JP2022143990A - thermoelectric conversion material - Google Patents

Abstract

To put a thermoelectric conversion material into practical use in a temperature range of 400 to 900K, and to provide a thermoelectric conversion material having a large Seebeck coefficient in the temperature range.SOLUTION: In a p-type controlled thermoelectric conversion material, in a basic structure of Ru2TiD, which has a Heusler alloy-type crystal structure and a total number of valence electrons per chemical formula of 24, an element D is at least one of Si and Ge, or in an n-type controlled thermoelectric conversion material, in a basic structure of Ru2TiSi, the other element M substituted for Ti is at least one of V, Nb, and Ta, and the substitution amount of the element M is adjusted within the range of 0<α<0.5 that satisfies the general formula Ru2(Ti 1-αMα)D.SELECTED DRAWING: Figure 2

Description

The present invention relates to thermoelectric conversion materials.

Thermoelectric conversion elements capable of interconverting thermal energy and electrical energy are known. A thermoelectric conversion element is composed of two types of thermoelectric conversion materials, p-type and n-type, and is configured such that the two types of thermoelectric conversion materials are electrically connected in series and thermally arranged in parallel. . When a voltage is applied between both terminals of a thermoelectric conversion element, holes and electrons move and a temperature difference occurs between both surfaces (Peltier effect). On the other hand, in a thermoelectric conversion element, when a temperature difference is applied between both surfaces, holes and electrons similarly move, generating an electromotive force between both terminals (Seebeck effect). For this reason, thermoelectric conversion elements are used as elements for cooling refrigerators, car air conditioners, etc., and as elements for power generation utilizing waste heat generated from refuse incinerators.

Conventionally, intermetallic compounds have been known as thermoelectric conversion materials constituting thermoelectric conversion elements. Among them, thermoelectric conversion materials containing Bi ₂ Te ₃ as a main component have long been known for their large Seebeck coefficients. It has the drawback of being difficult to process and containing toxic elements. Therefore, in order to compensate for these drawbacks, Fe ₂ VAl-based intermetallic compounds having a Heusler alloy type crystal structure have been developed. In this Fe ₂ VAl-based thermoelectric conversion material, by substituting at least part of one element constituting Fe ₂ VAl with another element, the Seebeck coefficient at room temperature or slightly higher than room temperature, 300 to 400 K, has a large absolute value. (Patent Document 1).

Then, it is possible to change the total number of valence electrons and change the sign of the Seebeck coefficient, that is, control to p-type or n-type by combining the element to be substituted and the substituting element. In addition, many combinations of elements to be substituted and substituting elements are possible. Specifically, substitution of Fe with Pt or Co, substitution of V with Mo or Ti, and substitution of Al with Si, Ge, or Sn are possible. is disclosed. More specifically, by substituting Si or Ge for Al, the Seebeck coefficient changes from positive to negative, and its absolute value increases. The Seebeck coefficient remains positive even when V is replaced with Ti, but when it is replaced with Mo, the Seebeck coefficient changes from positive to negative, and its absolute value increases.

Furthermore, in the Fe ₂ VAl system, the same element substitution as in Patent Document 1 is used to measure the thermal conductivity or electrical resistivity, which are factors other than the Seebeck coefficient that determines the thermoelectric figure of merit, at room temperature. It is disclosed that element substitution reduces the thermal conductivity and electrical resistivity, resulting in an increase in the figure of merit near room temperature (Patent Document 2).

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

However, Patent Documents 1 to 3 above do not disclose the Seebeck coefficient, electrical resistivity, etc. in a medium to high temperature range of 400 to 900K, which is higher than room temperature. Also in Document 1, in which measurements are made at temperatures higher than room temperature, the peak of the Seebeck coefficient is 300 to 400 K, and although the peak temperature may be higher, the absolute value is small. As described above, it was difficult to obtain a large Seebeck coefficient in the temperature range of 400 to 900K. Furthermore, in the Fe ₂ VAl system, there is a problem that the p-type Seebeck coefficient is extremely small compared to the n-type.

Japanese Patent No. 4035572 Japanese Patent Application Laid-Open No. 2004-253618 Japanese Patent Application Laid-Open No. 2004-119647

An object of the present invention is to put a thermoelectric conversion material into practical use in the temperature range of 400 to 900 K, and to provide a thermoelectric conversion material with a large Seebeck coefficient and a small electrical resistivity in that temperature range.

The present inventors have found that the above problems can be solved by substituting a part of Ti with another element in thermoelectric conversion materials having basic compositions of Ru ₂ TiSi and Ru ₂ TiGe. That is, according to the present invention, the following thermoelectric conversion material is provided.

[1] The basic structure of Ru ₂ TiD having a Heusler alloy type crystal structure is characterized in that the element D is at least one of Si and Ge.

[2] With respect to the basic structure of Ru ₂ TiSi having a Heusler alloy type crystal structure, another element N substituted for Ti is at least one of V, Nb and Ta, and the substitution amount of the element N is It is characterized in that it is controlled to be n-type by adjusting within the range of 0<α<0.5 that satisfies the general formula Ru ₂ (Ti _1-α N _α )Si.

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

According to the present invention, it is possible to provide a thermoelectric conversion material having a large Seebeck coefficient and a small electrical resistivity in the temperature range of 400 to 900K. As a result, it is possible to provide a thermoelectric conversion element for power generation using exhaust heat of 900 K or less.

It is a figure which shows each X-ray-diffraction pattern of the p-type and n-type thermoelectric conversion materials of this invention. FIG. 2 is a diagram showing temperature dependence of Seebeck coefficients of p-type and n-type thermoelectric conversion materials of the present invention. FIG. 2 is a diagram showing the temperature dependence of each electrical resistivity of p-type and n-type thermoelectric conversion materials of the present invention. FIG. 2 is a diagram showing the temperature dependence of each thermal conductivity of p-type and n-type thermoelectric conversion materials of the present invention. FIG. 2 is a diagram showing the temperature dependence of each figure of merit of p-type and n-type thermoelectric conversion materials of the present invention. FIG. 2 is a diagram showing the temperature dependence of each output factor of the p-type thermoelectric conversion material of the present invention; FIG. 2 is a diagram showing the temperature dependence of each output factor of the n-type thermoelectric conversion material of the present invention; FIG. 4 is a diagram showing the temperature dependence of each figure of merit of the p-type thermoelectric conversion material of the present invention and the p-type thermoelectric conversion material Fe ₂ V _0.69 Ti _0.34 Ta _0.05 Al _0.92 of the comparative example. . FIG. 2 is a diagram showing the temperature dependence of each figure of merit of the n-type thermoelectric conversion material of the present invention and the n-type thermoelectric conversion material Fe ₂ V _0.98 Ta _0.10 Al _0.92 of the comparative example. FIG. 4 is a diagram showing temperature dependence of each output factor of a p-type thermoelectric conversion material of a comparative example; FIG. 4 is a diagram showing temperature dependence of each output factor of an n-type thermoelectric conversion material of a comparative example;

BEST MODE FOR CARRYING OUT THE 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 made without departing from the scope of the invention.

The thermoelectric conversion material of the present invention is manufactured as follows. This production method includes a first step of preparing a raw material mixture having an elemental composition ratio that allows production of the thermoelectric conversion material, and melting or vaporizing the raw material mixture in a vacuum or in an inert gas, and then solidifying it. and a second step of obtaining a thermoelectric conversion material.

In the second step, for example, the raw material mixture is melted by arc melting or casting in vacuum or inert gas, and then solidified to produce an ingot. Although this ingot can be used as a thermoelectric conversion material as it is, a method of mechanically pulverizing this ingot in an inert gas or nitrogen gas atmosphere, a method of molten metal atomization or gas atomization, and an inert gas or mechanical alloying method. A powder having a substantially uniform grain size is obtained by a method such as repeating crimping and breaking of 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 energization method, or the like. For example, when the powder is sintered by the pulse energization method, the sintering proceeds under a uniaxial pressure of 50 MPa at 1600° C., 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 adopted. be.

Then, after the raw material mixture is melted and solidified, the powder obtained by pulverizing is subjected to X-ray diffraction measurement by the X-ray diffraction method. On the other hand, the powder obtained by the pulverization is sintered by a pulse energization method or the like, and then cut, etc., and the electrical resistivity, Seebeck coefficient, and thermal conductivity are measured in predetermined sizes. .

The Ru ₂ TiSi-based thermoelectric conversion material of the present invention is a thermoelectric conversion material having a stoichiometric composition of Ti and Si with respect to Ru, and is a thermoelectric conversion material controlled to be p-type. Further, the temperature at which the absolute value of the Seebeck coefficient peaks in the p-type thermoelectric conversion material of the present invention is preferably 600K or higher.

In addition, the Ru ₂ TiSi-based thermoelectric conversion material of the present invention has the general formula Ru ₂ (Ti _1-α N _α ) A thermoelectric conversion material that satisfies Si, satisfies 0≦α≦0.30, and is controlled to be n-type. Furthermore, in the thermoelectric conversion material of the present invention controlled to n-type, the temperature at which the absolute value of the Seebeck coefficient peaks is preferably 600K or higher.

EXAMPLES The present invention will be described in more detail below based on examples, but the present invention is not limited to these examples.

(Example)
Thermoelectric conversion characteristics were investigated for Ru ₂ TiSi having a stoichiometric composition. Ruthenium (Ru) with a purity of 99.98% by mass, titanium (Ti) with a purity of 99.9% by mass, and silicon (Si) with a purity of 99.99% by mass are weighed so as to have the above composition, and further mixed to obtain a raw material. A mixture was obtained. On the other hand, for the n-type composition Ru ₂ (Ti _1-α Ta _α )Si, α=0.03, 0.06 by mixing tantalum (Ta) with a purity of 99.99% by mass in addition to the above raw materials. , 0.12, 0.20.

Next, this raw material mixture was arc-melted in an argon atmosphere to produce a button-shaped ingot. To obtain a homogenous ingot, the ingot was remelted to obtain a homogenous ingot. The change in mass before and after dissolution was 0.1% or less, and it was assumed that the change in mass due to dissolution was negligible.

Thereafter, this ingot was homogenized in a high vacuum of 5×10 ⁻³ Pa or less at 1273 K for 48 hours, and then processed into strips, powders, and blocks for measurement. After that, strain removal treatment was performed in vacuum at 1273K for 1 hour. Thus, a thermoelectric conversion material was obtained.

<evaluation>
(1) X-ray diffraction measurement Using the obtained thermoelectric conversion material of the example as a powder, X-ray diffraction measurement was performed by a powder X-ray diffraction method. The results are shown in FIG. From the XRD pattern, it was confirmed that it was composed of a D0 ₃ (L2 ₁ ) single phase and had a Heusler alloy type crystal structure. Furthermore, it was confirmed from the fluorescent X-ray analysis that impurities were not present and that the composition was substantially the target composition.

(2) Measurement of Seebeck Coefficient An ingot of the thermoelectric conversion material of Example was cut with a dicing saw to obtain a prismatic test piece of 2.5 mm×2.5 mm×10 mm. Then, the Seebeck coefficient was measured at 300 to 1000K using ZEM-3 manufactured by ULVAC-RIKO. FIG. 2 shows the temperature variation of the Seebeck coefficient of p-type Ru ₂ TiSi (α=0) and n-type Ru ₂ (Ti _1-α Ta _α )Si. The Seebeck coefficient of the p-type material shows a positive peak at 700K, reaching 190 μV/K. On the other hand, the n-type material shows a negative peak around 600K to 900K, reaching -160 μV/K.

(3) Measurement of electrical resistivity (specific resistance) The electrical resistivity of the thermoelectric conversion materials of Examples was measured at 300 to 1000K using ZEM-3 manufactured by ULVAC-RIKO. FIG. 3 shows the temperature variation of the electrical resistivity of p-type Ru ₂ TiSi (α=0) and n-type Ru ₂ (Ti _{1 -α} Ta _α )Si. The electrical resistivity of the p-type material peaks at 600K and reaches 840 μΩcm. On the other hand, the peak of the electrical resistivity of the n-type material is 800K to 1000K, and the peak tends to shift to the high temperature side as α increases, but the peak value of the electrical resistivity decreases.

(4) Measurement of Thermal Conductivity Sintered bodies of p-type and n-type thermoelectric conversion materials of Examples were cut by electrical discharge machining to obtain disc-shaped test pieces of 10 mmφ×1 mm. Then, the thermal conductivity was measured by the laser flash method using LFA1000 manufactured by LINSEIS. The results are shown in FIG. 4, respectively. The thermal conductivity of p-type Ru ₂ TiSi (α=0) is 20 W/mK at 300 K, but decreases sharply as the temperature rises. On the other hand, in n-type Ru ₂ (Ti _1-α Ta _α )Si, the thermal conductivity decreases significantly as α increases, reaching approximately 5 W/mK at α = 0.20, and almost no change in temperature. is no longer visible.

FIG. 5 shows the result of obtaining a dimensionless thermoelectric figure of merit ((Seebeck coefficient) ² × temperature/electrical resistivity × thermal conductivity) from the above Seebeck coefficient measurements, electrical resistivity measurements, and thermal conductivity measurements. . It can be seen that the dimensionless figure of merit of p-type Ru ₂ TiSi (α=0) rapidly increases with increasing temperature and reaches 0.4 at 1000K. On the other hand, n-type Ru ₂ (Ti _{1 -α} Ta _α )Si exhibits a dimensionless figure of merit of 0.4 at 900K because the thermal conductivity is greatly reduced by Ta substitution. From this result, it can be seen that both the p-type and the n-type are remarkably superior to the conventional Fe ₂ VAl system in the figure of merit in the high temperature range.

(comparison test)
As an index for evaluating a thermoelectric conversion material, there is an output factor ((Seebeck coefficient) ² /electrical resistivity). The temperature dependence of the power factor was determined for Ru ₂ TiSi and Ru ₂ TiGe as the p-type thermoelectric conversion materials of Examples. The results are shown in FIG. It can be seen that both exhibit a high output factor in the temperature range of 400K to 900K. This result shows that the output factor is superior not only to the Fe ₂ VAl system, but also to the p-type half-Heusler alloy and the manganese silicide system thermoelectric conversion material, which are known as environmentally friendly types. Therefore, according to the thermoelectric conversion materials of the examples, it is possible to exhibit better power generation efficiency even in the medium to high temperature range of 400 to 900K compared to the conventional thermoelectric conversion materials.

(comparison test)
The output factor of Ru ₂ (Ti _1-α Nb _α )Si (α=0.12) and Ru ₂ (Ti _1-α Ta _α )Si (α=0.20) as n-type thermoelectric conversion materials of Examples Temperature dependence was obtained. The results are shown in FIG. Although the output factor is lower than that of the p-type, it is larger than the conventional Fe ₂ VAl system in the high temperature range of 600 K or higher, and can exhibit better power generation efficiency in the high temperature range. .

From FIG. 8, it can be seen that the Fe ₂ VAl-based p-type has a dimensionless figure of merit ZT of about 0.18 at 500 K, and the performance is degraded at high temperatures. On the other hand, it can be seen that the Ru system is more than twice as high. On the other hand, as shown in FIG. 9, when the Fe ₂ VAl system is subjected to a special treatment called HPT processing, the ZT of the n-type increases to about 0.3 at 400K. This is the highest performance of the Fe ₂ VAl system at present, but at high temperatures of 600K or higher, the performance of the Ru system is about 1.4 times higher.

As shown in FIGS. 10 and 11, the p-type half-Heusler alloy Zr _0.5 Hf _0.5 CoSb _0.8 Sn _0.2 has an output factor of 2.8×10 ⁻³ W/mK ² at 673 K, It is found to be 2.26×10 ⁻³ W/mK ² at 700K for manganese silicide Mn _0.98 V _0.02 Si.

INDUSTRIAL APPLICABILITY The material of the present invention can be used for thermoelectric conversion elements, particularly power generating elements that generate electricity using exhaust heat in the temperature range of 400 to 900K.

Claims (2)

A p-type thermoelectric conversion material characterized in that element D is at least one of Si and Ge in a basic structure of Ru ₂ TiD having a Heusler alloy type crystal structure. With respect to the basic structure of Ru ₂ TiSi having a Heusler alloy type crystal structure, another element M substituted for Ti is at least one of V, Nb and Ta, and the substitution amount of the element M is given by the general formula Ru _2. A thermoelectric conversion material characterized in that it is controlled to be n-type by adjusting within a range of 0<α<0.5 that satisfies (Ti _1-α M _α )D.

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