,
When one or more atoms having 10 valence electrons are included in the 4c site,
(4) Both the 4a site and the 4b site include one or more atoms having 4 valence electrons and one or more atoms having 5 valence electrons,
When the 4c site contains one or more atoms having 10 valence electrons and one or more atoms having 8 valence electrons,
This is the case.

  The type of atoms occupying each site is not particularly limited, and can be arbitrarily selected as long as the above-described conditions are satisfied. Among these, the following elements and combinations thereof are particularly preferable.

That is, the constituent element X of the 4a site includes group IIIa ( ₂₁ Sc, ₃₉ Y), group IVa ( ₂₂ Ti, ₄₀ Zr, ₇₂ Hf), group Va ( ₂₃ V, ₄₁ Nb, ₇₃ Ta), and rare earth elements One or more elements selected from ( ₅₇ La to ₇₁ Lu) are preferred.
Further, the constituent element Y of the 4c site includes group VIIIa ( ₂₆ Fe, ₂₇ Co, ₂₈ Ni, ₄₄ Ru, ₄₅ Rh, ₄₆ Pd, ₇₆ Os, ₇₇ Ir, ₇₈ Pt) and group Ib ( ₂₉ Cu, ₄₇ Ag, ₇₉ One or more elements selected from Au) are preferred.
Furthermore, constituent elements Z of 4b sites, IIIb group _{(5 B, 13 Al, 31} Ga, 49 In, 81 Tl), IVb group _{(6 C, 14 Si, 32} Ge, 50 Sn, 82 Pb) and Group Vb One or more elements selected from ( ₇ N, ₁₅ P, ₃₃ As, ₅₁ Sb, ₈₃ Bi) are preferred.
The 4a site and the 4b site are equivalent. Therefore, instead of the combination described above, the constituent element Z of the 4b site is one or more elements selected from the group IIIa, IVa group, Va group and rare earth element, and the constituent element X of the 4a site is the group IIIb, group IVb And one or more elements selected from the group Vb.
Of the IVb group elements, Si is often unevenly distributed at grain boundaries without entering the 4a site or the 4b site. Therefore, in the present invention, the amount of Si is limited to the amount described below or less.

Among these, the constituent element Y of the 4c site is preferably Ni and / or Co. Half-Heusler compounds containing Ni and / or Co at the 4c site exhibit relatively high thermoelectric properties.
Moreover, Sn and / or Sb are preferable as either the constituent element Z of the 4b site or the constituent element X of the 4a site. A half-Heusler compound containing Sn and / or Sb at either the 4b site or the 4a site exhibits relatively high thermoelectric properties.
Furthermore, Ti, Zr, and / or Hf are preferable as either the constituent element X of the 4a site or the constituent element Z of the 4b site. The half-Heusler compound containing Ti, Zr and / or Hf at either the 4a site or the 4b site exhibits relatively high thermoelectric properties.

In addition, the half-Heusler compound is better as the content of an element with a larger environmental load is smaller. Specific examples of elements having a large environmental load include Cd, Hg, Tl, Pb, As, Se, Te, and Sb.
Among these, since Cd, Hg, Tl, Pb, As, Se, and Te have a very large environmental load, their concentration is preferably 0.1 at% or less of all atoms constituting the half-Heusler compound. .
On the other hand, Sb has a smaller load on the environment than Cd or the like, so the concentration of Sb is preferably 5 at% or less of all atoms constituting the half-Heusler compound.

The ratio of atoms having different valence electrons in each site is not particularly limited, and can be arbitrarily selected. Here, of the atoms having different valence electrons in each site, those having a ratio of the atoms with respect to all atoms in the corresponding site of 50 at% or less are defined as “minority atoms”. The proportion of minority atoms may be 50 at%, but generally the thermoelectric properties tend to decrease as the proportion of minority atoms approaches 50 at%. This is presumably because of the difference in ionic radius, it is easy to separate the different phases without dissolving them, and the electrical conductivity is thereby reduced.
In order to obtain high thermoelectric properties, the proportion of minority atoms is preferably 20 at% or less, and more preferably 10 at% or less.
On the other hand, if the proportion of the minority atom is too small, the effects described later cannot be obtained, and the thermoelectric characteristics deteriorate. Accordingly, the proportion of minority atoms is preferably 0.1 at% or more.

The half-Heusler compounds that satisfy the various conditions described above include those having various compositions. Among these, in addition to the various conditions described above, a half-Heusler compound that further satisfies the following conditions exhibits high thermoelectric properties.
(1) The half-Heusler compound is represented by a general formula: A _1-x B _x (0.001 ≦ x ≦ 0.5).
(2) The compound A is a half-Heusler compound having the AgAsMg type crystal structure and having 6 valence electrons per atom.
(3) The compound B is composed of a half-Heusler compound having the AgAsMg type crystal structure and having 6 valence electrons per atom.
(4) Each site of the compound B contains only one or more atoms having the same number of valence electrons.
(5) The compound A includes an element having a valence electron number different from that of the compound B at at least two sites.

The “general formula: A _1-x B _x ” means that the compound A and the compound B form a solid solution. Moreover, the compound A should just have the valence electron number per atom at least 6, and the kind of element which occupies each site is not specifically limited. That is, each site of compound A may contain only one or two or more atoms having the same number of valence electrons, or one or two different valence electrons at one or more sites. It may contain two or more atoms. Furthermore, the upper limit of the content x of the compound B containing a minority atom is preferably 0.2 or less, more preferably 0.1 or less, and the lower limit of the content x of the compound B is 0.00. The point where 001 or more is preferable is as described above.

A first specific example of compound B is a half-Heusler compound represented by the general formula: RNiSb (where R = Sc, Y and / or rare earth elements (La to Lu)).
In this case, as the compound A used in combination with the compound B, specifically,
(Ti _1-xy Zr _x Hf _y ) NiSn (0 ≦ x ≦ 1, 0 ≦ y ≦ 1).

A second specific example of compound B is a half-Heusler compound represented by the general formula: NbR′Sn (where R ′ = Co and / or Rh).
In this case, as the compound A used in combination with the compound B, specifically,
(Ti _1-xy Zr _x Hf _y ) NiSn (0 ≦ x ≦ 1, 0 ≦ y ≦ 1).

A third specific example of compound B is a half-Heusler compound made of ScCuSn.
In this case, as the compound A used in combination with ScCuSn, specifically,
(Ti _1-xy Zr _x Hf _y ) NiSn (0 ≦ x ≦ 1, 0 ≦ y ≦ 1).

A fourth specific example of compound B is a half-Heusler compound made of TiCoSb.
In this case, as compound A used in combination with TiCoSb, specifically,
(Ti _1-xy Zr _x Hf _y ) NiSn (0 ≦ x ≦ 1, 0 ≦ y ≦ 1).

A fifth specific example of compound B is a half-Heusler compound made of TiRhAs.
In this case, as compound A used in combination with TiRhAs, specifically,
(Ti _1-xy Zr _x Hf _y ) NiSn (0 ≦ x ≦ 1, 0 ≦ y ≦ 1).

  Among these, RNiSb is particularly suitable as the compound B because it has relatively high thermoelectric properties when used in combination with the compound A composed of XNiSn (X = Ti, Zr, Hf) or the like. On the other hand, TiRhAs can obtain relatively high thermoelectric characteristics by combining with compound A composed of XNiSn (X = Ti, Zr, Hf) or the like, but is not preferable in that it contains As with a large environmental load.

  In the present invention, the half-Heusler compound constituting the thermoelectric material contains at least five kinds of elements (X atom, Y atom and Z atom, and a minority atom substituting at least two of these elements). These elements are preferably dispersed uniformly throughout the compound. Here, “uniformly dispersed” means that when X-ray diffraction is performed under the conditions of a scan speed of 2 ° / min or less and a step of 0.02 ° or less, a major diffraction peak (at least, Uniformity to the extent that no separation of the maximum diffraction peak is observed. Further, when surface analysis is performed by EDX, it is preferable that the uniformity of the composition is maintained in a region of 10 μm or less. Each element may be uniformly dispersed at the atomic level, or may be regularly arranged. In particular, a half-Heusler compound in which a minority atom constitutes a cluster exhibits high thermoelectric properties.

[1.4. Impurity amount]
In the present invention, the O concentration ([O] (at%)) and the Si concentration ([Si] (at%)) in the half-Heusler compound satisfy the following formula (a).
2.5 ≦ 3.305-5.10 [O] −0.540 [Si] (a)
The thermoelectric material according to the present embodiment is preferably made of only the half-Heusler compound described above, but may contain impurities inevitably included. However, fewer impurities that adversely affect thermoelectric properties are preferred.
Among impurities, O has a great influence on thermoelectric characteristics. Of the IVb group elements, Si is often unevenly distributed at the grain boundaries without entering the 4a site or the 4b site. Si unevenly distributed at the grain boundary causes a decrease in thermoelectric characteristics. Therefore, it is preferable that the O concentration and the Si concentration in the half-Heusler compound satisfy the formula (a).
In addition, (a) Formula can be calculated | required by multiple regression analysis using the output factor (PF) in measurement temperature 500 degreeC, O concentration (at%) and Si concentration (at%) contained in a sample. . The left side in the equation (a) corresponds to the output factor (mW / K ² m) at a measurement temperature of 500 ° C., and the equation (a) obtains an output factor of 2.5 (mW / K ² m) or more. Therefore, the upper limits of the O concentration and the Si concentration are specified. When the numerical value on the left side of the equation (a) is sequentially increased to 2.6, 2.7, 2.8..., Upper limits of O concentration and Si concentration necessary to obtain the corresponding output factor can be specified.
In order to obtain high thermoelectric characteristics, the O concentration is preferably less than 0.18 at% in addition to satisfying the formula (a). The O concentration is more preferably 0.15 at% or less, and still more preferably 0.10 at% or less. .
Further, the Si concentration is preferably 1.30 at% or less in addition to satisfying the formula (a). The Si concentration is more preferably 0.30 at% or less, further preferably 0.20 at% or less, and further preferably 0.10 at% or less.
Furthermore, the thermoelectric material according to the present embodiment may be a composite of the above-described half-Heusler compound and another material (for example, resin, rubber, etc.).

[2. Thermoelectric material (2)]
Next, a thermoelectric material according to the second embodiment of the present invention will be described. In the thermoelectric material according to the present embodiment, a dopant is further added to the above-described half-Heusler compound (first half-Heusler compound) having 6 valence electrons per atom, and the number of valence electrons per atom is It includes a half-Heusler compound (second half-Heusler compound) that is 5.9 or more and 6.1 or less (excluding those having 6 valence electrons per atom).

  Doping is performed by replacing a part of constituent elements of at least one site with another element having a different valence electron number. Doping may replace only the constituent elements at one site, or may replace constituent elements at two or more sites simultaneously. In addition, the doping may be one in which a part of the constituent element is replaced with two or more elements having the same or different valence electrons at one or two or more sites.

In general, the first half-Heusler compound is often an n-type thermoelectric material because electrons are the dominant carrier even if the number of valence electrons per atom is six. When a part of the constituent elements of the first half-Heusler compound is replaced with an element having a higher valence electron number (hereinafter referred to as “n-type dopant”), the valence electron number per atom is 6 A larger second half-Heusler compound is obtained. When the number of valence electrons per atom exceeds 6, an electron is doped and an n-type thermoelectric material having a higher electrical conductivity is obtained.
On the other hand, when a part of the constituent elements of the first half-Heusler compound is replaced with an element having a smaller valence number (hereinafter referred to as “p-type dopant”), the number of valence electrons per atom is more than 6. A small second half-Heusler compound is obtained. When the number of valence electrons per atom is less than 6, holes are doped. Further, when the amount of the p-type dopant exceeds a certain amount, the Seebeck coefficient S turns positive and becomes a p-type thermoelectric material.

  Furthermore, doping may be performed by simultaneously adding an n-type dopant and a p-type dopant. However, the increase in carriers mainly depends on the difference between the contribution from the increase in the number of valence electrons by the n-type dopant and the contribution from the increase in the number of valence electrons by the p-type dopant. Therefore, in terms of increasing carriers, simultaneous addition of an n-type dopant and a p-type dopant is not beneficial, and it is preferable to add one of them.

For example, in the case where the 4a site of the first half-Heusler compound is composed of Ti and Y, if a part of Ti is further substituted with a group Va element, electrons can be doped, and a part of Ti is further group IIIa element. And / or substitution with rare earth elements can dope holes.
Further, for example, in the case where the 4b site of the first half-Heusler compound is composed of Sn and Sb, if a part of Sn is further substituted with a Vb group element, electrons can be doped, and a part of Sn is further converted into IIIb. Substitution with a genus element can dope holes.
Further, for example, in the case where the 4c site of the first half-Heusler compound is made of Ni, if a part of Ni is substituted with a group Ib element, electrons can be doped, and a part of Ni is included in the group VIIIa element. If the element is substituted with an element (Fe, Co, etc.) having a lower valence electron number than Ni, holes can be doped.
Further, these may be used in combination.

The doping to the first half-Heusler compound is performed so that the number of valence electrons per atom after doping (that is, the second half-Heusler compound) is 5.9 or more and 6.1 or less. In both cases where the number of valence electrons per atom is less than 5.9 and exceeds 6.1, the second half-Heusler compound becomes metallic, and high thermoelectric properties cannot be obtained.
In general, the Seebeck coefficient S, the electrical conductivity σ, and the thermal conductivity κ that govern the thermoelectric characteristics are all functions of the carrier concentration. Therefore, in order to obtain high thermoelectric characteristics, it is preferable to select an optimal value for the number of valence electrons per atom according to the composition of the second half-Heusler compound.
The second half-Heusler compound may contain unavoidable impurities, it is preferable to have less impurities that adversely affect thermoelectric properties, and it may be used in combination with other materials. Is the same as in the first embodiment.

[3. Action of thermoelectric material]
Next, the operation of the thermoelectric material according to the present invention will be described. Half-Heusler compounds are known to be semiconductors when the number of valence electrons per atom is equal to 6. For example, in the case of TiNiSn, since (4 + 10 + 4) / 3 = 6, TiNiSn is a semiconductor. When such a semiconductor is doped with a small amount of electrons or holes, its electrical conductivity σ increases. However, since the Seebeck coefficient S generally has a negative correlation with the carrier concentration, the absolute value of the Seebeck coefficient S is reduced by doping electrons or holes. Therefore, in the conventional method of doping a single element to a half-Heusler compound, there is a limit to the figure of merit Z (or dimensionless figure of merit ZT) that can be reached.

  On the other hand, the first half-Heusler exhibiting high thermoelectric characteristics when a part of atoms occupying at least two sites is replaced with atoms having different valence electrons so that the number of valence electrons per atom is maintained at 6. A compound is obtained. Although the details of the reason are unclear, it is thought to be due to the following reasons.

  The first reason is that the thermal conductivity κ is reduced. This is achieved by replacing a part of atoms occupying at least two sites with atoms having different valence electrons (in other words, by compounding two kinds of half-Heusler compounds having different constituent elements). This is thought to be due to suppression of propagation.

  The second reason is that the Seebeck coefficient S increases. It is known that the Seebeck coefficient S is proportional to the density of states of the semiconductor. In the half-Heusler compound, when a part of atoms occupying at least two sites is substituted with atoms having different valence electrons, the electronic structure becomes complicated and the density of states increases. As a result, the Seebeck coefficient S is considered to increase.

The third reason is that the decrease in carrier mobility is small. In the case of conventional single element doping, ion sites generated by doping exert a Coulomb force over a long distance. Therefore, even if the carrier is far away from the ion site, it is affected by the ion site and lowers the mobility of the carrier. Since the electrical conductivity σ is proportional to the carrier mobility, a decrease in the carrier mobility causes a decrease in the electrical conductivity σ.
On the other hand, since the first half-Heusler compound according to the present invention has 6 valence electrons per atom (that is, neutral on average), the decrease in mobility is considered to be small.

The fourth reason is that, in particular, in the case of a so-called nanodot structure in which a relatively small amount of substitution element is formed and locally forms a cluster, the thermal conductivity κ is reduced without reducing the electrical conductivity σ. The effect of making it smaller is great.
If a relatively small amount of substitution elements locally form clusters, it is considered that the propagation of lattice waves is suppressed and the thermal conductivity κ is reduced.
When an atom occupying two or more sites is substituted so that the number of valence electrons per atom is maintained, at least one of the substituted atoms becomes a positive ion site, and the other becomes a negative ion site. When these substitution atoms form a cluster locally, for carriers far from the cluster, the cluster behaves electrically neutral. As a result, it is considered that the decrease in carrier mobility is reduced.
In order to easily form a cluster and maintain local electrical neutrality, it is preferable to substitute atoms at two sites. This is because one site cannot be arranged closest to each other, and the coulomb force due to the distribution of different ions such as +2, −1, and −1 at three sites reduces mobility.

Further, in the case where a dopant is further added to such a first half-Heusler compound, when the addition amount of the dopant is optimized, the electrical conductivity σ increases and the thermoelectric characteristics thereof are improved.
In addition, the thermoelectric material needs to have high electric conductivity and Seebeck coefficient and low thermal conductivity. However, when impurities are mixed in the synthesis process, it becomes a scattering source and the electric conductivity is lowered, so that the thermoelectric characteristics are lowered. In particular, O and Si present at the grain boundaries cause deterioration in thermoelectric properties. Since the thermoelectric material according to the present invention is obtained by rapid solidification using a boron nitride nozzle as described later, the amount of impurities (particularly O and Si) is small. Therefore, there is little decrease in electrical conductivity and a high figure of merit can be obtained. Moreover, since there is little composition shift, the reproducibility of high thermoelectric characteristics is also high.

[4. Method for manufacturing thermoelectric material]
Next, the manufacturing method of the thermoelectric material which concerns on this invention is demonstrated. The manufacturing method of the thermoelectric material according to the present invention includes a melting step and a rapid cooling step.

[4.1 Melting process]
The dissolution step is a step of dissolving the raw materials blended so as to be the first half-Heusler compound or the second half-Heusler compound described above.
The raw material may contain only a single element, or may be an alloy or compound containing two or more elements. Moreover, each raw material is mix | blended so that the target composition may be obtained. The method for dissolving the raw material is not particularly limited, and various methods can be used. In addition, it is preferable to dissolve the raw material in an inert atmosphere in order to prevent oxidation.

[4.2. Rapid cooling process]
The rapid cooling step is a step of rapidly solidifying the molten metal obtained in the melting step. In the present invention, the rapid cooling step is to spray or drop the molten metal onto the cooling medium using a boron nitride nozzle. This point is different from the conventional manufacturing method.
Specifically, as a rapid cooling method,
(1) A method (copper roll method) of spraying or dripping molten metal melted in a boron nitride nozzle onto a rotating copper roll (cooling medium),
(2) The molten metal dissolved in the boron nitride nozzle is sprayed or dripped from the nozzle hole, jet fluid (inert gas, water, air, etc.) is blown from the surroundings to the molten metal flow, and the generated droplet is dropped. Coagulation method (atomization method),
and so on.
The boron nitride nozzle may be used as it is, but it is preferable to use a nozzle that has been heat-treated at 600 ° C. or higher in an inert gas atmosphere (Ar, N _2, etc.) before melting the raw material. Immediately after the manufacture, boron nitride adsorbs gas and moisture, but if this is heat-treated under the prescribed conditions, the adsorbed gas and adsorbed water are removed, so the contamination of impurities (especially O) is minimized. It can be suppressed to the limit.

  The cooling rate during the rapid cooling is preferably 100 ° C./sec or more. When the cooling rate is less than 100 ° C./sec, the component elements are biased and a uniform solid solution may not be obtained. In order to obtain a uniform solid solution, the faster the cooling rate, the better.

[4.3. Sintering process]
When the molten metal is rapidly solidified using a copper roll method or the like, a ribbon-like or powdery half-Heusler compound is obtained. These ribbon-like or powdery half-Heusler compounds may be combined with other materials as they are or after being pulverized (pulverizing step) as necessary. Alternatively, the ribbon-shaped or powder-shaped half-Heusler compound may be pulverized (pulverizing step) and then sintered (sintering step) as necessary.

When a powdered half-Heusler compound is sintered, various methods can be used for the sintering method. Specific examples of the sintering method include atmospheric pressure sintering, hot pressing, HIP, and discharge plasma sintering (SPS). Among these, the discharge plasma sintering method is particularly suitable as a sintering method because a dense sintered body can be obtained in a short time.
Sintering conditions (for example, sintering temperature, sintering time, applied pressure during sintering, atmosphere during sintering, etc.) should be optimized according to the composition of the half-Heusler compound, the sintering method used, etc. select.
For example, when using the discharge plasma sintering method, the sintering temperature is preferably not higher than the melting point of the half-Heusler compound, and the applied pressure is preferably not less than 20 MPa. If the half-Heusler compound is melted during sintering, component elements may be biased during cooling. Further, when the applied pressure is 20 MPa or more, a dense sintered body can be obtained. As the sintering time, an optimal time is selected according to the sintering temperature so that a dense sintered body can be obtained.

[4.4. Annealing process]
Furthermore, after the powder is sintered, an annealing treatment for holding the sintered body at a predetermined temperature may be performed (annealing step). When annealing is performed on the sintered body, deviation of component elements, lattice defects at 4c sites, and the like can be removed.
The annealing temperature is preferably 700 ° C. or higher and the melting point of the half-Heusler compound or lower. If the annealing temperature is less than 700 ° C., sufficient effects cannot be obtained. On the other hand, when the annealing temperature exceeds the melting point of the half-Heusler compound, the component elements may be biased during cooling.
As the annealing time, an optimum time is selected according to the annealing temperature. In general, as the annealing temperature increases, deviations and lattice defects can be removed in a shorter time. Usually, it is several hours to several tens of hours.

[5. Operation of Thermoelectric Material Manufacturing Method]
Since the method for producing a thermoelectric material according to the present invention includes a rapid cooling step, a uniform solid solution can be obtained even when the half-Heusler compound is composed of a large number of elements. Further, when such rapidly solidified compound is pulverized and sintered, a dense sintered body with few defects such as a cast hole can be obtained. Furthermore, when annealing treatment is performed on such a sintered body, deviation, lattice defects, and the like are removed, and a thermoelectric material having high characteristics can be obtained.

Further, when phase separation occurs in the synthesis process of the thermoelectric material, it becomes a scattering source and the electric conductivity is lowered, so that the thermoelectric characteristics are lowered. Phase separation can be improved through a rapid cooling process. However, when a half-Heusler compound having a predetermined composition is produced using a rapid solidification method, when a conventionally used quartz nozzle is used, an easily oxidizable element in the constituent elements reacts with the quartz nozzle. As a result, impurities (especially O and Si) are likely to be mixed, and the figure of merit is lowered. In addition, it is difficult to produce a thermoelectric material with high reproducibility due to compositional deviation.
On the other hand, since boron nitride has low reactivity with many elements, when a boron nitride nozzle is used as a rapid solidification nozzle, reaction between the constituent elements and the nozzle is suppressed. Therefore, phase uniformity can be improved without causing a composition shift.
Furthermore, the nozzle surface is cleaned by preliminarily heat-treating the boron nitride nozzle at 600 ° C. or higher. As a result, mixing of impurities that cause a decrease in electrical conductivity is suppressed, and thermoelectric characteristics can be improved.

(Reference Example 1, Example 2 , Comparative Examples 1-3)
[1. Preparation of sample]
Ti, Zr, and Hf were weighed so that the composition ratio was Ti _0.5 Zr _0.25 Hf _0.25, and set in an arc melting furnace. After evacuating 10 ⁻⁶ Torr (1.33 × 10 ⁻⁴ Pa), Ar gas was introduced and replaced until the chamber pressure became 0.5 atm (5.07 × 10 ⁴ Pa). A Ti _0.5 Zr _0.25 Hf _0.25 alloy was obtained by arc melting. Next, Ni and Sn were weighed so that the overall composition was (Ti _0.5 Zr _0.25 Hf _0.25 ) NiSn, and was put together with the synthesized Ti _0.5 Zr _0.25 Hf _0.25 alloy into a boron nitride crucible and melted at high frequency. .
At this time, when the high-frequency melt was normally performed, the surface of the synthesized ingot was oxidized and turned blue or brown, and the ingot stuck to the crucible and became difficult to peel off. On the other hand, when the boron nitride crucible was heat-treated at 600 ° C. or higher, moisture and gas adsorbed on the surface of the boron nitride crucible were desorbed, and it was confirmed that the weight of the crucible was reduced. Further, it was found that when high-frequency dissolution was performed using the heat-treated crucible, the gas, adsorbed water, and the like were removed, so that oxidation of the synthesized ingot was suppressed and adhesion between the crucible and the ingot could be suppressed.

An ingot (using a crucible after heat treatment; the same applies hereinafter) was pulverized in an Ar atmosphere in a glove box and sintered in a discharge plasma apparatus at 1100 ° C. and 50 MPa (Comparative Example 1).
The ingot was pulverized in an Ar atmosphere in a glove box, and sintered under conditions of 1100 ° C. and 50 MPa in a hot press apparatus (Comparative Example 2).
The ingot was high-frequency melted again in a glass tube, and the molten metal was rapidly solidified on a Cu roll surface rotating at 3000 rpm to obtain a ribbon-like sample. This was pulverized and sintered in a discharge plasma apparatus at 1100 ° C. and 50 MPa (Comparative Example 3).
The ingot was pulverized and melted in a boron nitride nozzle, and the molten metal was rapidly solidified under the same conditions as in Comparative Example 3. The obtained ribbon-shaped sample was pulverized and sintered under the same conditions as in Comparative Example 3 using a discharge plasma apparatus (Reference Example 1) .
Further, a sintered body was produced according to the same procedure as in Example 1 except that the boron nitride nozzle was heat-treated at 600 ° C. or higher before high-frequency melting (Example 2).

[2. Test method]
X-ray diffraction measurement was performed on the obtained sample. In addition, component analysis was performed by ICP and SEM-EDX measurements. Furthermore, a rod-shaped sample was cut out from the obtained sintered body, and thermoelectric characteristics were measured.

[3. result]
[3.1. Generation phase]
When the quenching process was not performed and sintering was performed with a discharge plasma apparatus (Comparative Example 1), the strongest peak of X-ray diffraction of the half-Heusler phase was split into two. As a result of SEM-EDX measurement, it was found that this was separated into a Ti-rich phase and a Ti-poor phase.
Even in the case where the quenching step was not performed, peak separation of X-rays was suppressed when sintering was performed with a hot press apparatus (Comparative Example 2). However, when observed with SEM-EDX, it was found that the phase separation into Ti-rich phase and Ti-poor phase was not suppressed.
There is no separation in the X-ray diffraction peaks of the samples that have undergone the rapid cooling process (Comparative Example 3 , Reference Example 1, Example 2) , and even the SEM-EDX measurement results show phase separation into Ti-rich phase and Ti-poor phase. Was confirmed to be suppressed.

[3.2. component]
Table 2 shows the O concentration and Si concentration of each sample obtained by SEM-EDX measurement.
When the quenching process was performed with a quartz nozzle (Comparative Example 3), as a result of SEM-EDX measurement, it was found that impurities such as Si and O were mixed.
On the other hand, when quenching was performed with a boron nitride nozzle (Reference Example 1, Example 2) , the presence of Si was not confirmed within the EDX detection limit (<100 ppm). When quenching is performed with an untreated boron nitride nozzle (Reference Example 1) , oxygen is mixed into the grain boundary in the form of oxide, but when quenching is performed with a heat treated boron nitride nozzle (Example 2). ), It was confirmed that the oxygen concentration was greatly reduced.

FIG. 2 shows an EDX analysis result of a sample (Comparative Example 3) rapidly cooled using a quartz nozzle.
From FIG.
(1) The inside of the grains (region 16) has a composition almost as prepared.
(2) Part of the grain boundary (region 18) has an extremely high O concentration and a significant compositional deviation, and
(3) The other part of the grain boundary (regions 19 and 20) has a high Si concentration and a composition shift.
I understand.
FIG. 3A shows an SEM photograph of the sample obtained in Comparative Example 3. FIG. 3B shows an SEM photograph of the sample obtained in Example 2. When quartz nozzles are used (FIG. 3A), black spots (regions with high Si concentration) are scattered, whereas when boron nitride nozzles are used (FIG. 3B), black spots are scattered. It can be seen that the spots have disappeared.

The samples obtained in Comparative Example 3, Reference Example 1 and Example 2 were subjected to composition analysis by ICP. Table 3 shows the results. When the quenching process is performed with a quartz tube (Comparative Example 3) and when the quenching process is performed using an untreated boron nitride nozzle (Reference Example 1) , the actual composition is larger than the charged composition. It was confirmed that it was shifted. On the other hand, when the quenching step was performed after the boron nitride nozzle was pre-heated (Example 2), it was confirmed that the composition almost as prepared was maintained.

[3.3. Thermoelectric characteristics]
The electrical conductivity σ and Seebeck coefficient S of the samples obtained in Comparative Examples 1 and 3 and Reference Example 1 and Example 2 were measured, and the output factor (PF) was calculated. FIG. 4 shows the result. From FIG. 4, the effect of rapid cooling (Comparative Example 1 vs. Comparative Example 3), the effect of the nozzle material (Comparative Example 3 vs. Reference Example 1 ), and the effect of pre-heating treatment ( Reference Example 1 vs. Example 2) are clearly shown. became.

FIG. 5 shows the relationship between the O concentration and the output factor (measurement temperature: 500 ° C.). FIG. 6 shows the relationship between the Si concentration and the output factor (measurement temperature: 500 ° C.). 5 and 6 that a higher output factor is obtained as the O concentration is lower and / or as the Si concentration is lower.
Further, from FIGS. 5 and 6, the upper limits of the O concentration and the Si concentration necessary for obtaining the target output factor can be read. Table 4 shows an example.
For example, when the O concentration is 0.001 at% or less, it is understood that the Si concentration should be 1.30 at% or less in order to obtain an output factor of 2.6 mW / K ² m or more. It can also be seen that when the O concentration is 0.001 at% or less, the Si concentration should be 1.11 at% or less in order to obtain an output factor of 2.7 mW / K ² m or more.
Similarly, when the Si concentration is 0 at%, it can be seen that the O concentration should be 0.138 at% or less in order to obtain an output factor of 2.6 mW / K ² m or more. It can also be seen that when the Si concentration is 0 at%, in order to obtain an output factor of 2.7 mW / K ² m or more, the O concentration should be 0.118 at% or less. The same applies to other cases.

  Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the gist of the present invention.

  Thermoelectric materials according to the present invention include solar thermoelectric generators, seawater temperature difference thermoelectric generators, fossil fuel thermoelectric generators, various types of thermoelectric generators such as factory exhaust heat and automobile exhaust heat regenerative generators, photodetection elements, and laser diodes. As a thermoelectric material that constitutes thermoelectric elements used in precision temperature control devices such as field effect transistors, photomultiplier tubes, spectrophotometer cells, chromatography columns, thermostats, air conditioners, refrigerators, power supplies for watches, etc. Can be used.

It is a figure which shows the unit cell of AgAsMg type | mold crystal structure. The EDX analysis result of the sample (comparative example 3) rapidly cooled using the quartz nozzle is shown. FIG. 3A shows an SEM photograph of a sample (comparative example) that was quenched using a quartz nozzle. FIG. 3B shows an SEM photograph of a sample (Example 2) that was rapidly cooled using a boron nitride nozzle. It is a figure which shows the temperature dependence of the output factor of the sample obtained in Comparative Examples 1 and 3 and Reference Example 1 and Example 2 . It is a figure which shows the relationship between O concentration and an output factor (measurement temperature: 500 degreeC). It is a figure which shows the relationship between Si density | concentration and an output factor (measurement temperature: 500 degreeC).

Claims (7)

A thermoelectric material comprising a half-Heusler compound having the following configuration.
(1) The half-Heusler compound has an AgAsMg crystal structure.
(2) The half-Heusler compound has 6 valence electrons per atom.
(3) In the half-Heusler compound, at least two of the three sites of the AgAsMg type crystal structure each include two or more kinds of atoms having different valence electrons.
(4) The O concentration ([O]) and the Si concentration ([Si]) in the half-Heusler compound satisfy the following formula (a).
2.5 ≦ 3.305-5.10 [O] −0.540 [Si] (a)
(5) The O concentration in the half-Heusler compound is 0.10 at% or less, and the Si concentration is 0.10 at% or less.
(6) The thermoelectric material is obtained by rapidly cooling and solidifying the molten metal of the half-Heusler compound using a boron nitride nozzle heated at 600 ° C. or higher in an inert gas atmosphere.   The thermoelectric material according to claim 1, wherein the half-Heusler compound contains Ni and / or Co at a 4c (1/4, 1/4, 1/4) site.   The said half-Heusler compound contains Sn and / or Sb in either 4b (1/2, 1/2, 1/2) site or 4a (0, 0, 0) site. The thermoelectric material described.   The half-Heusler compound contains Ti, Zr and / or Hf at any one of 4a (0, 0, 0) site and 4b (1/2, 1/2, 1/2) site. The thermoelectric material according to any one of 3 to 3. The manufacturing method of the thermoelectric material provided with the following structures.
(1) The manufacturing method of the thermoelectric material is as follows:
A dissolution step of dissolving the raw materials that are blended so that the half-Heusler compound according to any one of 請 Motomeko 1 to 4,
A rapid cooling step for rapidly solidifying the molten metal obtained in the melting step;
With
The rapid cooling step is also to the molten metal is sprayed or dropped into the cooling medium by using a boron nitride-made nozzle.
(2) The boron nitride nozzle is obtained by heating at 600 ° C. or higher in an inert gas atmosphere. The method for producing a thermoelectric material according to claim 5, wherein the quenching step has a cooling rate of 100 ° C./sec or more. Method for producing a thermoelectric material according to claim 5 or 6 example further Bei sintering step of sintering the powder of quenched the half-Heusler compound.

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