The thermoelectric effect, which converts waste heat into electricity, holds promise as a renewable energy technology. Recently, bismuth telluride (Bi2Te3)-based alloys are being recognized as important materials for practical applications in the temperature range from room temperature to 500 K. However, conventional sintering processes impose limitations on shape-changeable and tailorable Bi2Te3 materials. To overcome these issues, three-dimensional (3D) printing (additive manufacturing) is being adopted. Although some research results have been reported, relatively few studies on 3D printed thermoelectric materials are being carried out. In this study, we utilize extrusion 3D printing to manufacture n-type Bi_1.7Sb_0.3Te₃ (N-BST). The ink is produced without using organic binders, which could negatively influence its thermoelectric properties. Furthermore, we introduce graphene oxide (GO) at the crystal interface to enhance the electrical properties. The formed N-BST composites exhibit significantly improved electrical conductivity and a higher Seebeck coefficient as the GO content increases. Therefore, we propose that the combination of the extrusion 3D printing process (Direct Ink Writing, DIW) and the incorporation of GO into N-BST offers a convenient and effective approach for achieving higher thermoelectric efficiency.

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• Exploring Thermoelectric Transport Properties and Band Parameters of n-Type Bi2-xSbxTe3 Compounds Using the Single Parabolic Band Model
  Linh Ba Vu, Soo-ho Jung, Jinhee Bae, Jong Min Park, Kyung Tae Kim, Injoon Son, Seungki Jo
  journal of Korean Powder Metallurgy Institute.2024; 31(2): 119.     CrossRef

Thermoelectric materials and devices are energy-harvesting devices that can effectively recycle waste heat into electricity. Thermoelectric power generation is widely used in factories, engines, and even in human bodies as they continuously generate heat. However, thermoelectric elements exhibit poor performance and low energy efficiency; research is being conducted to find new materials or improve the thermoelectric performance of existing materials, that is, by ensuring a high figure-of-merit (zT) value. For increasing zT, higher σ (electrical conductivity) and S (Seebeck coefficient) and a lower к (thermal conductivity) are required. Here, interface engineering by atomic layer deposition (ALD) is used to increase zT of n-type BiTeSe (BTS) thermoelectric powders. ALD of the BTS powders is performed in a rotary-type ALD reactor, and 40 to 100 ALD cycles of ZnO thin films are conducted at 100°C. The physical and chemical properties and thermoelectric performance of the ALD-coated BTS powders and pellets are characterized. It is revealed that electrical conductivity and thermal conductivity are decoupled, and thus, zT of ALD-coated BTS pellets is increased by more than 60% compared to that of the uncoated BTS pellets. This result can be utilized in a novel method for improving the thermoelectric efficiency in materials processing.

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• Enhancing Electrical Properties of N-type Bismuth Telluride Alloys through Graphene Oxide Incorporation in Extrusion 3D Printing
  Jinhee Bae, Seungki Jo, Kyung Tae Kim
  journal of Korean Powder Metallurgy Institute.2023; 30(4): 318.     CrossRef

High-temperature and high-pressure post-processing applied to sintered thermoelectric materials can create nanoscale defects, thereby enhancing their thermoelectric performance. Here, we investigate the effect of hot isostatic pressing (HIP) as a post-processing treatment on the thermoelectric properties of p-type Bi_0.5Sb_1.5Te_3.0 compounds sintered via spark plasma sintering. The sample post-processed via HIP maintains its electronic transport properties despite the reduced microstructural texturing. Moreover, lattice thermal conductivity is significantly reduced owing to activated phonon scattering, which can be attributed to the nanoscale defects created during HIP, resulting in an ~18% increase in peak zT value, which reaches ~1.43 at 100°C. This study validates that HIP enhances the thermoelectric performance by controlling the thermal transport without having any detrimental effects on the electronic transport properties of thermoelectric materials.

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• Exploring Thermoelectric Transport Properties and Band Parameters of n-Type Bi2-xSbxTe3 Compounds Using the Single Parabolic Band Model
  Linh Ba Vu, Soo-ho Jung, Jinhee Bae, Jong Min Park, Kyung Tae Kim, Injoon Son, Seungki Jo
  journal of Korean Powder Metallurgy Institute.2024; 31(2): 119.     CrossRef
• Enhancing Electrical Properties of N-type Bismuth Telluride Alloys through Graphene Oxide Incorporation in Extrusion 3D Printing
  Jinhee Bae, Seungki Jo, Kyung Tae Kim
  journal of Korean Powder Metallurgy Institute.2023; 30(4): 318.     CrossRef

Thermoelectric materials can reversely convert heat and electricity into each other; therefore, they can be very useful for energy harvesting from heat waste. Among many thermoelectrical materials, SnSe exhibits outstanding thermoelectric performance along the particular direction of a single crystal. However, single-crystal SnSe has poor mechanical properties and thus it is difficult to apply for mass production. Therefore, polycrystalline SnSe materials may be used to replace single-crystal SnSe by overcoming its inferior thermoelectric performance owing to surface oxidation. Considerable efforts are currently focused on enhancing the thermoelectric performance of polycrystalline SnSe. In this study, we briefly review various enhancement methods for SnSe thermoelectric materials, including doping, texturing, and nano-structuring. Finally, we discuss the future prospects of SnSe thermoelectric powder materials.

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• The Mechanism Behind the High zT of SnSe₂ Added SnSe at High Temperatures
  JunSu Kim, Seong-Mee Hwang, Hyunjin Park, Yinglu Tang, Won-Seon Seo, Chae Woo Ryu, Heesun Yang, Weon Ho Shin, Hyun-Sik Kim
  Korean Journal of Metals and Materials.2023; 61(11): 857.     CrossRef

Atomic layer deposition (ALD) is a promising technology for the uniform deposition of thin films. ALD is based on a self-limiting mechanism, which can effectively deposit thin films on the surfaces of powders of various sizes. Numerous studies are underway to improve the performance of thermoelectric materials by forming core-shell structures in which various materials are deposited on the powder surface using ALD. Thermoelectric materials are especially relevant as clean energy storage materials due to their ability to interconvert between thermal and electrical energy by the Seebeck and Peltier effects. Herein, we introduce a surface and interface modification strategy based on ALD to control the performance of thermoelectric materials. We also discuss the properties of the interface between various deposition materials and thermoelectric materials.

The SnSe single crystal shows an outstanding figure of merit (ZT) of 2.6 at 973 K; thus, it is considered to be a promising thermoelectric material. However, the mass production of SnSe single crystals is difficult, and their mechanical properties are poor. Alternatively, we can use polycrystalline SnSe powder, which has better mechanical properties. In this study, surface modification by atomic layer deposition (ALD) is chosen to increase the ZT value of SnSe polycrystalline powder. SnSe powder is ground by a ball mill. An ALD coating process using a rotary-type reactor is adopted. ZnO thin films are grown by 100 ALD cycles using diethylzinc and H₂O as precursors at 100°C. ALD is performed at rotation speeds of 30, 40, 50, and 60 rpm to examine the effects of rotation speed on the thin film characteristics. The physical and chemical properties of ALD-coated SnSe powders are characterized by scanning and tunneling electron microscopy combined with energy-dispersive spectroscopy. The results reveal that a smooth oxygenrich ZnO layer is grown on SnSe at a rotation speed of 30 rpm. This result can be applied for the uniform coating of a ZnO layer on various powder materials.

Nanoparticles of PbTe are prepared via chemical reaction of the equimolar aqueous solutions of Pb(CH₃COO)₂ and Te at 120°C. The size of the obtained particles is 100 nm after calcination in a hydrogen atmosphere. Dense specimens for the thermoelectric characterization are produced by spark plasma sintering of prepared powders at 400°C to 500°C under 80 MPa for 5 min. The relative densities of the prepared specimens reach approximately 97% and are identified as cubic based on X-ray diffraction analyses. The thermoelectric properties are evaluated between 100°C and 300°C via electrical conductivity, Seebeck coefficient, and thermal conductivity. Compared with PbTe ingot, the reduction of the thermal conductivities by more than 30% is verified via phonon scattering at the grain boundaries, which thus contributes to the increase in the figure of merit.

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• Improved Thermoelectric Performance of Cu3Sb1−x−ySnxInySe4 Permingeatites Double-Doped with Sn and In
  Ho-Jeong Kim, Il-Ho Kim
  Korean Journal of Metals and Materials.2023; 61(6): 422.     CrossRef
• Enhancing Electrical Properties of N-type Bismuth Telluride Alloys through Graphene Oxide Incorporation in Extrusion 3D Printing
  Jinhee Bae, Seungki Jo, Kyung Tae Kim
  journal of Korean Powder Metallurgy Institute.2023; 30(4): 318.     CrossRef

In this work, p-type Bi−Sb−Te alloys powders are prepared using gas atomization, a mass production powder preparation method involving rapid solidification. To study the effect of the sintering temperature on the microstructure and thermoelectric properties, gas-atomized powders are consolidated at different temperatures (623, 703, and 743 K) using spark plasma sintering. The crystal structures of the gas-atomized powders and sintered bulks are identified using an X-ray diffraction technique. Texture analysis by electron backscatter diffraction reveals that the grains are randomly oriented in the entire matrix, and no preferred orientation in any unique direction is observed. The hardness values decrease with increasing sintering temperature owing to a decrease in grain size. The conductivity increases gradually with increasing sintering temperature, whereas the Seebeck coefficient decreases owing to increases in the carrier mobility with grain size. The lowest thermal conductivity is obtained for the bulk sintered at a low temperature (603 K), mainly because of its fine-grained microstructure. A peak ZT of 1.06 is achieved for the sample sintered at 703 K owing to its moderate electrical conductivity and sustainable thermal conductivity.

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• Influence of the SPS heating rate on the optical and mechanical properties of Y2O3-MgO nanocomposites
  Seok-Min Yong
  Journal of Ceramic Processing Research.2019; 20(1): 59.     CrossRef

We fabricate fine (<20 μm) powders of Bi_0.5Sb_1.5Te₃ alloys using a large-scale production method and subsequently consolidate them at temperatures of 573, 623, and 673 K using a spark plasma sintering process. The microstructure, mechanical properties, and thermoelectric properties are investigated for each sintering temperature. The microstructural features of both the powders and bulks are characterized by scanning electron microscopy, and the crystal structures are analyzed by X-ray diffraction analysis. The grain size increases with increasing sintering temperature from 573 to 673 K. In addition, the mechanical properties increase significantly with decreasing sintering temperature owing to an increase in grain boundaries. The results indicate that the electrical conductivity and Seebeck coefficient (217 μV/K) of the sample sintered at 673 K increase simultaneously owing to decreased carrier concentration and increased mobility. As a result, a high ZT value of 0.92 at 300 K is achieved. According to the results, a sintering temperature of 673 K is preferable for consolidation of fine (<20 μm) powders.

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• Complex microstructure induced high thermoelectric performances of p-type Bi–Sb–Te alloys
  Eun-Ha Go, Babu Madavali, Min-Woo Shin, Sung Ho Song, Soon-Jik Hong
  Materials Chemistry and Physics.2023; 307: 128156.     CrossRef
• Role of sintering temperature on electronic and mechanical properties of thermoelectric material: A theoretical and experimental study of TiCoSb half-Heusler alloy
  Ajay Kumar Verma, Kishor Kumar Johari, Kriti Tyagi, Durgesh Kumar Sharma, Pawan Kumar, Sudhir Kumar, Sivaiah Bathula, S.R. Dhakate, Bhasker Gahtori
  Materials Chemistry and Physics.2022; 281: 125854.     CrossRef
• Enhanced thermoelectric properties of Li and Mg co−substituted Bi2Sr2Co2O fabricated by combined conventional sintering and spark plasma sintering
  K. Park, H.Y. Hong, S.Y. Gwon
  Inorganic Chemistry Communications.2022; 145: 110005.     CrossRef

Graphene oxide (GO) powder processed by Hummer's method is mixed with p-type Bi₂Te₃ based thermoelectric materials by a high-energy ball milling process. The synthesized GO-dispersed p-type Bi₂Te₃ composite powder has a composition of Bi_0.5Sb_1.5Te₃ (BSbT), and the powder is consolidated into composites with different contents of GO powder by using the spark plasma sintering (SPS) process. It is found that the addition of GO powder significantly decreases the thermal conductivity of the pure BSbT material through active phonon scattering at the newly formed interfaces. In addition, the electrical properties of the GO/BSbT composites are degraded by the addition of GO powder except in the case of the 0.1 wt% GO/BSbT composite. It is found that defects on the surface of GO powder hinder the electrical transport properties. As a result, the maximum thermoelectric performance (ZT value of 0.91) is achieved from the 0.1% GO/BSbT composite at 398 K. These results indicate that introducing GO powder into thermoelectric materials is a promising method to achieve enhanced thermoelectric performance due to the reduction in thermal conductivity.

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• Exploring Thermoelectric Transport Properties and Band Parameters of n-Type Bi2-xSbxTe3 Compounds Using the Single Parabolic Band Model
  Linh Ba Vu, Soo-ho Jung, Jinhee Bae, Jong Min Park, Kyung Tae Kim, Injoon Son, Seungki Jo
  journal of Korean Powder Metallurgy Institute.2024; 31(2): 119.     CrossRef
• Enhancing Electrical Properties of N-type Bismuth Telluride Alloys through Graphene Oxide Incorporation in Extrusion 3D Printing
  Jinhee Bae, Seungki Jo, Kyung Tae Kim
  journal of Korean Powder Metallurgy Institute.2023; 30(4): 318.     CrossRef
• Advancement of thermoelectric performances through the dispersion of expanded graphene on p-type BiSbTe alloys
  Eun-Ha Go, Rathinam Vasudevan, Babu Madavali, Peyala Dharmaiah, Min-Woo Shin, Sung Ho Song, Soon-Jik Hong
  Powder Metallurgy.2023; 66(5): 722.     CrossRef
• The role of edge-oxidized graphene to improve the thermopower of p-type bismuth telluride-based thick films
  Young Min Cho, Kyung Tae Kim, Gi Seung Lee, Soo Hyung Kim
  Applied Surface Science.2019; 476: 533.     CrossRef
• The Preparation and Growth Mechanism of the Recovered Bi2Te3 Particles with Respect to Surfactants
  Hyeongsub So, Eunpil Song, Yong-Ho Choa, Kun-Jae Lee
  Journal of Korean Powder Metallurgy Institute.2017; 24(2): 141.     CrossRef

The recent rise in applications of thermoelectric materials has attracted interest in studies toward the fabrication of thermoelectric materials using mass production techniques. In this study, we successfully fabricate n-type Bi₂Te_2.7Se_0.3 material by a combination of mass production powder metallurgy techniques, gas atomization, and spark plasma sintering. In addition, to examine the effects of hydrogen reduction in the microstructure, the thermoelectric and mechanical properties are measured and analyzed. Here, almost 60% of the oxygen content of the powder are eliminated after hydrogen reduction for 4 h at 360°C. Micrographs of the powder show that the reduced powder had a comparatively clean surface and larger grain sizes than unreduced powder. The density of the consolidated bulk using as-atomized powder and reduced atomized powder exceeds 99%. The thermoelectric power factor of the sample prepared by reduction of powder is 20% better than that of the sample prepared using unreduced powder.

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• Enhancing thermoelectric performance of K-doped polycrystalline SnSe through band engineering tuning and hydrogen reduction
  Nan Xin, Yifei Li, Guihua Tang, Longyun Shen
  Journal of Alloys and Compounds.2022; 899: 163358.     CrossRef
• The effect of powder pre-treatment on the mechanical and thermoelectric properties of spark plasma sintered N-type bismuth telluride
  Ahmed A. Abdelnabi, Vickram Lakhian, Joseph R. McDermid, James S. Cotton
  Journal of Alloys and Compounds.2021; 874: 159782.     CrossRef
• Investigation of Spark Plasma Sintering Temperature on Microstructure and Thermoelectric Properties of p-type Bi-Sb-Te alloys
  Jin-Koo Han, Dong-won Shin, Babu Madavali, Soon-Jik Hong
  Journal of Korean Powder Metallurgy Institute.2017; 24(2): 115.     CrossRef
• The Preparation and Growth Mechanism of the Recovered Bi2Te3 Particles with Respect to Surfactants
  Hyeongsub So, Eunpil Song, Yong-Ho Choa, Kun-Jae Lee
  Journal of Korean Powder Metallurgy Institute.2017; 24(2): 141.     CrossRef
• Enhanced thermoelectric cooling properties of Bi2Te3−xSex alloys fabricated by combining casting, milling and spark plasma sintering
  Seung Tek Han, Pradip Rimal, Chul Hee Lee, Hyo-Seob Kim, Yongho Sohn, Soon-Jik Hong
  Intermetallics.2016; 78: 42.     CrossRef

P-type ternary Bi_0.5Sb_1.5Te₃ alloys are fabricated via mechanical alloying (MA) and spark plasma sintering (SPS). Different ball sizes are used in the MA process, and their effect on the microstructure; hardness, and thermoelectric properties of the p-type BiSbTe alloys are investigated. The phases of milled powders and bulks are identified using an X-ray diffraction technique. The morphology of milled powders and fracture surface of compacted samples are examined using scanning electron microscopy. The morphology, phase, and grain structures of the samples are not altered by the use of different ball sizes in the MA process. Measurements of the thermoelectric (TE) transport properties including the electrical conductivity, Seebeck coefficient, and power factor are measured at temperatures of 300- 400 K for samples treated by SPS. The TE properties do not depend on the ball size used in the MA process.

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• Investigation of Spark Plasma Sintering Temperature on Microstructure and Thermoelectric Properties of p-type Bi-Sb-Te alloys
  Jin-Koo Han, Dong-won Shin, Babu Madavali, Soon-Jik Hong
  Journal of Korean Powder Metallurgy Institute.2017; 24(2): 115.     CrossRef
• Flexible Thermoelectric Device Using Thick Films for Energy Harvesting from the Human Body
  Han Ki Cho, Da Hye Kim, Hye Sun Sin, Churl-Hee Cho, Seungwoo Han
  Journal of the Korean Ceramic Society.2017; 54(6): 518.     CrossRef
• Investigation of the Microstructure and Thermoelectric Properties of P-Type BiSbTe Alloys by Usage of Different Revolutions Per Minute (RPM) During Mechanical Milling
  S.-M. Yoon, B. Madavali, Y.-N. Yoon, S.-J. Hong
  Archives of Metallurgy and Materials.2017; 62(2): 1167.     CrossRef
• Mechanical and thermoelectric properties of Bi2−xSbxTe3 prepared by using encapsulated melting and hot pressing
  Woo-Jin Jung, Il-Ho Kim
  Journal of the Korean Physical Society.2016; 69(8): 1328.     CrossRef

Bi2Te3 related compounds show the best thermoelectric properties at room temperature. However, n-type Bi2Te2.7Se0.3 showed no improvement on ZT values. To improve the thermolectric propterties of n-type Bi2Te2.7Se0.3, this research has Cu-doped n-type powder. This study focused on effects of Cu-doping method on the thermoelectric properties of n-type materials, and evaluated the comparison between the Cu chemical and mechanical doping. The synthesized powder was manufactured by the spark plasma sintering(SPS). The thermoelectric properties of the sintered body were evaluated by measuring their Seebeck coefficient, electrical resistivity, thermal conductivity, and hall coefficient. An introduction of a small amount of Cu reduced the thermal conductivity and improved the electrical properties with Seebeck coefficient. The authors provided the optimal concentration of Cu_0.1Bi_1.99Se_0.3Te_2.7. A figure of merit (ZT) value of 1.22 was obtained for Cu_0.1Bi_1.9Se_0.3Te_2.7 at 373K by Cu chemical doping, which was obviously higher than those of Cu_0.1Bi_1.9Se_0.3Te_2.7 at 373K by Cu mechanical doping (ZT=0.56) and Cu-free Bi₂Se_0.3Te_2.7 (ZT=0.51).