• 한국분말재료학회지
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• 제9권6호
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• pp.422-432
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• 2002

Mechanically driven decomposition of intermetallics during mechanical milling(MM 1 was investigated. This process for Fe-Ce and Fe-Sn system was studied using conventional XRD, DSC, magnetization and alternative current susceptibility measurements. Mechanical alloying and milling form products of the following composition (in sequence of increasing Gecontent): $\alpha$(${\alpha}_1$) bcc solid solution, $\alpha$+$\beta$-phase ($Fe_{2-x}Ge$), $\beta$-phase, $\beta$+FeGe(B20), FeGE(B20), FeGe(B20)+$FeGe_2$,$FeGe_2$,$FeGe_2$+Ge, Ge. Incongruently melting intermetallics $Fe_6Ge_5$ and $Fe_2Ge_3$ decompose under milling. $Fe_6Ge_5$ produces mixture of $\hat{a}$-phase and FeGe(B20), $Fe_2Ge_3$ produces mixture of FeGe(B20) and $FeGe_2$ phases. These facts are in good agreement with the model that implies local melting as a mechanism of new phase for-mation during medchanical alloying. Stability of FeGe(B20) phase, which is also incongruently melting compound, is explained as a result of highest density of this phase in Fe-Ge system. Under mechanical milling (MM) in planetary ball mill, FeSn intermetallic decomposes with formation $Fe_5Sn_3$ and $FeSn_2$ phases, which have the biggest density among the phases of Fe-Sn system. If decomposition degree of FeSn is relatively small(<60%), milled powder shows superparamagnetic behavior at room temperature. For this case, magnetization curves can be fitted by superposition of two Langevin functions. particle sizes for ferromagnetic $Fe_5Sn_3$ phase determined from fitting parameters are in good agreement with crystalline sizes determined from XRD data and remiain approximately chageless during MM. The decomposition of FeSn is attributed to the effects of local temperature and local pressure produced by ball collisions.

• 한국재료학회지
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• 제16권6호
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• pp.382-385
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• 2006

Ni-doped $CoSb_3$ was prepared by the hot pressing, and its doping effects on the thermoelectric properties were investigated. Single phase ${\delta}-CoSb_3$ was successfully obtained by encapsulated induction melting and the subsequent heat treatment at 773 K for 24 hrs, followed by the hot pressing under 60 MPa at 773 K for 2 hrs. Nickel atoms acted as electron donors by substituting cobalt atoms. Thermoelectric properties were remarkably improved by the Ni doping.

• Advances in materials Research
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• 제1권3호
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• pp.205-219
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• 2012

Co-Re alloy development is prompted by the search for new materials for future gas turbines which can be used at temperatures considerably higher than the present day single crystal Ni-based superalloys. The Co-Re based alloys are designed to have very high melting range. Although Co-alloys are used in gas turbine applications today, the Co-Re system was never exploited for structural applications and basic knowledge on the system is lacking. The alloy development strategy therefore is based on studying alloying additions on simple model alloy compositions of ternary and quaternary base. Various strengthening possibilities have been explored and precipitation hardening through fine dispersion of MC type carbides was found to be a promising route. In the early stages of the development we are mainly dealing with polycrystalline alloys and therefore the grain boundary embrittlement needed to be addressed and boron addition was considered for improving the ductility. In this paper recent results on the effect of boron on the strength and ductility and the stability of the fine structure of the strengthening TaC precipitates are presented. In the beginning the alloy development strategy is briefly discussed.

• 열처리공학회지
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• 제31권5호
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• pp.220-230
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• 2018

The objective of this study is to investigate the effect of solution treatment on the microstructure and corrosion behavior of cast AZ91-4%RE magnesium alloy. In the as-cast state, microstructure of the AZ91-4%RE alloy was characterized by intermetallic ${\beta}(Mg_{17}Al_{12})$, $Al_{11}RE_3$ and $Al_2RE$ phase particles distributed in ${\alpha}-(Mg)$ matrix. After solution treatment, the ${\beta}$ particles with low melting point dissolved into the matrix, but Al-RE phases still remained due to their high thermal stabilities. It was found from the immersion and potentiodynamic polarization tests that corrosion rate of the AZ91-4%RE alloy increased after the solution treatment. On the contrary, EIS tests and EDS compositional analyses on the surface corrosion products indicated that the stability of the corrosion product was improved after the solution treatment. Examinations on the corroded microstructures for the ascast and solution-treated samples revealed that dissolution of the ${\beta}$ particles which play a beneficial role in suppressing corrosion propagation, would be responsible for the deterioration of corrosion resistance after the solution treatment. This result implies that the microstructural features such as amount, size and distribution of secondary phases that determine corrosion mechanism, are more influential on the corrosion rate in comparison with the stability of surface corrosion product.

• 전기학회논문지
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• 제58권11호
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• pp.2209-2213
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• 2009

$REBa_{2}Cu_{3}O_{7-d}$(REBCO) coated conductors(REBCO CCs) have been studied for electric power applications which require high current density wires. As long as the critical transition temperature(Tc) is concerned, REBCO CCs with large $RE^{3+}$ ions have been expected to have better current transport properties than those with smaller $RE^{3+}$ ions. For this reason, REBCO's with large $RE^{3+}$ ions which include GdBCO, NdBCO and SmBCO have been mainly considered as the superconducting layer of CCs. On the other hand, REBCO's with smaller $RE^{3+}$ions are expected to have advantages in the fabrication process of CCs because of the lower melting temperature. But it has not yet been made clear which REBCO is the most suitable for the superconducting layer of CCs. In this study, we investigated the current transport properties of REBCO CCs with small $RE^{3+}$ ion and advantages of using that in the CC fabrication process. Thin films of TmBCO, which has smaller $RE^{3+}$ion than most other $RE^{3+}$ ions, were fabricated on buffered metal substrate as the superconducting layer of CC by PLD process. TmBCO CC shows critical current density (Jc (77 K, sf) = $2.3\;MA/cm^2$) high enough to be utilized for application in electric power devices. Compared with previous experiments using the same PLD system, deposition temperature was approximately $20^{\circ}C$ lower than NdBCO thin films on buffered metal substrates.

• 한국재료학회지
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• 제28권11호
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• pp.663-670
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• 2018

H13 tool steels are widely used as metallic mold materials due to their high hardness and thermal stability. Recently, many studies are undertaken to satisfy the demands for manufacturing the complex shape of the mold using a 3D printing technique. It is reported that the mechanical properties of 3D printed materials are lower than those of commercial forged alloys owing to micropores. In this study, we investigate the effect of microstructures and defects on mechanical properties in the 3D printed H13 tool steels. H13 tool steel is fabricated using a selective laser melting(SLM) process with a scan speed of 200 mm/s and a layer thickness of $25{\mu}m$. Microstructures are observed and porosities are measured by optical and scanning electron microscopy in the X-, Y-, and Z-directions with various the build heights. Tiny keyhole type pores are observed with a porosity of 0.4 %, which shows the lowest porosity in the center region. The measured Vickers hardness is around 550 HV and the yield and tensile strength are 1400 and 1700 MPa, respectively. The tensile properties are predicted using two empirical equations through the measured values of the Vickers hardness. The prediction of tensile strength has high accuracy with the experimental data of the 3D printed H13 tool steel. The effects of porosities and unmelted powders on mechanical properties are also elucidated by the metallic fractography analysis to understand tensile and fracture behavior.

• 한국자기학회:학술대회 개요집
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• 한국자기학회 2014년도 자성 및 자성재료 국제학술대회
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• pp.71-71
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• 2014

• 한국재료학회:학술대회논문집
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• 한국재료학회 2012년도 춘계학술발표대회
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• pp.93.1-93.1
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• 2012

세라믹 고온초전도체는 에너지 저장장치의 핵심소재로 사용된다. 초전도 플라이휠 에너지 저장장치(Superconductor flywheel energy storage system)는 전기 에너지를 운동 에너지로 변환하여 저장하는 친환경, 고효율 에너지 저장장치이다. 에너지를 최소화하는데 사용되는 초전도 베어링은 고온초전도체와 영구자석으로 구성된다. 베어링에는 희토류계 초전도 물질(RE-Ba-Cu-O, RE:Rare-earth elements)가 사용된다. 베어링의 효율은 영구자석의 자력크기, 초전도체의 자기부상력과 포획자력에 비례한다. 에너지 저장효율을 높이려면 고온 초전도체의 임계전류밀도(초전도체 내부에 흘릴 수 있는 전기량)를 높이고, 초전도 결정립의 크기를 키워야 한다. 결정크기를 키우는 공정으로 종자결정성장법(Seed growth process)이 사용된다. 초전도체 제조공정은 분말의 성형, incongruent melting을 포함하는 부분 용융, 액상에서의 입성장, 포정반응을 통한 초전도 결정의 성장과정을 포함한다. 본 발표에서는 초전도 에너지 저장장치의 기본 원리, 초전도 베어링의 구성, 베어링용 초전도체의 제조방법과 특성(자기부상력과 포획자력) 평가기술, 차세대 에너지 저장장치로서의 초전도 플라이휠 에너지 저장장치의 전망에 대해 요약하였다.

• 한국재료학회지
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• 제16권9호
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• pp.529-532
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• 2006

Sn-filled $Co_8Sb_{24}$ skutterudites were synthesized by the encapsulated induction melting process. Single ${\delta}-phase$ was successfully obtained by subsequent annealing and confirmed by X-ray diffraction analysis. Temperature dependences of Seebeck coefficient, electrical resistivity and thermal conductivity were examined from 300 K to 700 K. The positive Seebeck coefficient confirmed the p-type conductivity of the Sn-filled $Co_8Sb_{24}$. Electrical resistivity increased with increasing temperature, which shows that the Sn-filled $Co_8Sb_{24}$ skutterudite is a highly degenerate semiconductor. Thermal conductivity was reduced by Sn-filling because the filler atoms acted as phonon scattering centers in the skutterudite lattice. Thermoelectric figure of merit was enhanced by Sn filling and its optimum filling content was considered to be $z{\leq}0.5$ in the $Sn_zCo_8Sb_{24}$ system.

• 한국결정성장학회지
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• 제15권5호
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• pp.175-181
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• 2005

스컬용융법에 의해 루틸 단결정들을 성장시켰으며, 서로 다른 융액의 유지시간에 따른 ingot의 특성을 비교하였다. 스컬용융법은 교류전자기장(RF)에 의해 전기가 흐르는 융액의 직접유도가열에 근거하며, 가열은 RF 에너지의 흡수로 실행된다. $TiO_2$는 상온에서는 부도체이지만 온도가 올라갈수록 전기 전도성이 증가한다. 따라서, 초기 RF 유도가열을 위해 티타늄 금속 링(외경 : 6cm, 내경 : 4cm, 두께 : 0.2cm)을 $TiO_2$ 분말(아나타제상, CERAC, 3N)내부에 묻었다. 스컬용융법에 의한 산화물 용융에서 매우 중요한 것은 융점에서의 전기 저항 값, RF generator의 주파수 그리고 냉각도가니 크기이다. 본 연구에서는, $TiO_2$의 융점에서의 전기저항$(10^{-2}\~10^{-1}\;{\Omega}{\cdot}m)$은 알루미나$(10^{-1}\;{\Omega}{\cdot}m)$의 지르코니아$(10^{-3}\;{\Omega}{\cdot}m)$의 전기저항 데이터를 바탕으로 추정하였다. 냉각도가니의 내부직경은 11cm, 높이는 14cm였으며, 이것은 침투깊이$(0.36\~1.13cm)$와 RF generator 주파수를 고려하여 결정하였다