JP5528873B2 - Composite thermoelectric material and method for producing the same - Google Patents

Description

  The present invention relates to a composite thermoelectric material and a method for producing the same, and more particularly to a composite thermoelectric material having a half-Heusler compound as a parent phase and a method for producing the same.

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 a material capable of mutually converting thermal energy and electrical energy, ie, thermoelectric material, generally, a 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. Further, the output factor PF (= S ² σ) may be used as an index for evaluating the characteristics of the thermoelectric material.
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) Skutterudite compounds such as Zn-Sb, Co-Sb, and Fe-Sb,
(3) Half-Heusler compounds such as TiNiSn,
Etc. 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. In addition, Ge—Si-based compound semiconductors use a large amount of expensive Ge.

  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. 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.

On the other hand, the MNiSn-based (M = Ti, Zr, Hf) half-Heusler compound is characterized by exhibiting high thermoelectric properties in the middle / low temperature range and free of environmentally hazardous elements. 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. However, the MNiSn compound has a problem in that there is a limit in the figure of merit that can be reached due to its high thermal conductivity, despite having an essentially high output factor.

Various proposals have been made for methods for reducing the thermal conductivity of thermoelectric materials.
For example, Patent Document 1 discloses that
(1) Disperse alumina particles in an ethanol solution of cobalt chloride and antimony chloride,
(2) A reducing agent is added to the dispersion to precipitate Co particles and Sb particles,
(3) Hydrothermally treating the dispersion to form composite particles composed of aggregates of alumina particles and CoSb ₃ particles;
(4) A method for producing a thermoelectric material in which composite particles are subjected to spark plasma sintering (SPS) is disclosed.
This document describes that a thermoelectric material in which alumina particles having an average particle diameter of 1 to 100 nm are uniformly dispersed in a CoSb ₃ continuous phase is obtained by such a method.

In addition, in Patent Document 2,
(1) Weighing a predetermined amount of Fe, Al, V, and Si, and mechanically alloying this powder to produce a Fe ₂ VAl _0.9 Si _0.1 alloy powder;
(2) Bi is added to the alloy powder and further mixed,
(3) A method of manufacturing a thermoelectric material in which mixed powder is subjected to discharge plasma sintering is disclosed.
This document describes that a thermoelectric material in which a Bi layer is formed so as to surround Fe ₂ VAl _0.9 Si _0.1 alloy particles can be obtained by such a method.

Patent Document 3 discloses a method in which fine particles are added to a molten metal when a powder of a thermoelectric material is manufactured using an atomizing method.
This document describes that a foil in which fine particles are taken into a base material can be obtained by such a method.

Non-Patent Document 1 includes
(1) 2-6 vol% of ZrO ₂ nanoparticles are added to the ZrNiSn powder and mixed, followed by annealing for 3 days,
(2) A method for producing a thermoelectric material in which the obtained powder is subjected to spark plasma sintering is disclosed.
In the same document,
(A) ZrO ₂ nanoparticles are mainly present at grain boundaries, and
(B) ZT increments at temperatures above 650K consists respectively, about 10% ZrNiSn-2vol% ZrO _2, about 15% ZrNiSn-6vol% ZrO ₂ (about 0.22 in the absolute value of ZT) Points are listed.

Non-Patent Document 2 includes
(1) 3-9 vol% of ZrO ₂ nanoparticles were added to and mixed with Zr _0.5 Hf _0.5 Ni _0.8 Pd _0.2 Sn _0.99 Sb _0.01 powder, followed by annealing for 3 days.
(2) A method for producing a thermoelectric material in which the obtained powder is subjected to spark plasma sintering is disclosed.
In the same document,
(A) ZrO ₂ nanoparticles are mainly present at grain boundaries, and
(B) the ZT of the sample containing 6 vol% or 9 vol% ZrO ₂ exceeds 0.7 at 800 K and is about 30% in ZT increments;
Is described.
Furthermore, Non-Patent Document 3 describes that although it is not a composite material, the mean free path of phonons at a low temperature of the half-Heusler phase is about the particle size.

In order to achieve high performance of thermoelectric materials by compositing, it is necessary to control the size and interparticle distance of the compositing particles.
When a chemical method for reducing a metal salt in a solution is used, a nano-sized uniform composite material can be produced. However, when elements that are easily oxidized and difficult to reduce, such as rare earth, Ti, Zr, and Hf, are included as main constituent elements, it is extremely difficult to reduce these elements to produce the target phase.

  On the other hand, when a composite material is produced by mechanical alloying or simple ball mill mixing, compositing is relatively easy, but the partner particles to be composited may become coarse due to aggregation. In addition, impurities may be mixed in from a ball or pot during a long process, or phase separation may be promoted by a mechanochemical reaction, resulting in unexpected performance degradation. Furthermore, in this method, the oxide particles are segregated mainly at the crystal grain boundaries of the parent phase. For this reason, the distance between the oxide particles is limited by the degree of refinement of the crystal grains of the parent phase, and it is difficult to sufficiently reduce the distance between the particles. Further, some of the oxide particles are also precipitated in the grains, but the size of the oxide particles is about 500 nm, and the size cannot be sufficiently reduced.

JP 2008-305907 A JP 2008-192652 A JP 2003-225743 A

X.Y.Huang et al., Solid State Communications 130 (2004) 181-185 L.D.Chen et al., J.Appl.Phys. 99 (2006) 064305 S. Bhattacharya et al., Physical Review B77 (2008) 184203-1-8

A problem to be solved by the present invention is a composite thermoelectric material in which an MNiSn half-Heusler compound (M = Ti, Zr, Hf) is used as a parent phase, and oxide fine particles are uniformly dispersed in the mother phase. It is to provide a manufacturing method.
Another problem to be solved by the present invention is to reduce the thermal conductivity of such a composite thermoelectric material and improve the dimensionless figure of merit.

In order to solve the above problems, a composite thermoelectric material according to the present invention is summarized as having the following configuration.
(A) The composite thermoelectric material is
A matrix composed of thermoelectric materials;
One or more oxide particles precipitated in at least the grains of the matrix;
It has.
(B) The parent phase includes a half-Heusler compound having a composition represented by the formula (1).
(M _1-a A _a ) _{1 + x} (Ni _1-b B _b ) _{1 + y} (Sn _1-c C _c ) (1)
However,
0 ≦ a <0.1, 0 ≦ b <0.1, 0 ≦ c <0.1.
−0.1 ≦ x ≦ 0.2, 0 ≦ y ≦ 0.2.
M is one or more elements selected from Ti, Zr and Hf.
A is one or more elements selected from group IIIa elements, group IVa elements (excluding Ti, Zr, and Hf), group Va elements and rare earth elements,
B is one or more elements selected from Group VIIIa elements (excluding Ni) and Group Ib elements;
C is one or more elements selected from Group IIIb elements, Group IVb elements (excluding Sn), and Group Vb elements.
(C) The oxide particles contain at least one metal element selected from transition metal elements, rare earth elements, IIb group elements, IIIb group elements, and IVb group elements.
(D) The intragranular precipitation rate of the oxide particles is 15% or more.

The manufacturing method of the composite thermoelectric material according to the present invention is as follows:
A matrix phase dissolving step of dissolving the raw materials blended so as to have the composition represented by the formula (1) according to claim 1;
A mother phase solidification step for solidifying the molten metal obtained in the mother phase melting step;
A re-dissolution step of adding the oxide particles according to claim 1 to the solidified product obtained in the matrix solidification step and re-dissolving the solidified product,
A rapid solidification step of rapidly solidifying the molten metal obtained in the remelting step;
It has.

When oxide particles are added to an MNiSn-based half-Heusler compound and melted, and the molten metal is rapidly solidified, a composite powder in which 1 to 100 nm of oxide fine particles are uniformly dispersed inside the crystal grains made of the half-Heusler compound is obtained. It is done. When this composite powder is sintered, a sintered body in which oxide fine particles of 1 to 100 nm are uniformly dispersed inside crystal grains (parent phase) made of a half-Heusler compound is obtained. That is, a sintered body is obtained in which oxide fine particles are dispersed at a distance shorter than the phonon mean free path in the matrix. As a result, the thermal conductivity κ of the sintered body is reduced.
Further, the presence of oxide fine particles dispersed in the matrix phase forms impurity levels in the matrix phase. As a result, the Seebeck coefficient S is improved and the output factor PF is increased. As a result, the dimensionless figure of merit ZT is improved as compared with the case where oxide fine particles are not combined and the case where oxide fine particles are dispersed at the grain boundary of the matrix.

It is a figure which shows the unit cell of AgAsMg type | mold crystal structure. FIG. 2A is an SEM image of the composite thermoelectric material produced by the mixing method. FIG. 2B is a composition analysis result by O of EPMA in the same field of view as FIG. It is a SEM image of the composite thermoelectric material produced by the rapid solidification method. 4 is a low-magnification SEM photograph of the sample in FIG. 3. Fig.5 (a) shows the distribution map of the analysis object particle | grains extracted from the SEM photograph of FIG.5 (b). FIG. 5B is an SEM photograph used for image analysis. It is the particle size distribution of the particle | grains calculated | required with the image analysis software using the image of Fig.5 (a). ZrNiSn (no compounding), ZrNiSn / 1vol% ZrO ₂ composite thermoelectric material prepared in the mixing process, and the temperature dependence of the absolute value of the Seebeck coefficient of ZrNiSn / 1vol% ZrO ₂ composite thermoelectric material prepared in rapid solidification FIG. It is a figure which shows the output factor PF in 873K of a ZrNiSn type thermoelectric material and a ZrNiSn type composite thermoelectric material. FIG. 9A is a diagram showing the influence of the amount of ZrO ₂ on the thermal conductivity κ (@ 300K) of various composite thermoelectric materials. FIG. 9B is a diagram showing the influence of the amount of ZrO ₂ on the dimensionless figure of merit ZT (@ 873K) of various composite thermoelectric materials.

Hereinafter, an embodiment of the present invention will be described in detail.
[1. Composite thermoelectric material]
The composite thermoelectric material according to the present invention includes a parent phase made of a thermoelectric material and at least one or more oxide particles precipitated in the grains of the parent phase.

[1.1. Mother phase]
The parent phase includes a half-Heusler compound having a predetermined composition as a main phase.
[1.1.1. Crystal structure]
FIG. 1 shows a schematic diagram of a unit cell of an AgAsMg type crystal structure. The half-Heusler compound has an AgAsMg type crystal structure (space group F43m) and is represented by a general formula: XYZ. The MNiSn compound (M = Ti, Zr, Hf) constituting the main phase of the thermoelectric material according to the present embodiment is a kind of half-Heusler compound having an AgAsMg type crystal structure.
In FIG. 1, X atom and Z atom are respectively 4a (0, 0, 0) site (hereinafter simply referred to as “M site”) and 4b (1/2, 1/2, 1/2) site ( Hereinafter, it is simply referred to as “Sn site”), and the X atom and the Z atom form a rock salt structure. The M site and the Sn site are equivalent.
Y atoms are octahedrally coordinated pockets (centers of cubes composed of X atoms and Z atoms), that is, 4c (1/4, 1/4, 1/4) sites (hereinafter simply referred to as “Ni Site "). The other pockets, that is, 4d (3/4, 3/4, 3/4) sites (hereinafter simply referred to as “4d sites”) are usually empty.
As will be described later, when the Ni site atoms are excessive with respect to the Sn site atoms, the excess Ni site atoms enter the 4d site.

[1.1.2. Number of valence electrons]
The number of valence electrons per atom of the half-Heusler compound XYZ containing no dopant is 6. Half-Heusler compounds with 6 valence electrons per atom (or 18 total valence electrons) are known to exhibit semiconducting properties and have moderately large Seebeck coefficient S and electrical resistivity ρ. ing.
Since the half-Heusler compound XYZ is a compound of X: Y: Z = 1: 1: 1, the number of valence electrons #e per atom is represented by the following formula (a).
_{#E = (# e X + #} e Y + # e Z) / 3 ··· (a)
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.

When any one or more sites of the half-Heusler compound XYZ having 6 valence electrons per atom are doped with a homologous element having the same number of valence electrons as the main constituent element, phonon scattering increases. As a result, the thermal conductivity κ decreases.
On the other hand, when any one or more sites of the half-Heusler compound XYZ having 6 valence electrons per atom are doped with an element having a valence electron number different from that of the main constituent element, the valence electron number per atom changes. As a result, the electrical conductivity σ increases, the Seebeck coefficient S increases, or the thermal conductivity κ decreases.

  Doping may replace only the main constituent element of one site, or may replace main constituent elements of 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 main constituent element of such a half-Heusler compound is replaced with an element having a larger valence number (hereinafter referred to as “n-type dopant”), the number of valence electrons per atom is larger than 6. A half-Heusler compound is obtained. When the number of valence electrons per atom exceeds 6, an electron is doped and an n-type thermoelectric material having a higher electrical conductivity is obtained.
On the other hand, when a part of the main constituent element 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 count 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 than the main constituent element and a smaller element may be added simultaneously. Further, even when a part of the main constituent elements 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 changes and the slope of the density of states near the Fermi level becomes steep, the Seebeck coefficient S increases, or
(3) Since the p-type dopant and the n-type dopant form a local dipole, the Coulomb force due to the dopant is shielded and the decrease in carrier mobility is suppressed.
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 preferably 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.

[1.1.3. Constituent elements]
The parent phase includes a half-Heusler compound having a composition represented by the following formula (1).
(M _1-a A _a ) _{1 + x} (Ni _1-b B _b ) _{1 + y} (Sn _1-c C _c ) (1)
However,
0 ≦ a <0.1, 0 ≦ b <0.1, 0 ≦ c <0.1.
−0.1 ≦ x ≦ 0.2, 0 ≦ y ≦ 0.2.
M is one or more elements selected from Ti, Zr and Hf.
A is one or more elements selected from group IIIa elements, group IVa elements (excluding Ti, Zr, and Hf), group Va elements and rare earth elements,
B is one or more elements selected from Group VIIIa elements (excluding Ni) and Group Ib elements;
C is one or more elements selected from Group IIIb elements, Group IVb elements (excluding Sn), and Group Vb elements.

“A” represents an element (dopant) that replaces the M site. The element A is a group IIIa element ( ₂₁ Sc, ₃₉ Y), a group IVa element ( ₉₀ Th) excluding Ti, Zr, and Hf, a group Va element ( ₂₃ V, ₄₁ Nb, ₇₃ Ta), or a rare earth element ( ₅₇ Any of La to ₇₁ Lu) may be used.
“B” represents an element (dopant) that replaces the Ni site. Element B, VIIIa group elements excluding _{Ni (26 Fe, 27 Co,} 44 Ru, 45 Rh, 46 Pd, 76 Os, 77 Ir, 78 Pt), or Ib group element _{(29 Cu, 47 Ag, 79} Au) Either may be sufficient.
“C” represents an element (dopant) that replaces the Sn site. Element C is, IIIb group elements _{(5 B, 13 Al, 31} Ga, 49 In, 81 Tl), IVb group elements excluding _{Sn (6 C, 14 Si,} 32 Ge, 82 Pb), or Vb group element ₍₇ N, ₁₅ P, ₃₃ As, ₅₁ Sb, ₈₃ Bi).

Among these, the element A is preferably ₃₉ Y. The element B is preferably one or more selected from ₂₇ Co and ₂₉ Cu. Furthermore, the element C is preferably any one or more selected from ₁₃ Al, ₁₄ Si, and ₅₁ Sb.
Since these elements are relatively inexpensive and have a great effect of increasing the output factor PF without significantly increasing the thermal conductivity κ, they are suitable as elements for substituting each site.

“A” represents the amount of substitution of the M site by the element A. “B” represents the amount of substitution of Ni sites by element B. “C” represents the amount of substitution of Sn sites by element C.
In general, when the main constituent element of each site is replaced with an element having the same or different valence electron number, the carrier concentration increases or phonon scattering increases. However, if the amount of substitution at each site is excessive, the rate of heterogeneous formation increases, and the thermoelectric properties deteriorate. Therefore, a, b, and c must each be less than 0.1. Each of a, b, and c is more preferably 0.05 or less.

“X” represents a deviation from the stoichiometric composition of the element (M + A) occupying the M site. Even if the amount of (M + A) deviates from the stoichiometric composition, the influence on the lattice defects and thermoelectric properties of the half-Heusler phase is relatively small.
However, if the amount of (M + A) becomes too small compared to the stoichiometric composition, a problem that different phases such as full Heusler MNi ₂ Sn (M = Ti, Zr, Hf), Sn, Ni, Ni—Sn alloy precipitate. There is. Therefore, x needs to be −0.1 or more. x is more preferably −0.05 or more, and further preferably −0.01 or more.
On the other hand, when the amount of (M + A) is excessive as compared with the stoichiometric composition, a heterogeneous phase (for example, metal Ti phase, Ti ₆ Sn _5, etc.) mainly containing excess element M is precipitated in the material. Therefore, x needs to be 0.2 or less. x is more preferably 0.15 or less, and still more preferably 0.1 or less.

“Y” represents a deviation from the stoichiometric composition of the element (Ni + B) occupying the Ni site. The half-Heusler phase may have a stoichiometric composition (y = 0).
On the other hand, when the amount of (Ni + B) is excessive with respect to the amount of element (Sn + C) occupying the Sn site, excess (Ni + B) is introduced into the 4d site. (Ni + B) introduced into the 4d site has the effect of reducing the thermal conductivity κ of the half-Heusler phase without reducing the output factor PF. Therefore, y is preferably larger than 0.
On the other hand, when the amount of (Ni + B) is excessive as compared with the stoichiometric composition, a full Heusler phase is precipitated in the material. Since the full Heusler phase is metallic, the precipitation of the full Heusler phase causes a decrease in thermoelectric properties. Therefore, y needs to be 0.2 or less.

The optimum y value depends on the composition of the half-Heusler phase.
For example, when M = Ti, y is preferably 0.015 or more, more preferably 0.047 or more, in order to reduce the thermal conductivity κ.
On the other hand, in order to suppress the precipitation of the full Heusler phase, y is preferably 0.145 or less, more preferably 0.123 or less.
For example, when M = Zr, in order to reduce the thermal conductivity κ, y is preferably 0.01 or more, more preferably 0.031 or more, and further preferably 0.04 or more.
On the other hand, in order to suppress the precipitation of the full Heusler phase, y is preferably 0.10 or less, more preferably 0.06 or less, and still more preferably 0.05 or less.

[1.1.4. Lattice constant of half-Heusler compound]
When the amount of (Ni + B) is made to exceed the stoichiometric composition and the manufacturing conditions are optimized, the excess (Ni + B) enters the 4d site. As a result, the lattice constant of the MNiSn half-Heusler compound increases. That is, the increase in lattice constant correlates with the amount of (Ni + B) introduced into the 4d site.

The optimum lattice constant depends on the composition of the half-Heusler phase.
For example, when M = Ti, in order to reduce the thermal conductivity κ without reducing the output factor PF, the lattice constant of the half-Heusler compound (main phase) is preferably 0.5933 nm or more. The lattice constant is more preferably 0.5937 nm or more.
In the case of a TiNiSn sintered body containing no dopant, the lattice constant of the half-Heusler phase can be changed by controlling the Ni / Sn ratio. Specifically, a TiNiSn half-Heusler compound having a lattice constant of 0.5929 nm or more and 0.5942 nm or less can be obtained by changing the y value.
On the other hand, with a Y—Sb substitution material made to improve the output factor PF, a material having a lattice constant of 0.5947 nm can be synthesized. Therefore, the upper limit of the lattice constant of the TiNiSn half-Heusler compound having a high output factor PF is 0.5947 nm.

For example, when M = Zr, in order to reduce the thermal conductivity κ without reducing the output factor PF, the lattice constant of the half-Heusler compound (main phase) is preferably 0.6110 nm or more. . The lattice constant is more preferably 0.6115 nm or more, and still more preferably 0.6118 nm or more.
On the other hand, in order to improve the output factor PF, if any one of Zr sites, Ni sites and Sn sites is doped, a ZrNiSn half-Heusler compound having a maximum lattice constant of 0.6130 nm is obtained. .
In the case of a ZrNiSn sintered body containing no dopant, the lattice constant of the half-Heusler phase can be changed by controlling the Ni / Sn ratio. Specifically, by changing the y value, a ZrNiSn half-Heusler compound having a lattice constant of 0.6110 nm to 0.6130 nm can be obtained.

[1.1.5. impurities]
The composite thermoelectric material according to the present invention is preferably composed of only the above-described MNiSn half-Heusler compound and oxide fine particles described later, 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 composite thermoelectric material according to the present embodiment may be a composite of a composite particle composed of a matrix phase and oxide fine particles and another material (for example, resin, rubber, etc.).

Here, the “heterophase” refers to a phase different from the MNiSn half-Heusler compound and oxide fine particles. 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.
As described above, when the amount of (Ni + B) becomes excessive, a full Heusler phase may be precipitated in the parent phase. Since the full Heusler phase is a metal phase, the precipitation of the full Heusler phase causes a decrease in thermoelectric properties.

The amount of impurities allowed depends on the composition of the half-Heusler phase.
For example, when M = Ti, in order to obtain high thermoelectric characteristics, the strongest peak intensity ratio is preferably less than 18%. The strongest line peak intensity ratio is more preferably 10% or less, and still more preferably 5% or less.
For example, when M = Zr, in order to obtain high thermoelectric characteristics, the strongest peak intensity ratio is preferably less than 6%. The strongest line peak intensity ratio is more preferably 5% or less, and still more preferably 4% or less.
Here, the strongest line peak intensity ratio means a value represented by the following equation (b).
Strongest peak intensity ratio = I _{FULL (220)} x 100 / I _{HALF (220)} (b)
However, I _{HALF (220)} is the strongest peak intensity in the X-ray diffraction of the half-Heusler phase contained in the thermoelectric material. I _{FULL (220)} is the strongest line peak intensity in the X-ray diffraction of the full Heusler phase contained in the thermoelectric material.

[1.1.6. Thermoelectric characteristics]
As described above, if the (Ni + B) / (Sn + C) ratio is made larger than 1 (y> 0), or if a dopant is added to any one or more of the M site, Ni site, and Sn site, the thermoelectric of the parent phase Improved characteristics.
For example, in the case of M = Ti, if y> 0, the thermal conductivity κ of the matrix at room temperature can be 4 W / mK or less without adding heavy elements such as Zr and Hf. . Even when no dopant is added to any of the M site, Ni site, and Sn site, the ZT value of the matrix at room temperature can be 0.05 or more. Furthermore, the ZT value of the parent phase can be made 0.07 or more.
For example, in the case of M = Zr, if y> 0, the thermal conductivity κ of the parent phase at room temperature should be 6.7 W / mK or less without adding a heavy element such as Hf. Can do. Even when no dopant is added to any of the M site, Ni site, and Sn site, the ZT value of the parent phase at 773 to 873 K can be 0.35 or more.

[1.2. Oxide particles]
The oxide particles are precipitated at least in the grains of the matrix.
The oxide particles may be precipitated only within the parent phase grains, or may be precipitated both within the parent phase grains and at the grain boundaries. When the oxide particles are precipitated both in the grains of the matrix and in the grain boundaries, the oxide particles in the grains and the oxide particles in the grain boundaries may have the same composition or different compositions. There may be.

[1.2.1. composition]
The oxide particles include transition metal elements ( ₂₁ Sc to ₂₉ Cu, ₃₉ Y to ₄₇ Ag, ₇₂ Hf to ₇₉ Au), rare earth elements ( ₅₇ La to ₇₁ Lu), IIb group elements ( ₃₀ Zn, ₄₈ Cd, ₈₀ Hg). ), IIIb group elements _{(5 B, 13 Al, 31} Ga, 49 in, 81 Tl), and, IVb group element _{(6 C, 14 Si, 32} Ge, 50 Sn, 82 Pb) from at least one selected Contains metal elements.
Specific examples of oxides constituting the oxide particles include rare earth oxides, SiO ₂ , TiO ₂ , ZrO ₂ , HfO ₂ , AlO _1.5 and ZnO. Any one of these oxide particles may be precipitated in the mother phase grains, or two or more of them may be precipitated.
Among these, the oxide particles are preferably at least one selected from ZnO, TiO ₂ , ZrO ₂ , YO _1.5 , NdO _1.5 and LaO _1.5 . These oxides have a greater effect of improving thermoelectric properties than other oxides.

[1.2.2. Volume fraction and intragranular precipitation rate]
The total volume ratio of the oxide particles in the entire composite thermoelectric material affects the thermoelectric characteristics of the composite material. In general, as the total volume ratio of oxide particles increases, the thermal conductivity κ decreases or the Seebeck coefficient S increases. On the other hand, when the total volume ratio of the oxide particles becomes excessive, the electrical conductivity σ decreases. Further, when the oxide particles become excessive, the particles may aggregate and grow, and the intragranular precipitated particles may be relatively reduced. In order to obtain high thermoelectric properties, the larger the amount of oxide particles precipitated in the parent phase grains, the better.
Specifically, the volume fraction α of the oxide particles present in the parent phase grains is preferably in the range of 0 <α ≦ 0.10. The volume fraction α is more preferably 0 <α ≦ 0.07, more preferably 0 <α ≦ 0.05, and still more preferably 0.005 ≦ α ≦ 0.03.
Here, “the volume fraction of the oxide particles present in the grains of the parent phase” refers to a value obtained by the following procedure.
(1) A microphotograph of at least one field of view is taken so that a plurality of (preferably several to a dozen) crystal grains are included, and the area of the field of view is obtained.
(2) Count the number of oxide particles precipitated in the grains in the field of view.
(3) A plurality of (preferably dozens to several tens) oxide particles are selected from the field of view, or all the oxide particles in the field of view are selected, and an average particle diameter (described later “ The equivalent circle diameter ") is obtained, and the average area of the oxide particles assumed to be a circle is obtained.
(4) The number of oxide particles precipitated in the grains × the average area of the oxide particles is divided by the area of the visual field to obtain the particle area ratio.
(5) Assuming that the particle area ratio (particle distribution state) is also present in the depth direction of the image, the particle area is converted into a volume, and the obtained value is expressed as “the oxide particles present in the mother phase grains. Volume fraction ". Note that “the particle distribution state also exists in the depth direction of the screen” means that a model in which the precipitated particles are distributed on the lattice points of the three-dimensional lattice is assumed, and the cube of the region to be considered is in the depth direction. When several vertical cross sections (cross sections that are just on the lattice points) are extracted, the state of the area of the precipitated particles with respect to the cross sectional area is constant in any cross section.
Further, the ratio of the volume of the oxide particles precipitated in the parent phase grains to the total volume of the oxide particles (hereinafter referred to as “intragranular precipitation rate”) is preferably 15% or more. The intragranular precipitation rate is more preferably 20% or more.
The “intragranular precipitation rate” refers to a value obtained by the following procedure.
(1) A micrograph of at least one field of view is taken so that a plurality of (preferably several to a dozen) crystal grains are included.
(2) Assuming that the average particle size of the particles precipitated in the grains and the average particle size of the particles precipitated in the grain boundaries are the same, the total number of oxide particles in the field of view and the precipitates in the grains The number of oxide particles (excluding coarse particles having a particle size of 500 nm or more) is counted.
(3) A value obtained by dividing the number of oxide particles precipitated in the grain by the total number of oxide particles in the field of view is defined as “intragranular precipitation rate”.

[1.2.3. Average particle size]
The average particle size of the oxide particles affects the thermoelectric properties of the composite material. Generally, when the average particle size of oxide particles becomes too small, phonon scattering becomes insufficient. Therefore, the average particle diameter of the oxide particles is preferably 1 nm or more.
On the other hand, if the average particle diameter of the oxide particles becomes too large at a certain volume fraction α, the interparticle distance becomes too large and phonon scattering becomes insufficient. Therefore, the average particle size of the oxide particles is preferably 100 nm or less.
Here, the “particle diameter of the oxide particles” refers to the diameter of a circle having an area equivalent to the area of the oxide particles observed on a micrograph (equivalent diameter of a circle). The “average particle diameter of oxide particles” refers to the average value of the particle diameters of a plurality of (preferably 10 or more) oxide particles observed on a micrograph.
The average particle diameter can be determined by the following procedure.
(1) A micrograph of at least one field of view is taken so that a plurality of (preferably several to a dozen) crystal grains are included.
(2) The total area of the particles is estimated by image analysis software (for example, free software: ImageJ), and the total area of the particles is divided by the number of particles to obtain the average area per particle.
(3) Assuming that the particles are perfect spheres (circles), the diameter (average particle size) is calculated from the average area of one particle.

[1.2.4. Average interparticle distance]
The average distance between the oxide particles precipitated in the matrix phase particles affects the thermoelectric properties of the composite material. In general, if the average distance between oxide particles is too short, the scattering probability of carriers such as electrons becomes too high, and the electrical conductivity may be significantly reduced.
The carrier mean free path (MFP _car ) can be calculated by the following equation.
MFP _car = v × τ = v × (μ × m ^* / e)
v = (3 × k _B × T / m ^* ) ^0.5
Where, v: velocity of carrier, τ: relaxation time, μ: mobility, m ^* : effective mass of carrier, e: elementary charge of electron, k _B : Boltzmann constant, T: absolute temperature.
m ^* = 2-3 me (literature value: C. Uher et al., "Transport properties of pure and doped MNiSn (M = Zr, Hf), Physical Review B59 (13) p.8651-8621, me = free electron Mass), μ = 17.4 to 21.5 cm ² / Vs (actual measurement value), the average free path of the carrier is calculated to be 1.5 to 2.5 nm. 1.5 nm or more is preferable.
On the other hand, if the average distance between the oxide particles becomes too long, phonon scattering becomes insufficient. Therefore, the average distance between the oxide particles is preferably 1 μm or less.
Here, the “average distance between oxide particles” refers to a value obtained by the following procedure.
(1) According to the above-described procedure, the number of particles per field of view (average number of particles) and the average particle size of the particles are obtained using image analysis software.
(2) The average inter-particle distance (one side of the square lattice) assuming that the number of particles having an average particle size are arranged at equal intervals (square lattice shape) within the area of one field of view. Is calculated).

[2. Manufacturing method of composite thermoelectric material]
The method for producing a composite thermoelectric material according to the present invention includes a matrix phase melting step, a matrix phase solidification step, a remelting step, a rapid solidification step, and a sintering step.

[2.1. Mother phase dissolution process]
The matrix phase dissolution step is a step of dissolving the raw materials blended so as to have the composition represented by the above-described formula (1). Since the MNiSn half-Heusler compound has a melting point of Sn lower than that of other elements, it is preferable to prepare a uniform matrix having a predetermined composition in advance before dispersing the oxide.
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.

[2.2. Mother phase solidification process]
The mother phase solidification step is a step of solidifying the molten metal obtained in the mother phase melting step. In the mother phase solidification step, the molten metal may be slowly solidified or rapidly solidified. The rapid solidification method has the advantage that the crystal grains are refined and the thermal conductivity κ can be reduced. Moreover, precipitation of a different phase can also be suppressed by rapid cooling.

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. Since the boron nitride nozzle is poor in reactivity with the molten metal, if it is used for rapid solidification, compositional deviation, contamination of impurities, and deterioration of thermoelectric properties due to these can be suppressed.
Specifically, as a rapid solidification method,
(1) A method (copper roll method) in which a raw material is melted in a boron nitride nozzle and sprayed or dropped onto a copper roll (cooling medium) that rotates the molten metal,
(2) A method in which a raw material is melted in a boron nitride nozzle, a molten metal is sprayed or dripped from the nozzle hole, a jet fluid is blown from the surroundings to the molten metal flow, and the generated droplet is solidified while dropping (the 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.

[2.3. Remelting process]
The redissolving step is a step of adding the above-described oxide particles to the solidified product obtained in the parent phase solidifying step to re-dissolve the solidified product.
Since the re-dissolution process is the same as the mother-phase dissolution process except that a predetermined amount of oxide particles is added to the raw material, detailed description thereof is omitted.

[2.4. Rapid solidification process]
The rapid solidification process is a process in which the molten metal obtained in the remelting process is rapidly solidified.
Since the rapid solidification step is the same as the rapid solidification method of the matrix phase dissolution step except that a predetermined amount of oxide particles is added to the molten metal, detailed description thereof is omitted.

[2.5. Sintering process]
The sintering step is a step of forming and sintering the rapidly solidified powder obtained in the rapid solidification step. The rapidly solidified product can be used for various purposes as it is after being appropriately pulverized as required. Accordingly, the sintering process is not necessarily a necessary process, but when used in a bulk state, the sintering is usually performed.
When the powdered MNiSn half-Heusler compound is sintered, various methods can be used as the sintering method. Specific examples of the sintering method include a normal pressure sintering method, a hot press, a HIP, and a discharge plasma sintering (SPS) method. Among these, the SPS 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, pressure applied during sintering, atmosphere during sintering, etc.) are optimal depending on the composition of the MNiSn half-Heusler compound, the sintering method used, etc. Choose one.
For example, when using the SPS method, the sintering temperature is preferably equal to or lower than the melting point of the MNiSn half-Heusler compound, and the applied pressure is preferably equal to or greater 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.

[2.6. Other processes]
After the powder is sintered, an annealing treatment for holding the sintered body at a predetermined temperature may be performed. By subjecting the sintered body to annealing treatment, segregation of component elements, precipitated foreign phases, and the like can be removed.
The annealing temperature is preferably 700 ° C. or higher and below the melting point of the MNiSn half-Heusler compound. 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 the like can be removed in a shorter time. Usually, it is several hours to several tens of hours.

[3. Action of Composite Thermoelectric Material and Method for Producing the Same]
In general, the conversion efficiency of a thermoelectric material has a one-to-one correspondence with the dimensionless figure of merit ZT, and the greater the ZT, the greater the thermoelectric conversion efficiency. The figure of merit ZT is expressed by the following equation.
ZT = [(σ × S ² ) / κ] × T = PF / κ × T
(Σ: electrical conductivity, S: Seebeck coefficient, κ: thermal conductivity, T: absolute temperature)
From this equation, it can be seen that in order to improve ZT, it is sufficient to improve PF (output factor) or reduce κ.

Usually, the thermal conductivity κ is given by the sum of the contribution from the carrier (κ _carr ) and the contribution from the lattice vibration (κ _ph ) as follows.
κ = κ _carr + κ _ph
However, in the thermoelectric material, usually, ZT although the carrier concentration such that maximum is selected, the carrier concentration thereof, since the κ _carr «κ _ph, after all, the thermal conductivity, the size of the kappa _ph Dependent. The lattice thermal conductivity is scattered by various imperfections (impurities, structural defects, etc.) of the material.

That is, phonons are mainly responsible for heat conduction in thermoelectric materials. Therefore, if the structure can be controlled so that nano-sized heterogeneous phases are dispersed in the thermoelectric material at intervals less than the mean free path of phonons, phonons will be scattered by the heterogeneous phases during propagation, reducing thermal conductivity. Can do. As a method for dispersing such a different phase, a chemical method or a mixing method using mechanical alloying or simple ball mill mixing is known.
However, the chemical method cannot be applied to a thermoelectric material containing an element that is difficult to reduce as a main constituent element.

On the other hand, in the mixing method, different phases can be easily dispersed in the matrix phase regardless of the composition of the matrix phase. However, in the mixing method, there is a possibility that the partner particles to be complexed by aggregation are coarse.
Further, the mixing method has a fine structure in which oxide fine particles are segregated mainly at the crystal grain boundaries of the matrix, and the inter-particle distance is limited by the degree of refinement of the matrix crystal grains. Therefore, with this method, it is difficult to sufficiently reduce the interparticle distance. Some of the oxide particles are also precipitated in the crystal grains, but the size of the oxide particles is about 500 nm, and the size of the oxide particles cannot be sufficiently reduced.
Furthermore, when a long-time treatment is performed to refine the mother phase particles and oxide particles, the amount of impurities mixed from the balls and pots increases. In addition, the long-time treatment may promote phase separation by a mechanochemical reaction, resulting in unexpected performance degradation. Actually, when the composition of the present invention was produced by the same method as in Non-Patent Documents 1 and 2, there was a problem that the Seebeck coefficient was lowered due to the reaction between the oxide and the parent phase.

In contrast, when oxide particles are added to the MNiSn half-Heusler compound and melted, and the molten metal is rapidly solidified, oxide fine particles of 1 to 100 nm are uniformly dispersed inside the crystal grains made of the half-Heusler compound. A composite powder is obtained. When this composite powder is sintered, a sintered body in which oxide fine particles of 1 to 100 nm are uniformly dispersed inside crystal grains (parent phase) made of a half-Heusler compound is obtained. That is, a sintered body is obtained in which oxide fine particles are dispersed at a distance shorter than the phonon mean free path in the matrix. As a result, the thermal conductivity κ of the sintered body is reduced.
Further, the presence of oxide fine particles dispersed in the matrix phase forms impurity levels in the matrix phase. As a result, the Seebeck coefficient S is improved and the output factor PF is increased. As a result, the dimensionless figure of merit ZT is improved as compared with the case where oxide fine particles are not combined and the case where oxide fine particles are dispersed at the grain boundary of the matrix.

Further, in addition to uniformly dispersing the oxide particles in the matrix phase, the thermal conductivity κ of the composite material can be further reduced by replacing a part of the elements constituting the matrix phase with a heavy element.
For example, if a part of Ti in TiNiSn is replaced with Zr or Hf, or a part of Ni in ZrNiSn is replaced with Pd or Pt, the thermal conductivity κ of the composite material can be further reduced.

In addition, MnNiSn has a crystal structure in which M-Sn forms a rock salt type lattice, Ni occupies half of the body center position, and the other half exists as unoccupied vacancies. When Ni is excessive relative to the stoichiometric MNiSn, the excess Ni randomly occupies the vacancies. A lattice occupied by Ni has an increased lattice constant compared to a lattice not occupied. Therefore, phonons are scattered and κ _ph is reduced due to lattice constant mismatch (ie, crystal imperfection).
Such imperfections can simultaneously scatter carriers and reduce electrical conductivity. However, since the mean free path of carriers is generally smaller than that of phonons, the degree of phonon reduction is greater and the dimensionless figure of merit increases.

The method of introducing defects into the Ni site is low in cost because the thermal conductivity κ of the parent phase can be reduced without using a large amount of expensive heavy elements.
In addition, the introduction of Ni site atoms into the vacancies (4d sites) also affects the Seebeck coefficient S and the electrical conductivity σ of the parent phase. Therefore, when the y value is optimized, the thermal conductivity κ of the parent phase can be reduced, and at the same time, the output factor PF can be increased. In particular, when a Ni interstitial defect is introduced into a ZrNiSn half-Heusler compound, the output factor (3.8 mW / K ² m) is higher than that of the TiNiSn half-Heusler compound by optimizing the carrier concentration ( 4.6 mW / K ² m) is obtained.

Furthermore, in addition to dispersing the oxide particles in the grains of the matrix and introducing excess Ni site atoms between the lattices, adding an appropriate dopant to the M site, Ni site and / or Sn site, The output factor PF can be further increased while keeping the thermal conductivity κ of the matrix phase low.
Furthermore, if the composition and manufacturing conditions are optimized so that the strongest peak intensity ratio of the full-Heusler phase / half-Heusler phase is less than a predetermined value, the deterioration of the thermoelectric properties of the matrix due to the precipitation of the full-Heusler phase is suppressed. be able to.

(Example 1, Comparative Example 1)
[1. Preparation of sample]
Total composition _{(Zr 1-a Y a)} Ni (Sn 1-a Sb a), (Ti 0.8 Zr 0.2) NiSn, _{or, [(Ti 0.5 Zr 0.25 Hf} 0.25) 0.99 Y 0.01] Ni (Sn 0.99 Sb _0.01 )), Y, Ni, Sb, Ti, Zr, Hf, and Sn were weighed. This raw material was put into a boron nitride crucible and melted at high frequency to obtain an ingot. The ingot was pulverized with a mortar or ball mill, and a mother phase powder was obtained through a 20 μm mesh sieve.
TiO ₂ , ZrO ₂ , ZnO, SiO ₂ , Y ₂ O ₃ , Nd ₂ O ₃ , or La ₂ O ₃ nanoparticles (10 to 100 nm) are added to the parent phase powder using ethanol as a solvent. 1-10 vol% was mixed. Thereafter, the solvent was removed by holding the sample at room temperature to 400 ° C. under reduced pressure to obtain a half-Heusler powder coated with nanoparticles (hereinafter referred to as “coated powder”) (Comparative Example 1). .
Furthermore, the coating powder was redissolved and rapidly solidified to obtain a half-Heusler powder (hereinafter referred to as “dispersed powder”) in which oxide particles were dispersed in crystal grains (Example 1).
The obtained mother phase powder, coating powder or dispersion powder was sintered using an SPS apparatus. The sintering temperature was 1100 ° C., the sintering time was 15 minutes, and the pressure during sintering was 50 MPa.

[2. Test method]
[2.1. Tissue observation]
The structure of the sintered body was observed by SEM. In addition, the average interparticle distance, average particle diameter, volume fraction α, and intragranular precipitation rate of the oxide particles were calculated from the SEM photograph using image analysis software (ImageJ).
[2.2. Thermoelectric characteristics]
A sample was cut out from the sintered body, and the Seebeck coefficient S, electrical conductivity σ, and thermal conductivity κ were measured. In addition, the output factor PF and the dimensionless figure of merit ZT were calculated using the measured Seebeck coefficient S, electrical conductivity σ, and thermal conductivity κ.

[3. result]
[3.1. Organization]
2 (a) and 2 (b), SEM photographs of composite thermoelectric materials (Comparative Example 1: ZrNiSn + 1 vol% ZrO ₂ ) produced using the coating powder, respectively, and the same field of view as FIG. 2 (a) The composition analysis result of EP of O by EPMA is shown. 3, a composite thermoelectric material prepared by using a dispersion powder (Example _{1: (Ti 0.8 Zr 0.2)} NiSn + 3vol% ZrO 2) shows the SEM photograph of. Further, FIG. 4 shows a low-magnification SEM photograph of the sample of FIG.
As shown in FIG. 2, when a composite thermoelectric material was produced using a coating powder, the structure was a core-shell type microstructure in which oxide nanoparticles (about 100 nm) were segregated at the grain boundaries of the half-Heusler phase. . In the case of the core-shell type, there was only a slight oxide particle-like appearance in the grain, and even when this was taken into consideration, both the volume fraction α and the intragranular precipitation rate were almost 0%.
On the other hand, as shown in FIG. 3 and FIG. 4, when a composite thermoelectric material is produced using a dispersed powder, the structure is a dispersed microstructure in which oxide particles of 100 nm or less are dispersed in the crystal grains of the mother phase. Met. Further, it was found that when the amount of ZrO ₂ was increased to 3 vol%, not only nanoparticles were precipitated in the grains, but also aggregated and grain-grown oxide coarse particles were formed at the grain boundaries.
FIG. 5A shows a distribution diagram of analysis target particles (black spots) extracted from the SEM photograph of FIG. FIG. 5B shows an SEM photograph (same SEM photograph as FIG. 3) used for image analysis. Further, FIG. 6 shows the particle size distribution of the particles obtained by the image analysis software using the image of FIG.
In the case of the sample of FIG. 3, the average particle size = 54 nm (maximum 264 nm, minimum 20 nm or less (limit of photographic resolution)), volume fraction α = 0.005 (0.85 vol%), intragranular precipitation rate = 30% The average interparticle distance was 238 nm.
The average interparticle distance and the volume fraction of oxide particles present in the parent phase grains varied depending on the charged composition of the oxide particles. On the other hand, when the method according to the present invention was used, the intragranular precipitation rate exceeded 0% regardless of the charged composition.

  The mean free path of phonons in the half-Heusler phase varies depending on the production method, but according to Non-Patent Document 3, it is about the particle size at low temperatures. Therefore, the mean free path of the uncomplexed sample is estimated to be on the order of μm below room temperature. In the sample prepared by the rapid solidification method, it is considered that the mean free path of phonons can be reduced because nanoparticles that can be phonon scattering sources are dispersed in the grains.

[3.2. Thermoelectric characteristics]
Figure 7, ZrNiSn (no compounding), ZrNiSn / 1vol% ZrO ₂ composite thermoelectric material prepared in the mixing process, and, for the absolute value of the Seebeck coefficient of ZrNiSn / 1vol% ZrO ₂ composite thermoelectric material prepared in rapid solidification Shows temperature dependence.
FIG. 7 shows that the absolute value of the Seebeck coefficient in the case of the sample prepared by the mixing method is lower than that of the uncomplexed ZrNiSn, whereas the absolute value of the Seebeck coefficient in the case of the sample prepared by the rapid solidification method. Was found to increase.

FIG. 8 shows an output factor (@ 873K) of a composite thermoelectric material in which Zr _0.99 Y _0.01 NiSn _0.99 Sb _0.01 is used as a parent phase and 1 vol% of YO _1.5 or ZrO ₂ is added by a mixing method or a rapid solidification method. In FIG. 8, the output factor of the parent phase (Zr _0.99 Y _0.01 NiSn _0.99 Sb _0.01 ) is also shown.
In the composite thermoelectric material produced by the mixing method, the absolute value of the Seebeck coefficient was reduced, so that the output factor was reduced as compared with the sample without the composite ((1) → (2) in FIG. 8). On the other hand, when a composite thermoelectric material having the same composition was produced by the rapid solidification method, the output factor did not decrease compared to the sample without composite ((1) → (3) in FIG. 8). Furthermore, in the case of producing a composite thermoelectric material by the rapid solidification method, it was found that when the oxide was changed from YO _1.5 to ZrO ₂ , the output factor increased as compared with the sample without composite ((1) in FIG. 8). → (4)).

FIG. 9A shows the thermal conductivity of a ZrNiSn / ZrO ₂ composite thermoelectric material, (Ti _0.8 Zr _0.2 ) NiSn / ZrO ₂ composite thermoelectric material, and (Ti _0.5 Zr _0.25 Hf _0.25 ) NiSn composite thermoelectric material. The influence of the amount of ZrO ₂ on κ (@ 300K) is shown. FIG. 9B shows the influence of the amount of ZrO ₂ on the dimensionless figure of merit ZT (@ 873K) of these composite thermoelectric materials.
FIG. 9 shows the following.
(1) The thermal conductivity decreases as the amount of oxide added increases. Also, up to at least 10 vol%, the addition of oxide is effective in reducing thermal conductivity.
(2) In the case of (Ti _0.5 Zr _0.25 Hf _0.25 ) NiSn composite thermoelectric material, the output factor slightly decreased as the mixing ratio of oxide particles increased. However, due to the effect of reducing the thermal conductivity, as shown in FIG. 9B, the dimensionless figure of merit ZT increased from that of the sample without compounding up to about 7 vol%.
(3) Depending on the composition of the parent phase, there is a difference in the performance improvement effect by compounding. In particular, in the case of (Ti _0.8 Zr _0.2 ) NiSn / ZrO ₂ composite thermoelectric material, ZT of 0.85 or more was obtained by adding 0.5 vol% or more of ZrO ₂ .

Table 2 shows the influence of the type of oxide on the thermoelectric properties of a dispersion type composite thermoelectric material (1 vol% oxide composite) having a ZrNiSn-based half-Heusler compound as a parent phase.
Table 2 shows the following.
(1) Addition of 1 vol% oxide with respect to the parent phase causes a decrease in thermal conductivity. In particular, compounding with rare earth oxides such as NdO _1.5 and LaO _1.5 (Nos. 5 to 7) has a high effect of reducing thermal conductivity.
(2) 1 vol% of oxide is added to the matrix except when ZrO ₂ is added by the mixing method (No. 3) and when SiO ₂ is added by the rapid solidification method (No. 9) As a result, the output factor is increased. As a result, these dimensionless figure of merit increased by about 30% or more as compared with the case of no oxide composite (No. 1).

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

Claims (8)

A composite thermoelectric material having the following configuration.
(A) The composite thermoelectric material is
A matrix composed of thermoelectric materials;
One or more oxide particles precipitated in at least the grains of the matrix;
It has.
(B) The parent phase includes a half-Heusler compound having a composition represented by the formula (1).
(M _1-a A _a ) _{1 + x} (Ni _1-b B _b ) _{1 + y} (Sn _1-c C _c ) (1)
However,
0 ≦ a <0.1, 0 ≦ b <0.1, 0 ≦ c <0.1.
−0.1 ≦ x ≦ 0.2, 0 ≦ y ≦ 0.2.
M is one or more elements selected from Ti, Zr and Hf.
A is one or more elements selected from group IIIa elements, group IVa elements (excluding Ti, Zr, and Hf), group Va elements and rare earth elements,
B is one or more elements selected from Group VIIIa elements (excluding Ni) and Group Ib elements;
C is one or more elements selected from Group IIIb elements, Group IVb elements (excluding Sn), and Group Vb elements.
(C) The oxide particles contain at least one metal element selected from transition metal elements, rare earth elements, IIb group elements, IIIb group elements, and IVb group elements.
(D) The intragranular precipitation rate of the oxide particles is 15% or more. _2. The composite thermoelectric material according to claim 1, wherein the oxide particles include one or more selected from rare earth oxides, SiO ₂ , TiO ₂ , ZrO ₂ , HfO ₂ , AlO _1.5 and ZnO. 3. The composite thermoelectric material according to claim 1, wherein the oxide particles include one or more selected from ZnO, TiO ₂ , ZrO ₂ , YO _1.5 , NdO _1.5, and LaO _1.5 .   4. The composite thermoelectric material according to claim 1, wherein a volume fraction α of the oxide particles existing in the grains of the matrix phase is in a range of 0 <α ≦ 0.10. The composite thermoelectric material according to any one of claims 1 to 4 , wherein an average particle diameter of the oxide particles is in a range of 1 to 100 nm. The composite thermoelectric material according to any one of claims 1 to 5, wherein an average distance between the oxide particles precipitated in the mother phase grains is 1.5 nm or more and 1 µm or less. A matrix phase dissolving step of dissolving the raw materials blended so as to have the composition represented by the formula (1) according to claim 1;
A mother phase solidification step for solidifying the molten metal obtained in the mother phase melting step;
A re-dissolution step of adding the oxide particles according to claim 1 to the solidified product obtained in the matrix solidification step and re-dissolving the solidified product,
A rapid solidification step of rapidly solidifying the molten metal obtained in the remelting step;
A method for producing a composite thermoelectric material comprising: The method for producing a composite thermoelectric material according to claim 7, further comprising a sintering step of forming and sintering a powder of the rapidly solidified product obtained in the rapid solidification step.

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