JP5671985B2 - 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.

  In order to improve the drawbacks of the conventional thermoelectric materials as described above, it has been proposed to make the thermoelectric materials into nanocomposites. It has been theoretically suggested that when a thermoelectric material is made into a nanocomposite, thermoelectric properties are improved over the corresponding bulk material due to size effect, dimensional effect, and grain boundary scattering of phonons and carriers. Various proposals have been made for such nanocomposite techniques.

For example, Non-Patent Document 1 is not a thermoelectric material,
(1) A BaTiO ₃ precursor solution is prepared by dissolving metal Ba and Ti (O ⁱ Pr) ₄ in a mixed solvent of i-PrOH and 2-methoxyethane,
(2) Ni particles coated with BaTiO ₃ obtained by adding Ni to a BaTiO ₃ precursor solution to cause hydrolysis are disclosed.
This document describes that this method can improve the large sintering shrinkage difference between the Ni electrode and the dielectric and the oxidation resistance of the Ni electrode in the co-sintering process of the multilayer ceramic capacitor. Has been.

In Patent Document 1,
(1) Silica particles are produced using the Stover method,
(2) 3-mercaptopropyl-modified silica particles are produced by adding mercaptopropyltrimethoxysilane (MPS) to the silica particle dispersion and reacting them.
(3) A Bi-EDTA complex solution is dropped into the modified silica particle dispersion, the surface of the modified silica particles is complexed with Bi,
(4) Silica core / Bi ₂ Te ₃ shell type particles obtained by adding a NaHTe solution to a dispersion of modified silica particles whose surface is complexed with Bi are disclosed.
This document describes that a thermoelectric material containing such core / shell type particles has improved thermoelectric properties compared to a bulk material.

Non-Patent Document 2 includes
(1) Pb _0.75 Sn _0.25 Te particles are produced using a CVD method,
(2) Disclosed is a two-dimensional coated nanostructure obtained by dispersing Pb _0.75 Sn _0.25 Te particles and Se in ethylenediamine, placing the dispersion in an autoclave, and performing hydrothermal treatment at 130 ° C. for 20 hours. .
In the same document,
(A) By such a method, a Pb _0.75 Sn _0.25 Se crystal layer of about 30 nm can be formed on the surface of micron-sized Pb _0.75 Sn _0.25 Te particles; and
(B) Since both have similar electronic structures and different lattice constants, such nanostructures are expected to have low carrier scattering and high phonon scattering,
Is described.

Non-Patent Document 3 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.

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.

Furthermore, in Patent Document 3, although it is not a thermoelectric material,
(1) Put the mixed powder of alumina and ceria in a rotating container,
(2) A method for surface modification of a powdery substrate is disclosed in which a target made of a Pt plate is irradiated with laser light while rotating a container, and scattered particles are attached to the rolling powder.

As described above, wet methods such as a chemical solution method and dry methods such as a gas phase method have been reported as methods for coating the particle surface. Among these, the wet method has an advantage that a relatively uniform coating is possible.
However, since the wet method needs to dissolve the raw material of the material to be coated in a solvent, the coating materials that can be used are limited. Further, since the raw material is dissolved in a solvent in the form of ions, there is a problem that the counter ions, the solvent, etc. remain and the particles are contaminated. Furthermore, since the heat treatment is performed in the solution, oxidation of the matrix material is also a problem. In addition, when a nanoparticle is generated in a solution and coating is performed (particularly when a matrix powder composed of an intermetallic compound is used), a reduction reaction is necessary, so that applicable substances are considerably limited.

  Further, for example, when the mother phase powder and the nanoparticles are wet mixed using a ball mill, the nanoparticles adhere around the mother phase powder, but it is difficult to form a uniform thin film. Actually, a coating method using a matrix powder composed of a thermoelectric material, an oxide sol, and a ball mill was examined, but a uniform film was not formed. Moreover, the solvent contained in oxide sol remained, the mother phase decomposed | disassembled by reaction etc. by a residual solvent, and the problem that Seebeck coefficient fell occurred.

On the other hand, the dry method has an advantage that there is no restriction on the coating substance because it is not necessary to dissolve the raw material in the solvent. In particular, the method of coating the surface of the mother phase powder using laser ablation can coat the surface of the mother phase powder with various materials relatively uniformly without being restricted by the material. Moreover, there is little possibility that the matrix powder is decomposed by the residual solvent or the like.
However, in the conventional laser ablation method, there is a limit to uniformly and uniformly coat the surface of the matrix powder with a different material.

JP 2008-147625 A JP 2008-192652 A JP 2009-024196 A

T. Hatano et al., Journal of the European Ceramic Society 24 (2004) 507-510 B. Zhang et al., Applied Physics Letter 89 163114 (2006) X.Y.Huang et al., Solid State Communication 130 (2004) 181-185

  The problem to be solved by the present invention is that a MNiSn half-Heusler compound (M = Ti, Zr, Hf) is used as a parent phase, and the periphery of the parent phase is covered with a thin and uniform oxide layer made of a predetermined metal oxide. An object of the present invention is to provide a composite thermoelectric material and a manufacturing method thereof.

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 a thermoelectric material and consisting of one crystal grain or an aggregate of a plurality of crystal grains,
An oxide layer comprising a metal oxide having a band gap equal to or larger than the matrix, and covering the periphery of the matrix in a continuous and layered manner;
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 composite thermoelectric material has an interface coverage represented by the formula (2) of 50% or more.
Interface coverage = Σ (L _i × 100 / L _0i ) / n (2)
However,
L _0i is, i-th (1 ≦ i ≦ n) the field surface length of the section of the parent phase of.
L _i is the above oxide layer covering the ambient of the cross section of the i-th of said matrix, the aspect ratio is 10 or more and the length of the part thickness is 1nm or more 500nm or less total.
n is the number of the parent phases (n ≧ 5).

The manufacturing method of the composite thermoelectric material which concerns on this invention makes it a summary to have the following structures.
(A) The method for producing the composite thermoelectric material is as follows:
A powder production step of producing a matrix powder composed of a thermoelectric material and comprising one crystal grain or an aggregate of a plurality of crystal grains;
By irradiating the surface of the target made of a substance capable of generating a metal oxide having a band gap larger than that of the parent phase while rolling the parent phase powder, the periphery of the parent phase powder is surrounded by the metal. And a PLD step of continuously and layer-covering with an oxide layer made of an oxide.
(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) In the PLD process, rolling of the matrix powder and irradiation of the laser beam are performed so that the interface coverage represented by the formula (2) is 50% or more.
Interface coverage = Σ (L _i × 100 / L _0i ) / n (2)
However,
L _0i is, i-th (1 ≦ i ≦ n) the field surface length of the section of the parent phase of.
L _i is the above oxide layer covering the ambient of the cross section of the i-th of said matrix, the aspect ratio is 10 or more and the length of the part thickness is 1nm or more 500nm or less total.
n is the number of the parent phases (n ≧ 5).

When the mother phase powder composed of the MNiSn-based half-Heusler compound is put in a rotatable container and the surface of the mother phase powder is coated with an oxide layer by laser ablation, the mother phase powder is sufficiently rolled. A powder composed of a composite thermoelectric material in which the surface of the powder is coated with a thin and uniform oxide layer is obtained. When such a powder is molded and sintered, a sintered body made of a composite thermoelectric material in which the periphery of the matrix phase is coated with a thin and uniform oxide layer is obtained.
In the obtained composite thermoelectric material, the band gap of the oxide layer is larger than that of the parent phase, so that an energy barrier is formed at the interface between the two. This barrier hinders the conduction of low energy carriers and minority carriers (filter effect). As a result, the Seebeck coefficient S of the composite thermoelectric material increases without significantly reducing the electrical conductivity σ.
In addition, phonon conduction is hindered by lattice mismatch at the interface between the parent phase and the oxide layer. As a result, the thermal conductivity κ of the composite thermoelectric material is reduced.

It is a figure which shows the unit cell of AgAsMg type | mold crystal structure. It is a schematic block diagram of a pulse laser deposition (PLD) apparatus. It is a schematic block diagram of a rotary container. FIG. 4A is a schematic diagram showing the behavior of the powder when the powder is rolled using a rotary container without protrusions. FIG. 4B is a schematic diagram showing the behavior of the powder when the powder is rolled using a rotary container having protrusions. FIG. 5A is an SEM photograph of the ZrNiSn / 1 vol% ZrO ₂ sintered body obtained in Example 1 (PLD method, with protrusions). FIG. 5B is an SEM photograph of the ZrNiSn / 1 vol% ZrO ₂ sintered body obtained in Comparative Example 4 (PLD method, no projection). FIG. 6A is an SEM photograph of the ZrNiSn / 1 vol% ZrO ₂ sintered body obtained in Comparative Example 2 (solution method). FIG. 6B is an SEM photograph of the ZrNiSn / 5 vol% ZrO ₂ sintered body obtained in Comparative Example 3 (solution method). It is a figure which shows the temperature dependence of the Seebeck coefficient of the ZrNiSn type sintered compact from which a composition and preparation methods differ.

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 is
A matrix composed of a thermoelectric material and consisting of one crystal grain or an aggregate of a plurality of crystal grains,
An oxide layer comprising a metal oxide having a band gap equal to or larger than the matrix, and covering the periphery of the matrix in a continuous and layered manner;
It has.

[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.
The Y atom has four body centers on the diagonal, that is, 4c (1/4, 1/4, 1/4) sites (hereinafter referred to as “8”) among the body centers of 8 cubes composed of X atoms and Z atoms. , Simply called “Ni site”). The other body center positions, 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 preferably includes only the above-described MNiSn half-Heusler compound and an oxide layer 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 composite particles composed of a parent phase and an oxide layer and other materials (for example, resin, rubber, etc.).

Here, the “different phase” refers to a phase different from the MNiSn half-Heusler compound and the oxide layer. 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.

[1.1.7. Size of parent phase]
The parent phase may be composed of one crystal grain, or may be an aggregate of a plurality of crystal grains.
When the parent phase is composed of one crystal grain, the “size of the parent phase” refers to the equivalent circle diameter of the crystal grain. Further, when the parent phase is composed of an aggregate of a plurality of crystal grains, the “matrix phase size” refers to the equivalent circle diameter of the aggregate of crystal grains. “Equivalent circle diameter” refers to the diameter of a circle having the same area as the projected area of a crystal grain or an aggregate of crystal grains.

In general, the smaller the size of the matrix, the greater the phonon scattering effect. In order to obtain such an effect, the size of the parent phase is preferably 20 μm or less. The size of the mother phase is more preferably 1 μm or less.
On the other hand, the size of the parent phase is preferably as small as possible. However, if the size of the matrix is too small, the matrix particles aggregate and it becomes difficult to form an oxide layer uniformly on the surface of the matrix. Therefore, the size of the parent phase is preferably 0.1 μm or more.

[1.2. Oxide layer]
The oxide layer is made of a metal oxide having a band gap larger than that of the parent phase, and continuously covers the periphery of the mother phase in a layered manner.

[1.2.1. composition]
The metal oxide constituting the oxide layer is made of a material having a band gap larger than that of the parent phase. When an oxide layer having a larger band gap than the parent phase is formed on the surface of the parent phase, the Seebeck coefficient S increases due to the filter effect.
Examples of such metal oxides include rare earth oxides, ZrO ₂ , TiO ₂ , HfO ₂ , ZnO, alkaline earth oxides, SiO ₂ , Al ₂ O ₃ , Ta ₂ O ₅ , Nb ₂ O ₅ , There are NiO, MnO, WO ₃ , SnO, CoO and the like. The oxide layer may be composed of any one of these metal oxides, or may be a mixture or a solid solution of two or more metal oxides.
Among these, the metal oxide is preferably one or more selected from Al ₂ O ₃ , alkaline earth oxide, rare earth oxide, ZrO ₂ , TiO ₂ , HfO ₂ , and ZnO. This is because these oxides are less reactive with the parent phase than other oxides. When the oxide layer reacts with the parent phase, a defect level may be generated at the interface to change the carrier concentration of the parent phase, or the insulating properties of the oxide layer may be reduced, and the filter effect may not be obtained.

[1.2.2. thickness]
In general, the thinner the oxide layer formed around the matrix, the greater the filter effect and phonon scattering effect. However, if the oxide layer becomes too thin, it becomes difficult to uniformly coat the periphery of the matrix. Further, low energy carriers pass through the oxide layer, and the effect of increasing the Seebeck coefficient cannot be obtained. Therefore, the thickness of the oxide layer is preferably 1 nm or more. The thickness of the oxide layer is more preferably 5 nm or more.
On the other hand, if the thickness of the oxide layer becomes too thick, the electrical resistance between the parent phases increases, and the thermoelectric characteristics deteriorate. Therefore, the thickness of the oxide layer is preferably 500 nm or less. The thickness of the oxide layer is more preferably 100 nm or less.

Here, “the thickness of the oxide layer” refers to a value obtained by the following procedure.
(1) Photograph the cross section of the parent phase with a microscope, and connect the surface of the parent phase or the interface between the mother phases with a smooth curve on the cross-sectional photograph.
(2) A perpendicular line is drawn so as to be perpendicular to the tangent line passing through one point (contact point) on the curve and to pass through the contact point.
(3) Measure the length of the perpendicular across the oxide layer (= thickness of the oxide layer).

[1.2.3. aspect ratio]
The oxide layer must be continuously and layered around the matrix. “Coating continuously and in layers” means that the surface of the parent phase or the interface of the parent phase is two-dimensionally covered with a thin oxide layer having a certain area or more.
Therefore,
(1) granular metal oxides scattered discontinuously along the periphery of the matrix, and
(2) Of the oxide layer, a region having a thickness exceeding 500 nm,
Are not included in the “oxide layer covering the periphery of the matrix phase continuously and in layers”.
The degree of “continuous and layered” can be represented by the aspect ratio of the oxide layer covering the surface of the parent phase when the cross section of the parent phase is observed. In order to obtain high thermoelectric characteristics, the aspect ratio of the oxide layer is preferably 5 or more. The aspect ratio is more preferably 10 or more.

Here, the “aspect ratio of the oxide layer” refers to a value obtained by the following procedure.
(1) Photograph the cross section of the parent phase with a microscope, and connect the surface of the parent phase or the interface of the mother phase with a smooth curve on the cross-sectional photograph.
(2) The length of a curve (= the length L of the oxide layer) that is substantially parallel to the continuous oxide layer is measured using an image analyzer or the like. The “continuous oxide layer” is a region where metal oxides are continuously present and refers to a region from one discontinuous point (start point) to the next discontinuous point (end point). “Discontinuous point” refers to a point where the metal oxide is interrupted or a point where the thickness exceeds 500 nm.
(3) The thickness d of the oxide layer is measured at a plurality of locations (preferably 5 or more) selected at random from the continuous oxide layer, and the average value dm is calculated.
(4) L / dm (= aspect ratio) is calculated.

[1.2.3. Interface coverage]
In general, the greater the ratio of the matrix phase covered with a continuous and layered oxide layer, the greater the filter effect and phonon scattering effect. This ratio can be expressed by the interface coverage expressed by the following equation (2).
Interface coverage = Σ (L _i × 100 / L _0i ) / n (2)
However,
L _0i is, i-th (1 ≦ i ≦ n) the field surface length of the section of the parent phase of.
L _i is the above oxide layer covering the ambient of the cross section of the i-th of said matrix, the aspect ratio is 10 or more and the length of the part thickness is 1nm or more 500nm or less total.
n is the number of the parent phases (n ≧ 5).

Here, the "field surface length of the cross section of the parent phase", in the case of observing the cross-section of the matrix phase refers to the entire length of the smooth curve connecting the interface surface or matrix of the matrix phase. The interface length can be measured using an image analyzer or the like.
In order to obtain high thermoelectric properties, the interface coverage needs to be 50% or more. The interface coverage is more preferably 70% or more.

[1.2.4. Metal oxide content]
The content of metal oxide contained in the entire composite thermoelectric material and the optimum value thereof vary depending on the thickness of the oxide layer, the aspect ratio, and the interface coverage.
In general, the thinner and more uniform the oxide layer, the higher the filter effect and / or the higher phonon scattering effect with a small amount of metal oxide. However, if the metal oxide is too small, a sufficient effect cannot be obtained.
On the other hand, when the content of the metal oxide becomes excessive, the electrical specific resistance of the composite thermoelectric material increases, and on the contrary, the thermoelectric characteristics deteriorate.
The optimum metal oxide content varies depending on the composition and shape of the oxide layer, but is usually about 0.1 to 5 vol%.

[1.3. Shape of composite thermoelectric material]
The composite thermoelectric material according to the present invention may be either a powder or a sintered body. The powdery composite thermoelectric material can be used in a state where it is combined with other materials (resin, rubber, etc.), for example.

[2. PLD device]
[2.1. Constitution]
In FIG. 2, the schematic block diagram of the pulse laser deposition (PLD) apparatus used for manufacture of the composite thermoelectric material which concerns on this invention is shown.
In FIG. 2, the PLD device 10 includes a laser device 12, a reflecting mirror 14, a condenser lens 16, a rotary container 18, and a rolling motor 20.

  The laser device 12 is a laser light source for irradiating a target 22 fixed inside the rotary container 18 with laser light. The laser device 12 may be any device that can generate scattered particles from the target 22. Specifically, the laser device 12 is preferably capable of irradiating pulsed laser light having a wavelength of 150 nm to 11 μm, a pulse width of 50 fs to 1 μs, and energy per pulse: 0.01 J to 100 J. Examples of such a laser device 12 include a femtosecond laser device, an excimer laser device, and a YAG laser device.

The reflecting mirror 14 reflects the laser light emitted from the laser device 12 and guides it to the target 22. Although the reflecting mirror 14 is not necessarily required, when the reflecting mirror 14 is used, the arrangement of the laser device 12 and the rotary container 18 can be freely selected.
The condensing lens 16 is for condensing the laser light emitted from the laser device 12. The condenser lens 16 is disposed in the middle of the optical path of the laser light. The condensing lens 16 is preferably one that can make the irradiation intensity of the laser light 10 ⁹ W / cm ² or more.

The rotary container 18 is for accommodating an object to be processed (matrix phase powder) 24 therein. The rotary container 18 is rotatable and is provided with an incident window 26 for introducing laser light in the direction of the rotation axis. The entrance window 26 is made of a material that can transmit laser light (for example, quartz). A target 22 is fixed to the bottom surface of the rotary container 18.
The rolling motor 20 is for rotating the rotary container 18 and rolling the parent phase powder 24 inserted into the rotary container 18. The rolling motor 20 is preferably capable of rotating the rotary container 18 0.1 to 10 times per minute. Examples of the rolling motor 20 include a stepping motor.

FIG. 3 shows an example (partial sectional view) of the rotary container. In the example shown in FIG. 3, the rotary container 18 a includes a lower lid 32, an upper lid 34, an outer container 36, and an inner container 38.
The lower lid 32 and the upper lid 34 are for closely contacting the both ends of the outer container 36 to seal the inside of the outer container 36. Rubber packings 40 and 40 are inserted between the lower lid 32 and the upper lid 34 and the outer container 36.
A target 22 a is fixed to the inner surface of the lower lid 32.
An entrance window 26a for guiding the laser beam to the target 22a is provided in the approximate center of the upper lid 34. Further, the upper lid 34 is provided with a valve 42 for controlling the atmosphere in the outer container 36. At the tip of the valve 42, there is a vacuum pump (not shown) for evacuating the inside of the outer container 36, and an inert gas supply source (not shown) for making the inside of the outer container 36 an inert gas atmosphere. It is connected.

Both ends of the outer container 36 are open, and the target 22a can be irradiated with laser light incident from the incident window 26a, and scattered particles scattered from the target 22a can be introduced into the outer container 36. Yes.
An inner container 38 for holding an object to be processed (matrix phase powder) 24 is fixed inside the outer container 36. Both ends of the inner container 38 are open, so that the target 22a can be irradiated with laser light incident from the incident window 26a, and scattered particles scattered from the target 22a can be introduced into the inner container 38. Yes.

At both ends of the inner container 38, stoppers 38a and 38b are provided. The stoppers 38a and 38b are for making both ends of the inner container 38 higher than the center part (or forming holes having an inner diameter smaller than the inner diameter of the inner container 38 at both ends of the inner container 38). By providing the stoppers 38 a and 38 b at both ends of the inner container 38, the movement of the workpiece 24 toward the lower lid 32 and the upper lid 34 can be prevented without hindering the laser light irradiation.
The inner container 38 and the stoppers 38a and 38b are not necessarily required. However, if the container has a double structure, or if the stoppers 38a and 38b are provided, the insertion of the workpiece 24 into the container and the inside of the container Cleaning can be facilitated.

When the object to be processed 24 is inserted into the rotary container 18a and the target 22a is irradiated while rotating the rotary container 18a, scattered particles can be uniformly attached around the object 24 to be processed. .
Further, when the rotary container 18a is provided with a structure (rolling promotion means) for promoting the rolling of the workpiece 24, the scattered particles are uniformly attached around the workpiece 24 in a short time. Can be made. In this case, it is preferable that the rolling promotion means is less likely to cause the workpiece 24 to scatter in the container. When the object to be processed 24 is scattered in the container, the laser light is blocked by the object to be processed 24 that is floating, and it becomes difficult to irradiate the target 22a with the laser light.

As a method for promoting such rolling of the workpiece 24,
(1) A method of providing protrusions on the inner surface of the container (inner container 38),
(2) A method of making the inner surface structure of the container (inner container 38) into a prismatic shape,
(3) A method of increasing the friction coefficient by roughening the inner wall of the container (inner container 38),
(4) A method of giving an impact to the container when the container (inner container 38) rotates,
and so on.
Among these, the method of providing protrusions on the inner surface of the container is particularly suitable as a rolling acceleration means because the object to be processed 24 can be efficiently rolled without scattering the object to be processed 24 inside the container. is there.

The shape, number, arrangement, and the like of the protrusions are not particularly limited as long as the object to be processed 24 can be efficiently rolled.
As a protrusion, for example,
(1) Columnar, hemispherical, conical or frustum-shaped protrusions provided regularly or irregularly on the inner surface of the rotary container 18a,
(2) an elongated protrusion provided substantially parallel to the rotation axis of the rotary container 18a;
(3) an elongated protrusion provided spirally with respect to the rotation axis of the rotary container 18a,
and so on.
Further, instead of the protrusion, a rotary blade may be attached to the inner surface of the rotary container 18a.
In the present invention, the term “projection” refers to both a convex-shaped projection (positive projection) and a concave-shaped depression (negative projection, for example, a cylindrical or spike-shaped depression) unless otherwise specified. means.

The cross-sectional shape of the protrusion affects the rolling efficiency of the workpiece 24. In order to efficiently roll the object to be processed without scattering the object to be processed 24, a cross section of the protrusion (positive protrusion) when viewed from a direction parallel to the rotation axis (with respect to the rotation axis). The shape of the vertical cross section) is preferably a shape having a rising angle of 60 ° or less.
On the other hand, if the rising angle is too small, a sufficient rolling effect cannot be obtained. Accordingly, the rising angle of the positive protrusion is preferably 20 ° or more.
In the case of a negative projection, for the same reason, the rising angle is preferably −60 ° to −20 °. That is, the absolute value of the rising angle is preferably 20 to 60 °.
The “rising angle” is an angle formed by a tangent line of the inner wall surface of the container passing through the contact point between the inner wall surface of the container and the side surface of the protrusion, and a tangent line of the side surface of the protrusion passing through the contact point, for example When the cross-sectional shape of the protrusion is trapezoidal, it means the base angle). Further, the rising angle measured from the tangent to the inner wall surface of the container toward the center of the container is defined as “+”.

[2.2. How to use PLD device]
First, the workpiece 24 is placed in the rotary container 18 of the PLD apparatus 10 shown in FIG. Next, the inside of the rotary container 18 is evacuated and an inert gas is introduced as necessary. Since the MNiSn half-Heusler compound is easily oxidized, laser ablation is preferably performed in an inert gas atmosphere.
Next, while rotating the rotary container 18 using the rolling motor 20, laser light is emitted using the laser device 12 and the target 22 fixed to the bottom surface of the rotary container 18 is irradiated with the laser light.
When the target 22 is irradiated with the laser light, the material constituting the target 22 is scattered and the scattered particles are uniformly attached to the surface of the workpiece 24.

FIG. 4 shows a cross-sectional view of the rotary container 18 as viewed from the direction of the rotation axis. As shown in FIG. 4 (a), when the inner surface of the rotary container 18 is cylindrical and there are no protrusions on the inner surface, and the rotation speed is slow, the workpiece 24 is placed under the rotary container 18. It is difficult to roll just by shaking left and right. Therefore, if laser ablation is performed in this state, scattered particles adhere only to the surface layer portion of the object to be processed 24, and the scattered particles cannot be uniformly adhered around the object to be processed 24.
On the other hand, when the rotational speed of the rotary container 18 is increased, the rolling of the workpiece 24 can be promoted. However, if the rotational speed is increased too much, the object to be processed 24 scatters and floats inside the rotary container 18. For this reason, the laser beam is blocked and the laser ablation of the target cannot be performed sufficiently.

On the other hand, as shown in FIG. 4B, when the protrusion 44 (positive protrusion) is provided on the inner surface of the rotary container 18, the workpiece 24 is lifted to a predetermined height by the protrusion 44 along with the rotation. It is done. When the rotation angle of the rotary container 18 reaches a certain angle, the workpiece 24 gets over the protrusion 44 and falls downward. Therefore, even when the inner surface shape of the rotary container 18 is cylindrical or when the rotational speed of the rotary container 18 is low, the workpiece 24 can be sufficiently rolled.
Further, if the cross-sectional shape (particularly the rising angle), the arrangement, the number, etc. of the protrusions 44 are optimized, the workpiece 24 will not be lifted higher than necessary. As a result, the workpiece 24 can be efficiently rolled without being scattered. In addition, scattered particles can be uniformly attached around the object to be processed 24.

[3. Manufacturing method of composite thermoelectric material]
The manufacturing method of the composite thermoelectric material which concerns on this invention is equipped with the powder preparation process, the PLD process, and the sintering process.

[3.1. Powder production process]
The powder production step is a step of producing a parent phase powder made of a thermoelectric material and comprising one crystal grain or an aggregate of a plurality of crystal grains.

[3.1.1. Mother phase]
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.
The details of the parent phase are as described above, and thus the description thereof is omitted.

[3.1.2. Preparation method of mother phase powder]
The method for producing the matrix phase powder is not particularly limited, and various methods can be used. Specifically, as a method for producing the mother phase powder,
(1) A method of producing an ingot made of a parent phase alloy having a predetermined composition using a melting / casting method, and physically pulverizing the ingot using means such as a ball mill, a bead mill, a jet mill,
(2) A method of preparing a molten metal whose components are adjusted so as to become a matrix alloy having a predetermined composition, and rapidly solidifying the molten metal using a copper roll method, an atomizing method,
and so on.
Among these, the rapid solidification method is particularly suitable as a powder preparation method because a matrix powder having a uniform composition can be obtained.
The pulverization and rapid solidification are preferably performed so that the size of the matrix phase is 20 μm or less.

[3.2. PLD process]
In the PLD process, a laser beam is irradiated on the surface of a target made of a material capable of forming a metal oxide having a band gap equal to or larger than that of the parent phase while rolling the parent phase powder. This is a step of continuously and layer-coating with an oxide layer made of an oxide.

[3.2.1. Oxide layer]
Since the details of the oxide layer are as described above, description thereof is omitted.
[3.2.2. target]
As the target, a substance capable of generating a metal oxide having a band gap larger than that of the parent phase is used. For example, in the case where the oxide layer is a metal oxide containing one kind of metal element, a target made of a target metal oxide may be used. Further, when the oxide layer is a mixture or solid solution of two or more kinds of metal oxides, laser ablation may be alternately performed using two or more kinds of targets having different compositions.

[3.2.3. Interfacial coverage]
In the PLD process, the matrix powder is rolled and irradiated with laser light so that the interface coverage represented by the formula (2) is 50% or more.
Interface coverage = Σ (L _i × 100 / L _0i ) / n (2)
However,
L _0i is, i-th (1 ≦ i ≦ n) the field surface length of the section of the parent phase of.
L _i is the above oxide layer covering the ambient of the cross section of the i-th of said matrix, the aspect ratio is 10 or more and the length of the part thickness is 1nm or more 500nm or less total.
n is the number of the parent phases (n ≧ 5).
Since the details of the interface coverage are as described above, the description is omitted.

[3.2.4. Coating method and coating conditions]
The coating method and coating conditions for the oxide layer are not particularly limited as long as they are methods and conditions that provide a predetermined interface coverage.
The coating method is preferably a method using the above-described PLD apparatus. In particular, in the PLD process, a mother phase powder is inserted into a container provided with protrusions on the inner wall, is rotatable, and is provided with an incident window for introducing laser light in the direction of the rotation axis. It is preferable to irradiate the laser beam in the direction of the rotation axis of the container while fixing the target to the container and rotating the container. In this case, the absolute value of the rising angle of the protrusion is preferably 20 ° or more and 60 ° or less.
The details of the PLD apparatus and the details of the coating method using the same are as described above, and thus the description thereof is omitted.
In general, the longer the coating process is performed and / or the more sufficient the rolling of the matrix powder is, the easier it is to coat the periphery of the matrix phase continuously and in layers. On the other hand, if the coating treatment is performed more than necessary, the thickness of the oxide layer increases, and a sufficient effect cannot be obtained.

[3.3. Sintering process]
A sintering process is a process of shape | molding and sintering the powder obtained at the PLD process. The powder obtained in the PLD process can be used for various purposes 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 sintering a powdery composite thermoelectric material, various methods can be used for 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.
Select the optimum sintering conditions (for example, sintering temperature, sintering time, applied pressure during sintering, sintering atmosphere, etc.) according to the composition of the parent phase, the sintering method used, etc. To do.
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.

[3.4. 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.

[4. Action of Composite Thermoelectric Material and Method for Producing the Same]
When the mother phase powder composed of the MNiSn-based half-Heusler compound is put in a rotatable container and the surface of the mother phase powder is coated with an oxide layer by laser ablation, the mother phase powder is sufficiently rolled. A powder composed of a composite thermoelectric material in which the surface of the powder is coated with a thin and uniform oxide layer is obtained. In particular, when the mother phase powder is rolled using a container having protrusions on the inner wall surface, the mother phase powder can be efficiently rolled without being scattered.
In addition, when rolling and laser ablation of the matrix powder in a container with controlled atmosphere, the matrix powder can be uniformly coated with a nano-sized oxide layer without oxidizing the matrix powder. it can.
Furthermore, when such a powder is molded and sintered, a sintered body made of a composite thermoelectric material in which the periphery of the matrix phase is coated with a thin and uniform oxide layer is obtained.

In the obtained composite thermoelectric material, the band gap of the oxide layer is larger than that of the parent phase, so that an energy barrier is generated at the interface between the two. This barrier hinders the conduction of low energy carriers and minority carriers (filter effect). As a result, the Seebeck coefficient S of the composite thermoelectric material increases without significantly reducing the electrical conductivity σ.
In addition, phonon conduction is hindered by lattice mismatch at the interface between the parent phase and the oxide layer. As a result, the thermal conductivity κ of the composite thermoelectric material is reduced.

(Examples 1-2, Comparative Examples 1-5)
[1. Preparation of sample]
[1.1. Preparation of matrix phase powder]
Y, Ni, so that the total composition is ZrNiSn (Example 1, Comparative Examples 1 to 4), or (Zr _0.99 Y _0.01 ) Ni (Sn _0.99 Sb _0.01 ) (Example 2, Comparative Example 5). Sb, Zr, and Sn were weighed. This raw material was put into a boron nitride crucible and melted at high frequency to obtain an ingot.
In order to improve the composition uniformity, this ingot was redissolved and sprayed onto a copper roll rotating at 3000 rpm to rapidly cool and solidify the molten metal. The rapidly solidified product was pulverized with a mortar or a ball mill, and a matrix powder was obtained through a 20 μm mesh sieve.

[1.2. Preparation of composite powder by PLD method (Examples 1-2, Comparative Example 4)]
A powder made of a composite thermoelectric material was produced using the PLD apparatus 10 shown in FIG. As the rotary container 18, a double-structure rotary container 18a shown in FIG. 3 was used. In addition, two types of inner containers 38 were prepared, one having an inner surface provided with a protrusion 44 (Examples 1 and 2) and one without a protrusion 44 (Comparative Example 4).
After the mother phase powder was put into the inner container 38, both ends of the outer container 36 were sealed with the lower lid 32 and the upper lid 34. A target 22a made of an oxide that constitutes an oxide layer was fixed almost at the center of the lower lid 32. ZrO ₂ (Example 1, Comparative Example 4) or Y ₂ O ₃ (Example 2)) was used as the target 22a.
In order to prevent oxidation of the mother phase powder, a pump device was attached to the valve 42 and the pressure was reduced to 10 ⁻³ Pa. Thereafter, Ar gas was introduced into the rotary container 18a.

  While rotating the rotary container 18a at 10 to 50 rpm, laser light was irradiated from the incident window 26a toward the target 22a to ablate the target material. As the laser light, a triple harmonic Nd: YAG laser (wavelength 355 nm, 400 mJ) was used. The treatment time was 1 to 3 hours.

[1.3. Preparation of composite powder by solution method (Comparative Examples 2-3)]
1-10 vol% of ZrO ₂ nanoparticles (10-100 nm) were added to the powder after rapid solidification and pulverization, and these were wet mixed using ethanol as a solvent. After mixing, the solvent was removed by holding the sample at room temperature to 400 ° C. under reduced pressure to obtain half-Heusler powder coated with nanoparticles (Comparative Examples 2-3).

[1.4. Preparation of sintered body]
Powder after high-frequency dissolution and rapid solidification (high-frequency dissolution method: Comparative Examples 1 and 5), composite powder by PLD method (Examples 1-2 and Comparative Example 4), and composite powder by solution method (Comparative Examples 2-3) ) To prepare a sintered body. A discharge plasma apparatus was used for sintering. The sintering conditions were 1100 ° C. and 50 MPa.

[2. Test method]
[2.1. Tissue observation]
Using an electron microscope, structure observation and composition analysis by EDX were performed.
[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 dimensionless figure of merit ZT was calculated using the measured Seebeck coefficient S, electrical conductivity σ, and thermal conductivity κ.

[3. result]
[3.1. Organization]
FIG. 5A shows an SEM photograph of the ZrNiSn / 1 vol% ZrO ₂ sintered body obtained in Example 1 (PLD method, with protrusions). Figure 5 (b), Comparative Example 4 (PLD method, without protrusions) shows a SEM photograph of ZrNiSn / 1vol% ZrO ₂ sintered body obtained in the.
As a result of composition analysis by EDX, it was confirmed that the black portion in the figure was an oxide. In addition, the sintered body obtained in Example 1 was confirmed to have a structure in which the grain boundary portion of the parent phase was covered with an oxide layer. In the case of Example 1, the interface coverage was 56%. Although not shown, the interface coverage of Example 2 was 52%.
On the other hand, in the case of Comparative Example 4 in which PLD was performed using a container having no protrusion, the structure in which the oxide layer was coated around the matrix phase was not observed, and the oxide was present as a coarse aggregate of several μm. Was. In the case of Comparative Example 4, the interface coverage was 0%.

In a container without protrusions, the powder tends to slip when the container rotates. Therefore, when the conditions are not appropriate, the surface of the powder exposed to the ablation particles is unlikely to change, and the target material is likely to be thickly deposited on a specific surface.
On the other hand, in a container having protrusions, when the powder moves over the protrusions by rotation, a new uncoated surface is likely to be exposed to the ablation particles. Therefore, it is considered that a more uniform coating was possible.

FIG. 6A shows an SEM photograph of the ZrNiSn / 1 vol% ZrO ₂ sintered body obtained in Comparative Example 2 (solution method). FIG. 6B shows an SEM photograph of the ZrNiSn / 5 vol% ZrO ₂ sintered body obtained in Comparative Example 3 (solution method).
In the case of the comparative example 2 produced by the solution method, the structure which the oxide segregated to the grain boundary part can be confirmed. However, the oxide adhered as particles and was not a continuous thin film. In the case of Comparative Example 2, the interface coverage was 0%.
The same applies to Comparative Example 3 in which the amount of oxide was increased to 5 vol%, and the nanoparticles were segregated at the grain boundaries in an aggregated state, and a uniform oxide film could not be obtained. In the case of Comparative Example 3, the interface coverage was 0%.

[3.2. Thermoelectric characteristics]
In FIG. 7, Seebeck coefficient S of the sintered compact obtained in Examples 1-2 and Comparative Examples 1, 2, and 5 is shown. Table 2 shows the thermal conductivity κ and dimensionless figure of merit ZT of each sample.
The thermal conductivity κ tended to decrease with compositing (Comparative Example 1 → Example 1, Comparative Example 1 → Comparative Example 2, Comparative Example 5 → Example 2).

However, the Seebeck coefficient S differs depending on the manufacturing method. As a result, the dimensionless figure of merit changed even with the same composition.
First, compared to Comparative Example 1 that was not composited, Comparative Example 2 that was composited by the solution method had a significant decrease in the Seebeck coefficient S due to the reaction between the oxide and the parent phase alloy.
On the other hand, the decrease in Seebeck coefficient S was also observed in Example 1 combined with the PLD method, but the decrease rate was suppressed as compared with Comparative Example 2. Furthermore, when the parent phase alloy composition is (Zr _0.99 Y _0.01 ) Ni (Sn _0.99 Sb _0.01 ), the Seebeck coefficient S is lower in Example 2 combined by the PLD method than in Comparative Example 5 that is not combined. There wasn't.

In general, the Seebeck coefficient S varies depending on the carrier concentration. In this case, the decrease in the Seebeck coefficient S results from an increase in the carrier concentration accompanying the reaction between the parent phase and the oxide. The reason why the decrease rate of the Seebeck coefficient S is suppressed in Examples 1 and 2 is presumed that the filter effect is added to the effect accompanying the increase in the carrier concentration.
Usually, when a thermoelectric material is put into practical use, the carrier concentration is optimized. Table 2 also shows the dimensionless figure of merit ZT when the carrier concentration is optimized. The optimum carrier concentration was determined by Jonker-plot from the measured values of electrical conductivity and Seebeck coefficient. From Table 2, it can be seen that the difference (ΔZT) in the dimensionless figure of merit ZT before and after optimization of the carrier concentration is larger when the PLD method is used than when the solution method is used.

  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 a thermoelectric material and consisting of one crystal grain or an aggregate of a plurality of crystal grains,
An oxide layer comprising a metal oxide having a band gap equal to or larger than the matrix, and covering the periphery of the matrix in a continuous and layered manner;
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 composite thermoelectric material has an interface coverage represented by the formula (2) of 50% or more.
Interface coverage = Σ (L _i × 100 / L _0i ) / n (2)
However,
L _0i is, i-th (1 ≦ i ≦ n) the field surface length of the section of the parent phase of.
L _i is the above oxide layer covering the ambient of the cross section of the i-th of said matrix, the aspect ratio is 10 or more and the length of the part thickness is 1nm or more 500nm or less total.
n is the number of the parent phases (n ≧ 5). _2. The composite according to claim 1, wherein the oxide layer includes at least one selected from Al ₂ O ₃ , alkaline earth oxide, rare earth oxide, ZrO ₂ , TiO ₂ , HfO ₂ , and ZnO. Thermoelectric material.   The composite thermoelectric material according to claim 1 or 2, wherein a size of the matrix phase is 20 µm or less.   The composite thermoelectric material according to any one of claims 1 to 3, wherein the composite thermoelectric material is a powder or a sintered body. The manufacturing method of the composite thermoelectric material provided with the following structures.
(A) The method for producing the composite thermoelectric material is as follows:
A powder production step of producing a matrix powder composed of a thermoelectric material and comprising one crystal grain or an aggregate of a plurality of crystal grains;
By irradiating the surface of the target made of a substance capable of generating a metal oxide having a band gap larger than that of the parent phase while rolling the parent phase powder, the periphery of the parent phase powder is surrounded by the metal. And a PLD step of continuously and layer-covering with an oxide layer made of an oxide.
(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) In the PLD process, rolling of the matrix powder and irradiation of the laser beam are performed so that the interface coverage represented by the formula (2) is 50% or more.
Interface coverage = Σ (L _i × 100 / L _0i ) / n (2)
However,
L _0i is, i-th (1 ≦ i ≦ n) the field surface length of the section of the parent phase of.
L _i is the above oxide layer covering the ambient of the cross section of the i-th of said matrix, the aspect ratio is 10 or more and the length of the part thickness is 1nm or more 500nm or less total.
n is the number of the parent phases (n ≧ 5).   In the PLD process, the matrix phase powder is inserted into a container provided with a protrusion on an inner wall, is rotatable, and is provided with an incident window for introducing the laser beam in the direction of the rotation axis. The method of manufacturing a composite thermoelectric material according to claim 5, wherein the target is fixed to the bottom surface of the substrate and the laser beam is irradiated in the rotation axis direction of the container while rotating the container.   The method for producing a composite thermoelectric material according to claim 6, wherein an absolute value of a rising angle of the protrusion is 20 ° or more and 60 ° or less.   The manufacturing method of the composite thermoelectric material in any one of Claim 5-7 further equipped with the sintering process which shape | molds the powder obtained by the said PLD process, and sinters it.

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