JP2015216280A - Thermoelectric conversion material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To put a thermoelectric conversion material into practical use in a temperature region of 300-600 K; and to provide a thermoelectric conversion material which has a large Seebeck coefficient in the temperature region.SOLUTION: A thermoelectric conversion material has a Heusler alloy crystal structure, and comprises, in regard to a basic structure of FeVAl of which the total number of valence electrons per chemical formula is 24, a composition satisfying the following general formula: FeVTaAl, provided that the thermoelectric conversion material is controlled to be p-type when -0.10≤x≤-0.03 and 0.08≤y≤0.12, and n-type when 0.03≤x≤0.10 and 0.08≤y≤0.12.

Description

  The present invention relates to a thermoelectric conversion material.

  Thermoelectric conversion elements capable of mutual conversion between thermal energy and electrical energy are known. The thermoelectric conversion element is composed of two types of p-type and n-type thermoelectric conversion materials, and the two types of thermoelectric conversion materials are electrically connected in series, and are configured to be thermally arranged in parallel. . When a voltage is applied between both terminals of the thermoelectric conversion element, movement of holes and electrons occurs and a temperature difference occurs between both surfaces (Peltier effect). On the other hand, if a temperature difference is given between both surfaces of the thermoelectric conversion element, the movement of holes and electrons similarly occurs, and an electromotive force is generated between both terminals (Seebeck effect). For this reason, the thermoelectric conversion element is used as an element for cooling such as a refrigerator or a car air conditioner, and is also used as an element for power generation utilizing exhaust heat generated from a waste incinerator.

Conventionally, as a thermoelectric conversion material constituting a thermoelectric conversion element, an intermetallic compound is known, and among them, a thermoelectric conversion material mainly composed of Bi ₂ Te ₃ has been known for a long time because it has a large Seebeck coefficient. It has the disadvantages of being difficult to process and high raw material costs. Therefore, in order to compensate for these drawbacks, development of Fe ₂ VAl-based intermetallic compounds having a Heusler alloy type crystal structure has been performed. Regarding this Fe ₂ VAl-based thermoelectric conversion material, the absolute value of the Seebeck coefficient at 300 to 400 K, which is slightly higher than room temperature or slightly higher than room temperature, is large by substituting at least a part of one element constituting Fe ₂ VAl with another element. (Patent Document 1).

The total valence electron number is changed by the combination of the element to be substituted and the element to be substituted, and the sign of the Seebeck coefficient is changed, that is, it can be controlled to be p-type or n-type. Many combinations of elements to be substituted and substitution elements are possible. Specifically, Fe is substituted with Pt or Co, V is substituted with Mo or Ti, and Al is substituted with Si, Ge, or Sn. Is disclosed. More specifically, Al is replaced by Si or Ge, the Seebeck coefficient changes from positive to negative, and the absolute value is increased. Further, when V is replaced with Ti, the Seebeck coefficient remains positive, but when it is replaced with Mo, the Seebeck coefficient changes from positive to negative, and its absolute value is increased.

Further, in the Fe ₂ VAl system, the thermal conductivity or electrical resistivity, which is a factor other than the Seebeck coefficient that determines the thermoelectric performance index, is measured at room temperature by the same element substitution as in Patent Document 1. It is disclosed that thermal conductivity and electric resistivity are reduced by element substitution, and as a result, a figure of merit near room temperature is increased (Patent Document 2).

On the other hand, in the same Fe ₂ VAl system, Seebeck coefficient and thermal conductivity near room temperature are disclosed for an example in which Al is substituted with Ga or B and further shifted from the stoichiometric composition (Patent Document 3). ).

However, Patent Documents 1 to 3 do not disclose Seebeck coefficient, electrical resistivity, or the like that is higher than room temperature, that is, a medium to high temperature range of 300 to 600K. And also in the literature 1 measured at a temperature higher than room temperature, the peak of the Seebeck coefficient is 300 to 400 K, and the absolute value is small although the peak temperature may be high. From the above, it was difficult to obtain a large Seebeck coefficient in the temperature range of 300 to 600K.

Japanese Patent No. 4035572 JP 2004-253618 A JP 2004-119647 A

  An object of the present invention is to provide a thermoelectric conversion material having a large Seebeck coefficient and a low electrical resistivity in the temperature range, with the objective of putting the thermoelectric conversion material to practical use in a temperature range of 300 to 600K.

In the thermoelectric conversion material of the basic composition Fe ₂ VAl system, the inventors shifted the compounding ratio of V and Al with respect to Fe from the stoichiometric composition and substituted at least part of V with Ta. It has been found that the above problems can be solved. That is, according to the present invention, the following thermoelectric conversion materials are provided.

[1] The general structure of Fe ₂ V _{1 + xy} Ta _y Al _1-x is satisfied with respect to the basic structure of Fe ₂ VAl having a Heusler alloy type crystal structure and a total number of valence electrons of 24 per chemical formula A thermoelectric conversion material that is -0.10 ≦ x ≦ −0.03, 0.08 ≦ y ≦ 0.12, and is controlled to be p-type.

[2] The thermoelectric conversion material according to [1], wherein x and y are −0.05 ≦ x ≦ −0.03 and 0.10 ≦ y ≦ 0.12, respectively.

[3] The thermoelectric conversion material according to [1] or [2], wherein the temperature at which the absolute value of the Seebeck coefficient reaches a peak is 400K or higher.

[4] The general structure of Fe ₂ V _{1 + xy} Ta _y Al _1-x is satisfied with respect to the basic structure of Fe ₂ VAl having a Heusler alloy type crystal structure and a total number of valence electrons of 24 per chemical formula Thermoelectric conversion material, 0.03 ≦ x ≦ 0.10, 0.08 ≦ y ≦ 0.12, and controlled to n-type.

[5] The thermoelectric conversion material according to [4], wherein x and y are 0.03 ≦ x ≦ 0.05 and 0.10 ≦ y ≦ 0.12, respectively.

[6] The thermoelectric conversion material according to [4] or [5], wherein the temperature at which the absolute value of the Seebeck coefficient reaches a peak is 360K or higher.

It is a figure which shows the manufacture flow of the thermoelectric conversion material of this invention. It is a figure which shows the composition dependence of each X-ray-diffraction pattern and each lattice constant of the p-type and n-type thermoelectric conversion material of this invention. It is a figure which shows the temperature dependence of each electrical resistivity of the p-type and n-type thermoelectric conversion material of this invention. It is a figure which shows the temperature dependence of each Seebeck coefficient of the p-type and n-type thermoelectric conversion material of this invention. It is a figure which shows the temperature dependence of each heat conductivity of the p-type and n-type thermoelectric conversion material of this invention. It is a figure which shows the temperature dependence of each figure of merit of the p-type and n-type thermoelectric conversion material of this invention.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. The present invention is not limited to the following embodiments, and changes, modifications, and improvements can be added without departing from the scope of the invention.

  The thermoelectric conversion material of the present invention is produced as follows. In this production method, a first step of preparing a raw material mixture having an elemental composition ratio capable of producing the thermoelectric conversion material, and the raw material mixture is melted or vaporized in a vacuum or an inert gas, and then solidified. A second step of obtaining a thermoelectric conversion material.

  In the second step, for example, the ingot is prepared by melting and solidifying the raw material mixture in a vacuum or in an inert gas by arc melting or casting. Although this ingot can be used as a thermoelectric conversion material, a method of mechanically pulverizing the ingot in an inert gas or nitrogen gas atmosphere, a method using molten metal atomization or gas atomization, and an inert gas such as mechanical alloying or A powder having a substantially uniform particle diameter is obtained by a method of repeatedly pressing and breaking the raw material mixture in a nitrogen gas atmosphere. The powder thus obtained can be sintered by a hot press method, a HIP method (hot isostatic pressing method), a discharge plasma sintering method, a pulse current method, or the like. For example, when a powder is sintered by the HIP method, for example, compression molding and sintering can be simultaneously performed in an argon gas atmosphere at 800 ° C. and 150 MPa, and solidification can be performed at substantially true density. In addition, in order to make the p-type or n-type thermoelectric conversion material as small as possible, a method such as performing strain processing such as hot rolling or quenching the molten raw material is appropriately employed. The

  And after melt-solidifying a raw material mixture, the X-ray-diffraction measurement is performed by the X-ray-diffraction method for the powder obtained by grinding | pulverizing. On the other hand, the powder obtained by the pulverization is sintered by the HIP method or the like, further cut, etc., and the electrical resistivity, the Seebeck coefficient, and the thermal conductivity are each measured at a predetermined size.

The Fe ₂ VAl-based thermoelectric conversion material of the present invention has the general formula Fe ₂ V _{1 + x − in} which the compounding ratio of V and Al to Fe is shifted from the stoichiometric composition and a part of V is substituted with Ta. _a thermoelectric conversion material satisfying _y Ta _y Al _1-x , wherein −0.10 ≦ x ≦ −0.03, 0.08 ≦ y ≦ 0.12, and a p-type thermoelectric conversion material Preferably there is. More preferably, x and y are −0.05 ≦ x ≦ −0.03 and 0.10 ≦ y ≦ 0.12, respectively. Furthermore, the p-type thermoelectric conversion material of the present invention preferably has a temperature at which the absolute value of its Seebeck coefficient peaks is 400K or higher. In the thermoelectric conversion material controlled to be p-type of the present invention, the total number of valence electrons per chemical formula is 23.7 or more and less than 24.0.

The Fe ₂ VAl-based thermoelectric conversion material of the present invention has a general formula Fe ₂ V _{1+ in} which the mixing ratio of V and Al to Fe is shifted from the stoichiometric composition, and a part of V is substituted with Ta. _A thermoelectric conversion material satisfying _xy Ta _y Al _1-x , wherein 0.03 ≦ x ≦ 0.10, 0.08 ≦ y ≦ 0.12, and a thermoelectric conversion material controlled to be n-type Preferably there is. More preferably, x and y are 0.03 ≦ x ≦ 0.05 and 0.10 ≦ y ≦ 0.12, respectively. Further, the n-type thermoelectric conversion material of the present invention preferably has a temperature at which the absolute value of its Seebeck coefficient peaks is 310K or higher. In the n-type thermoelectric conversion material of the present invention, the total number of valence electrons per chemical formula is more than 24.0 and not more than 24.2.

  EXAMPLES Hereinafter, although this invention is demonstrated further in detail based on an Example, this invention is not limited to these Examples.

(Example)
In the p-type composition Fe ₂ V _0.88-a Ta _0.12 Al _{1 + a} , the characteristics of factors affecting the thermoelectric conversion characteristics of materials having a = 0.03, 0.05, and 0.08 were examined. 99,99% by mass of iron (Fe), 99.99% by mass of aluminum (Al), 99.9% by mass of vanadium (V), and 99.9% of Ta by the same composition. Were further mixed to obtain a raw material mixture. On the other hand, for n-type composition Fe ₂ V _{0.90 + a} Ta _0.10 Al _1-a , a mixture of materials having a = 0.03, 0.05, 0.08 was obtained.

  Next, according to the manufacturing flow shown in FIG. 1, this raw material mixture was arc-melted in an argon atmosphere to form a button-like ingot. In order to obtain a homogeneous ingot, the ingot was redissolved to obtain a homogeneous ingot. The mass change before and after dissolution was 0.1% or less, and the mass change due to dissolution was assumed to be negligible.

Thereafter, this ingot was homogenized for 48 hours at 1273 K in a high vacuum of 5 × 10 ⁻³ Pa or less, and then processed into strips, powders, and block shapes. After that, 1273K × 1Hr strain removal processing and 673K × 4Hr ordering processing were performed in vacuum. In this way, a thermoelectric conversion material was obtained.

<Evaluation>
X-ray diffraction measurement Using the obtained thermoelectric conversion material of the example as powder, X-ray diffraction measurement was performed by a powder X-ray diffraction method. The result is shown in FIG. From the XRD pattern, it was confirmed that it was composed of a DO ₃ (L2 ₁ ) single phase and had a Heusler alloy type crystal structure. Moreover, it confirmed that the amount of V / Al changed continuously from the composition dependence of a lattice constant. Furthermore, from X-ray fluorescence analysis, it was confirmed that there was no impurity and the target composition was almost the same.

-Measurement of electrical resistivity (specific resistance) The ingot of the thermoelectric conversion material of the example was cut with a dicing saw to obtain a strip-shaped test piece of 1.0 mm x 1.0 mm x 15 mm. The electrical resistivity of this test piece was measured at a temperature of 4.2 to 600 K by a direct current four-terminal method. The temperature change of the electrical resistivity of the p-type material and the n-type material is shown in FIGS. 3 (a) and 3 (b), respectively. The electrical resistivity of the p-type material had a peak at over 500K. On the other hand, the peak of the electrical resistivity of the n-type material was 250K to 500K, but the peak shifted to the higher temperature side as a increased.

-Measurement of Seebeck coefficient The ingot of the thermoelectric conversion material of the example was cut with a dicing saw to obtain a strip-shaped test piece of 0.5 mm x 0.5 mm x 5.0 mm. And the Seebeck coefficient was measured by 100K-600K using "SB100" by MMR-Technologies. The temperature change of the Seebeck coefficient of the p-type material and the n-type material is shown in FIGS. 4 (a) and 4 (b). The p-type material showed a positive peak above 400K, while the n-type material showed a negative peak around 250K-500K.

(4) Measurement of thermal conductivity The ingots of the p-type and n-type thermoelectric conversion materials of the examples were cut with a dicing saw to obtain 3.5 mm × 3.5 mm × 4 mm block-shaped test pieces. Then, in a vacuum of 4 × 10 ⁻⁴ Pa, the thermal conductivity was measured using a stationary comparative measurement method using a heat flow method. The results are shown in FIGS. 5 (a) and 5 (b), respectively. Both p-type and n-type were about 8 to 10 (W / mK) at 400K to 600K.

FIG. 6 shows the result of determining the dimensionless thermoelectric performance index ((Seebeck coefficient) ² × temperature / electric resistivity × thermal conductivity) from the above electrical resistivity measurement, Seebeck coefficient measurement, and thermal conductivity measurement. From this result, at least at room temperature, both the p-type material and the n-type material of the present invention perform better than materials in which V and Al are complementarily shifted from the stoichiometric composition without replacing V with Ta. It can be seen that the index is excellent. In addition, in the temperature dependence of the electrical resistivity and Seebeck coefficient, there is not much difference between a material in which V and Al are complementarily shifted from the stoichiometric composition and the p-type material and n-type material of the present invention. Since the thermal conductivity is greatly reduced, it can be inferred that the performance index is excellent for both the p-type material and the n-type material even at 300K to 600K.

  From the above results, the reason why the performance index of Ta substitution of the present invention is large is not certain, but in a material in which V and Al are simply shifted from the stoichiometric composition in a complementary manner, the heat conduction by the non-stoichiometric composition While only a reduction in the rate can be expected, it can be inferred that the Ta substitution is due to a significant reduction in thermal conductivity due to the combined effect of heavy element substitution and non-stoichiometric composition.

The material of the present invention can be used for a thermoelectric conversion element, particularly a power generation element that generates power using exhaust heat in a temperature range of 300K to 600K.

Claims (6)

Thermoelectric conversion material satisfying the general formula Fe ₂ V _{1 + xy} Ta _y Al _1-x with respect to the basic structure of Fe ₂ VAl having a Heusler alloy type crystal structure and a total number of valence electrons of 24 per chemical formula A thermoelectric conversion material in which −0.10 ≦ x ≦ −0.03 and 0.08 ≦ y ≦ 0.12 is controlled to be p-type. 2. The thermoelectric conversion material according to claim 1, wherein x and y satisfy −0.05 ≦ x ≦ −0.03 and 0.10 ≦ y ≦ 0.12, respectively. The thermoelectric conversion material according to claim 1 or 2, wherein the temperature at which the absolute value of the Seebeck coefficient reaches a peak is 400K or higher. Thermoelectric conversion material satisfying the general formula Fe ₂ V _{1 + xy} Ta _y Al _1-x with respect to the basic structure of Fe ₂ VAl having a Heusler alloy type crystal structure and a total number of valence electrons of 24 per chemical formula A thermoelectric conversion material that is 0.03 ≦ x ≦ 0.10 and 0.08 ≦ y ≦ 0.12 and is controlled to be n-type. The thermoelectric conversion material according to claim 4, wherein x and y are 0.03 ≦ x ≦ 0.05 and 0.10 ≦ y ≦ 0.12, respectively. The thermoelectric conversion material according to claim 4 or 5, wherein the temperature at which the absolute value of the Seebeck coefficient reaches a peak is 360K or higher.