JP2010199193A - Thermoelectric material, and method of manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve such a problem that it is difficult to manufacture thermoelectric material with reduced cost owing to a short mold life and that thermoelectric material of high performance index can not be manufactured efficiently. <P>SOLUTION: The manufacturing method for thermoelectric material includes the process to solidify the powder of the alloy containing at least one element selected from the group consisting of Bi and Sb and at least one element selected from the group consisting of Te and Se. After that, extrusion treatment is performed by using a mold, whose pressurization axis of a pressurization passage and extrusion axis of an extrusion passage do not exist on one axis, on the solidified alloy, without giving back pressure in the direction opposite to the extrusion direction of the alloy being extruded inside the extrusion passage. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a thermoelectric material.

  BiTe-based thermoelectric materials can be manufactured with a high performance index thermoelectric material by extruding with a mold configured so that the pressure shaft of the pressure passage and the extrusion shaft of the extrusion passage do not exist on one axis. It is known that Various methods for producing thermoelectric materials using such a mold have been proposed, and the technology of performing extrusion while applying a back pressure opposite to the extrusion direction to the alloy of the material during the extrusion process is known. (For example, Patent Document 1 and Patent Document 2). Moreover, the technique which uses an ingot as a starting material of the said extrusion process is known (for example, nonpatent literature 1).

Japanese Patent No. 3842873 Japanese Patent No. 4161681

Jae-Taek Im, K. Ted. Hartwig, Jeff Sharp: “Microstructural refinement of cast p-type Bi2Te3-Sb2Te3 by equal channel angular extrusion”, Acta Materialia, Vol. 52, 2004, p. 49-55

In the prior art, it is difficult to produce a thermoelectric material at a low cost because the mold life is short. Moreover, a thermoelectric material having a high figure of merit could not be produced efficiently.
That is, when a back pressure is applied to the material alloy during the extrusion process, it is necessary to increase the pressure on the pressure shaft by about 2 to 8 with respect to the back pressure 1. For this reason, compared with the case where a back pressure is not applied to the alloy of material, the pressure which should be made to act on a pressurization axis | shaft becomes very large, and a metal mold | die lifetime will be shortened.

Further, when the starting material of the extrusion process is an ingot, the recrystallization of the alloy is not sufficiently promoted during the extrusion process, and the crystal grain size of the alloy obtained as a result of the extrusion process becomes non-uniform. Therefore, the characteristics of the thermoelectric material are not stable.
The present invention has been made in view of the above problems, and an object of the present invention is to use a mold for a long period of time and efficiently produce a thermoelectric material having a high figure of merit.

  In order to solve at least one of the above objects, in the present invention, an alloy obtained by solidifying powder in a BiTe-based thermoelectric material is used as a starting material, and the pressure shaft of the pressure passage and the extrusion shaft of the extrusion passage do not exist on one axis. Extrusion is performed by a mold. Further, when the extrusion process is performed by the mold, the extrusion process is performed without applying a back pressure opposite to the extrusion direction to the alloy extruded in the extrusion passage.

  That is, since the back pressure is not applied to the alloy during the extrusion process, it is not necessary to apply an excessive pressure to the alloy when pressurizing in the pressurizing passage. Therefore, the mold can be used for a long period of time as compared with the case where the back pressure is applied to the alloy. Further, since the alloy obtained by solidifying the powder is used as a starting material, the extrusion process can be started from a state in which the crystal grain size in the alloy is small, and the alloy can be uniformly recrystallized by the extrusion process. Therefore, the performance of the manufactured thermoelectric material is uniform, and a high performance index thermoelectric material can be efficiently manufactured.

Here, the thermoelectric material may be a BiTe-based thermoelectric material. That is, when at least one element selected from the group consisting of Bi and Sb is expressed as (Bi, Sb), and at least one element selected from the group consisting of Te and Se is expressed as (Te, Se), (Bi, Sb) _x (Te, Se) specific x in _y, when a combination of y, the material is rhombohedral crystal structure (space group R3-m (- is usually expressed above 3)) It becomes a thermoelectric material. Note that in this specification, crystal axes such as c-axis and a-axis, and crystal planes such as c-plane are crystal axes and crystal planes when the crystal of space group R3-m is expressed in hexagonal crystal form.

  Further, in the mold, the pressurizing passage and the extrusion passage are connected, and a pressurizing shaft that passes through the center of gravity of the region surrounded by the inner diameter of the pressurizing passage in a plane perpendicular to the direction in which the pressurizing passage extends, The extrusion shaft passing through the center of gravity of the region surrounded by the inner diameter of the extrusion passage in a plane perpendicular to the direction in which the extrusion passage extends may not exist on one axis. For example, the pressure passage and the extrusion passage may be formed so that the pressure shaft and the extrusion shaft do not coincide with each other, for example, the pressure shaft and the extrusion shaft are orthogonal to each other.

  Furthermore, the starting material may adopt a configuration in which the powder is solidified in a mold as a structural example for solidifying the powder. For example, it is possible to adopt a configuration in which the alloy is solidified by pressurizing the alloy powder in the pressurizing passage while the push-out passage is closed by inserting an insertion tool into the extrusion passage. That is, in the metal mold according to the present invention, the pressurizing passage and the extrusion passage are oriented in different directions, and therefore the extrusion passage can be closed by inserting the insertion tool into the extrusion passage. Therefore, the alloy powder is put into the pressurizing passage in a state where the extrusion passage is closed by the insertion tool, and the powder is pressurized in the pressurizing passage so that the powder does not move to the extrusion passage.

  According to this configuration, the powder can be solidified in the mold for performing the extrusion process. Moreover, if an insertion tool is pulled out from an extrusion channel | path, an extrusion process can be performed immediately. Therefore, a thermoelectric material can be manufactured with high work efficiency, and a thermoelectric material with a high figure of merit can be manufactured efficiently.

  In addition, the insertion tool should just be able to block an extrusion channel | path, and can be comprised by the columnar body etc. provided with the outer diameter of substantially the same shape as the internal diameter of an extrusion channel | path. Further, it is only necessary to apply a pressure for solidifying the powder to the alloy powder in the pressurizing passage in a state where the extrusion passage is closed with the insertion tool. Therefore, it is only necessary that the insertion tool is inserted into the extrusion passage so that the powder can be prevented from moving in the direction of the extrusion passage.

  Further, the particle size of the alloy powder is 30 μm or less (preferably 1 to 10 μm or more), and it is preferable to press and solidify so that the alloy has a theoretical density of 98% or more. That is, when the crystal grains in the material are refined by pulverization or the like, the mechanical strength increases (Hall Petch rule). In addition, the density in this specification is a mass density.

  However, when a powder having a particle size of 30 μm or less is solidified and subjected to extrusion processing, the crystal grain size becomes coarse to some extent by recrystallization during the extrusion processing, but the thermoelectric material maintains high mechanical strength or mechanical characteristics. Can be manufactured. The powder may be obtained by roll-type liquid quenching or gas atomization of the molten alloy, or may be powdered by scattering the molten alloy using the rotation of a rotating disk, or obtained by melting after weighing. The obtained alloy may be pulverized.

Furthermore, it is possible to manufacture a thermoelectric material so that most of the manufactured thermoelectric material is comprised by the target composition by performing an extrusion process after pressing until it becomes 98% or more of theoretical density. That is, the theoretical density is the theoretical density of the target composition that can be specified by the JCPDS card (for example, Isao Kinkei, Saichi Katayama, “Bi _1-x Sb _x ) ₂ (Te _1-y Se _y ) High lattice constants and densities of ₃ systems ", Applied Physics, Japan Society of Applied Physics, 1970, Vol. 39, No. 11, p.1028-1033), solidification by pressurization using the theoretical density as an index It is possible to manufacture a thermoelectric material having a target composition.

  Further, since the thermoelectric material manufactured as described above has a high figure of merit in a specific direction, the thermoelectric material is set so that the specific direction (the specific direction in which the longitudinal directions of the crystal grains are aligned) is the energization direction. The thermoelectric element is manufactured by cutting the material. And if the thermoelectric element obtained is combined and it is set as a thermoelectric conversion module, a high-performance thermoelectric conversion module can be manufactured.

It is a flowchart which shows the manufacturing method of a thermoelectric material. It is a schematic diagram which shows an example of a metal mold | die. It is sectional drawing which shows a manufacturing process.

Here, embodiments of the present invention will be described in the following order.
(1) Thermoelectric material manufacturing method:
(2) Comparison of mold life, relative density, and crystal grain size:
(3) Other embodiments:

(1) Thermoelectric material manufacturing method:
FIG. 1 is a flowchart showing an embodiment of a method for producing a thermoelectric material. In the present embodiment, first, an element that is a raw material of the BiTe-based thermoelectric material is weighed and melted, and then quenched by a roll-type liquid quenching method to produce a thin-film alloy powder (step S100). That is, an ingot of at least one element selected from the group consisting of Bi and Sb and at least one element selected from the group consisting of Te and Se is weighed and melted, and (Bi, Sb) _x A powder alloy having a composition of (Te, Se) _y is prepared. x and y are 2 and 3, for example.

  According to the roll type liquid quenching method, the particle size of the crystals in the powder can be 30 μm. The liquid quenching method may be a single roll method or a twin roll method. Moreover, as long as the crystal grain size in the powder can be reduced to 30 μm, the powder may be prepared by a gas atomizing method or ingot grinding.

  Once the alloy powder is prepared, the alloy powder is solidified before the extrusion process. In the present embodiment, a configuration is adopted in which the powder is pressed and solidified without applying a shearing force to the powder in the extrusion mold, so that the powder can be pressed in the mold. Prepare to. FIG. 2 is a schematic diagram showing an example of a mold. In the example shown in FIG. 2, the mold 10 is a rectangular parallelepiped, and a rectangular opening 11 a is formed on the surface 11, and a rectangular opening 12 a is formed on a surface 12 adjacent to the surface 11. In the mold 10, a hole extending in a direction perpendicular to the surface 11 from the opening 11a is formed, and the hole constitutes a pressurizing passage 11b. Further, a hole extending in a direction perpendicular to the surface 12 from the opening 12a is formed, and the hole constitutes the extrusion passage 12b.

  Further, the pressurizing passage 11 b and the extrusion passage 12 b are connected inside the mold 10. Further, a straight line that passes through the center of gravity of the rectangle formed by the opening 11a and is perpendicular to the surface 11 is a pressure axis that passes through the center of the pressure passage 11b, and passes through the center of gravity of the rectangle formed by the opening 12a. A vertical straight line is a pressure axis passing through the center of the extrusion passage 12b. In the present embodiment, the pressure shaft and the extrusion shaft are orthogonal to each other in the mold 10.

  In the present embodiment, an insertion tool is inserted into the extrusion passage 12b so that a powder alloy can be set and solidified in the mold 10 (step S110). The insertion tool 30 is a quadrangular prism having an outer diameter slightly smaller than the inner diameter of the extrusion passage 12b. In step S110, the insertion tool 30 is inserted into the extrusion passage 12b in advance to close the extrusion passage 12b. . That is, the insertion tool 30 is inserted into the extrusion passage 12b until the extension line of the inner wall of the pressure passage 11b is positioned on the surface of the distal end of the insertion tool 30, and the extrusion passage 12b is blocked when viewed from the pressure passage 11b side. It is assumed that it is peeled off. In this state, the pressurizing passage 11b is a quadrangular columnar hole that opens on the surface 11 side and is closed at the other portion.

  Therefore, the alloy powder prepared in step S100 is set in the pressurizing passage 11b (step S120). FIG. 3 is a cross-sectional view of the mold 10 showing a state in which the insertion tool 30 is inserted into the extrusion passage 12b. FIG. 3A shows a state in which the powdered alloy 20 is set in the pressurization passage 11b. In this embodiment, the thermoelectric material remains in the mold 10 after the extrusion process, and the mold 10 has a structure (not shown) for taking out the thermoelectric material (for example, the mold 10 can be assembled). ).

  When the powder is set in the mold 10, the inside of the chamber is evacuated, and after the evacuation is completed, argon gas is introduced into the chamber (step S130). That is, the atmosphere of the mold 10 is replaced with argon gas. Thereafter, the mold 10 is heated by a heater (not shown) (step S140), and the mold 10 is set to a predetermined set temperature.

  When the mold 10 reaches the set temperature, the powdered alloy 20 in the pressurizing passage 11b is solidified (step S150). FIG. 3B is a diagram showing the solidification process. In this example, the plunger 31 is a quadrangular column having an outer diameter slightly smaller than the inner diameter of the pressurizing passage 11b, and the alloy 20 is solidified by pressurizing the alloy 20 with the plunger 31 in a state where the insertion tool 30 is fixed. .

In the present embodiment, pressurization is performed until it reaches 98% of the theoretical density at room temperature in the composition of (Bi, Sb) _x (Te, Se) _y . That is, the geometric shape in the pressurizing passage 11b is specified in advance, and the weight of the powdered alloy 20 is also measured. Therefore, the density can be specified by specifying the volume in the pressure passage 11b by the position of the plunger 31 and dividing the weight of the powdered alloy 20 by the volume. Therefore, when the relative density of the specified density with respect to the theoretical density at room temperature becomes 98% or more, the pressurization by the plunger 31 is terminated.

  Next, the insertion tool 30 is removed from the extrusion passage 12b (step S160), and thereafter, extrusion processing is performed by the plunger 31 (step S170). Here, for example, the extrusion process is performed so that the plunger 31 moves at a speed of about 1 mm / min to 12 mm / min by pressurization. When the plunger 31 reaches a specific position, the extrusion process ends, and thereafter, the mold is cooled (step S180) and the thermoelectric material is taken out (step S190). A material processing step of cutting out the thermoelectric element is performed on the manufactured thermoelectric material. Moreover, when manufacturing a thermoelectric material continuously, the process after step S120 is further repeated.

(2) Comparison of mold life, relative density, and crystal grain size:
Next, the life of the mold 10, the relative density, and the crystal grain size of the powdered alloy 20 will be described. In Table 1, it shows the life of the die 10 in the case of performing extrusion process by the mold 10 against alloy 20 having a composition of Bi ₂ Te _2.7 Se _0.3. That is, the relationship between the number of repetitions when the mold 10 is damaged by repeating the extrusion process, the back pressure (force opposite to the extrusion direction), and the extrusion pressure is shown. In this specification, ECAP is Equal-Channnel Angular Pressing, and is an example in which the opening 11a and the opening 11a have the same shape in the mold 10, and the shear imparting extrusion is performed in the mold 10 with the opening 11a. This is an example where the opening 11a has a different shape. Further, the extrusion direction is a direction in which the alloy 20 moves by pressurization in the extrusion passage 12b, and in FIG. 2, is a direction facing the opening 12a side along the pressurization axis.

  That is, Table 1 shows the results when a plurality of molds 10 are prepared and the extrusion process is repeated until each mold 10 is damaged. Table 1 shows the back pressure and extrusion pressure when extruding the alloy at the same extrusion speed. According to Table 1, when the extrusion process is performed by applying a back pressure of 100 MPa to the alloy 20, the extrusion pressure by the plunger 31 increases as compared with the case of performing the extrusion process without applying the back pressure.

  Further, when the back pressure is applied and when the back pressure is not applied, although the back pressure increment is 100 MPa, the extrusion pressure increment is 240 MPa or 600 MPa, and the extrusion pressure increment is the back pressure increment. Is greatly exceeded. In both ECAP and shearing extrusion, when the back pressure is applied, the number of times until the mold 10 is damaged is extremely small compared to the case where the back pressure is not applied. That is, when a back pressure is applied, the number of times until the mold 10 is damaged due to an increase in the extrusion pressure is extremely reduced. Therefore, it is possible to extend the life of the mold 10 by performing the extrusion process without applying back pressure, and to suppress the cost when manufacturing the thermoelectric material.

Table 2 shows the relative density after solidifying the alloy 20 and the relative density after performing the extrusion treatment by ECAP in the case of performing the extrusion treatment by ECAP after solidifying the powdered alloy 20 in the mold 10. Show. In the example shown in Table 2, the composition of the alloy 20 was Bi ₂ Te _2.7 Se _0.3 , and the relative density after ECAP was specified by actually measuring the size and weight.

  As shown in Table 2 above, if the relative density of the solidified alloy is set to 98% or more, the relative density after ECAP is also maintained at 98%, and a thermoelectric material having a desired composition can be produced. Further, if the relative density of the solidified alloy is 96% or less, it is not preferable in production with a decrease in density. In particular, if the relative density of the solidified alloy is 92% or less, cracking occurs after ECAP. Therefore, a configuration in which the solid is pressed and solidified until the relative density becomes 98% is preferable.

Table 3 shows that the powdered alloy 20 (Bi ₂ Te _2.7 Se _0.3 ) is solidified beforehand by hot pressing, SPS (Spark Plasma Sintering) method or the like so that the relative density becomes 98% or more. On the other hand, the relative density after performing the extrusion process by ECAP with the metal mold | die 10 is shown. Again, the relative density after ECAP is specified based on actual measurements of size and weight.

  As shown in Table 3, when the crystal grain size of the alloy 20 after solidification is 30 μm or less, the relative density does not decrease even when the extrusion treatment by ECAP is performed. Density decreases. Accordingly, the crystal grain size of the alloy 20 after solidification is preferably 30 μm or less. In addition, when the crystal grain size of the thermoelectric material is 30 μm or less, the mechanical strength increases due to the refinement of crystal grains in the thermoelectric material.

  An alloy having a crystal grain size of 30 μm or less as described above can be easily realized by a configuration in which a powdered alloy is solidified. For example, when an alloy is formed by pressurizing an ingot, it is difficult to form an alloy with crystals having a desired particle diameter in both ECAP and shearing extrusion. On the other hand, according to the configuration in which the powder is solidified, an alloy can be configured with crystals having a desired particle diameter very easily. For example, when an ingot is extruded, the proportion of crystals having a particle size of 500 μm or less is at most about 10% to 60%. However, when the powdered alloy 20 is extruded, the particle size is 500 μm or less very easily. The proportion of the crystals can be made 100%.

When alloy Bi ₂ Te _2.7 Se _0.3 having a crystal grain size of 7 μm is solidified by pressurizing at 50 MPa in an argon atmosphere for 30 minutes, the crystal grain size is set to 7 μm by adjusting the temperature range to about 320 ° C. to 420 ° C. It can be set to ˜14 μm. On the other hand, by applying pressure at 440 ° C., the crystal grain size becomes 36 μm. Therefore, the crystal grain size can be made smaller than 30 μm by applying pressure in the temperature range of 320 ° C. to 420 ° C. Furthermore, the alloy Bi ₂ Te _2.7 Se _0.3 is pressurized in an argon atmosphere at 420 ° C. for 30 minutes, and in other cases, the relative density can be increased to 98% or more by pressing at 50 MPa to 100 MPa.

  In the present specification, when the crystal axes are defined for the atomic structure of the alloy 20, the same crystal grains as long as the inclination of specific crystal axes (for example, a-axis facing different directions) is within 15 °. The structure having an inclination exceeding 15 ° is assumed to be different crystal grains. The inclination of the crystal axis can be obtained, for example, by measuring an arbitrary cross section of a thermoelectric material with an EBSD (Electron Back Scatter Diffraction) apparatus manufactured by TSL, and analyzing the measurement result with analysis software. . Further, in this specification, the size of a crystal grain (crystal grain size) is defined by the radius of a circle having the same area as the crystal grain area in a certain cross section. The area of the crystal grains can also be measured with the above-described EBSD apparatus.

(3) Other embodiments:
In the present invention, it is only necessary to solidify the alloy powder and perform the extrusion process without applying back pressure to the alloy in the extrusion passage of the mold. It can be adopted. For example, the mold 10 is an example, and as long as the pressure shaft and the extrusion shaft do not exist on one axis, the angles of both axes are not limited, and the embodiment in which both are orthogonal to each other is provided. It is not limited to. Moreover, the diameters of the holes may be different between the pressure shaft and the extrusion shaft. In other words, the hole along the extrusion axis may be smaller than the hole along the pressure axis. Furthermore, it is good also as a structure which plugs the extrusion channel | path 12b with a part of pressurization channel | path 11b by inserting the insertion part 30 until it contacts the inner wall of the pressurization channel | path 11b.

Furthermore, it is good also as a structure which solidifies a powdery alloy previously and performs an extrusion process, without making a back pressure act on an alloy within an extrusion channel | path with respect to the solidified alloy. Table 4 shows the relative density after solidification of the alloy 20 and the extrusion by ECAP in the case where the powdery alloy 20 is solidified outside the mold 10 by hot pressing and then the extrusion treatment by ECAP is performed in the mold 10. The relative density after processing is shown. Again, the composition of the alloy 20 was Bi ₂ Te _2.7 Se _0.3 , and the relative density after ECAP was specified by measuring the size and weight.

  As shown in Table 4 above, when the relative density of the solidified alloy is set to 98% or more, the relative density after ECAP is also maintained at 98%, and a thermoelectric material having a desired composition can be produced. Further, if the relative density of the solidified alloy is 97% or less, it is not preferable in production with a decrease in density. In particular, if the relative density of the solidified alloy is 95% or less, cracking occurs after ECAP. Therefore, a configuration in which the solid is pressed and solidified until the relative density becomes 98% is preferable.

DESCRIPTION OF SYMBOLS 10 ... Mold 11a ... Opening part 11b ... Pressurization channel | path 12a ... Opening part 12b ... Extrusion channel | path 20 ... Alloy 30 ... Insertion tool 31 ... Plunger

Claims (4)

Solidifying a powder of an alloy of at least one element selected from the group consisting of Bi and Sb and at least one element selected from the group consisting of Te and Se;
With respect to the solidified alloy, the pressing direction of the pressurizing passage and the extruding shaft of the extruding passage with respect to the alloy extruded in the extruding passage by a mold that does not exist on one axis, Extrusion without reverse pressure acting
Thermoelectric material manufacturing method. Solidifying the alloy by pressurizing the alloy powder in the pressure passage in a state where the extrusion passage is closed by inserting an insertion tool into the extrusion passage;
The manufacturing method of the thermoelectric material of Claim 1. The particle size of the alloy powder is 30 μm or less, and the solidified alloy is 98% or more of the theoretical density of the alloy.
The manufacturing method of the thermoelectric material in any one of Claim 1 or Claim 2. It is manufactured by the method for manufacturing a thermoelectric material according to any one of claims 1 to 3.
Thermoelectric material.

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