JP2011054850A - Thermoelectric material and manufacturing method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a thermoelectric material that exhibits relatively high thermoelectric characteristics in the temperature range of 100 to 600 K, and additionaly, has high-level environmental harmoniousness, and is low-cost, and to provide a manufacturing method for the material. <P>SOLUTION: Presented herein are a thermoelectric material which has an orthorhombic TiSi<SB>2</SB>type structure and contains an MnAlSi-system compound expressed by the formula (1) and a manufacturing method thereof: (Mn<SB>1-x</SB>A<SB>x</SB>)(Al<SB>1-y</SB>Si<SB>y</SB>)<SB>2(t-z)</SB>B<SB>2z</SB>(1) where 0≤x≤0.2, 0.45≤y≤0.55, 0≤z≤0.1, 0.94≤t≤1.06, and A and B are metallic elements of one or two kinds or more, respectively (excluding alkali metals and alkaline-earth metals). <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

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

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

As an index for evaluating the characteristics of 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.

Among thermoelectric materials, Bi ₂ Te ₃ series thermoelectric materials are known to exhibit a high dimensionless figure of merit ZT in the temperature range of 100 to 600K. However, the Bi ₂ Te ₃ series thermoelectric material has a problem that Te is a rare element and an element having a large environmental load, so that the cost is high and the environmental harmony is low. Therefore, a thermoelectric material having high thermoelectric characteristics and not containing a rare element and an environmental load element is desired.

Various proposals have been made for materials exhibiting high thermoelectric characteristics in the temperature range of 100 to 600K.
For example, Non-Patent Document 1 discloses a thermoelectric material made of RuAl ₂ having a TiSi ₂ type structure.
This document describes that RuAl ₂ exhibits an output factor of ² mW / mK ² and a thermal conductivity of 20 W / mK near room temperature.

In Non-Patent Document 2, although not a thermoelectric material, the structure of an Al—Mn—Si ternary alloy at 550 ° C. or 700 ° C. is examined over the entire composition range.
In the same document,
(1) Al-Mn-Si ternary alloy has 10 kinds of stable ternary phases (τ _{1 to} τ ₁₀ ), and
(2) τ ₃ has an Al ₃₄ Mn ₃₄ Si ₃₂ composition and has a TiSi ₂ type crystal structure,
Is described.

Patent Document 1 discloses a thermoelectric material made of a quasicrystal of Al ₇₄ Mn ₂₀ Si ₆ , Al ₇₂ Mn ₂₀ Si ₈ , Al ₅₅ Mn ₂₅ Si ₂₀ , or Al ₅₃ Mn ₂₀ Si ₂₇ .
In this document, the Seebeck coefficient α of these materials is 70 to 72 (μV / K), the thermal conductivity κ is 1.1 to 1.3 (W / mK), and the figure of merit Z is 1. The point which is 9-2.5 * 10 < ^-3 > (1 / K) is described.

Further, Patent Document 2 discloses a thermoelectric device comprising a composite of Al ₇₄ Mn ₂₀ Si ₆ , Al ₇₂ Mn ₂₀ Si ₈ , Al ₅₅ Mn ₂₅ Si ₂₀ , or Al ₅₃ Mn ₂₀ Si ₂₇ and a composite of Al. A material is disclosed.
In the same document, the absolute value | α | of the Seebeck coefficient at room temperature of a composite of 3 to 39% by weight of Al and these quasicrystals is 4 to 70 (μV / K), and the specific resistance ρ at room temperature is It is 0.05 to 0.8 (μΩm), and the performance index Z at room temperature is 0.01 to 3.1 × 10 ⁻³ (1 / K).

JP-A-8-181357 JP-A-8-178758

Hiroaki Hamada, Journal of the Thermoelectric Society of Japan, Vol.5, No.3, 13 (2009) N. Krdelsberger et al., Metall. Mater. Trans. 33A, 3311 (2002)

RuAl ₂ does not contain an environmental load element and exhibits a relatively high output factor near room temperature. However, RuAl ₂ is difficult to put into practical use because Ru is expensive. Moreover, since the thermal conductivity is large, a practical figure of merit Z is not obtained.
In addition, the Al-rich Al—Mn—Si compound and the composite of Al and Mn—Si have low thermoelectric properties.

  The problem to be solved by the present invention is to provide a thermoelectric material that exhibits relatively high thermoelectric characteristics in a temperature range of 100 K to 600 K, has high environmental harmony, and is low in cost, and a method for manufacturing the thermoelectric material. is there.

In order to solve the above problems, a thermoelectric material according to the present invention has an orthorhombic TiSi ₂ type structure and includes a MnAlSi compound represented by the formula (1).
(Mn _1-x A _x ) (Al _1-y Si _y ) _{2 (tz)} B _2z (1)
However,
0 ≦ x ≦ 0.2, 0.45 ≦ y ≦ 0.55, 0 ≦ z ≦ 0.1,
0.94 ≦ t ≦ 1.06,
A and B are each one or more metal elements (excluding alkali metals and alkaline earth metals).

The first of the methods for producing a thermoelectric material according to the present invention is:
A melting and casting process for melting and casting the raw materials blended so as to be the MnAlSi-based compound according to the present invention;
The ingot obtained in the melting and casting process is heated at a temperature of 500 ° C. or higher and 1000 ° C. or lower in a vacuum atmosphere or an inert gas to generate the MnAlSi compound having the orthorhombic TiSi ₂ type structure. And an annealing step.

Furthermore, the second method for producing a thermoelectric material according to the present invention is as follows.
A rapid cooling step of rapidly solidifying a molten metal obtained by dissolving a raw material blended to be a MnAlSi-based compound according to the present invention;
The MnAlSi having the orthorhombic TiSi ₂ type structure is obtained by pressure-sintering the rapidly solidified powder obtained in the quenching step at a temperature of 500 ° C. to 1000 ° C. in a vacuum atmosphere or an inert gas. And a sintering step of obtaining a sintered body containing a system compound.

In order to realize a high-performance thermoelectric material, it is necessary to achieve both a large Seebeck coefficient S and a high electrical conductivity σ. Both of them depend greatly on the electronic state in the vicinity of the Fermi surface. The former requires a large energy dependency of the density of states, and the latter requires a large bandwidth. An MnAlSi compound having an Mn: Al: Si atomic ratio of approximately 1: 1: 1 has an electronic state that combines these two elements. That is, the d orbitals of Mn and the p orbitals of Al and Si coexist in the vicinity of the Fermi surface, and a large change in state density due to d electrons and a large bandwidth due to p orbitals are compatible. It is considered that high-performance thermoelectric characteristics are realized from the characteristics of the electronic state.
Moreover, the position of the Fermi surface can be optimized so that PF = σS ² is maximized by various dopings. Furthermore, by replacing the elements with different masses, the thermal conductivity κ is reduced and the figure of merit Z = PF / κ is improved.

Sample No. 1 to 3 and A X-ray diffraction patterns. Sample No. 4 is an X-ray diffraction pattern of 4 to 6 and B. It is a figure which shows the temperature dependence of Seebeck coefficient S (FIG. 3 (A)) of the sample A, specific resistance (rho) (FIG. 3 (B)), and thermal conductivity (kappa) (FIG. 3 (C)). It is a figure which shows the temperature dependence of Seebeck coefficient S (FIG. 4 (A)) of the sample B, specific resistance (rho) (FIG. 4 (B)), and thermal conductivity (kappa) (FIG. 4 (C)). Sample No. 7 to 10 X-ray diffraction patterns. Sample No. It is a figure which shows the temperature dependence of specific resistance (rho) of 9 and 10 (FIG. 6 upper figure), Seebeck coefficient S (middle figure of FIG. 6), and output factor PF (FIG. 6 lower figure). Illustrates formation energy (FE) when the substitution of one of the Mn atoms in the element X of Mn ₈ Al ₈ Si _8. Formation energy (FE) when one Al atom or one Si atom of Mn ₈ Al ₈ Si ₈ is replaced by element X, and formation energy when Al or Si of MnAlSi is replaced by element X ( It is a figure which shows FE.

Hereinafter, an embodiment of the present invention will be described in detail.
[1. Thermoelectric material]
The thermoelectric material according to the present invention includes a MnAlSi compound having an orthorhombic TiSi ₂ type structure and having a predetermined composition.

[1.1. Crystal structure]
Among MnAlSi compounds having an Mn: Al: Si atomic ratio of approximately 1: 1: 1, those having an orthorhombic TiSi ₂ type structure are low-temperature phases, and these low-temperature phases have high thermoelectric properties. On the other hand, a high temperature phase also exists in the MnAlSi compound. Details of the crystal structure and composition of the high temperature phase are unknown, but the high temperature phase has metallic properties. Therefore, the MnAlSi compound constituting the thermoelectric material is preferably as the low-temperature phase content is high, and is preferably substantially composed of only the low-temperature phase. In order to obtain high thermoelectric properties, the proportion of the low temperature phase in the MnAlSi compound is preferably 90 vol% or more. The ratio of the low temperature phase is more preferably 95 vol% or more, and more preferably 99 vol% or more.

[1.2. composition]
In the present invention, the MnAlSi compound has a composition represented by the formula (1). The MnAlSi compound is based on a compound having an equiatomic ratio (Mn: Al: Si = 1: 1: 1), but the ratio of atoms occupying each site may be slightly deviated from the equiatomic ratio.
(Mn _1-x A _x ) (Al _1-y Si _y ) _{2 (tz)} B _2z (1)
However,
0 ≦ x ≦ 0.2, 0.45 ≦ y ≦ 0.55, 0 ≦ z ≦ 0.1,
0.94 ≦ t ≦ 1.06,
A and B are each one or more metal elements (excluding alkali metals and alkaline earth metals).

  (1) In formula, x represents the atomic ratio of the element A which substitutes a Mn site. Although doping of the element A to the Mn site is not always necessary, optimizing the kind of the element A improves the thermoelectric characteristics as compared with the case of no doping. On the other hand, excessive doping reduces the thermoelectric properties. Therefore, x is preferably 0 or more and 0.2 or less.

In the formula (1), y represents the atomic ratio of Si to (Al + Si). The MnAlSi compound is based on Al: Si = 1: 1 (y = 0.5), but the Al: Si ratio may be slightly deviated from the equiatomic ratio.
In both cases where y is too small and too large, either Al or Si becomes excessive, which causes a heterogeneous phase. Therefore, y is preferably 0.45 or more and 0.55 or less.

In the formula (1), t represents an atomic ratio between an atom occupying a Mn site and an element occupying an Al site + Si site. The MnAlSi compound is based on Mn: (Al + Si) = 1: 2 (t = 1), but the Mn: (Al + Si) ratio may be slightly different from 1: 2.
In both cases where t is too small and too large, either Mn or (Al + Si) becomes excessive, causing a heterogeneous phase. Therefore, t is preferably 0.96 or more and 1.06 or less.

  (1) In formula, z represents the atomic ratio of the element B which substitutes an Al site or a Si site. Although doping of the element B to the Al site or Si site is not necessarily required, optimizing the type of the element B improves the thermoelectric characteristics compared to the case of no doping. On the other hand, excessive doping reduces the thermoelectric properties. Therefore, z is preferably 0 or more and 0.1 or less.

(1) In formula, A represents the element which substitutes a Mn site. Element A consists of a metal element (however, excluding alkali metals and alkaline earth metals). The Mn site may be substituted with any one of these elements A, or may be substituted with two or more elements A.
In particular, the element A is preferably at least one element selected from Cr, Fe, Co, Mo, Ru, Rh, W, Re, Os, and Ir. Since these elements are surely introduced into the Mn site, they are particularly suitable as the element A.

(1) In formula, B represents the element which substitutes an Al site or Si site. The element B is composed of a metal element (excluding alkali metals and alkaline earth metals). One of the Al site and the Si site may be substituted with the element B, or both may be substituted with the element B. Moreover, the Al site and / or the Si site may be substituted by any one of these elements B, or may be substituted by two or more kinds of elements B.
In particular, the element B is preferably at least one element selected from Sc, Ti, Zn, Ga, Ge, Al, and Si. Since these elements are surely introduced into the Al site or Si site, they are particularly suitable as the element A.

Here, in the present invention, the term “metal element” includes so-called metalloid elements such as Si and Ge. That is, the “metal element” refers to a transition metal element or a typical metal element (excluding an alkali metal element and an alkaline earth metal element).
In the present invention, the “transition metal element” refers to a Group 3 to Group 11 element (Sc to Cu, Y to Ag, La to Au, Ac to Rg).
“Typical metal element” refers to a metal element or a semi-metal element belonging to Group 12 to Group 18.

[1.3. impurities]
The thermoelectric material according to the present invention is preferably composed of only the above-described MnAlSn-based compound having the TiSi ₂ type crystal structure, but may contain inevitable impurities (for example, heterogeneous phase). However, it is preferable that the number of heterogeneous phases that adversely affect thermoelectric properties is small.
Furthermore, the thermoelectric material according to the present embodiment may be a composite of the above-described MnAlSn-based compound and other materials (for example, resin, rubber, etc.).

[2. Thermoelectric Material Manufacturing Method (1)]
The method for manufacturing a thermoelectric material according to the first embodiment of the present invention includes a melting casting process and an annealing process.

[2.1. Melting and casting process]
The melt casting step is a step of melting and casting the raw materials blended so as to be the MnAlSi compound according to the present invention.
As the raw material, pure metal may be used, or an alloy containing two or more constituent elements may be used. Moreover, the MnAlSi type compound which passed through processes, such as melt | dissolution, casting, and sintering, or its scrap can also be used for a raw material. These raw materials are blended so as to obtain a MnAlSi compound having a desired composition.
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 and a high frequency melting method. In addition, it is preferable to dissolve the raw material in an inert atmosphere in order to prevent oxidation.
The method for casting the molten metal is not particularly limited, and various methods (for example, a mold casting method, a sand mold casting method, etc.) can be used.

[2.2. Annealing process]
In the annealing process, the ingot obtained in the melt casting process is heated in a vacuum atmosphere or in an inert gas at a temperature of 500 ° C. or higher and 1000 ° C. or lower to produce an MnAlSi compound having an orthorhombic TiSi ₂ type structure. It is a process to make.
As described above, the MnAlSi compound has a low temperature phase and a high temperature phase. Therefore, the high temperature phase may remain in a part of the ingot simply by melting and casting. Since the high temperature phase is metallic, the larger the ratio of the high temperature phase, the lower the thermoelectric properties. The annealing process is performed to transform the high temperature phase remaining in the ingot to the low temperature phase.

In general, if the annealing temperature is too low, it becomes difficult to transform the high temperature phase into the low temperature phase within a practical time. Therefore, the annealing temperature is preferably 500 ° C. or higher.
On the other hand, if the annealing temperature is too high, the ratio of the high temperature phase increases. Accordingly, the annealing temperature is preferably 1000 ° C. or lower.
Select the optimal annealing time according to the annealing temperature. In general, the higher the annealing temperature, the faster the high temperature phase can be transformed into the low temperature phase.
Further, the annealing is preferably performed in a vacuum atmosphere or in an inert gas in order to suppress oxidation of the sample and composition change resulting therefrom.
The optimum annealing conditions vary depending on the raw material composition and the heat history of the ingot. Therefore, it is preferable to select optimum annealing conditions according to the raw material composition and thermal history so that the ingot becomes a low-temperature single phase.

[3. Thermoelectric Material Manufacturing Method (2)]
The method for manufacturing a thermoelectric material according to the second embodiment of the present invention includes a rapid cooling process, a sintering process, and an annealing process.

[3.1. Rapid cooling process]
The rapid cooling step is a step of rapidly solidifying a molten metal obtained by melting a raw material blended so as to be the MnAlSi compound according to the present invention.
As a raw material used in the rapid cooling step, pure metal may be used, or an alloy containing two or more constituent elements may be used. Moreover, the MnAlSi type compound which passed through processes, such as melt | dissolution, casting, and sintering, or its scrap can also be used for a raw material. These raw materials are blended so as to obtain a MnAlSi compound having a desired composition.

Rapid solidification is performed by spraying or dripping molten metal onto a cooling medium using a nozzle. The material of the nozzle used in the rapid solidification is not particularly limited, and various materials can be used. As the nozzle, a quartz nozzle is generally used, but a boron nitride nozzle may be 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 the molten metal melted in the nozzle is sprayed or dripped onto a rotating copper roll (cooling medium),
(2) A method in which the molten metal melted in the nozzle is sprayed or dripped from the nozzle hole, a jet fluid is blown from the surroundings to the flow of the molten metal, and the generated droplet is solidified while dropping (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.

When a boron nitride nozzle is used as the nozzle, the nozzle may be used as it is, but before the raw material is dissolved, heat treatment is performed at 600 ° C. or higher in an inert gas atmosphere (for example, Ar, N _2, etc.) in advance. It is preferable to use the above. Immediately after production, boron nitride adsorbs gas and moisture, but if this is heat-treated under specified conditions, the adsorbed gas and adsorbed water are removed, so that the contamination of impurities (especially O) is minimized. Can be suppressed.
The cooling rate during the rapid cooling is preferably 100 ° C./sec or more. When the cooling rate is less than 100 ° C./sec, the component elements are segregated and a uniform solid solution may not be obtained. In order to obtain a uniform solid solution, the faster the cooling rate, the better.

[3.2. Sintering process]
In the sintering process, the rapidly solidified powder obtained in the rapid cooling process is pressure-sintered at a temperature of 500 ° C. or higher and 1000 ° C. or lower in a vacuum atmosphere or in an inert gas to form an orthorhombic TiSi ₂ type structure. This is a step of obtaining a sintered body containing the MnAlSi-based compound.
The rapidly solidified product is pulverized as necessary to obtain an appropriate particle size. This powder is filled as it is or a compact of this powder is filled in a mold, and pressure sintering is performed.
The pressure sintering method is not particularly limited, and various methods can be used. Specific examples of the pressure sintering method include hot pressing, HIP, and discharge plasma sintering (SPS). Among these, the discharge plasma sintering method is particularly suitable as a sintering method because a dense sintered body can be obtained in a short time.

When the molten metal is rapidly solidified, a low temperature phase comprising fine crystal grains is obtained although it contains a high temperature phase. When this is sintered at an appropriate temperature, the sintered body is densified and at the same time a transformation from a high temperature phase to a low temperature phase occurs.
Generally, when the sintering temperature is too low, a dense sintered body cannot be obtained within a practical time. Therefore, the sintering temperature is preferably 500 ° C. or higher.
On the other hand, if the sintering temperature is too high, the low temperature phase generated in the sintering process is transformed again into the high temperature phase. Therefore, the sintering temperature is preferably 1000 ° C. or less.

Select the optimum sintering time according to the sintering temperature. Generally, as the sintering temperature increases, a dense sintered body can be obtained in a short time.
In addition, the sintering is preferably performed in a vacuum atmosphere or an inert gas in order to suppress the oxidation of the sample and the composition change resulting therefrom.
Furthermore, the optimum pressure for sintering is selected according to the composition, the sintering method used, and the like. Generally, as the pressure during sintering increases, a dense sintered body can be obtained under low temperature and short time conditions. On the other hand, more pressure than necessary has no real benefit. For example, when the discharge plasma sintering method is used, the applied pressure is preferably 20 MPa or more.
The optimum sintering condition varies depending on the raw material composition. Therefore, it is preferable to select optimum annealing conditions according to the raw material composition and thermal history so that a dense sintered body can be obtained.

[3.3. Annealing process]
In the annealing process, the sintered body obtained in the sintering process is heated in a vacuum atmosphere or in an inert gas at a temperature of 500 ° C. or higher and 1000 ° C. or lower to obtain an MnAlSi compound having an orthorhombic TiSi ₂ type structure. It is a process of generating.
If the sintering conditions are appropriate, the transformation from the high temperature phase to the low temperature phase occurs simultaneously with the sintering in the sintering process. Therefore, when the sintering conditions are appropriate, the annealing step is not always necessary.
However, depending on the sintering conditions, the high-temperature phase and strain generated during rapid solidification may remain as they are, or the low-temperature phase generated in the initial stage of sintering may transform again into the high-temperature phase in the later stage of sintering. is there. In such a case, it is preferable to anneal the sintered body to transform the high temperature phase in the sintered body into a low temperature phase.
The details of the annealing process are the same as those in the first embodiment, and thus the description thereof is omitted.

[4. Operation of thermoelectric material and manufacturing method thereof]
In order to realize a high-performance thermoelectric material, it is necessary to achieve both a large Seebeck coefficient S and a high electrical conductivity σ. Both of them depend greatly on the electronic state in the vicinity of the Fermi surface. The former requires a large energy dependency of the density of states, and the latter requires a large bandwidth. The MnAlSi-based compound has an electronic state that combines these two elements. That is, the d orbitals of Mn and the p orbitals of Al and Si coexist in the vicinity of the Fermi surface, and a large change in state density due to d electrons and a large bandwidth due to p orbitals are compatible. It is considered that high-performance thermoelectric characteristics are realized from the characteristics of the electronic state.
Moreover, the position of the Fermi surface can be optimized so that PF = σS ² is maximized by various dopings. Furthermore, by replacing the elements with different masses, the thermal conductivity κ is reduced and the figure of merit Z = PF / κ is improved.

Example 1
[1. Sample Preparation (Sample Nos. 1-6, A, B)]
The raw materials were weighed according to the composition ratio and packed in a BN crucible. This was melted by high frequency melting in an Ar atmosphere and solidified in a BN crucible. The obtained ingot was vacuum-sealed in a glass tube and annealed at 800 ° C. for 24 hours. Table 1 shows the composition ratio of each sample and the parameters x, y, z, and t of the general formula (formula (1)). Table 1 shows sample No. described later. The composition ratios and parameters of 7 to 10 and C to G were also shown.

[2. Test method]
[2.1. XRD]
The crystal phase and structure of the sample were evaluated by an X-ray diffraction (XRD) apparatus.
[2.2. Thermoelectric characteristics]
The sample was processed into a rectangle, and thermoelectric properties of 600K or less were evaluated. The thermoelectric characteristics in the low temperature region were evaluated by Quantum Design, Inc., a thermal transport measuring device (TTO), and the thermoelectric properties in the high temperature region were evaluated by a Seebeck effect measuring device (MMR) manufactured by MMR.

[3. result]
[3.1. XRD]
1 and 2 show the XRD measurement results of the annealed sample. From these results, sample no. In the compositions of 2, 3, 4, A, and B, a compound close to a MnAlSi single phase was obtained. Sample No. When away from the composition of A, heterogeneous components as indicated by ↓ increased.
[3.2. Thermoelectric characteristics]
Sample No. near MnAlSi single phase. Regarding A and B, thermoelectric properties were evaluated. 3 and FIG. 4, sample No. The thermoelectric characteristics of A and B are shown. Sample No. In A, the Seebeck coefficient S showed a negative n-type. Sample No. In B, the Seebeck coefficient was positive p-type. In both cases, the absolute value of the Seebeck coefficient S was as large as 300 μV / K or more in the vicinity of 400K. On the other hand, the specific resistance ρ was 10 to 20 mΩcm. The thermal conductivity was 10 W / K / m near room temperature, about half that of RuAl ₂ of the prior art.

(Example 2)
[1. Preparation of sample (sample Nos. 7 to 10)]
The raw material is sample no. It weighed according to the composition ratio of A and packed in a BN crucible. The raw material was melted by high-frequency melting in an Ar atmosphere and solidified in a BN crucible (Sample No. 7). The obtained ingot was again melted in a BN crucible with a nozzle by high-frequency melting in an Ar atmosphere, and rapidly solidified by a single roll method (sample No. 8). The recovered material was sintered by a SPS sintering apparatus under the conditions of 800 ° C. × 15 minutes × 50 MPa (sample No. 9). Further, the sample was vacuum sealed in a glass tube and annealed at 800 ° C. for 24 hours (Sample 10). Table 1 shows the composition of each sample and the parameters of the general formula.

[2. Test method]
[2.1. XRD]
The crystal phase and structure of the sample were evaluated by an X-ray diffraction (XRD) apparatus.
[2.2. Thermoelectric characteristics]
The sample was processed into a rectangular shape, and the thermoelectric characteristics at 100 ° C. (373 K) to 300 ° C. (573 K) were evaluated.

[3. result]
[3.1. XRD]
In FIG. The XRD measurement result of 7-10 is shown. Sample No. melted and cast 7 and sample No. 8 was an XRD pattern different from MnAlSi having a TiSi ₂ type structure. Probably the high temperature phase. Sample No. after SPS sintering 9 and the sample No. after annealing. No. 10, an XRD pattern almost similar to a MnAlSi single phase was obtained.
[3.2. Thermoelectric characteristics]
Sample No. close to single phase. 9 and sample no. 10 was evaluated for thermoelectric properties. FIG. 6 shows the result. Sample No. 1 of Example 1 Like A, the Seebeck coefficient S showed a negative n-type. In addition, the absolute value of the Seebeck coefficient S was as large as 250 μV / K in the vicinity of 373K. On the other hand, the specific resistance ρ was 7 mΩcm.
From the above results, it was found that the sample produced by the method according to the present invention showed good thermoelectric properties.

(Example 3)
[1. First-principles calculation (Sample Nos. C to G)]
The formation energy at the time of element substitution at each site of Mn site, Al site, and Si site was evaluated by first principle calculation. The smaller the formation energy, the more stable it is, and it is 0 or less, indicating that substitution can exist even in the ground state. Although depending on the process conditions, if the formation energy is 1 eV or less, the element substitution is considered possible.
Table 1 shows the composition of each sample and the parameters of the general formula.

[2. result]
FIG. 7 shows the result of 12.5 at% element substitution at the Mn site.
From FIG.
(1) The formation energy of these elements (Cr, Fe, Co, Mo, Ru, Rh, W, Re, Os, Ir) is as low as 1 eV or less, and element substitution to Mn sites is possible.
(2) In order to introduce (n-type) electron carriers, Ru, Rh, Os, Ir, Fe, and Co are effective as substitution elements.
(3) To introduce hole carriers (p-type), Mo, Cr, and W are effective as substitution elements.
(4) As a group element for reducing the thermal conductivity κ, Re is effective as a substitution element.
I understand that.

FIG. 8 shows the result of element substitution at 12.5 at% for the Al site (●: sample No. D) and the result of substitution of 12.5 at% element for the Si site (■: sample No. E).
From FIG.
(1) Sc, Zn, and Ga are suitable as substitution elements for the Al site.
(2) Ga, Ti, and Ge are suitable as substitution elements for the Si site.
(3) In order to introduce (p-type) hole carriers, Zn is effective as a substitution element for the Al site, and Ga is effective as a substitution element for the Si site.
(4) As a cognate element for reducing the thermal conductivity κ, Sc and Ga are effective as substitution elements for Al sites, and Ti and Ge are effective as substitution elements for Si sites.
I understand that.

Further, FIG. 8 shows the result of substituting 100 at% element for the Al site (◯: sample No. F) and the result of substituting 100 at% element for the Si site (□: sample No. G).
As shown in FIG. 8, when the Al site is doped with Sc, Zn or Ga, and when the Si site is doped with Ge, the difference in formation energy between the 100 at% element substitution and the 12.5 at% element substitution. It turns out that there is not much. For this reason, it is considered that high-concentration substitution is possible when doping them.

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

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

Claims (5)

A thermoelectric material having an orthorhombic TiSi ₂ type structure and containing a MnAlSi-based compound represented by the formula (1).
(Mn _1-x A _x ) (Al _1-y Si _y ) _{2 (tz)} B _2z (1)
However,
0 ≦ x ≦ 0.2, 0.45 ≦ y ≦ 0.55, 0 ≦ z ≦ 0.1,
0.94 ≦ t ≦ 1.06,
A and B are each one or more metal elements (excluding alkali metals and alkaline earth metals). A is at least one element selected from Cr, Fe, Co, Mo, Ru, Rh, W, Re, Os, and Ir;
2. The thermoelectric material according to claim 1, wherein B is at least one element selected from Sc, Ti, Zn, Ga, Ge, Al, and Si. A melting and casting step of melting and casting a raw material blended so as to be the MnAlSi compound according to claim 1 or 2;
The ingot obtained in the melting and casting process is heated at a temperature of 500 ° C. or higher and 1000 ° C. or lower in a vacuum atmosphere or an inert gas to generate the MnAlSi compound having the orthorhombic TiSi ₂ type structure. A method of manufacturing a thermoelectric material comprising an annealing step. A rapid cooling step for rapidly solidifying a molten metal obtained by dissolving a raw material blended so as to be the MnAlSi-based compound according to claim 1 or 2;
The MnAlSi having the orthorhombic TiSi ₂ type structure is obtained by pressure-sintering the rapidly solidified powder obtained in the quenching step at a temperature of 500 ° C. to 1000 ° C. in a vacuum atmosphere or an inert gas. The manufacturing method of the thermoelectric material provided with the sintering process of obtaining the sintered compact containing a system compound. The sintered body obtained in the sintering step is heated in a vacuum atmosphere or in an inert gas at a temperature of 500 ° C. or higher and 1000 ° C. or lower to produce the MnAlSi compound having the orthorhombic TiSi ₂ type structure. The method of manufacturing a thermoelectric material according to claim 4, further comprising an annealing step.

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