JP2009111357A - Thermoelectric material and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a thermoelectric material which is high in thermoelectric characteristics, does not contain a large amount of harmful elements, is inexpensive and excellent in oxidation resistance and stability, and to provide its manufacturing method. <P>SOLUTION: The thermoelectric material contains a half-Heusler compound having the following structure. (1) The half-Heusler compound has a crystal structure of AgAsMg type. (2) The number of valence electrons per atom of the half-Heusler compound is six. (3) The half-Heusler compound contains any two or more of Ni, Pd, and Pt in a 4c site. (4) The half-Heusler compound contains Bi in either of a 4b site or a 4a site. (5) The half-Heusler compound contains one or more elements selected from rare earth elements in either of the 4a site or the 4b site. (6) A shift amount to a logic value of a diffraction angle 2θ of the strongest beam peak of the half-Heusler compound is ±0.1 deg. <P>COPYRIGHT: (C)2009,JPO&INPIT

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 800K, it is expected that a highly efficient exhaust heat recovery system can be obtained by using such a thermoelectric element using a skutterudite compound.

Furthermore, the n-type half-Heusler compound is an n-type thermoelectric material that exhibits relatively high thermoelectric properties in the middle / low temperature range. 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 n-type 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 used in combination with the skutterudite compound.
However, many of the skutterudite compounds exhibiting high thermoelectric properties in the middle / low temperature range have a problem that they contain 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 contrast, many half-Heusler compounds do not contain a large amount of environmentally hazardous elements. Further, the n-type half-Heusler compound has extremely high thermoelectric properties. Therefore, if a p-type half-Heusler compound having high thermoelectric properties is used as the p-type thermoelectric material used in combination with the n-type half-Heusler compound, there is a possibility that a high-performance thermoelectric element having a low environmental load may be obtained. However, there are few reports of p-type half-Heusler compounds that are paired with n-type half-Heusler compounds.

Examples of reported p-type half-Heusler compounds include:
For example, in Non-Patent Document 1,
(1) A constituent element is arc-melted in Ar,
(2) Remelt the ingot at least 5 times for homogenization,
(3) The obtained ingot is pulverized into powder, and the powder is SPS sintered.
(4) Annealing the obtained sample at 1173K for 1 week;
Discloses TiCoSn _x Sb _1-x (x = 0.01, 0.05).
In the same document,
(A) The point that the electrical resistivity at room temperature is increased by doping Sn in the annealed sample,
(B) In the case of the annealed sample of x = 0.05, the Seebeck coefficient S changes from negative to positive at around 400K, and the Seebeck coefficient S reaches the maximum value (+222.7 μV / K) at 843K. ,as well as,
(C) The maximum value of the dimensionless figure of merit ZT of the p-type thermoelectric material (x = 0.05) is 0.03 (988K),
Is described.

Non-Patent Document 2 discloses a FeVSb half-Heusler compound obtained by inductively dissolving constituent elements in an Ar atmosphere and annealing the obtained sample in He-hydrogen at 650 ° C. for 1 week. ing.
In the same document,
(A) Ti-doped FeVSb has a positive Seebeck coefficient S at room temperature, a maximum Seebeck coefficient (+180 μV / K) with an FeV _0.95 Ti _0.05 Sb composition, and
(B) The Zr-doped FeVSb has a point where the Seebeck coefficient S changes from negative to positive as the amount of Zr increases,
Is described.

Non-Patent Document 3 discloses LaPdSb, GdPdSb, ErPdSb, LaPdBi, GdPdBi obtained by arc melting and pulverizing metal pieces in an Ar atmosphere, SPS sintering, and annealing in vacuum. ErPdBi is disclosed.
This document describes that the maximum ZT = 0.28 is obtained at 420 K for LaPdSb.

Further, Patent Document 1 is obtained by arc-melting a weighed raw material, heat-treating the metal mass at 1073 K for 72 hours, pulverizing the metal mass, and pressure-sintering the powder (Ti, Zr, Hf). ) CoSb-based half-Heusler compounds are disclosed.
This document uses 60 (Y _0.5 Er _0.5 ) Ni _0.99 Co _0.01 Sb as p-type thermoelectric materials and 60 {(Ti _0.3 Zr _0.35 Hf _0.35 ) _0.997 Ta _0.003 } CoSb as n-type thermoelectric materials. When the high temperature side is 570 ° C. and the low temperature side is 55 ° C., the voltage is 5.0 V, the current is 3.24 A, and the power is 16.2 W. The point to be described is described.

Non-Patent Document 4 reports the electrical conductivity of RBiPt (R = Y, Nd, Tm, Gd, Tb, Dy, Ho, Er, Yb) single crystals.
Non-Patent Document 5 reports a band structure based on first-principles calculations such as YNiBi and YbNiSb.
Non-Patent Document 6 discloses RPdBi (R = Y, Dy, Ho, Er) obtained by arc melting of constituent elements in an Ar atmosphere and annealing.
This document describes that the Seebeck coefficient S of RPdBi at room temperature is 40 to 80 μV / K.

Furthermore, Non-Patent Document 7 includes
(1) An ErPdX (x = Sb and Bi) ingot is produced using an arc melting method,
(2) After compensating for the loss of Sb and Bi, arc melting again,
(3) redissolving the resulting ingot at least 5 times for homogenization,
(4) A sample is sealed in a quartz tube and annealed at 1173K in a vacuum for 1 week.
(5) A half-Heusler compound obtained by pulverizing a sample and subjecting it to SPS sintering is disclosed.
In the same document,
(A) both ErPdSb and ErPdBi contain an unknown impurity phase, and
(B) The electrical resistivity ρ and Seebeck coefficient S of ErPdSb are larger than ErPdBi, and the output factor exceeds 10 μW / K ² at a temperature higher than room temperature,
Is described.

J. Alloys and comp. 407 (2006) 326-329 Physical Review B 70 (2004) 184207-1-11 138th Annual Meeting of the Japan Institute of Metals (2006) 330 Japanese Laid-Open Patent Publication No. 2005-116746 J.Appl.Phys. 70 (1991) 5800 Physical Review B 59 (1999) 15661-15668 Physical Review B 72 (2005) 094409 J.Appl.Phy., 99 (2006) 103701-1-7

Among n-type half-Heusler compounds, those having high thermoelectric properties are known. However, in order to improve the performance of the thermoelectric element, it is desired to further improve the thermoelectric characteristics of the n-type half-Heusler compound.
On the other hand, conventionally known p-type half-Heusler compounds have a problem that their thermoelectric properties are generally low. Many of the p-type half-Heusler compounds contain harmful Sb. LaPdBi, GdPdBi, ErPdBi, and the like have been reported as p-type half-Heusler compounds that do not contain harmful Sb, but there is a problem that they contain a large amount of expensive Pd. Moreover, the Bi type half Heusler compound obtained by the conventional method contains many different phases, and there is no report example in which the single phase alloy was synthesize | combined. The presence of the heterogeneous phase increases the electrical conductivity of the thermoelectric material, and the high electrical conductivity increases the thermal conductivity, thus causing a problem that the thermoelectric performance is degraded.
On the other hand, RENiBi (RE is a rare earth element) -based compound is inexpensive because it contains Ni as a main component, and does not contain harmful elements. However, this system has problems in oxidation resistance and stability because the binding force between Ni and RE or Bi is weak.
Furthermore, the half-Heusler compound is generally produced using an arc melting method or a high-frequency melting method. However, since Bi has high volatility, when a half-Heusler compound containing Bi is produced by a conventional method, it is difficult to control the composition, which may cause deterioration in reproducibility and characteristics.

The problem to be solved by the present invention is to provide a thermoelectric material mainly composed of a p-type or n-type half-Heusler compound having relatively high thermoelectric properties and a method for producing the same.
Another object of the present invention is to provide a thermoelectric material that does not contain a large amount of harmful elements and that is low in cost, and a method for manufacturing the thermoelectric material.
Another problem to be solved by the present invention is to provide a thermoelectric material excellent in oxidation resistance and stability and a method for producing the thermoelectric material.
Another problem to be solved by the present invention is to provide a method for producing a thermoelectric material in which the composition is easily controlled and the reproducibility and characteristics are hardly deteriorated.
Furthermore, another problem to be solved by the present invention is to provide a thermoelectric material containing a Bi-based half-Heusler compound with a low content of heterogeneous phase and a method for producing the thermoelectric material.

In order to solve the above problems, the first of the thermoelectric materials according to the present invention includes a half-Heusler compound having the following configuration.
(1) The half-Heusler compound has an AgAsMg type crystal structure.
(2) The half-Heusler compound has 6 valence electrons per atom.
(3) The half-Heusler compound contains any two or more of Ni, Pd, and Pt at the 4c (1/4, 1/4, 1/4) site.
(4) The half-Heusler compound contains Bi at either the 4b (1/2, 1/2, 1/2) site or the 4a (0, 0, 0) site.
(5) The half-Heusler compound includes at least one selected from rare earth elements at either the 4a (0, 0, 0) site or the 4b (1/2, 1/2, 1/2) site. Contains elements.
(6) The half-Heusler compound has a shift amount of ± 0.1 deg with respect to the theoretical value of the diffraction angle 2θ of the strongest line peak.
The half-Heusler compound preferably has a heterogeneous phase content of 10% or less.

The second aspect of 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 type crystal structure.
(2) The number of valence electrons per atom of the half-Heusler compound is 5.9 or more and 6.1 or less.
(3) The half-Heusler compound contains any two or more of Ni, Pd, and Pt at the 4c (1/4, 1/4, 1/4) site.
(4) The half-Heusler compound contains Bi at either the 4b (1/2, 1/2, 1/2) site or the 4a (0, 0, 0) site.
(5) The half-Heusler compound includes at least one selected from rare earth elements at either the 4a (0, 0, 0) site or the 4b (1/2, 1/2, 1/2) site. Contains elements.
(6) The half-Heusler compound includes the 4a (0, 0, 0) site, the 4b (1/2, 1/2, 1/2) site, and the 4c (1/4, 1/4, 1). / 4) At least one site of the site further includes one or more kinds of atoms having a different valence electron number from the main constituent element.
(7) The half-Heusler compound has a shift amount of ± 0.1 deg with respect to the theoretical value of the diffraction angle 2θ of the strongest line peak.
The half-Heusler compound preferably has a heterogeneous phase content of 10% or less.

Furthermore, the manufacturing method of the thermoelectric material according to the present invention includes:
A first alloy production process for producing a first alloy composed of two or more elements including Bi and not including a rare earth element among the constituent elements of the half-Heusler compound according to the present invention;
A second melting / casting step of mixing and melting / casting two or more elements including the rare earth element and not containing Bi among the constituent elements of the half-Heusler compound;
A blending step of blending raw materials so as to be the half-Heusler compound using at least the first alloy and the second alloy obtained in the second melting / casting step;
A calcining step of calcining the blend obtained in the blending step;
The main point is that

The first alloy manufacturing process
(1) a first melting / casting step of mixing and melting / casting two or more elements containing Bi and not containing the rare earth element, or
(2) a first encapsulation / heat treatment step of vacuum-sealing and heat-treating two or more elements containing Bi and not containing the rare earth element;
Either may be sufficient.
In the blending step, Bi may be further blended in addition to the first alloy and the second alloy.
Further, the calcining step may be a second enclosing / heat treatment step of vacuum-sealing and heat-treating the compound.

Furthermore, the manufacturing method of the thermoelectric material according to the present invention includes:
(1) A rapid cooling step in which the calcined product obtained in the calcining step is re-dissolved and the molten metal is rapidly solidified, and / or
(2) A sintering process in which the calcined product obtained in the calcining process or the solidified product obtained in the quenching process is pulverized, molded and sintered,
May be further provided.
The quenching step is preferably one in which the calcined material is re-dissolved using a boron nitride nozzle.

In the half-Heusler compound represented by the general formula: RENiBi, if all or part of the Ni site (4c site) is substituted with Pd and / or Pt, oxidation resistance and stability are improved. This is probably because Pd and Pt have a stronger binding force with RE and Bi than Ni, and thus phase separation is suppressed. In particular, when a part of the Ni site is replaced with Pd and / or Pt, not only the thermoelectric characteristics are improved, but also the amount of expensive Pd and Pt used can be reduced.
In addition, when any one or more of the RE site (4a site or 4b site), Ni site, and Bi site (4b site or 4a site) is replaced with a similar element having different ionic radii, thermoelectric characteristics are improved. This is presumably because the band structure is changed by substituting one of the sites with a homologous element having a different ionic radius.
Similarly, when any one or more of the RE site, Ni site, and Bi site is replaced with an element having a different valence, thermoelectric characteristics are improved. This is probably because the carrier concentration is changed by substituting one of the sites with an element having a different valence.
Furthermore, when synthesizing a p-type half-Heusler compound containing highly volatile Bi, two or more kinds of alloys that are easy to control the composition are prepared, mixed at a predetermined ratio, and sample synthesis is performed by solid phase reaction. If done, the composition controllability is improved and the properties can be improved.

In the case of producing the thermoelectric material according to the present invention, when the rapid solidification method is used, a thermoelectric material having a very small proportion of heterogeneous phases can be obtained. Further, when a boron nitride nozzle is used during rapid solidification, the reaction between the molten metal and the nozzle is suppressed. Therefore, compositional deviation and contamination of impurities can be further suppressed, and a decrease in Seebeck coefficient S can be suppressed. Moreover, the rapid solidification method also has the effect of reducing the crystal particle size and reducing the thermal conductivity κ.
Further, when the first alloy containing Bi is manufactured, the vacuum encapsulation method is used when the vacuum encapsulation method is used instead of the melting / casting method or when the compound of the first alloy and the second alloy is calcined. If so, compositional deviation due to Bi volatilization can be further suppressed.

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 type crystal structure.
(2) The half-Heusler compound has 6 valence electrons per atom.
(3) The half-Heusler compound contains any two or more of Ni, Pd, and Pt at the 4c (1/4, 1/4, 1/4) site.
(4) The half-Heusler compound contains Bi at either the 4b (1/2, 1/2, 1/2) site or the 4a (0, 0, 0) site.
(5) The half-Heusler compound includes at least one selected from rare earth elements at either the 4a (0, 0, 0) site or the 4b (1/2, 1/2, 1/2) site. Contains elements.
(6) The half-Heusler compound has a shift amount of ± 0.1 deg with respect to the theoretical value of the diffraction angle 2θ of the strongest line peak.

[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. X atom and Z atom are respectively 4a (0, 0, 0) sites (hereinafter simply referred to as “4a sites”) and 4b (1/2, 1/2, 1/2) sites (hereinafter simply referred to as “ It is located at “4b site”), and the X and Z atoms form a rock salt structure. The 4a site and the 4b site are equivalent. 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 thermoelectric material according to the present embodiment, the half-Heusler compound has 6 valence electrons per atom. 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]
In the thermoelectric material according to the present embodiment, the half-Heusler compound contains any two or more of Ni, Pd, and Pt at the 4c site (Ni site). Further, either 4b site or 4a site (Bi site) contains Bi, and the other (RE site) contains one or more elements selected from rare earth elements. In the present invention, “rare earth element” refers to ₃₉ Y, ₂₁ Sc, and a lanthanoid element ( ₅₇ La to ₇₁ Lu).

Specifically, the half-Heusler compound contained in the thermoelectric material according to the present embodiment preferably has a composition represented by the following formula (1). The half-Heusler compound represented by the formula (1) is a p-type thermoelectric material.
_{RE (Ni 1-xy Pd x} Pt y) Bi ··· (1)
However, RE is any one or more elements selected from rare earth elements.
0 ≦ x <1, 0 ≦ y <1, x + y ≦ 1,
(1-xy) x + xy + y (1-xy)> 0
In the formula (1), x and y represent the amount of Pd and the amount of Pt that replace the Ni site, respectively. When all or part of Ni is replaced with Pd and / or Pt,
(A) The band gap changes and the Seebeck coefficient changes.
(B) The radius of curvature of the band gap tip changes, and the carrier mobility changes.
(C) Phonon scattering increases and thermal conductivity decreases,
There is an effect. Therefore, thermoelectric characteristics can be improved by optimizing x and y.

For example, in the case of a half-Heusler compound in which a part of Ni is substituted with Pd, those having a composition represented by the following formula (2) are preferable.
RE (Ni _1-x Pd _x ) Bi (2)
However, RE is any one or more elements selected from rare earth elements.
0.4 ≦ x <1
When the Pd substitution amount x is 0.4 ≦ x <1, an output factor PF higher than RENiBi or REPdBi is obtained. The Pd substitution amount x is more preferably 0.5 ≦ x ≦ 0.9, and more preferably 0.6 ≦ x ≦ 0.8.

Moreover, when the ion radius of the rare earth element occupying the RE site is changed,
(A) The band gap changes and the Seebeck coefficient changes.
(B) The radius of curvature of the band gap tip changes, and the carrier mobility changes.
(C) Phonon scattering increases and thermal conductivity decreases,
There is an effect. Therefore, the thermoelectric characteristics can be improved by optimizing the rare earth element occupying the RE site.

For example, in the case of a half-Heusler compound in which 50 at% of Ni is substituted with Pd, it has a composition represented by the following formula (3), and the average ionic radius of the rare earth element RE is 0.102 nm or more and 0.108 nm or less Are preferred. “Average ionic radius” refers to the average value of the ionic radii of rare earth elements occupying the RE site. The average ionic radius of the rare earth element RE is more preferably 0.103 nm or more and 0.107 nm or less, and further preferably 0.104 nm or more and 0.106 nm or less. Specific examples of the rare earth element RE satisfying such conditions include Er, Y, Dy, Tb, Gd, Gd _0.5 Y _0.5 , Y _0.5 Dy _0.5 , Y _0.5 Er _0.5 , Y _0.5 Tb _0.5 , Gd _0.5 Er. _0.5 and so on.
RE (Ni _0.5 Pd _0.5 ) Bi (3)
However, RE is any one or more elements selected from rare earth elements.

  The Bi site is the same as the Ni site and the RE site. When the Bi site is replaced with a homologous element having different ionic radii, thermoelectric characteristics can be improved by changing the band structure and increasing phonon scattering.

[1.4. Uniformity]
Since Bi is highly volatile, compositional deviation tends to occur when it is dissolved together with other elements. In addition, since the affinity between the rare earth elements RE and Bi is high, a vigorous reaction occurs at the moment when the two elements are dissolved and mixed, and the constituent elements are scattered, and further composition deviation and non-uniformity are likely to occur. On the other hand, when the method according to the present invention described later is used, composition deviation caused by volatilization or intense reaction of Bi can be suppressed.
The degree of composition deviation can be expressed by a shift amount with respect to the theoretical value of the diffraction angle 2θ of the strongest line peak in the X-ray diffraction pattern. In order to obtain a thermoelectric material excellent in oxidation resistance and stability and having high thermoelectric properties, the half-Heusler compound preferably has a shift amount of ± 0.1 deg with respect to the theoretical value of the diffraction angle 2θ of the strongest peak. . The shift amount is more preferably ± 0.07 deg.

[1.5. impurities]
The thermoelectric material according to the present embodiment is preferably made of only the half-Heusler compound described above, but may contain inevitable impurities (heterophase). However, it is preferable that the number of heterogeneous phases that adversely affect thermoelectric properties is small.
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.).

Here, the “heterophase” refers to a phase different from the half-Heusler compound. Among the different phases, those having a high electric conductivity σ cause an increase in the electric conductivity σ of the entire system. In general, the electrical conductivity σ and the thermal conductivity κ have a positive correlation, and the higher the electrical conductivity σ, the higher the thermal conductivity κ. Therefore, when the content of the different phase having a high electric conductivity σ is increased and the increase in the thermal conductivity κ is larger than the increase in the electric conductivity σ, the figure of merit Z of the entire system is lowered.
In order to suppress the lowering of the figure of merit Z of the whole system, the lower the proportion of different phases, the better. “Existence ratio of heterogeneous phase” refers to a value represented by the following formula (a).
Presence ratio of heterogeneous phase (%) = Σ [I _impurity / (I ₂₂₀ + I _impurity )] × 100 (a)
However, I ₂₂₀ is the strongest peak intensity of the half-Heusler compound, and I _impurity is the strongest peak intensity of the different phase.
When a method described later is used, a thermoelectric material having a heterogeneous phase content of 10% or less, 5% or less, or 2% or less can be obtained.

[2. Thermoelectric material (2)]
The thermoelectric material according to the second embodiment of the present invention includes a half-Heusler compound having the following configuration.
(1) The half-Heusler compound has an AgAsMg type crystal structure.
(2) The number of valence electrons per atom of the half-Heusler compound is 5.9 or more and 6.1 or less.
(3) The half-Heusler compound contains any two or more of Ni, Pd, and Pt at the 4c (1/4, 1/4, 1/4) site.
(4) The half-Heusler compound contains Bi at either the 4b (1/2, 1/2, 1/2) site or the 4a (0, 0, 0) site.
(5) The half-Heusler compound includes at least one selected from rare earth elements at either the 4a (0, 0, 0) site or the 4b (1/2, 1/2, 1/2) site. Contains elements.
(6) The half-Heusler compound includes the 4a (0, 0, 0) site, the 4b (1/2, 1/2, 1/2) site, and the 4c (1/4, 1/4, 1). / 4) At least one site of the site further includes one or more kinds of atoms having a different valence electron number from the main constituent element.
(7) The half-Heusler compound has a shift amount of ± 0.1 deg with respect to the theoretical value of the diffraction angle 2θ of the strongest line peak.

[2.1. Number of valence electrons and constituent elements]
The thermoelectric material according to the present embodiment includes a half-Heusler compound having a valence electron number of 5.9 or more and 6.1 or less. Control of the number of valence electrons is performed by adding a dopant to a half-Heusler compound having 6 valence electrons per atom, and at least one of the 4a site, 4b site, and 4c site. This is performed by substituting a part of Ni, Pd, Pt, Bi) with one or more elements having different valence electrons.

  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, half-Heusler compounds often have n-type thermoelectric materials 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 such a half-Heusler compound is replaced with an element having a larger valence electron number (hereinafter referred to as “n-type dopant”), the half valence electron number per atom is larger than six. A Heusler compound is obtained. When the number of valence electrons per atom exceeds 6, an electron is doped, resulting in an n-type thermoelectric material having higher electrical conductivity.
On the other hand, when a part of the constituent elements of the half-Heusler compound having 6 valence electrons per atom is replaced with an element having a smaller valence electron number (hereinafter referred to as “p-type dopant”), A half-Heusler compound having a valence electron number of less than 6 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. That is, at the same or different sites, an element having a larger valence electron number and a smaller element than the main constituent element may be added simultaneously. Further, even when a part of atoms occupying at least two sites is substituted with atoms having different valence electrons so that the number of valence electrons per atom is maintained at 6, the thermoelectric characteristics are improved. this is,
(1) Since the thermal conductivity κ is reduced by element substitution,
(2) Since the electronic structure becomes complicated and the density of states increases, the Seebeck coefficient S increases, or
(3) Since the number of valence electrons per atom is 6 (on average, neutral), the decrease in carrier mobility is small.
it is conceivable that.
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.

The half-Heusler compound is doped so that the number of valence electrons per atom after doping is 5.9 or more and 6.1 or less. When the number of valence electrons per atom is less than 5.9 and exceeds 6.1, the half-Heusler compound becomes metallic and high thermoelectric characteristics cannot be obtained.
In general, the Seebeck coefficient S, electrical conductivity σ, and thermal conductivity κ that govern thermoelectric properties are all functions of carrier concentration. Therefore, in order to obtain high thermoelectric properties, it is preferable to select an optimal value for the number of valence electrons per atom according to the composition of the half-Heusler compound.

Specifically, the half-Heusler compound contained in the thermoelectric material according to the present embodiment preferably has a composition represented by the following formula (4). The half-Heusler compound represented by the formula (4) is an n-type or p-type thermoelectric material depending on the type and amount of the dopant.
_{(RE 1-u M u)} {(Ni 1-xy Pd x Pt y) 1-v M 'v} (Bi 1-w M "w) ··· (4)
However, RE is any one or more elements selected from rare earth elements.
0 ≦ x <1, 0 ≦ y <1, x + y ≦ 1,
(1-xy) x + xy + y (1-xy)> 0
M is an element having a valence different from that of RE.
M ′ is an element having a valence different from that of Ni, Pd, and Pd.
M ″ is an element having a different valence from Bi.
0 ≦ u ≦ 0.3, 0 ≦ v ≦ 0.3, 0 ≦ w ≦ 0.3, u + v + w> 0
When the addition amounts u, v, and w of the elements M, M ′, and M ″ are 0.3 or less, respectively, an output factor PF that is equal to or higher than that of a half-Heusler compound not containing them is obtained. , W are more preferably 0.001 or more and 0.03 or less, and still more preferably 0.01 or more and 0.03 or less.

Specifically, the element M substituting the RE site is preferably Ti, Zr, Hf, Mg, Ca, Sr, Ba or the like.
Specifically, Cu, Ag, Au, Co, Rh, Ir, and the like are preferable as the element M ′ that replaces the Ni site.
Specifically, Si, Ge, Se, Te, Sn, etc. are preferable as the element M ″ for substituting the Bi site.
The addition amount of the dopant that substitutes each site is selected in accordance with the type of dopant. For example, when the dopant (element M) for substituting the RE site is Zr, the addition amount u is preferably 0.001 ≦ u ≦ 0.03, and more preferably 0.01 ≦ u ≦ 0.03. . For example, when the dopant (element M ′) that replaces the Ni site is Cu, the addition amount v is preferably 0.001 ≦ v ≦ 0.03, and more preferably 0.01 ≦ v ≦ 0. 03.

[2.2. Others]
Other points regarding the crystal structure, constituent elements, uniformity, impurities, and the like are the same as those in the first embodiment, and thus description thereof is omitted.

[3. Method for manufacturing thermoelectric material]
The method for manufacturing a thermoelectric material according to the present invention includes a first alloy manufacturing process, a second melting / casting process, a blending process, a calcination process, a rapid cooling process, and a sintering process.

[3.1 First Alloy Manufacturing Process]
A 1st alloy manufacturing process is a process of manufacturing the 1st alloy which consists of 2 or more types of elements which contain Bi and do not contain rare earth elements among the structural elements of the half-Heusler compound concerning the present invention. Methods for producing the first alloy include a melting / casting method (first melting / casting step), a vacuum sealing method (first sealing / heat treatment step), and the like. Any method may be used in the present invention.

[3.1.1 First melting / casting process]
The first melting / casting step is a step of mixing and melting and casting two or more elements that contain Bi and do not contain a rare earth element among the constituent elements of the half-Heusler compound according to the present invention.
The ingot (first alloy) produced in the first melting / casting process may be any one that does not contain a rare earth element. When synthesizing a half-Heusler compound to which a dopant is added, the first alloy may or may not contain a dopant. In the first melting / casting step, one kind of first alloy having a specific composition may be produced, or two or more kinds of first alloys having different compositions may be produced.
For example, when synthesizing a half-Heusler compound represented by the general formula: RE (Ni _1-x Pd _x ) Bi, the raw materials are blended so as to be (Ni _{1-x ′} Pd _{x ′} ) Bi _2, and melt casting do it. In this case, two or more kinds of alloys having different x ′ may be produced.
The method for dissolving the raw material is not particularly limited, and various methods can be used. Specific examples of the melting method include an arc melting method, a high frequency melting method, and a glass tube enclosing annealing method. In addition, it is preferable to dissolve the raw material in an inert atmosphere in order to prevent oxidation.

[3.1.2 First encapsulation / heat treatment process]
The first encapsulation / heat treatment step is a step of vacuum-sealing and heat-treating two or more elements containing Bi and not containing a rare earth element. When the vacuum encapsulation method is used instead of the melting / casting method as the manufacturing method of the first alloy, the slight composition deviation due to the volatilization of Bi, the coexistence of Bi after synthesis, and the deterioration of the thermoelectric properties due to these are suppressed. be able to.
The material of the sealing tube for vacuum-sealing the raw materials blended at a predetermined ratio is not particularly limited as long as it has poor reactivity with the raw materials. A quartz tube is usually used as the enclosing tube.
The heat treatment temperature may be a temperature at which the raw material in the sealed tube reacts within a realistic processing time and a first alloy having a predetermined composition is formed. The heat treatment temperature is not necessarily a temperature at which the entire raw material becomes a melt. For example, when the first alloy is (Ni _{1-x ′} Pd _{x ′} ) Bi ₂ , the heat treatment temperature is preferably 300 to 700 ° C.
The other points regarding the first alloy are the same as those in the first melting / casting process, and thus the description thereof is omitted.

[3.2. Second melting / casting process]
The second melting / casting step is a step of mixing and melting and casting two or more elements including rare earth elements and not containing Bi among the constituent elements of the half-Heusler compound according to the present invention.
The ingot (second alloy) produced in the second melting / casting process only needs to contain no Bi. When synthesizing a half-Heusler compound to which a dopant is added, the second alloy may or may not contain a dopant. In the second melting / casting step, one kind of second alloy having a specific composition may be produced, or two or more kinds of second alloys having different compositions may be produced.
For example, when synthesizing a half-Heusler compound represented by the general formula: RE (Ni _1-x Pd _x ) Bi, the raw materials are blended and dissolved so as to be RE ₂ (Ni _{1-x "} Pd _x" ). Cast it. In this case, two or more kinds of alloys having different x ″ may be produced. The value of x ″ is determined by the first alloy ((Ni _{1-x ′} Pd _{x ′} ) produced in the first melting and casting process. It is preferred to select such that when mixed with Bi ₂ ), the desired half-Heusler compound RE (Ni _1-x Pd _x ) Bi is obtained.
The method for dissolving the raw material is not particularly limited, and various methods can be used. Specific examples of the melting method include an arc melting method, a high frequency melting method, and a glass tube enclosing annealing method. In addition, it is preferable to dissolve the raw material in an inert atmosphere in order to prevent oxidation.

[3.3. Compounding process]
In the blending step, at least the first alloy obtained in the first alloy manufacturing step and the second alloy obtained in the second melting / casting step are used to blend the raw materials so as to be the target half-Heusler compound. It is a process.
For example, when synthesizing a half-Heusler compound represented by the general formula: RE (Ni _1-x Pd _x ) Bi, (Ni _{1-x ′} Pd _{x ′} ) Bi ₂ is produced in the first alloy manufacturing process, When RE ₂ (Ni _{1-x "} Pd _x" ) is produced in the second melting / casting process, they may be blended at a ratio of 1: 1. The same applies when two or more kinds of first alloys are produced in the first alloy manufacturing process and / or when two or more kinds of second alloys are produced in the second melting / casting process. What is necessary is just to mix | blend these so that a half-Heusler compound may be obtained.
In the blending step, raw materials other than the first alloy and the second alloy may be blended. For example, in the case of synthesizing a half-Heusler compound to which a dopant is added, a first alloy containing the dopant is produced in the first alloy manufacturing process, and / or a second alloy containing the dopant is produced in the second melting / casting process. Although it may be produced, a dopant may be added separately in the blending step regardless of whether the first alloy and the second alloy contain the dopant.

  Furthermore, when the melting / casting method is used in the first alloy manufacturing process, composition deviation due to the volatilization of Bi is likely to occur. In such a case, Bi may be further blended in addition to the first alloy and the second alloy. The Bi compounding amount may be an amount that can supplement at least Bi that has volatilized during the production of the first alloy. Even if the amount of Bi is excessive, excess Bi volatilizes during heating in the post-process, but adding more than necessary is not beneficial. The optimum Bi blending amount is selected according to the degree of Bi volatilization in the first alloy manufacturing process, the conditions of the subsequent process, and the like.

[3.4. Calcination process]
The calcining step is a step of calcining the blend obtained in the blending step. Thereby, a solid phase reaction occurs between the powders, and a half-Heusler compound having a target composition is obtained.
The calcination is performed by molding the blend obtained in the blending step and heating it at a predetermined temperature. As the heating temperature, an optimum temperature is selected according to the composition of the half-Heusler compound. The calcination is preferably performed in an inert atmosphere in order to prevent oxidation.

Further, as the calcination method, a method (second encapsulation / heat treatment step) in which the compound is vacuum-encapsulated and heat-treated may be used. When the vacuum encapsulation method is used as the calcination method, Bi volatilization during calcination can be suppressed.
The material of the sealing tube for vacuum-sealing the compound is not particularly limited as long as it has poor reactivity with the raw material. A quartz tube is usually used as the enclosing tube.

[3.5 Rapid cooling process]
The rapid cooling process is a process in which the calcined product obtained in the calcining process is remelted and the molten metal is rapidly solidified. The calcined product obtained in the calcining step can be used for various purposes as it is after being appropriately pulverized as necessary. Therefore, the rapid cooling process is not necessarily a necessary process, but if the calcined product is further rapidly solidified by solidification, a thermoelectric material with a small proportion of different phases can be obtained.

Rapid solidification is performed by spraying or dripping molten metal onto a cooling medium using a nozzle. In the rapid solidification, a quartz nozzle is generally used as the nozzle, but in the present invention, a boron nitride nozzle is preferably used. Boron nitride nozzles are poorly reactive with molten metal containing rare earth elements, and rapid solidification using them can suppress compositional deviations and contamination of impurities, and deterioration of thermoelectric properties caused by these. .
Specifically, as a rapid solidification method,
(1) A method (copper roll method) in which molten metal melted in a boron nitride nozzle is sprayed or dropped on a rotating copper roll (cooling medium),
(2) A method in which molten metal melted in a boron nitride nozzle is sprayed or dripped from a nozzle hole, a jet fluid is blown from the surroundings to the molten metal flow, and the generated liquid droplets are solidified while being dropped (atomizing method).
and so on.
When the atomizing method is used as the rapid solidification method, it is preferable to use an inert gas (for example, Ar) for the jet fluid in order to prevent oxidation of the molten metal.

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 (eg, Ar, N _2, etc.) in advance before the raw material is dissolved. . Immediately after production, boron nitride adsorbs gas and moisture, but if this is heat-treated under specified conditions, the adsorbed gas and adsorbed water are removed, so that the contamination of impurities (especially O) is minimized. Can be suppressed.
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 segregated and a uniform solid solution may not be obtained. In order to obtain a uniform solid solution, the faster the cooling rate, the better.

[3.6. Sintering process]
The sintering step is a step of pulverizing, molding and sintering the calcined product obtained in the calcining step or the solidified product obtained in the rapid cooling step. The calcined product or the solidified product can be used for various applications as it is after being appropriately pulverized as necessary. Accordingly, the sintering process is not necessarily a necessary process, but when used in a bulk state, the sintering is usually performed.
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. 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.

The existence ratio of the heterogeneous phase contained in the sintered body depends not only on the presence or absence of the vacuum sealing process or the rapid cooling process but also on the sintering temperature. The heterogeneous phase contained in the sintered body is roughly classified into a first heterogeneous phase that disappears by reacting with the matrix and a second heterogeneous phase that is generated when the matrix decomposes. In general, if the sintering temperature is too low, the first heterogeneous phase contained in the powder tends to remain in the sintered body as it is. In order to cause the first heterogeneous phase contained in the powder to react with the matrix and reduce the proportion of the heterogeneous phase, the sintering temperature is preferably 800 ° C. or higher.
In general, the higher the sintering temperature, the more the reaction between the first heterogeneous phase contained in the powder and the matrix is promoted, and the heterogeneous ratio is reduced. However, if the sintering temperature becomes too high, the matrix decomposes and a new second heterogeneous phase is generated. Therefore, the sintering temperature is preferably 950 ° C. or lower.

[3.7 Other steps]
After the powder is sintered, an annealing treatment for holding the sintered body at a predetermined temperature may be performed. When annealing is performed on the sintered body, segregation of component elements, lattice defects at the 4c site, 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.
As the annealing time, an optimum time is selected according to the annealing temperature. In general, as the annealing temperature increases, segregation and lattice defects can be removed in a shorter time. Usually, it is several hours to several tens of hours.

Moreover, after calcining a calcined product, a rapidly solidified product, or a sintered body to form a powder, it may be combined with another material. Furthermore, instead of the calcined product, the sintered body may be melted and rapidly solidified.
Or
(1) Bi flux method in which excessive Bi is mixed into the first alloy and the second alloy, and a single crystal is grown using Bi as a flux, or
(2) Floating zone (FZ) method in which calcined material, rapidly solidified material or sintered body is used as a starting material, and the material is partially melted and single-crystallized.
A single crystal made of a half-Heusler compound may be produced by, for example.

[4. Operation of thermoelectric material and manufacturing method thereof]
Next, the effect | action of the thermoelectric material which concerns on this invention, and its manufacturing method is demonstrated.
The half-Heusler compound represented by the general formula: RENiBi is a p-type thermoelectric material that does not contain expensive elements and harmful elements and can be used in an intermediate temperature range. However, since this system has a low binding force between Ni and RE or Bi, it is likely to be decomposed into RE + NiBi and REBi + Ni when produced using a normal melting / casting method. Since RE and REBi are very easy to oxidize, oxidation resistance and stability are significantly reduced when phase separation occurs. Moreover, since RE and REBi have low thermoelectric characteristics, phase separation also causes deterioration of thermoelectric characteristics.
On the other hand, in the half-Heusler compound represented by the general formula: RENiBi, when all or part of the Ni site (4c site) is substituted with Pd and / or Pt, oxidation resistance and stability, and thermoelectric properties are obtained. improves. This is probably because Pd and Pt have a stronger binding force with RE and Bi than Ni, and thus phase separation is suppressed. In particular, when a part of the Ni site is replaced with Pd and / or Pt, not only the thermoelectric characteristics are improved, but also the amount of expensive Pd and Pt used can be reduced.

Moreover, ErPdBi etc. are reported as a p-type half-Heusler compound which does not contain a harmful element. However, since the conventionally known p-type half-Heusler compound has low thermoelectric properties, it is necessary to improve the thermoelectric properties.
The properties of thermoelectric materials vary depending on the band structure and carrier concentration. In general, when the band gap is small, the carrier mobility is large, but the thermoelectric performance deteriorates at a high temperature. Conversely, if the band gap is too large, it becomes an insulator and is not suitable as a thermoelectric material. The band structure can be controlled to some extent by the ionic radius and valence of the element occupying each site.
In a RENiBi-based half-Heusler compound based on a rare earth element, the band gap is changed when any one or more of the RE site, Ni site, and Bi site are replaced with a homologous element having different ionic radii. In particular, the band gap can be easily controlled by changing the ion radius of the rare earth element occupying the RE site. Therefore, thermoelectric characteristics can be improved by optimizing the ionic radius of the element occupying each site (particularly, the rare earth element occupying the RE site).

  Also, the carrier concentration can be controlled by substituting the elements occupying each site with elements having different valences. Therefore, in the RENiBi half-Heusler compound, if any one or more of the RE site, Ni site, and Bi site are replaced with elements having different valences, thermoelectric characteristics are improved. In addition, by optimizing the type and amount of the substitution element, not only a p-type half-Heusler compound having high thermoelectric properties but also an n-type half-Heusler compound can be obtained.

Furthermore, when all the elements constituting the RENiBi half-Heusler compound are dissolved at the same time, composition deviation is likely to occur due to the volatilization of Bi. In addition, since the affinity between RE and Bi is high, if they are dissolved at the same time, a vigorous reaction occurs at the moment when they are dissolved and mixed, the constituent elements are scattered, and further compositional deviation and non-uniformity occur. .
On the other hand, when synthesizing a p-type half-Heusler compound containing highly volatile Bi, two or more kinds of alloys that are easy to control the composition are prepared, mixed at a predetermined ratio, and sampled by solid-phase reaction When the synthesis is performed, the composition controllability is improved and the characteristics can be improved.

In addition, RENiBi-based half-Heusler compounds easily generate heterogeneous phases in the production process, and there has been no example in which a material with few heterogeneous phases has been obtained. The coexistence of different phases having a high electric conductivity σ increased the electric conductivity σ of the entire system. This high electrical conductivity σ causes an increase in thermal conductivity κ, and an extreme increase in thermal conductivity κ causes a decrease in the figure of merit Z.
On the other hand, when rapid solidification is performed on the calcined product, the thermal conductivity κ can be reduced without excessively reducing the electrical conductivity σ.
this is,
(1) Due to the rapid solidification, the proportion of foreign phases decreases, and
(2) The crystal grain size is reduced by rapid cooling, and the area of grain boundaries that are phonon scattering sources increases.
it is conceivable that.
Further, in the case of performing rapid solidification, if a commonly used quartz nozzle is used, the molten metal reacts with the nozzle, resulting in a composition shift or mixing of impurities. Compositional deviation and contamination with impurities cause a decrease in Seebeck coefficient. In contrast, when rapid solidification is performed using a boron nitride nozzle, reaction with the molten metal is suppressed, and the Seebeck coefficient can be increased.

Further, when the melting and casting method is used in manufacturing the first alloy, it is not possible to suppress the volatilization of Bi during melting and the coexistence of Bi after solidification. This coexisting Bi volatilizes during heating for the solid-phase reaction (calcination), causing a composition shift.
On the other hand, when the vacuum sealing method is used in manufacturing the first alloy, Bi volatilization and Bi phase separation are suppressed. Therefore, the composition shift at the time of calcination is suppressed, and the performance and reproducibility can be improved.
Similarly, when the compound is calcined and the vacuum encapsulation method is used, the volatilization of Bi at the time of calcining and the composition shift due to this can be suppressed.

(Example 1, Comparative Examples 1-4)
[1. Preparation of sample]
[1.1. Production Method 1 (Comparative Example 1)]
RE, Ni, Pd, and Bi were mixed so that the composition was RE (Ni _1-x Pd _x ) Bi, and a vacuum of 10 ⁻⁶ Torr (1.33 × 10 ⁻⁴ Pa) was applied. Ar gas was introduced and replaced until the chamber pressure became 0.5 atm (0.5 × 10 ⁵ Pa) and dissolved by an arc melting method or a high-frequency melting method to produce a target half-Heusler phase.
[1.2. Production Method 2 (Comparative Example 2)]
Ni, Pd, and Bi are weighed so that the composition is (Ni _1-x Pd _x ) Bi ₂ , dissolved by arc melting method or high frequency melting method, and (Ni _1-x Pd _x ) Bi ₂ alloy is obtained. Obtained. RE was added to this and again melted by an arc melting method or a high-frequency melting method to obtain a target half-Heusler phase.

[1.3. Production Method 3 (Comparative Example 3)]
Ni, Pd, and Bi are weighed so that the composition is (Ni _{1-x ′} Pd _{x ′} ) Bi ₂ , dissolved by arc melting method or high frequency melting method, and (Ni _{1-x ′} Pd _{x ′} ) A Bi ₂ alloy was obtained. The volatilization amount of Bi is estimated from the weight loss after synthesis, and the total composition combined with the synthesized (Ni _{1-x ′} Pd _{x ′} ) Bi ₂ is RE (Ni _1-x Pd _x ) Bi. Thus, RE, Ni, and Pd were weighed and dissolved in the same manner to obtain a RE ₂ (Ni _{1-x "} Pd _x" ) alloy.
These synthesized alloys were blended and melted by an arc melting method or a high frequency melting method to obtain a target half-Heusler phase.
[1.4. Production Method 4 (Comparative Example 4)]
RE, Ni, and Pd were weighed so that the composition would be RE (Ni _1-x Pd _x ), and melted by an arc melting method or a high frequency melting method to form an alloy. This alloy was pulverized in an Ar atmosphere in a glove box, and Bi was added to the pulverized alloy and mixed so that the total composition would be RE (Ni _1-x Pd _x ) Bi. This mixture was sintered to obtain the desired half-Heusler phase.

[1.5. Production Method 5 (Example 1)]
Ni, Pd, and Bi are weighed so that the composition is (Ni _{1-x ′} Pd _{x ′} ) Bi ₂ , dissolved by arc melting method or high frequency melting method, and (Ni _{1-x ′} Pd _{x ′} ) A Bi ₂ alloy was obtained. The volatilization amount of Bi is estimated from the weight loss after synthesis, and the total composition combined with the synthesized (Ni _{1-x ′} Pd _{x ′} ) Bi ₂ is RE (Ni _1-x Pd _x ) Bi. Thus, RE, Ni, and Pd were weighed and dissolved in the same manner to obtain a RE ₂ (Ni _{1-x "} Pd _x" ) alloy.
These synthesized alloys were pulverized in a glove box and in an Ar atmosphere, respectively, weighed so that the total composition would be RE (Ni _1-x Pd _x ) Bi, and mixed. The mixed powder was molded at a pressure of 50 MPa in the same glove box. The molded body was calcined at 600 ° C. in an Ar flow to obtain a target half-Heusler phase.

[2. experimental method]
The X-ray diffraction pattern of the obtained sample was measured.

[3. result]
In the production methods 1 to 3 using only the arc melting method or the high frequency melting method, there was a weight loss of about 3 to 5% due to the volatilization of Bi, and a compositional deviation occurred. Further, since the affinity between RE such as Y and Bi is high, a vigorous reaction occurs at the moment when RE and Bi are dissolved and mixed, the constituent elements are scattered, and further compositional deviation and non-uniformity occur.
On the other hand, in the two-step synthesis method (preparation methods 4 and 5) in which a solid-phase reaction is combined by an arc melting method or a high-frequency melting method and heat treatment, it was possible to suppress such intense reaction of RE and Bi. However, in the sample synthesized by the production method 4, Bi having a low melting point is first dissolved and exudes out of the mixed powder molded body and volatilizes as the temperature rises.
On the other hand, in the sample synthesized by the production method 5, the volatilization amount of Bi could be suppressed to 0.5 wt% or less.

FIG. 2 shows an X-ray diffraction pattern of YNiBi produced by the production methods 1 to 5. In the samples synthesized by the production methods 1 to 3, the peak shift was confirmed at the strongest peak due to the composition shift. On the other hand, it was confirmed that the sample synthesized by the production method 5 has no peak shift or splitting and can suppress compositional deviation and nonuniformity.
Table 2 shows the peak shift of the strongest peak (I ₂₂₀ ) of YNiBi synthesized by the production methods 1 to 5. In the samples synthesized by the production methods 1 to 3, a shift of 0.30 ° or more occurred as compared with the literature value. On the other hand, when synthesized by the production method 5, the peak shift amount was ± 0.010 ° or less.

Table 3 shows the peak shift of the strongest peak (I ₂₂₀ ) of RE (Ni _0.5 Pd _0.5 ) Bi synthesized by the production method 5. By synthesizing by the production method 5, the strongest peak of RE (Ni _0.5 Pd _0.5 ) Bi was within a shift within a range of ± 0.07 ° as compared with the calculated value.

(Example 2)
The lattice constant, band structure, formation energy, and cohesive energy of RENiBi, REPdBi, and REPtBi were calculated by first-principles calculation. FIG. 3 shows the relationship between the ionic radius of the rare earth element RE and the lattice constant of REMMi (M = Ni, Pd, Pt). FIG. 4 shows the relationship between the ion radius and the formation energy of REMBi. 5 to 7 show the relationship between the ionic radius of REPtBi, REPdBi, and RENiBi and the band gap at the gamma point, respectively.
From the first principle calculation, it was confirmed that the elements at the Ni site stabilize the system in the order of Ni <Pd <Pt (see FIG. 4). Furthermore, it was confirmed that the band gap increases as the ion radius of the rare earth element RE increases and increases in the order of Pt <Pd <Ni (see FIGS. 5 to 7).

Example 3
[1. Preparation of sample]
Using production method 5 (Example 1), Y (Ni _1-x Pd _x ) Bi (where x = 0 to 1.0) was synthesized. The synthesized half-Heusler was pulverized in an Ar atmosphere in a glove box, and sintered in a discharge plasma apparatus at 950 ° C. and 50 MPa.
[2. experimental method]
[2.1. Oxidation test]
In Y (Ni _1-x Pd _x ) Bi, in order to confirm the effect of improving the oxidation resistance accompanying the substitution of Ni sites with Pd, the sintered body was subjected to a heat treatment at 400 ° C. for 1 hour in an oxidizing atmosphere. went.
After the oxidation test, the presence ratio of the heterogeneous phase was measured by XRD. The presence ratio of the different phase was determined as a ratio of the strongest peak intensity of the different phase to the sum of the strongest peak intensity of the half-Heusler phase and the highest peak intensity of the different phase according to the equation (a).
Furthermore, elemental analysis was performed by SEM-EDX analysis.
[2.2. Thermoelectric characteristics]
A rod-shaped sample was cut out from the sintered body, and the Seebeck coefficient S and electrical conductivity σ were measured. Further, the output factor PF was calculated using the measured Seebeck coefficient S and the electrical conductivity σ.

[3. result]
[3.1. Oxidation test]
Table 4 shows the appearance and the ratio of different phases of the sample after the oxidation test. In YNiBi, the sample surface after the oxidation test was completely black. On the other hand, it was confirmed that the oxidation resistance was improved and the metallic luster was maintained as the Pd substitution amount increased.
Further, when the existence ratio of the heterogeneous phase was determined, it was confirmed that the phase separation was suppressed as the Pd substitution amount increased.
8A and 8B show SEM photographs of YNi _0.5 Pd _0.5 Bi and YPdBi, respectively. By SEM-EDX analysis, Y (Ni _0.5 Pd _0.5 ) Bi was separated into a half-Heusler phase and a different phase, and it was confirmed that the decomposed Ni phase was distributed over the entire surface. Moreover, it was confirmed that the coexisting Y phase and Y-Bi phase are bonded to oxygen, and the phase separation is the cause of oxidation. On the other hand, it was revealed that YPdBi is difficult to oxidize because there are few phase separations.

[3.2. Thermoelectric characteristics]
FIG. 9 shows the temperature dependence of the Seebeck coefficient S of Y (Ni _1-x Pd _x ) Bi. FIG. 10 shows the relationship between the Pd substitution amount x and the output factor PF. As shown in FIG. 9, the temperature coefficient of the Seebeck coefficient turned from negative to positive as the temperature increased regardless of the Pd substitution amount x. It was confirmed that the peak temperature of the Seebeck coefficient S tends to shift to the low temperature side as the Pd substitution amount x increases. Since the change in the sign of the temperature coefficient is considered to occur when the temperature region shifts from the impurity region to the intrinsic region, it is considered that the change in the band gap is reflected. This is consistent with the decreasing tendency of the band gap accompanying Pd substitution, as predicted by the first principle calculation.
However, the value of the Seebeck coefficient itself tended to increase as the Pd substitution amount increased. This is thought to be due to the change in the band structure and the decrease in phase separation. As a result, it was confirmed that the output factor PF was maximized when the Pd substitution amount x was about 70%, as shown in FIG.

Example 4
[1. Preparation of sample]
REPdBi and RE (Ni _0.5 Pd _0.5 ) Bi were synthesized using Production Method 5 (Example 1). The synthesized half-Heusler was pulverized in an Ar atmosphere in a glove box, and sintered in a discharge plasma apparatus at 950 ° C. and 50 MPa.
[2. experimental method]
A rod-shaped sample was cut out from the sintered body, and the Seebeck coefficient S and electrical conductivity σ were measured. Further, the output factor PF was calculated using the measured Seebeck coefficient S and the electrical conductivity σ.

[3. result]
FIG. 11 shows the temperature dependence of the Seebeck coefficient S of RE (Ni _0.5 Pd _0.5 ) Bi. FIG. 12 shows changes in the output factor PF at 200 ° C. for RE (Ni _0.5 Pd _0.5 ) Bi and REPdBi accompanying rare earth substitution. As shown in FIG. 11, the temperature dependence of the Seebeck coefficient S changed as the ionic radius of the rare earth element RE increased. It has been confirmed that the temperature coefficient of the Seebeck coefficient S tends to change from negative to positive as the ion radius of the rare earth element RE increases. This is a result reflecting the increase in the band gap accompanying the increase in the ion radius of the rare earth element RE, which was predicted by the first principle calculation. As the ionic radius of the rare earth element RE increased, the phase separation was promoted.
As shown in FIG. 12, the output factor PF of RE (Ni _0.5 Pd _0.5 ) Bi changes depending on the ion radius of the rare earth element RE, and a good output factor PF is maintained at an ion radius of about 0.105 nm. It was. In addition, even when two or more rare earth elements are combined such as Y _0.5 Er _0.5 so that the average ionic radius is 0.105 nm, it is confirmed that a good output factor PF value is maintained. It was done.

(Example 5)
[1. Preparation of sample]
(Y _1-u Zr _u ) (Ni _0.5 Pd _0.5 ) Bi and Y {(Ni _0.5 Pd _0.5 ) _1-v Cu _v } Bi were synthesized using the production method 5 (Example 1). Zr was added when manufacturing the second alloy containing no Bi. Cu was added when the first alloy containing no RE was produced.
The synthesized half-Heusler was pulverized in an Ar atmosphere in a glove box, and sintered in a discharge plasma apparatus at 950 ° C. and 50 MPa.
[2. experimental method]
A rod-shaped sample was cut out from the sintered body, and the Seebeck coefficient S and electrical conductivity σ were measured. Further, the output factor PF was calculated using the measured Seebeck coefficient S and the electrical conductivity σ.

[3. result]
FIG. 13 shows the temperature dependence of the output factor PF. FIG. 14 shows the relationship between the dopant amount for Y (Ni _0.5 Pd _0.5 ) and the output factor PF at 100 ° C.
Both Y (Ni _0.5 Pd _0.5 ) Bi Y site substituted with Zr and Ni site partially substituted with Cu are output by substituting about 1 to 3 at% of trace elements. It was confirmed that the factor PF was improved. This is because the carrier concentration was changed by substituting the Y site element or Ni site element with an element having a different valence.

(Example 6)
[1. Preparation of sample]
[1.1 Production Method 1A (Samples 1, 2, 12, 13): Production of First Alloy by Melt Casting Method]
Y, Ni, and Pd are weighed so that the composition ratio is Y (Ni _1-x Pd _x ) _0.5 and melted by arc melting method or high frequency melting method, and Y (Ni _1-x Pd _x ) _0.5 alloy is obtained. Obtained. Next, Bi, Ni, and Pd are weighed so that the composition ratio is Bi ₂ (Ni _1-y Pd _y ), dissolved by the arc melting method or the high frequency melting method, and Bi ₂ (Ni _1-y Pd _y). ) An alloy was obtained.
The obtained two types of alloys were each pulverized in a glove box and in an Ar atmosphere, weighed so that the total composition would be Y (Ni _0.5 Pd _0.5 ) Bi, and mixed. The mixed powder was molded and calcined at 600 ° C. to obtain the desired half-Heusler phase (Production Method 1A, Sample 1).
The obtained calcined product was pulverized in an Ar atmosphere in a glove box. The pulverized powder was sintered in a discharge plasma apparatus under conditions of 950 ° C. and 50 MPa (Sample 2).

In addition, a sintered body having a Dy (Ni _0.5 Pd _0.5 ) Bi composition was prepared according to the same procedure as Sample 2 except that the entire amount of Y was replaced with Dy (Sample 12). Similarly, a sintered body having a (Y _0.5 Gd _0.5 ) (Ni _0.5 Pd _0.5 ) Bi composition was prepared in accordance with the same procedure as Sample 2 except that 50 at% of Y was replaced with Gd (Sample 13).

[1.2 Production Method 2A (Sample 3): Production of First Alloy by Vacuum Encapsulation Method]
Bi, Ni and Pd were weighed so that the composition ratio would be Bi (Ni _1-y Pd _y ) _0.5, and this was put into a quartz tube with one end closed. After vacuuming to 4 × 10 ⁻³ Pa with a vacuum pump, the other mouth of the quartz tube was heated with a burner and sealed in a vacuum. This vacuum sealed sample was heated at 850 ° C. for 2 hours to obtain a Bi (Ni _1-y Pd _y ) _0.5 alloy.
This and Y (Ni _1-x Pd _x ) _0.5 alloy produced by the production method 1A are pulverized in a glove box and in an Ar atmosphere, respectively, and weighed so that the total composition becomes Y (Ni _0.5 Pd _0.5 ) Bi. These were mixed. The mixed powder was molded and calcined at 600 ° C. in an Ar atmosphere to obtain a target half-Heusler phase (Production Method 2A).
The obtained calcined product was pulverized in an Ar atmosphere in a glove box. The pulverized powder was sintered in a discharge plasma apparatus under the conditions of 950 ° C. and 50 MPa (Sample 3).

[1.3 Production method 3A: Calcination by vacuum encapsulation method]
[1.3.1 Vacuum sealing (samples 4 and 5)]
In fabrication method 2A, a compact obtained by weighing, mixing and molding the first alloy and the second alloy so that the total composition is Y (Ni _0.5 Pd _0.5 ) Bi is vacuum-sealed in a quartz tube, and 600 ° C. or 1000 ° C. (Preparation method 3A).
The obtained calcined material was pulverized in an Ar atmosphere in a glove box. The pulverized powder was sintered in a discharge plasma apparatus under the conditions of 950 ° C. and 50 MPa (sample 4 (calcination temperature: 600 ° C.), sample 5 (calcination temperature: 1000 ° C.)).

[1.3.2 Rapid solidification (samples 6 to 10)]
The calcined material obtained by the production method 3A was again melted by high frequency in a quartz nozzle, and the molten metal was rapidly cooled on a Cu roll surface rotating at 3000 rpm, thereby obtaining a ribbon-shaped sample (sample 6).
The obtained ribbon-shaped sample was pulverized and sintered in a discharge plasma apparatus under conditions of 800 ° C. (sample 7), 900 ° C. (sample 8), 950 ° C. (sample 9), and 50 MPa.
Furthermore, the calcined product obtained by the production method 3A was pulverized and molded in a glove box and in an Ar atmosphere. This molded body was melted in a boron nitride nozzle, and the molten metal was rapidly cooled on a Cu roll surface rotating at 3000 rpm to obtain a ribbon-shaped sample. The obtained ribbon-like sample was pulverized and sintered under the conditions of 900 ° C. and 50 MPa using a discharge plasma apparatus (Sample 10).

[1.3.3 Bi Dope (Sample 11)]
In fabrication method 2A, the first alloy and the second alloy are weighed, mixed and molded so that the total composition is Y (Ni _0.5 Pd _0.5 ) Bi, and Bi corresponding to 1 wt% of the molded body is converted into a quartz tube. The mixture was vacuum sealed and calcined at 600 ° C.
The obtained calcined product was pulverized in an Ar atmosphere in a glove box. The pulverized powder was sintered in a discharge plasma apparatus at 950 ° C. and 50 MPa (Sample 11).

Table 5 shows the manufacturing conditions of each sample.

[2. Test method]
[2.1 X-ray diffraction]
The X-ray diffraction pattern of the obtained sample was measured. In addition, the presence ratio of the different phase was calculated from the strongest peak intensity of the half-Heusler phase and the different phase.
[2.2 Thermoelectric properties]
The Seebeck coefficient S, electrical conductivity σ, and thermal conductivity κ of the sintered body were measured. Further, the output factor PF was calculated using the measured Seebeck coefficient S and the electrical conductivity σ.

[3. result]
[Composition of 3.1 Bi (Ni _1-y Pd _y ) _0.5 alloy (first alloy)]
XRD measurement was performed on the Bi (Ni _1-y Pd _y ) _0.5 alloy produced by the production methods 1A to 3A. In the Bi (Ni _1-y Pd _y ) _0.5 alloy synthesized by the production method 1A (melting casting method), about 20% of the Bi phase coexists in addition to the target phase, which causes the compositional deviation of the half-Heusler phase. It was.
On the other hand, the Bi (Ni _1-y Pd _y ) _0.5 alloy synthesized by the manufacturing methods 2A and 3A (vacuum sealing method) was almost single phase. As a result of ICP analysis, it was found that the same composition as the charged composition was maintained.

[3.2 Different phases]
XRD measurement was performed on Y (Ni _0.5 Pd _0.5 ) Bi sintered bodies produced by the production methods 1A to 3A. FIG. 15 shows an X-ray diffraction pattern of each sample. In these samples, two types of yttrium bismuth oxide phases (Y-Bi-O phases) and Ni phases having different constituent element ratios were confirmed in addition to the target half-Heusler phase.

From the XRD peak intensity ratio, the ratio of these different phases to the half-Heusler phase was determined. Table 6 shows the results.
In the state before sintering, in Sample 1 that was not subjected to rapid solidification, a total of 20% or more of these different phases were detected. On the other hand, it was found that in the sample 6 subjected to rapid solidification, the existence ratio of the heterogeneous phase was reduced to about 10%.
In sample 2 sintered at 950 ° C. without rapid solidification, the total proportion of the different phases is about 11%. This is because the Ni phase and the Y—Bi—O (1) phase decreased during the sintering process. At this time, the proportion of the Y—Bi—O (2) phase is increasing. This means that between room temperature and 950 ° C., the disappearance temperature of the Ni phase and the Y—Bi—O (1) phase and the generation temperature of the Y—Bi—O (2) phase exist. .

In the sample 7 sintered at 800 ° C. after the rapid solidification, the existence ratio of the heterogeneous phase is 10%, which is the same as that before the sintering (sample 6), and the abundance ratio is hardly changed. This means that there is no heterogeneous decomposition / generation temperature between room temperature and 800 ° C.
When the sintering temperature is increased to 900 ° C. (sample 8), the Ni phase and the Y—Bi—O (1) phase disappear, and between 800 and 900 ° C., these react with the matrix phase and disappear. It can be seen that there is a temperature to perform. At the same time, since the existence ratio of the Y-Bi-O (2) phase is hardly increased, the temperature at which the matrix phase decomposes and the Y-Bi-O (2) phase is generated is higher than this. I understand.
In the sample 9 sintered at 950 ° C., the heterogeneous ratio is increased again. This means that the decomposition temperature of the matrix phase (the formation temperature of the Y—Bi—O (2) phase) is between 900 and 950 ° C.
Therefore, it can be seen that the sintering temperature of this system is preferably 800 ° C. or higher and preferably 950 ° C. or lower. In Sample 8, which was rapidly solidified and sintered at around the optimum temperature, the presence ratio of the heterogeneous phase decreased to 4.7%.

  Furthermore, in the sample 10 sintered after rapid solidification with a boron nitride nozzle, the presence ratio of the different phases is further reduced compared to the samples 7 to 9 subjected to rapid solidification and sintering using the quartz nozzle, and almost single phase. was gotten. This is considered to be because when a boron nitride nozzle is used during rapid solidification, the reaction between the molten metal and the nozzle is suppressed, and it is difficult for impurities to be mixed and compositional deviation to be the core of heterogeneous phase formation.

  FIG. 16 shows SEM photographs of Sample 4 and Sample 8. In the case of Sample 4 that does not undergo rapid solidification, the presence of a heterogeneous phase is significant and the crystal grains are large. On the other hand, in the case of the sample 8 subjected to rapid solidification, it can be seen that the heterogeneous phase is greatly reduced and the crystal grains are also refined.

[3.3 Thermoelectric characteristics]
17, 18 and 19 show the electrical conductivity σ, Seebeck coefficient S, and output factor PF of Samples 2 to 5 and 11, respectively. Compared with the sample 2 produced by the production method 1A for the first alloy, in the samples 3, 4 and 5 produced by the production method 2A or 3A, both the Seebeck coefficient S and the electrical conductivity σ are increased. As a result, the output factor PF is Increased. Production methods 2A and 3A differ in the temperature or atmosphere at which Y (Ni _0.5 Pd _0.5 ) Bi is calcined. Samples 4 and 5 calcined by the vacuum encapsulation method have higher electrical conductivity than sample 3 calcined in an Ar pressurized atmosphere. When using the vacuum encapsulation method, the sample 5 calcined at 1000 ° C. has higher electrical conductivity than the sample 4 calcined at 600 ° C. This is a defect due to the volatilization of Bi at the time of calcination, and it is presumed that the carrier concentration has increased.
Furthermore, in the case of the sample 11 doped with 1 wt% Bi in order to compensate for the volatilization of Bi during calcination, the Seebeck coefficient S hardly changed, but the electrical conductivity σ further increased, so that the output factor PF was Increased.

20, FIG. 21 and FIG. 22 show the electrical conductivity σ, Seebeck coefficient S, and output factor PF of Samples 2, 4, 8, and 10, respectively. The electrical conductivities σ of the samples 8 and 10 subjected to rapid solidification were significantly reduced as compared with the samples 2 and 4 which were not subjected to rapid solidification. Further, in the sample 8 that was rapidly solidified using a quartz nozzle, the Seebeck coefficient S did not increase, so that the output factor PF was significantly reduced as compared with the samples 2 and 4.
On the other hand, the sample 10 subjected to rapid solidification using a boron nitride nozzle has a reduced electrical conductivity σ but an increased Seebeck coefficient S, and therefore its output factor PF is higher than that of the sample 4. Only about 10% decrease. This value is about 5% lower than that of Sample 2 which is not vacuum-sealed when the first alloy is manufactured.
However, the thermal conductivity κ of the sample 2 is about 5 W / Km at room temperature because the electrical conductivity σ is large, whereas the electrical conductivity of the sample 10 is lowered due to the rapid solidification effect, and the crystal grains are also reduced. As a result of miniaturization, the thermal conductivity κ of the sample 10 is about 30% lower than that of the sample 2 (see FIG. 23). Therefore, the dimensionless figure of merit ZT of sample 10 increased by about 25% compared to sample 2.

  Regarding the thermal conductivity κ, even when the rapid solidification is not performed, when the rare earth site is replaced with an element (for example, Dy) that has an effect of reducing the electrical conductivity (sample 12), Y is partially substituted with another element. It can also be reduced when replaced (sample 13) (see FIG. 23). Therefore, further improvement in characteristics can be achieved by combining such element substitution and rapid solidification treatment.

  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.

  The thermoelectric material and the manufacturing method thereof according to the present invention include a solar thermoelectric generator, a seawater temperature difference thermoelectric generator, a fossil fuel thermoelectric generator, various thermoelectric generators such as factory exhaust heat and automobile exhaust heat regenerative generator, and light detection. Constructs thermoelectric elements used for precision temperature control devices such as elements, laser diodes, field effect transistors, photomultiplier tubes, spectrophotometer cells, chromatographic columns, thermostats, air conditioners, refrigerators, clock power supplies, etc. It can be used as a thermoelectric material and a manufacturing method thereof.

It is a figure which shows the unit cell of AgAsMg type | mold crystal structure. It is an X-ray diffraction pattern of YNiBi produced by Production Methods 1-3. It is a figure which shows the relationship between the ionic radius of rare earth elements RE calculated | required by the first principle calculation, and the lattice constant of REMBi (RE = rare earth elements, M = Ni, Pd, Pt). It is a figure which shows the relationship between the ion energy of the rare earth element RE calculated | required by the first principle calculation, and the formation energy of REMBi (RE = rare earth element, M = Ni, Pd, Pt). It is a figure which shows the relationship between the ion gap of rare earth elements RE and the band gap in the gamma point of REPtBi calculated | required by the first principle calculation. It is a figure which shows the relationship between the ionic radius of rare earth elements RE calculated | required by the first principle calculation, and the band gap in the gamma point of REPdBi. It is a figure which shows the relationship between the ion gap of rare earth elements RE calculated | required by the first principle calculation, and the band gap in the gamma point of RENiBi. FIG. 8A and FIG. 8B are SEM photographs of the YNi _0.5 Pd _0.5 Bi sintered body and the YPdBi sintered body, respectively, produced using the production method 5. It is a graph showing the temperature dependence of the _{Y (Ni 1-x Pd x} ) Seebeck coefficient of Bi S. It is a graph showing changes in Y associated with Pd substitution (Ni _1-x Pd x) the power factor of Bi PF. RE (Ni _0.5 Pd _0.5) is a graph showing the temperature dependence of the Seebeck coefficient S of Bi. Is a diagram showing changes in power factor PF at 200 ° C. of RE (Ni _0.5 Pd _0.5) Bi and REPdBi with rare earth element substitution. Y a (Ni _0.5 Pd _0.5) Bi to Zr or Cu is a diagram showing changes in power factor PF of adding trace amounts. Is a diagram showing the relationship between Y (Ni _0.5 Pd _0.5) power factor at the dopant amount and 100 ° C. for Bi PF. It is an X-ray diffraction pattern of Samples 1, 2, 6, 7, 8, and 10. 16A is an SEM photograph of sample 4, and FIG. 16B is an SEM photograph of sample 8. It is a figure which shows the temperature dependence of the electrical conductivity (sigma) of samples 2-5, and 11. It is a figure which shows the temperature dependence of Seebeck coefficient S of samples 2-5, 11. It is a figure which shows the temperature dependence of the output factor PF of the samples 2-5, and 11. It is a figure which shows the temperature dependence of the electrical conductivity (sigma) of samples 2, 4, 8, and 10. FIG. It is a figure which shows the temperature dependence of Seebeck coefficient S of the samples 2, 4, 8, and 10. It is a figure which shows the temperature dependence of the output factor PF of the samples 2, 4, 8, and 10. It is a figure which shows the temperature dependence of the thermal conductivity (kappa) of sample 2, 8, 10, 12, 13.

Claims (21)

A thermoelectric material comprising a half-Heusler compound having the following configuration.
(1) The half-Heusler compound has an AgAsMg type crystal structure.
(2) The half-Heusler compound has 6 valence electrons per atom.
(3) The half-Heusler compound contains any two or more of Ni, Pd, and Pt at the 4c (1/4, 1/4, 1/4) site.
(4) The half-Heusler compound contains Bi at either the 4b (1/2, 1/2, 1/2) site or the 4a (0, 0, 0) site.
(5) The half-Heusler compound includes at least one selected from rare earth elements at either the 4a (0, 0, 0) site or the 4b (1/2, 1/2, 1/2) site. Contains elements.
(6) The half-Heusler compound has a shift amount of ± 0.1 deg with respect to the theoretical value of the diffraction angle 2θ of the strongest line peak. The thermoelectric material according to claim 1, wherein the half-Heusler compound has a composition represented by the following formula (1).
_{RE (Ni 1-xy Pd x} Pt y) Bi ··· (1)
However, RE is any one or more elements selected from rare earth elements.
0 ≦ x <1, 0 ≦ y <1, x + y ≦ 1,
(1-xy) x + xy + y (1-xy)> 0 2. The thermoelectric material according to claim 1, wherein the half-Heusler compound has a composition represented by the following formula (2).
RE (Ni _1-x Pd _x ) Bi (2)
However, RE is any one or more elements selected from rare earth elements.
0.4 ≦ x <1 The half-Heusler compound has a composition represented by the following formula (3):
An average ionic radius of the rare earth element RE occupying either the 4a (0, 0, 0) site or the 4b (1/2, 1/2, 1/2) site is 0.102 nm or more and 0.108 nm or less. 2. The thermoelectric material according to claim 1, wherein
RE (Ni _0.5 Pd _0.5 ) Bi (3)
However, RE is any one or more elements selected from rare earth elements. A thermoelectric material comprising a half-Heusler compound having the following configuration.
(1) The half-Heusler compound has an AgAsMg type crystal structure.
(2) The number of valence electrons per atom of the half-Heusler compound is 5.9 or more and 6.1 or less.
(3) The half-Heusler compound contains any two or more of Ni, Pd, and Pt at the 4c (1/4, 1/4, 1/4) site.
(4) The half-Heusler compound contains Bi at either the 4b (1/2, 1/2, 1/2) site or the 4a (0, 0, 0) site.
(5) The half-Heusler compound includes at least one selected from rare earth elements at either the 4a (0, 0, 0) site or the 4b (1/2, 1/2, 1/2) site. Contains elements.
(6) The half-Heusler compound includes the 4a (0, 0, 0) site, the 4b (1/2, 1/2, 1/2) site, and the 4c (1/4, 1/4, 1). / 4) At least one site of the site further includes one or more kinds of atoms having a different valence electron number from the main constituent element.
(7) The half-Heusler compound has a shift amount of ± 0.1 deg with respect to the theoretical value of the diffraction angle 2θ of the strongest line peak. The thermoelectric material according to claim 5, wherein the half-Heusler compound has a composition represented by the following formula (4).
_{(RE 1-u M u)} {(Ni 1-xy Pd x Pt y) 1-v M 'v} (Bi 1-w M "w) ··· (4)
However, RE is any one or more elements selected from rare earth elements.
0 ≦ x <1, 0 ≦ y <1, x + y ≦ 1,
(1-xy) x + xy + y (1-xy)> 0
M is an element having a valence different from that of RE.
M ′ is an element having a valence different from that of Ni, Pd, and Pt.
M ″ is an element having a different valence from Bi.
0 ≦ u ≦ 0.3, 0 ≦ v ≦ 0.3, 0 ≦ w ≦ 0.3, u + v + w> 0   The thermoelectric material according to claim 6, wherein M is Zr.   The thermoelectric material according to claim 7, wherein 0.001 ≦ u ≦ 0.03.   The thermoelectric material according to any one of claims 6 to 8, wherein the M 'is Cu.   The thermoelectric material according to claim 9, wherein 0.001 ≦ v ≦ 0.03.   The thermoelectric material according to any one of claims 1 to 10, wherein an existing ratio of a phase (heterophase) different from the half-Heusler compound is 10% or less. The 1st alloy manufacture which manufactures the 1st alloy which consists of 2 or more types of elements which contain Bi among the constituent elements of the half-Heusler compound as described in any one of Claim 1-11 and do not contain a rare earth element Process,
A second melting / casting step of mixing, melting and casting two or more elements including the rare earth element and not containing Bi among the constituent elements of the half-Heusler compound;
A blending step of blending raw materials so as to be the half-Heusler compound using at least the first alloy and the second alloy obtained in the second melting / casting step;
A calcining step of calcining the blend obtained in the blending step;
A method for producing a thermoelectric material comprising:   13. The thermoelectric material according to claim 12, wherein the first alloy manufacturing process is a first melting / casting process in which two or more elements including Bi and not including the rare earth element are mixed and melted / cast. Production method.   The thermoelectric material manufacturing method according to claim 12, wherein the first alloy manufacturing process is a first enclosing / heat-treating process in which two or more elements including Bi and not including the rare earth element are vacuum-encapsulated and heat-treated. Method.   The method of manufacturing a thermoelectric material according to any one of claims 12 to 14, wherein the blending step further blends Bi in addition to the first alloy and the second alloy.   The method for producing a thermoelectric material according to any one of claims 12 to 15, wherein the calcining step is a second enclosing / heat treatment step of vacuum-sealing and heat-treating the compound.   The method for producing a thermoelectric material according to any one of claims 12 to 16, further comprising a quenching step in which the calcined product obtained in the calcining step is re-dissolved and the molten metal is rapidly solidified.   The method of manufacturing a thermoelectric material according to claim 17, wherein the quenching step is to re-dissolve the calcined product using a boron nitride nozzle.   The method for producing a thermoelectric material according to claim 17 or 18, wherein the quenching step has a cooling rate of the molten metal of 100 ° C / sec or more.   The thermoelectric device according to any one of claims 12 to 19, further comprising a sintering step of pulverizing, forming and sintering the calcined product obtained in the calcining step or the solidified product obtained in the quenching step. Material manufacturing method.   The method for producing a thermoelectric material according to claim 20, wherein the sintering step has a sintering temperature of 800 ° C or higher and 950 ° C or lower.

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