JP2007051345A - Clathrate compound, and thermoelectric conversion element using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a new clathrate compound; and to provide a thermoelectric conversion element using the same. <P>SOLUTION: The clathrate compound is expressed by Sr<SB>8-x</SB>Ba<SB>x</SB>Ga<SB>y</SB>Ge<SB>46-y</SB>(0<x≤2, 12≤y<18). The thermoelectric conversion element is obtained by using the sintered compact thereof. The clathrate compound has the properties as those of an N type semiconductor. When x lies at the outside of the above range, the electric conductivity thereof is made low, thus its figure-of-merit can be made low. Further, when y lies at the outside of the above range, the carrier density thereof is too high, thus its Seebeck coefficient can become low. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a clathrate compound and a thermoelectric conversion element using the same.

  Thermoelectric conversion elements using the Seebeck effect can convert thermal energy into electrical energy. Utilizing this property, it is possible to convert exhaust heat exhausted from industrial / consumer processes and mobile objects into effective electric power, and thermoelectric conversion elements are attracting attention as energy-saving technologies in consideration of environmental problems.

  The figure of merit ZT of the thermoelectric conversion material used for the thermoelectric conversion element utilizing the Seebeck effect can be expressed by the following formula (A).

ZT = α ² σT / κ (A)

  Here, α, σ, κ, and T represent the Seebeck coefficient, electrical conductivity, thermal conductivity, and measurement temperature, respectively.

  As apparent from the above formula (A), in order to improve the performance of the thermoelectric conversion element, it is necessary to increase the Seebeck coefficient and electrical conductivity of the material used for the element, and to decrease the thermal conductivity. is important.

A clathrate compound is known as a compound having both high electrical conductivity and low thermal conductivity (see, for example, Patent Document 1). The clathrate compound has a structure in which alkali and / or alkaline earth metal atoms are encapsulated in a polyhedral cage having group III and / or group IV atoms at the top, and has attracted attention as a novel thermoelectric conversion material. .
Japanese Patent Laying-Open No. 2005-136032

In an example of Patent Document 1, Ba ₈ Ga ₁₆ Ge ₃₀ is disclosed. However, the electrical conductivity of Ba ₈ Ga ₁₆ Ge ₃₀ is smaller than that of existing thermoelectric conversion materials, and further improvement is necessary.

  The present invention has been made in view of the above problems, and an object thereof is to provide a novel clathrate compound suitable for a thermoelectric conversion element and a thermoelectric conversion element using the same.

That is, the present invention
<1> A clathrate compound represented by the following composition formula (1).

Sr _8-x Ba _x Ga _y Ge _46-y (0 <x ≦ 2, 12 ≦ y <18) (1)

  <2> A thermoelectric conversion element using the sintered body of the clathrate compound according to <1>.

  ADVANTAGE OF THE INVENTION According to this invention, the novel clathrate compound suitable for a thermoelectric conversion element and a thermoelectric conversion element using the same can be obtained.

  Hereinafter, the clathrate compound of the present invention and the thermoelectric conversion element using the same will be described in detail.

<Clathrate compound>
The clathrate compound of the present invention is represented by the following composition formula (1).

Sr _8-x Ba _x Ga _y Ge _46-y (0 <x ≦ 2, 12 ≦ y <18) (1)

  The clathrate compound of the present invention has properties as an n-type semiconductor. When x in the composition formula (1) is out of the above range, the electrical conductivity becomes low, and the figure of merit ZT may be small. If y is out of the above range, the Seebeck coefficient may be small because the carrier density is too high.

  In the composition formula (1), a more preferable range of x is 0.5 ≦ x ≦ 2, and a more preferable range of y is 15 ≦ y ≦ 17.

  The clathrate compound of the present invention can be synthesized through a melting step of melting the constituent elements constituting the clathrate compound, but is not limited to this method. As a melting temperature, 1000-1500 degreeC is preferable, 1000-1400 degreeC is more preferable, 1200-1400 degreeC is especially preferable. The melting time is preferably 10 to 100 minutes, more preferably 10 to 60 minutes, and particularly preferably 20 to 60 minutes. As a melting method, an arc melting method, a high-frequency heating method, or the like can be used.

<Thermoelectric conversion element>
The thermoelectric conversion element of the present invention uses a sintered body of the clathrate compound of the present invention. The thermoelectric conversion element of the present invention is subjected to a fine particle forming step of making the clathrate compound of the present invention synthesized through the melting step into fine particles, and a sintering step of sintering the fine particles obtained in the fine particle forming step. Although it can manufacture, it is not limited to this method.

  In the fine particle formation step, fine particles can be obtained by pulverizing the clathrate compound using a ball mill or a mortar. The particle diameter of the fine particles is preferably 150 μm or less, and more preferably 90 μm or less. Further, a so-called flowing gas evaporation method can be used in which vapor of a clathrate compound is generated in vacuum and the vapor is blown off with a high-pressure inert gas to obtain fine particles. Details of the flowing gas evaporation method are described in Japanese Patent Publication No. 5-9483. In the sintering step, the fine particles can be sintered using a discharge plasma sintering method, a hot press sintering method, a hot isostatic pressing method, or the like.

  As a sintering condition in the case of using the discharge plasma sintering method, the temperature is preferably 650 to 950 ° C, and more preferably 700 to 900 ° C. The sintering time is preferably 20 to 120 minutes, more preferably 30 to 90 minutes. The pressure is preferably 25 to 40 MPa, more preferably 30 to 40 MPa.

  The formation of the clathrate compound of the present invention can be confirmed by X-ray diffraction. Specifically, if the sample after firing shows only the clathrate phase by X-ray diffraction, it can be confirmed that the clathrate compound has been synthesized.

  EXAMPLES Hereinafter, although this invention is demonstrated further in detail using an Example, this invention is not limited by the following Example.

[Manufacture of clathrate compounds and thermoelectric conversion elements]
As raw materials, Sr (99.9%), Ba (99.9%), Ga (99.9999%) and Ge (99.9999%) were weighed in a predetermined amount under an argon atmosphere, and this was measured under an argon atmosphere. It was melted and cooled by arc melting (arc temperature 1200 ° C., arc melting time 20 minutes) to obtain the desired clathrate compound. Table 1 summarizes the charged amounts of Sr, Ba, Ga and Ge used for each clathrate compound.

  Each obtained clathrate compound was pulverized with an agate mortar so as to have a particle size of less than 106 μm under an argon atmosphere in a glove box to obtain fine particles. Fine particles of each clathrate compound were baked by a discharge plasma sintering apparatus at a temperature of 680 to 710 ° C., a holding time of 30 minutes, and a sintering pressure of 40 MPa to obtain thermoelectric conversion elements using the respective clathrate compounds. Each clathrate compound was cut into a width of 1 to 2 mm, a thickness of 1 to 2 mm, and a length of 15 to 20 mm and used as a measurement sample.

-Measurement of Seebeck coefficient-
FIG. 1 is a diagram for explaining a method of measuring the Seebeck coefficient. The thermocouple 40 and the thermocouple 50 were bonded to the thermoelectric conversion element 30 placed on the stage 20 provided in the heating furnace 10 with an interval of about 10 mm. Adhesion of the thermocouple was performed by baking at 180 ° C. for 15 minutes using a silver paste (Tokuriku head office, conductive paste (Sylbest) PS-769). A thermometer 42 and a thermometer 52 were connected to the thermocouple 40 and the thermocouple 50, respectively. Moreover, the voltmeter 60 was connected to the plus leg side of the thermocouple 40 and the thermocouple 50. A heater 70 is arranged on the surface of the stage 20 opposite to the surface on which the thermoelectric conversion element 30 is placed so that one side of the thermoelectric conversion element 30 can be heated.

  The heating furnace 10 is heated to set the temperature in the furnace to T, and the heater 70 is heated on the side where the thermocouple 50 is bonded, and the thermocouple 40 is bonded to the portion where the thermocouple 50 is bonded. A temperature difference ΔT was generated between the film and the portion where the film is bonded. ΔT was obtained by the thermometer 42 and the thermometer 52, and the voltage (thermal electromotive force (V)) when ΔT reached about 10 ° C. was measured. The Seebeck coefficient (α) was calculated from V / ΔT using the obtained V and ΔT. The obtained results are shown in FIGS.

-Measurement of electrical conductivity-
Electrical conductivity (σ) was measured by the four probe method. The obtained results are shown in FIGS.

-Calculation of power factor-
The power factor (α ² σ) was calculated using the obtained Seebeck coefficient and electrical conductivity. The obtained results are shown in FIGS. Here, the value of the power factor includes an error of ± 10%. The obtained results show that the thermoelectric conversion element of the present invention is an excellent n-type thermoelectric semiconductor. 4, 7 and 13, the thermoelectric conversion element using the clathrate compound according to x = 0 (outside the present invention) of the composition formula (1) uses the clathrate compound according to x = 2 (the present invention). Although the power factor higher than a thermoelectric conversion element may be shown, these show an equivalent power factor within an error range.

It is a figure for demonstrating the measuring method of a Seebeck coefficient. Is a diagram showing the value of the Seebeck coefficient of the thermoelectric conversion element using the clathrate compound represented by _{Sr 8-x Ba x Ga 16} Ge 30. Is a diagram showing the value of the electrical conductivity of the thermoelectric conversion element using the clathrate compound represented by _{Sr 8-x Ba x Ga 16} Ge 30. Is a diagram showing the value of the power factor of the thermoelectric conversion element using the clathrate compound represented by _{Sr 8-x Ba x Ga 16} Ge 30. It is a diagram showing the value of the Seebeck coefficient of the thermoelectric conversion element using the clathrate compound represented by _{Sr 8-x Ba x Ga 16.5} Ge 29.5. Is a diagram showing the value of the electrical conductivity of the thermoelectric conversion element using the clathrate compound represented by _{Sr 8-x Ba x Ga 16.5} Ge 29.5. It is a diagram showing the value of the power factor of the thermoelectric conversion element using the clathrate compound represented by _{Sr 8-x Ba x Ga 16.5} Ge 29.5. Is a diagram showing the value of the Seebeck coefficient of the thermoelectric conversion element using the clathrate compound represented by _{Sr 8-x Ba x Ga 15} Ge 31. Is a diagram showing the value of the electrical conductivity of the thermoelectric conversion element using the clathrate compound represented by _{Sr 8-x Ba x Ga 15} Ge 31. Is a diagram showing the value of the power factor of the thermoelectric conversion element using the clathrate compound represented by _{Sr 8-x Ba x Ga 15} Ge 31. Is a diagram showing the value of the Seebeck coefficient of the thermoelectric conversion element using the clathrate compound represented by _{Sr 8-x Ba x Ga 13} Ge 33. Is a diagram showing the value of the electrical conductivity of the thermoelectric conversion element using the clathrate compound represented by _{Sr 8-x Ba x Ga 13} Ge 33. Is a diagram showing the value of the power factor of the thermoelectric conversion element using the clathrate compound represented by _{Sr 8-x Ba x Ga 13} Ge 33.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Heating furnace 20 Stage 30 Thermoelectric conversion element 40, 50 Thermocouple 42, 52 Thermometer 60 Voltmeter 70 Heater

Claims (2)

A clathrate compound represented by the following composition formula (1).
Sr _8-x Ba _x Ga _y Ge _46-y (0 <x ≦ 2, 12 ≦ y <18) (1)   The thermoelectric conversion element using the sintered compact of the clathrate compound of Claim 1.

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