KR102435413B1 - Thermoelectric materials of Skinnerite Cu3SbS3 and method for preparing the same - Google Patents

Abstract

A method of manufacturing a thermoelectric material according to various embodiments of the present invention includes mixing Cu, Sb, and S powder as raw materials; synthesizing a powder by mechanically alloying the raw material; and hot pressing (HP) the synthesized powder.

Description

Skinnerite thermoelectric material and its manufacturing method {Thermoelectric materials of Skinnerite Cu3SbS3 and method for preparing the same}

The present invention relates to a skinnerite thermoelectric material and a method for manufacturing the same.

As environmental problems due to depletion of fossil fuels and greenhouse gas emissions are emerging as global issues, the development of new and renewable energy is necessary. In addition, with the recent development of IoT sensors and small smart devices, a new type of energy source is needed. Accordingly, energy harvesting technology that harvests or uses wasted energy and regenerates it into electrical energy is being researched and developed. Among them, thermoelectric energy conversion technology that can utilize waste heat from industrial and transportation equipment and body temperature, which are mostly discarded, into electrical energy is attracting attention. The efficiency of the thermoelectric module largely depends on the dimensionless thermoelectric figure of merit ( ZT ) of the thermoelectric material, ZT = α ² σκ ^-1 T ( α is Seebeck coefficient, σ is electrical conductivity, κ is thermal conductivity, and T is absolute temperature) ) is indicated. Therefore, in order to improve the performance of the thermoelectric material, it is necessary to increase the output factor (α ² σ) and decrease the thermal conductivity.

Recently, there is an increasing need for the search for eco-friendly materials composed of non-toxic elements with abundant reserves and low cost. Accordingly, Cu-Sb-S compounds including Cu ₃ SbS ₄ (famatinite), Cu ₁₂ Sb ₄ S ₁₃ (tetrahedrite), Cu ₃ SbS ₃ (skinnerite), and Cu ₃ SbS ₂ (chalcostibite) are used as p-type thermoelectric materials. are interested Except for Cu ₃ SbS ₄ , the three compounds are known to have low lattice thermal conductivity as Cu atoms vibrate due to the repulsive force caused by the lone electron pair of Sb atoms.

Among them, Cu ₃ SbS ₃ has the largest S-Sb-S bonding angle and thus has the lowest lattice thermal conductivity. In addition, it has been reported as a promising material in the photovoltaic field due to its high absorption coefficient and moderate bandgap (1.40-1.84 eV), but there are few studies on the thermoelectric properties of Cu ₃ SbS ₃ .

The present invention is skinnerite Cu ₃ SbS ₃ To a thermoelectric material and a method for manufacturing the same, and to provide optimal process conditions for solid-phase synthesis of Cu ₃ SbS ₃ phase using mechanical alloying and hot compression. Another object of the present invention is to provide a skinnerite Cu ₃ SbS ₃ thermoelectric material with improved thermoelectric performance by maximizing an output factor and a manufacturing method thereof.

A method of manufacturing a thermoelectric material according to various embodiments of the present invention includes mixing Cu, Sb, and S powder as raw materials; synthesizing powder by mechanically alloying the raw material; and hot pressing (HP) the synthesized powder.

The thermoelectric material manufactured by the manufacturing method according to various embodiments of the present invention is Cu ₃ SbS ₃ It is Skinnerite.

Various embodiments of the present invention may provide a thermoelectric material with improved thermoelectric performance and a method for manufacturing the same.

Skinnerite thermoelectric material of the present invention can improve the thermoelectric figure of merit by maximizing the output factor. According to an embodiment of the present invention, in the case of a specimen sintered at 623 K, a maximum output factor of 0.61 mWm ^-1 K ^-2 and a ZT of 0.51 at 623 K, and 0.58 mWm ^-1 K in the case of a specimen sintered at 673 K It showed an output factor of ^-2 and a maximum ZT of 0.57.

The manufacturing method of the present invention can successfully synthesize cubic skinnerite Cu ₃ SbS ₃ by using mechanical alloying and hot compression molding. In addition, it is an effective process because it can produce homogeneous Cu ₃ SbS ₃ by solid-state synthesis within a relatively short time.

1 shows the XRD pattern after MA and HP of Cu ₃ SbS ₃ .
2 is a TG-DSC analysis result of the MA powder of Example 1.
3 is a TG-DSC analysis result of the HP specimen of Example 1.
4 is a FESEM image of the surface and fracture surface of Cu ₃ SbS ₃ according to the HP temperature.
5 is a graph showing the electron mobility of Cu ₃ SbS ₃ according to the HP temperature.
6 shows the electrical conductivity of Cu ₃ SbS ₃ according to the HP temperature.
7 shows the Seebeck coefficient according to the HP temperature of Cu ₃ SbS ₃ .
8 shows an output factor according to the HP temperature of Cu ₃ SbS ₃ .
9 is a graph showing the thermal conductivity of Cu ₃ SbS ₃ according to the HP temperature.
10 shows the dimensionless thermoelectric figure of merit ( ZT ) according to the HP temperature of Cu ₃ SbS ₃ .

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 thermoelectric material according to various embodiments of the present invention includes mixing Cu, Sb, and S powder as raw materials; synthesizing powder by mechanically alloying the raw material; and hot pressing (HP) the synthesized powder.

In the step of preparing the raw material, the raw material may be prepared by weighing it according to the compositional formula Cu ₃ SbS ₃ . 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 S 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 (MA). Mechanical alloying is a method in which alloying proceeds while crushing and pressure welding are repeated between the component element powder and the ball as the container rotates. In the present invention, the prepared powder and a steel ball having a diameter of 4 to 6 mm may be mixed in a ratio of 1:15 to 1:20 for ball milling. Specifically, it may be ball milled at 200 rpm to 2000 rpm for 1 hour to 100 hours. If the ball milling speed is less than 200 rpm, the Cu ₃ SbS ₃ phase may not be properly synthesized, and if it is greater than 2000 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 1 hour, the Cu ₃ SbS ₃ phase may not be properly synthesized, and if it is longer than 100 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 24 hours. Next, the hot compression molding is preferably performed in a temperature range of 473 K to 873 K and a pressure range of normal pressure to 100 MPa. At a condition lower than this range, a desired sintering result may not 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 573 K to 673 K. More preferably, it may be sintered in a temperature range of 623 K to 673 K and a pressure of 70 Mpa.

The manufacturing method of the present invention can synthesize a homogeneous cubic skinnerite phase by using mechanical alloying and hot compression molding. In addition, the high energy ball mill process at room temperature consumes less energy and time, so it can be easily manufactured at low cost. In addition, it is an effective process because it can produce homogeneous Cu ₃ SbS by solid-state synthesis within a relatively short time.

The thermoelectric material according to various embodiments of the present invention is represented by Cu ₃ SbS ₃ . According to various embodiments of the present invention, it is possible to provide a thermoelectric material with improved thermoelectric performance by reducing thermal conductivity and optimizing an output factor.

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

Example 1: Skinnerite Cu ₃ SbS ₃ manufacture of

To synthesize Cu ₃ SbS ₃ , the stoichiometric composition of Cu (purity 99.9%, < 45 μm), Sb (purity 99.999%, < 75 μm), and S (purity 99.99%, < 75 μm) in elemental powder state After weighing, the mixed powder and stainless-steel balls (diameter 5 mm) were charged into a hardened steel jar at a ratio of 1:20. Mechanical alloying (MA) was performed at 350 rpm for 6 to 24 hours using a planetary ball mill (Fritsch, Pulverisette 5). The synthesized powder was charged into a graphite mold with an inner diameter of 10 mm, and vacuum hot compression molding (HP) was performed at 573 to 673 K for 2 hours at a pressure of 70 MPa.

Experimental Example 1: X-ray diffraction pattern and DSC analysis of Example 1

1 shows the XRD pattern after MA and HP of Cu ₃ SbS ₃ . After milling at 350 rpm for 6 hours, cubic Cu ₃ SbS ₃ ( I 43 m ) phase (PDF# 01-075-1574) was synthesized, and a trace amount of monoclinic Cu ₃ SbS ₃ ( P 2 ₁ / c ) phase (PDF# 01-082-0851) was detected. As the MA time increased, the diffraction peak intensity of monoclinic decreased.

In order to confirm the phase change in more detail, TG-DSC analysis was performed and is shown in FIG. 2 . All MA powders showed a large endothermic peak at 873 K. It is known that Cu ₃ SbS ₃ melts at about 880 K, and the endothermic peak at 873 K is due to the melting of Cu ₃ SbS ₃ , and the other small endothermic peak is due to the phase change due to sulfur volatilization, which is the mass loss of the TG curve. also coincides with From the XRD and TG-DSC results, the optimal MA condition for synthesizing Cu ₃ SbS ₃ was determined for 18 hours, and then phase analysis and thermoelectric properties according to the HP temperature were evaluated.

Figure 1 (b) is a phase analysis result according to the HP temperature condition. All specimens maintained cubic Cu ₃ SbS ₃ phase even after HP, but Cu ₂ S secondary phase was formed.

3 shows the results of TG-DSC analysis of the HP specimen. The HP specimen showed a weak endothermic peak between 798-808 K and a large endothermic peak at 865-876 K. At 795±2 K, tetrahedrite (Cu ₁₂ Sb ₄ S ₁₃ ) + chalcostibite (CuSbS ₂ ) -> famatinite (Cu ₃ SbS ₄ ) + skinnerite (Cu ₃ SbS ₃ ), at 804±2 K, chalcostibite + Sb -> It has been reported that the reaction of skinnerite + liquid occurs. The melting point of Cu ₂ S detected in the XRD result was 1143 K, which did not appear in the measurement range. Therefore, as in the TG-DSC result of MA powder, a large endothermic peak is judged to be the melting point of Cu ₃ SbS ₃ , and other small endothermic peaks are interpreted as a phase change due to sulfur volatilization.

Experimental Example 2: Analysis of the shape and composition of Example 1

4 is a FESEM image of the surface and fracture surface of Cu ₃ SbS ₃ according to the HP temperature. In the case of HP573K2H, a tissue with many pores and not densely can be seen, which is consistent with the low relative density in Table 1 below.

Specimen Composition Relative density
[%] Lattice constant [nm] Nominal Actual MA350R18H Cu ₃ SbS ₃ Cu _2.99 Sb _1.06 S _2.95 - 1.0343 HP573K2H Cu ₃ SbS ₃ Cu _3.34 Sb _0.89 S _2.77 94.4 1.0365 HP623K2H Cu ₃ SbS ₃ Cu _3.32 Sb _0.89 S _2.79 99.7 1.0374 HP673K2H Cu ₃ SbS ₃ Cu _3.32 Sb _0.89 S _2.79 99.2 1.0394

In the case of HP623K2H and HP673K2H, relative densities higher than 99% compared to the theoretical density of skinnerite (5.1 gcm ^-3 ) were obtained, and it was confirmed that there were no pores or cracks through FESEM. Although Cu ₂ S impurity was detected in the XRD result of the HP specimen, the Cu ₂ S phase could not be significantly distinguished from the surface image observed with the backscattered electron mode.

Table 1 shows the nominal composition and the actual composition through EDS analysis. In the case of MA powder, the nominal composition and the actual composition are almost identical, while the HP specimen has a slight difference in the actual composition, which is thought to be due to the volatilization of Sb and S elements with high vapor pressure during the sintering process. The lattice constant of the MA powder was 1.0343 nm, and as the HP temperature increased, the lattice constant increased, and it appeared in the range of 1.0365-1.0394 nm.

Experimental Example 3: Electron migration characteristic analysis of Example 1

5 is a graph showing the electron mobility of Cu ₃ SbS ₃ according to the HP temperature. As a result of measuring the Hall coefficient at room temperature, it was shown that it is a p -type semiconductor having a carrier concentration of (3.25-3.87) × 10 ¹⁸ cm ^-3 and a mobility of 146-153 cm ² V ^-1 s ^-1 . It is a frequently observed phenomenon that Cu vacancies with low formation energy are easily formed in Cu chalcogenide compounds, contributing to the p -type behavior. A high carrier concentration of 9.1 × 10 ¹⁹ cm ^-3 and a low mobility of 3.7 cm ² V ^-1 s ^-1 were reported in cubic Cu ₃ SbS ₃ synthesized by MA (mechanical alloying)-SPS (discharge plasma sintering). have. Since the SPS method maintains a small average particle size of the MA powder with a short sintering time, it appears that the SPS method exhibits lower mobility due to more carrier scattering.

Experimental Example 4: Electrical conductivity analysis of Example 1

6 shows the electrical conductivity of Cu ₃ SbS ₃ according to the HP temperature. All specimens showed non-degenerate semiconductor behavior in which the electrical conductivity increased as the temperature increased, and the electrical conductivity at high temperature decreased as the HP temperature increased. An increase in the sintering temperature causes a small variation in composition, which can lead to a decrease in carrier concentration. Therefore, as the HP temperature increases, it is judged to be due to the change in carrier concentration due to volatilization of sulfur with high vapor pressure. According to a previous study, a very low electrical conductivity of about 10 Sm ^-1 was obtained from Cu ₃ SbS ₃ fabricated with MA-SPS (at 623 K for 3 min), which was due to the low carrier concentration. In addition, according to another study, Cu ₃ SbS ₃ was fabricated with MA-SPS (at 673 K for 5 min), and electrical conductivity of 540 Sm ⁻¹ was reported at room temperature. In this experimental example, electrical conductivity of (7.6-9.5) × 10 ³ Sm ^-1 at 323 K and (1.5-2.5) × 10 ³ Sm ^-1 at 623 K was obtained. This is considered to be due to the increase in mobility due to grain growth during the process due to the slow temperature increase rate and long holding time of HP compared to SPS.

Experimental Example 5: Seebeck coefficient and output factor analysis of Example 1

7 shows the Seebeck coefficient according to the HP temperature of Cu ₃ SbS ₃ . The Seebeck coefficient had a positive value indicating the p -type conductivity in which the majority carriers were holes, and this was consistent with the positive sign of the Hall coefficient. In general, the Seebeck coefficient shows a maximum value as the temperature increases and then decreases again because of intrinsic conduction due to thermal excitation. In Cu ₃ SbS ₃ in this experimental example, the Seebeck coefficient continued to increase in the measurement temperature range, and the start of intrinsic conduction was not observed. Equation α = (8π ² k _B ² T / 3eh ² )m ^* (π/3n) ^2/3 According to ( k _B : Boltzmann constant, e : electron charge, h : Planck constant, m ^* : effective carrier mass and n : carrier concentration), the Seebeck coefficient has an inverse relationship with the carrier concentration. Therefore, it seems that the increase in the Seebeck coefficient according to the increase in the HP temperature is due to the change in carrier concentration such as the electrical conductivity of FIG. 6 . The output factor (PF = α ² σ ) obtained from the electrical conductivity and Seebeck coefficient is shown in FIG. 8 . For all specimens, the output factor increased as the temperature increased, and the maximum value was 0.58-0.61 mWm ^-1 K ^-2 at 623 K.

Experimental Example 6: Thermal conductivity analysis of Example 1

/116]을 사용하여 계산하였고, 323 K에서 (1.75-1.82)×10^-8 V²K^{- 2}을, 623 K에서 (1.68-1.76)×10^-8 V²K^-2의 값을 얻었다. 이는 전기전도도의 결과와 같이 Cu₃SbS₃는 비축퇴 반도체임을 의미한다. 이론적으로 로렌츠 상수는 (1.45-2.44)×10^-8 V²K^-2의 범위를 가지며, 값이 작을수록 비축퇴 반도체 거동을, 값이 클수록 축퇴 반도체(또는 금속성) 거동을 의미한다. 모든 시편은 측정 온도 범위에서 0.78 Wm^-1K^-1 이하의 낮은 열전도도를 보였다. 온도가 증가할수록 전기전도도가 증가하여 전자 열전도도가 증가한 반면, 격자 열전도도는 온도에 거의 비의존적인 거동을 보였다. 이전 연구에서는 group A^V chalcogenide 화합물에서 나타나는 격자 열전도도의 차이가 Sb 5s lone pair electrons와 인접 원자간의 상호작용으로 인한 격자 비조화성에 의한 포논 산란 메커니즘 때문이며, 그 강도는 X-Sb-X (X: chalcogen atom)의 결합각과 인접 원자의 배위수의 관계에 의해 결정됨을 증명하였다. 그러므로 lone pair electrons를 갖는 세 Cu-Sb-S 화합물 중 가장 큰 S-Sb-S 결합각 (99.3°)을 갖는 Cu₃SbS₃가 온도 비의존적이면서, 가장 낮은 격자 열전도도를 갖는다고 보고하였다.9 is a graph showing the thermal conductivity of Cu ₃ SbS ₃ according to the HP temperature. Thermal conductivity (κ) can be expressed as the sum of thermal conductivity due to lattice vibration ( κ _L ) and thermal conductivity due to electrons ( κ _{E )} . number) can be calculated by In this case, the Lorentz constant is expressed as L = 1.5 + exp[

/116], and obtained a value of (1.75-1.82)×10 ^{-8 V 2 K -2 at 323 K and (1.68-1.76)×10 -8 V 2 K -2 at 623 K.} This means that Cu ₃ SbS ₃ is a non-degenerate semiconductor as the result of electrical conductivity. Theoretically, the Lorentz constant has a range of (1.45-2.44)×10 ^-8 V ² K ^-2 , and a smaller value means a non-degenerate semiconductor behavior, and a larger value means a degenerate semiconductor (or metallic) behavior. All specimens showed low thermal conductivity of 0.78 Wm ^-1 K ^-1 or less in the measurement temperature range. As the temperature increased, the electrical conductivity increased and the electronic thermal conductivity increased, while the lattice thermal conductivity showed a behavior that was almost independent of the temperature. In a previous study, the difference in lattice thermal conductivity in group A ^V chalcogenide compounds was due to the phonon scattering mechanism caused by the lattice dissonance caused by the interaction between Sb 5s lone pair electrons and adjacent atoms, and the intensity was X-Sb-X (X: It was proved that it is determined by the relationship between the bond angle of the chalcogen atom) and the coordination number of the adjacent atom. Therefore, it was reported that Cu ₃ SbS ₃ , which has the largest S-Sb-S bond angle (99.3°) among the three Cu-Sb-S compounds with one pair electrons, is temperature-independent and has the lowest lattice thermal conductivity.

Experimental Example 7: Thermoelectric figure of merit (ZT) analysis of Example 1

0.01 @ 623 K 와 cubic Cu₃SbS₃의 ZT

0.2 @ 625 K 보다도 높은 값이며, 높은 전기전도도로 인한 높은 출력인자 때문이다. 본 실시예에서 MA-HP로 제작한 cubic skinnerite phase를 갖는 Cu₃SbS₃가 열전재료로서 잠재성이 있음을 확인하였고, 도핑에 의한 상 안정화 및 캐리어농도 최적화를 통해 성능지수를 향상시킬 수 있을 것으로 기대된다.10 shows the dimensionless thermoelectric figure of merit ( ZT ) according to the HP temperature of Cu ₃ SbS ₃ . As the HP temperature increased, the ZT increased. For all specimens, the ZT increased as the temperature increased, showing a maximum value of 0.57 at 623 K. This is the ZT of orthorhombic Cu ₃ SbS ₃ synthesized with MA-SPS.

0.01 @ 623 K with ZT of cubic Cu ₃ SbS ₃

It is higher than 0.2 @ 625 K, and it is because of the high output factor due to high electrical conductivity. In this example, it was confirmed that Cu ₃ SbS ₃ having a cubic skinnerite phase made of MA-HP has potential as a thermoelectric material, and the figure of merit can be improved through phase stabilization and carrier concentration optimization by doping. It is expected.

Features, structures, effects, etc. described in the above 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 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 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 (6)

mixing Cu, Sb and S powder as raw materials;
synthesizing powder by mechanically alloying the raw material; and
Comprising the step of hot compression molding (Hot Pressing, HP) the synthesized powder,
The step of hot compression molding is a method of manufacturing a thermoelectric material, characterized in that it is performed in a temperature range of 623 K to 673 K. According to claim 1,
In the step of synthesizing the powder, the method for manufacturing a thermoelectric material, characterized in that the prepared raw material is ball milled at 200 rpm to 2000 rpm. According to claim 1,
In the step of synthesizing the powder, the method for producing a thermoelectric material, characterized in that the prepared raw material is ball milled for 1 hour to 100 hours. delete According to claim 1,
The method of manufacturing a thermoelectric material, characterized in that the hot compression molding is performed in a pressure range of normal pressure to 100 MPa. Cu ₃ SbS ₃ Skinnerite thermoelectric material prepared by the method of any one of claims 1 to 3, and claim 5.

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