JP5448942B2 - Thermoelectric conversion material - Google Patents

Description

  The present invention relates to a thermoelectric conversion material using a clathrate compound.

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 dimensionless figure of merit ZT of the thermoelectric conversion element using the Seebeck effect can be expressed by the following mathematical formula (1).

ZT = S ² ρT / κ (1)
(Here, S, ρ, κ, and T represent the Seebeck coefficient, electrical resistivity, thermal conductivity, and measurement temperature, respectively.)

As is clear from the above formula (1), in order to improve the performance of the thermoelectric conversion element, it is important to increase the Seebeck coefficient of the element, increase the electrical conductivity, and decrease the thermal conductivity. It is.

As thermoelectric materials with high performance index, bismuth and tellurium materials, silicon
Thermoelectric conversion elements using germanium materials, lead / tellurium materials, and the like are known.

By the way, the conventional thermoelectric conversion element has the problem which should be solved, respectively. For example, bismuth-tellurium-based materials have a large ZT value at room temperature.
The T value becomes small and cannot be used as a thermoelectric material. Bismuth / tellurium and lead / tellurium contain the environmentally hazardous substances lead and tellurium. Silicon-germanium-based materials have large ZT at a high temperature range of 700 ° C. or higher, but have low performance at medium and low temperatures.

Therefore, there is a demand for new thermoelectric materials that have good thermoelectric performance, low environmental burden, and lighter weight. As one of such new thermoelectric materials, a clathrate compound based on Si, which is a light element, has attracted attention.

The composition of several types of Si clathrate compounds and methods for synthesizing polycrystals have already been disclosed.

For example, a clathrate compound formed of a group 14 element and a group 13 element to form a clathrate lattice and encapsulate a group 2 element (see Patent Document 1), a Ba ₈ Cu ₆ Si ₄₀ clathrate compound (
Patent Document 2), Ba ₈ (Al, Ga) _x Si _46-x , 10.8 ≦ x ≦ 12.
2 (see Patent Document 3), La ₂ Ba ₆ A added with rare earth elements
A clathrate compound such as u ₆ Si ₄₀ (see Patent Document 4) is disclosed.

Japanese Patent No. 4372276 JP 2001-064006 A JP 2004-067425 A JP 2006-057124 A

  By the way, the following subjects are mentioned in these Si clathrate compounds.

In the technique described in Patent Document 1, ZT = 1 at 700 K in Ba ₈ Si ₂₆ Al ₂₀ .
. Although it is disclosed that it is 05, the characteristics at other temperatures are not clear, and there is a concern that the performance is low. In the technique described in Patent Document 2, it is disclosed that in Ba ₈ Al ₁₆ Si ₃₀ , ZK = 0.5 at 1000 K, but the characteristics are low at medium and low temperatures. In the technique described in Patent Document 3, the thermoelectric characteristics are unknown. In the technique described in Patent Document 4, the prediction that the band gap increases and the ZT increases due to the addition of rare earth elements is disclosed, but the thermoelectric conversion performance is unknown.

Then, an object of this invention is to provide the thermoelectric conversion material using the Si clathrate compound which is suitable for a thermoelectric conversion element, and has the outstanding thermoelectric conversion characteristic.

That is, the present invention relates to a clathrate compound A _{a Mb} Q _c Si _46-bc (A = Ba, one or more elements selected from Sr, M = Cu, Ag, Au, one or more elements selected from Element, Q = one or more elements selected from Al, Ga and In), the a of the clathrate compound A _a M _b Q _c Si _46- bc is 7.7 or more and 8.0 or less, and b 3.4 to 3.9, c is 2.2 to 3.6, and D value defined by D = −2a + 3b + c is −3 or more. The

Further, the present invention is more preferably characterized in that the sintered density is a sintered body exceeding 95% of the theoretical density and the strongest peak ratio of the clathrate compound phase is 85% or more.

The thermoelectric conversion material according to the present invention includes those containing a clathrate compound as a main component and a small amount of other additives.

  ADVANTAGE OF THE INVENTION According to this invention, the thermoelectric conversion element which has the outstanding thermoelectric conversion characteristic can be provided.

An embodiment of the present invention will be described.
A preferred embodiment of the present invention comprises a clathrate compound A _a M _b Q _c Si _46-bc (A
= Thermoelectric conversion using one or more elements selected from Ba and Sr, M = one or more elements selected from Cu, Ag and Au, and Q = one or more elements selected from Al, Ga and In) The D value defined by D = -2a + 3b + c is -3 or more.

The clathrate compound used in the embodiment of the present invention can be produced, for example, through a dissolution step for dissolving the raw material mixture of the clathrate compound. The component of the raw material mixture may be a single element, or may be an alloy or compound containing two or more elements. Further, the raw material mixture is blended so as to obtain a predetermined composition.

As the melting temperature of the raw material mixture, the melting point of Si, which is the raw material with the highest melting point (1414 ° C.)
Although the above is necessary, it is desirable that the melting temperature be as low as possible in consideration of energy saving during production. The dissolution temperature is more preferably 1414 ° C to 1500 ° C,
1420 ° C. is particularly preferred.

In addition, as the dissolution time, it is necessary to mix all the raw materials uniformly in a liquid state, but it is desired that the time is as short as possible in consideration of energy saving during production, and preferably 1 to 100 minutes. Is more preferable, and 1 to 5 minutes is particularly preferable.

The method for dissolving the raw material mixture is not particularly limited, and various methods can be used. For example, it may be melted by arc melting, high frequency melting, a vacuum induction device using an alumina crucible, a cold crucible melting device or the like.

In addition, in order to prevent oxidation, the raw material mixture is dissolved in a vacuum or near vacuum,
It is preferable to carry out in an inert gas atmosphere.

In the production of the clathrate compound used in the embodiment of the present invention, an annealing treatment may be performed after the dissolution step. In consideration of energy saving at the time of manufacture, it is desirable that the time is as short as possible. However, considering the annealing effect, a long time is required, and the treatment time of the annealing treatment is preferably 1 hour or more, and more preferably 1 to 10 hours.

700-930 degreeC is preferable and the process temperature of annealing treatment has more preferable 850-930 degreeC.

A preferred embodiment of the thermoelectric conversion element of the present invention is a sintered body of the above clathrate compound. A preferred embodiment of the thermoelectric conversion element of the present invention undergoes a fine particle forming step of making the clathrate compound of the preferred embodiment of the present invention 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 the like.

The particle size of the fine particles is desired to be small in order to improve the sinterability, and 15
It is preferably 0 μm or less, and more preferably 75 μm or less.

Further, a so-called flowing gas evaporation method can be used in which a vapor of a clathrate compound is generated in a vacuum and the vapor is blown off with a high-pressure inert gas to obtain fine particles. For details of the flowing gas evaporation method, see Japanese Patent Publication No. 5-9483.
Detailed information on issue gazettes.

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 when using the discharge plasma sintering method, the temperature is preferably 600 to 900 ° C, more preferably 800 to 900 ° C. The sintering time is preferably 1 to 10 minutes, and more preferably 3 to 7 minutes. The pressure is preferably 40 to 80 MPa, and more preferably 50 to 70 MPa.

(Characteristic evaluation test)
The formation of the clathrate compound in a preferred embodiment of the present invention can be confirmed by X-ray diffraction. Specifically, if the calcined sample shows only a type I clathrate phase by X-ray diffraction, it can be confirmed that a type I clathrate compound has been synthesized.

In the preferred embodiment of the present invention, the strongest peak ratio of the clathrate compound phase is preferably 85% or more, and more preferably 90% or more. The peak ratio is the strongest peak (IHS) of the clathrate compound phase measured in the powder X-ray diffraction measurement,
From the strongest peak intensity (IA) of impurity phase A and the strongest peak intensity (IB) of impurity phase B,
It is defined as IHS / (IHS + IA + IB) × 100 (%).

Next, a description will be given of a characteristic investigation conducted for calculating the dimensionless figure of merit ZT for the thermoelectric material according to the present invention manufactured by the above method. Characteristic survey items are powder X-ray diffraction (XR
D), microstructure observation, electron beam microanalyzer (EPMA), Seebeck coefficient S,
The electrical resistivity ρ and the thermal conductivity κ were used.

The Seebeck coefficient and the electrical resistivity were measured by a four-terminal method using a thermoelectric property evaluation apparatus ZEM-3 manufactured by ULVAC-RIKO.

Thermal conductivity κ is
κ = cδα (2)
It was calculated by the following formula. Here, c is specific heat, δ is density, and α is thermal diffusivity.

The specific heat c was measured by DSC (Differential Scanning Calorimetry) method. As a measuring device, differential scanning calorimeter EXSTAR6 made by SII Nano Technology Co., Ltd.
000 DSC was used.
The density δ was measured by the Archimedes method. As a measuring device, a precision electronic balance LIBBROR AEG-320 manufactured by Shimadzu Corporation was used.
The thermal diffusivity α was measured by a laser flash method. As a measuring device, a thermal constant measuring device TC-7000 manufactured by ULVAC-RIKO was used.
The sintered density was evaluated by the ratio δ / δt between the density δ and the theoretical density δt. Theoretical density δt
Is a density of an ideal sintered body that does not include pores or defects, and is theoretically calculated.

Based on the measurement results, the dimensionless performance index ZT, which is an index for evaluating the thermoelectric conversion performance, is calculated using the above-described formula (1), and the characteristics of the obtained thermoelectric conversion material are evaluated.

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

One or more elements selected from high-purity Ba and Sr having a purity of 2N or higher, one or more elements selected from high-purity Cu, Ag, and Au having a purity of 3N or higher, and high-purity Al having a purity of 3N or higher,
One or more elements selected from Ga and In and high-purity Si having a purity of 3N or more were weighed to prepare a raw material mixture.

This raw material mixture was arc-melted in an Ar atmosphere, and then cooled to obtain an ingot. The obtained ingot was pulverized using an agate planetary ball mill to obtain a pulverized product.
The obtained pulverized product was sintered using a discharge plasma sintering method (SPS method). The sintered product thus obtained was subjected to the above characteristic evaluation test. The “EPMA quantitative value” in Table 1 is the clathrate compound A _{a Mb} Q _c Si _46-bc (element A is selected from Ba and Sr)
The element more than the component, the element M is one or more elements selected from Cu, Ag, and Au, and the element Q is one or more elements selected from Al, Ga, and In).

Regarding the dimensionless figure of merit ZT, the one showing a value of ZT of 0.3 or higher at a temperature of 500 ° C. is judged as “good” and the table is marked with “O” and shows a value of less than 0.3 Was determined to be “impossible” and the table was marked with “x”. In addition, the clathrate compound A _a M _b Q _c Si _46-bc (
Element A is one or more elements selected from Ba and Sr, Element M is one or more elements selected from Cu, Ag, and Au, and Element Q is one or more elements selected from Al, Ga, and In In the table, the D value defined by D = -2a + 3b + c is shown in the table.

  The obtained results are shown in Table 1.

As shown in Table 1, all the samples having a D value of −3 or more, which is a preferred embodiment of the present invention, exhibited a good value of ZT of 0.3 or more at a temperature of 500 ° C. On the other hand, it was found that in all the samples in which the D value was smaller than −3, ZT at a temperature of 500 ° C. was less than 0.3, and the thermoelectric conversion efficiency deteriorated.

Claims (3)

Clathrate compound _{A a M b Q c Si 46} -b-c ( element A, Ba, 1-component or more elements selected from Sr, the element M, Cu, Ag, 1-component or more elements selected from Au, Element Q is a thermoelectric conversion material using one or more elements selected from Al, Ga, and In,
Wherein a clathrate compound _{A a M b Q c Si 46} -b-c is 7.7 to 8.0, and b is 3.4 or 3.9 or less, and c is 2.2 or more 3.6 And
A D value defined by D = −2a + 3b + c is −3 or more. The thermoelectric conversion material according to claim 1, wherein the sintered density is a sintered body exceeding 95% of the theoretical density.   The clathrate compound defined by the strongest peak (IHS) of the clathrate compound phase, the strongest peak intensity (IA) of the impurity phase A, and the strongest peak intensity (IB) of the impurity phase B, measured in powder X-ray diffraction measurement Strongest peak ratio of phase
      IHS / (IHS + IA + IB) × 100 (%)
The thermoelectric conversion material according to claim 1, wherein the value of is at least 85%.