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대한금속재료학회(구 대한금속학회)> 대한금속재료학회지> 캐리어 산란 메커니즘에 따른 열전반도체의 이론 전자수송 특성 변화

KCI등재SCISCOUPUS

캐리어 산란 메커니즘에 따른 열전반도체의 이론 전자수송 특성 변화

Calculated Electric Transport Properties of Thermoelectric Semiconductors Under Different Carrier Scattering Mechanisms

김상일 ( Sang-il Kim ) , 김현식 ( Hyun-sik Kim )
  • : 대한금속재료학회
  • : 대한금속재료학회지 59권2호
  • : 연속간행물
  • : 2021년 02월
  • : 127-134(8pages)
대한금속재료학회지

DOI


목차

1. 서 론
2. 실험 방법
3. 결과 및 고찰
4. 결 론
감사의 글
REFERENCE

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초록 보기

The widespread application of thermoelectric devices in cooling and waste heat recovery systems will be achieved when materials achieve high thermoelectric performance. However, improving thermoelectric performance is not straightforward because the Seebeck coefficient and electrical conductivity of the materials have opposite trends with varying carrier concentration. Here, we demonstrate that carrier scattering mechanism engineering can improve the power factor, which is the Seebeck coefficient squared multiplied by electrical conductivity, by significantly improving the electrical conductivity with a decreased Seebeck coefficient. The effect of engineering the carrier scattering mechanism was evaluated by comparing the band parameters (density-of-states effective mass, non-degenerate mobility) of Te-doped and Te, transition metal co-doped n-type Mg2Sb3 fitted via the single parabolic band model under different carrier scattering mechanisms. Previously, it was reported that co-doping transition metal with Te only changed the carrier scattering mechanism from ionized impurity scattering to mixed scattering between ionized impurities and acoustic phonons, compared to Te-doped samples. The approximately three times enhancement in the power factor of Te, transition metal co-doped samples reported in the literature have all been attributed to a change in the scattering mechanism. However, here it is demonstrated that Te, transition metal co-doping also increased the density-of-states effective mass. Here, the impact of the scattering mechanism change on the electric transport properties of n-type Mg2Sb3 without an effective mass increase was studied. Even without the effective mass increase, carrier scattering mechanism engineering improved the power factor, and its effect was maximized by appropriate carrier concentration tuning.
(Received December 17, 2020; Accepted December 22, 2020)

UCI(KEPA)

간행물정보

  • : 공학분야  > 금속공학
  • : KCI등재
  • : SCI,SCOPUS
  • : 월간
  • : 1738-8228
  • : 2288-8241
  • : 학술지
  • : 연속간행물
  • : 1963-2021
  • : 6952


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59권10호(2021년 10월) 수록논문
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KCI등재 SCI SCOPUS

1저온 템퍼링을 이용한 1.2 GPa급 페라이트계 경량철강 개발

저자 : 배효주 ( Hyo Ju Bae ) , 고광규 ( Kwang Kyu Ko ) , 박형석 ( Hyoung Seok Park ) , 정재석 ( Jae Seok Jeong ) , 김정기 ( Jung Gi Kim ) , 성효경 ( Hyokyung Sung ) , 설재복 ( Jae Bok Seol )

발행기관 : 대한금속재료학회 간행물 : 대한금속재료학회지 59권 10호 발행 연도 : 2021 페이지 : pp. 683-691 (9 pages)

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Previously reported low-Mn ferritic-based lightweight steels are potential candidates for industrial applications, however, they typically exhibit lower strength, with < 1 GPa and lower strength-ductility balance, than medium- and high-Mn austenitic lightweight steels. Herein, we introduce a low-temperature tempering-induced partitioning (LTP) treatment that avoids the strength-ductility dilemma of low-Mn ferritic-based steels. When the LTP process was performed at 330 ℃ for 665 s, the strength of typical ferritic base Fe-2.8Mn5.7Al0.3C (wt%) steel with heterogeneously sized metastable austenite grains embedded in a ferrite matrix, exceeded 1.1 GPa. Notably, the increased strength-ductility balance of the LTP-processed ferritic steel was comparable to that of the high-Mn based austenitic lightweight steel series. Using microscale to nearatomic scale characterization we found that the simultaneous improvement in strength and total elongation could be attributed to size-dependent dislocation movement, and controlled deformation-induced martensitic transformation.
(Received May 14 2021; Accepted July 5, 2021)

KCI등재 SCI SCOPUS

2나노구조의 ZrB2-Al2O3 복합재료 합성과 소결

저자 : 손인진 ( In-jin Shon )

발행기관 : 대한금속재료학회 간행물 : 대한금속재료학회지 59권 10호 발행 연도 : 2021 페이지 : pp. 692-697 (6 pages)

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ZrB2 is considered a candidate material for ultra-high temperature ceramics because of its high thermal conductivity, high melting point, and low coefficient of thermal expansion. Despite these attractive properties, ZrB2 applications are limited by its low fracture toughness below the brittle-ductile transition temperature. To improve its ductile properties, the approach universally utilized has been to add a second material to form composites, and to fabricate nanostructured materials. In this study a dense nanostructured ZrB2-Al2O3 composite was rapidly sintered using the pulsed current activated heating (PCAH) method within 3 min in one step, from mechanically synthesized powders of ZrB2 and Al2O3. Consolidation was accomplished using an effective combination of current and mechanical pressure. A highly dense ZrB2- Al2O3 composite with a relative density of up to 97.4% was fabricated using the simultaneous application of 70 MPa pressure and a pulsed current. The fracture toughness and hardness of the ZrB2-Al2O3 composite were 3.9 MPa.m1/2 and 1917 kg/㎟, respectively. The fracture toughness of the composite was higher than that of monolithic ZrB2.
(Received May 7 2021; Accepted July 20, 2021)

KCI등재 SCI SCOPUS

3ERNiFeCr-2 용가재 적용에 따른 CM247LC 초내열합금 용접부 응고균열 민감도 변화 거동

저자 : 김경민 ( Kyeong-min Kim ) , 정혜은 ( Hye-eun Jeong ) , 정예선 ( Ye-seon Jeong ) , 이의종 ( Uijong Lee ) , 이형수 ( Hyungsoo Lee ) , 서성문 ( Seong-moon Seo ) , 천은준 ( Eun-joon Chun )

발행기관 : 대한금속재료학회 간행물 : 대한금속재료학회지 59권 10호 발행 연도 : 2021 페이지 : pp. 698-708 (11 pages)

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The metallurgical aspects of weld solidification cracking in Ni-based superalloys (with Ti+Al > 5 mass%) have not been widely investigated thus far. Herein, the solidification cracking susceptibility of the CM247LC superalloy and its welds with ERNiFeCr-2 filler wire was quantitatively evaluated using a novel modified Varestraint testing method, for the successful manufacturing of CM247LC superalloy gas turbine blades. It was found that the solidification brittle temperature range (BTR) of the CM247LC superalloy was 400 K. This measurement was obtained with a high-speed thermo-vision camera. The BTR increased to 486 K for the CM247LC/ERNiFeCr-2 welds (dilution ratio: 74%). Theoretical calculations (i.e., the Scheil equation, performed using Thermo-Calc software) were conducted to determine the temperature range in which both solid and liquid phases coexist, together with the microstructural characterization of the solidification cracking surfaces. The greater increase in BTR for the CM247LC/ERNiFeCr-2 welds than that for CM247LC was attributed to the enlargement of the solid-liquid coexistence temperature range. This correlated with the formation of a low-temperature Laves phase during the terminal stage of solidification, and was affected by the diluted Nb and Fe components in the ERNiFeCr-2 filler metal. Based on the experimental and theoretical results, the proposed modified Varestraint testing method for dissimilar welds is expected to be an effective testing process for solidification cracking behavior in the manufacturing of high-soundness CM247LC superalloy welds.
(Received June 24 2021; Accepted July 16, 2021)

KCI등재 SCI SCOPUS

4수소 분위기에서 Mg와 VCl3의 밀링에 의한 Mg의 수소 흡수 방출 특성의 향상

저자 : 송명엽 ( Myoung Youp Song ) , 이성호 ( Seong Ho Lee ) , 곽영준 ( Young Jun Kwak )

발행기관 : 대한금속재료학회 간행물 : 대한금속재료학회지 59권 10호 발행 연도 : 2021 페이지 : pp. 709-717 (9 pages)

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VCl3 (vanadium (III) chloride) was selected as an additive to Mg to increase the hydrogenation and dehydrogenation rates and the hydrogen storage capacity of Mg. Instead of MgH2, Mg was used as a starting material since Mg is cheaper than MgH2. Samples with a composition of 95 wt% Mg + 5 wt% VCl3 (named Mg-5VCl3) were prepared by milling in hydrogen atmosphere (reactive milling). In the first cycle (n=1), Mg-5VCl3 absorbed 5.38 wt% H for 5 min and 5.95 wt% H for 60 min at 573 K in 12 bar hydrogen. The activation of Mg-5VCl3 was completed after three hydrogenation-dehydrogenation cycles. During milling in hydrogen, β-MgH2 and γ-MgH2 were produced. The formed β-MgH2 and γ-MgH2 are considered to have made the effects of reactive milling stronger as β-MgH2 and γ-MgH2 themselves were being pulverized. The introduced defects and the interfaces between the Mg and the phases formed during the reactive milling and during hydrogenation-dehydrogenation cycling are believed to serve as heterogeneous active nucleation sites for MgH2 and Mg-H solid solution. The phases generated during hydrogenation-dehydrognation cycling are also believed to prevent the particles from coalescing during hydrogenation-dehydrognation cycling.
(Received June 13 2021; Accepted July 28, 2021)

KCI등재 SCI SCOPUS

5RF sputtering으로 증착된 ZnO 박막을 전자 수송층으로 사용한 양자점 발광 다이오드에 관한 연구

저자 : 강명석 ( Myoungsuk Kang ) , 김지완 ( Jiwan Kim )

발행기관 : 대한금속재료학회 간행물 : 대한금속재료학회지 59권 10호 발행 연도 : 2021 페이지 : pp. 718-723 (6 pages)

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We report a highly efficient quantum dot light emitting diode (QLEDs) with a radio frequency (RF) sputtered ZnO thin film as an electron transport layer (ETL) instead of the conventional ZnO nanoparticles (NPs) by solution process. ZnO NPs have been used as a key material to improve the performance of QLEDs, but the charge imbalance in ZnO NPs resulting from fast electron injection, and their limited uniformity are significant disadvantages. In this study, ZnO layers were deposited by RF sputtering with various O2 partial pressures. All of the ZnO films showed preferential growth along the (002) direction, smooth morphology, and good optical transmittance. To test their feasibility for QLEDs, we fabricated devices with RF sputtered ZnO layers as an ETL, which has the inverted structure of ITO/RF sputtered ZnO/QDs/CBP/MoO3/Al. The optical/electrical characteristics of two devices, comprised of RF sputtered ZnO and ZnO NPs, were compared with each other. QLEDs with the sputtered ZnO ETL achieved a current efficiency of 11.32 cd/A, which was higher than the 8.23 cd/A of the QLEDs with ZnO NPs ETL. Next, to find the optimum ZnO thin film for highly efficient QLEDs, deposition conditions with various O2 partial pressures were tested, and device performance was investigated. The maximum current efficiency was 13.33 cd/A when the ratio of Ar/O2 was 4:3. Additional oxygen gas reduced the O vacancies in the ZnO thin film, which resulted in a decrease in electrical conductivity, thereby improving charge balance in the emission layer of the QLEDs. As a result, we provide a way to control the ZnO ETL properties and to improve device performance by controlling O2 partial pressure.
(Received June 14 2021; Accepted July 9, 2021)

KCI등재 SCI SCOPUS

6Charge Transport and Thermoelectric Properties of Sn-Doped Tetrahedrites Cu12Sb4-ySnyS13

저자 : Hee-jae Ahn , Il-ho Kim

발행기관 : 대한금속재료학회 간행물 : 대한금속재료학회지 59권 10호 발행 연도 : 2021 페이지 : pp. 724-731 (8 pages)

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In this study, tetrahedrite compounds doped with Sn were prepared by mechanical alloying and hot pressing, and their charge transport and thermoelectric properties were analyzed. X-ray diffraction analysis revealed that both the synthetic powders and sintered bodies were synthesized as a single tetrahedrite phase without secondary phases. Densely sintered specimens were obtained with relatively high densities of 99.5%-100.0% of the theoretical density, and the component elements were distributed uniformly. Sn was successfully substituted at the Sb site, and the lattice constant increased from 1.0348 to 1.0364 nm. Positive signs of the Hall and Seebeck coefficients confirmed that the Sn-doped tetrahedrites were p-type semiconductors. The carrier concentration decreased from 1.28 × 1019 to 1.57 × 1018 cm-3 as the Sn content decreased because excess electrons were supplied by doping with Sn4+ at the Sb3+ site of the tetrahedrite. The Seebeck coefficient increased with increasing Sn content, and Cu12Sb3.6Sn0.4S13 exhibited maximum values of 238-270 μVK-1 at temperatures of 323-723 K. However, the electrical conductivity decreased as the amount of Sn doping increased. Thus, Cu12Sb3.9Sn0.1S13 exhibited the highest electrical conductivity of (2.24-2.40) × 104 Sm-1 at temperatures of 323-723 K. A maximum power factor of 0.73 mWm-1K-2 was achieved at 723 K for Cu12Sb3.9Sn0.1S13. Sn substitution reduced both the electronic and lattice thermal conductivities. The lowest thermal conductivity of 0.49-0.60Wm-1K-1 was obtained at temperatures of 323-723 K for Cu12Sb3.6Sn0.4S13, where the lattice thermal conductivity was dominant at 0.49-0.57 Wm-1K-1. As a result, a maximum dimensionless figure of merit of 0.66 was achieved at 723 K for Cu12Sb3.9Sn0.1S13.
(Received July 12, 2021; Accepted July 28, 2021)

KCI등재 SCI SCOPUS

7Glycothermally Synthesized Self-aggregated ZnS Spherical Particles for Methyl Orange Photodecomposition

저자 : Sang-jun Park , Jeong-hwan Song

발행기관 : 대한금속재료학회 간행물 : 대한금속재료학회지 59권 10호 발행 연도 : 2021 페이지 : pp. 732-740 (9 pages)

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Using ethylene glycol (C2H6O2) as the solvent, ZnS particles were synthesized in high yield at a relatively low temperature of 125 ℃ via the glycothermal method. We report a facile method for preparing spherical self-aggregated ZnS particles from ZnS nanocrystals, using zinc acetate as the Zn2+ source and thiourea as a sulfur source, without mineralization or other agents. The crystal phase structure, morphology, size, surface chemical composition, and optical properties of the self-aggregated ZnS particles were characterized using XRD, FE-SEM, TEM, XPS, BET, and UV-Vis absorption. The ZnS particles had a cubic phase zinc blende structure without any other impurities. The average crystallite size of the synthesized primary nanocrystal, estimated from XRD peak width and TEM images, was nearly 4 nm. FE-SEM images showed that all of the ZnS consisted of self-aggregated particles with a spherical morphology and a size of approximately 0.2 μm~0.5 μm, and contained many tiny primary nanocrystals. The prepared ZnS exhibited strong photoabsorption in the UV region. The optical band gap decreased from 3.85 eV to 3.62 eV as the glycothermal reaction temperature was increased, due to improvement in particle size and crystallization. The effects of the glycothermal reaction temperature on the photocatalytic activity of the synthesized self-aggregated ZnS particles were investigated by the photodecomposition of methyl orange (MO) dye under UV illumination (λ = 365 nm). The prepared ZnS exhibited excellent photocatalytic degradation with increasing reaction temperature, of 125 ℃ (5%), 150 ℃ (10%), 175 ℃ (60%), and 200 ℃ (90%) after irradiation for 60 min. It was found that the ZnS particle prepared at 200 ℃ achieved the highest photocatalytic degradation, with nearly 100% MO decomposition after 90 min, by various photogenerated radical scavengers.
(Received June 9 2021; Accepted July 6, 2021)

KCI등재 SCI SCOPUS

8고분자 전해질 연료전지의 산소환원반응에 대한 Pt-M 합금 촉매의 최근 연구 동향

저자 : 심유진 ( Yu-jin Shim ) , 정원석 ( Won Suk Jung )

발행기관 : 대한금속재료학회 간행물 : 대한금속재료학회지 59권 10호 발행 연도 : 2021 페이지 : pp. 741-752 (12 pages)

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Due to environmental pollution and global warming, research on new energy sources that can replace fossil fuels is important. A fuel cell is an eco-friendly energy conversion system that discharges water, and uses hydrogen as fuel. Although platinum is a widely used catalyst in PEMFCs, it has commercial limitations because of its low stability and high cost. Pt-based bimetal catalysts are being studied to improve performance and reduce the cost of fuel cell catalysts. Pt-M is excellent in terms of performance, stability, and cost, avoiding the disadvantages of the Pt catalyst. Studies on various bimetallic catalysts have been conducted, and among them, studies on Pt-Ni, Pt-Co, and Pt-Fe have been the most active. This review summarizes reports of fuel cell catalysts using Pt-M from 2014 to 2020. In recent studies, in order to improve the Pt-M performance, there have been attempts to change the pretreatment, the type of support, and the composition of Pt and M. There have also been studies that have applied new synthetic methods, which are different from traditional synthetic methods. Many Pt-M catalysts have shown better performance than commercial Pt/C, and exhibited stable performance in durability tests.
(Received May 29 2021; Accepted July 7, 2021)

KCI등재 SCI SCOPUS

9질소 5N 가스 분위기에서 다공질 ZrFe 합금의 산화 안정화

저자 : 김광배 ( Kwangbae Kim ) , 진새라 ( Saera Jin ) , 임예솔 ( Yesol Lim ) , 이현준 ( Hyunjun Lee ) , 김성훈 ( Seonghoon Kim ) , 노윤영 ( Yunyoung Noh ) , 송오성 ( Ohsung Song )

발행기관 : 대한금속재료학회 간행물 : 대한금속재료학회지 59권 10호 발행 연도 : 2021 페이지 : pp. 753-759 (7 pages)

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A porous ZrFe alloy specimen was prepared as a 6 × 3 mm (diameter × thickness) disk. The reaction of the ZrFe alloy was confirmed while the whole system was maintained at a target temperature, which was increased from 150 ℃ to 950 ℃ in a 99.999% low purity nitrogen atmosphere, consisting of 10 ppm of impurity gas. Surface color, pore size, stabilized layer, and phase change were confirmed with optical microscopy, scanning electron microscopy-energy dispersive X-ray spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and Micro-Raman, according to temperature. The surface color of the ZrFe alloy changed from metallic silver to dark gray as the temperature increased. In the EDS and XPS results, nitrogen component was not observed, and oxygen content increased on each surface at the elevated temperatures. In this way, the ZrFe alloy was stabilized in a low purity nitrogen atmosphere, preventing rapid nitride reactions.
(Received June 1 2021; Accepted July 16, 2021)

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1알루미늄 합금의 표면처리에 있어서 2단계 양극산화의 젖음성 영향

저자 : 어재동 ( Jae Dong Eo ) , 김진규 ( Jingyu Kim ) , 정용석 ( Yongsug Jung ) , 이종항 ( Jong-hang Lee ) , 김욱배 ( Wook Bae Kim )

발행기관 : 대한금속재료학회 간행물 : 대한금속재료학회지 59권 2호 발행 연도 : 2021 페이지 : pp. 73-80 (8 pages)

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Industrial anodizing of aluminum alloys is widely employed for various products, to improve corrosion and contamination protection as well as aesthetic appearance. At the same time, nanostructure fabrication using highly ordered porous aluminum oxides has been increasingly investigated in academic research for diverse micro-/nano applications. This approach is based on two-step anodization with limited process conditions, such as extended process time and low temperature. In this study, two-step anodizing was employed to anodize hairline-finished Al 1050 with sulfuric acid considering industrial processing conditions. The method is particularly suited for anodized products that require post-processing such as printing, dyeing and/or bonding. Porous anodized layers that were fabricated using conventional single anodizing, and twostep anodizing under identical processing conditions were compared. Variations in porosity, pore diameter, and inter-pore distance were examined in relation to the anodizing parameters, such as temperature and voltage. The results showed that two-step anodizing caused an increase in all measured pore-related measurements, and produced a much more uniform porous layer than the conventional anodizing process. Water contact angles were evaluated on the anodized surface of the previously machined hairline specimen. It was found that the water contact angles clearly decreased on the surfaces treated by two-step anodization, compared to the conventional anodizing process.
(Received September 23, 2020; Accepted December 17, 2020)

KCI등재SCISCOUPUS

2고에너지 직접 적층법으로 제조된 경사조성재료의 미세조직 및 경도 변화에 미치는 후열처리 영향 연구

저자 : 신기승 ( Giseung Shin ) , 박용호 ( Yongho Park ) , 김대환 ( Dae Whan Kim ) , 윤지현 ( Ji Hyun Yoon ) , 김정한 ( Jeoung Han Kim )

발행기관 : 대한금속재료학회 간행물 : 대한금속재료학회지 59권 2호 발행 연도 : 2021 페이지 : pp. 81-98 (18 pages)

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In this work, the effects of post weld heat treatment (PWHT) on the microstructure and mechanical properties of functionally gradient materials (FGM) was investigated. The FGM consisted of five different layers which were mixtures of austenitic stainless steel (type 316L) and ferritic steel (LAS). The ratio of type 316L and LAS powder in each deposition layer was 100:0, 75:25, 50:50, 25:75, and 0:100. FGM samples were successfully fabricated without cracks or delamination by a direct energy deposition process. The sensitization phenomenon of the FGM samples was investigated after PWHT. The PWHTs were conducted at 700℃, 900℃, and 1100℃ for 4 hours and the samples were then air-cooled. After PWHT, the annealed specimens were observed by optical and scanning electron microscopy to analyze their microstructure. The occurrence of sensitization was found in the specimen annealed at 700℃. The contents of Cr and C increased substantially along grain boundaries. However, the sensitization did not occur in other samples annealed at 900℃ and 1100℃. In the C and D layers of the 1100℃ annealed sample, micro-hardness was measured to be very high due to the formation of bainitic ferrite and a lath martensite structure. In addition, a reduction of the austenite fraction was confirmed by Electron Back Scatter Diffraction.
(Received September 25, 2020; Accepted November 24, 2020)

KCI등재SCISCOUPUS

3Optimization of Oil Adsorption Capacity by Aerogel Powder Synthesized Using Emulsion Droplets as Micro-reactors in Ambient Conditions from Sodium Silicate as Precursor

저자 : Young-sang Cho , Sohyeon Sung , Seung Hee Woo , Young Seok Kim

발행기관 : 대한금속재료학회 간행물 : 대한금속재료학회지 59권 2호 발행 연도 : 2021 페이지 : pp. 99-112 (14 pages)

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In this study, silica aerogel particles were synthesized from emulsion droplets as micro-reactors at room temperature under ambient pressure. An economical precursor, sodium silicate, was used as the starting material for silica, and an emulsification technique was applied to form droplets in continuous phase. By controlling the composition of the dispersed phase using ammonium hydroxide, the effect of pH on the morphologies of the final aerogel particles was studied by SEM observation. As a demonstrative application, hydrophobic silica aerogel particles were produced by modification using a silane coupling agent, for oil adsorption. The amount of oil adsorbed by the aerogel particles was optimized by adjusting the concentration of precursors in the emulsion droplets, the composition of the dispersed phase, and the concentration of the coupling agent during surface treatment of the particles. The resulting aerogel particles were characterized using BET, TGA, and the contact angle of water droplets after modification using silane coupling agents with different carbon numbers. The optimized value of adsorbed silicone oil (100 CS) was measured to be about 250 % relative to the weight of the aerogel particles. For comparison, other types of porous silica particles were also prepared from emulsion-assisted self-assembly routes to quantify the amount of adsorbed oil.
(Received October 5, 2020; Accepted December 24, 2020)

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4Effect of Ultrasonic Cleaning of Titanium Turning Scraps Immersed in Alkaline Solution and Subsequent Preparation of Ferrotitanium Ingots

저자 : Suhwan Yoo , Jikwang Chae , Jung-min Oh , Jae-won Lim

발행기관 : 대한금속재료학회 간행물 : 대한금속재료학회지 59권 2호 발행 연도 : 2021 페이지 : pp. 113-120 (8 pages)

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Ti has excellent properties but is more expensive than other materials due to its high melting point, high reactivity and difficult processability. One way to lower the production cost of Ti products is to recycle Ti scraps. Before recycling Ti scraps, pretreatment to remove contamination is essential. Ti scraps can be pretreated in an alkaline solution, thus eliminating the use of acidic solutions. However, a pretreatment only involving immersion in alkaline solution requires a high concentration solution, and has low efficiency. Therefore, in this study, an optimized ultrasonic cleaning process is introduced to pretreat Ti scraps in a lowconcentration alkaline solution. The carbon content of Ti scraps before pretreatment was 6,800 ppm, and showed a sharp decrease with pretreatment. Using this pretreatment process, C, O, and N impurities were removed by 97.6%, 58.8%, and 29.2%, respectively. Ferrotitanium ingots were then produced by vacuum arc melting (VAM) and plasma arc melting (PAM) using the pretreated Ti scraps and electrolytic iron. Differences in ingots melted by VAM and PAM were investigated. As the time was increased, the content of impurities decreased. Subsequently, a ferrotitanium ingot prepared using the pretreated Ti scraps showed that the carbon concentration in the ingot was less than 200 ppm.
(Received August 13, 2020; Accepted December 29, 2020)

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5Fluorine-based Inductively Coupled Plasma Etching of α-Ga2O3 Epitaxy Film

저자 : Ji Hun Um , Byoung Su Choi , Woo Sik Jang , Sungu Hwang , Dae-woo Jeon , Jin Kon Kim , Hyun Cho

발행기관 : 대한금속재료학회 간행물 : 대한금속재료학회지 59권 2호 발행 연도 : 2021 페이지 : pp. 121-126 (6 pages)

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α-Ga2O3 has the largest bandgap (~5.3 eV) among the five polymorphs of Ga2O3 and is a promising candidate for high power electronic and optoelectronic devices. To fabricate various device structures, it is important to establish an effective dry etch process which can provide practical etch rate, smooth surface morphology and low ion-induced damage. Here, the etch characteristics of α-Ga2O3 epitaxy film were examined in two fluorine-based (CF4/Ar and SF6/Ar) inductively coupled plasmas. Under the same source power, rf chuck power and process pressure, an Ar-rich composition of CF4/Ar and an SF6-rich composition of SF6/Ar produced the highest etch rates. Monotonic increase in the etch rate was observed as the source power and rf chuck power increased in the 2CF4/13Ar discharges, and a maximum etch rate of ~855 A/min was obtained at a 500 W source power, 250 W rf chuck power, and 2 mTorr pressure. A smooth surface morphology with normalized roughness of less than ~1.38 was achieved in the 2CF4/13Ar and 13SF6/2Ar discharges under most of the conditions examined. The features etched into the α-Ga2O3 layer using a 2CF4/ 13Ar discharge with 2 mTorr pressure showed good anisotropy with a vertical sidewall profile.
(Received December 15, 2020; Accepted January 14, 2021)

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6캐리어 산란 메커니즘에 따른 열전반도체의 이론 전자수송 특성 변화

저자 : 김상일 ( Sang-il Kim ) , 김현식 ( Hyun-sik Kim )

발행기관 : 대한금속재료학회 간행물 : 대한금속재료학회지 59권 2호 발행 연도 : 2021 페이지 : pp. 127-134 (8 pages)

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The widespread application of thermoelectric devices in cooling and waste heat recovery systems will be achieved when materials achieve high thermoelectric performance. However, improving thermoelectric performance is not straightforward because the Seebeck coefficient and electrical conductivity of the materials have opposite trends with varying carrier concentration. Here, we demonstrate that carrier scattering mechanism engineering can improve the power factor, which is the Seebeck coefficient squared multiplied by electrical conductivity, by significantly improving the electrical conductivity with a decreased Seebeck coefficient. The effect of engineering the carrier scattering mechanism was evaluated by comparing the band parameters (density-of-states effective mass, non-degenerate mobility) of Te-doped and Te, transition metal co-doped n-type Mg2Sb3 fitted via the single parabolic band model under different carrier scattering mechanisms. Previously, it was reported that co-doping transition metal with Te only changed the carrier scattering mechanism from ionized impurity scattering to mixed scattering between ionized impurities and acoustic phonons, compared to Te-doped samples. The approximately three times enhancement in the power factor of Te, transition metal co-doped samples reported in the literature have all been attributed to a change in the scattering mechanism. However, here it is demonstrated that Te, transition metal co-doping also increased the density-of-states effective mass. Here, the impact of the scattering mechanism change on the electric transport properties of n-type Mg2Sb3 without an effective mass increase was studied. Even without the effective mass increase, carrier scattering mechanism engineering improved the power factor, and its effect was maximized by appropriate carrier concentration tuning.
(Received December 17, 2020; Accepted December 22, 2020)

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7Nucleation and Growth-Controlled Morphology Evolution of Cu Nanostructures During High-Pressure Thermal Evaporation

저자 : Eunji Lee , Woomi Gwon , Sangwoo Ry

발행기관 : 대한금속재료학회 간행물 : 대한금속재료학회지 59권 2호 발행 연도 : 2021 페이지 : pp. 135-141 (7 pages)

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The formation of porous material structures has been widely investigated for the development of high-performance energy materials, catalysts, and chemical sensing devices. Various nanoporous structure fabrication methods are based on wet-chemical processes, which require precise control of the process parameters. Physical vapor deposition such as thermal evaporation utilizes high vacuum so that the deposition process is relatively simple, free of contamination, and easily reproduced. However, because of the long mean-free-path of the evaporated atoms in high vacuum, heterogeneous nucleation and the growth of adatoms occurs on the substrate surface, which results in the formation of dense and compact thin films. But by changing the working pressure, various morphologies of porous nanostructures can be obtained. As applied to copper, with increasing pressure the thin film evolves from a dense structure to a coral-like nanoporous structure through a porous columnar structure. All of the porous structures consist of nanoparticle aggregates, where copper nanoparticles are connected to each other, and many nano-gaps are found inside the aggregates. A surface plasmonic effect is expected. The porous copper nanostructured films demonstrated high surfaceenhanced Raman spectroscopy activity.
(Received December 16, 2020; Accepted December 31, 2020)

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8Fabricating Elastomeric Photomask with Nanosized-Metal Patterns for Near-Field Contact Printing

저자 : Sangyoon Paik , Gwangmook Kim , Dongchul Seo , Wooyoung Shim

발행기관 : 대한금속재료학회 간행물 : 대한금속재료학회지 59권 2호 발행 연도 : 2021 페이지 : pp. 142-148 (7 pages)

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When an elastomeric photomask is used for near-field contact printing, the high deformability of the elastomer mask plate enables gap-free full contact with the substrate, minimizing the effect of diffraction. This image-transfer technique provides sub-50 nm resolution and depth-of-focus-free lithographic capability with cost-efficient equipment. However, the method's application is limited due to the lack of a wellestablished protocol for fabricating a nanoscale mask pattern on an elastomeric substrate, which remains a major technical challenge in the field of near-field contact printing. In this study, we present a reliable protocol for fabricating a metal-embedded polydimethylsiloxane (PDMS) photomask. Our fabrication protocol uses conventional nanofabrication processes to fabricate nanosized chromium mask patterns and then transfers the chromium patterns to an elastomeric mask plate using a sacrificial Ni layer. Our protocol provides a high flexibility mask pattern design, and highly stable metal patterns during transferring process. By careful optimizing the experimental parameters, we determined a perfect pattern transfer ratio, which avoided any mechanical failure of the metal pattern, such as debonding or wrinkling. We then fabricated a PDMS photomask and confirmed its nanoscale patterning resolution, with the smallest feature 51 nm in width under a 400-nm light source. We anticipate that our fabrication protocol will enable the application of cost-efficient and high-resolution near-field photolithography.
(Received September 23, 2020; Accepted December 13, 2020)

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