JP2013089882A - Thermoelectric conversion material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an FeVal-based thermoelectric conversion material allowing a small environmental load, a low cost, and a large thermoelectric conversion performance index.SOLUTION: P-type and n-type thermoelectric conversion materials having a chemical composition ratio of Fe and V being adjusted with an adjustment amount n relative to a basic structure of FeVal and comprising a compound with some constituent elements replaced by other elements are adjusted in the range of an adjustment amount allowing improved thermoelectric conversion efficiency compared with a compound having a chemical composition ratio of Fe and V being not adjusted with an adjustment amount n and in the range of the total valence electron number.

Description

    The present invention relates to a thermoelectric conversion material.

  There is a thermoelectric conversion material as a material that converts thermal energy into electric energy. This thermoelectric conversion material is roughly classified into two types, n-type and p-type. A thermoelectric conversion element can be obtained by alternately arranging and connecting n-type thermoelectric conversion materials and p-type thermoelectric conversion materials so as to be electrically in series and thermally parallel. If a temperature difference is given between both surfaces of the thermoelectric conversion element, power generation can be performed. Moreover, if a voltage is applied between the both terminals of a thermoelectric conversion element, a temperature difference will generate | occur | produce between both surfaces.

  As a general thermoelectric conversion material, there is a Bi-Te-based intermetallic compound, which is widely used because it has a high Seebeck coefficient and relatively good power generation efficiency (Non-patent Document 1). Further, as other general thermoelectric conversion materials, there are Pb—Te-based intermetallic compounds and Zn—Sb-based intermetallic compounds, and as thermoelectric conversion materials other than intermetallic compounds, there are oxide-based thermoelectric conversion materials ( Non-patent document 1).

The inventors of the present application have proposed a thermoelectric conversion material described in Patent Document 1. This thermoelectric conversion material has a Heusler alloy type crystal structure, and the chemical composition ratio is adjusted with respect to the basic structure Fe ₂ VAl having a total number of valence electrons of 24 per chemical formula. The total number of valence electrons per chemical formula is controlled by substituting with elements.

The inventors have also proposed a thermoelectric conversion material described in Patent Document 2. This thermoelectric conversion material is obtained by substituting a part of the constituent element with another element and controlling the atomic weight of the element to be substituted with respect to the basic structure of Fe ₂ VAl.

The thermoelectric conversion materials according to these proposals have a total valence electron number of more than 24, n-type, and less than 24 p when the constituent elements in the stoichiometric composition of Fe ₂ VAl are replaced with other elements. It becomes a mold (FIG. 13 of Patent Document 2). For example, n-type Fe ₂ V (Al _1−α M _α ) (M = Si, Ge or Sn, 0 <α <1) having a total valence electron number exceeding 24 has a Seebeck coefficient of about −120 μV / K. The high absolute value is shown (patent documents 1, 2, nonpatent literature 2). On the other hand, p-type Fe ₂ (V _1−α M _α ) Al (M = Ti, 0 <α <1) having a total valence electron number of less than 24 has a Seebeck coefficient of about +80 μV / K (patent document) 1, 2, Non-Patent Document 3).

Moreover, a thermoelectric conversion material with better power generation efficiency is desired as the thermoelectric conversion material. For this reason, the inventors have proposed a thermoelectric conversion material in which at least a part of each of Fe and V is substituted with another element for the basic structure of Fe ₂ VAl in Patent Document 3.

In this thermoelectric conversion material, when the other element that replaces Fe is M ₁ , the element M ₁ is selected from the group consisting of groups 7 to 10 of the 4th to 6th periods in the periodic table, and V When the other element to be substituted in place of is M ₂ , the element M ₂ is selected from the group consisting of groups 4 to 6 of the 4th to 6th periods in the periodic table. Further, in this thermoelectric conversion material, the substitution amount of the element M ₁ and the element M ₂ is 0 <α <1 and 0 <where the general formula (Fe _1-α M _1α ) ₂ (V _1-β M _2β ) Al is satisfied. Adjustment is made within the range of β <1. Furthermore, this thermoelectric conversion material is controlled to be p-type so that the total number of valence electrons per chemical formula is less than 24, and is controlled to be n-type so as to exceed 24. For example, a thermoelectric conversion material in which M ₁ is Ir and M ₂ is Ti has a Seebeck coefficient of about +90 μV / K, and the output factor and the figure of merit are improved as compared with a thermoelectric conversion material in which M ₂ is simply replaced with Ti. .

WO2003 / 019681 Japanese Patent Laid-Open No. 2004-253618 WO2007 / 108176

"New Textbook Series Thermoelectric Conversion-Fundamentals and Applications-" pp. 96-97, Hanafusa (2005) Ryo Sakata "Thermoelectric properties of pseudogap-type Heusler compounds" Materia 44, No. 8 (2005) pp. 648-653, Yoichi Nishino "Element substitution effect on thermoelectric properties of pseudogap Fe2VA1 alloy" Journal of the Japan Institute of Metals, Vol. 66, No. 7 (2002), pp. 767-771, Hitoshi Matsuura, Yoichi Nishino, Yuichiro Mizutani, Shigeru Asano

  By the way, in order to apply thermoelectric conversion materials to power generation devices that use waste heat, such as automobile engines, motorcycle engines, household fuel cells, and gas cogeneration, the thermoelectric conversion materials only exhibit good power generation efficiency. It is also necessary that the load on the environment is small and that the oxidation resistance and mechanical strength are also high.

It is known that a thermoelectric conversion material composed of the above-described Bi—Te intermetallic compound exhibits a large Seebeck coefficient near room temperature. However, since Bi and Te, which are constituent elements, are expensive metals, not only the cost is increased, but Se is added when forming an n-type thermoelectric conversion material, but Se and Te are toxic. Therefore, this thermoelectric conversion material also has a large environmental load. Furthermore, thermoelectric conversion materials made of Bi-Te based intermetallic compounds have low oxidation resistance and mechanical strength, and therefore, when applied to actual waste heat in the middle temperature range such as 500 to 700K. Advanced peripheral technology is required to guarantee durability. For this reason, it is difficult to apply Bi-Te based intermetallic compounds to intermediate temperature waste heat.
Further, in the Pb-Te intermetallic compound described above, the constituent element Pb is harmful to the human body as well as the Bi-Te intermetallic compound. From the viewpoint of the global environment, the Pb-Te intermetallic compound is also used. The use of compounds is not preferred.

  In addition, the Zn—Sb-based intermetallic compounds described above have been studied as thermoelectric semiconductors for a long time since ZnSb, which is a p-type semiconductor, has a high Seebeck coefficient of +200 μV / K and low electrical resistance. However, because it has low strength and low toughness as a material, it is difficult to put it into practical use unless the mechanical properties are significantly improved when considering the incorporation into modules. As a result, the manufacturing cost of thermoelectric conversion elements increases. Invite

In addition, the oxide-based thermoelectric conversion material has an advantage of exhibiting a high Seebeck coefficient in a high temperature region around 1000K. However, the oxide-based thermoelectric conversion material, like the Bi—Te-based thermoelectric conversion material, has the property of being brittle and difficult to process. For this reason, when a thermoelectric conversion element is manufactured using an oxide-based thermoelectric conversion material, a cutting allowance for cutting is still necessary, and the ingot is easily broken at the time of cutting, and the yield is very poor. This leads to an increase in device manufacturing costs.

On the other hand, the Fe ₂ VAl-based thermoelectric conversion material described in Patent Documents 1-3 is composed of relatively inexpensive elements such as Fe, V, and Al, and each element has no toxicity, and is Bi-Te-based. It is a thermoelectric conversion material superior to intermetallic compounds and Pb—Te-based intermetallic compounds. In addition, as described in Patent Documents 1-3, Fe ₂ VAl, which is a basic structure, is a semimetal positioned between a semiconductor and a metal, and thus has higher toughness and higher mechanical properties than semiconductors and oxides. Is excellent. Therefore, the Fe ₂ VAl-based thermoelectric conversion material described in Patent Documents 1-3 is a thermoelectric conversion material having superior mechanical characteristics as compared to Zn—Sb-based intermetallic compounds and oxide-based thermoelectric conversion materials.

However, as described in the comparative test of Patent Document 2, a part of the constituent elements replaced with another element without adjusting the composition ratio of the three elements with respect to the basic structure of Fe ₂ VAl is Seebeck The relationship between the coefficient and the amount of substitution varies depending on the type of element to be replaced, but the manner of change with respect to the amount of substitution differs, but when organized by the total number of valence electrons, the Seebeck coefficient is independent of the type of element to be substituted. It is a way of change that can be described by a master curve. For this reason, since the sign of the Seebeck coefficient can be controlled by selecting the type of element to be replaced, it is possible to produce p-type and n-type thermoelectric conversion materials. It is difficult to greatly increase the absolute value of the Seebeck coefficient simply by selecting the type of the.

Therefore, as in the described in Patent Document _{1 Fe 2-n V 1 +} n (Al 1-y Si y), the basic structure of Fe ₂ VAl, adjusted by the adjustment amount n the chemical composition ratio of Fe and V In addition, if a thermoelectric conversion material in which at least a part of Al is replaced with Si is used, the Seebeck coefficient is further increased as compared with a thermoelectric conversion material in which at least a part of Al is replaced with Si, and good power generation is achieved. It is desired that efficiency be exhibited.
However, the compound of Patent Document 1 that adjusts the chemical composition ratio of Fe and V by the adjustment amount n does not disclose the range of the adjustment amount that exhibits good power generation efficiency, and is not only n-type but also p-type. Also, there is a need for a thermoelectric conversion material that exhibits better power generation efficiency than a compound that is not adjusted with the adjustment amounts described in Patent Documents 1 to 3.

The present invention has been made in view of the above-described conventional situation, and with respect to the basic structure of Fe ₂ VAl, the chemical composition ratio of Fe and V is adjusted by an adjustment amount n, and a part of the constituent elements is adjusted. It is composed of a compound substituted with other elements, and Fe and V described in Patent Document 1 are adjusted with an adjustment amount n, and excellent power generation efficiency is exhibited as compared with a compound in which at least a part of Al is substituted with Si. It is set as the problem which should be solved to provide the thermoelectric conversion material. Further, the present invention comprises a compound in which the chemical composition ratio of Fe and V is adjusted with an adjustment amount n with respect to the basic structure of Fe ₂ VAl, and a part of the constituent elements is substituted with another element, and Fe and V P-type and n-type thermoelectric conversions in which the range of the adjustment amount and the total number of valence electrons are controlled so that the thermoelectric conversion efficiency can be improved as compared with a compound in which the chemical composition ratio of V is not adjusted by the adjustment amount n Providing materials is another issue to be solved.

As the inventors have confirmed in a previous application (Patent Document 1), the basic structure of Fe ₂ VAl having a Heusler alloy type crystal structure has 24 total valence electrons per chemical formula. That is, when the average electron concentration per atom is 24/4 = 6, the thermoelectric conversion material has a sharp pseudogap at the Fermi level. If the chemical composition ratio of Fe and V of the basic structure is adjusted with respect to this basic structure, and the total number of valence electrons per chemical formula is controlled by substituting some of the constituent elements with other elements, Fermi The level can be shifted from the center of the pseudogap, and the sign and magnitude of the Seebeck coefficient can be changed.

This time, the inventors adjusted the chemical composition ratio of Fe and V to the basic structure of Fe ₂ VAl by an adjustment amount within a specific range, and further replaced some of the constituent elements with other elements. In addition, it was found that the absolute value of the Seebeck coefficient can be greatly increased compared to the case where element substitution is performed on a compound whose chemical composition ratio of Fe and V is not adjusted, and the present invention has been completed. It was. The total number of valence electrons when element substitution is performed on the basic structure of Fe ₂ VAl is less than 24 in the case of p-type, while it is more than 24 in the case of n-type. It has been found that the total valence electrons of the p-type or n-type thermoelectric conversion material of the present invention do not have a distribution centered at 24, and are different from the total valence electrons of the basic structure.

That is, the invention described in claim 1 has a chemical composition ratio adjustment amount x and a basic structure with respect to a basic structure of Fe ₂ VAl having a Heusler alloy type crystal structure and a total valence electron number of 24 per chemical formula. The substitution amount x of the element D that substitutes for Al in the range of 0.02 ≦ x ≦ 0.08 satisfying the general formula Fe _2-x V _{1 + x} (Al _1-y D _y ) In the thermoelectric conversion material for selecting, when the other element substituted for Al is D, the element D is an element selected from the group consisting of groups 14 to 16 of the 3rd to 6th periods in the periodic table And the total number of valence electrons per chemical formula is controlled to be n-type so that it exceeds 23.7 and is 24.1 or less.
The invention according to claim 2 has an adjustment amount x of the chemical composition ratio and V of the basic structure with respect to the basic structure of Fe ₂ VAl having a Heusler alloy type crystal structure and having 24 total valence electrons per chemical formula. The adjustment amount x is selected within the range of 0.02 ≦ x ≦ 0.08 where the substitution amount y of the element N to be replaced with satisfies the general formula Fe _2-x (V _{1 + xy} N _y ) Al. In the thermoelectric conversion material, when the other element to be substituted in place of V is N, the element N is an element selected from the group consisting of Group 6 of the 4th to 6th periods in the periodic table, and per chemical formula The total number of valence electrons is controlled to be n-type so as to exceed 23.7 and not more than 24.1.
The invention according to claim 3 has an adjustment amount x of the chemical composition ratio and V of the basic structure with respect to the basic structure of Fe ₂ VAl having a Heusler alloy type crystal structure and a total valence electron number of 24 per chemical formula. In which the substitution amount y of the element N to be replaced with satisfies the general formula Fe _2-x (V _{1 + xy} N _y ) Al and the adjustment amount x is selected within a range of −0.07 ≦ x <0. In the conversion material, when the other element to be substituted in place of V is N, the element N is an element selected from the group consisting of groups 4 to 6 in the periodic table, and the total per chemical formula The p-type is controlled so that the number of valence electrons is more than 24 and 24.3 or less.
The invention according to claim 4 has an adjustment amount x of the chemical composition ratio and Fe of the basic structure with respect to the basic structure of Fe ₂ VAl having a Heusler alloy type crystal structure and a total valence electron number of 24 per chemical formula. The substitution amount y of the element M to be substituted instead of and the substitution amount z of the element Ta to substitute instead of V satisfy the general formula (Fe _2−x− M _y ) (V _{1 + x−z} Ta _z ) Al In the thermoelectric conversion material in which the adjustment amount x is selected within the range of 0.02 ≦ x ≦ 0.08, when the other element to be substituted in place of Fe is M, the element M is the fourth to fourth in the periodic table. It is an element selected from the group consisting of Group 9 and Group 10 in 6 cycles, and the total valence electrons per chemical formula is controlled to be n-type so as to exceed 23.7 and not more than 24.1 Features.
The invention according to claim 5 has an adjustment amount x of the chemical composition ratio and Fe of the basic structure with respect to the basic structure of Fe ₂ VAl having a Heusler alloy type crystal structure and a total valence electron number of 24 per chemical formula. The adjustment amount x is selected within a range of 0.02 ≦ x ≦ 0.08 where the substitution amount y of the element M to be substituted satisfies the general formula (Fe _{2- xy My} ) V _{1 + x} Al. In the thermoelectric conversion material, when the other element to be substituted in place of Fe is M, the element M is an element selected from the group consisting of groups 9 and 10 of the 4th to 6th periods in the periodic table, The total number of valence electrons per chemical formula is controlled to be n-type so as to be over 23.7 and below 24.1.
The invention according to claim 6 has a chemical composition ratio adjustment amount x and a basic structure V with respect to the basic structure of Fe ₂ VAl having a Heusler alloy type crystal structure and a total valence electron number of 24 per chemical formula. The substitution amount y of the element N substituted for V and the substitution amount z of the element Ta substituted for V _{satisfy the} general formula Fe _2-x (V _{1 + x−yZ} N _y Ta _z ) Al In the thermoelectric conversion material in which the adjustment amount x is selected within the range of 02 ≦ x ≦ 0.08, when the other element to be substituted in place of V is N, the element N is the fourth to sixth periods in the periodic table. The total number of valence electrons per chemical formula is controlled to be n-type so as to be more than 23.7 and not more than 24.1.
The invention according to claim 7 has an adjustment amount x of the chemical composition ratio and V of the basic structure 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. The substitution amount y of the element N substituted for V and the substitution amount z of the element Ta substituted for V satisfies the general formula Fe _2-x (V _{1 + x−yz} N _y Ta _z ) Al−0 In the thermoelectric conversion material in which the adjustment amount x is selected within the range of .07 ≦ x <0,
When the other element substituted for V is N, the element N is an element selected from the group consisting of Group 4 of the 4th to 6th periods in the periodic table, and the total number of valence electrons per chemical formula is , 24, and 24.3 or less, and the p-type control is performed.

In relation to the thermoelectric conversion material of Example 1 (Fe _1.95 V _1.05 (Al _1-y Si _y ) (y = 0, 0.03, 0.05, 0.10)), the temperature, the Seebeck coefficient, It is a graph which shows the relationship. In relation to the thermoelectric conversion material (Fe _1.95 V _1.05 (Al _1-y Si _y ) (y = 0, 0.03, 0.05, 0.10)) of Example 1, temperature and electrical resistivity It is a graph which shows the relationship. Thermoelectric conversion material of Example _{1 (Fe 2-x V 1} + x (Al 1-y Si y) (x = -0.07, -0.04,0.02,0.05,0.08) ( y = 0, 0.01, 0.03, 0.05, 0.07, 0.10)) and the thermoelectric conversion material of Comparative Example 1 (Fe ₂ V (Al _1-y Si _y ) (y = 0, 0.03, 0.05, 0.10)), and is a graph showing the relationship between the Si composition and the Seebeck coefficient at 300K. The thermoelectric conversion material of Example 2 (Fe _2.04 (V _0.96-y Ti _y ) Al (y = 0, 0.01, 0.03, 0.05, 0.10)), It is a graph which shows the relationship with a Seebeck coefficient. The thermoelectric conversion material of Example 2 ((Fe _2.04 (V _0.96-y Ti _y ) Al (y = 0, 0.01, 0.03, 0.05, 0.10))), temperature It is a graph which shows the relationship between and electrical resistivity. Thermoelectric conversion material of Example 2 (Fe _2−x (V _{1 + xy} Ti _y ) Al (x = −0.07, −0.04, 0.02, 0.05) (y = 0, 0 0.01, 0.02, 0.03, 0.05, 0.10)) and the thermoelectric conversion material of Comparative Example 2 (Fe ₂ (V _1-y Ti _y ) Al (y = 0, 0.01, ₀ ) .02, 0.03, 0.05, 0.10)), and is a graph showing the relationship between the Ti composition and the Seebeck coefficient at 300K. Thermoelectric conversion material of Example 1 (Fe _1.98 V _1.02 (Al _0.90 Si _0.10 )), Thermoelectric conversion material of Comparative Example 1 (Fe ₂ V (Al _0.90 Si _0.10 )) The thermoelectric conversion material of Example 2 (Fe _2.04 (V _0.86 Ti _0.10 ) Al) and the thermoelectric conversion material of Comparative Example 2 (Fe ₂ (V _0.90 Ti _0.10 ) Al). It is a graph which shows the relationship between temperature and an output factor. The thermoelectric conversion material of Example 3 ((Fe _1.95-y Ir _y ) (V _0.95 Ta _0.10 ) Al (y = 0, 0.03, 0.05, _0.10 )), It is a graph which shows the relationship between temperature and a Seebeck coefficient. The thermoelectric conversion material of Example 3 ((Fe _1.95-y Ir _y ) (V _0.95 Ta _0.10 ) Al (y = 0, 0.03, 0.05, _0.10 )), It is a graph which shows the relationship between temperature and an electrical resistivity. It relates to the thermoelectric conversion material of Example 3 ((Fe _2−x−y Ir _y ) (V _{0.90 + x} Ta _0.10 ) Al (x = 0.02, 0.05)), Ir composition and 300K It is a graph which shows the relationship with the heat conductivity in. Thermoelectric conversion material (Fe _1.98-y Ir _y ) (V _0.92 Ta _0.10 ) Al of Example 3 and thermoelectric conversion material (Fe _1.98-y Ir _y ) V _{1.02 of} Example 4 It is a graph which shows the relationship between Ir composition and a figure of merit in 300K concerning Al. Thermoelectric conversion material of Example 1 (Fe2 _{-xV1 + x} (Al1 _{- ySiy} ), Thermoelectric conversion material of Example 2 (Fe2 _-x (V1 _{+ xyTiy} ) Al, Implementation Thermoelectric conversion material of Example 3 ((Fe _2-xy Ir _y ) (V _{0.90 + x} Ta _0.10 ) Al, Thermoelectric conversion material of Example 4 ((Fe _1.98-y Ir _y ) V _1.02 Al), thermoelectric conversion material of Example 5 (Fe _2-x (V _{1 + xy} Mo _y ) Al, thermoelectric conversion material of Example 6 (Fe _2-x (V _{0.90 + x− y} Ti _y Ta _0.10 ) Al, thermoelectric conversion material of Comparative Example 1 (Fe ₂ V (Al _1-y Si _y ), thermoelectric conversion material of Comparative Example 2 (Fe ₂ (V _1-y Ti _y ) Al), thermoelectric conversion material of Comparative example _{3 (Fe 2-y Ir y} ) VAl and thermoelectric conversion material of Comparative example _{4 (Fe 2 (V 1-} y Mo y) a It relates to a graph showing the relationship between the Seebeck coefficient in the total number of valence electrons and 300K.

  The thermoelectric conversion material of the present invention can be produced, for example, by the following production method. This manufacturing method includes a first step of preparing a raw material mixture having an element capable of manufacturing the thermoelectric conversion material and a composition ratio, and melting or vaporizing and solidifying the raw material mixture in a vacuum or an inert gas, A second step of obtaining a conversion material.

  If the said thermoelectric conversion material is manufactured with this manufacturing method, the Seebeck coefficient of the thermoelectric conversion material obtained will become comparatively high. For this reason, according to this manufacturing method, a thermoelectric conversion material with high waste heat recovery efficiency and less risk of environmental pollution can be manufactured at low cost.

  As a 2nd process, the method of cooling, after melt | dissolving a raw material mixture in a vacuum or an inert gas is employable, for example. In order to make an n-type thermoelectric conversion material or a p-type thermoelectric conversion material as an aggregate of powders having as small a particle size as possible, first, the raw material mixture is melted by arc melting or the like and then solidified. A method for obtaining a substantially uniform powder by mechanically pulverizing it in an inert gas or nitrogen gas atmosphere, and a method for obtaining a substantially uniform powder by molten metal atomization or gas atomization. Further, a method of obtaining a substantially uniform powder by repeatedly pressing and breaking the raw material mixture in an inert gas or nitrogen gas atmosphere by a mechanical alloying method can be employed. The powder thus obtained can be sintered by a hot press method in a vacuum, a HIP (hot isostatic pressing) method, a discharge plasma sintering method, a pulse current method or the like. When the powder is sintered by the HIP method, for example, compression molding and sintering can be simultaneously performed with an argon gas of 150 MPa at 800 ° C. to solidify at a true density. Further, according to the pseudo-HIP method, the true density can be solidified at low cost by using a molding press. In addition, in order to make an n-type thermoelectric conversion material or a p-type thermoelectric conversion material as an aggregate of crystal grains having a particle size as small as possible, strain processing such as hot rolling is performed, or a molten raw material is used. A method of reducing the crystal grains by rapid cooling or the like can be employed.

  It is possible to manufacture a thermoelectric conversion element with the thermoelectric conversion material of the present invention. In the thermoelectric conversion element thus obtained, the thermoelectric conversion material having a positive sign of the Seebeck coefficient exhibits a behavior as a p-type, and the thermoelectric conversion material having a negative sign of the Seebeck coefficient exhibits a behavior as an n-type. These thermoelectric conversion elements have high thermoelectric conversion efficiency, can be manufactured at low cost, and are less likely to cause environmental pollution.

[Examples 1 to 6]
In the thermoelectric conversion materials of Examples 1 to 6, the adjustment amount x of the general formula Fe _2−x V _{1 + x} Al is within the range of −0.07 ≦ x ≦ 0.08 with respect to Fe ₂ VAl having the basic structure. In addition, some of the constituent elements are replaced with other elements.

The total number of valence electrons per chemical formula of the basic structure of Fe ₂ VAl is 24 by the following calculation. That is, the number of valence electrons of Fe is 16, which is obtained by multiplying a total of 8 of 2 of the 4s orbit and 6 of the 3d orbit by the coefficient 2. Further, the number of valence electrons of V is a total of 5, 2 of 4s orbitals and 3 of 3d orbitals. Further, the number of valence electrons of Al is 3 in total, 2 of 3s orbitals and 1 of 3p orbitals. The total number of valence electrons 24 of Fe, V and Al is the total number of valence electrons per chemical formula of the basic structure.

With respect to this basic structure, in the compound represented by the general formula Fe _2−x V _{1 + x} Al, by setting 0 <x ≦ 0.08, the total number of valence electrons becomes less than 24, and the n-type thermoelectric conversion material It becomes. Further, by setting −0.07 ≦ x <0, the total number of valence electrons exceeds 24, and a p-type thermoelectric conversion material is obtained.

  This thermoelectric conversion material is manufactured as follows. First, three kinds of elements of Fe, V, and Al were weighed so as to satisfy the above composition condition. These elements were melted using an argon arc to produce a button-like ingot. In order to obtain a homogeneous ingot, the obtained ingot was redissolved. This re-dissolution was performed twice or more to obtain a homogeneous ingot. Since the change in weight before and after dissolution is within 0.1%, it was assumed that the change in composition due to dissolution was negligible.

The produced ingot was homogenized at 1273 K × 48 hr in a high vacuum of 5 × 10 ⁻³ Pa or less, and molded into strips, powders and blocks. Thereafter, a distortion removing process of 1273 K × 1 hr and a regularizing process of 673 K × 4 hr were performed in a vacuum. Thus, thermoelectric conversion materials of Examples 1 to 6 were obtained. The thermoelectric conversion materials of Examples 1 and 2 are obtained by shifting the total number of valence electrons from 24 of the basic structure by slightly adjusting the composition ratio between Fe and V. Here, the smaller the adjustment amount of the composition ratio from the stoichiometric composition, the more the form of the solid solution is maintained.When the adjustment amount of the composition ratio from the stoichiometric composition is large, precipitates are generated and all of the constituent elements are formed. No longer exists in the form of a solid solution.
For this reason, since the adjustment amount of the thermoelectric conversion materials of Examples 1 to 6 is small, the basic structure is maintained.

[Comparative Examples 1-4]
The thermoelectric conversion materials of Comparative Examples 1 to 4 are obtained by substituting a part of the constituent elements with other elements for the basic structure Fe ₂ VAl.

[Evaluation method]
The following evaluation was performed about the thermoelectric conversion material of Examples 1-6 and Comparative Examples 1-4.

(X-ray diffraction)

In order to determine the structure of each material, X-ray diffraction was performed using the powder produced by the above method. CuKα rays were used for the evaluation. Since these are alloy systems containing Fe, a monochromator was used for the purpose of removing the background. As a result, all the produced materials had a Heusler structure.

(Measurement of Seebeck coefficient)

Using a test piece of 0.5 × 0.5 × 5.0 mm ³ , the Seebeck coefficient S (μV / K) was measured in a temperature range of 100K to 400K with “SB-100” manufactured by MMR-Technologies.

(Measurement of electrical resistivity)
An electrical resistivity ρ (μΩcm) was measured by a DC four-terminal method using a 1 × 1 × 15 mm ³ strip sample. The measurement temperature range is from liquid He temperature (4.2K) to 800K. From 4.2K to room temperature, the temperature was naturally raised and measured. From room temperature to 800 K, an electric furnace was used and the temperature was increased at 0.05 K / second in a vacuum atmosphere of 5 × 10 ⁻³ Pa or less and measurement was performed.

(Measurement of thermal conductivity)
Each thermoelectric conversion material is cut with a silicon carbide cutting blade to obtain a 3.5 × 3.5 × 4.0 (mm ³ ) prismatic test piece. Then, in a vacuum of 4 × 10 ⁻⁴ Pa, the thermal conductivity κ (W / mK) of each test piece is measured using a stationary comparative measurement method using a heat flow method.

(Output factor and figure of merit)
As an index for evaluating the thermoelectric conversion material, there are an output factor: P = S ² / ρ and a performance index: Z = S ² / (ρκ). Here, S is the Seebeck coefficient, ρ is the electrical resistivity, and κ is the thermal conductivity. These values were taken as the above measured values, and the output factor P (10 ⁻³ W / mK ² ) and the figure of merit Z (10 ⁻⁴ / K) were determined.

Regarding the thermoelectric conversion material of Example 1 (Fe _1.95 V _1.05 (Al _1-y Si _y ) (y = 0, 0.03, 0.05, 0.10)), the temperature and Seebeck coefficient The relationship is shown in FIG. 1, and the relationship between temperature and electrical resistivity is shown in FIG. Further, the thermoelectric conversion material of Example 1 (Fe _2-x V _{1 + x} (Al _1-y Si _y ) (x = −0.07, −0.04, 0.02, 0.05, 0.08) ) (Y = 0, 0.01, 0.03, 0.05, 0.07, 0.10)) and the thermoelectric conversion material of Comparative Example 1 (Fe ₂ V (Al _1-y Si _y ) (y = (0, 0.03, 0.05, 0.10)), the relationship between the Si composition and the Seebeck coefficient at 300K is shown in FIG.

As shown in FIG. 1, the thermoelectric conversion material of Example 1 has an Si addition amount of 0.03 and an absolute value of the Seebeck coefficient of 180 μV / K which is the maximum around 300K. Further, the absolute value of the Seebeck coefficient decreases as the addition amount of Si further increases. In addition, as shown in FIG. 2, it can be seen that the thermoelectric conversion material of Example 1 has an electric resistivity of 500 K or less that decreases as the amount of Si added increases. In particular, the thermoelectric conversion material with y = 0.10 shows a low electrical resistivity of about 300 μΩcm at 4.2 K, and the electrical resistivity increases with increasing temperature below 500 K, and the semiconductor It showed a metallic behavior from a typical one. As shown in FIG. 3, the thermoelectric conversion material of Example 1 has a negative sign of the Seebeck coefficient regardless of the Si composition in the range of the chemical composition ratio adjustment amount x = 0.02 to 0.08, and n It turns out that it is a type. Furthermore, compared with the thermoelectric conversion material of Comparative Example 1, the absolute value of the Seebeck coefficient at 300 K is increased in all Si compositions. Among them, the thermoelectric conversion material in which x = 0.05 has the maximum Seebeck coefficient. In Example 1, Al was replaced by Si, but it may be replaced by Ge, Sn, or P.

Regarding the thermoelectric conversion material of Example 2 (Fe _2.04 (V _0.96-y Ti _y ) Al (y = 0, 0.01, 0.03, 0.05, 0.10)), the temperature and Seebeck FIG. 4 shows the relationship with the coefficient, and FIG. 5 shows the relationship between the temperature and the electrical resistivity. Further, the thermoelectric conversion material of Example 2 (Fe _2−x (V _{1 + xy} Ti _y ) Al (x = −0.04, −0.07, 0.02, 0.05) (y = 0) , 0.01, 0.02, 0.03, 0.05, 0.10)) and the thermoelectric conversion material of Comparative Example 2 (Fe ₂ (V _1-y Ti _y ) Al (y = 0, 0.01) , 0.02, 0.03, 0.05, 0.10)), the relationship between the Ti composition and the Seebeck coefficient at 300K is shown in FIG. In Example 2, V is substituted with Ti, but Zr may be substituted.

As shown in FIG. 4, the thermoelectric conversion material of Example 2 has a maximum addition amount of 110 μV / K around 300 K with an addition amount of Ti of 0.03 and a Seebeck coefficient. Further, the absolute value of the Seebeck coefficient decreases as the addition amount of Ti further increases. Further, as shown in FIG. 5, it can be seen that the thermoelectric conversion material of Example 2 has an electrical resistivity of 700 K or less that decreases as the addition amount of Ti increases. In particular, the thermoelectric conversion material with y = 0.10 shows a low electric resistivity of about 130 μΩcm at 4.2 K, and the electric resistivity increases with increasing temperature below 800 K, and the semiconductor It showed a metallic behavior from a typical one. Further, as shown in FIG. 6, the thermoelectric conversion material of Example 2 has a positive sign of the Seebeck coefficient regardless of the Ti composition in the range of the chemical composition ratio adjustment amount x = −0.07 to 0, and is p-type. It can be seen that it is. Furthermore, as compared with the thermoelectric conversion material of Comparative Example 2, the thermoelectric conversion materials having x = −0.04 and −0.07 have a large Seebeck coefficient at 300 K in all Ti compositions. Among them, the thermoelectric conversion material in which x = −0.04 has the maximum Seebeck coefficient when the Ti composition y is 0.03.

Thermoelectric conversion material of Example 1 (Fe _1.98 V _1.02 (Al _0.90 Si _0.10 )) and thermoelectric conversion material of Comparative Example 1 (Fe ₂ V (Al _0.90 Si _0.10 )) , And the thermoelectric conversion material (Fe _2.04 (V _0.86 Ti _0.10 ) Al) of Example 2 and the thermoelectric conversion material (Fe ₂ (V _0.90 Ti _0.10 ) Al) of Comparative Example 2 The temperature dependence of the output factor was obtained. The result is shown in FIG.

FIG. 7 shows that the output factor of the thermoelectric conversion material of Example 1 reaches 7 × 10 ⁻³ W / mK ^{2 at} 300 K, which is larger than the thermoelectric conversion material of Comparative Example 1 having the same Si composition. Furthermore, it is the magnitude | size exceeding the typical value 4.5 * 10 < ^-3 > W / mK < ² > of the output factor of the conventional typical thermoelectric conversion material Bi-Te type n-type thermoelectric conversion material. On the other hand, although the thermoelectric conversion material of Example 2 is p-type, it reaches a maximum of 4 × 10 ⁻³ W / mK ² , which is larger than the thermoelectric conversion material of Comparative Example 2 having the same Ti composition. Furthermore, it is the magnitude | size exceeding the typical value 3 * 10 < ^-3 > W / mK < ² > of the output factor of the Bi-Te type p-type thermoelectric conversion material which is a conventional typical thermoelectric conversion material.

Regarding the thermoelectric conversion material ((Fe _1.95-y Ir _y ) (V _0.95 Ta _0.10 ) Al (y = 0, 0.03, 0.05, _0.10 )) of Example 3, the temperature 8 and FIG. 9 show the relationship between the temperature and the electrical resistivity.

  FIG. 8 shows that the thermoelectric conversion material in which the chemical composition ratio of Fe and V is adjusted and a part of V is substituted with Ta has a negative Seebeck coefficient at 200 K when a part of Fe is not substituted with Ir. It can be seen that the value is as large as −200 μV / K. In addition, the absolute value of the Seebeck coefficient decreases as the Ir composition increases. In addition, as shown in FIG. 9, in the thermoelectric conversion material in which the chemical composition ratio of Fe and V is adjusted and a part of V is replaced with Ta, a large electrical resistivity is exhibited at a low temperature and a semiconductor negative temperature is obtained. Dependence appears. However, the electrical resistivity of 400K or less rapidly decreases with the increase of the Ir substitution amount, and shows metallic temperature dependence at y = 0.10.

Regarding the thermoelectric conversion material of Example 3 ((Fe _2−xy Ir _y ) (V _{0.90 + x} Ta _0.10 ) Al (x = 0.02, 0.05)), the Ir composition and 300K The relationship with thermal conductivity is shown in FIG.

The thermoelectric conversion material of Example 3 having a basic structure (substitution amount y = 0) has a relatively small value of 10.5 W / mK at 300 K because V is already substituted by 10% with Ta. However, when Fe is replaced with Ir, the thermal conductivity is remarkably reduced. In particular, at the substitution amount of y = 0.10, the value decreases to 6.5 W / mK regardless of the adjustment amount x. Therefore, it can be seen that the decrease in thermal conductivity becomes significant by substitution with an element having a large atomic weight.

Further, it is known that the thermal conductivity is the sum of a component due to carriers and a component due to lattice vibration. When the thermal conductivity due to carriers is estimated from the electrical resistivity of FIG. 9 using the Wiedemann-Franz rule, it can be seen that the overall thermal conductivity shown in FIG. Therefore, in the thermoelectric conversion material of Example 3, the contribution of the lattice vibration and the carrier to the thermal conductivity is about the same, and substitution with an element having a large atomic weight is effective in significantly reducing the thermal conductivity due to the lattice vibration. is there. For this reason, if the thermoelectric conversion material of Example 3 is used, it turns out that a heat conductivity is small and by extension the thermoelectric conversion element excellent in the performance of thermoelectric conversion can be obtained. That is, when Fe is replaced with Ir, the absolute value of the Seebeck coefficient is reduced, but the electrical resistivity and thermal conductivity are significantly reduced, resulting in an increase in the figure of merit. The substitution amount y of Fe by Ir is particularly preferably 0.10 or more. In Example 3, Fe was replaced with Ir, but Pd, Co, or Ni may be used.

Thermoelectric conversion material (Fe _1.98-y Ir _y ) (V _0.92 Ta _0.10 ) Al of Example 3 and thermoelectric conversion material (Fe _1.98-y Ir _y ) V _{1.02 of} Example 4 FIG. 11 shows the relationship between the Ir composition and the figure of merit at 300 K for Al.

From FIG. 11, it can be seen that the figure of merit is greatly increased by replacing part of Fe with Ir. In particular, in the thermoelectric conversion material of Example 3 in which V was previously substituted with 10% Ta, the figure of merit increased remarkably, and reached 1 × 10 ⁻³ / K at an Ir composition y = 0.15. The figure of merit due to simultaneous substitution with Ta as well as substitution with Ir is large, and when a thermoelectric conversion element is manufactured using a thermoelectric conversion material substituted with an element having a large atomic weight, a thermoelectric characteristic exhibiting a large figure of merit is shown. It can be seen that a conversion element is obtained.

Further, as Example 4, a thermoelectric conversion material (Fe _2-xy Ir _y ) V _{1 + x} Al (x = −0.04, 0.02, 0.05), and as Example 5, a thermoelectric conversion material (Fe _{2− x} (V _{1 + xy} Mo _y ) Al (x = −0.04, 0.02), and thermoelectric conversion material Fe _2-x (V _{0.90 + xy} Ti _y Ta ₀ as Example 6) _.10 ) The Seebeck coefficient of Al (x = −0.04) at 300 K was measured.

Thermoelectric conversion material Fe _2-x V _{1 + x} (Al _1-y Si _y ) of Example 1, thermoelectric conversion material Fe _2-x (V _{1 + xy} T _y ) Al of Example 2, and implementation Thermoelectric conversion material (Fe _2-xy Ir _y ) (V _{0.90 + x} Ta _0.10 ) Al of Example 3 and thermoelectric conversion material (Fe _2-xy Ir _y ) V _{1 + x of} Example 4 Al and thermoelectric conversion materials _{Fe 2-x} of example _{5 (V 1 + x-y} Mo y) Al and thermoelectric conversion materials _{Fe 2-x} of example _{6 (V 0.90 + x-y} Ti y Ta _0.10 ) Al, the above six examples, the thermoelectric conversion material Fe ₂ V (Al _1-y Si _y ) of Comparative Example 1, and the thermoelectric conversion material Fe ₂ (V _1-y Ti _y ) of Comparative Example 2 ) Al, thermoelectric conversion material (Fe _2-y Ir _y ) VAl of Comparative Example 3, and thermoelectric conversion material Fe ₂ (V _{1-y of} Comparative Example 4) Mo _y ) Al, and the relationship between the Seebeck coefficient (μV / K) at 300 K and the total number of valence electrons is determined for the above four comparative examples. The results are shown in FIG. In addition, Table 1 describes the compositions of the samples of the examples and comparative examples.

In FIG. 12, when the chemical composition ratio is not adjusted by the adjustment amount x (x = 0) as in the thermoelectric conversion materials of Comparative Examples 1 to 4, the total valence electron number of the basic structure Fe ₂ VAl is 24.0. The absolute value of the Seebeck coefficient is greatly increased both when the total valence electron number becomes less than 24.0 due to element substitution and when the total valence electron number exceeds 24.0. Such a change in the Seebeck coefficient is particularly remarkable in the vicinity where the total number of valence electrons is 24.0. Further, the thermoelectric conversion materials of Comparative Examples 1, 3, and 4 have a total valence electron number exceeding 24.0, and all Seebeck coefficients are negative values, and therefore show n-type thermoelectric conversion characteristics. I understand. On the other hand, the thermoelectric conversion material of Comparative Example 2 has a total valence electron number of less than 24.0, and all Seebeck coefficients are positive values, indicating that p-type thermoelectric conversion characteristics are exhibited.

When organized by the total number of valence electrons as shown in FIG. 12, the Seebeck coefficient changes in such a way that it can be described by one master curve regardless of the type of element to be replaced. For this reason, as clarified in the present invention, it is possible to optimize the energy position of the Fermi level in the pseudogap by selecting the type of element to be replaced, and thus control the sign of the Seebeck coefficient. In addition to making it possible to fabricate p-type and n-type thermoelectric conversion materials based on the basic structure Fe ₂ VAl, by greatly increasing the absolute value of the Seebeck coefficient, A thermoelectric conversion material that can exhibit excellent thermoelectric properties can be produced.

In FIG. 12, when the chemical composition ratio is adjusted by the adjustment amount x as in the thermoelectric conversion materials of Examples 1 to 6, the master curve has a total valence electron number of 24 or less at x = 0.02 to 0.08. Further, the shift is larger as the adjustment amount is larger. On the other hand, at x = −0.04 and −0.07, the master curve shifts toward the total valence electron number of 24 or more, and the master curve shifts larger as the absolute value of the adjustment amount increases. Here, it is understood that the absolute value of the n-type thermoelectric conversion material having a negative Seebeck coefficient greatly increases when the adjustment amount x is a positive value. Specifically, it is possible to produce an n-type thermoelectric conversion material that can exhibit excellent thermoelectric characteristics when the adjustment amount x is in the range of 0 <x ≦ 0.08. On the other hand, the p-type thermoelectric conversion material having a positive Seebeck coefficient has a large increase in absolute value when the adjustment amount x is a negative value. Specifically, it is possible to manufacture a p-type thermoelectric conversion material that can exhibit excellent thermoelectric characteristics when the adjustment amount x is within a range of −0.07 ≦ x <0.

Moreover, a thermoelectric conversion element is manufactured by one set which selected p-type and n-type from the thermoelectric conversion material of Examples 1-6, or the combination of the thermoelectric conversion material of Examples 1-6 and other well-known thermoelectric conversion materials. can do. Since the thermoelectric conversion materials of Examples 1 to 6 can be manufactured at low cost using a general-purpose metal, the manufacturing cost of these thermoelectric conversion elements is also low. Furthermore, since the thermoelectric conversion materials of Examples 1 to 6 are composed of only extremely toxic components, these thermoelectric conversion elements are less likely to cause environmental pollution.

INDUSTRIAL APPLICABILITY The present invention can be used for a power generation device or the like using waste heat in an intermediate temperature range, such as an engine of a car or a motorcycle, a household fuel cell or a gas cogeneration.

Claims (7)

For 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, the chemical composition ratio adjustment amount x and the substitution of element D that replaces the basic structure Al In the thermoelectric conversion material in which the adjustment amount x is selected within the range of 0.02 ≦ x ≦ 0.08 where the amount y satisfies the general formula Fe _2−x V _{1 + x} (Al _1−y D _y ), it is replaced with Al. When the other element to be substituted is D, the element D is an element selected from the group consisting of groups 14 to 16 of the third to sixth periods in the periodic table, and the total number of valence electrons per chemical formula is A thermoelectric conversion material which is controlled to be n-type so as to exceed 23.7 and not more than 24.1. Substitution of element N which replaces 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 in place of the adjustment amount x of the chemical composition ratio and V of the basic structure In the thermoelectric conversion material in which the adjustment amount x is selected in the range of 0.02 ≦ x ≦ 0.08 where the amount y satisfies the general formula Fe _2-x (V _{1 + xy} N _y ) Al, When the other element to be substituted is N, the element N is an element selected from the group consisting of Group 6 of the 4th to 6th periods in the periodic table, and the total number of valence electrons per chemical formula is 23.7. And a n-type thermoelectric conversion material that is controlled to be 24.1 or less. Substitution of element N which replaces 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 in place of the adjustment amount x of the chemical composition ratio and V of the basic structure In the thermoelectric conversion material in which the adjustment amount x is selected within the range of −0.07 ≦ x <0 where the amount y satisfies the general formula Fe _2-x (V _{1 + xy} N _y ) Al, substitution is performed for V When the other element is N, the element N is an element selected from the group consisting of Group 4 of the 4th to 6th periods in the periodic table, and the total number of valence electrons per chemical formula exceeds 24, A thermoelectric conversion material which is controlled to be p-type so as to be 24.3 or less. For 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, the chemical composition ratio adjustment amount x and the substitution of the element M substituted for the basic structure Fe The substitution amount z of the element Ta to be substituted in place of the amounts y and V satisfies 0.02 ≦ x ≦ 0.08 satisfying the general formula (Fe _2−x− M _y ) (V _{1 + x−z} Ta _z ) Al. In the thermoelectric conversion material in which the adjustment amount x is selected within the above range, when the other element to be substituted for Fe is M, the element M is from the 9th and 10th groups of the 4th to 6th periods in the periodic table. A thermoelectric conversion material, which is an element selected from the group consisting of, and is controlled to be n-type so that the total number of valence electrons per chemical formula exceeds 23.7 and is 24.1 or less. For 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, the chemical composition ratio adjustment amount x and the substitution of the element M substituted for the basic structure Fe In the thermoelectric conversion material in which the adjustment amount x is selected within the range of 0.02 ≦ x ≦ 0.08 where the amount y satisfies the general formula (Fe _2−xy M _y ) V _{1 + x} Al, instead of Fe When the other element to be substituted is M, the element M is an element selected from the group consisting of groups 9 and 10 of the 4th to 6th periods in the periodic table, and the total number of valence electrons per chemical formula is 23 A thermoelectric conversion material that is controlled to be n-type so as to exceed .7 and not more than 24.1. Substitution of element N which replaces 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 in place of the adjustment amount x of the chemical composition ratio and V of the basic structure The range of 0.02 ≦ x ≦ 0.08 where the substitution amount z of the element Ta to be substituted in place of the amounts y and V satisfies the general formula Fe _2-x (V _{1 + x−yZ} N _y Ta _z ) Al In the thermoelectric conversion material in which the adjustment amount x is selected, when the other element to be substituted in place of V is N, the element N is selected from the group consisting of Group 6 of the 4th to 6th periods in the periodic table. A thermoelectric conversion material characterized in that the total number of valence electrons per chemical formula is controlled to be n-type so as to be more than 23.7 and not more than 24.1. Substitution of element N which replaces 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 in place of the adjustment amount x of the chemical composition ratio and V of the basic structure In the range of −0.07 ≦ x <0, the substitution amount z of the element Ta to be substituted in place of the amounts y and V satisfies the general formula Fe _2-x (V _{1 + x−yz} N _y Ta _z ) Al. In the thermoelectric conversion material for selecting the adjustment amount x in the above, when the other element to be substituted in place of V is N, the element N is selected from the group consisting of Group 4 of the 4th to 6th periods in the periodic table. A thermoelectric conversion material which is an element and is controlled to be p-type so that the total number of valence electrons per chemical formula exceeds 24 and is 24.3 or less.