• 한국재료학회지
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• 제17권2호
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• pp.91-95
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• 2007

Electromigration characteristics of Sn-3.5Ag flip chip solder bump were analyzed using flip chip packages which consisted of Si chip substrate and electroplated Cu under bump metallurgy. Electromigration test temperatures and current densities peformed were $140{\sim}175^{\circ}C\;and\;6{\sim}9{\times}10^4A/cm^2$ respectively. Mean time to failure of solder bump decreased as the temperature and current density increased. The activation energy and current density exponent were found to be 1.63 eV and 4.6, respectively. The activation energy and current density exponent have very high value because of high Joule heating. Evolution of Cu-Sn intermetallic compound was also investigated with respect to current density conditions.

• Bulletin of the Korean Chemical Society
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• 제14권1호
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• pp.21-29
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• 1993

For the study of relaxation processes in complex spin system, a general master equation, which can be used to simulate a vast range of pulse experiments, has been formulated using the Liouville representation of quantum mechanics. The state of a nonequilibrium spin system in magnetic field is described by a density vector in Liouville space and the time evolution of the system is followed by the application of a linear master operator to the density vector in this Liouville space. In this master equation the nuclear spin relaxation due to intramolecular dipolar interaction or randomly fluctuating field interaction is explicitly implemented as a relaxation supermatrix for a strong coupled two-spin (1/2) system. The whole dynamic information inherent in the spin system is thus contained in the density vector and the master operator. The radiofrequency pulses are applied in the same space by corresponding unitary rotational supertransformations of the density vector. If the resulting FID is analytically Fourier transformed, it is possible to represent the final nonstationary spectrum using a frequency dependent spectral vector and intensity determining shape vector. The overall algorithm including relaxation interactions is then translated into an ANSIFORTRAN computer program, which can simulate a variety of two dimensional spectra. Furthermore a new strategy is tested by simulation of multiple quantum signals to differentiate the two relaxation interaction types.

• 한국분말재료학회지
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• 제26권4호
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• pp.311-318
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• 2019

The objective of this study is to investigate the influence of powder shape and densification mechanism on the microstructure and mechanical properties of Ti-6Al-4V components. BE powders are uniaxially and isostatically pressed, and PA ones are injection molded because of their high strengths. The isostatically compacted samples exhibit a density of 80%, which is higher than those of other samples, because hydrostatic compression can lead to higher strain hardening. Owing to the higher green density, the density of BE-CS (97%) is found to be as high as that of other samples (BE-DS (95%) and P-S (94%)). Furthermore, we have found that BE powders can be consolidated by sintering densification and chemical homogenization, whereas PA ones can be consolidated only by simple densification. After sintering, BE-CS and P-S are hot isostatically pressed and BE-DS is hot forged to remove residual pores in the sintered samples. Apparent microstructural evolution is not observed in BE-CSH and P-SH. Moreover, BE-DSF exhibits significantly fine grains and high density of low-angle grain boundaries. Thus, these microstructures provide Ti-6Al-4V components with enhanced mechanical properties (tensile strength of 1179 MPa).

• Geomechanics and Engineering
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• 제16권5호
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• pp.483-493
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• 2018

Experimental study of the deterioration of high-temperature rock subjected to rapid cooling is essential for thermal engineering applications. To evaluate the influence of thermal shock on heated granite with different temperatures, laboratory tests were conducted to record the changes in the physical properties of granite specimens and the dynamic mechanical characteristics of granite after rapid cooling were experimentally investigated by using a split Hopkinson pressure bar (SHPB). The results indicate that there are threshold temperatures ($500-600^{\circ}C$) for variations in density, porosity, and P-wave velocity of granite with increasing treatment temperature. The stress-strain curves of $500-1000^{\circ}C$ show the brittle-plastic transition of tested granite specimens. It was also found that in the temperature range of $200-400^{\circ}C$, the through-cracks induced by rapid cooling have a decisive influence on the failure pattern of rock specimens under dynamic load. Moreover, the increase of crack density due to higher treatment temperature will result in the dilution of thermal shock effect for the rocks at temperatures above $500^{\circ}C$. Eventually, a fitting formula was established to relate the dynamic peak strength of pretreated granite to the crack density, which is the exponential function.

• 한국표면공학회지
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• 제53권6호
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• pp.312-321
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• 2020

Morphology of porous cobalt electro-deposits was systematically investigated as functions of cobalt precursors in the plating bath and applied cathodic current density with a special focus on cobalt nano-rod formation. It was proved that the concentration of cobalt precursor plays little effect on the morphology of cobalt electro-deposits at relatively low plating current density while it significantly affects the morphology with increasing plating current density. Such a dependence was discussed in terms of the kinetics of two competitive reactions of cobalt reduction and hydrogen evolution. Cobalt nano-rod structure was created at specific ranges of cobalt precursor content and applied cathodic current density, and its diameter and length varied with plating time without notable formation of side branches which is usually found during dendrite formation. Specifically, the nano-rod length was preferentially increased in relative short plating time (<15 s), resulting in higher aspect ratio of nano-rod with plating time. Whereas, both the nano-rod length and diameter were increased nearly at the same level in a prolonged plating time, making the aspect ratio unchanged. From the analysis of crystal structure, it was confirmed that the cobalt nano-rod preferentially grew in the form of single crystal on a dense poly-crystalline cobalt thin film initially formed on the substrate.

• 한국통신학회논문지
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• 제39A권3호
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• pp.127-139
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• 2014

멀티 레벨 낸드 플래시 메모리는 한 셀에 2 비트 이상의 정보를 저장하는 구조이고, 비트 위치별 채널 LLR의 밀도 함수 l-밀도가 비대칭 특성을 가지고 있다. 이런 특성은 이진 무기억 대칭 채널 조건에서 설계된 오류 정정부호의 성능이 제대로 발휘되지 못하게 할 뿐만 아니라, 멀티 레벨 낸드 플래시 메모리용 연판정 복호를 수행하는 이진 오류 정정 부호의 설계도 어렵게 한다. 본 논문에서 밀도 미러링과 EM 알고리즘을 이용하여 오류 정정 부호 설계를 위한 차선책을 소개한다. 밀도 미러링은 EM 알고리즘을 적용하기 전에 0 부호어를 전송한 경우로 가정할 수 있도록 하기 위해서 채널 LLR을 처리하는 과정이고, 이후 채널 LLR l-밀도를 EM 알고리즘을 적용하여 K개의 성분으로 이루어진 대칭 가우시안 혼합 밀도로 근사화하는 방법을 소개한다.

• 공업화학
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• 제9권1호
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• pp.52-57
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• 1998

알루미늄-공기전지의 4M KOH전해질에 아연화합물과 같은 첨가제를 넣어 수소발생 및 알루미늄의 부식에 미치는 영향을 검토하였다. 첨가제중의 아연화합물은 수소발생과전압을 증가시키고, TPC(tripotasium citrate)와 CaO는 알루미늄표면에 치밀한 막을 형성하여 수소발생속도와 알루미늄부식속도를 감소시켰다. 이들 첨가제들에 의해 고순도알루미늄(순도, 99.999%)의 개회로전위는 양의 방향으로, 알루미늄 No 1050(순도, 99.5%)의 개회로전위는 음의 방향으로 약간 이동했다. 개회로전위에서 첨가제는 수소발생속도와 알루미늄 부식속도를 감소시켰으며, 과전압이 증가할수록 수소발생속도가 감소하여 알루미늄의 이용율이 증가하였다. 높은 전류밀도$(>100mA/cm^2)$에서는 TPC/CaO/ZnO 첨가제에 의해 고순도 알루미늄의 이용율이 In,Ga,Tl 합금 알루미늄의 이용율과 비슷하였다.

• 대한전자공학회논문지SD
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• 제46권10호
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• pp.53-60
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• 2009

반도체 공정의 발달로 집적도가 증가하고 문턱전압이 감소하면서, 반도체 집적회로 소모 전력에서 누설전류(leakage current)의 비중이 점차 증가하고 있다. 대기 상태에서 CMOS 조합 회로(combinational circuit)는 입력 값에 따라 누설전류가 크게 달라진다. 본 연구에서는 누설전류로 인한 소모전력을 줄이기 위해 대기 상태 (standby state) 회로의 입력 신호를 제어하며, 작은 누설전류를 갖는 입력 신호를 찾기 위한 새로운 효율적인 알고리즘을 개발하였다. 이 방법을 벤치마크 예제에 실험적으로 적용하여 누설전류 평균값에 비해 15.7%, simulated evolution 방법에 비해 6.7% 누설전류를 줄일 수 있음을 보였다. 또한 순차 회로에서도 idle 입력을 이용하여 누설전류 평균값에 비해 6.8%, simulated evolution 방법에 비해 3.2% 누설전류를 줄일 수 있었다.

• 천문학회보
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• 제41권1호
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• pp.35.1-35.1
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• 2016

Nearby galaxy clusters and their surrounding regions represent the current endpoint of evolution galaxy cluster evolution. We present a new catalog of 1589 galaxies, what we call Extended Virgo Cluster Catalog (EVCC), in wider area of the Virgo cluster based on the Sloan Digital Sky Survey (SDSS) Data Release 7. The EVCC covers an area 5.2 times larger than the footprint of the classical Virgo Cluster Catalog, and reaches out to 3.5 times the virial radius of the Virgo cluster. The EVCC contains fundamental information such as membership, morphology, and photometric parameters of galaxies. The EVCC defines a comprehensive galaxy sample covering a wider range in galaxy density that is significantly different from the inner region of the Virgo cluster. It will be the foundation for forthcoming galaxy evolution studies in the extended Virgo cluster region, complementing ongoing and planned Virgo cluster surveys at various wavelengths. We also present the large scale structures in the field around the Virgo cluster. We identified seven galaxy filaments and one possible sheet in three dimensions of super-galactic coordinates based on the HyperLEDA database. By examining spatial distribution and Hubble diagram of galaxies, we found that six filaments are directly associated with the main body of the Virgo cluster. On the other hand, one filament and one sheet are structures located at background of the main body of Virgo cluster. The EVCC and the filament structures will be the foundation for forthcoming studies of galaxy evolution in various environments as well as buildup of the galaxy cluster at z ~ 0, complementing ongoing and planned Virgo cluster surveys at various wavelengths.

• 한국세라믹학회지
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• 제49권4호
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• pp.279-286
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• 2012

Microstructure tailoring of filler loaded preceramic polymer systems offers a high potential for property improvement of Si-based ceramics and composites. Advancements in manufacturing of bulk materials by controlling microstructure evolution during thermal induced polymer-ceramic transforma-tion and polymer-filler reactions will be presented. Rate controlled pyrolysis, multilayer gradient laminate design and surface modification by gas solid reaction are demonstrated to yield ceramic components of high fractional density and superior mechanical properties. Emerging fields of applications are presented.