• Korean Chemical Engineering Research
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• v.51 no.4
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• pp.411-417
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• 2013

To use as an anode material of lithium secondary battery, graphite-silicon composite powders are prepared by ball-milling with silicon nanoparticles (average diameter 100 nm, 0~50 wt%) and graphite powder (average diameter $15{\mu}m$) and their electrochemical properties are examined. As the silicon content increases, the graphite becomes smaller by the ball-milling and amorphous phase appears whereas the silicon do not suffer the change of nanocrystalline phases and embeds within the amorphous phase of graphite. Cyclic voltammetry at low scan rate reveals that typical oxidation peaks of graphite and silicon appear at 0.2~0.35 and 0.55~0.6 V, respectively, with higher reversibility for repeated cycles. In contrast, the high-scan-rate redox behavior is very irreversible for repeated cycles. High irreversible capacity is exhibited in the initial charging-discharging cycles, but it diminishes as the cycle number increases. The saturated discharge capacity achieves about 485 mAh $g^{-1}$ at 50th cycle for the composite of Si 20 wt%. This is due to the formation of amorphous graphite morphology by the adequate composition (C:Si=8:2 w/w), which efficiently buffers the volume change during alloying/dealloying between silicon and lithium.

• Proceedings of the Korea Association of Crystal Growth Conference
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• 1996.06b
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• pp.311-352
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• 1996

Peltier 효과를 이용한 열전소자는 열응담 감도가 좋고 선택적 냉각이 가능하며 무소음, 무진동 및 소형화의 장점으로 각종 전자부품의 국부냉각소자로 응용되고 있다. 또한 최근 냉매의 사용없이 냉각이 가능한 열전재료를 이용한 자동차나 가정용 에어컨 및 냉장고 등의 각종 냉방시스템의 개발도 크게 주목을 받고 있다. 기존의 Bi2Te3계 단결정 열전재료는 성능지수는 우수하나, 기계적 취약성에 기인하여 소자가공시 수율 저하가 가장 큰 문제점으로 지적되고 있다. 이와 같은 문제점을 해결하기 위해 최근 단결정에 비해 기계적 강도가 우수한 다결정 열전재료의 제조공정에 관한 연구가 활발히 이루어지고 있으며, 그 일환으로 기계적 합금화법을 이용한 열전재료의 제조공정이 연구되고 있다. 원료금속이 고 에너지 볼-밀 내에서의 연쇄적인 파괴와 압접에 의해 합금분말로 변화되는 기계적 합금화 공정은 상온공정으로 이를 사용하여 다결정 열전재료를 제조시 기존의 다결정 열전재료의 제조공정인 "용해 및 분쇄법'과 비교하여 제조단가를 낮출 수 있는 장점이 있다. 본 연구에서는 전자냉각소자용 열전재료로서 상온부근에서 성능지수가 가장 우수한 p형 (Bi,Sb)2Te3 및 n형 Bi2(Te,Se)3 합금분말을 기계적 합금화 공정으로 제조하여 분말 특성을 분석하였으며, 가압소결 후 열전특성의 변화거동을 연구하였다. 순도 99.99% 이상인 Bi, Sb, Te, Se granule을 (Bi1-xSbx)2Te3 및 Bi2(Te1-ySey)3 조성에 맞게 칭량하여 불과 분말의 무게비 5:1로 강구와 함께 공구강 vial에 장입 후, Spex mixer/mill을 이용하여 기계적 합금화 하였다. 기계적 합금화 공정으로 제조한 분말에 대한 X-선 회절분석과 시차 열분석으로 합금화 정도를 분석하였다. (Bi1-xSbx)2Te3 및 Bi2(Te1-ySey)3 합금분말을 10-5 torr의 진공중에서 300℃∼550℃의 온도로 30분간 가압소결하였다. 가압소결체의 파단면에서의 미세구조를 주사전자현미경으로 관찰하였으며, 상온에서 가압소결체의 열전특성을 측정하였다. (Bi1-xSbx)2Te3의 기계적 합금화에 요구되는 공정시간은 Sb2Te3 함량에 따라 증가하여 x=0.5 조성에서는 4 시간 45분, x=0.75 조성에서는 5 시간, x=1 조성에서는 6 시간 45분의 vibro 밀링이 요구되었다. n형 Bi2(Te1-ySey)3 합금분말의 제조에 요구되는 밀링시간 역시 Bi2Se3 함량 증가에 따라 증가하였으며 Bi2(Te0.95Se0.05)3 합금분말의 제조에는 2시간, Bi2(Te0.9Se0.1)3 및 Bi2(Te0.85Se0.15)3 합금분말의 형성에는 3시간의 bivro 밀링이 요구되었다. 기계적 합금화로 제조한 p형 (Bi0.2Sb0.8)2Te3 및 n형 Bi2(Te0.9Se0.1)3 가압 소결체는 각기 2.9x10-3/K 및 2.1x10-3/K 의 우수한 성능지수를 나타내었다.

• Journal of the Korean Vacuum Society
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• v.20 no.2
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• pp.77-85
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• 2011

The silicon-germanium (SiGe) alloy, which is compatible with silicon semiconductor technology and has a smaller band gap and a lower thermal conductivity than silicon, has been used to fabricate electronic devices such as transistors, photodetectors, solar cells, and thermoelectric devices. This paper reviews the application of SiGe alloys to electronic devices and related technical issues. Since the SiGe alloy comprises germanium whose band gap is smaller than silicon, its band gap is also smaller than that of silicon irrespective of the ratio of silicon to germanium. This narrow band gap of SiGe enables the base thickness of bipolar transistors to decrease without a loss in current gain so that it is possible to improve the speed of bipolar transistors by adopting the SiGe-base. In addition, the conversion efficiency of solar cells is enhanced by the absorption of long-wavelength light in the SiGe absorption layer. Phonon scattering caused by the irregular distribution of alloying elements induces the lower thermal conductivity of SiGe than those of pure silicon and germanium. Because a thin film layer with a low thermal conductivity suppresses thermal conduction through a thermal sink, the SiGe alloy is considered to be a promising material for silicon-based thermoelectric systems.

• Korean Journal of Materials Research
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• v.10 no.11
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• pp.760-769
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• 2000

Mechanical properties of HSLA cast steels alloyed with 0.15% Nb, Ti or V were tested as variations of austenizing temperatures and tempering times. The test results are as follows. The hardness of HSLA cast steels austenized for 2hrs at 115$0^{\circ}C$ was shown the highest value regardless of alloying elements and then decreased as the temperature decreased below 110$0^{\circ}C$. The hardness of HSLA cast steels with 0.15% Ti austenized for 2 hrs at $1150^{\circ}C$ was higher than that of any other HSLA cast steels, and chich was mainly attributed to the relatively high amount of bainite, and solid solution hardening. Charpy impact energy of HSLA cast steels was comparable to the C-Mn cast steel except HSLA cast steels with 0.15% Ti austenized at 115$0^{\circ}C$. The hardness of HSLA cast steels austenized for 2 hrs at $1150^{\circ}C$ increased at a ten-minute tempering, and after that, the hardness kept almost sililar level except HSLA cast steels with 0.15% V.

• Korean Journal of Materials Research
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• v.21 no.7
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• pp.357-363
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• 2011

In this paper, high temperature oxidation behavior of newly developed alloys, Ti-6Al-4Fe and Ti-6Al-1Fe, is examined. To understand the effect of Fe on the air oxidation behavior of the Ti-Al-Fe alloy system, thermal oxidation tests are carried out at $700^{\circ}C$ and $800^{\circ}C$ for 96 hours. Ti-6Al-4V alloy is also prepared and tested under the same conditions for comparison with the developed alloys. The oxidation resistance of the Ti-Al-Fe alloy system is superior to that of Ti-6Al-4V alloy. Ti-6Al-4V shows the worst oxidation resistance for all test conditions. This is not a result of the addition of Fe, but rather it is due to the elimination of V, which has deleterious effects on high temperature oxidation. The oxidation of the Ti-Al-Fe alloy system follows the parabolic rate law. At $700^{\circ}C$, Fe addition does not have a noticeable influence on the amount of weight gain of all specimens. However, at $800^{\circ}C$, Ti-6Al-4Fe alloy shows remarkable degradation compared to Ti-6Al-1Fe and Ti-6Al. It is discovered that the formation of $Al_2O_3$, a diffusion resistance layer, is remarkably hindered by a relative decrease of the ${\alpha}$ volume fraction. This is because Fe addition increases the volume fraction of ${\beta}$ phase within the Ti-6Al-xFe alloy system. Activities of Al, Ti, and Fe with respect to the formation of oxide layers are calculated and analyzed to explore the oxidation mechanism.

• Applied Chemistry for Engineering
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• v.25 no.1
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• pp.1-13
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• 2014

Lithium ion batteries (LIBs) are the state-of-the-art technology among electrochemical energy storage and conversion cells, and are still considered the most attractive class of battery in the future due to their high specific energy density, high efficiency, and long cycle life. Rapid development of power-hungry commercial electronics and large-scale energy storage applications (e.g. off-peak electrical energy storage), however, requires novel anode materials that have higher energy densities to replace conventional graphite electrodes. Germanium (Ge) and silicon (Si) are thought to be ideal prospect candidates for next generation LIB anodes due to their extremely high theoretical energy capacities. For instance, Ge offers relatively lower volume change during cycling, better Li insertion/extraction kinetics, and higher electronic conductivity than Si. In this focused review, we briefly describe the basic concepts of LIBs and then look at the characteristics of ideal anode materials that can provide greatly improved electrochemical performance, including high capacity, better cycling behavior, and rate capability. We then discuss how, in the future, Ge anode materials (Ge and Ge oxides, Ge-carbon composites, and other Ge-based composites) could increase the capacity of today's Li batteries. In recent years, considerable efforts have been made to fulfill the requirements of excellent anode materials, especially using these materials at the nanoscale. This article shall serve as a handy reference, as well as starting point, for future research related to high capacity LIB anodes, especially based on semiconductor Ge and Si.

• Electronic Materials Letters
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• v.14 no.6
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• pp.689-699
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• 2018

Nanostructured Ni doped $Bi_2S_3$ ($Bi_{2-x}Ni_xS_3$, $0{\leq}x{\leq}0.07$) is explored as a candidate for telluride free thermoelectric material, through a combination process of mechanical alloying with subsequent consolidation by cold pressing followed with a sintering process. The cold pressing method was found to impact the thermoelectric properties in two ways: (1) introduction of the dopant atom in the interstitial sites of the crystal lattice which results in an increase in carrier concentration, and (2) introduction of a porous structure which reduces the thermal conductivity. The electrical resistivity of $Bi_2S_3$ was decreased by adding Ni atoms, which shows a minimum value of $2.35{\times}10^{-3}{\Omega}m$ at $300^{\circ}C$ for $Bi_{1.99}Ni_{0.01}S_3$ sample. The presence of porous structures gives a significant effect on reduction of thermal conductivity, by a reduction of ~ 59.6% compared to a high density $Bi_2S_3$. The thermal conductivity of $Bi_{2-x}Ni_xS_3$ ranges from 0.31 to 0.52 W/m K in the temperature range of $27^{\circ}C$ (RT) to $300^{\circ}C$ with the lowest ${\kappa}$ values of $Bi_2S_3$ compared to the previous works. A maximum ZT value of 0.13 at $300^{\circ}C$ was achieved for $Bi_{1.99}Ni_{0.01}S_3$ sample, which is about 2.6 times higher than (0.05) of $Bi_2S_3$ sample. This work show an optimization pathway to improve thermoelectric performance of $Bi_2S_3$ through Ni doping and introduction of porosity.

• Journal of Korea Foundry Society
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• v.41 no.5
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• pp.411-418
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• 2021

In order to experimentally investigate the effect of Ni or Cu addition on microstructure and mechanical properties of high Si Solution Strengthened Ferritic Ductile cast Iron (SSF DI), a series of lab-scale sand casting experiment were conducted by changing initial concentration of Ni up to 3.0wt% or Cu up to 0.9wt% in the alloy. It was found that increase in Ni or Cu content in the alloy leads to increase in strength properties and hardness as well as decrease in ductility. The higher Ni or Cu content the SSF DI has, the higher fraction of pearlite was observed. At similar levels of Ni or Cu contents in the alloy, higher pearlite area fraction was observed in the Cu-containing SSF DI than that in the Ni-containing SSF DI. When the effect of the microstructure on the mechanical properties of Ni-containing SSF DI was considered, Ni-containing SSF DI was found to have excellent strength and hardness as well as good elongation when the pearlite fraction was controlled less than 10%. As the pearlite fraction in the Ni-containing SSF DI exceeds 10%, however, it shows drastic decrease in elongation. Meanwhile, gradual increase in strength and hardness, and decrease in elongation with respect to increase in pearlite fraction were observed in Cu-containing SSF DI. The different microstructure-mechanical property relationships between Ni-containing and Cu-containing SSF DI were due to the combined effect of the relatively weak pearlite stabilizing effect of Ni compared to that of Cu in high Si SSF DI, and matrix strengthening effect caused by the different amounts of those alloying elements required for similar pearlite fraction.

• Journal of Conservation Science
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• v.37 no.5
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• pp.525-535
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• 2021

This study identified material properties through scientific analysis on Jikgeumdan(satin with gold threads) from Donggungbi-Wonsam and the gold threads used in the embroidery. The Donggungbi-Wonsam's base of gold threads were estimated to have used mulberry fiber's Korean paper(Hanji) because non-wood-based fibers were observed. The X-ray spectrometer showed that the Tongsuseulan of Donggungbi-Wonsam was a flat gold thread of pure gold and Jikgeumdan of flat silver thread of its Saekdong and Hansam. High sulfur levels were detected in the flat silver thread, which appeared to have formed silver sulfide by either manufacturing process using sulfur or conservation environment. he dragon insignia's embroidery is also described as two types twisted gold threads; pure gold and alloying-gold and silver. while dragon insignia's border line is decorated with a twisted gold thread of pure gold. In particular, it was investigated that adhesives such as an animal glue, a protein-based compound by gas chromatography mass spectrometry. Additionally, XRF and Raman spectroscopy analysis on the mixture substances between the metal surface and the base paper of gold threads identified talc and quartz in the gold threads and Seokganju(hematite) in the flat silver threads.

• Journal of Korea Foundry Society
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• v.41 no.3
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• pp.252-259
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• 2021

The magnesium is one of the important alloying elements in the conventional aluminum alloys. The addition of magnesium to aluminum is well known to increase the mechanical strength of the aluminum without the trade-off of the decreased elongation. However, the content of magnesium in aluminum alloys has been limited to be lower than about 5wt.% because of the high oxidation tendency of magnesium element during the manufacturing processes such as casting, hot-forming and post heat-treatments, which can deteriorate the quality and properties of the final products. In this study, new 'ECO-Almag6~9' (containing 6~9wt%Mg) alloys were investigated to be made of the ECO-Mg master alloy, which has been invented to reduce the oxidation tendency of itself. It was successfully demonstrated that ECO-Almag6~9 alloys can be fabricated through the mass-production facilities of DC casting and extrusion routes without the problems of magnesium oxidation. In addition, it was confirmed that the strength and ductility were simultaneously improved due to the addition of high magnesium contents.