• 열처리공학회지
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• 제17권5호
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• pp.283-292
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• 2004

The effects of the annealing parameters on microstructures were examined in a cold-rolled high strength steel containing 0.1% C, 0.5% Si, 1.5% Mn, and 0.04% Nb. It was impossible to avoid martensite in the microstructure even though the continuous annealing parameters were controlled. This indicates that the alloying elements such as silicon and manganese contributing to manganese equivalent($Mn_{eq}$) should be reduced to produce the ferrite-pearlite microstructure for the solid solution and precipitation hardened steel. It was found that a decrease in the rapid cooling temperature to $520^{\circ}C$ was effective to change the microstructure from ferrite-martensite to ferrite-pearlite-martensite. Typical dual-phase properties exhibiting a low yield ratio and a continuous yielding behavior were obtained when the rapid cooling temperature was in the range of $680^{\circ}C$ to $600^{\circ}C$. The critical volume fraction of martensite for the typical properties of dual-phase steel was about 11 percent.

• 청정기술
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• 제24권2호
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• pp.112-118
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• 2018

온도 변화에 따라 상 전이를 나타내는 열 감응성 고분자는 외부 온도 감응으로 태양광 투과 조절이 가능하므로 스마트 윈도우용 소재로 적용 가능하다. 넒은 온도 범위에서 사용 가능한 스마트 윈도용 열감응성 고분자의 개발은 바람직하다. 고 성능스마트 윈도우용 소재를 얻기 위하여, 단량체 N-isopropylacrylamide, 가교제 N, N'-methylenebisacrylamide (MBAm), 산화개시제 ammonium persulfate (APS)/촉매 tetramethylene diamine 및 혼합용매(물/글리세롤)을 사용하여 3차원의 열감응성(thermoresponsive) poly(N-isopropylacrylamide) (PNIPAm) 겔을 제조하였다. 본 연구에서는 혼합용매 중의 글리세롤의 함량이 가교된 PNIPAm 겔 필름의 하한임계온도(low critical solution temperature, LCST), 어는점 및 태양광의 투광도에 미치는 영향을 조사하였다. 글리세롤 함량이 0 wt%에서 10 wt%로 증가하면 PNIPAm 겔 필름의 LCST/어는점은 각각 $34.3/6.3^{\circ}C$에서 $28.2/-6.5^{\circ}C$로 감소함을 알 수 있었다. LCST보다 낮은 $25^{\circ}C$에서는 본 연구에서 합성한 모든 PNIPAm 겔 필름은 투명(광 투과)하지만 LCST보다 높은 $45^{\circ}C$에서는 불투명하다는 것을 알 수 있었다. 이러한 결과는 본 연구에서 합성한 PNIPAm 겔 소재는 $-6.5^{\circ}C$ 부근에서도 스마트 윈도우용 소재로 활용할 가능성이 높음을 알 수 있다.

• 한국세라믹학회지
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• 제55권6호
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• pp.527-555
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• 2018

High-temperature thermal insulation materials challenge extensive oxide candidates such as porus $SiO_2$, $Al_2O_3$, yttria-stabilized zirconia, and mullite, due to the needs of good mechanical, thermal, and chemical reliabilities at high temperatures simultaneously. Recently, porous rare earth (RE) silicates have been revealed to be excellent thermal insulators in harsh environments. These materials display attractive properties, including high porosity, moderately high compressive strength, low processing shrinkage (near-net-shaping), and very low thermal conductivity. The current critical challenge is to balance the excellent thermal insulation property (extremely high porosity) with their good mechanical properties, especially at high temperatures. Herein, we review the recent developments in processing techniques to achieve extremely high porosity and multiscale strengthening strategy, including solid solution strengthening and fiber reinforcement methods, for enhancing the mechanical properties of porous RE silicate ceramics. Highly porous RE silicates are highlighted as emerging high-temperature thermal insulators for extreme environments.

• 대한기계학회논문집B
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• 제29권12호
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• pp.1291-1298
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• 2005

Since the interior shape of a pressure regulator is complex and the change of fluid resistance at each operation condition is rapid and big, the pressure regulator can become the major factor that causes big loss in pipelines. So the suitable pressure regulator modeling by each operation condition is important to obtain reliable results especially in small scale pipeline network analysis. And in order to prevent the condensation and freezing problems, it is needed to confirm both whether temperature recovery is achieved after passing by the pressure regulator's narrow neck and how much amount of low temperature area that can cause condensate accumulation is distributed by various PCV models at every inlet-outlet pressure ratio. In this research, the numerical model resembling P company pressure regulator that is used widely for high pressure range in commercial, is adopted as the base model of CFD analysis to investigate pressure regulator's flow characteristics at each pressure ratio. Additionally it is also introduced to examine pressure regulator's critical flow characteristics and possibility of condensation or freezing at each pressure ratio. Furthermore, the comparison between the results of CFD analysis and the results of analytic solution obtained by compressible fluid-dynamics theory is attempted to validate the results of CFD modeling in this study and to estimate the accuracy of theoretical approach at each pressure ratio too.

• 대한기계학회논문집B
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• 제29권12호
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• pp.1299-1306
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• 2005

The suitable pressure regulator modeling at each opening ratio and pressure ratio is very important to obtain reliable results, especially in small scale pipeline network analysis such as a pressure regulator system. And it is needed to confirm both whether temperature recovery is achieved after passing by the pressure regulator's narrow neck and how much amount of low temperature area that can cause condensate accumulation is distributed by various PCV models and driving conditions. In this research, the numerical model resembling P company pressure regulator that is used widely for high pressure range in commercial, is adopted as the base model of CFD analysis to investigate pressure regulator's flow characteristics at each pressure ratio and opening ratio. And it is also introduced to examine pressure regulator's critical flow characteristics and possibility of condensation or freezing at each pressure ratio and opening ratio. Additionally, the comparison between the results of CFD analysis and the results of analytic solution obtained by compressible fluid-dynamics theory is attempted to validate the results of CFD modeling in this study and to estimate the accuracy of theoretical approach at each pressure ratio and opening ratio too.

• 한국전기전자재료학회:학술대회논문집
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• 한국전기전자재료학회 2003년도 추계학술대회 논문집 Vol.16
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• pp.587-590
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• 2003

The first attention in designing a transformer for low temperature rise should be to reduce losses. Leakage inductance and temperature rise are two of the more impotent problems facing the magnetic core technology of today's high frequency transformers. Excessive leakage inductance increases the stress on the switching transistors and limits the duty-cycle, and excessive temperature rise can lead the design limitation of high frequency transformer with high current. The flat transformer technology provides a very good solution to the problems of leakage inductance and thermal management for high frequency power. The critical magnetic components and windings are optimized and packaged within a completely assembled module. The turns ratio in a flat transformer is determined as the product of the number of elements or modules times the number of primary turns. The leakage inductance increase proportionately to the number of elements, but since it is reduced as the square of the turns, the net reduction can be very significant. The flat transformer modules use cores which have no gap. This eliminates fringing fluxes and stray flux outside of the core. The secondary windings are formed of flat metal and are bonded to the inside surface of the core. The secondary winding thus surrounds the primary winding, so nearly all of the flux is captured.

• 한국전기전자재료학회논문지
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• 제24권2호
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• pp.156-161
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• 2011

The present study introduces a novel wet etching technique for nanostructure fabrications which usually requires low surface roughness. Using the current method, acquired profiles were smooth even in the nanoscale, which cannot be easily achieved with conventional wet or dry etching methods. As one of the most popular single crystal silicon etchant, potassium hydroxide (KOH) solution was used as a base solvent and two additives, antimony trioxide (Sb2O3) and ethyl alcohol were employed in. Four experimental parameters, concentrations of KOH, Sb2O3, and ethyl alcohol and temperature were optimized at 60 wt.%, 0.003 wt.%, 10 v/v%, and $23^{\circ}C$, respectively. Effects of additives in KOH solution were investigated on the profiles in both (110) and (111) planes of single crystal silicon wafer. The preliminary results show that additives play a critical role to decrease etch rate significantly down to ~2 nm/min resulting in smooth side wall profiles on (111) plane and enhanced surface roughness.

• 공업화학
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• 제16권3호
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• pp.397-402
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• 2005

Core-shell 형의 Poly(styrene-co-N-isopropylacrylamide) (poly(St-co-NIPAm)) 마이크로겔은 과항상칼륨(KPS)을 개시제로 사용하여 styrene (St)과 N-isopropylacrylamide (NIPAm)의 무유화 유화중합으로 제조하였다. St/NIPAm의 비, 모노머의 농도, 가교제의 농도에 따른 입자크기의 영향을 조사하였으며 고분자 마이크로겔의 온도에 따른 열감응성을 조사하였다. 마이크로겔의 입자 크기는 NIPAm의 몰비에 따라 증가하였으며 마이크로겔 분산용액의 투과도가 하한임계온도 이상($32{\sim}34^{\circ}C$)에서는 급격히 감소였다. 수용액 중에서 마이크로겔의 팽윤성은 모노머인 NIPAm의 농도가 증가할수록 증가하였으며 가교제의 농도에 따라서 감소하였다.

• Macromolecular Research
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• 제17권4호
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• pp.265-270
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• 2009

Noble thermosensitive nanoparticles, based on a PNIPAAm-co-AA core and a chitosan shell structure, were designed and synthesized for the controlled release of the loaded drug. PNIPAAm nanoparticles containing a carboxylic group on their surface were synthesized using emulsion polymerization. The carboxylic groups were conjugated with the amino group of a low molecular weight, water soluble chitosan. The particle size of the synthesized nanoparticles was decreased from 380 to 25 nm as the temperature of the dispersed medium was increased. Chitosan-conjugated nanoparticles with $2{\sim}5$ wt% MBA, a crosslinking monomer, induced a stable aqueous dispersion at a concentration of 1mg/1mL. The chitosan-conjugated nanoparticles showed thermo sensitive behaviors such as LCST and size shrinkage that were affected by the PNIPAAm core and induced some particle aggregation around LCST, which was not shown in the NIPAAm-co-AA nanoparticles. These chitosan-conjugated nanoparticles are also expected to be more biocompatible than the PNIPAAm core itself through the chitosan shell structures.

• Bulletin of the Korean Chemical Society
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• 제35권10호
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• pp.3001-3004
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• 2014

A series of hydroxyl-derivatized quaternized polymers were successfully synthesized by atom transfer radical polymerization (ATRP) and Cu(I)-catalyzed 1,3-dipolar cycloaddition of azide and alkynes (click chemistry), followed by quaternization reactions. ATRP was employed to synthesize poly(2-hydroxyethyl methacrylate) (PHEMA), followed by introduction of alkyne groups using pentynoic acid, leading to HEMA-Alkyne. 2-Azido-1-ethanol and 3-azido-1-propanol were combined with the HEMA-Alkyne backbone via click reaction, resulting in triazole-ring containing hydroxyl-derivatized polymers. Quaternization reactions with methyl iodide were conducted on the triazole ring of each polymer. Molecular weight, molecular weight distribution, and the degree of quaternization (DQ) were determined by gel permeation chromatography (GPC) and $^1H$ NMR spectroscopy. The average molecular weight ($M_n$) of the resulting polymers ranged from $5.9{\times}10^4$ to $1.05{\times}10^5g/mol$ depending on the molecular architecture. The molecular weight distribution was low ($M_w/M_n$ = 1.26-1.38). The transmission spectra of the 0.1 wt % aqueous solutions of the resulting quaternized polymers at 650 nm were measured as a function of temperature. Results showed that the upper critical solution temperature (UCST) could be finely controlled by the level of DQ.