• Composites Research
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• v.32 no.3
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• pp.148-157
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• 2019

In this study, $C_f/SiC$, $SiC_f/SiC$ and $C_f-SiC_f/SiC$ ceramic composites reinforcing carbon fiber, SiC fiber and hybrid fiber were fabricated by hybrid TGCVI and PIP process. After the thermal shock cycle, 3-point bending and Oxy-Acetylene torch test, their mechanical behavior, oxidation and erosion resistance were evaluated. The $C_f/SiC$ composite showed a decrease in mechanical property along with increasing temperature, a pseudo-ductile fracture mode and a large quantity of erosion. The $SiC_f/SiC$ composite exhibited stronger mechanical property and lower erosion rate compared to the $C_f/SiC$, but brittle fracture mode. On the other hand, hybrid type of $C_f-SiC_f/SiC$ composite gave the best mechanical property, more ductile failure mode than the $SiC_f/SiC$, and lower erosion rate than the $C_f/SiC$. During the Oxy-Acetylene torch test, the $SiO_2$ formed by reaction of the SiC matrix with oxygen prevented further oxidation or erosion of the fibers for $C_f-SiC_f/SiC$ and $SiC_f/SiC$ composites particularly. In conclusion, if a hybrid composite with low porosity is prepared, this material is expected to have high applicability as a high temperature thermo-structural composite under high temperature oxidation atmosphere by improving low mechanical property due to the oxidation of $C_f/SiC$ and brittle fracture mode of $SiC_f/SiC$ composite.

• Journal of the Korean Electrochemical Society
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• v.17 no.1
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• pp.49-56
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• 2014

As an electrolyte additive, the effects of lithium bis(oxalate)borate (LiBOB) on the electrochemical properties of a carbon-coated silicon monoxide (C-coated SiO) negative electrode are investigated. The used electrolyte is 1.3M $LiPF_6$ that is dissolved in ethylene carbonate (EC), fluoroethylene carbonate (FEC), and diethyl carbonate (DEC) (5:25:70 v/v/v) with or without 0.5 wt. % LiBOB. In the LiBOB-free electrolyte, the film resistance is not so high in the initial period of cycling that lithiation is facilitated to generate the crystalline $Li_{15}Si_4$ phase. Due to repeated volume change that is caused by such a deep charge/discharge, cracks form in the active material to cause a resistance increase, which eventually leads to capacity fading. When LiBOB is added into the electrolyte, however, more resistive surface film is generated by decomposition of LiBOB in the initial period. The crystalline $Li_{15}Si_4$ phase does not form, such that the volume change and crack formation are greatly mitigated. Consequently, the C-coated SiO electrode exhibits a better cycle performance in the later cycles. At an elevated temperature ($45^{\circ}C$), wherein the effect of film resistance is less critical, the alloy ($Li_{15}Si_4$ phase) formation is comparable for the LiBOB-free and added cell to give a similar cycle performance.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2009.10a
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• pp.467-469
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• 2009

In this study, fatigue samples were prepared from cylinder head parts that are actually used in domestic (A) and foreign (B) automobiles; high-temperature, high-cycle, and low-cycle fatigue characteristics were then evaluated and compared. A study on the correlation between the microstructural factor and high temperature fatigue characteristic was attempted. The chemical compositions of the heat resistant aluminum alloys above represented A356 (A) and A319 (B), respectively. The result of the tensile strength test on material B at $250^{\circ}C$ was higher by 30.8MPa compared to material A. On the other hand, elongation was 8.5% higher for material A. At $130{\circ}C$, material B exhibited high fatigue life given high cycle fatigue under high stress, whereas material A showed high fatigue life when stress was lowered. With regard to the low-cycle fatigue result ($250^{\circ}C$) showing higher fatigue life as ductility is increased, material A demonstrated higher fatigue life. Through the observation of the differences in microstructure and the fatigue fracture surface, an attempt to explain the high-temperature fatigue deformation behavior of the materials was made.

• Journal of the Korean Association of Geographic Information Studies
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• v.14 no.2
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• pp.82-95
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• 2011

The objective of this paper is to analyze the GIS-based water cycle system: rainfall, evapotranspiration, surface run-off of Gyeongsanam-do for the effective rainwater management. The rainfall(1999~2008) analyzed by a spatial interpolation method, showed relatively higher amount in Hadong-gun, Sanchung-gun, and Sacheon-gun on the southwest coast than in Changnyeong-gun, Miryang-si, and Changwon-si in the mideast inland. The evapotranspiration was calculated by the three independent variables: air temperature, landuse, and NDVI(normalized difference vegetation index). The analysis showed that Namhae-gun had the highest evapotranspiration of 93.71mm, and Jinhae-si and Changwon-si had the lowest values of 81.78mm and 84.37mm. The surface run-off was analysed by a run-off equation based on the SCS hydrologic soil classification and landuse. The amount of surface run-off showed that Hadong-gun had the highest value, of 90.40mm, and Geochang-gun had the lowest, of 46.69mm. The analysis results of the GIS-based water cycle system will be used to support the establishment of the effective rainwater management plan in Gyeongasngnam-do.

• Journal of the Korean Ceramic Society
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• v.34 no.3
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• pp.296-302
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• 1997

The effect of cold cyclic compaction on densification of SiC whisker/Al2O3 composite was investigated. Re-lative density of the compact increased as the number of cycle and the compaction pressure increased and the bias pressure decreased. The rate of loading and unloading and the frequency of cold cyclic compaction did not affect much on sliding and rearrangement of the particles. Fracture of SiC whisker was hardly ob-served during cold cyclic compaction and the direction of whisker was randomly oriented throughout the compact regardless of the direction of compaction. Thus, cold cyclic compaction may be an efficient method to densify SiC whisker/Al2O3 composite.

• Proceedings of the Materials Research Society of Korea Conference
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• 2003.11a
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• pp.209-209
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• 2003

The adventages of Li alloys have attracted the attention of many research groups, many of which have investigated tin-based alloys [1-2], Despite interesting performances of these, the irreversible capacity loss systematically observed on the first cycle for these compounds is a main drawback for their use as anode materials in lithium ion cells. Not only Sn is efficient in forming alloys with Li, Si can also react with Li to form alloys with a high Li/Si ratio, like Li$\_$22/Si$\_$5/ at 400$^{\circ}C$. It corresponds to a capacity of 4200mAh/g. Electrochemical Li-Si reaction occurs between 0 and 0.3 V against Li/Li$\^$+/, so that high-energy density battery can be realized. Despite the high theoretical capacity of elements like Si, however, particles of the alloys crack and fragment due to the repeated alloying and do-alloying which occurs as cell are charged and discharged. The research groups of Muggins [3] and Besenhard [4] have proposed that the volume expansion due to the insertion of Li can be reduced in micro- and submicro-structured matrix alloys. For this reason, the research group of J.R. Dahn investigated Sn/Mo sequential sputter deposition to prepare nanocomposites [5]. In this study, we investigated the characterization and the electrochemical characteristics of sequentially sputtered Si/Mo multilayer for microbattery anode.

• Proceedings of the Korean Vacuum Society Conference
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• 2014.02a
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• pp.186.2-186.2
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• 2014

최근 친환경 에너지에 대한 관심이 증폭되면서 리튬이차전지에 대한 연구가 활발히 진행되고 있다. 특히 음극(anode) 물질의 경우 기존의 흑연(graphite)보다 이론적 용량이 약 10배 이상 높은 실리콘(Silicon)에 대한 관심이 매우 높다. 하지만 Si의 경우 리튬 충전거동 시 400% 이상의 부피팽창으로 몇 번의 충전/방전 싸이클(cycle)에 전극이 파괴되는 문제점을 지니고 있다. 이를 극복하기 위해 Si 나노선이 고려되고 있다. 우수한 전극특성을 갖는 Si 소재를 개발하기 위해서는 원자단위에서 Si 나노선의 리튬 충전 메커니즘을 살펴보는 것이 매우 중요하다. 하지만 기존의 시뮬레이션 기법으로는 Si 나노선의 볼륨팽창에 관한 메커니즘과 리튬 충전과정에서의 상변화(결정질에서 비정질) 과정을 설명하기는 기술적으로 매우 힘들다. 고전적인 분자동역학 방법의 경우 실제 나노스케일을 고려할 수 있지만, empirical potential로는 원자들간의 화학반응을 제대로 묘사할 수 없다. 한편 양자역학에 기반을 둔 제일원리방법의 경우 계산의 복잡성으로 현재의 컴퓨터 환경에서는 나노스케일에서 원자들의 동역학적인 거동을 연구하기 매우 힘들다. 우리는 이러한 문제를 해결하기 위해 실제 나노스케일에서 원자간 화학반응을 예측할 수 있는 Si-Li 시스템의 Reactive force field를 개발하였고, 분자동역학 계산방법을 이용하여 Si 나노선의 Li 충전 메커니즘을 규명하였다.

• Journal of the Korean Electrochemical Society
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• v.15 no.3
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• pp.154-159
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• 2012

Structural volume change occurring on the Si-based anode battery materials during alloying/dealloying with lithium is noticed to be a major drawback responsible for a limited cycle life. Silicon monoxide has been reported to show relatively improved cycling performance compared to Si-containing materials for rechargeable lithium batteries, due to the structural buffering role of in-situ formed $Li_2O$ and lithium silicate during the reaction of silicon monoxide and lithium. Here we report improved cycling ability of interfacially stabilized Si-$SiO_2$-graphite composite anode using silane-based electrolyte additive for rechargeable lithium batteries, which includes low cost silicon dioxide for structural stabilization and graphite for enhanced conductivity.

• Journal of the Korean Ceramic Society
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• v.39 no.10
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• pp.948-954
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• 2002

Porous reaction bonded SiC with high fracture strength was developed using Si melt infiltration method for use of the support layer in high temperature gas filter that is essential to develop the next generation power system such as integrated gasification combined cycle system. The porosity and pore size of porous RBSC developed in this study were in the range of 32∼36% and 37∼90 ${\mu}m$ respectively and the maximum fracture strength of porous RBSC fabricated was 120 MPa. The fracture strength and thermal shock resistance of porous RBSC fabricated by Si melt infiltration were much improved compared to those of commercially available porous clay bonded SiC due to the formation of the strong SiC/Si interface between SiC particles. The characteristics of pore structure of porous RBSC was varied depending on the amounts of residual Si as Well as the size of SiC particle used in green body.

• Journal of Electrochemical Science and Technology
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• v.2 no.3
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• pp.146-151
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• 2011

The formation of Li-Si-O phases, $Li_4SiO_4$ and $Li_2SiO_3$ from the starting materials SiO and $Li_2O$ are analyzed using Vienna Ab-initio Simulation (VASP) package and the total energies of Li-Si-O compounds are evaluated using Projector Augmented Wave (PAW) method and correlated the structural characteristics of the binary system SiO-$Li_2O$ with experimental data from electrochemical method. Despite $Li_2SiO_3$ becomes stable phase by virtue of lowest formation energy calculated through VASP, the experimental method shows presence of $Li_4SiO_4$ as the only product formed when SiO and $Li_2O$ reacts during slow heating to reach $550^{\circ}C$ and found no evidence for the formation of $Li_2SiO_3$. Also, higher density of $Li_4SiO_4$(2.42 g $ml^{-1}$) compared to the compositional mixture $1SiO_2-2Li_2O$ (2.226 g $ml^{-1}$) and better cycle capacity observed through experiment proves that $Li_4SiO_4$ as the most stable anode supported by better cycleabilityfor lithium ion battery remains as paradox from the point of view of VASP calculations.