KR100786633B1 - METHOD FOR MANUFACTURING Bi-Te BASED THERMOELECTRIC MATERIALS - Google Patents

Abstract

The present invention relates to a method of manufacturing a Bi-Te-based n-type thermoelectric material, and an object thereof is to mix coarse powder and fine powder at a predetermined mixing ratio, and then sinter through a discharge plasma sintering process capable of sintering in a short time in a few minutes. Therefore, the present invention provides a method for producing a Bi-Te-based n-type thermoelectric material which is excellent in mechanical strength and advantageous in terms of performance and manufacturing cost by improving crystallographic orientation and facilitating densification and control of tissues. To this end, the present invention weighs Bi, Te, Se elements and SbI ₃ dopants to fit into n-type composition, loads them into quartz tubes, loads them in a vacuum furnace, prepares ingots, and breaks the manufactured ingots. Afterwards, the coarse powder of 200 ~ 300㎛ and fine powder of 45㎛ or less, classify the reduced powder of hydrogen, and then mixed in a predetermined mixing ratio of coarse powder and fine powder to sinter using the discharge plasma sintering process The technical gist of the method for manufacturing a Bi-Te-based n-type thermoelectric material consisting of

Discharge Plasma Sintering, Orientation, Thermoelectric Materials, Coarse Powder, Fine Powder

Description

METHOD FOR MANUFACTURING Bi-Te BASED THERMOELECTRIC MATERIALS}

1 is a graph showing the change in relative density according to the change of the fine powder mixing ratio of the n-type thermoelectric material prepared by the embodiment of the present invention,

2 is a pole figure of the (0 0 6) plane of an n-type thermoelectric material drawn by an embodiment of the present invention,

3 is a graph showing the change in bending strength according to the change of the fine powder mixing ratio of the n-type thermoelectric material prepared by the embodiment of the present invention,

4 is a graph showing a change in thermoelectricity according to the change of the fine powder mixing ratio of the n-type thermoelectric material prepared by the embodiment of the present invention,

5 is a graph showing a change in carrier concentration and mobility converted through measurement of the Hall coefficient according to the change of the fine powder mixing ratio of the n-type thermoelectric material prepared by the embodiment of the present invention,

6 is a graph showing a change in electrical resistivity according to the change of the fine powder mixing ratio of the n-type thermoelectric material prepared by the embodiment of the present invention,

7 is a graph showing a change in thermal conductivity according to the change of the fine powder mixing ratio of the n-type thermoelectric material prepared by the embodiment of the present invention,

8 is a graph showing a change in the performance index according to the change of the fine powder mixing ratio of the n-type thermoelectric material prepared by the embodiment of the present invention.

The present invention relates to a method for manufacturing a Bi-Te-based n-type thermoelectric material, and in particular, to prepare a thermoelectric material by mixing coarse powder and fine powder in a predetermined mixing ratio and sintering through a discharge plasma small crystal crystal. It relates to a method for producing a thermoelectric material.

In general, Bi-Te-based thermoelectric materials are used as a means for solving the heat dissipation problems of high-integration devices and various sensors due to their excellent thermoelectric performance in the vicinity of room temperature, and are mainly manufactured by unidirectional solidification or single crystal growth. However, the thermoelectric element by casting method such as unidirectional solidification method or single crystal growth method is a unit crystal rhombohedron despite its excellent thermoelectric performance.Te-Te bond is composed of Van der waals bond with the weakest bonding force among atomic bonds. Due to the increase in manufacturing cost due to the reduced recovery rate during processing and manufacturing of the module has a disadvantage of high cost. In addition, this property has a strong anisotropy in the mechanical and electrical properties of Bi ₂ Te ₃ system thermoelectric material, and the electrical conductivity is about three times the difference between the a-axis and c-axis. In addition, due to the anisotropy, in order to improve the energy conversion efficiency, it should be used in the a-axis direction, but in the case of single crystal, since the material is easily broken along the cleaved surface, there is a difficulty in processing, and there is a considerable loss of material.

In order to overcome the problems of the single crystal as described above, the research on powder metallurgy, which is excellent in mechanical strength and expected to be advantageous in terms of performance and manufacturing cost, is being actively conducted. In this paper, there is a problem in that it is difficult to control the organization of the grains such as grain size and crystallographic anisotropy and thus control of thermoelectricity.

The present invention has been made in consideration of the above problems, and an object of the present invention is to crystallize by mixing coarse powder and fine powder in a predetermined mixing ratio, and then sintering through a discharge plasma sintering process capable of sintering in a short time in water. The present invention provides a method for producing a Bi-Te-based n-type thermoelectric material which is excellent in mechanical strength and advantageous in terms of performance and manufacturing cost by improving orientation and facilitating densification and control of tissues.

Bi-Te-based n-type thermoelectric material of the present invention to achieve the object as described above and to perform the problem to eliminate the conventional defects, Bi, Te, Se element and SbI ₃ dopant in the n-type composition A vacuum sealing step (S1) which is weighed to fit and charged in a quartz tube and then vacuum sealed;

An ingot manufacturing step (S2) of adding a quartz tube encapsulated through the encapsulation step (S1) into a stirring furnace and stirring the furnace to produce an ingot;

A shredding step (S3) of shredding the ingot manufactured through the step of manufacturing the ingot (S2);

Grinding step (S4) for grinding the crushed powder through the crushing step (S3);

A classification step (S5) of separating the powder ground through the grinding step into a coarse powder having a particle diameter of 200 to 300 µm and a fine powder having a particle diameter of 20 to 45 µm;

Charging the powder classified through the classification step (S5) into a Pyrex tube, filled with hydrogen, sealed, and then heated and maintained at a temperature of 340 to 380 ° C. to reduce hydrogen of the powder (S6);

Mixing step of mixing 60 ~ 80wt.% Coarse powder and 20 ~ 40wt.% Coarse powder through the hydrogen reduction step (S6) into an ampoule, charged in an ampoule, and then mixing using a three-dimensional mixer ( S7);

It consists of a sintering step (S8) for sintering the powder mixed through the mixing step (S7) using the discharge plasma sintering equipment.
The vacuum encapsulation step (S1) is a preparatory step for manufacturing an ingot, and an element such as Bi, Te, Se, etc. having a purity of 99.999% and SbI ₃ dopant are weighed to fit the n-type composition of the thermoelectric material and loaded into a quartz tube. After dissolution, vacuum sealing is carried out at 10 ⁻⁴ torr or less to suppress oxidation of component elements. On the other hand, the n-type composition is not limited to a specific composition ratio and all the composition of the Bi-Te-based thermoelectric material may be the object, but preferably made of Bi ₂ Te _2.85 Se _0.15 .

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The ingot manufacturing step (S2) is a step of manufacturing an ingot using a prepared material, and the ingot is manufactured by injecting a quartz tube filled with the elements into a stirring furnace. This ingot manufacturing step (S2) is stirred for 2 hours at a rate of 10 times / min at 700 ~ 750 ℃, then proceeds to the furnace cooling. Bi ₂ Te melt temperature range of _{0 .15 2 .85} Se is performed at 590 ~ 595 ℃ and the preparation temperature is generally 100 ~ 150 ℃ higher than the melting point temperature range. At higher temperatures, the volatilization of the element is increased, and the longer the melting time is, the more favorable it is.

The crushing step (S3) is a step of crushing the solidified ingots, the crushing is made by using alumina induction in the grove box in a nitrogen (inert) atmosphere to prevent the oxidation of the powder to the maximum.

The grinding step (S4) is to crush the crushed ingot using a ball mill in an argon atmosphere, wherein the powder to be pulverized to have a particle size of 300㎛ or less.

The classification step (S5) is a step of separating the pulverized powder into coarse powder and fine powder, it is classified into coarse powder having a particle diameter of 200 ~ 300㎛, and fine powder having a particle diameter of 20 ~ 45㎛ or less. At this time, the coarse powder is to increase the crystallographic orientation, it is preferable that the powder is made of powder having a particle size of 200 ~ 300㎛ while maintaining a thin plate-like form, the fine powder is densified and improved density of the tissue and The increase in the mechanical strength is preferably 45㎛ or less, but if the particle size is too fine it is made of powder having a particle size of 20 ~ 45㎛ because the performance of the thermoelectric element is degraded by the oxidation effect.

The hydrogen reduction step (S6) is charged to the classified powder in a pyrex tube, filled with more than 99.999% of hydrogen 300 ~ 500torr and sealed, and proceeds for 10 to 15 hours at 340 ~ 380 ℃ within the powder Hydrogen is reduced. Here, if the amount of hydrogen is insufficient, a sufficient ring reduction reaction cannot be made, and if the amount of hydrogen is excessive, there is a risk of expansion and explosion at high temperatures. In addition, at temperatures below 340 ° C., the activation energy required for the reaction is small and hydrogen reduction is required for a very long time. If the temperature exceeds 380 ° C., there is a risk of volatilization of components in hydrogen reduction. The hydrogen reduction process time depends on temperature, and 10-15 hours are preferable in the temperature range of 340-380 degreeC.

The mixing step (S7) is to mix the coarse powder and the fine powder in a predetermined ratio to charge in an ampoule, and vacuum-packed to 10 ^-4 torr or less, and then using a three-dimensional mixer for 2 to 3 hours at 90 rpm It is mixed. In this case, when the mixing ratio of the coarse powder is increased, the electrical properties are improved by the increase of the orientation, while the defects increase in the material, resulting in poor thermal conductivity. On the other hand, if the mixing ratio of the fine powder is excessively increased, the electrical properties are drastically reduced due to the decrease in the orientation and the increase of oxidation, so that the coarse powder of 60 to 80 wt.% And the fine powder of 20 to 40 wt.% Are preferably mixed. In addition, if the mixing time is short, the mixing of the powder is not uniform. If the mixing time is too long, the powder is pulverized due to friction and collision between the powders, so the particle size is reduced, and 2-3 hours is preferable. It may vary depending on the conditions.

The sintering step (S8) is to produce a thermoelectric material by performing a discharge plasma sintering process at a temperature of 380 ~ 460 ℃ and a pressure of 20 ~ 40MPa in a vacuum of 10 ^-3 torr or less. Higher sintering pressure is advantageous, but in this embodiment, carbon mold was used as the sintering mold, and the maximum compressive strength of the carbon mold was 40 MPa, so it was limited to 40 MPa. If the sintered mold is replaced with a steel mold or tungsten mold with high compressive strength, it may be possible to pressurize to the maximum compressive strength of these molds.

Example

Bi, Te, Se and SbI ₃ having a purity of 99.999% have a composition of Bi ₂ Te _2.85 Se _0.15 + 0.05 wt.% SbI ₃ to form a base material of the n-type thermoelectric material. The base material was charged into a quartz tube and then vacuum sealed at 2 × 10 ⁻⁵ torr. The quartz tube was put in a stirring furnace, stirred at a rate of 10 times / minute at 750 ° C. for 2 hours, and then cooled to prepare an ingot. The ingot prepared as described above is crushed using alumina induction in a grove box under a nitrogen atmosphere. The crushed ingots are ground for 10 hours using a ball mill in an argon atmosphere. The pulverized powder is classified into a coarse powder of 200 to 300 µm and a fine powder of 20 to 45 µm. The classified powder is charged into a pyrex tube, filled with 380 torr of 99.999% hydrogen, and then charged at 380 ° C. for 10 hours to reduce the hydrogen in the powder. The fine powder is added to the coarse powder in 10% increments, charged in an amplifier, vacuum-sealed at 2 × 10 ^-5 torr, and mixed at 90 rpm for 3 hours using a three-dimensional mixer. The mixed powder was subjected to a discharge plasma sintering process at a temperature of 420 ° C. and a pressure of 40 MPa in a vacuum of 1 × 10 ⁻³ torr to produce a Bi-Te-based n-type thermoelectric material of Φ 20 × 8 mm.

Figure 1 shows the change in relative density according to the mixing ratio of the fine powder, referring to Figure 1, when 20wt.% Of the fine powder is mixed showed a relative density of more than 97%, these results are found between the coarse powder This is because the sinterability is improved by filling the gaps with fine powder.

2 shows a pole figure of the (0 0 6) plane of the n-type thermoelectric material. 2 (a) sintered only with coarse powder and 2 (b) with 20wt.% Of fine powder showed higher orientation than 2 (c) sintered only with fine powder.

Figure 3 shows the change in bending strength according to the mixing ratio of the fine powder, the bending strength showed a tendency to increase almost linearly with increasing the mixing ratio of the fine powder and the maximum value of 90.8MPa in the sintered compact sintered only with fine powder Indicated. Even when 20 wt. The cast material here exhibits a bending strength of about 18 MPa.

Figure 4 shows the change in the thermal power according to the mixing ratio of the fine powder, the thermal power was reduced as the mixing ratio of the fine powder increases. This is due to the increase in electrons due to oxidation of the surface of the powder generated during the crushing process, resulting in a decrease in thermoelectricity, which can be confirmed by a change in the carrier concentration in terms of the measurement of the hole coefficient.

Figure 6 shows the change in the electrical resistivity according to the mixing ratio of the fine powder, the change in the electrical resistivity showed a tendency to decrease gradually after the sharp decrease as the fine powder increases to 20wt.%. As shown in FIG. 5, the rapid decrease in specific resistance is because both carrier concentration and mobility increase as the mixing ratio of the fine powder increases, and when the fine powder is mixed, the electrical resistivity decreases due to the decrease of carrier mobility. The rate of decline slowed down.

Figure 7 shows the change in thermal conductivity according to the mixing ratio of the fine powder, slightly increased as the mixing ratio of the fine powder. Thermal conductivity is expressed as the sum of the contributions by the carrier and the lattice vibration, and the contributions by the carrier are inversely proportional to the electrical resistivity by the Wiedemann-Franz law. On the other hand, since the contribution by lattice vibration is constant regardless of the carrier concentration, the overall thermal conductivity depends on the contribution by the carrier. Therefore, the thermal conductivity was inversely proportional to the electrical resistivity, which can be seen as a change in carrier concentration and mobility of FIG. 5.

8 shows the change of the performance index according to the mixing ratio of the fine powder, when 20wt.% Of the fine powder is mixed, showed the maximum performance index as 2.62 × 10 ^-3 / K, which increases the orientation by the coarse powder And the improvement of the sinterability by the fine powder increased the carrier mobility.

The present invention is not limited to the above-described specific preferred embodiments, and various modifications can be made by any person having ordinary skill in the art without departing from the gist of the present invention claimed in the claims. Of course, such changes will fall within the scope of the claims.

The present invention mixes coarse powder with high crystallographic orientation and fine powder with relatively low orientation at a predetermined mixing ratio as described above, and then sinters using a discharge plasma sintering process capable of sintering in a short period of time. It is configured to make it easier to densify the thermoelectric material structure and control the grain size and thermoelectric performance. In addition, it is possible to realize the excellent mechanical strength of the thermoelectric material, it is more advantageous in terms of performance and manufacturing cost.

Claims (7)

delete A vacuum encapsulation step (S1) in which elements of Bi, Te, Se and SbI ₃ dopant are weighed to fit the n-type composition, charged into a quartz tube, and then vacuum-sealed; An ingot manufacturing step (S2) of adding a quartz tube encapsulated through the encapsulation step (S1) into a stirring furnace and stirring the furnace to produce an ingot; A shredding step (S3) of shredding the ingot manufactured through the step of manufacturing the ingot (S2); Grinding step (S4) for grinding the crushed powder through the crushing step (S3); A classification step (S5) of separating the powder ground through the grinding step into a coarse powder having a particle diameter of 200 to 300 µm and a fine powder having a particle diameter of 20 to 45 µm; Charging the powder classified through the classification step (S5) into a Pyrex tube, filled with hydrogen, sealed, and then heated and maintained at a temperature of 340˜380 ° C. to reduce hydrogen of the powder (S6); Mixing step of mixing 60 ~ 80wt.% Coarse powder and 20 ~ 40wt.% Coarse powder through the hydrogen reduction step (S6) into an ampoule, charged in an ampoule, and then mixing using a three-dimensional mixer ( S7); Sintering step (S8) of the powder mixed through the mixing step (S7) at a temperature of 420 ~ 460 ℃ and a pressure of 20 ~ 40MPa in a vacuum of 10 ^-3 torr or less using a discharge plasma sintering equipment; Method for producing a Bi-Te-based n-type thermoelectric material, characterized in that. The method of claim 2, The crushing step (S3), Bi-Te-based n-type thermoelectric material characterized in that the crushing using alumina induced in the glove box under a nitrogen atmosphere. The method of claim 2, The grinding step (S4), Bi-Te-based n-type thermoelectric material, characterized in that the grinding using a ball mill in an argon atmosphere. delete The method of claim 2, The ingot manufacturing step (S2), after stirring for 2 hours at a rate of 10 times / min at 700 ~ 750 ℃, the method for producing a Bi-Te-based n-type thermoelectric material, characterized in that the furnace cooling. The method of claim 2, The hydrogen reduction step (S6), Bi-Te-based system characterized in that the Pyrex tube filled with the classified powder is filled with 300 ~ 500 torr of hydrogen, and maintained for 10 to 15 hours at 340 ~ 380 ℃ Method for producing n-type thermoelectric material.

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