KR102268703B1 - Fabrication method of thermoelectric permingeatite materials - Google Patents

Abstract

According to various embodiments of the present invention, a method for manufacturing a thermoelectric material comprises: a step of preparing Cu, Sb and Se which are raw materials; a step of synthesizing powder by mechanically alloying the prepared raw materials; and a step of hot pressing (HP) and molding the powder.

Description

Manufacturing method of firming gear tight thermoelectric material {FABRICATION METHOD OF THERMOELECTRIC PERMINGEATITE MATERIALS}

The present invention relates to a method for manufacturing a thermoelectric material, and more particularly, to a method for manufacturing a permingeatite thermoelectric material based on copper (Cu), antimony (Sb) and selenium (Se).

In the 21st century, the conservation of the global environment and the depletion of energy resources have emerged, and alternative energy development is required. Thermoelectric energy conversion technology is attracting attention as an alternative energy technology because it can directly convert thermal energy into electric energy or electric energy into thermal energy. Thermoelectric material is an eco-friendly energy conversion material that can obtain electrical energy by using heat energy that is discarded or left unattended, such as automobile and industrial waste heat and solar heat, and does not generate pollution such as noise, vibration, and waste.

Thermoelectric conversion technology can be classified into thermoelectric cooling technology and thermoelectric power generation technology. Thermoelectric cooling technology has the advantages of being able to replace refrigerant gas that causes global warming, and it does not require a compressor, so there is no vibration and noise, and precise temperature control is possible. On the other hand, thermoelectric power generation technology is the only power generation method that can directly convert low-temperature thermal energy that is difficult to recycle and small-scale distributed thermal energy into electrical energy. It has a long lifespan and can reduce greenhouse gases by converting waste heat into electricity. has the advantage of

The energy conversion efficiency of thermoelectric materials is evaluated by a dimensionless figure of merit (ZT) ^{, defined as ZT = α 2} σ T/κ. where α, σ, κ, and T are Seebeck coefficient, electrical conductivity, thermal conductivity, and absolute temperature, respectively. The Seebeck coefficient is defined as the change in electromotive force per unit temperature difference, and the product of the square of the Seebeck coefficient and the electrical conductivity is called the ^{output factor (α 2 σ).} Therefore, in order to obtain a thermoelectric material with high performance, a high output factor and low thermal conductivity are required. However, since there is a trade-off relationship between Seebeck coefficient and electrical conductivity, electrical conductivity depends on carrier concentration, and as electrical conductivity increases, electronic thermal conductivity increases and total thermal conductivity increases. Electrical and thermal properties are required. To this end, studies have been conducted on improving electrical properties through optimizing carrier concentration or reducing thermal conductivity due to vibration of elements filled in the pores of the crystal structure.

On the other hand, recently, interest in the search for materials composed of inexpensive and non-toxic elements is increasing. _{Bi 2} Te ₃ , PbTe, and skutterudite compounds known to have good performance to date have a problem consisting of toxic heavy metals or rare elements with limited reserves. On the other hand, copper-based chalcogenide (Cu-based chalcogenide) compounds are attracting attention as promising materials because they are non-toxic and inexpensive.

2m space group)를 갖는 화합물이다. 작은 밴드갭 (0.29 - 0.4 eV) 과 큰 캐리어 유효질량(~1.1 m_e)의 장점을 갖고 있어, 중온 영역의 p형 열전재료로서 적합하다. 지금까지 대부분의 Cu₃SbSe₄-based 화합물은 direct melting of elements에 의해 합성되었다. 그러나 이와 같은 방법은 오랜 승온 시간과 균질성을 위한 열처리가 필요하다.Cu ₃ SbSe ₄ (permingeatite) is a zinc blende-type tetragonal lattice (I

2m space group). It has the advantages of a small bandgap (0.29 - 0.4 eV) and a large effective carrier mass (~1.1 m _e ), making it suitable as a p-type thermoelectric material in the mid-temperature region. So far, most Cu ₃ SbSe ₄ -based compounds have been synthesized by direct melting of elements. However, this method requires a long heating time and heat treatment for homogeneity.

An object of the present invention is to provide a manufacturing method capable of producing a _{homogeneous Cu 3} SbSe _{4 thermoelectric material within a short time.}

A method of manufacturing a thermoelectric material according to various embodiments of the present invention includes preparing raw materials Cu, Sb and Se; synthesizing the powder by mechanical alloying the prepared raw material; and hot pressing (HP) the powder.

Various embodiments of the present invention may provide a thermoelectric material having improved mechanical properties and thermoelectric performance, and a method for manufacturing the same. Through the manufacturing method of the present invention _{, it is possible to obtain Cu 3} SbSe ₄ having thermoelectric properties equal to or higher than that _{of Cu 3} SbSe _{4 prepared by the conventional dissolution method, and homogeneous Cu 3} SbSe ₄ by solid-state synthesis within a relatively short time. It is an effective process to manufacture.

Figure 1 (a) is the analysis result of _{the Cu 3} SbSe ₄ phase according to the ball milling time in MA. Figure 1 (b) is a Cu ₃ SbSe ₄ phase analysis results according to the HP condition.
2 is a TG-DSC analysis result for confirming the phase change.
3 is a TG-DSC analysis result of the HP specimen.
4 is a FESEM image of the surface and fracture surface of _{Cu 3} SbSe ₄ according to the HP temperature.
5 is an EDS line scan and element mapping result of the HP573K2H specimen.
6 shows the electron mobility characteristics of _{Cu 3} SbSe ₄ according to the HP temperature.
7 is a result of electrical conductivity according to the HP temperature.
8 shows the Seebeck coefficient of _{Cu 3} SbSe ₄ according to the HP temperature.
9 shows an output factor (PF = α ² σ ) obtained from electrical conductivity and Seebeck coefficient.
10 shows the thermal conductivity of _{Cu 3} SbSe ₄ according to the HP temperature.
11 shows the dimensionless thermoelectric figure of merit ( ZT ) according to the HP temperature.

Hereinafter, various embodiments of the present document will be described with reference to the accompanying drawings. The examples and terms used therein are not intended to limit the technology described in this document to specific embodiments, and should be understood to include various modifications, equivalents, and/or substitutions of the embodiments.

A method of manufacturing a permingeatite thermoelectric material based on copper (Cu), antimony (Sb) and selenium (Se) according to various embodiments of the present invention includes preparing Cu, Sb and Se as raw materials; synthesizing the powder by mechanical alloying the prepared raw material; and hot pressing (HP) the powder.

In the step of preparing the raw material, Cu, Sb, and Se, which are raw materials, may be weighed and prepared according _{to the compositional formula Cu 3} SbSe _{4 .} At this time, Cu may prepare an elemental powder having a particle size of less than 45 μm, Sb may prepare an elemental powder having a particle size of less than 75 μm, and Se may prepare an elemental powder having a particle size of less than 75 μm.

Next, in the step of synthesizing the powder, the prepared raw material may be mechanically alloyed. The prepared mixed powder and a steel ball having a diameter of 5 mm can be mixed in a ratio of 1:15 to 1:20 to perform ball milling. Specifically, it may be ball milled at 250 rpm to 450 rpm for 4 to 20 hours. If the ball milling speed is less than 250 rpm, the Cu ₃ SbSe ₄ phase may not be properly synthesized, and if it is greater than 450 rpm, phase decomposition may occur or a secondary phase may be formed by excessive energy. On the other hand, if the ball milling time is shorter than 4 hours, the Cu ₃ SbSe ₄ phase may not be properly synthesized, and if it is longer than 20 hours, phase decomposition may occur or a secondary phase may be formed due to excessive energy. Preferably, it can be ball milled at 350 rpm for 6 hours to 18 hours.

Next, the hot compression molding is preferably performed in a temperature range of 473 K to 823 K and a pressure range of normal pressure to 100 MPa. At a condition lower than this range, a desired sintering result cannot be obtained, and at a condition higher than this range, the manufacturing cost may increase. On the other hand, preferably, in the case of sintering by hot pressing (HP), it may be sintered at a pressure of 70 Mpa in a temperature range of 523 K to 623 K. More preferably, it may be sintered at a temperature of 573 K and a pressure of 70 Mpa.

Through the manufacturing method of the present invention _{, it is possible to obtain Cu 3} SbSe ₄ having thermoelectric properties equal to or higher than that _{of Cu 3} SbSe _{4 prepared by the conventional dissolution method, and homogeneous Cu 3} SbSe ₄ by solid-state synthesis within a relatively short time. It is an effective process to manufacture.

_{Cu 3} SbSe ₄ obtained by the method of the present invention may be used alone, or solid solution formation and doping may be performed to further improve thermoelectric properties.

Hereinafter, various embodiments of the present invention will be described in detail with reference to the accompanying drawings.

Example

To synthesize Cu ₃ SbSe ₄ compound, stoichiometric composition of Cu (purity 99.9%, <45 μm), Sb (purity 99.999%, <75 μm) and Se (purity 99.99%, <75 μm) in elemental powder state After weighing, the mixed powder and steel balls (diameter 5 mm) were charged into a hardened steel jar at a ratio of 1:20. Mechanical alloying (hereinafter, 'MA') was performed using a planetary ball mill (Fritsch, Pulverisette5) at 350 rpm at different times for 6 hours, 12 hours, 18 hours, and 24 hours. The synthesized powder was charged into a graphite mold having an inner diameter of 10 mm, and the temperature was changed to 523 K, 573 K, and 623 K, and vacuum hot compression molding was performed at a pressure of 70 MPa for 2 hours (hereinafter, 'HP').

= 0.15405 nm)을 사용하여 MA 분말 및 HP 시편의 상을 분석하였다. MA 분말과 소결 시편의 질량 및 상 변화는 열중량분석 및 시차열분석기를 (TG-DSC; Mettler Toledo TG/DSC1) 이용하여 Ar 분위기에서 973 K까지 5 K/min의 승온속도로 분석하였다. 전계방사형 주사전자현미경 (FESEM; JEOL, JSM-7610F)으로 원료 및 MA 분말과 HP 시편의 미세조직을 관찰하였고, 에너지분산형 분광기 (EDS; Oxford, X-Max50)로 성분원소의 분포 및 조성을 확인하였다. _{Cu K α} by X-ray diffractometer (XRD; Bruker, D2-Phaser) radiation (

= 0.15405 nm) was used to analyze the phases of MA powder and HP specimens. The mass and phase changes of the MA powder and the sintered specimen were analyzed using a thermogravimetric analysis and a differential thermal analyzer (TG-DSC; Mettler Toledo TG/DSC1) in an Ar atmosphere at a temperature increase rate of 5 K/min to 973 K. The microstructure of the raw material, MA powder, and HP specimen was observed with a field emission scanning electron microscope (FESEM; JEOL, JSM-7610F), and the distribution and composition of the component elements were confirmed with an energy dispersive spectrometer (EDS; Oxford, X-Max50). did.

Experimental Example 1: Phase analysis according to ball milling time during MA

Figure 1 (a) is the analysis result of _{the Cu 3} SbSe ₄ phase according to the ball milling time in MA. Referring to Figure 1 (a), MA350R6H is the result of milling at 350 rpm for 6 hours, MA350R12H is the result of milling at 350 rpm for 12 hours, and MA350R18H is the result of milling at 350 rpm for 18 hours. and MA350R24H is the result of synthesis by milling at 350 rpm for 24 hours.

Referring to (a) of FIG. 1 , in the case of MA350R6H, a Cu ₃ SbSe ₄ phase was synthesized, and a secondary phase was not found in MA350R12H and MA350R18H. That is, it can be seen that the secondary phase was not found until the milling time was 18 hours. However, in the case of MA350R24H with a longer milling time, a CuSbSe ₂ secondary phase was formed. Mechanical collision energy, which is the main driving force for chemical reactions in ball milling systems such as MA, can increase structural periodicity and breakage of chemical bonds, free ions/electrons and new surfaces, and when the mechanical energy is high enough, the material's free energy A chemical reaction may be induced to reduce the Therefore, in the case of MA350R24H, it seems that the phase decomposition was caused by excessive energy due to the long milling time.

Meanwhile, FIG. 2 is a TG-DSC analysis result for confirming the phase change. The endothermic peak near 650 K in the MA powder appears to be related to the stagnation reaction. The endothermic peak near 735 K coincides with the melting point _{of Cu 3} SbSe _{4 . Stable Cu 3} SbSe ₄ from XRD and TG-DSC results It was confirmed that the preferred MA time for synthesizing the phase was 12 hours, and thereafter, phase analysis and thermoelectric properties according to the HP temperature were evaluated.

Experimental Example 2: Phase analysis and thermoelectric characteristics observation according to HP temperature

Figure 1 (b) is a Cu ₃ SbSe ₄ phase analysis results according to the HP condition. Referring to Figure 1 (b), HP523K2H is a specimen hot compression molded at 523 K, HP573K2H is a specimen hot compression molded at 573 K, and HP623K2H is a specimen hot compression molded at 623 K. The diffraction patterns of all specimens were _{in good agreement with the Cu 3} SbSe ₄ standard diffraction data (PDF #01-085-0003), indicating that the permingeatite phase was well maintained even after sintering, and no secondary phase was found.

Meanwhile, Table 1 below shows the results of evaluating the chemical composition and physical properties of each specimen.

Psalter
(Specimen) Composition Relative density
[%] Lattice constant
[nm] Half width (FWHM ₍₁₁₂₎ )
[deg] Lorenz number
at 323K
[10 ^-8 V ² K ^-2 ] Nominal Actual a c MA350R12H Cu ₃ SbSe ₄ Cu _3.31 Sb _0.97 Se _3.72 - 0.5651 1.1252 0.355 - HP523K2H Cu ₃ SbSe ₄ Cu _3.27 Sb _0.93 Se _3.80 89.9 0.5646 1.1243 0.234 1.62 HP573K2H Cu ₃ SbSe ₄ Cu _3.44 Sb _0.67 Se _3.89 97.8 0.5649 1.1243 0.203 1.57 HP623K2H Cu ₃ SbSe ₄ Cu _3.29 Sb _0.93 Se _3.78 97.7 0.5650 1.1243 0.171 1.54

As shown in Table 1 above, regardless of the HP temperature, all specimens had similar lattice constants (a = 0.5646 - 0.5650 nm and c = 1.124 nm). Also, the full width at half maximum (FWHM) of the diffraction peak for the (112) plane decreased from 0.355° of the MA350R6H powder to 0.171 to 0.234° after HP. The broad diffraction peak is due to the refinement of the powder due to mechanical collision during the MA process, and it seems that the width of the diffraction peak decreased due to grain growth and crystallinity improvement after sintering.

3 is a TG-DSC analysis result of the HP specimen. An endothermic peak of about 735 K, such as TG-DSC results of MA powders and the melting point of the Cu ₃ SbSe _4, particular phase change to a temperature below it can be seen that the bore that the Cu ₃ SbSe ₄ different stabilization of sintered without.

4 is a FESEM image of the surface and fracture surface of _{Cu 3} SbSe ₄ according to the HP temperature. In the case of the HP523K2H specimen, a non-dense texture was observed, which is consistent with the low relative density results in Table 1. On the other hand, the HP5733K2H and HP623K2H specimens had relatively no cracks or pores, and obtained a relative density higher than 97% compared to the ^{theoretical density (5.86 gcm -3 ).}

5 is an EDS line scan and element mapping result of the HP573K2H specimen. Referring to FIG. 5 , it can be seen that all constituent elements are homogeneously distributed without secondary phase or segregation.

6 shows the electron mobility characteristics of _{Cu 3} SbSe ₄ according to the HP temperature. As a result of measuring the Hall coefficient at room temperature, it was found that it is a p- type semiconductor having a carrier concentration of ^{about 4.5x10 18} - 5.7x10 ¹⁸ cm ^{-3 .} In Cu ₃ SbSe ₄ , Cu vacancies with low formation energy are easily formed and contribute to the p- type behavior, a phenomenon frequently observed in other Cu chalcogenide compounds. In other studies so far, the undoped Cu ₃ SbSe ₄ compound has been reported as an intrinsic semiconductor with a carrier concentration of ~10 ¹⁸ cm ^{-3 .} As the HP temperature increased, the carrier mobility was slightly decreased, and it showed a value of ^{35 - 62 cm 2} V ^-1 s ^{-1 .}

7 is a result of electrical conductivity according to the HP temperature. Referring to FIG. 7 , as the HP temperature increased, the electrical conductivity decreased, which is due to the decrease in mobility and carrier concentration as shown in FIG. 6 . As the temperature increases, it seems that the carrier concentration is changed due to the volatilization of Se with high vapor pressure, and the Se vacancy can act as an n- type dopant to reduce the carrier concentration. At 323 K, it was 2.55x10 ³ - 5.71x10 ³ Sm ^-1 , and at 623 K it was 3.66x10 ³ - 4.87x10 ³ Sm ^-1, and the temperature dependence of electrical conductivity was not large.

8 shows the Seebeck coefficient of _{Cu 3} SbSe ₄ according to the HP temperature. Referring to FIG. 8 , the Seebeck coefficient exhibited a positive value indicating p- type conductivity in which the majority carriers are holes, which also coincided with the sign of the Hall coefficient. Except for the HP523K2H specimen, the Seebeck coefficient increased and then decreased as the temperature increased, which is due to intrinsic excitation. Also, as the HP temperature increased, the Seebeck coefficient increased. Based on the Mott equation, the Seebeck coefficient ( α ) is α = (8 π ² k _B ² T / 3eh ² )m ^* (π/3n) ^2/3 ( k _B : Boltzmann constant, e : electron charge, h : Planck constant, m ^* : effective carrier mass and n : carrier concentration). According to the formula, the Seebeck coefficient is proportional to the effective mass m ^* and inversely proportional to the carrier concentration. have a relationship Therefore, it seems that the increase in Seebeck coefficient according to the increase in HP temperature is due to the difference in carrier concentration such as the change in electrical conductivity. Referring to FIG. 8 , at 323 K, it ^{was found to be 242 to 380 μVK −1} , and at 623 K it was shown to be from 321 to 330 μVK ⁻¹ , resulting in similar results to those of the previous study.

9 shows an output factor (PF = α ² σ ) obtained from electrical conductivity and Seebeck coefficient. The HP523K2H and HP573K2H specimens showed a maximum output factor of ^{about 0.5 mWm -1} K ^{-2 @ 573 - 623 K.} The HP623K2H specimen had low electrical conductivity and showed a low output factor as the Seebeck coefficient decreased as the temperature increased.

] 으로 계산하여, 323 K에서 (1.54 - 1.62)

10^-8 V²K^{- 2}을, 623 K에서 1.56

10^-8 V²K^-2의 값을 얻었으며, 표 1에는 각 시편의 323 K에서의 로렌츠상수를 나타내었다. 모든 시편의 열전도도는 323 K에서 0.97 - 1.3 Wm^-1K^-1, 623 K에서 0.76 - 0.78 Wm^-1K^-1의 값을 가졌으며, 전자 열전도도의 기여는 거의 없었다. 이는 용해-소결로 제작한 Cu₃SbSe₄의 상온 열전도도 값인 2.27 - 3.19 Wm^-1K^-1 보다도 매우 낮은 값을 보였다. 또한 MA-SPS로 제작한 Cu_2.95SbSe₄의 격자 열전도도 1.1 Wm^-1K^-1 @ 673 K 와 rapid-injection route로 합성하고 HP한 나노파티클 Cu₃SbSe₄의 열전도도 0.91 - 1.15 Wm^-1K^-1 @ 300 - 570 K의 값과 비슷하거나 낮았다. 이는 MA에 의해 grain size가 작아져 고밀도의 grain boundary가 형성되고, 그로 인한 포논산란 증가가 격자 열전도도 감소에 기여한 것으로 판단된다. Cu₃SbSe₄의 이론적인 최소 격자 열전도도의 값은 275 - 700 K에서 약 0.47 Wm^-1K^-1이다. 따라서 고용체 형성 또는 도펀트 첨가를 통해 격자 열전도도를 더 감소시킬 수 있다. 10 shows the thermal conductivity of _{Cu 3} SbSe ₄ according to the HP temperature. For the calculation of thermal conductivity, ^{a value of 0.32 Jg -1} K ^-1 was used. Thermal conductivity can also be expressed as the sum of thermal conductivity due to lattice vibration (κ _L ) and thermal conductivity due to electrons ( κ _E ), and electronic thermal conductivity is determined by the Wiedemann-Franz law (κ _E = LσT , L : Lorenz number). ) can be calculated by In this case, the Lorentz constant is expressed as L = 1.5 + exp[

] at 323 K (1.54 - 1.62)

10 ^-8 V ² K ^{- 2} , at 623 K 1.56

A value of 10 ^-8 V ² K ^-2 was obtained, and Table 1 shows the Lorentz constant at 323 K of each specimen. The thermal conductivity of all specimens was 0.97 - 1.3 Wm ^-1 K ^-1 at 323 K and 0.76 - 0.78 Wm ^-1 K ^-1 at 623 K, with little contribution to the electronic thermal conductivity. This was much lower than the room temperature thermal conductivity of 2.27 - 3.19 Wm ^-1 K ^-1 _{of Cu 3} SbSe ₄ prepared by dissolution-sintering. In addition, the lattice thermal conductivity of _{Cu 2.95} SbSe ₄ prepared by MA-SPS was ^{1.1 Wm -1} K ^-1 @ 673 K and the thermal conductivity of _{Cu 3} SbSe ₄ nanoparticles synthesized by rapid-injection route was HP ^{0.91 - 1.15 Wm -1} It was similar to or lower than the value of K ^{-1 @ 300 - 570 K.} It is considered that the grain size is reduced by MA, forming a high-density grain boundary, and the increase in phonon scattering thereby contributed to the decrease in lattice thermal conductivity. The theoretical minimum lattice thermal conductivity of Cu ₃ SbSe ₄ ^{is about 0.47 Wm -1} K ^{-1 at 275 - 700 K.} Therefore, the lattice thermal conductivity can be further reduced through solid solution formation or dopant addition.

11 shows the dimensionless thermoelectric figure of merit ( ZT ) according to the HP temperature. As the temperature increased, ZT increased, showing a maximum ZT of 0.39 at 623 K. In the case of the HP523K2H specimen, ZT = 0.41 at 573 K, but there may be problems in the application because of the low sintering density condition.

Through the above experimental examples, the embodiment of the present invention can obtain results comparable to the conventional melting method even with the mechanical alloying method, and it is an effective process for producing _{homogeneous Cu 3} SbSe _{4 by solid-state synthesis within a relatively short time.} Confirmed. In addition, the optimum sintering temperature was determined to be 573 K in consideration of the sintering density and thermoelectric characteristics through this Example and Experimental Examples.

Features, structures, effects, etc. described in the above-described embodiments are included in at least one embodiment of the present invention, and are not necessarily limited to one embodiment. Furthermore, features, structures, effects, etc. illustrated in each embodiment can be combined or modified for other embodiments by those of ordinary skill in the art to which the embodiments belong. Accordingly, the contents related to such combinations and modifications should be interpreted as being included in the scope of the present invention.

In addition, although the embodiments have been mainly described above, these are merely examples and do not limit the present invention, and those of ordinary skill in the art to which the present invention pertains are exemplified above in a range that does not depart from the essential characteristics of the present embodiment. It can be seen that various modifications and applications that have not been made are possible. For example, each component specifically shown in the embodiments may be implemented by modification. And the differences related to these modifications and applications should be construed as being included in the scope of the present invention defined in the appended claims.

Claims (4)

Preparing raw materials Cu, Sb and Se;
synthesizing _{Cu 3} SbSe ₄ powder by mechanically alloying the prepared raw material; and
Method _{for producing a Cu 3} SbSe ₄ thermoelectric material comprising the step of preparing a _{Cu 3 SbSe 4} sintered specimen by hot pressing the powder (Hot Pressing, HP).
According to claim 1,
The method of manufacturing _{a Cu 3} SbSe ₄ thermoelectric material, characterized in that the raw material is prepared by weighing it according to the compositional formula Cu _{3 SbSe 4 .}
According to claim 1,
The method of the steps of synthesizing the powder Cu ₃ SbSe ₄ thermoelectric material to the prepared raw material characterized in that the ball milled for 4 hours to 20 hours at 250 rpm to 450 rpm.
According to claim 1,
The hot compression molding is a method of manufacturing _{a Cu 3} SbSe ₄ thermoelectric material, characterized in that it is performed in a temperature range of 473 K to 823 K and a pressure range of normal pressure to 100 MPa.

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