JPH09214006A - Manufacturing method of thermoelectric material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To sinter a thermoelectric material even in the atmosphere by hot pressing raw material metal powder at pressure of a specific value or higher. SOLUTION: Raw material powder is pressurized and sintered with hot press. The hot press is performed with the conditions of pressure of 1000kg/cm<2> or more. The sintering with the hot press is desired to be performed at higher pressure within a range where no trouble is caused for the strength of a metal mold, etc. More specifically, the pressure of 2000kg/cm<2> or higher is more desirous, and 3000kg/cm<2> or higher is further more desirous. The raw material powder being relatively small in its diameter is oxidized when it makes contact with the atmosphere and hence allows an oxide film to be formed on a powder surface, but large pressure of 1000kg/cm<2> or higher is applied to the raw material powder upon sintering with the hot press so that the oxide film on the powder surface is destroyed at a stretch and a sintered product not substantially affected by the oxide film is formed. Accordingly, there is no need of the use of expensive atmospheric gas such as Ar gas, and hence the running cost is reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a thermoelectric material, and more particularly to improvement of a powder sintering method by hot pressing.

[0002]

2. Description of the Related Art Thermoelectric materials, which are intermetallic compound semiconductors such as bismuth and tellurium, have been widely used as materials for thermoelectric cooling and thermoelectric power generation. The optimum thermoelectric efficiency obtained with a thermoelectric material depends on the thermoelectric figure of merit of the material. This figure of merit is represented by the following equation. Z = α ² / ρκ (1 / K) where α is the Seebeck coefficient (V / K) and ρ is the electrical resistance (Ω ・
m) and κ are thermal conductivity (W / m · K). This figure of merit Z
The greater the value, the greater the thermoelectric efficiency, so the thermoelectric material should have a large figure of merit. By the way, the figure of merit depends not only on the type and amount of the elements constituting the thermoelectric material but also on the manufacturing method of the thermoelectric material.

As one of the methods for producing a thermoelectric material, there is a sintering method by hot pressing. In this sintering method, a powder obtained by crushing an ingot obtained by mixing and dissolving a predetermined amount of component element powder or a mixed powder obtained by mixing a predetermined amount of each component element powder is put in a press die of a hot press machine and heated And simultaneously pressurize and sinter. In this hot press sintering method, since heating and pressurization are performed at the same time, a high density sintered body can be obtained without sublimating and scattering elements such as selenium and tellurium having a high vapor pressure during sintering. The high-density sintered body has low electric resistance and can have a large figure of merit.

Sintering by hot pressing generally involves
About 2 in vacuum or in an atmosphere of an inert gas such as Ar gas or in an atmosphere containing a reducing gas such as H ₂ or CO
It is carried out for about 20 minutes or longer under the conditions of a pressure of 00 to 450 kg / cm ² and a temperature at which crystallization is possible (depending on the material system, for example, about 350 to 600 ^° C.). The reason why sintering is performed in a vacuum or an atmosphere of an inert gas is that the raw material powder has a very small particle size, and is oxidized when contacted with the atmosphere to form an oxide film having a large electric resistance on the surface. This is because the electric resistance of the sintered body increases, and as a result, the thermoelectric figure of merit decreases. However, on the other hand, there is a problem that the running cost for maintaining an atmosphere such as a vacuum or an inert gas becomes high, which causes an increase in manufacturing cost.

[0005]

DISCLOSURE OF THE INVENTION The present invention provides a thermoelectric material having a large thermoelectric figure of merit in an atmosphere without introducing an inert gas or a reducing gas in a method for producing a thermoelectric material by hot pressing. It is an object to provide a method that can be manufactured.

[0006]

In order to achieve the above object, in the method for producing a thermoelectric material of the present invention, sintering by hot pressing is carried out under a condition of a pressure of 1000 kg / cm ² or more so that the material is burned in the atmosphere. It was made possible to tie.

Since the raw material powder has a relatively small particle size, it is oxidized when exposed to the atmosphere and an oxide film is formed on the powder surface.
Since a large pressure of 00 kg / cm ² or more acts, the oxide film on the powder surface is destroyed at once, and a sintered body that is substantially unaffected by the oxide film is formed.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION As the thermoelectric material of the present invention, for example, a structure composed of three or four elements selected from the group consisting of Bi, Te, Se and Sb can be shown. The elements of the desired components are stoichiometrically weighed, sealed in a quartz tube or the like, melted and cooled, and the ingot thus obtained is crushed to obtain a raw material powder. Alternatively, the raw material powder may be obtained by weighing the elements of the desired components stoichiometrically and uniformly mixing them in a high energy type ball mill or a rolling ball mill.

The raw material powder having the above structure is pressure-sintered by hot pressing. The hot pressing can be performed under a condition of a pressure of 1000 kg / cm ² or more, for example, at a temperature of 350 to 550 ° C. in an air atmosphere. Pressure is 1000kg / c
The reason for setting m ² or more is to destroy the oxide film formed on the powder surface, prevent the electric resistance from increasing, and obtain a thermoelectric material having a high figure of merit. It is desirable that the sintering by hot pressing is performed at a higher pressure within a range that does not hinder the strength of the mold or the like. This is because the sintering time can be shortened, the amount of oxide film generated on the powder surface can be reduced, and the force for destroying the oxide film can be increased. Specifically, 2000
kg / cm ² or more is more preferable, and 3000 kg / cm ² or more is further preferable.

As for the pressing time in the hot press, the longer the time, the greater the density ratio of the thermoelectric material can be obtained. In the present invention, however, since the hot press is carried out at a high pressure, it is about 20.
It is possible to obtain a sintered product having a sufficiently high density (for example, a density ratio of about 97% or more) in a short pressing time of not less than 2 seconds and not more than about 3 minutes.
These times are based on the type of raw material powder, the pressing temperature,
The time may be shorter than 20 seconds, for example, as long as a dense sintered body can be obtained, depending on conditions such as pressing pressure.

[0011]

EXAMPLE The elements of Bi, Te and Sb were stoichiometrically weighed so that the composition was (Bi ₂ Te ₃ ) _0.25 (Sb ₂ Te ₃ ) _0.75 , and these were sealed in a quartz tube and melted. The resulting ingot is crushed to obtain an average particle size of about 40
A μm raw material powder was obtained. As shown in Table 1, the raw material powder was
Various thermoelectric materials were prepared by changing the treatment conditions of the hot press. The density ratio and physical property values of the obtained thermoelectric material are also shown in Table 1.

[0012]

[Table 1]

In Table 1, Test No. 1 is an example in which hot pressing was performed under conventional pressure conditions in an Ar gas atmosphere, No. 2
Is an example of hot pressing under the conventional pressure conditions in the atmosphere, and No. 3 to No. 6 are examples of hot pressing under the pressure conditions of the present invention in the atmosphere.

Regarding the density ratio, all the thermoelectric materials under test had sufficiently high values and were sufficiently densified. The density ratio represents the ratio of the density of a molded article to the density of a completely dense body, and the density of the molded article was measured by the Archimedes method.

Since the sample No. 1 was hot-pressed in Ar gas, a good figure of merit was obtained even at a pressure of 450 kg / cm ² , but the sample was hot-pressed in the atmosphere. Test No. 2 had a large electric resistance and a small figure of merit. The increase in the electrical resistance of sample No. 2 is
It is considered that the influence of the oxide film is great. On the other hand, as is clear from the comparison of the test samples No. 2 to No. 6, it is shown that the electrical resistance decreases and the figure of merit increases with increasing pressure even in the air atmosphere.
This is because the processing time of hot pressing becomes shorter as the pressure increases, the amount of oxide film generated decreases, and the destructive force of the oxide film increases, so that the influence of the oxide film having high electric resistance becomes smaller. It is thought to be because.

From the above results, (Bi ₂ Te ₃ ) _0.25 (Sb ₂
In the case of Te ₃ ) _0.75 thermoelectric material, by performing hot pressing at a pressure of 1000 kg / cm ² or more, about 2.5 x 1
It can be seen that a high figure of merit of 0 ^-3 / K or more can be obtained.
Also, to obtain a larger figure of merit, the pressure is 2000
It can be seen that kg / cm ² or more is desirable, and 3000 kg / cm ² is even more desirable.

[0017]

According to the manufacturing method of the present invention, the hot pressing is carried out in the atmosphere, so that it is not necessary to use an expensive atmosphere gas such as Ar gas, and the running cost can be reduced. Further, since a high density sintered product can be obtained in a much shorter time than the processing time of the conventional hot press, the improvement of the production efficiency can be achieved. According to the manufacturing method of the present invention, a thermoelectric material having a small electric resistance and a large figure of merit can be obtained with almost no influence of an oxide film. Therefore, it can be effectively used in a wide range of fields such as thermoelectric power generation, thermoelectric cooling, temperature sensors, thermostatic devices in semiconductor processes, and cooling of electronic devices.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Michio Yamaguchi 1-1-1 Hama, Amagasaki City, Hyogo Prefecture Kubota Technology Development Laboratory Co., Ltd.

Claims (1)

[Claims] 1. A method for producing a thermoelectric material, comprising press-sintering a raw metal powder by hot pressing, wherein the hot pressing is performed at a pressure of 1000 kg / cm ² or more.