• Composites Research
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• 제29권4호
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• pp.173-178
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• 2016

Graphene oxide (GO) was concurrently reduced and functionalized using long alkyl chain dodecyl amine (DA). The DA functionalized GO (DA-G) was assumed to disperse homogenously in linear low density polyethylene (LLDPE). Subsequently, DA-G was used to fabricate DA-G/LLDPE composites by melt mixing technique. Fourier transform infrared spectra analysis was performed to ascertain the simultaneous reduction and functionlization of GO. Field emission scanning electron microscopy analysis was performed to ensure the homogenous distribution and dispersion of DA-G in LLDPE matrix. The enhanced storage modulus value of the composites validates the homogenous dispersion of DA-G and its good interfacial interaction with LLDPE matrix. An increased in tensile strength value by ~ 64% also confirms the generation of good interface between the two constituents, through which efficient load transfer is possible. However, no significant improvement in glass transition temperature was observed. This simple technique of fabricating LLDPE composites following industrially viable melt mixing procedure could be realizable to developed mechanically strong graphene based LLDPE composites for future applications.

• Journal of Mechanical Science and Technology
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• 제18권8호
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• pp.1271-1287
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• 2004

In this article, vapor bubble nucleation in liquid and the evaporation process of a liquid droplet at its superheat limit were discussed from the viewpoint of molecular clustering (molecular cluster model for bubble nucleation). For the vapor bubble formation, the energy barrier against bubble nucleation was estimated by the molecular interaction due to the London dispersion force. Bubble nucleation by quantum tunneling in liquid helium under negative pressure near the absolute zero temperature and bubble nucleation on cavity free micro heaters were also presented as the homogenous nucleation processes.

• Coupled systems mechanics
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• 제5권3호
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• pp.203-221
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• 2016

The present study deals with the numerical modelling for the one dimensional contaminant transport through saturated homogeneous and stratified porous media using meshfree method. A numerical algorithm based on element free Galerkin method is developed. A one dimensional form of the advectivediffusive transport equation for homogeneous and stratified soil is considered for the analysis using irregular nodes. A Fortran program is developed to obtain numerical solution and the results are validated with the available results in the literature. A detailed parametric study is conducted to examine the effect of certain key parameters. Effect of change of dispersion, velocity, porosity, distribution coefficient and thickness of layer is studied on the concentration of the contaminant.

• 한국재료학회지
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• 제25권8호
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• pp.386-390
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• 2015

Fe-base superalloy powders with $Y_2O_3$ dispersion were prepared by high energy ball milling, followed by spark plasma sintering for consolidation. High-purity elemental powders with different Fe powder sizes of 24 and 50 mm were used for the preparation of $Fe-20Cr-4.5Al-0.5Ti-O.5Y_2O_3$ powder mixtures (wt%). The milling process of the powders was carried out in a horizontal rotary ball mill using a stainless steel vial and balls. The milling times of 1 to 5 h by constant operation (350 rpm, ball-to-powder ratio of 30:1 in weight) or cycle operation (1300 rpm for 4 min and 900 rpm for 1 min, 15:1) were applied. Microstructural observation revealed that the crystalline size of Fe decreased with an increase in milling time by cyclic operation and was about 15 nm after 3 h, forming a FeCr alloy phase. The cyclic operation had an advantage over constant milling in that a smaller-agglomerated structure was obtained. The milled powders were sintered at $1100^{\circ}C$ for 30 min in vacuum. With an increase in milling time, the sintered specimen showed a more homogeneous microstructure. In addition, a homogenous distribution of Y-compound particles in the grain boundary was confirmed by EDX analysis.

• 한국재료학회지
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• 제27권8호
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• pp.445-450
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• 2017

YSZ (Yttria-stabilized zirconia) is a ceramic material that is used for electronic and structural materials due to its excellent mechanical properties and specific electrical characteristics according to the Yttrium addition. Hydrothermal synthesis has several advantages such as fine particle size, uniform crystalline phase, fast reaction time, low process temperature and good dispersion condition. In order to synthesize YSZ nanoparticles with high crystallinity, hydrothermal synthesis was performed at various concentrations of NaOH. The hydrothermal process was held at a low temperature ($100^{\circ}C$), with a short process time (2,4,8 hours); the acidity or alkalinity of solution was controlled in a range of pH 2~12 by addition of NaOH. The optimum condition was found to be pH 12, at which high solubility levels of Y(OH) and Zr(OH) were reported. The synthesized nano powder showed high crystallinity and homogenous composition, and uniform particle size of about 10 nm.

• 대한기계학회논문집
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• 제14권3호
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• pp.581-593
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• 1990

본 연구에서는 교반기술에 의하여 얻어진 반응고상태의 금속에 단섬유를 첨가 하여 복합재료를 제조하였다. 그리고 제조되어진 복합재료에 있어서 섬유의 분산상 태및 기지재와의 접합관계를 조사하여 압연가공에 필요한 반응고상태인 금속복합재료 의 제조방법을 확립하였다. 균일하게 분실되어진 반용융상태의 단섬유강화형 금속복 합재료를 직접 압연하여 박판을 제조할 수 있는 가능성을 검토하였으며, 또한 제조되 어진 박판의 인장시험에 의하여 기계적 성질을 조사하였다.

• 대한비뇨기종양학회지
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• 제15권3호
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• pp.178-186
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• 2017

Purpose: Poloxamer 407 (P407) thermo-sensitive hydrogel formulations were developed to enhance the retention time in the urinary bladder after intravesical instillation. Materials and Methods: P407 hydrogels (P407Gels) containing 0.2 w/w% fluorescein isothiocyanate dextran (FD, MW 4 kDa) as a fluorescent probe were prepared by the cold method with different concentrations of the polymer (20, 25, and 30 w/w%). The gel-forming capacities were characterized in terms of gelation temperature (G-Temp), gelation time (G-Time), and gel duration (G-Dur). Homogenous dispersion of the probe throughout the hydrogel was observed by using fluorescence microscopy. The in vitro bladder simulation model was established to evaluate the retention and drug release properties. P407Gels in the solution state were administered to nude mice via urinary instillation, and the in vivo retention behavior of P407Gels was visualized by using an in vivo imaging system (IVIS). Results: P407Gels showed a thermo-reversible phase transition at $4^{\circ}C$ (refrigerated; sol) and $37^{\circ}C$ (body temperature; gel). The G-Temp, G-Time, and G-Dur of FD-free P407Gels were approximately $10^{\circ}C-20^{\circ}C$, 12-30 seconds, and 12-35 hours, respectively, and were not altered by the addition of FD. Fluorescence imaging showed that FD was spread homogenously in the gelled P407 solution. In a bladder simulation model, even after repeated periodic filling-emptying cycles, the hydrogel formulation displayed excellent retention with continuous release of the probe over 8 hours. The FD release from P407Gels and the erosion of the gel, both of which followed zero-order kinetics, had a linear relationship ($r^2=0.988$). IVIS demonstrated that the intravesical retention time of P407Gels was over 4 hours, which was longer than that of the FD solution (<1 hour), even though periodic urination occurred in the mice. Conclusions: FD release from P407Gels was erosion-controlled. P407Gels represent a promising system to enhance intravesical retention with extended drug delivery.

• 헤리티지:역사와 과학
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• 제44권1호
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• pp.72-105
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• 2011

본 연구는 전북 완주군 동상면 대아리 산1-2번지의 도유림인 속칭 가막골 금낭화군락지의 천연보호구역적 가치를 검토하는 한편 금낭화군락지의 적정 생육환경을 구명하고 이를 바탕으로 금낭화군락지의 보전을 위한 기초자료를 제공할 목적으로 시도 되었다. 이를 위해 문헌 및 인터넷조사를 통해 '금낭화의 형태적 생태적 특성'과 '국내 천연기념물 초본군락의 지정현황 및 문제점'을 살펴보는 한편 현지조사를 통해 금낭화군락의 광환경, 토양환경, 식생조건 등을 조사 분석하여 가막골 금낭화군락지의 천연보호구역적 가치를 파악하고자 한 연구 결과는 아래와 같다. 국내 금낭화 자생군락지를 분석한 결과, 금낭화는 전국의 산지에 고루 분포되고 있으며 이 중 완주 가막골 금낭화군락지의 서식규모는 매우 큰 것으로 나타났다. 또한 금낭화군락지 분포에 대한 문헌 및 현지조사 결과, 준고냉지에서 생육이 월등하며, 생육왕성기인 5월의 풍부한 강수량은 금낭화 생육에 매우 필수적인 것으로 보아 가막골 금낭화군락지는 매우 전형적 기상여건을 보이는 서식지라 판단된다. 한편 금낭화 고밀도분포 군락의 환경 분석 결과, 전석지(轉石地) 토양을 기반으로 고도 300~375m의 범위에 있었으며 평균고도는 344.5m이고 자생지의 좌향(坐向)은 북향 또는 북서향도 포함되나 북동향(NE)이 압도적이었으며 평균 경사는 $19.5^{\circ}$로 나타났다. 또한 광조건 분석 결과, 4월부터 8월까지 생육기간인 5개월간 군락 내 광도는 평균 30,810Lux 이었으며, 생육 초기에는 낮았고 후기로 갈수록 광도는 상승하는 경향이었다. 금낭화 자생조건은 4~5월 생육기에는 약 14,000~18,000Lux 정도의 광조건이었으며, 개화 이후에는 상대적으로 광도 증가와는 관련 없이 결실이 이루어지는 것으로 판단된다. 토양환경 분석 결과, 토양산도(pH)는 표토, 5.2~6.1, 심토, 5.2~6.2로 거의 유사한 범위를 보였으며 표토의 평균 토양pH는 5.54, 심토는 5.58로 이 조건은 비교대상 군락지와 거의 유사하고 전형성이 큰 것으로 추정됨에 따라 가막골의 토양 안정성은 매우 높다고 판단된다. 식생환경 분석 결과, 조사구별 금낭화의 개체수는 13~52개체까지 심한 편차를 보였으나 군락내 발견된 분류군(taxa)은 총 126종류로 순수군락을 유지하면서도 높은 종다양성이 확인되었다. 그리고 대아수목원 금낭화군락지는 금낭화, 미나리냉이, 거북꼬리, 이삭여귀, 물봉선 등이 우점하는 식물사회로 확인되었으며 군락내 식별종(Differential species)은 물봉선, 꼭두서니, 칡, 산딸기와 고추나무, 광대싸리, 개다래로 밝혀짐에 따라 지형 및 토양습도에 따라 전형적 하위군락으로 구분되는 것으로 판단된다. 이와 같은 결과를 종합할 때, 완주군 동상면 대아리 가막골 금낭화군락지는 탁월한 관상성을 가진 자생식물의 서식처임을 확인할 수 있고, 본 군락은 전국분포상의 금낭화 자생군락으로는 국내 최대 규모로 추정되며 기상 및 지형, 토양조건과 식생환경이 군락지의 지속적인 생육 환경을 보장할 수 있는 매우 탁월한 서식지로 판단된다. 따라서 전형적 서식조건과 지속가능성이 큰 가막골 금낭화군락은 당연히 천연보호구역적 가치가 매우 큰 것으로 사료된다.