JP2003258322A - Lanthanum sulfide sintered compact for thermoelectric conversion material and method of manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To develop a material for thermoelectric conversion device having a big Seebeck coefficient and a big figure of merit which is acquired by adding electric conductivity. <P>SOLUTION: A lanthanum sulfide sintered compact comprises a β type crystal structure, in which β type lanthanum sulfide powder and metallic palladium particle are mixed and sintered as major constituents, and a trace of γ type ingredient, wherein the sintered compact contains 1-5 percent by mass of metallic palladium and specific resistance is 100 kΩcm or below. It can be manufactured by mixing high purity β type lanthanum sulfide powder containing oxygen as an impurity of 1.1 weight percent or below and metallic palladium particle with particle diameter of 10 μm or below, and after compression molding with a pressure of 25 MPa or over at ordinary temperatures, sintering it at temperatures ranging from 1200°C to 1500°C in a vacuum or an inert gas for more than 30 minutes. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a lanthanum sulfide sintered body having a small specific resistance and a large thermoelectromotive force, which is particularly useful as a thermoelectric conversion material, and a method for producing the same.

[0002]

2. Description of the Related Art Applications of thermoelectric conversion materials are wide-ranging. It is most expected to be used as a clean energy source that converts heat energy into electric energy, but a small refrigerator that utilizes the Peltier effect,
Heat sinks, thermostats, electric heaters, etc. have been conceived and realized.

Thermoelectromotive force is a voltage V generated by a temperature difference ΔT between two contacts when two kinds of electric conductors are joined, and there is a relation of V = αΔT between them. This α is called the Seebeck coefficient. When this thermal electromotive force is used to convert thermal energy into electrical energy, the electrical conductivity σ and thermal conductivity κ are used as an index showing the effectiveness of the thermoelectric material, and the equation Z = α ² σ is used. A performance index Z represented by / κ is used. The larger the value of Z, the better the thermoelectric material.

Many thermoelectric materials have been reported or used, and the largest figure of merit at present is the Bi-Te type substance, which has a value of about 3 × 10 ^-3 (/ K). As shown, the Seebeck coefficient values for these substances are
It is about 200 (μV / K) ("Practical New Material and Technology Handbook", Intermittent Document Survey Committee, 1996, 904).

Rare earth element sulfides have a large Seebeck coefficient, and among lanthanoid tridisulfides, sulfides from La to Nd are rhombic α-phase to tetragonal β-phase, which are stable phases at low temperature, Furthermore, cubic Th, which is a stable phase at high temperature,
₃ irreversible transformation to P ₄ type γ-phase, in particular, La ₂ S
₃ is +373 μK at +354 μv · deg ⁻¹ , Ce ₂ S ₃
Is a thermoelectric material having a Seebeck coefficient of +574 μv · deg ⁻¹ at 373 K (Ge ·
Ve Samsonov and others: "Sulfide Handbook", Sino-Japan News Agency,
1974, p108).

Also, lanthanum sulfide La _3-x S ₄ and L
It has been reported that a maximum of 2.9 × 10 ⁻⁴ (/ K) figure of merit was obtained in the aAS system (A is Ca or Ba) (Shigeru Katsuyama et al. “Thermoelectric conversion symposium '9.
9 articles ”, 1999, 56). However, the Seebeck coefficient reported there has a maximum value of about 100 (μV /
K).

The inventors of the present invention have found a substance having a Seebeck coefficient which is about an order of magnitude larger than the Seebeck coefficient reported in the lanthanum sulfide system, and applied for a patent (Japanese Patent Laid-Open No. 2001-335367). This substance is
A lanthanum sulfide sintered body having a composition represented by La ₂ S ₃ , a crystal structure consisting of a mixed phase of β and γ, and a Seebeck coefficient having a larger value when it is a γ single phase,
The Seebeck coefficient has a value of 1000 (μV / K) or more at 60 ° C.

[0008]

In the equation for determining the above-mentioned figure of merit Z of the thermoelectric conversion material, three physical properties determine the value, but the value of the Seebeck coefficient α is the square of Z. Since the value is increased, the substance having a large α value can be a better thermoelectric material. The above-mentioned lanthanum sulfide sintered body invented by the present inventor is a material having a large Seebeck coefficient α and a high figure of merit Z, but has a large electric resistance and a small figure of merit.

The thermoelectric conversion material has a wide range of applications such as cooling of electronic devices and sensors as a heat dissipation plate, and it is ultimate to use the temperature difference existing not only on the earth but also in artificial satellites as electric energy. It is a clean energy source, and its development is urgent when considering the deterioration of the global environment.

If the practical application of clean energy using a thermoelectric conversion element is realized, the effect will be very large. Further, application to a heat sink or the like solves the heat generation problem due to the dense integration of ICs, and further progress in this direction such as miniaturization and high integration is expected. Therefore, an object of the present invention is to develop a material for a thermoelectric conversion element that has a large Seebeck coefficient and electrical conductivity, and has a small specific resistance and a large figure of merit.

[0011]

Means for Solving the Problems As a result of studying improvement of electric conductivity by adding various metal elements, the present inventors have found that
It has been found that the above problems can be solved by adding palladium metal.

That is, according to the present invention, the crystal structure obtained by mixing and sintering β-type lanthanum sulfide powder and metallic palladium particles is β
A lanthanum sulfide sintered body containing a mold as a main component and a slight amount of a γ-type component, containing 1 to 5 mass% of metallic palladium, and having a specific resistance of 100 kΩ · cm or less. It is a lanthanum sulfide sintered body for materials.

Regarding the content of palladium metal in the lanthanum sulfide sinter, when the content is low, the electrical resistance is high, and when the content is high, the electrical resistance is low, but the Seebeck coefficient is low.
The figure of merit also decreases. Palladium is β of lanthanum sulfide
It has a melting point above 1300 ° C, which is considered to be the transition temperature from the phase to the γ phase, and does not form compounds with sulfur.

Further, according to the present invention, high-purity β-type lanthanum sulfide powder having an oxygen content of 1.1% by weight or less as an impurity and metallic palladium particles having a particle size of 10 μm or less are mixed, and at room temperature, it is 25 MPa or more. 1 after compression molding with pressure
A method for producing a lanthanum sulfide sintered body for a thermoelectric conversion material, which comprises firing in a temperature range of 200 ° C. to 1500 ° C. in vacuum or in an inert gas for 30 minutes or more.

When the oxygen concentration of the lanthanum sulfide powder exceeds 1.1% by weight, the γ-phase is not generated and it becomes an electrical insulator. When the particle size of the metal palladium particles exceeds 10 μm, the palladium segregates and the electric resistance is 1 × 10 ⁶ Ω.
・ It is not preferable because it becomes larger than cm. If the pressure for compression molding is less than 25 MPa, the sintered body is porous and has an electric resistance of 1 × 10 ⁶ Ω · cm or more, which is inappropriate. The upper limit of the pressure is not particularly limited, but it is not necessary to increase it more than necessary and is preferably about 150 MPa or less.

If the firing temperature is less than 1200 ° C., the γ phase is not generated, and the sintered body remains an electrically insulating material.
If the temperature exceeds 0 ° C, the β phase decreases and the Seebeck coefficient decreases, which is unsuitable. If the firing time is less than 30 minutes, the production of the γ phase of the sintered body or the firing and solidification is not sufficient, which is unsuitable. The firing time may be up to 2 hours at the longest.
Thus, by adopting the normal pressure sintering, a lanthanum sulfide sintered body having a β-type crystal structure as a main component and a slight amount of a γ-type component can be obtained. By this method, the brittleness and electrical properties of the sintered body are improved and a specific resistance of 100 kΩ · cm or less can be realized.

The lanthanum sulfide sintered body of the present invention is, for example,
As a clean energy power generation material using temperature difference, auxiliary power source in spacecraft etc., thermocouple thermometer, electric heater, small refrigerator, heat absorption plate, heat dissipation plate using Peltier effect,
Used for constant temperature baths.

[0018]

EXAMPLES Example 1 La ₂ having a purity of 99.99 mass% and an average particle size of 1.77 μm.
Place the O ₃ powder on a quartz boat and insert it into an electric furnace.
In an atmosphere, heated to a temperature of 1023K, the CS ₂ gas vaporized from the CS ₂ solution is introduced using an Ar carrier gas,
Sulfurization was carried out for 8 hours. The powder after the reaction was confirmed to be a β-phase single phase by an X-ray diffraction method using MgO as an internal standard. Regarding the composition, a rare earth metal was determined by a chelate titration method, and sulfur, carbon, and oxygen were determined by a simultaneous analyzer made by LECO. As a result, their composition was La _2.13 S.
The value was ₃ O _0.13 . The carbon content was below the detection limit.

1.75% by mass of palladium metal particles having a particle size of 1.0 μm or less was mixed with this powder, which was placed in a graphite jig whose inside was coated with hexagonal boron nitride, and the mixture was allowed to stand at room temperature for 5 minutes.
A pressure of 0 MPa was applied for compression molding. Firing was performed by holding this molded body in vacuum at 1500 ° C. for 1 hour. As a result of measuring this sample by the X-ray diffraction method, the β phase was mainly present, and a trace amount of the γ type component was observed. According to the results of local analysis by SEM-EDX, although some palladium is present as grains, it is also present along the grain boundaries of lanthanum sulfide, which is presumed to cause a change in electrical properties. To be done. Seebeck coefficient, resistivity,
The figures of merit are 1570 (μV / K) and 41, respectively.
It was kΩ · cm and 0.1 × 10 ⁻³ .

Example 2 Palladium metal particles were mixed with the same method and conditions as in Example 1.
The sample mixed by mass% was fired. The Seebeck coefficient, the specific resistance and the figure of merit of this sample are 940 (μV
/ K) was 3.44 kΩ · cm and 0.04 × 10 ⁻³ .

Comparative Example 1 A lanthanum sulfide powder produced under the same conditions as in Example 1 was placed in a graphite jig whose inside was coated with hexagonal boron nitride, and compression molded at room temperature under a pressure of 50 MPa. Firing was performed by holding this molded body in vacuum at 1500 ° C. for 1 hour. As a result of measuring this sample by an X-ray diffraction method, it was a sintered body in which the β phase was mainly present and the γ phase coexisted, and the Seebeck coefficient was about 2000 (μV / K) and the specific resistance was 1 × 10 ⁶ Ω.
・ It was cm.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) H01L 35/34 H02N 11/00 A H02N 11/00 C01F 17/00 E // C01F 17/00 C04B 35 / 00 T (72) Inventor Satoshi Nishimura 3-106-205 Takezono, Tsukuba-shi, Ibaraki (72) Inventor Shinji Hirai 1-1-13-13, Kotobuki, Muroran-shi, Hokkaido (72) Inventor Kazunori Shimakage 31 Mizumoto-cho, Muroran-shi, Hokkaido -1-203 F term (reference) 4G030 AA55 AA61 BA01 BA12 BA21 CA04 GA11 GA22 GA24 GA27 4G076 AA03 AA18 AB03 AB16 BA38 DA30 4K018 AD20 DA21 KA32

Claims (2)

[Claims] 1. A lanthanum sulfide sintered body having a β-type main component with a crystal structure obtained by mixing and sintering β-type lanthanum sulfide powder and metallic palladium particles, which comprises a trace amount of γ-type component.
Contains ~ 5% by mass of metallic palladium and has a specific resistance of 100.
A lanthanum sulfide sintered body for thermoelectric conversion material, which is kΩ · cm or less. 2. A high-purity β-type lanthanum sulfide powder having an oxygen content of 1.1% by weight or less as an impurity and a particle size of 10 μm.
The following metal palladium particles are mixed, and at room temperature, 25
After compression molding at a pressure of MPa or higher, 1200 ° C to 1
3 in vacuum or inert gas in the temperature range of 500 ° C
A method for producing a lanthanum sulfide sintered body for a thermoelectric conversion material, which comprises firing for 0 minutes or more.