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Babu Madavali

National Institute for Materials Science, Research Center for Magnetic and Spintronic Materials (CMSM), Post-Doc

Kongju National University, Advanced Materials Engineering & System Engineering, Graduate Student

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My research interests includes Thermoelectric(TE) materials (Direct conversion of waste heat in to useful electric energy), specially, synthesis of nanocomposites and characterization of novel thermoelctric materials for enhancing TE performance. Currently, I am working on the fabrication of hierarchical -nanostructured p-type BiSbTe based thermoelectric materials and their transport properties at ambient temperature.

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Francisco Lerín

University of Derby

Kyung Hee University

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Papers by Babu Madavali

Reduction of Thermal Conductivity through the Dispersion of TIC Nanoparticles into a p-Type BI0.5SB1.5TE3 Alloy by Ball Milling and Spark Plasma Sintering

Archives of Metallurgy and Materials

The dispersion of nanoparticles in the host matrix is a novel approach to enhance the thermoelect... more The dispersion of nanoparticles in the host matrix is a novel approach to enhance the thermoelectric performance. In this work, we incorporate the TiC (x = 0, 1 and 2 wt.%) nanoparticles into a p-type Bi 0.5 Sb 1.5 Te 3 matrix, and their effects on microstructure and thermoelectric properties were systematically investigated. The existence of TiC contents in a base matrix was confirmed by energy dispersive X-ray spectroscopy analysis. The grain size decreases with increasing the addition of TiC content due to grain boundary hardening where the dispersed nanoparticles acted as pinning points in the entire matrix. The electrical conductivity significantly decreased and the Seebeck coefficient was slightly enhanced, which attributes to the decrease in carrier concentration by the addition of TiC content. Meanwhile, the lowest thermal conductivity of 0.97 W/mK for the 2 wt.% TiC nanocomposite sample, which is ~16% lower than 0 wt.% TiC sample. The maximum figure of merit of 0.90 was obtained at 350 K for the 0 wt.% TiC sample due to high electrical conductivity. Moreover, the Vickers hardness was improved with increase the addition of TiC contents.

Effect of Milling Time Parameter on the Microstructure and the Thermoelectric Properties of n-Type BI2TE2.7SE0.3 Alloys

Archives of Metallurgy and Materials

EFFECT OF MILLING TIME PARAMETER ON THE MICROSTRUCTURE AND THE THERMOELECTRIC PROPERTIES OF N-TYP... more EFFECT OF MILLING TIME PARAMETER ON THE MICROSTRUCTURE AND THE THERMOELECTRIC PROPERTIES OF N-TYPE Bi 2 Te 2.7 Se 0.3 ALLOYS Nanostructured thermoelectric materials receiving great attention for its high thermoelectric performance. In this research, nanostructured n-type Bi 2 Te 2.7 Se 0.3 alloys have prepared using high energy ball milling and followed by spark plasma sintering. Also, we have varied ball milling time to investigate milling time parameter on the thermoelectric properties of n-type Bi 2 Te 2.7 Se 0.3 powder. The powders were discrete at 10 min milling and later particles tend to agglomerate at higher milling time due to cold welding. The bulk fracture surface display multi-scale grains where small grains intersperse in between large grains. The maximum Seebeck coefficient value was obtained at 20-min milling time due to their lower carrier density. The κ values were decreased with increasing milling time due to the decreasing trend observed in their κ L values. The highest ZT of 0.7 at 350 K was observed for 30-min milling time which was ascribed to its lower thermal conductivity. The Vickers hardness values also greatly improved due to their fine microstructure.

Investigation of graphene dispersion on thermoelectric, magnetic, and mechanical properties of p-type Bi0.5Sb1.5Te3 alloys

Materials Chemistry and Physics, 2021

Fabrication of ultrafine powder using processing control agent, and investigation of its effect on microstructure and thermoelectric properties of p-type (Bi, Sb)2Te3 alloys

Advanced Powder Technology, 2022

SUK-MIN YOON*, SOON-JIK HONG*# REDUCTION OF THERMAL CONDUCTIVITY THROUGH THE DISPERSION OF TiC NANOPARTICLES INTO A P-TYPE Bi0.5Sb1.5Te3 ALLOY BY BALL MILLING AND SPARK PLASMA SINTERING

The dispersion of nanoparticles in the host matrix is a novel approach to enhance the thermoelect... more The dispersion of nanoparticles in the host matrix is a novel approach to enhance the thermoelectric performance. In this work, we incorporate the TiC (x = 0, 1 and 2 wt.%) nanoparticles into a p-type Bi0.5Sb1.5Te3 matrix, and their effects on microstructure and thermoelectric properties were systematically investigated. The existence of TiC contents in a base matrix was confirmed by energy dispersive X-ray spectroscopy analysis. The grain size decreases with increasing the addition of TiC content due to grain boundary hardening where the dispersed nanoparticles acted as pinning points in the entire matrix. The electrical conductivity significantly decreased and the Seebeck coefficient was slightly enhanced, which attributes to the decrease in carrier concentration by the addition of TiC content. Meanwhile, the lowest thermal conductivity of 0.97 W/mK for the 2 wt.% TiC nanocomposite sample, which is ~16% lower than 0 wt.% TiC sample. The maximum figure of merit of 0.90 was obtained...

Investigation of homogeneity in microstructure and thermoelectric properties at various positions in high-thickness sintered bulks of p-type 20%Bi2Te3–80%Sb2Te3 alloys

Journal of Materials Science: Materials in Electronics, 2021

Thermoelectric devices are environmentally friendly renewable energy sources that use waste heat ... more Thermoelectric devices are environmentally friendly renewable energy sources that use waste heat to generate electricity. So far, p-type Bi–Sb–Te-based alloys with high thermoelectric properties were fabricated using low-dimensional and single growth methods as small-scale, laboratory-grown samples. However, large-scale fabrication processes are required for commercial applications of thermoelectric devices. In this work, large amounts (2–3 kg) of p-type 20%Bi2Te3–80%Sb2Te3 alloy powders were fabricated using the gas atomization (GA) process under an inert gas atmosphere. Subsequently, a large-scale sample, 64 mm thick and 25 mm in diameter, was consolidated from the as-fabricated GA powders using spark plasma sintering at 673 K. The homogeneity of the microstructure, density, thermoelectric properties, and mechanical properties of the high-thickness sintered bulk were systematically investigated at various positions. The EBSD texture analysis revealed that an almost similar microstructure existed in all positions of the bulk sample, which was comprised of a mixture of coarse grains and fine grains randomly distributed throughout the matrix. The peak electrical conductivity of 1100 Ω−1 cm−1 was obtained from the top position of the thick sample, which had a relatively higher carrier mobility. The maximum power factor of 3.96 mW/mK2 was achieved by the top part of the specimen, because of its higher electrical conductivity. The lowest thermal conductivity of 0.964 W/mK at 350 K was obtained from the bottom part of the specimen. As a result, the maximum figure of merit, ZT of 1.25 was achieved in the bottom position of the high-thickness sample. Homogeneous (within 5% of variation) thermoelectric transport properties were observed throughout the fabricated high-thickness sintered sample, regardless of position.

Worn surface and subsurface layer structure formation behavior on wear mechanism of CoCrFeMnNi high entropy alloy in different sliding conditions

Applied Surface Science, 2021

Abstract In this study, equiatomic CoCrFeMnNi high entropy alloy (HEA) was fabricated by inductio... more Abstract In this study, equiatomic CoCrFeMnNi high entropy alloy (HEA) was fabricated by induction melting and subsequent thermomechanical treatments were carried out to achieve single FCC phase with equiaxed grains. The friction and wear behavior of HEA was investigated using ball-on-disc configuration in dry and ambient conditions under different sliding time, normal load and velocity. The detailed microscopic characterizations were invested to examine the worn surface and subsurface deformation mechanisms to identify the kinds of wear involved during dry sliding process. Results revealed the hardness of deformed layer showed 63% higher than matrix owing to grain refinement induced by sliding friction. The friction coefficient stabilized at longer sliding time due to oxidized wear debris acting as as lubricant behavior during sliding. While wear rate significantly decreased with increasing sliding time due to oxidation of wear debris on worn surface and formation of deformed layer with grain refinement resists the plastic deformation by strengthening the subsurface layers. On the other hand, wear rate stabilized for 6 and 8 N due to worn surface oxidation and subsurface hardening. Moreover, wear rate stabilized at higher sliding velocity owing to balance between subsurface hardening and delamination behavior.

$Research paper thumbnail of Investigation of the magnetic properties and fracture behavior of Nd–Fe–B alloy powders during high-energy ball milling$

Investigation of the magnetic properties and fracture behavior of Nd–Fe–B alloy powders during high-energy ball milling

Materials Research Express, 2020

In this work, Nd–Fe–B alloy powders are fabricated via high energy ball milling and their fractur... more In this work, Nd–Fe–B alloy powders are fabricated via high energy ball milling and their fracture behavior and magnetic properties are investigated at different milling times. The powder characteristics (particle shape, size, size distribution, and microstructure) and magnetic properties of the Nd–Fe–B alloys are systematically analyzed. The initial spherical Nd–Fe–B powder is cracked/crushed even after a short period of milling (30 s) due to the high impact energy generated during high energy ball milling. These randomly cracked polygonal-shaped powders are fragmented after 5 min of milling and converted to fine powders. Interestingly, the fine particles after milling for 5 min are conducive to agglomeration at longer milling times (20–60 min) and formed spherically-shaped agglomerated powder. Additionally, the sphericity of the powder increased to 92% by re-pulverization and agglomeration at 60 min. The milled powder shows the Nd2Fe14B phase, and the x-ray diffraction patterns ex...

Fabrication of large-scale p-type 75%Sb2Te3-25%Bi2Te3 thermoelectric materials by gas atomization and hot isostatic pressing

Materials Research Bulletin, 2020

Abstract For economic viability of thermoelectric modules, issues of scalability and sturdiness t... more Abstract For economic viability of thermoelectric modules, issues of scalability and sturdiness to drive down production and manufacturing cost must be adequately addressed. In this study, we report a large-scale production of 3 Kg/batch of 75%Sb2Te3-25%Bi2Te3 powder by gas atomization. The as-produced large-scale atomized powder was subsequently sintered by hot isostatic pressing (HIP) at two different temperatures. The microstructure of the HIPed fracture surface reveals a transgranular mode of fracture in the basal plane of the grains perpendicular to the contact surface. The texture analysis showed that grains were randomly distributed and indicating no preferred alignment in any direction. The sample HIPed at 450 °C shows the highest electrical conductivity of 1350 Ω−1 cm−1 at room temperature due to their high carrier mobility. The highest ZT of 1.15 at 375 K was observed for the sample HIPed at 450 °C owing to their high electrical conductivity, and reasonable low thermal conductivity.

Oxide formation mechanism and its effect on the microstructure and thermoelectric properties of p-type Bi0.5Sb1.5Te3 alloys

Intermetallics, 2018

Bismuth antimony telluride based thermoelectric (TE) materials have been intensively developed an... more Bismuth antimony telluride based thermoelectric (TE) materials have been intensively developed and synthesized using different mechanisms, for the room temperature TE applications. In particular, bismuth antimony telluride based TE alloys are very sensitive to deviations in their composition, and to contamination during the materials synthesis. Oxygen contamination during Bi-Sb-Te based materials synthesis is one of the critical factors that alters or diminishes thermoelectric-transport properties. Thus, in this study, how the oxide formation mechanism on the powder surface and bulks of p-type Bi 0.5 Sb 1.5 Te 3 alloys affected the microstructural features and thermoelectric properties were elucidated quantitatively. While applying heat treatment (HT) to Bi 0.5 Sb 1.5 Te 3 powder, the constituent elements interacted with the ambient atmosphere and formed a new oxide phase which acted as a barrier to carrier transport. At the initial stage (300°C) of heat treatment, only the powder surface was oxidized due to the reaction of outer surface atoms with atmospheric air and moisture. While increasing in temperature during HT, this surface oxygen contamination diffused further inside the powder through the grain boundaries. More diffusion and spreading occurred throughout the matrix at 450°C. The increment of oxygen content from 0.05 to 0.82 wt% drastically decreased the electrical conductivity by 67%, and thermal conductivity by 7% at the heat treatment temperature of 450°C. This reduction behavior is mainly due to severe scattering of the carriers/phonons at the new formation of oxide (Sb 2 O 3) phase near grain boundaries and within the matrix. At a glance, a small increase in the oxygen content wouldn't significantly influence the thermoelectric properties; however, at a certain level of oxide formation (0.82 wt%), severe effects could occur due to the intensified scattering or trapping of carriers by the oxide barrier formation at the grain boundaries.

Effect of Reduction Temperature on the Microstructure and Thermoelectric Properties of TAGS-85 Compounds

Korean Journal of Materials Research, 2017

In this work, the effects of hydrogen reduction on the microstructure and thermoelectric properti... more In this work, the effects of hydrogen reduction on the microstructure and thermoelectric properties of (GeTe) 0.85 (AgSbTe 2) 0.15 (TAGS-85) were studied by a combination of gas atomization and spark plasma sintering. The crystal structure and microstructure of TAGS-85 were characterized by X-ray diffraction(XRD) and scanning electron microscopy (SEM). The oxygen content of both powders and bulk samples were found to decrease with increasing reduction temperature. The grain size gradually increased with increasing reduction temperature due to adhesion of fine grains in a temperature range of 350 to 450 o C. The electrical resistivity was found to increase with reduction temperature due to a decrease in carrier concentration. The Seebeck coefficient decreased with increasing reduction temperature and was in good agreement with the carrier concentration and carrier mobility. The maximum power factor, 3.3 × 10 −3 W/mK 2 , was measured for the non-reduction bulk TAGS-85 at 450 o C.

Influence of milling atmosphere on thermoelectric properties of p-type Bi-Sb-Te based alloys by mechanical alloying

Journal of Korean Powder Metallurgy Institute, 2017

In this study, Bi-Sb-Te thermoelectric materials are produced by mechanical alloying (MA) and spa... more In this study, Bi-Sb-Te thermoelectric materials are produced by mechanical alloying (MA) and spark plasma sintering (SPS). To examine the influence of the milling atmosphere on the microstructure and thermo-electric (TE) properties, a p-type Bi-Sb-Te composite powder is mechanically alloyed in the presence of argon and air atmospheres. The oxygen content increases to 55% when the powder is milled in the air atmosphere, compared with argon. All grains are similar in size and uniformly, distributed in both atmospheric sintered samples. The Seebeck coefficient is higher, while the electrical conductivity is lower in the MA (Air) sample due to a low carrier concentration compared to the MA (Ar) sintered sample. The maximum figure of merit (ZT) is 0.91 and 0.82 at 350 K for the MA (Ar) and MA (Air) sintered samples, respectively. The slight enhancement in the ZT value is due to the decrease in the oxygen content during the MA (Ar) process. Moreover, the combination of mechanical alloying and SPS process shows a higher hardness and density values for the sintered samples.

Reduction of thermal conductivity in Al2O3 dispersed p-type bismuth antimony telluride composites

Materials Chemistry and Physics, 2019

In this work, BiSbTe/Al 2 O 3 composites were prepared by incorporation of alumina (with 2, 4, an... more In this work, BiSbTe/Al 2 O 3 composites were prepared by incorporation of alumina (with 2, 4, and 6 wt%) particles into the p-type Bi-Sb-Te gas atomized powders via mechanical milling and, subsequently consolidated by spark plasma sintering (SPS). The Seebeck coefficient was significantly improved (over 41%) at 325 K, while the electrical conductivity was abruptly decreased for 6 wt% Al 2 O 3 added BiSbTe/Al 2 O 3 composites, which was mainly due to the decreases in carrier concentration, and energy filtering of carriers at interfaces. The thermal conductivity was eminently decreased over 27.5% for 6 wt% Al 2 O 3 added BiSbTe/Al 2 O 3 composites, was mainly attributed to the scattering of carriers and phonons at enormous grain boundaries as well as newly formed interfaces. The Vickers hardness of the BiSbTe/Al 2 O 3 composites was improved compared to that of base matrix, and this was attributed to refinement of grains, and to a dispersion strengthening mechanism.

Densification mechanism and its effect on the magnetic properties of Nd-Fe-B bonded magnets through the new high-energy compaction method

Journal of Magnetism and Magnetic Materials, 2019

Gas-atomized Nd-Fe-B magnetic powders were subjected to magnetic pulse compaction, which is very ... more Gas-atomized Nd-Fe-B magnetic powders were subjected to magnetic pulse compaction, which is very highenergy compaction method, can be densified in a short time with various binder contents at different compaction pressures. The powders were rapidly forced together with in an abbreviated period, and formed a well-packed dense sample with full density during the magnetic pulse compaction process. The influence of density on the magnetic properties of the Nd-Fe-B magnet was investigated. The relative density increased with pressure and highest density of 6.9 g/cm 3 was achieved at a consolidation pressure of 3 GPa. The optimized binder content and pressure were found to be 1 wt% and 3 GPa, respectively, for achieving the highest density. The remanence and maximum energy product of the Nd-Fe-B magnets gradually increased with increasing density. The significant improvement in density can effectively offers high remanence of 6.635 kG and maximum energy product of 8.5 MGOe.

High Thermoelectric Figure of Merit in p-Type (Bi2Te3)x − (Sb2Te3)1−x Alloys Made from Element-Mechanical Alloying and Spark Plasma Sintering

Journal of Electronic Materials, 2018

Bi 2 Te 3) x À (Sb 2 Te 3) 1Àx alloys with high thermoelectric properties were fabricated for was... more Bi 2 Te 3) x À (Sb 2 Te 3) 1Àx alloys with high thermoelectric properties were fabricated for waste heat energy recovery by mechanical alloying followed by spark plasma sintering. The samples' diffraction peaks, such as the (015) positions, were slightly shifted from high to low 2h angles with decreasing Sb 2 Te 3 content due to the occupation of Sb sites by Bi atoms in the crystal lattice. The electrical conductivity increased with (Sb 2 Te 3) content due to an increase in carrier concentration. The sample with the nominal composition of (Bi 2 Te 3) 0.15 + (Sb 2 Te 3) 0.85 exhibited a maximum thermoelectric figure of merit, ZT of 1.3 ± 0.06 at 400 K, and 1.07 ± 0.06 at 300 K. This enhanced ZT was successfully achieved by increasing (Sb 2 Te 3) content, which reduces intrinsic conduction at higher temperatures by increasing carrier concentration and band gaps. The enhanced thermoelectric performance of the (Bi 2 Te 3) 0.15 + (Sb 2 Te 3) 0.85 TE materials can provide exceptional benefits for power generation and cooling applications around 400 K.

Enhanced thermoelectric figure-of-merit in Bi-Sb-Te nanocomposites with homogenously dispersed oxide ceramic ZrO2 nanoparticles

Journal of Applied Physics, 2017

In this research, p-type BiSbTe/ZrO2 nanocomposite powders were fabricated by high-energy ball mi... more In this research, p-type BiSbTe/ZrO2 nanocomposite powders were fabricated by high-energy ball milling. Different weight percentages of ZrO2 (2, 4, and 6 wt. %) nanoparticles were incorporated into the bulk (BiSbTe) matrix by consolidation of as-synthesized nanocomposites (NCs) powder by spark plasma sintering at 673 K. The phase and existence of ZrO2 nano-inclusions was confirmed by X-ray diffraction and transmission electron microscopy-selected area electron diffraction analysis. The Seebeck coefficient of the BiSbTe/ZrO2 NCs was significantly improved (∼36% for 4 wt. % added NCs) by a decrease in the carrier concentration and energy filtering effect, whereas the thermal conductivity was much reduced via strong scattering of carriers/phonons. The peak thermoelectric figure-of-merit (1.34 ± 0.06) was obtained for BiSbTe into which 2 wt. % ZrO2 was dispersed, which was approximately 20% greater than that of the undispersed sample. The hardness of the nanocomposites was significantly...

Investigation of Spark Plasma Sintering Temperature on Microstructure and Thermoelectric Properties of p-type Bi-Sb-Te alloys

Journal of Korean Powder Metallurgy Institute, 2017

Large-scale production of (GeTe) (AgSbTe2)100− (x=75, 80, 85, 90) with enhanced thermoelectric properties via gas-atomization and spark plasma sintering

Acta Materialia, 2017

Abstract (GeTe)x(AgSbTe2)100−x: TAGS thermoelectrics are an attractive class of materials due to ... more Abstract (GeTe)x(AgSbTe2)100−x: TAGS thermoelectrics are an attractive class of materials due to their combination of non-toxicity and good conversion efficiency at mid-temperature ranges. In the present work, we have utilized energy and time efficient high-pressure gas atomization and spark-plasma sintering techniques for large-scale preparation of samples with varying composition (i.e., (GeTe)x(AgSbTe2)100−x where x = 75, 80, 85, and 90). High-temperature x-ray diffraction was used to understand the phase transformation mechanism of the as-atomized powders. Detailed high-resolution transmission electron microscopy of the sintered samples revealed the presence of nanoscale precipitates, antiphase, and twin boundaries. The nanoscale twins and antiphase boundaries serve as phonon scattering centers, leading to the reduction of total thermal conductivity in TAGS-80 and 90 samples. The maximum ZT obtained was 1.56 at 623 K for TAGS-90, which was ∼94% improvement compared to values previously reported. The presence of the twin boundaries also resulted in a high fracture toughness (KIC) of the TAGS-90 sample due to inhibition of dislocation movement at the twin boundary.

Enhanced Seebeck coefficient by energy filtering in Bi-Sb-Te based composites with dispersed Y2O3 nanoparticles

Intermetallics, 2017

The incorporation of ceramic nanoparticles in the bulk thermoelectric matrix is one of the new st... more The incorporation of ceramic nanoparticles in the bulk thermoelectric matrix is one of the new strategies to boost the Seebeck coefficient. In this research, different weight percentages of Y 2 O 3 (2, 4, and 6) nanoparticles (NPs) were incorporated into the pre-alloyed BiSbTe powder for making nanocomposites (NCs) by mechanical milling. The resultant NCs powders were subsequently consolidation by spark plasma sintering (SPS) at 450 C. The existence of Y 2 O 3 nano-inclusions was confirmed by x-ray diffraction and TEM-SAED analysis. The hardness of the nanocomposites was significantly improved (>49%) compared to that of pure BiSbTe, and this was attributed to grain-boundary hardening and to a dispersion strengthening mechanism. The electrical conductivity decreased while the Seebeck coefficient significantly improved (45%) at room temperature for the NCs to which 2 wt% Y 2 O 3 was added. This was due to the scattering of carriers through the energy filtering effect. The electronic component of the thermal conductivity greatly contributed to the reduction of total thermal conductivity (22%) in BiSbTe NCs to which 6 wt% Y 2 O 3 was added. A peak ZT of 1.24 was achieved for BiSbTe/(2 wt%) Y 2 O 3 NCs due to reduction in their thermal conductivity and improved Seebeck coefficient values.

Studies on Microstructure and Thermoelectric Properties of p-Type Bi-Sb-Te Based Alloys by Gas Atomization and Hot Extrusion Processes

Journal of Electronic Materials, 2016

p-Type Bi 2 Te 3 + 75% Sb 2 Te 3 based thermoelectric materials were fabricated via gas atomizati... more p-Type Bi 2 Te 3 + 75% Sb 2 Te 3 based thermoelectric materials were fabricated via gas atomization and the hot extrusion process. The gas atomized powder showed a clean surface with a spherical shape, and expanded in a wide particle size distribution (average particle size 50 lm). The phase of the fabricated extruded and R-extruded bars was identified using x-ray diffraction. The relative densities of both the extruded and R-extruded samples were measured by Archimedes principle with $98% relative density. The R-extruded bar exhibited finer grain microstructure than that of single extrusion process, which was attributed to a recrystallization mechanism during the fabrication. The R-extruded sample showed improved Vickers hardness compared to the extruded sample due to its fine grain microstructure. The electrical conductivity improved for the extruded sample whereas the Seebeck coefficient decreases due to its high carrier concentration. The peak power factor, $4.26 9 10 À3 w/mK 2 was obtained for the single extrusion sample, which is higher than the R-extrusion sample owing to its high electrical properties.

Reduction of Thermal Conductivity through the Dispersion of TIC Nanoparticles into a p-Type BI0.5SB1.5TE3 Alloy by Ball Milling and Spark Plasma Sintering

Archives of Metallurgy and Materials

The dispersion of nanoparticles in the host matrix is a novel approach to enhance the thermoelect... more The dispersion of nanoparticles in the host matrix is a novel approach to enhance the thermoelectric performance. In this work, we incorporate the TiC (x = 0, 1 and 2 wt.%) nanoparticles into a p-type Bi 0.5 Sb 1.5 Te 3 matrix, and their effects on microstructure and thermoelectric properties were systematically investigated. The existence of TiC contents in a base matrix was confirmed by energy dispersive X-ray spectroscopy analysis. The grain size decreases with increasing the addition of TiC content due to grain boundary hardening where the dispersed nanoparticles acted as pinning points in the entire matrix. The electrical conductivity significantly decreased and the Seebeck coefficient was slightly enhanced, which attributes to the decrease in carrier concentration by the addition of TiC content. Meanwhile, the lowest thermal conductivity of 0.97 W/mK for the 2 wt.% TiC nanocomposite sample, which is ~16% lower than 0 wt.% TiC sample. The maximum figure of merit of 0.90 was obtained at 350 K for the 0 wt.% TiC sample due to high electrical conductivity. Moreover, the Vickers hardness was improved with increase the addition of TiC contents.

Effect of Milling Time Parameter on the Microstructure and the Thermoelectric Properties of n-Type BI2TE2.7SE0.3 Alloys

Archives of Metallurgy and Materials

EFFECT OF MILLING TIME PARAMETER ON THE MICROSTRUCTURE AND THE THERMOELECTRIC PROPERTIES OF N-TYP... more EFFECT OF MILLING TIME PARAMETER ON THE MICROSTRUCTURE AND THE THERMOELECTRIC PROPERTIES OF N-TYPE Bi 2 Te 2.7 Se 0.3 ALLOYS Nanostructured thermoelectric materials receiving great attention for its high thermoelectric performance. In this research, nanostructured n-type Bi 2 Te 2.7 Se 0.3 alloys have prepared using high energy ball milling and followed by spark plasma sintering. Also, we have varied ball milling time to investigate milling time parameter on the thermoelectric properties of n-type Bi 2 Te 2.7 Se 0.3 powder. The powders were discrete at 10 min milling and later particles tend to agglomerate at higher milling time due to cold welding. The bulk fracture surface display multi-scale grains where small grains intersperse in between large grains. The maximum Seebeck coefficient value was obtained at 20-min milling time due to their lower carrier density. The κ values were decreased with increasing milling time due to the decreasing trend observed in their κ L values. The highest ZT of 0.7 at 350 K was observed for 30-min milling time which was ascribed to its lower thermal conductivity. The Vickers hardness values also greatly improved due to their fine microstructure.

Investigation of graphene dispersion on thermoelectric, magnetic, and mechanical properties of p-type Bi0.5Sb1.5Te3 alloys

Materials Chemistry and Physics, 2021

Fabrication of ultrafine powder using processing control agent, and investigation of its effect on microstructure and thermoelectric properties of p-type (Bi, Sb)2Te3 alloys

Advanced Powder Technology, 2022

SUK-MIN YOON*, SOON-JIK HONG*# REDUCTION OF THERMAL CONDUCTIVITY THROUGH THE DISPERSION OF TiC NANOPARTICLES INTO A P-TYPE Bi0.5Sb1.5Te3 ALLOY BY BALL MILLING AND SPARK PLASMA SINTERING

The dispersion of nanoparticles in the host matrix is a novel approach to enhance the thermoelect... more The dispersion of nanoparticles in the host matrix is a novel approach to enhance the thermoelectric performance. In this work, we incorporate the TiC (x = 0, 1 and 2 wt.%) nanoparticles into a p-type Bi0.5Sb1.5Te3 matrix, and their effects on microstructure and thermoelectric properties were systematically investigated. The existence of TiC contents in a base matrix was confirmed by energy dispersive X-ray spectroscopy analysis. The grain size decreases with increasing the addition of TiC content due to grain boundary hardening where the dispersed nanoparticles acted as pinning points in the entire matrix. The electrical conductivity significantly decreased and the Seebeck coefficient was slightly enhanced, which attributes to the decrease in carrier concentration by the addition of TiC content. Meanwhile, the lowest thermal conductivity of 0.97 W/mK for the 2 wt.% TiC nanocomposite sample, which is ~16% lower than 0 wt.% TiC sample. The maximum figure of merit of 0.90 was obtained...

Investigation of homogeneity in microstructure and thermoelectric properties at various positions in high-thickness sintered bulks of p-type 20%Bi2Te3–80%Sb2Te3 alloys

Journal of Materials Science: Materials in Electronics, 2021

Thermoelectric devices are environmentally friendly renewable energy sources that use waste heat ... more Thermoelectric devices are environmentally friendly renewable energy sources that use waste heat to generate electricity. So far, p-type Bi–Sb–Te-based alloys with high thermoelectric properties were fabricated using low-dimensional and single growth methods as small-scale, laboratory-grown samples. However, large-scale fabrication processes are required for commercial applications of thermoelectric devices. In this work, large amounts (2–3 kg) of p-type 20%Bi2Te3–80%Sb2Te3 alloy powders were fabricated using the gas atomization (GA) process under an inert gas atmosphere. Subsequently, a large-scale sample, 64 mm thick and 25 mm in diameter, was consolidated from the as-fabricated GA powders using spark plasma sintering at 673 K. The homogeneity of the microstructure, density, thermoelectric properties, and mechanical properties of the high-thickness sintered bulk were systematically investigated at various positions. The EBSD texture analysis revealed that an almost similar microstructure existed in all positions of the bulk sample, which was comprised of a mixture of coarse grains and fine grains randomly distributed throughout the matrix. The peak electrical conductivity of 1100 Ω−1 cm−1 was obtained from the top position of the thick sample, which had a relatively higher carrier mobility. The maximum power factor of 3.96 mW/mK2 was achieved by the top part of the specimen, because of its higher electrical conductivity. The lowest thermal conductivity of 0.964 W/mK at 350 K was obtained from the bottom part of the specimen. As a result, the maximum figure of merit, ZT of 1.25 was achieved in the bottom position of the high-thickness sample. Homogeneous (within 5% of variation) thermoelectric transport properties were observed throughout the fabricated high-thickness sintered sample, regardless of position.

Worn surface and subsurface layer structure formation behavior on wear mechanism of CoCrFeMnNi high entropy alloy in different sliding conditions

Applied Surface Science, 2021

Abstract In this study, equiatomic CoCrFeMnNi high entropy alloy (HEA) was fabricated by inductio... more Abstract In this study, equiatomic CoCrFeMnNi high entropy alloy (HEA) was fabricated by induction melting and subsequent thermomechanical treatments were carried out to achieve single FCC phase with equiaxed grains. The friction and wear behavior of HEA was investigated using ball-on-disc configuration in dry and ambient conditions under different sliding time, normal load and velocity. The detailed microscopic characterizations were invested to examine the worn surface and subsurface deformation mechanisms to identify the kinds of wear involved during dry sliding process. Results revealed the hardness of deformed layer showed 63% higher than matrix owing to grain refinement induced by sliding friction. The friction coefficient stabilized at longer sliding time due to oxidized wear debris acting as as lubricant behavior during sliding. While wear rate significantly decreased with increasing sliding time due to oxidation of wear debris on worn surface and formation of deformed layer with grain refinement resists the plastic deformation by strengthening the subsurface layers. On the other hand, wear rate stabilized for 6 and 8 N due to worn surface oxidation and subsurface hardening. Moreover, wear rate stabilized at higher sliding velocity owing to balance between subsurface hardening and delamination behavior.

$Research paper thumbnail of Investigation of the magnetic properties and fracture behavior of Nd–Fe–B alloy powders during high-energy ball milling$

Investigation of the magnetic properties and fracture behavior of Nd–Fe–B alloy powders during high-energy ball milling

Materials Research Express, 2020

In this work, Nd–Fe–B alloy powders are fabricated via high energy ball milling and their fractur... more In this work, Nd–Fe–B alloy powders are fabricated via high energy ball milling and their fracture behavior and magnetic properties are investigated at different milling times. The powder characteristics (particle shape, size, size distribution, and microstructure) and magnetic properties of the Nd–Fe–B alloys are systematically analyzed. The initial spherical Nd–Fe–B powder is cracked/crushed even after a short period of milling (30 s) due to the high impact energy generated during high energy ball milling. These randomly cracked polygonal-shaped powders are fragmented after 5 min of milling and converted to fine powders. Interestingly, the fine particles after milling for 5 min are conducive to agglomeration at longer milling times (20–60 min) and formed spherically-shaped agglomerated powder. Additionally, the sphericity of the powder increased to 92% by re-pulverization and agglomeration at 60 min. The milled powder shows the Nd2Fe14B phase, and the x-ray diffraction patterns ex...

Fabrication of large-scale p-type 75%Sb2Te3-25%Bi2Te3 thermoelectric materials by gas atomization and hot isostatic pressing

Materials Research Bulletin, 2020

Abstract For economic viability of thermoelectric modules, issues of scalability and sturdiness t... more Abstract For economic viability of thermoelectric modules, issues of scalability and sturdiness to drive down production and manufacturing cost must be adequately addressed. In this study, we report a large-scale production of 3 Kg/batch of 75%Sb2Te3-25%Bi2Te3 powder by gas atomization. The as-produced large-scale atomized powder was subsequently sintered by hot isostatic pressing (HIP) at two different temperatures. The microstructure of the HIPed fracture surface reveals a transgranular mode of fracture in the basal plane of the grains perpendicular to the contact surface. The texture analysis showed that grains were randomly distributed and indicating no preferred alignment in any direction. The sample HIPed at 450 °C shows the highest electrical conductivity of 1350 Ω−1 cm−1 at room temperature due to their high carrier mobility. The highest ZT of 1.15 at 375 K was observed for the sample HIPed at 450 °C owing to their high electrical conductivity, and reasonable low thermal conductivity.

Oxide formation mechanism and its effect on the microstructure and thermoelectric properties of p-type Bi0.5Sb1.5Te3 alloys

Intermetallics, 2018

Bismuth antimony telluride based thermoelectric (TE) materials have been intensively developed an... more Bismuth antimony telluride based thermoelectric (TE) materials have been intensively developed and synthesized using different mechanisms, for the room temperature TE applications. In particular, bismuth antimony telluride based TE alloys are very sensitive to deviations in their composition, and to contamination during the materials synthesis. Oxygen contamination during Bi-Sb-Te based materials synthesis is one of the critical factors that alters or diminishes thermoelectric-transport properties. Thus, in this study, how the oxide formation mechanism on the powder surface and bulks of p-type Bi 0.5 Sb 1.5 Te 3 alloys affected the microstructural features and thermoelectric properties were elucidated quantitatively. While applying heat treatment (HT) to Bi 0.5 Sb 1.5 Te 3 powder, the constituent elements interacted with the ambient atmosphere and formed a new oxide phase which acted as a barrier to carrier transport. At the initial stage (300°C) of heat treatment, only the powder surface was oxidized due to the reaction of outer surface atoms with atmospheric air and moisture. While increasing in temperature during HT, this surface oxygen contamination diffused further inside the powder through the grain boundaries. More diffusion and spreading occurred throughout the matrix at 450°C. The increment of oxygen content from 0.05 to 0.82 wt% drastically decreased the electrical conductivity by 67%, and thermal conductivity by 7% at the heat treatment temperature of 450°C. This reduction behavior is mainly due to severe scattering of the carriers/phonons at the new formation of oxide (Sb 2 O 3) phase near grain boundaries and within the matrix. At a glance, a small increase in the oxygen content wouldn't significantly influence the thermoelectric properties; however, at a certain level of oxide formation (0.82 wt%), severe effects could occur due to the intensified scattering or trapping of carriers by the oxide barrier formation at the grain boundaries.

Effect of Reduction Temperature on the Microstructure and Thermoelectric Properties of TAGS-85 Compounds

Korean Journal of Materials Research, 2017

In this work, the effects of hydrogen reduction on the microstructure and thermoelectric properti... more In this work, the effects of hydrogen reduction on the microstructure and thermoelectric properties of (GeTe) 0.85 (AgSbTe 2) 0.15 (TAGS-85) were studied by a combination of gas atomization and spark plasma sintering. The crystal structure and microstructure of TAGS-85 were characterized by X-ray diffraction(XRD) and scanning electron microscopy (SEM). The oxygen content of both powders and bulk samples were found to decrease with increasing reduction temperature. The grain size gradually increased with increasing reduction temperature due to adhesion of fine grains in a temperature range of 350 to 450 o C. The electrical resistivity was found to increase with reduction temperature due to a decrease in carrier concentration. The Seebeck coefficient decreased with increasing reduction temperature and was in good agreement with the carrier concentration and carrier mobility. The maximum power factor, 3.3 × 10 −3 W/mK 2 , was measured for the non-reduction bulk TAGS-85 at 450 o C.

Influence of milling atmosphere on thermoelectric properties of p-type Bi-Sb-Te based alloys by mechanical alloying

Journal of Korean Powder Metallurgy Institute, 2017

In this study, Bi-Sb-Te thermoelectric materials are produced by mechanical alloying (MA) and spa... more In this study, Bi-Sb-Te thermoelectric materials are produced by mechanical alloying (MA) and spark plasma sintering (SPS). To examine the influence of the milling atmosphere on the microstructure and thermo-electric (TE) properties, a p-type Bi-Sb-Te composite powder is mechanically alloyed in the presence of argon and air atmospheres. The oxygen content increases to 55% when the powder is milled in the air atmosphere, compared with argon. All grains are similar in size and uniformly, distributed in both atmospheric sintered samples. The Seebeck coefficient is higher, while the electrical conductivity is lower in the MA (Air) sample due to a low carrier concentration compared to the MA (Ar) sintered sample. The maximum figure of merit (ZT) is 0.91 and 0.82 at 350 K for the MA (Ar) and MA (Air) sintered samples, respectively. The slight enhancement in the ZT value is due to the decrease in the oxygen content during the MA (Ar) process. Moreover, the combination of mechanical alloying and SPS process shows a higher hardness and density values for the sintered samples.

Reduction of thermal conductivity in Al2O3 dispersed p-type bismuth antimony telluride composites

Materials Chemistry and Physics, 2019

In this work, BiSbTe/Al 2 O 3 composites were prepared by incorporation of alumina (with 2, 4, an... more In this work, BiSbTe/Al 2 O 3 composites were prepared by incorporation of alumina (with 2, 4, and 6 wt%) particles into the p-type Bi-Sb-Te gas atomized powders via mechanical milling and, subsequently consolidated by spark plasma sintering (SPS). The Seebeck coefficient was significantly improved (over 41%) at 325 K, while the electrical conductivity was abruptly decreased for 6 wt% Al 2 O 3 added BiSbTe/Al 2 O 3 composites, which was mainly due to the decreases in carrier concentration, and energy filtering of carriers at interfaces. The thermal conductivity was eminently decreased over 27.5% for 6 wt% Al 2 O 3 added BiSbTe/Al 2 O 3 composites, was mainly attributed to the scattering of carriers and phonons at enormous grain boundaries as well as newly formed interfaces. The Vickers hardness of the BiSbTe/Al 2 O 3 composites was improved compared to that of base matrix, and this was attributed to refinement of grains, and to a dispersion strengthening mechanism.

Densification mechanism and its effect on the magnetic properties of Nd-Fe-B bonded magnets through the new high-energy compaction method

Journal of Magnetism and Magnetic Materials, 2019

Gas-atomized Nd-Fe-B magnetic powders were subjected to magnetic pulse compaction, which is very ... more Gas-atomized Nd-Fe-B magnetic powders were subjected to magnetic pulse compaction, which is very highenergy compaction method, can be densified in a short time with various binder contents at different compaction pressures. The powders were rapidly forced together with in an abbreviated period, and formed a well-packed dense sample with full density during the magnetic pulse compaction process. The influence of density on the magnetic properties of the Nd-Fe-B magnet was investigated. The relative density increased with pressure and highest density of 6.9 g/cm 3 was achieved at a consolidation pressure of 3 GPa. The optimized binder content and pressure were found to be 1 wt% and 3 GPa, respectively, for achieving the highest density. The remanence and maximum energy product of the Nd-Fe-B magnets gradually increased with increasing density. The significant improvement in density can effectively offers high remanence of 6.635 kG and maximum energy product of 8.5 MGOe.

High Thermoelectric Figure of Merit in p-Type (Bi2Te3)x − (Sb2Te3)1−x Alloys Made from Element-Mechanical Alloying and Spark Plasma Sintering

Journal of Electronic Materials, 2018

Bi 2 Te 3) x À (Sb 2 Te 3) 1Àx alloys with high thermoelectric properties were fabricated for was... more Bi 2 Te 3) x À (Sb 2 Te 3) 1Àx alloys with high thermoelectric properties were fabricated for waste heat energy recovery by mechanical alloying followed by spark plasma sintering. The samples' diffraction peaks, such as the (015) positions, were slightly shifted from high to low 2h angles with decreasing Sb 2 Te 3 content due to the occupation of Sb sites by Bi atoms in the crystal lattice. The electrical conductivity increased with (Sb 2 Te 3) content due to an increase in carrier concentration. The sample with the nominal composition of (Bi 2 Te 3) 0.15 + (Sb 2 Te 3) 0.85 exhibited a maximum thermoelectric figure of merit, ZT of 1.3 ± 0.06 at 400 K, and 1.07 ± 0.06 at 300 K. This enhanced ZT was successfully achieved by increasing (Sb 2 Te 3) content, which reduces intrinsic conduction at higher temperatures by increasing carrier concentration and band gaps. The enhanced thermoelectric performance of the (Bi 2 Te 3) 0.15 + (Sb 2 Te 3) 0.85 TE materials can provide exceptional benefits for power generation and cooling applications around 400 K.

Enhanced thermoelectric figure-of-merit in Bi-Sb-Te nanocomposites with homogenously dispersed oxide ceramic ZrO2 nanoparticles

Journal of Applied Physics, 2017

In this research, p-type BiSbTe/ZrO2 nanocomposite powders were fabricated by high-energy ball mi... more In this research, p-type BiSbTe/ZrO2 nanocomposite powders were fabricated by high-energy ball milling. Different weight percentages of ZrO2 (2, 4, and 6 wt. %) nanoparticles were incorporated into the bulk (BiSbTe) matrix by consolidation of as-synthesized nanocomposites (NCs) powder by spark plasma sintering at 673 K. The phase and existence of ZrO2 nano-inclusions was confirmed by X-ray diffraction and transmission electron microscopy-selected area electron diffraction analysis. The Seebeck coefficient of the BiSbTe/ZrO2 NCs was significantly improved (∼36% for 4 wt. % added NCs) by a decrease in the carrier concentration and energy filtering effect, whereas the thermal conductivity was much reduced via strong scattering of carriers/phonons. The peak thermoelectric figure-of-merit (1.34 ± 0.06) was obtained for BiSbTe into which 2 wt. % ZrO2 was dispersed, which was approximately 20% greater than that of the undispersed sample. The hardness of the nanocomposites was significantly...

Investigation of Spark Plasma Sintering Temperature on Microstructure and Thermoelectric Properties of p-type Bi-Sb-Te alloys

Journal of Korean Powder Metallurgy Institute, 2017

Large-scale production of (GeTe) (AgSbTe2)100− (x=75, 80, 85, 90) with enhanced thermoelectric properties via gas-atomization and spark plasma sintering

Acta Materialia, 2017

Abstract (GeTe)x(AgSbTe2)100−x: TAGS thermoelectrics are an attractive class of materials due to ... more Abstract (GeTe)x(AgSbTe2)100−x: TAGS thermoelectrics are an attractive class of materials due to their combination of non-toxicity and good conversion efficiency at mid-temperature ranges. In the present work, we have utilized energy and time efficient high-pressure gas atomization and spark-plasma sintering techniques for large-scale preparation of samples with varying composition (i.e., (GeTe)x(AgSbTe2)100−x where x = 75, 80, 85, and 90). High-temperature x-ray diffraction was used to understand the phase transformation mechanism of the as-atomized powders. Detailed high-resolution transmission electron microscopy of the sintered samples revealed the presence of nanoscale precipitates, antiphase, and twin boundaries. The nanoscale twins and antiphase boundaries serve as phonon scattering centers, leading to the reduction of total thermal conductivity in TAGS-80 and 90 samples. The maximum ZT obtained was 1.56 at 623 K for TAGS-90, which was ∼94% improvement compared to values previously reported. The presence of the twin boundaries also resulted in a high fracture toughness (KIC) of the TAGS-90 sample due to inhibition of dislocation movement at the twin boundary.

Enhanced Seebeck coefficient by energy filtering in Bi-Sb-Te based composites with dispersed Y2O3 nanoparticles

Intermetallics, 2017

The incorporation of ceramic nanoparticles in the bulk thermoelectric matrix is one of the new st... more The incorporation of ceramic nanoparticles in the bulk thermoelectric matrix is one of the new strategies to boost the Seebeck coefficient. In this research, different weight percentages of Y 2 O 3 (2, 4, and 6) nanoparticles (NPs) were incorporated into the pre-alloyed BiSbTe powder for making nanocomposites (NCs) by mechanical milling. The resultant NCs powders were subsequently consolidation by spark plasma sintering (SPS) at 450 C. The existence of Y 2 O 3 nano-inclusions was confirmed by x-ray diffraction and TEM-SAED analysis. The hardness of the nanocomposites was significantly improved (>49%) compared to that of pure BiSbTe, and this was attributed to grain-boundary hardening and to a dispersion strengthening mechanism. The electrical conductivity decreased while the Seebeck coefficient significantly improved (45%) at room temperature for the NCs to which 2 wt% Y 2 O 3 was added. This was due to the scattering of carriers through the energy filtering effect. The electronic component of the thermal conductivity greatly contributed to the reduction of total thermal conductivity (22%) in BiSbTe NCs to which 6 wt% Y 2 O 3 was added. A peak ZT of 1.24 was achieved for BiSbTe/(2 wt%) Y 2 O 3 NCs due to reduction in their thermal conductivity and improved Seebeck coefficient values.

Studies on Microstructure and Thermoelectric Properties of p-Type Bi-Sb-Te Based Alloys by Gas Atomization and Hot Extrusion Processes

Journal of Electronic Materials, 2016

p-Type Bi 2 Te 3 + 75% Sb 2 Te 3 based thermoelectric materials were fabricated via gas atomizati... more p-Type Bi 2 Te 3 + 75% Sb 2 Te 3 based thermoelectric materials were fabricated via gas atomization and the hot extrusion process. The gas atomized powder showed a clean surface with a spherical shape, and expanded in a wide particle size distribution (average particle size 50 lm). The phase of the fabricated extruded and R-extruded bars was identified using x-ray diffraction. The relative densities of both the extruded and R-extruded samples were measured by Archimedes principle with $98% relative density. The R-extruded bar exhibited finer grain microstructure than that of single extrusion process, which was attributed to a recrystallization mechanism during the fabrication. The R-extruded sample showed improved Vickers hardness compared to the extruded sample due to its fine grain microstructure. The electrical conductivity improved for the extruded sample whereas the Seebeck coefficient decreases due to its high carrier concentration. The peak power factor, $4.26 9 10 À3 w/mK 2 was obtained for the single extrusion sample, which is higher than the R-extrusion sample owing to its high electrical properties.