• 대한기계학회논문집A
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• 제36권2호
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• pp.187-194
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• 2012

일반적으로 복합재의 강도에 대한 크기 효과는 입자강화 알루미늄 복합재 제조시, 입자와 기지재를 압밀한 후 냉각할 때 입자와 기지재 사이의 열팽창계수 차에 의하여 기지재에 펀칭되는 기하적 필수 전위와, 변형 중 입자와 기지재사이의 탄소성 강성도 차로 인해 발생하는 변형률 구배 소성으로 인한 기하적 필수 전위가 주로 영향을 미치는 것으로 알려져 있다. 본 논문에서는 이러한 두 종류의 기하적 필수 전위를 전위 소성 이론에 입각하여 강도로 환산한 후 계층적으로 입자 주위 유한요소 영역에 할당하여 동일한 체적비에서 입자의 크기에 따라 변화하는 복합재의 파손 거동을 효과적으로 예측하였다. 이 방법을 적용함으로써 구형입자의 경우 간단한 축대칭 유한요소 모델링과 실험데이터를 연계하여 입자강화 복합재의 입자 크기 의존 강도 및 파손 효과를 수월하게 예측할 수 있음을 보였다. 또한 서로 다른 입자의 체적비 및 크기에 대하여SiC강화 알루미늄 2124-T4 복합재의 강도와 파손 거동이 분명한 차이가 있음을 보인다.

• Journal of Yeungnam Medical Science
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• 제10권1호
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• pp.114-126
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• 1993

최근 6년간 영남대학교 의과대학병원에서 경추골절로 진단받은 100명의 환자를 대상으로 임상 및 수술소견과 방사선학적 소견을 조사하여 다음과 같은 결과를 얻었다. 1. 가장 흔히 침범되는 곳은 하부 경추중 $C_5$와 $C_6$, 상부경추중 $C_2$였다. 2. 가장 흔히 침범되는 부위는 몸체, 추궁판, 치상돌기의 순으로 나타났다. 3. 경추골절을 일으키는 손상의 기전으로는 과굴곡 손상에 의한 경우가 53%로 가장 많았고 부위별로는 과굴곡 손상때는 몸체, 골극돌기, 부횡돌기의 골절이 많았고 과신전 손상때는 상대적으로 추궁근의 골절이 많았다. 4. 과신전(1.68 fractures/patients)때보다는 과굴곡(2.26 fractures/patients) 때 좀더 심한 손상을 입는 것으로 나타났다. 5. 추궁판만의 단독골절은 드물었고 대부분이 다른 부위의 골절과 동반되어 나타났으며 이중 70%는 과굴곡 손상때 나타났다. 6. 몸체골절은 50% 이상에서 다른 부위의 골절을 동반하고 있었는데 이들 다른 부위의 골절 중에서는 추궁판골절이 가장 많았다. 7. 두개 이상의 여러 경추를 동시에 침범한 경우의 40%가 인접하지 않은 다른 부위의 골절을 보여 어떤 한 부위의 골절이 발견되었다 하더라도 반드시 전 경추에 대한 세심한 관찰로 또 다른 골절의 유무를 살펴야겠다. 8. 골전과 동반된 전위는 대부분 과굴곡 손상에서 나타났으며 약 70%가 전방전위를 보였으며 $C_{5-6}$과 $C_{6-7}$ 사이에 가장 많았다.

• 대한금속재료학회지
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• 제47권7호
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• pp.391-396
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• 2009

Tensile, low cycle fatigue, and fatigue crack growth rate tests were conducted at RT and $300^{\circ}C$ for type 304 stainless steel. Tensile was tested under displacement control and low cycle fatigue was tested under strain control. Fatigue crack growth rate test was conducted under load control and crack was measured by DCPD method. Yield strength and elongation decreased at $300^{\circ}C$. Dynamic strain aging was not detected at $300^{\circ}C$. Low cycle fatigue life increased but fatigue strength decreased at $300^{\circ}C$. Fatigue crack growth rate increased at $300^{\circ}C$. Dislocation structures were mixed with cell and planar and did not change with temperature. Grain size did not change but plastic strain increased at $300^{\circ}C$. Strain induced martensite after low cycle fatigue test increased at RT but decreased at $300^{\circ}C$. It was concluded that the increase of low cycle fatigue life at $300^{\circ}C$ was due to the decrease of strain induced martensite at which crack was initiated.

• 열처리공학회지
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• 제6권1호
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• pp.26-36
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• 1993

In this study, the ausformed martensite and marformed martensite obtained from austenite with various deformation degrees in Fe-31% Ni-0.2%C alloy were transformed to revesed austenite at $510^{\circ}C$ by cyclic reverse martensite transformation. The effect of prior deformation, the rapid heating rate of reversion and number of cyclic transformation on the microstructure, mechanical properties of reversed austenite were investigated. The reverse austenite transformation is accomplised by the mechanism of shear type transformation. The structure of reversed austenite formed from ausformed martensite and marformed martensite with high deformation degrees is a fine structure of nearly equiaed grain containg a high density of dislocation tangles and was largely affected by the prior deformation applied before reversal transformation. The strength of reversed austenite is more increased with of cyclic transformation especially it is strength at the first cyclic transformation. The yield stress of revesed austenite of ausformed martensite is lower than that of marformed martensite.

• 대한의생명과학회지
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• 제12권4호
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• pp.439-442
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• 2006

To evaluate the postmortem changes in activities of oxygen free radical metabolizing enzymes, the rats were sacrificed with cervical dislocation and were kept in an incubator at $25^{\circ}C$, 70% of humidity for 12 hours. The activities of aniline hydroxylase, catalase, glutathione-S-transferase and superoxlde dismutase were decreased with the time. On the other hand, the activity and type conversion ratio (type D ${\to}$type O) of hepatic xanthine oxidase (XO) were gradually increased. From these changes of XO, the estimation of death time (mathematical equation) could be determined with the least square method. To clarify the cause of increasing XO activity, enzyme kinetics were examined. The Km values of XO were decreased with the time. In conclusion, the determination of liver XO activity might be used for the estimation of death time in the early postmortem period.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 1995년도 추계학술대회논문집
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• pp.167-171
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• 1995

The formation of preferred orientations in cold rolled and recrystallized Inconel 690 sheets was studied by the x-ray texture measurements and TEM observations. The increasing{220} pole intensity in the plane normal at the higher reductions was related to the{110}<112> texture component. The rolling texture of the Inconel 690 was the pure metal type. THe dislocation cells were found in the near{110}<112> oriented grains. The onset of deformation twins in the {112}<111>oriented grains. The onset of deformation twins in the {112}<111> oriented grains. The onset of deformation twins in the {112}<111> oriented grains caused the weakening of {112}<111> and the development of {552}<115> in the rolling texture. The annealing texture of the Inconel 690 sheets was dependent on the annealing temperature. The annealing texture of 750$^{\circ}C$ annealed sheets was similar to the cold rolling texture. The major preferred orientations of the 950$^{\circ}C$ annealed specimens were {112}<110> and {001}<110>. The formation of fine and closely spaced annealing twins in the specimen annealed at 1150$^{\circ}C$ led to the randomization of the annealing texture.

• Applied Science and Convergence Technology
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• 제26권4호
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• pp.79-85
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• 2017

The crack-free AlGaN template has been successfully grown by using selective area growth with triangular GaN facet. The triangular GaN stripe structure was obtained by vertical growth rate enhanced mode with low growth temperature of $950^{\circ}C$ and high growth pressure of 500 torr. The lateral growth rate enhanced mode of AlGaN for crack-free and flat surface was also investigated. Low pressure of 30 torr and high V/III ratio of 4400 were favorable for lateral growth of AlGaN. It was confirmed that the $4{\mu}m$ -thick $Al_{0.2}Ga_{0.8}N$ was crack-free over entire 2-inch wafer. The dislocation density of $Al_{0.2}Ga_{0.8}N$ was as low as ${\sim}7.6{\times}10^8/cm^2$ measured by cathodoluminescence. Based on the high quality AlGaN with low dislocation density, the ultraviolet laser diode epitaxy with cladding, waveguide and GaN/AlGaN multiple quantum well (MQW) was grown by metalorganic chemical vapor deposition. The stimulated emission at 349 nm with full width at half maximum of 1.8 nm from the MQW was observed through optical pumping experiment with 193 nm KrF laser. We also have fabricated the deep ridge type ultraviolet laser diode (UV-LD) with $5{\mu}m-wide$ and $700{\mu}m-long$ cavity for electrical properties. The turn on voltage was below 5 V and the resistance was ${\sim}55{\Omega}$ at applied voltage of 10 V. The amplified spontaneous emission spectrum of UV-LD was also observed from pulsed current injection.

• 한국재료학회지
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• 제24권12호
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• pp.645-651
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• 2014

GaN is most commonly used to make LED elements. But, due to differences of the thermal expansion coefficient and lattice mismatch with sapphire, dislocations have occurred at about $109{\sim}1010/cm^2$. Generally, a low temperature GaN buffer layer is used between the GaN layer and the sapphire substrate in order to reduce the dislocation density and improve the characteristics of the thin film, and thus to increase the efficiency of the LED. Further, patterned sapphire substrate (PSS) are applied to improve the light extraction efficiency. In this experiment, using an AlN buffer layer on PSS in place of the GaN buffer layer that is used mainly to improve the properties of the GaN film, light extraction efficiency and overall properties of the thin film are improved at the same time. The AlN buffer layer was deposited by using a sputter and the AlN buffer layer thickness was determined to be 25 nm through XRD analysis after growing the GaN film at $1070^{\circ}C$ on the AlN buffer CPSS (C-plane Patterned Sapphire Substrate, AlN buffer 25 nm, 100 nm, 200 nm, 300 nm). The GaN film layer formed by applying a 2 step epitaxial lateral overgrowth (ELOG) process, and by changing temperatures ($1020{\sim}1070^{\circ}C$) and pressures (85~300 Torr). To confirm the surface morphology, we used SEM, AFM, and optical microscopy. To analyze the properties (dislocation density and crystallinity) of a thin film, we used HR-XRD and Cathodoluminescence.

• 한국재료학회지
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• 제7권8호
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• pp.679-686
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• 1997

P-type Si(111)기판 위에 3C-SiC박막 성장시 TMS(tetramethylsilane)유량, 반응온도, 반응압력, 가스공급방법등 다양한 성장변수에 따른 박막의 결정성변화를 연구하였다. 증착된 막은 모두 (111)방향성만을 나타내었고, free Si, C의 존재는 관찰할 수 없었다. TMS 유량 0.5 sccm에서, 1100-120$0^{\circ}C$의 반응온도에서 , 반응압력 12-50Torr 조건에서 비록 dislocation과 twin등이 발결되었으나 단결정 3C-SiC 박막을 성장시킬 수 있었으며, 박막의 결정성은 기판에 흡착된 Si-종과 C-종이migration할 수 있는 시간과 에너지에 크게 영향 받음을 확인할 수 있었다. 또한 SiC/Si계면에서 carbonization공정에서 관찰되는 것으로 알려진 void를 관찰할 수 있었으며, 이러한 void의 발생은 기체공급방법을 달리함으로서 제거할 수 있음을 확인할 수 있었다.

• 열처리공학회지
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• 제4권3호
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• pp.1-6
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• 1991

In this study, intermediate thermo-mechanical treated Al-2.0 wt%Li, and Al-2.0 wt%Li-1.2 wt%Cu-1.0 wt%Mg-0.12 wt%Zr alloys were tested in tension at $10^{\circ}C$ and elevated temperature(100, 200 and $300^{\circ}C$). The results are follows : The tensile strength of Al-Li-Cu-Mg-Zr alloy is the highest but the elongation of Al-Li alloy is the highest(106%) among the all alloys in tension at $300^{\circ}C$. The Portervin-LeChartlier effect is showed in AI-Li-Cu-Mg-Zr alloy at 10 and $100^{\circ}C$, because of tangled dislocation by Mg and Cu. In the true stress-strain curves of all alloy, the peaks of stress at $300^{\circ}C$ are showed at the strain less than 0.1. In the binary alloy, the dynamic restoration process at 200 and $300^{\circ}C$ is nearly similar to dynamic recovery type. The hot deformation stress is decreased with increase of dynamic recovery degree, but the elongation is increased. When the strain the strain rate are constant, the temperature dependence of hot deformation stress is increased with increase of deformation temperature. The elongation and degree of dynamic recovery are decreased with increase of hot deformation activation energy, but the deformation stresses slightly increased.