JP2006269870A - Method of manufacturing thermoelectric material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a manufacturing method employing a liquid quenching method which can acquire an N-type Bi<SB>2</SB>Te<SB>3</SB>-based thermoelectric material having an excellent performance index. <P>SOLUTION: The method of manufacturing a thermoelectric material is provided with a thin-piece powder forming process of supplying a molten metal containing Bi and Te onto a rotating roll to form the metal into a thin-piece powder by the liquid quenching method; a preparatory molding process of pressing a dice in which the thin-piece powder is filled so that a surface (c) may be aligned substantially in parallel in a height direction in at least the center of a widthwise direction, in the axial direction same as the height direction of the dice at a temperature lower than a baking temperature of the thermoelectric material, to preparatorily mold the powder; and a hotpress process for applying hotpress to the preparatorily molded thin-piece powder, while pressing the powder in the axial direction same as the pressing direction in the preparatory molding process. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

The present invention relates to a method for manufacturing a thermoelectric material that is a temperature control element, and more particularly to a method for manufacturing an N-type Bi ₂ Te ₃ -based thermoelectric material.

There is a Bi ₂ Te ₃ series thermoelectric material as one of the thermoelectric materials used in a Peltier module that generates a temperature difference on the surface by energization or generates an electromotive force due to the temperature difference of the surface.

  As a manufacturing method of this thermoelectric material, a molten metal in which each element is melted is stirred, a solidification method is performed, a solid powder is prepared by pulverizing a raw ingot, and the obtained solid powder is sized. There is a sintering method to sinter. Although the material produced by the latter sintering method has higher strength than the melting method, contact with oxygen at the time of production causes oxidation or solid solution of oxygen, and the thermoelectric characteristics are likely to be deteriorated.

  For this reason, a powder forming step of melting the ingot as a raw material in an inert gas atmosphere and supplying the molten metal on a rotating roll to perform rapid solidification to form a flaky powder; Has been proposed (for example, Patent Document 1), which includes a hot press step of solidifying and sintering using sinter.

In addition, thermoelectric materials produced by the sintering method are required to have high crystal orientation in order to increase the figure of merit. Therefore, in the hot pressing process, the crystal orientation (a-axis) is grown in the direction perpendicular to the pressing direction, the c-plane orientation is increased, and the electrodes are joined so that current flows in this direction to form a thermoelectric element. In addition, the orientation is enhanced by an extrusion process.
JP 2003-163385 A

  By the way, in general, when a P-type thermoelectric element and an N-type thermoelectric element are compared, the performance index of the N-type tends to be lower due to the characteristics of the material. One of the reasons for this is that the oxygen concentration in the P-type and N-type thermoelectric materials has different effects on the figure of merit, and even if the oxygen concentration is controlled in the same way, the N-type thermoelectric material will contain more oxygen. This is because of the sensitivity. According to the study by the present inventors, the performance index did not decrease even when the oxygen concentration exceeded 200 ppm in the P-type thermoelectric material, but the performance index extremely decreased when the oxygen concentration exceeded 100 ppm in the N-type thermoelectric material. Was confirmed to decrease. For this reason, there exists a problem that the performance of a module will fall when it uses in combination with the N type thermoelectric material produced by the method similar to a P type thermoelectric material.

The flaky powder obtained by the above liquid quenching method is formed of polycrystals in which each powder is oriented, and since there are crystal grain boundaries that do not touch the external atmosphere, oxygen that tends to adhere to the interface of the crystal grain boundaries is reduced. The N-type Bi ₂ Te ₃ thermoelectric material has a larger performance index drop than the P-type thermoelectric material. It was considered that the performance is likely to be affected by oxidation when the powder is moved from the forming process to the hot pressing process. In this case, it is conceivable to perform the flaky powder forming process by the liquid quenching method and the hot pressing process in the same apparatus, but it is difficult to make the continuous process due to the apparatus structure of each process, and each process is performed simultaneously. Can not be expensive.

The present invention aims to solve the above-mentioned problems, and in a method for producing a thermoelectric material using a liquid quenching method, an N-type Bi ₂ Te ₃ thermoelectric material having an excellent figure of merit is obtained. Is to provide.

The invention according to claim 1 of the present invention includes a flaky powder forming step of supplying a molten metal containing Bi and Te onto a rotating roll to form a flaky powder by a liquid quenching method, and at least a central portion in the width direction of the die. Then, the die filled with the flaky powder is preliminarily formed by pressing it in the same direction as the die height direction at a temperature lower than the sintering temperature of the thermoelectric material so that the c-plane is arranged substantially parallel to the height direction. Production of N-type Bi ₂ Te ₃ thermoelectric material comprising a preforming step and a hot press process in which the preformed flaky powder is hot pressed while being pressed from the same direction as the pressing direction in the preforming step Is the method.

  By providing the preforming step between the flaky powder forming step and the hot pressing step, the powder filled in the die is closely spaced to suppress the penetration of outside air, especially oxygen, and the hot pressing step is performed. In doing so, the amount of oxygen present between the powders to be sintered can be greatly reduced. That is, the flaky powder formed by a liquid quenching method using a rotating roll is a thin flat powder having a polycrystal composed of a plurality of single crystals, and the c-plane is the thickness direction of the powder in the single crystal. It is in a state of being lined up in parallel. For this reason, when the flaky powder is filled into the die, the flaky powder is overlapped in the thickness direction of the powder at least in the central portion in the width direction of the die, and the c-plane is filled in substantially parallel to the height direction of the die, By pressing this die in the height direction and the coaxial direction, the filling of the powder in the die becomes high density while maintaining the directionality of the c-plane, and the filling state obtained in the flaky powder forming process with high orientation As a result, the amount of oxygen between the powders can be reduced. In addition, by pressing the highly filled powder obtained in the preforming process in the hot press process in the same direction as the pressing direction in the preforming process, molding can be performed while maintaining the dense filling state in the preforming process. Therefore, it is possible to obtain a molded body of the thermoelectric material while the state in which the crystals are in contact with oxygen is reduced.

  The invention according to claim 2 of the present invention is the invention according to claim 1, wherein the pressure in the preforming step is 10 MPa or more and 50 MPa or less.

  By preforming at a pressure in this range, the packing density and orientation of the flaky powder can be further increased, and the figure of merit can be improved.

  According to a third aspect of the present invention, in the first or second aspect of the present invention, the aspect ratio (of the major axis diameter and thickness of the powder) is added to the flaky powder filled in the die of the preforming step. A ratio) of 50 or more is used.

  By using a flaky powder having a high aspect ratio, the c-plane is easily aligned in the filling height direction at the time of filling, so that high orientation can be easily obtained, and the figure of merit can be further improved.

According to the manufacturing method of the first aspect of the present invention, the die filled with the flaky powder so that the c-plane is arranged substantially parallel to the height direction at least in the center portion in the width direction of the die is sintered with the thermoelectric material. Since a preforming process is performed between the flaky powder forming process by liquid quenching and the hot pressing process, the oxygen concentration in the thermoelectric material is reduced by pressing in the height direction of the die coaxially with the die at a temperature lower than the temperature. It can be extremely reduced, and a highly oriented thermoelectric material with a high packing density can be produced. Therefore, even if it is an N-type Bi ₂ Te ₃ thermoelectric material that is easily deteriorated by the contained oxygen, the oxygen concentration can be controlled to 100 ppm or less, and a thermoelectric material having an excellent performance index can be obtained.

  According to the manufacturing method of the second aspect of the present invention, since the packing density and the orientation can be improved without causing the flaky powder to be crushed, a thermoelectric material having further excellent performance index can be obtained.

  According to the manufacturing method of the third aspect of the present invention, since the flat flaky powder having an aspect ratio of 50 or more is used, the flaky powder is easily aligned and filled at the time of die filling, and the orientation is further improved. And the performance index can be further improved.

  Hereinafter, the manufacturing method of the present invention will be described with reference to the drawings.

  FIG. 1 is a schematic view showing a rapid solidification apparatus 10 by a rotary roll type liquid quenching method, which is an example of a flaky powder forming process used in the production method of the present invention. This apparatus includes a rotating roll 11 for obtaining a flaky powder, a chamber 13 for placing the rotating roll 11 in an inert gas atmosphere or a reduced-pressure atmosphere, and the like.

As a raw material of the thermoelectric material used in the present invention, an N-type Bi ₂ Te ₃ ingot is used. For example, in addition to Bi ₂ Te ₃ , a thermoelectric material in which a part of these is replaced with Sb, Se, etc. Can do. Such a thermoelectric material may be used by adding Sb, Se, etc. together with a Bi ₂ Te ₃ -based ingot. Moreover, 1 type, or 2 or more types of halogen elements, such as I, Cl, and Br, may be added to the molten metal. These halogen elements may be added in the form of SbI ₃ , AgI or HgBr ₂ . The molten metal can be obtained, for example, by adding an ingot containing each element with a predetermined composition and, if necessary, an additive, and heating and melting it.

  The chamber 13 is provided with an exhaust device 19 for exhausting the air in the chamber 13 and a gas cylinder 16 for replacing the interior of the chamber 13 with an inert gas such as nitrogen gas or argon gas. A constant pressure is maintained by these inert gases. The inert gas can be introduced into the chamber 13 by supplying the inert gas after evacuating the chamber 13 to 0.1 Pa or less. Further, the chamber 13 is provided with a guide 14 so that the collected flaky powder 2 is filled in the collecting device 15. The collecting device 15 is detachably connected to the guide 14.

  The rotating roll 11 is made of, for example, stainless steel or copper, and is rotatably arranged in a chamber 13 held in an inert gas atmosphere. In FIG. 1, the rotating roll 11 is a disk-shaped roll arranged vertically, but may be arranged horizontally and rotated horizontally.

  Above the rotating roll 11, a nozzle 12 is provided for ejecting molten molten metal at a certain distance from the surface of the roll 11. The nozzle 12 is made of quartz glass, and the nozzle 12 is filled with a raw material 1 containing an ingot and other additives. Further, an injection gas cylinder 18 is connected to the nozzle 12. The melted raw material 1 is supplied as a molten metal from the jet nozzle part toward the rotary roll 11 by the jet gas. The melting temperature is detected by a temperature sensor attached to the nozzle 12 and is adjusted to a constant temperature.

  In the present invention, first, a flaky powder is produced by such a rotating roll liquid quenching method. The temperature of the molten metal at the time of injecting the raw material 1 in the nozzle 12 melted by the high-frequency heating device 17 is set to be higher in the range of 20 ° C. or more and 100 ° C. or less than the complete melting temperature of the thermoelectric material. By making the molten metal temperature 20 ° C. or higher, the supply from the nozzle is made smooth at the time of injection, and it becomes easy to obtain a uniform flaky powder. By making it 100 ° C. or lower, the oxygen concentration contained in the powder can be further reduced. In addition, a powder having stable properties can be obtained without changing the components in the thermoelectric material.

The inside of the chamber 13 is maintained at about the same temperature as the room temperature, and the molten metal supplied onto the rotating roll 11 by means such as spraying is rapidly cooled and solidified. The quenching condition varies depending on the type of thermoelectric material, but is usually 10 ³ K / sec or more and 10 ⁶ K / sec or less. The distance between the nozzle tip and the rotary roll 11 is set to about 1 to several mm. The flaky powder 2 rapidly solidified on the surface of the rotating roll 11 is peeled off from the surface by the centrifugal force, scattered in the rotating direction of the rotating roll 11, passed through the guide 14, and filled into the recovery unit 15. The collecting device 15 may be a die used in a preforming process and a hot press process described later.

  The size of the flaky powder obtained in the flaky powder forming step has a flat shape, and the width and length are 1 mm or more and 10 mm or less, respectively, and the thickness is 10 μm or more and 20 μm or less. It is preferable to make it. The aspect ratio (ratio of the major axis of the powder to the thickness) is preferably 10 or more and 1000 or less, and it is particularly preferable to use only flaky powder having an aspect ratio of 50 or more. By setting the size of the powder to 1 mm or more in the major axis and the aspect ratio to 10 or more, the flaky powder can be more easily aligned in the thickness direction of the powder, and the orientation can be further enhanced. By setting the ratio to 1000 or less, variation in the shape of the flaky powder can be reduced and the performance can be improved. The size of the powder is based on an average value when 10 to 100 flaky powders are measured with an optical microscope. The flaky powder having a high aspect ratio as described above can be obtained, for example, by fractionating a flaky powder which has been rapidly solidified into a powder having a small particle diameter with a mesh having a predetermined diameter. By adopting such a flaky powder form, the c-plane of the crystal is easy to line up substantially parallel to the filling height direction when filling the die, and therefore the gap between the powders is reduced when pressing in the preforming process. It becomes possible to do.

  The speed of the rotating roll for forming the powder as described above is preferably 5 to 20 m / sec at the peripheral speed, and 0.15 to 2 MPa as the jetting pressure of the molten metal.

  Next, in the present invention, the obtained flaky powder is preformed before sintering in the hot pressing step. By performing the hot pressing process through this preforming step, the amount of oxygen present between the powders during hot pressing in which the powder is sintered is reduced, and a thermoelectric material having a low oxygen concentration can be obtained. In the pre-molding step, the flaky powder 2 is first filled in a die in order to perform molding by pressing.

  FIG. 4 shows the filling state of the flaky powder of the present invention into a die, where (a) is a die filled only with a powder having a low aspect ratio, and (b) is filled only with a powder having a high aspect ratio. It is the schematic which shows the state of dice | dies. Actually, if a separation step using a mesh or the like is not provided, flaky powders having different aspect ratios are mixed. In the figure, in (a), since the flaky powder is overlapped in the thickness direction of the powder at the center in the width direction of the die, the c-plane in the single crystal in each powder is substantially parallel to the height direction of the die. There are also flaky powders that are packed side by side but are not juxtaposed on the side wall of the die because of the low aspect ratio. On the other hand, in (b), since the aspect ratio is high, all powders are filled so as to overlap in the thickness direction of the powder, and the c-plane is further arranged in parallel with the height direction of the die. When only the flaky powder having an aspect ratio of 50 or more is used, the filling state (b) is likely to occur. By filling such c-planes so that the c-planes are arranged substantially parallel to the height direction of the die, the oxygen concentration is reduced, and a highly oriented thermoelectric material can be obtained with high filling.

  The atmosphere during pressing in the preforming step needs to be an inert gas atmosphere or a reduced pressure atmosphere, and it is desirable that the oxygen concentration be 0.01 ppm or less. Therefore, the inside of the die filled with the flaky powder may be replaced with an inert gas again. For this reason, the preforming step is performed in a closed system such as a glove box.

  FIG. 2 is a schematic view of the preforming machine 20 showing an example of the preforming process. In the glove box 21, a die 25 filled with the flaky powder 2 produced in the flaky powder forming step is installed so that the c-plane is arranged substantially parallel to the height direction, and from the opening end of the die 25. It has a structure capable of pressing the filled flaky powder 2.

  The preforming machine 20 is a normal one, and includes, for example, a lower support base 22 that supports the die 25 so as to be movable up and down, a pressing portion 23, and a hydraulic cylinder 24 that moves the lower support base 22 up and down. Connected to the glove box 21 are a gas cylinder 26 that can contain an inert gas such as argon gas and an exhaust device 27 that exhausts the air inside. The lower support 22 is formed with a recess for storing and holding the lower end of the die 25 at the upper end. Although not shown, the glove box 21 is provided with a carry-in portion capable of carrying the dice 25 into the glove box. Then, after replacing the inside of the glove box 21 including the carry-in portion with an inert gas, the dice 25 can be installed on the lower support base 22. Then, the lower support base 22 is positioned downward by operating the hydraulic cylinder 24. Next, the flaky powder 2 filled in the die 25 is pressed by gradually lifting the lower support 22 by the hydraulic cylinder 24.

  The pressing pressure in the preforming step of the present invention is preferably 10 MPa or more and 50 MPa or less. By setting it to 10 MPa or more, the packing density of the powder can be further increased and the gaps between the powders can be reduced, and preferably it is 15 MPa or more. On the other hand, by setting the pressure to 50 MPa or less, the packing density can be reduced, and the powder can be prevented from being crushed at the time of pressing, which causes a decrease in orientation, and preferably 30 MPa or less. The pressing time in the pre-forming step is sufficient for a short time of 10 seconds or less, and the flaky powder 2 is gradually pressed, and the pressure can be released when a predetermined pressure is reached. If the pressing time is too long, the flaky powder is crushed and the filling property tends to be lowered.

  In addition, as shown in the figure, the direction of pressing the die in the preforming step needs to be performed from the height direction when the flaky powder is filled and from the coaxial direction. By making this pressing direction coincide with the height direction during filling, filling can be enhanced without crushing the powder.

The temperature at the time of pressing may be lower than the temperature at which the thermoelectric material does not sinter, and is preferably room temperature. When the temperature in the preforming step approaches the temperature at the time of sintering, oxygen remains at the crystal interface due to the sintering, and the effect of reducing the oxygen concentration becomes difficult to obtain. The sintering temperature varies depending on the type of thermoelectric material used, but is 400 to 500 ° C. for a thermoelectric material made of Bi ₂ Te ₃ , for example.

  Next, the preformed body obtained as described above is subjected to hot press treatment to obtain a molded body. FIG. 3 is a schematic view of a hot press machine 30 showing an example of a hot press process.

  The hot press machine 30 is a normal one and is similar to the molding machine used in the preliminary molding process, but differs in that the heater 35 is arranged around the die 25.

In the hot press process, first, the inside of the chamber 31 is evacuated by, for example, the exhaust device 37 and then the chamber 31 is filled with an inert gas such as argon gas by the gas cylinder 36. Next, the heater 35 is turned on to hold the die 25 in a heated state. Next, the lower support 32 is gradually lifted by the hydraulic cylinder 34 to press the flaky powder 2 of the thermoelectric material filled in the die 25. Thereby, the hot pressing process which heats and pressurizes the flaky powder 2 is performed, the flaky powder 2 is sintered, and a molded object is formed. The pressurizing pressure is set to 5 MPa or more and 50 MPa or less. The heating temperature varies depending on the thermoelectric material used. In the case of a thermoelectric material made of Bi ₂ Te ₃ , the heating temperature only needs to be equal to or higher than the sintering temperature of the thermoelectric material described above. The treatment time is usually 0.5 to 3 hours, preferably 1 to 2 hours.

  The direction of pressing in the hot press process needs to be coaxial with the pressing direction in the preforming step. Thus, by matching the pressing direction, it is possible to perform molding while maintaining high filling and high orientation by preliminary molding. Accordingly, in the present invention, the gap between the powders is reduced by the preforming process as described above, and sintering is performed in a highly oriented state, so that the amount of oxygen at the crystal interface is reduced and the oxygen concentration in the thermoelectric material is 100 ppm or less. In addition, an N-type thermoelectric material having a high packing density and a high orientation can be obtained, whereby a thermoelectric element having an excellent figure of merit can be obtained.

  Since the thermoelectric material obtained by the production method of the present invention is highly filled and highly oriented, it can be cut into a predetermined size after the hot press treatment to obtain a thermoelectric element molded body as it is. If desired, the molded body can be further formed into a secondary molded body by an extrusion method or the like. As the secondary molded body by this extrusion method, for example, in biaxial machining, an extrusion temperature of 300 to 500 ° C., a stem speed of 0.1 to 10 mm / s, and an extrusion ratio of 5 to 40 are desirable.

  Hereinafter, the present invention will be described in more detail by way of examples. However, the following examples are merely examples and are not intended to limit the present invention, and any design changes based on the gist of the present invention will be described below. It is included in the technical scope.

Experiment 1
Using the rotary roll type rapid solidification apparatus shown in FIG. 1, flaky powder of N-type Bi ₂ Te ₃ series thermoelectric material was prepared. A Bi ₂ Te ₃ ingot containing Se 3 parts by weight was used as a raw material. After the inside of the apparatus was evacuated to 0.1 Pa or less, argon gas was injected to make an inert atmosphere. The raw material (melting temperature: 585 ° C.) charged into the nozzle 12 on the rotating roll 11 was melted at a temperature of 620 ° C., and this molten metal was jetted onto the metallic rotating roll 11. At this time, the rotational speed of the rotary roll 11 for rapidly cooling the liquid was 10 m / sec, and the ejection pressure was 0.1 MPa (nozzle diameter: φ0.3 mm).

  The jetted molten metal was cooled and scattered by a rotating roll so that the flaky powder 2 was recovered in the recovery device 15 through a guide. The obtained flaky powder was a plate-like powder having an average side of 5 mm and an average thickness of 20 μm.

  Next, after the recovered container containing the obtained flaky powder was sealed, it was removed from the rapid solidification apparatus, and was immediately carried into a glove box for preforming replaced with argon gas in the atmosphere. The glove box was evacuated and the inside was replaced again with argon gas, and then the flaky powder was filled into a die (inner diameter: φ28 mm) to a height of about 50 mm in the glove box. At the time of filling, the powder was piled up so that the c-plane was aligned substantially parallel to the height direction of the die.

  Using the preforming machine shown in FIG. 2, pressing was performed for 30 seconds up to a pressure of 15 MPa in the same direction as the filling height direction at room temperature, and when the pressure reached 15 MPa, the pressure was released.

  The die containing the obtained preform was taken out of the glove box and immediately carried into the chamber for hot pressing.

  The apparatus shown in FIG. 3 is used for hot pressing, and after the chamber is evacuated and the inside is replaced with argon gas, pressing is performed at a pressure of 50 MPa and a pressing time of 1.5 hours with the heater heated to a furnace temperature of 450 ° C. Then, heating and pressure sintering were performed to produce a molded body.

Experiment 2
In Experiment 1, a molded body was produced in the same manner as in Experiment 1 except that the pressing pressure in the preforming step was set to 30 MPa.

Experiment 3
In Experiment 1, a molded body was produced in the same manner as in Experiment 1 except that the pressing pressure in the preforming step was 10 MPa.

Experiment 4
In Experiment 1, the powder obtained in the flaky powder forming step is preliminarily sieved (1 mm mesh) to remove the powder whose major axis is less than 1 mm, and only flaky powder larger than 1 mm (aspect ratio: 50 or more) is used. A molded body was produced in the same manner as in Experiment 1 except that the above was.

Experiment 5
In Experiment 1, a molded body was produced in the same manner as in Experiment 1 except that the preforming was not performed.

Experiment 6
In Experiment 1, flaky powder was pulverized with a mortar before pre-molding, and the powder was randomly filled in a die using a powder having a particle size of 53 μm or less (aspect ratio: 3 or less). A molded body was produced in the same manner as in Experiment 1 except that was preformed.

Table 1 shows the results of measuring the oxygen concentration and the figure of merit of each thermoelectric material obtained as described above.

  As shown in Table 1, the thermoelectric materials of Experiments 1 to 3 produced by the production method having the preforming step of the present invention have a preforming step, and therefore the oxygen concentration is suppressed to 100 ppm or less, and the performance index is excellent. ing. In particular, Experiments 1 and 2, which were preformed at pressures of 15 MPa and 30 MPa, had an oxygen concentration of 50 ppm, and the thermoelectric material of Experiment 4 using only flaky powder with an aspect ratio of 50 or more further improved the figure of merit. Has been. In contrast, in Experiment 5 in which the preforming step was not performed, the oxygen concentration was high, and it was found that the figure of merit deteriorated at about 200 ppm. In addition, in Experiment 6 in which the preforming step was performed and the powder was randomly filled and pressed, the decrease in oxygen concentration was slight, and the performance index was lowered due to poor orientation.

It is the schematic which shows the rotary roll type rapid solidification apparatus used for the flaky powder shaping | molding process of this invention. It is the schematic which shows the preforming machine used for the preforming process of this invention. It is the schematic which the hot press machine used for the hot press process of this invention shows. (A) And (b) is the schematic which shows the filling state in the die | dye of the flaky powder of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Raw material 2 Flaky powder 10 Rapid solidification apparatus 11 Rotating roll 20 Preforming machine 25 Die 30 Hot press machine

Claims (3)

A method for producing an N-type Bi ₂ Te ₃ thermoelectric material,
A flaky powder forming step of supplying a molten metal containing Bi and Te onto a rotating roll to form a flaky powder by a liquid quenching method;
The die filled with the flaky powder is pressed in a direction coaxial with the die height direction at a temperature lower than the sintering temperature of the thermoelectric material so that the c-plane is arranged substantially parallel to the height direction at least in the center in the width direction. And a preforming step for preforming,
A hot press step of hot pressing while pressing the preformed flaky powder in a direction coaxial with the pressing direction in the preforming step;
A method for producing a thermoelectric material comprising:   The method for producing a thermoelectric material according to claim 1, wherein the pressure in the preforming step is 10 MPa or more and 50 MPa or less.   The method for producing a thermoelectric material according to claim 1 or 2, wherein an aspect ratio (ratio of the major axis of the powder to the thickness) of the flaky powder filled in the die in the preforming step is 50 or more.

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