JP5773483B2 - Thermoelectric conversion material - Google Patents

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. This 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 arranged in thermal parallel. ing. In this thermoelectric conversion element, when a voltage is applied between both terminals, movement of holes and electrons occurs and a temperature difference occurs between both surfaces (Peltier effect). On the other hand, if this thermoelectric conversion element gives a temperature difference between both surfaces, the movement of holes and electrons also occurs, and an electromotive force is generated between both terminals (Seebeck effect). For this reason, using a thermoelectric conversion element as an element for cooling, such as a refrigerator and a car air conditioner, or using it as an element for electric power generation using exhaust heat generated from a garbage incinerator is examined.

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. On the other hand, recently, in order to compensate for the disadvantages of Bi ₂ Te ₃ -based materials such as difficulty of processing or high raw material costs, development of Fe ₂ VAl-based intermetallic compounds having a Heusler alloy type crystal structure has been carried out. 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).

Then, 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, from p-type to n-type, or from n-type to p-type. Table 1 lists a number of combinations of elements to be substituted and substitution elements. Specifically, it is disclosed that Fe is replaced with Pt or Co, V is replaced with Mo or Ti, and Al is replaced with Si, Ge, or Sn. 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.

Furthermore, 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 conversion performance index, is measured at room temperature by element substitution similar to that of 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, in the middle to high temperature range of 400 to 700K. 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 500 to 700K.

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

  An object of the present invention is to put a thermoelectric conversion material into practical use in a temperature range of 300 to 700 K, and to provide a thermoelectric conversion material having a large Seebeck coefficient and a low electrical resistivity in the temperature range. To do.

The present inventors have found that the above-described problems can be solved by substituting a part of Al with Ta in the Fe ₂ VAl-based thermoelectric conversion material. That is, according to the present invention, the following thermoelectric conversion materials are provided.

[1] Fe ₂ VAl-based thermoelectric conversion material having a Heusler alloy type crystal structure, Fe ₂ V (Al _1−α Ta _α ) in which a part of Al is substituted with Ta , and 0.03 ≦ α ≦ 0.15 thermoelectric conversion material.

[2] The thermoelectric conversion material according to [1] , wherein the Seebeck coefficient is negative at a temperature of 300 K to 600 K, and the absolute value at a temperature at which the Seebeck coefficient reaches a peak is 100 (μV / K) or more.

  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 a temperature range of 300 to 700K. Thereby, the thermoelectric conversion element for electric power generation using especially the exhaust heat of 700K or less can be provided.

It is a figure which shows the temperature dependence of the electrical resistivity of this invention Examples 1-4. It is a figure which shows the temperature dependence of the Seebeck coefficient of this invention Examples 1-4. It is a figure which shows the element substitution amount dependence of the thermal conductivity in room temperature of this invention Examples 1-4 and Comparative Examples 1-3. It is a figure which shows the element substitution amount dependence of the thermoelectric conversion performance index at room temperature of this invention Examples 1-4 and Comparative Examples 1-3. It is a figure which shows the temperature dependence of the electrical resistivity of Comparative Examples 1-3. It is a figure which shows the temperature dependence of the Seebeck coefficient of Comparative Examples 1-3.

  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 the like. 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 a method of repeatedly pressing and breaking a raw material mixture in an inert gas or nitrogen gas atmosphere called mechanical alloying Etc. to obtain a powder having a substantially uniform particle diameter. 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. In the case of sintering the powder by the HIP method, for example, compression molding and sintering can be simultaneously performed with an argon gas of 150 MPa at 800 ° 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 straining such as hot rolling or quenching the melted 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, further cut and the like, 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 is Fe _{2 + x} V _{1 + y} (Al _1−α Ta _α ) _{1 + z} in which a part of Al is substituted with Ta, and −1 <x <1, −1 < It is a thermoelectric conversion material with y <1, −1 <z <1, 0 <α <1. From the viewpoint of decreasing the electrical resistivity, decreasing the thermal conductivity, and increasing the Seebeck coefficient, α is more preferably 0.01 ≦ α ≦ 0.15, and 0.03 ≦ α ≦ 0.12. More preferably.

In addition, the Fe ₂ VAl-based thermoelectric conversion material of the present invention has −0.2 ≦ x ≦ 0.2, −0.2 ≦ y ≦ 0, in Fe _{2 + x} V _{1 + y} (Al _1−α Ta _α ) _{1 + z} . More preferably, −0.2 ≦ z ≦ 0.2. Further, the total number of valence electrons per chemical formula is preferably 23.2 to 24.8 (the total number of valence electrons is 24 in the stoichiometric composition).

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

Example 1
The characteristics of factors affecting the thermoelectric conversion characteristics by substituting part of Al in the stoichiometric composition Fe ₂ VAl with Ta were investigated. Iron (Fe) with a purity of 99,99% by mass, 99.99% by mass of aluminum (Al), 99.9% by mass of vanadium (V), and 99.9% of Ta with Fe ₂ VAl _{0. 97} Ta _0.03 was weighed and further mixed to obtain a raw material mixture.

  Next, this raw material mixture was arc-melted under 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 Example 1 as powder, X-ray diffraction measurement was performed by a powder X-ray diffraction method. As a result, the thermoelectric conversion material of Example 1 was composed of a DO ₃ (L2 ₁ ) single phase and had a Heusler alloy type crystal structure.

-Measurement of electrical resistivity (specific resistance) The ingot of the thermoelectric conversion material of Example 1 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 1000 K by the DC four-terminal method.

  As a result of the measurement, the electric resistivity had a peak at around 400K and was about 500 μΩcm.

-Measurement of Seebeck coefficient The ingot of the thermoelectric conversion material of Example 1 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 at 100-700K using "SB100" by MMR-Technologies.

The relationship between the measured temperature and Seebeck coefficient is shown in FIG. The Seebeck coefficient was negative, had a peak around 350K, and was about 170 μV / K.

(4) Measurement of thermal conductivity The ingot of the thermoelectric conversion material of Example 1 was cut with a dicing saw to obtain a 3.5 mm × 3.5 mm × 4 mm block-shaped test piece. 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 thermal conductivity at 300K was 12 (W / mK).

(Examples 2 to 4)
Similar to Example 1, iron (Fe) having a purity of 99,99% by mass, aluminum (Al) of 99.99% by mass, vanadium (V) of 99.9% by mass, and Ta of 99.9% by mass. Fe ₂ VAl _0.95 Ta _0.05 (Example 2), Fe ₂ VAl _0.92 Ta _0.08 (Example 3), and Fe ₂ VAl _0.88 Ta _0.12 (Example 4). _The raw material mixture was obtained.

  The raw material mixture was melted and annealed under the same conditions as in Example 1. In X-ray diffraction measurement, electrical resistivity measurement, Seebeck coefficient measurement, and thermal conductivity measurement, a test piece having the same size as that of Example 1 was obtained and measured under the same conditions.

  The temperature dependence of the electrical resistivity, the temperature dependence of the Seebeck coefficient, and the Ta substitution amount dependence at the thermal conductivity (room temperature) of the test pieces of Examples 1 to 4 are shown in FIG. 1, FIG. 2, and FIG. 3 shows.

(Comparative Examples 1-3)
As a comparative example, in the stoichiometric composition Fe ₂ VAl, a sample piece in which a part of Al was substituted with Si was obtained, and the same characteristics as in Example 1 were measured. Fe ₂ VAl _0.97 Si _0.03 (Comparative Example 1), Fe ₂ VAl _0.95 Si _0.05 (Comparative Example 2), and Fe ₂ VAl _0.90 Si _0.10 (Comparative Example 3) Test pieces having different compositions were prepared and measured in the same manner as in Examples 1 to 4. The electrical resistivity measurement result is shown in FIG. 5, the Seebeck coefficient measurement result is shown in FIG. 6, and the Si substitution amount dependency in thermal conductivity (room temperature) is shown in FIG. 3 together with Ta substitution.

FIG. 4 shows the result of obtaining the thermoelectric conversion performance index (room temperature) from the above electrical resistivity measurement, Seebeck coefficient measurement, and thermal conductivity measurement. From this result, in the stoichiometric composition Fe ₂ VAl, when a part of Al is substituted with Ta (Ta substitution), compared with a case where a part of Al is substituted with Si (Si substitution), It can be seen that the figure of merit is excellent when the substitution amount is at least 0.03 to 0.1 at room temperature. From 300K (room temperature) to 600K, the absolute value of the Ta substitution Seebeck coefficient is larger than the Si substitution Seebeck coefficient, and the thermal conductivity of Ta substitution is smaller than the thermal conductivity of Si substitution. Therefore, it can be said that the performance index of Ta substitution is larger.

  The reason why the Ta substitution Seebeck coefficient is larger than the Si substitution Seebeck coefficient is not clear, but the Si structure does not significantly change the electronic structure because it is doped with free-electron 3p electrons. It is presumed that the Ta substitution is caused by a slight change in the electronic structure because it is doped with localized 5d electrons.

  The reason why the thermal conductivity of Ta substitution is smaller than that of Si substitution is that the thermal conductivity of the lattice by using Ta having an atomic number of 73 and a large average atomic weight as compared with Si having an atomic number of 14 It is presumed to be caused by the decline in

The material of the present invention can be used for thermoelectric conversion elements, particularly power generation elements that generate power using exhaust heat in a temperature range of 300 to 700K.

Claims (2)

In an Fe ₂ VAl-based thermoelectric conversion material having a Heusler alloy type crystal structure, Fe ₂ V (Al _1−α Ta _α ) in which a part of Al is substituted with Ta , and 0.03 ≦ α ≦ 0.0. 15 thermoelectric conversion material. The thermoelectric conversion material according to claim 1 , wherein the Seebeck coefficient is negative at a temperature of 300K to 600K, and the absolute value at a temperature at which the Seebeck coefficient reaches a peak is 100 (μV / K) or more.