KR20220052495A - Method for manufacturing thermoelectric material having porous structure - Google Patents

Abstract

The manufacturing method of a thermoelectric material comprises the following steps of: forming composite powder in which heterogeneous material particles are dispersed in a thermoelectric material by mixing and pressing the thermoelectric powder and a heterogeneous material having a dissolution property different from that of the thermoelectric powder; forming porous thermoelectric powder by selectively dissolving and removing the heterogeneous material particles of the composite powder; and sintering the thermoelectric powder to obtain a sintered body of a porous structure.

Description

Method for manufacturing a thermoelectric material having a porous structure

The present invention relates to a thermoelectric material, and more particularly, to a method of manufacturing a thermoelectric material having a porous structure.

Thermoelectric power generation and cooling technology has the Seebeck effect, which generates power when different temperatures are applied to both ends of the material, and the Peltier effect, where heat is absorbed and generated by the application of an electric field to both ends of the material. use it A thermoelectric element manufactured using a thermoelectric material has advantages of simple structure, long-term stability, rapid power generation and cooling.

Due to these advantages, thermoelectric technology is widely used for temperature control of small refrigerators, chillers, and electronic components. Thermoelectric power generation technology using the temperature difference between both ends of a thermoelectric semiconductor material is a technology that converts waste heat discarded from automobiles, industrial facilities, ships, etc. into effective electrical energy. In addition, thermoelectric power generation technology can be widely used as an independent power source for driving micro devices such as IoT sensors where there is a heat source.

The cooling ability or power generation performance of the thermoelectric element is greatly dependent on the thermoelectric performance index of the thermoelectric material itself constituting the element. As described above, the performance of a thermoelectric material largely depends on the thermal and electrical conductivity of the material, and the physical variables of these two factors are mutually dependent.

Most of the techniques to improve the thermoelectric performance of materials reported so far are based on techniques that reduce the thermal conductivity in a range that does not significantly decrease the electrical conductivity of the thermoelectric material. In other words, by inducing scattering of phonons in charge of thermal conductivity through nanopowdering of thermoelectric materials, artificial fine precipitates, and formation of dislocations, the thermal conductivity is ultimately reduced. The artificial interfaces and precipitates for phonon scattering are disadvantageous in that they disappear easily in the process of manufacturing the thermoelectric material into a sintered body, or are not stably maintained even in a high-temperature operating environment. In addition, when artificial precipitates are added to control thermal conductivity, they exist as impurities in the material, reducing electrical conductivity, causing local composition changes at the interface, etc., and may lead to deterioration of other properties of the thermoelectric material .

Research results to improve the performance of thermoelectric materials by forming pores in the material are also widely known. Air has a thermal conductivity of 0.025 Wm ^-1 K ^-1 at room temperature, which is very low compared to conventional solid thermoelectric materials. In addition, when the pore size is adjusted, selective wavelength phonon control is possible, and thermal conductivity can be decreased without a significant decrease in electrical conductivity, which can ultimately be a method to improve the thermoelectric performance index of the material. there is. In addition, the thermoelectric power generation device made of a thermoelectric material containing pores can be easily utilized to maintain the temperature difference between the low temperature part and the high temperature part due to low thermal conductivity.

In relation to the above technology, Korean Patent Laid-Open No. 10-1983626 discloses a porous thermoelectric material having a three-dimensional network shape. In the cited technology, a method of forming a three-dimensional insulating structure first and then filling it with a thermoelectric material is accompanied by difficulties in the process of controlling the size of micropores and filling the thermoelectric material.

The Biao Xu research group at Iowa State University in the United States succeeded in synthesizing a thermoelectric material with a porosity of ~30% by synthesizing a hollow nano thermoelectric wire. [Biao Xu et al, Angewandte Chemie International Edition, 56, 13, 3546 (2017)]. The above researchers realized low thermal conductivity based on a large porosity and achieved a maximum figure of merit of 1.1 around 450 K. The cited technology has a disadvantage in that a manufacturing method for making a hollow nanowire structure is complicated.

[G. Jeff Snyde et al., Advanced Materials 30, 34, 1802016 (2018)] The thermoelectric material manufactured by this citation method obtained the maximum figure of merit 1.2 near room temperature, and showed lattice thermal conductivity 50% lower than that of conventional materials. there is. This method requires a centrifugal separator equipped with a special heating furnace, and since the porosity of the upper and lower portions of the material is different, a problem may occur due to non-uniformity of thermoelectric performance within the same material.

Patent Document 1: Republic of Korea Patent No. 10-1983626

1. G. Jeff Snyder, et al. "Melt-Centrifuged (Bi,Sb)2Te3: Engineering Microstructure toward High Thermoelectric Efficiency." Advanced Materials, 30, 34, 1802016 (2018). 2. Biao Xu, et al. "Highly Porous Thermoelectric Nanocomposites with Low Thermal Conductivity and High Figure of Merit from Large-Scale Solution-Synthesized Bi2Te2.5Se0.5 Hollow Nanostructures." Angewandte Chemie International Edition, 56, 13, 3546 (2017).

An object of the present invention is to provide a method for efficiently manufacturing a porous thermoelectric material having low thermal conductivity and high thermoelectric performance by forming pores.

In a method for manufacturing a thermoelectric material according to an embodiment for realizing the object of the present invention, a composite in which particles of a different material are dispersed in a thermoelectric material by mixing and pressurizing a thermoelectric powder and a heterogeneous material having a dissolution property different from the thermoelectric powder Forming a powder, selectively dissolving and removing the heterogeneous material particles of the composite powder to form a porous thermoelectric powder, and sintering the thermoelectric powder to obtain a sintered body having a porous structure.

According to an embodiment, the thermoelectric powder includes at least one of a chalcogenide-based, antimonide-based, silicide-based, and Haf-Heusler-based thermoelectric material.

According to an embodiment, the heterogeneous material includes a metal salt.

According to an embodiment, the heterogeneous material includes a metal halide.

According to an embodiment, the heterogeneous material includes at least one of potassium chloride, calcium chloride, and sodium chloride.

According to an embodiment, the solvent for selectively dissolving the heterogeneous material includes a polar solvent or a non-polar solvent.

According to an embodiment, the sintering of the thermoelectric powder may include spark plasma sintering, hot pressing, or extrusion.

According to an embodiment, the porosity of the thermoelectric material is 10% or less.

According to an embodiment, the pore size of the thermoelectric material is 0.5 μm or less.

According to embodiments of the present invention, it is possible to manufacture a porous thermoelectric material having low thermal conductivity and high thermoelectric performance, and it is easy to control the porosity and pore size of the thermoelectric material.

The thermoelectric material manufactured as described above has overall low thermal conductivity due to the phonon scattering effect occurring at the pore interface and the low thermal conductivity of the pores themselves. Accordingly, the thermoelectric figure of merit is improved.

1 is a flowchart schematically illustrating a method of manufacturing a thermoelectric material according to an embodiment of the present invention.
2 is a cross-sectional view illustrating a thermoelectric element unit of a thermoelectric module according to an embodiment of the present invention.
3 is an electron micrograph (a) of the thermoelectric powder used in an embodiment of the present invention and an electron micrograph (b) of a porous thermoelectric powder formed from the thermoelectric powder.
4 is an electron micrograph (a) of the thermoelectric material obtained in Comparative Example and an electron micrograph (b) of the thermoelectric material obtained in Example of the present invention.
5 is a graph showing the porosity of the thermoelectric material according to the volume ratio (compared to the thermoelectric powder) of the potassium chloride (KCl) powder used in the embodiment of the present invention.
6 is a graph showing the thermal conductivity (a) of the thermoelectric material (Pure BST) obtained in Comparative Example and the thermal conductivity (b) of the thermoelectric material (porosity: 4.74%, 10.08%) obtained in Examples of the present invention.
7 shows the Seebeck coefficient (a), electrical resistivity (b) and thermoelectric figure of merit (c) of the thermoelectric material (Pure BST) obtained in Comparative Example and the thermoelectric material (porosity: 4.74%, 10.08%) obtained in Examples of the present invention. ) is a graph showing
8 is an electron micrograph (a) of a porous thermoelectric material whose pore size is controlled according to an embodiment of the present invention, a thermoelectric material (BST) obtained in Comparative Example, and a thermoelectric material obtained in an embodiment of the present invention (small size pore, It is a graph showing lattice thermal conductivity (b) and thermoelectric figure of merit (c) of big size pores).

In the present application, terms such as first, second, etc. may be used to describe various elements, but the elements should not be limited by the terms. The above terms are used only for the purpose of distinguishing one component from another. For example, without departing from the scope of the present invention, a first component may be referred to as a second component, and similarly, a second component may also be referred to as a first component.

The terms used in the present application are only used to describe specific embodiments, and are not intended to limit the present invention. The singular expression includes the plural expression unless the context clearly dictates otherwise. In the present application, terms such as "comprise" or "have" are intended to designate that a feature, number, step, operation, component, part, or a combination thereof described in the specification exists, but one or more other features It is to be understood that it does not preclude the possibility of the presence or addition of numbers, steps, operations, components, parts, or combinations thereof.

1 is a flowchart schematically illustrating a method of manufacturing a thermoelectric material according to an embodiment of the present invention.

Referring to FIG. 1 , the thermoelectric powder 10 and the heterogeneous material 20 having different dissolution properties from the thermoelectric powder 10 are mixed and pressed to form a composite powder 30 in which the heterogeneous material particles are dispersed in the thermoelectric material. do (S10).

For example, the thermoelectric powder 10 may include a chalcogenide, an antimonide-based, a silicide-based, a Half-Heusler-based, and an oxide-based thermoelectric material. can For example, the thermoelectric powder 10 may be a bi-element system such as Bi-Te-based, Sb-Te-based, Bi-Te-Se-based, Bi-Te-Sb-based, Bi-Sb-Te-Se-based or the like; It may include at least one of ternary and quaternary materials. The thermoelectric powder may be, for example, N-type by adding selenium (Se) to the Bi-Te system, or P-type by adding antimony (Sb), etc., SbI ₃ , N, P type through the implantation of impurities such as Cu, Ag, CuCl ₂ is also possible. In addition, a thermoelectric material such as Pb-Te-based or Mg-Sb-based thermoelectric material may be included for medium temperature, or a Si-Ge-based or Sn-Te-based thermoelectric material may be included for high temperature use.

A known method may be used to obtain the thermoelectric powder 10 . For example, from the raw material of each component constituting the thermoelectric powder, it can be obtained through various methods such as a ball-milling method, a mechanical alloying method, a melt-spin method, a melting method, and a hydrothermal synthesis method. there is. The size of the thermoelectric powder may be several nm to several hundred μm, but is not limited thereto.

The heterogeneous material 20 has a dissolution characteristic different from that of the thermoelectric powder 10 . For example, the heterogeneous material 20 may be water-soluble. According to an embodiment, the heterogeneous material 20 may include a metal salt, specifically, a metal halide. For example, the metal halide may include potassium chloride, calcium chloride, sodium chloride, and the like. However, embodiments of the present invention are not limited thereto. For example, the heterogeneous material 20 may include a metal compound soluble in an aprotic organic solvent.

In order to obtain the composite powder 30 of the thermoelectric powder 10 and the heterogeneous material 20, milling may be used. For example, the composite powder 30 may be obtained by mixing the thermoelectric powder 10 and the dissimilar material 20 and performing milling using a high energy ball milling equipment. The milling process may be performed in an inert gas condition such as nitrogen to prevent oxidation.

For example, the composite powder 30 may have a form in which the heterogeneous material 20 is dispersed in a thermoelectric material.

Next, the heterogeneous material is removed from the composite powder 30 to obtain a porous thermoelectric powder 40 .

In order to selectively remove the heterogeneous material, an appropriate solvent may be provided to the composite powder 30 . For example, when the heterogeneous material is a water-soluble material such as potassium chloride, water may be provided. However, embodiments of the present invention are not limited thereto, and a polar solvent such as methanol, ethanol, N-methylpyrrolidone, etc. or a non-polar solvent such as methylethylketone and heptane may be selected according to the dissolution characteristics of the heterogeneous material. can

According to an embodiment, water may be used to remove the potassium chloride, and ultrasonic waves may be applied for rapid removal.

According to the particle size and content of the heterogeneous material 20 dispersed in the composite powder 30, the pore size and porosity may vary in the porous structure from which the heterogeneous material is removed, which is the purpose of use of the thermoelectric material, the operating temperature range, etc. can be adjusted accordingly.

According to an embodiment, the volume ratio of the heterogeneous material 20 to the thermoelectric powder 10 may be 5% or less. When the content of the heterogeneous material 20 is excessively high, the porosity may excessively increase and the thermoelectric figure of merit may decrease.

Next, the porous thermoelectric powder 40 may be sintered to form a thermoelectric material. For example, the porous thermoelectric powder 40 may be sintered under high temperature and high pressure conditions, and various methods such as spark plasma sintering, hot pressing, and extrusion may be used to form a sintered body.

For example, in the sintering step, the temperature may be 300 °C to 600 °C, and the pressure may be 5 MPa to 500 MPa. However, embodiments of the present invention are not limited thereto, and sintering conditions may vary depending on the thermoelectric material and the purpose of the thermoelectric material.

The thermoelectric material may have a porous structure. Accordingly, the porous structure may have improved thermoelectric performance.

According to an embodiment, the porosity of the thermoelectric material may be within 10%. For example, the porosity of the thermoelectric material may be 1% to 10%. When the porosity of the thermoelectric material is excessive, the electrical resistivity is greatly increased compared to the decrease in thermal conductivity, so that the output factor may be decreased. However, embodiments of the present invention are not limited thereto, and in the case of a thermoelectric material for medium and high temperature with a large temperature difference between both ends, the optimum porosity may exceed 10%.

In addition, the thermoelectric material may have different thermoelectric properties according to pore sizes. For example, the average pore size (diameter) of the thermoelectric material may be less than 1 μm, and more specifically, 0.5 μm or less. When the pore size of the thermoelectric material is 0.5 μm or less, thermoelectric performance may be greatly increased in a high temperature region.

The thermoelectric material may be used for manufacturing a thermoelectric module. 2 is a cross-sectional view illustrating a thermoelectric element unit of a thermoelectric module according to an embodiment of the present invention.

Referring to FIG. 2 , the thermoelectric element unit 100 includes a first substrate 210 , a second substrate 220 spaced apart from the first substrate, and the first substrate 210 and the second substrate 220 . and a first electrode 12 and a second electrode 14 disposed therebetween and spaced apart from each other, and a thermoelectric leg 16 disposed between the first electrode 12 and the second electrode 14 . there is.

According to an embodiment, a first barrier layer 18a may be disposed between the first electrode 12 and the thermoelectric leg 16 . Also, a second barrier layer 18b may be disposed between the second electrode 14 and the thermoelectric leg 16 . The barrier layers may protect the thermoelectric leg 16 .

The first substrate 210 and the second substrate 220 may each include an electrically insulating material. For example, the first substrate 210 and the second substrate 220 may include alumina, sapphire, silicon, silicon nitride, silicon carbide, silicon aluminum carbide, quartz, a polymer, or the like. The polymer may include polyimide, polyamide, polycarbonate, polystyrene, polyethylene terephthalate, polyacrylic resin, and the like.

The first substrate 210 and the second substrate 220 may be made of the same material or may be made of different materials.

According to an embodiment, the thermoelectric element unit 10 may include a pair of thermoelectric legs 16 . The thermoelectric legs 16 may be doped with different types. For example, the thermoelectric element unit 10 may include a first thermoelectric leg N doped with an n-type and a second thermoelectric leg P doped with a p-type. One end of the first thermoelectric leg N and the second thermoelectric leg P may be electrically connected to the first electrode 12 in common. The other ends of the first thermoelectric leg N and the second thermoelectric leg P may be electrically connected to second electrodes 14 spaced apart from each other, respectively.

For example, the thermoelectric leg 16 may have a cylindrical shape or a polygonal column shape.

For example, the first electrode 12 may include a metal such as nickel (Ni), titanium (Ti), copper (Cu), platinum (Pt), gold (Au), or silver (Ag). . In addition, the first electrode 12 may further include a metal compound such as NiP, TiN, ZnO, or the like. Each of these may be used alone or in combination. The second electrode 14 may include the same material as the first electrode or a different material.

The barrier layers may include a material different from that of the first electrode 12 or the second electrode 14 . For example, the barrier layers may include nickel (Ni), titanium (Ti), copper (Cu), platinum (Pt), gold (Au), silver (Ag), molybdenum (Mo), tin (Sn), and zirconium. (Zr), niobium (Nb), a metal such as tungsten (W), an alloy thereof, or a metal compound thereof may be included. The metal compound may further include a metal compound such as NiP, TiN, or ZnO. According to an embodiment, the barrier layers may include a material having a lower coefficient of thermal expansion than a material constituting the first electrode 12 . For example, when the first electrode 12 or the second electrode 14 includes copper, the barrier layers may include a material other than copper, specifically, nickel, titanium, tin, It may include zirconium, an alloy thereof, or a metal compound thereof.

According to an embodiment, the thermoelectric leg 16 includes a thermoelectric material. The thermoelectric material has a porous structure. The thermoelectric material may be the same as the previously described thermoelectric material.

According to an embodiment of the present invention, a porous thermoelectric material having low thermal conductivity and high thermoelectric performance can be manufactured, and the porosity and pore size of the thermoelectric material can be easily controlled.

The thermoelectric material manufactured as described above has overall low thermal conductivity due to the phonon scattering effect occurring at the pore interface and the low thermal conductivity of the pores themselves. Accordingly, the thermoelectric figure of merit may be improved, and the thermoelectric performance of a thermoelectric module using the same may be improved.

Hereinafter, the effects of the embodiments of the present invention will be examined through specific experiments.

Example

A P-type thermoelectric powder having a size of 200 to 400 nm having a composition of Bi _0.4 Sb _1.6 Te ₃ was synthesized using a mechanical alloying method.

After putting the thermoelectric powder and potassium chloride powder in a stainless bell jar, a ball used for milling such as zirconia was put, and mixed/pressurized using a high energy ball milling equipment having a rotation speed of 1060 rpm per minute to obtain a composite powder . At this time, the inside was filled with high-purity nitrogen to prevent oxidation, and milling was performed.

Next, the composite powder was placed in ultrapure water and stirred with an ultrasonic generator to remove (dissolve) potassium chloride from the composite powder to obtain a porous thermoelectric powder.

Next, a thermoelectric material was obtained by sintering the porous thermoelectric powder (spark plasma sintering, 450° C., 40 MPa, 5 minutes) to form a sintered body.

comparative example

As a comparative example, a thermoelectric material was obtained by sintering the P-type thermoelectric powder used in Examples (spark plasma sintering) to form a sintered body.

3 is an electron micrograph (a) of the thermoelectric powder used in an embodiment of the present invention and an electron micrograph (b) of a porous thermoelectric powder formed from the thermoelectric powder.

Referring to FIG. 3 , it can be confirmed that a thermoelectric powder having a porous structure is formed through an embodiment of the present invention.

4 is an electron micrograph (a) of the thermoelectric material obtained in Comparative Example and an electron micrograph (b) of the thermoelectric material obtained in Example of the present invention.

Referring to FIG. 4 , it can be confirmed that by using the porous thermoelectric powder of the present invention, a porous structure can be maintained even in a thermoelectric material formed through a high-temperature and high-pressure sintering process, and pores can be uniformly distributed.

5 is a graph showing the porosity of the thermoelectric material according to the volume ratio (relative to the thermoelectric powder) of the potassium chloride (KCl) powder used in an embodiment of the present invention.

Referring to FIG. 5 , as the volume of the potassium chloride powder increased, the porosity of the final thermoelectric material increased. Therefore, it can be seen that the porosity of the final thermoelectric material can be controlled by adjusting the content of the heterogeneous material.

6 is a graph showing the thermal conductivity (a) of the thermoelectric material (Pure BST) obtained in Comparative Example and the thermal conductivity (b) of the thermoelectric material (porosity: 4.74%, 10.08%) obtained in Examples of the present invention. The thermal conductivity was measured using a laser flash system.

Referring to FIG. 6 , it can be confirmed that a thermoelectric material having a reduced thermal conductivity of about 25% compared to the conventional thermoelectric material is obtained through the present invention, and thus the performance of the thermoelectric material can be expected to be improved.

7 shows the Seebeck coefficient (a), electrical resistivity (b) and thermoelectric figure of merit (c) of the thermoelectric material (Pure BST) obtained in Comparative Example and the thermoelectric material (porosity: 4.74%, 10.08%) obtained in Examples of the present invention. ) is a graph showing Referring to FIG. 7 , although the electrical resistivity of the porous thermoelectric material increased as the porosity increased, the performance index of the thermoelectric material increased compared to the existing material due to low thermal conductivity, and it was confirmed that it had a value of 1.045 at 353 K. . In addition, referring to the measurement result of the thermoelectric figure of merit, it can be seen that when the porosity exceeds 10%, the increase in the thermoelectric figure of merit is weak or may be lowered depending on the temperature.

8 is an electron micrograph (a) of a porous thermoelectric material with controlled pore size and lattice heat conduction of a thermoelectric material (BST) obtained in Comparative Example and a thermoelectric material (small size pore, big size pore) obtained in Example of the present invention; It is a graph showing the figure (b) and the thermoelectric figure of merit (c). In the above embodiment, the pore size was adjusted to about 1 μm (big pore size) and about several hundred nm (small pore size) by varying the size of the potassium chloride powder.

Referring to FIG. 8 , the thermoelectric material having a relatively large pore size showed a high thermoelectric performance index at room temperature, and in the case of the thermoelectric material having a relatively small pore size, the thermoelectric performance at a high temperature. index was high. As a result of the analysis of lattice thermal conductivity shown in (b), the material having small pores in the high-temperature part is consistent with the result that the lattice thermal conductivity is lower than that of the thermoelectric material having a large pore size, which is the result of having small pores. This may mean that the phonon is effectively controlled in the high temperature part. Therefore, it can be seen that by adjusting the pore size, it is possible to obtain a thermoelectric material for an appropriate use (use temperature).

Although the above has been described with reference to the embodiments, those skilled in the art can variously modify and change the present invention within the scope without departing from the spirit and scope of the present invention described in the claims below you will understand

The present invention can be used in the manufacture of thermoelectric modules for small-scale power generation, cooling and temperature control of various devices, and the like.

Claims (9)

forming a composite powder in which particles of a different material are dispersed in a thermoelectric material by mixing and pressing the thermoelectric powder and a heterogeneous material having a dissolution property different from that of the thermoelectric powder;
forming a porous thermoelectric powder by selectively dissolving and removing the heterogeneous material particles of the composite powder; and
and sintering the thermoelectric powder to obtain a sintered body having a porous structure. The method of claim 1 , wherein the thermoelectric powder comprises at least one of a chalcogenide-based, antimonide-based, silicide-based, and Haf-Heusler-based thermoelectric material; The method of claim 1 , wherein the heterogeneous material includes a metal salt. The method of claim 1 , wherein the heterogeneous material includes a metal halide. The method of claim 1 , wherein the heterogeneous material comprises at least one of potassium chloride, calcium chloride, and sodium chloride. The method of claim 1 , wherein the solvent for selectively dissolving the heterogeneous material comprises a polar solvent or a non-polar solvent. The method of claim 1 , wherein the sintering of the thermoelectric powder comprises performing spark plasma sintering, hot pressing, or extrusion. The method of claim 1, wherein the porosity of the thermoelectric material is 10% or less. The method of claim 7, wherein the pore size of the thermoelectric material is 0.5 μm or less.

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