• 한국재료학회지
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• 제27권1호
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• pp.39-42
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• 2017

E-glass (electrical glass) fiber is the widely used as a reinforced composite material of PCBs (printed circuit boards). However, E-glass fiber is not stable because it has a dielectric constant of 6~7. On the other hand, D-glass (dielectric glass) fiber has a low dielectric constant of 3~4.5. Thus, it is adaptable for use as a reinforcing material of PCBs. In this study, we fabricated D-glass compositions with low dielectric constant, and measured the electrical and optical properties. In the glass composition, the boron content was changed from 9 to 31 wt%. To confirm the dependence of the dielectric constant on melting properties, D-glass with 22 wt% boron was melted at $1550^{\circ}C$ and $1650^{\circ}C$ for 2hrs. The glass melted at $1650^{\circ}C$ had a lower dielectric constant than the glass melted at $1550^{\circ}C$. Therefore, the D-glass with boron of 9~31 wt% was fabricated by melting at $1650^{\circ}C$ for 2hrs, and transparent clear glass was obtained. We identified the non-crystalline nature of the glass using an XRD (x-ray diffractometer) graph. The visible light transmittance values depending on the boron contents were measured and found to be 88.6 % ~ 82.5 %. Finally, the dielectric constant of the D-glass with 31 wt% boron was found to have decreased from 4.18 to 3.93.

• Elastomers and Composites
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• 제48권3호
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• pp.216-220
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• 2013

본 연구에서는 목분-고분자 복합체(Wood Flour-Polymer Composite, WPC)의 상분리 특성을 조사하고 이를 토대로 목분함량에 따른 WPC의 기계적 물성 변화에 관한 원인을 규명하였다. 이를 위해 고분자 매트릭스로 폴리프로필렌을 사용하여 서로 다른 함량의 목분을 갖는 시편을 제조하였다. 서로 다른 목분 함량을 갖는 시편들의 DSC thermogram으로부터 목분 함량에 따른 고분자 PP의 결정화 경향과 $T_m$ 경향을 분석하였다. 목분 함량이 증가함에 따라 고분자 PP의 결정량 및 $T_m$값 모두 감소하다 다시 증가하는 결과를 나타내었다. 이로부터 낮은 목분 함량에서는 목분이 매트릭스인 고분자 내에 분산되어 있으나 일정 목분 함량이상에서는 WPC의 매트릭스인 PP와 목분사이에 상분리가 일어남을 확인할 수 있었다. 한편, WPC 시편의 인장강도는 목분 함량이 증가할수록 감소하는 결과를 나타내었다. 낮은 목분 함량에서는 고분자에 분산된 목분과 고분자 사이에서의 낮은 계면 결합력과 시편내에서 강도를 높이는 역할을 하는 결정량의 감소가, 높은 목분 함량에서는 PP와 목분사이에서의 상분리로 인한 계면 결함이 WPC 시편의 인장강도를 목분함량 증가에 따라 지속적으로 감소시키는 원인이 되는 것으로 파악되었다. WPC 시편의 충격강도는 목분 함량이 증가함에 따라 증가하다가 목분 함량이 20%인 경우 최고값을 나타내며, 그 이후 다시 감소하는 경향을 나타내었는데, 낮은 목분 함량에서는 목분 함량이 증가할수록 시편의 결정량이 감소하므로 시편의 취성 감소에 의해 충격강도가 향상되나, 높은 목분 함량에서는 고분자 매트릭스와 목분 사이의 상분리로 인해 다시 충격강도가 저하되기 때문인 것으로 판단된다.

• Elastomers and Composites
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• 제37권2호
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• pp.79-85
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• 2002

2몰% 에틸가지를 포함하는 에틸 가지화 폴리에틸렌[PE(2)]과 에틸렌-프로필렌 몰비가 50:50 으로 같지만 입체규칙성이 다른 랜덤-에틸렌-프로필렌 고무(r-EPR), 교호-에틸렌-프로필렌고무(alt-EPR) 및 이소탁틱-교호-에틸렌-프로필렌 고무(iso-alt-EPR)를 에틸렌-프로필렌 고무의 입체규칙성의 차이에 대한 혼화성과 물성의 차이를 조사하기 위하여 혼합하였다. 혼합된 블렌드의 결정화도는 고무상 EPR 성분의 증가에 따라 감소되었으며, EPR의 입체규칙성이 작을수록, 혼합조성의 증가할수록 감소하였다. 열역학적 interaction parameter(x) 값은 3가지 블렌드 모두 거의 영에 가까운 값을 나타내어 본 블렌드계는 PE(2)의 녹는점 가까이 또는 그 이상에서 상호간 섞일 것으로 판단되었다. 녹는점($T_m$)과 결정화온도($T_c$)의 측정에서부터 세가지 블렌드는 혼합조성과 입체규칙성에 관계없이 직선관계를 보여주어 이 블렌드계의 PE(2)의 용융거동은 주로 희석효과(diluent effect)에 기인함을 알 수 있었다. Small angle light scattering(SALS) 방법에 의한 PE(2)의 spherulite의 크기는 세가지 블렌드계가 EPR의 혼합비 증가에 따라 증가하였으며, 같은 혼합비에서는 r-EPR>alt-EPR>iso-alt-EPR> 순서로 증가하였다.

• Bulletin of the Korean Chemical Society
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• 제30권11호
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• pp.2691-2696
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• 2009

Effects of representative group II and transition metal ions on the stability of the $poly(dA){\cdot}[poly(dT)]_2$ triplex were investigated by the van’t Hoff plot constructed from a thermal melting curve. The transition, $poly(dA){\cdot}[poly(dT)]_2\;{\rightarrow}\;poly(dA){\cdot}poly(dT)\;+\;poly(dT)$, was non-spontaneous with a positive Gibb’s free energy, endothermic (${\Delta}H^{\circ}$ > 0), and had a favorable entropy change (${\Delta}S^{\circ}$ > 0), as seen from the negative slope and positive y-intercept in the van’t Hoff plot. Therefore, the transition is driven by entropy change. The $Mg^{2+}$ ion was the most effective at stabilization of the triplex, with the effect decreasing in the order of $Mg^{2+}\;>\;Ca^{2+}\;>\;Sr^{2+}\;>\;Ba^{2+}$. A similar stabilization effect was found for the duplex to single strand transition: $poly(dA){\cdot}poly(dT)\;+\;poly(dT)\;→\;poly(dA)\;+\;2poly(dT)$, with a larger positive free energy. The transition metal ions, namely $Ni_{2+},\;Cu_{2+},\;and\;Zn_{2+}$, did not exhibit any effect on triplex stabilization, while showing little effect on duplex stabilization. The different effects on triplex stabilization between group II metal ions and the transition metal ions may be attributed to their difference in binding to DNA; transition metals are known to coordinate with DNA components, including phosphate groups, while group II metal ions conceivably bind DNA via electrostatic interactions. The $Cd_{2+}$ ion was an exception, effectively stabilizing the triplex and melting temperature of the third strand dissociation was higher than that observed in the presence of $Mg_{2+}$, even though it is in the same group with $Zn_{2+}$. The detailed behavior of the $Cd_{2+}$ ion is currently under investigation.

• 한국재료학회지
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• 제28권11호
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• pp.663-670
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• 2018

H13 tool steels are widely used as metallic mold materials due to their high hardness and thermal stability. Recently, many studies are undertaken to satisfy the demands for manufacturing the complex shape of the mold using a 3D printing technique. It is reported that the mechanical properties of 3D printed materials are lower than those of commercial forged alloys owing to micropores. In this study, we investigate the effect of microstructures and defects on mechanical properties in the 3D printed H13 tool steels. H13 tool steel is fabricated using a selective laser melting(SLM) process with a scan speed of 200 mm/s and a layer thickness of $25{\mu}m$. Microstructures are observed and porosities are measured by optical and scanning electron microscopy in the X-, Y-, and Z-directions with various the build heights. Tiny keyhole type pores are observed with a porosity of 0.4 %, which shows the lowest porosity in the center region. The measured Vickers hardness is around 550 HV and the yield and tensile strength are 1400 and 1700 MPa, respectively. The tensile properties are predicted using two empirical equations through the measured values of the Vickers hardness. The prediction of tensile strength has high accuracy with the experimental data of the 3D printed H13 tool steel. The effects of porosities and unmelted powders on mechanical properties are also elucidated by the metallic fractography analysis to understand tensile and fracture behavior.

• Design & Manufacturing
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• 제14권1호
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• pp.49-55
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• 2020

In this study, the fatigue behavior and fatigue life characteristics of PA2200 specimens fabricated by SLS 3D printer were studied. Fatigue tests were performed according to the standard specification (ASTM E468) and fatigue life curves were obtained. In order to perform the fatigue test, mechanical properties were measured according to the test speed of the simple tensile test, and the self-heating temperature of the specimen according to the test speed was measured using an infrared temperature measuring camera in consideration of heat generation due to plastic deformation. There was no significant difference within the set test speed range and the average self-heating temperature was measured at 38.5 ℃. The mechanical strength at the measured temperature showed a relatively small difference from the mechanical strength at room temperature. Fatigue test conditions were established through the preceding experiments, and the loading conditions below the tensile strength at room temperature 23 ℃ were set as the cyclic load. The maximum number of replicates was less than 100,000 cycles, and the fracture behavior of the specimens with the repeated loads showed the characteristics of Racheting. It was confirmed that SLS 3D printing PA2200 material could be applied to the Basquin's S-N diagram for the fatigue life curve of metal materials. SEM images of the fracture surface was obtained to analyze the relationship between the characteristics of the fracture surface and the number of repetitions until failure. Brittle fracture, crazing fracture, grain melting, and porous fracture surface were observed. It was shown that the larger the area of crazing damage, the longer the number of repetitions until fracture.

• Journal of Pharmaceutical Investigation
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• 제38권4호
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• pp.241-247
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• 2008

Paclitaxel is a taxane diterpene amide, which was first extracted from the stem bark of the western yew, Taxus brevifolia. This natural product has proven to be useful in the treatment of a variety of human neoplastic disorders, including ovarian cancer, breast and lung cancer. Paclitaxel is a highly hydrophobic drug that is poorly soluble in water. It is mainly given by intravenous administration. Therefore, The pharmaceutical formulation of paclitaxel ($Taxol^{(R)}$; Bristol-Myers Squibb) contains 50% $Cremophor^{(R)}$ EL and 50% dehydrated ethanol. However the ethanol/Cremophor EL vehicle required to solubilize paclitaxel in $Taxol^{(R)}$ has a pharmacological and pharmaceutical problems. To overcome these problems, new formulations for paclitaxel that do not require solubilization by $Cremophor^{(R)}$ EL are currently being developed. Therefore this study utilized a supercritical fluid antisolvent (SAS) process for cremophor-free formulation. To select hydrophilic polymers that require solubilization for paclitaxel, we evaluated polymers and the ratio of paclitaxel/polymers. HP-${\beta}$-CD was used as a hydrophilic polymer in the preparation of the paclitaxel solid dispersion. Although solubility of paclitaxel by polymers was increased, physical stability of solution after paclitaxel/polymer powder soluble in saline was unstable. To overcome this problem, we investigated the use of surfactants. At 1/20/40 of paclitaxel/hydrophilic polymer/ surfactant weight ratio, about 10 mg/mL of paclitaxel can be solubilized in this system. Compared with the solubility of paclitaxel in water ($1\;{\mu}g/mL$), the paclitaxel solid dispersion prepared by SAS process increased the solubility of paclitaxel by near 10,000 folds. The physicochemical properties was also evaluated. The particle size distribution, melting point and amophorization and shape of the powder particles were fully characterized by particle size distribution analyzer, DSC, SEM and XRD. In summary, through the SAS process, uniform nano-scale paclitaxel solid dispersion powders were obtained with excellent results compared with $Taxol^{(R)}$ for the physicochemical properties, solubility and pharmacokinetic behavior.

• 한국세라믹학회지
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• 제49권4호
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• pp.318-324
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• 2012

Reaction-bonded silicon nitrides containing rare-earth oxide sintering additives were densified by gas pressure sintering. The sintering behavior, microstructure and mechanical properties of the resultant specimens were analyzed. For that purpose, $Lu_2O_3-SiO_2$ (US), $La_2O_3$-MgO (AM) and $Y_2O_3-Al_2O_3$ (YA) additive systems were selected. Among the tested compositions, densification of silicon nitride occurred at the lowest temperature when using the $La_2O_3$-MgO system. Since the $Lu_2O_3-SiO_2$ system has the highest melting temperature, full densification could not be achieved after sintering at $1950^{\circ}C$. However, the system had a reasonably high bending strength of 527 MPa at $1200^{\circ}C$ in air and a high fracture toughness of 9.2 $MPa{\cdot}m^{1/2}$. The $Y_2O_3-Al_2O_3$ system had the highest room temperature bending strength of 1.2 GPa.

• 한국결정성장학회지
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• 제9권2호
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• pp.149-156
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• 1999

초크랄스키 결정성장계의 산소농도에 영향을 주는 유동거동에 대한 정보를 얻기 위해 저온모델을 이용하여 실험적으로 초크랄스키 멜트에 내의 유속을 측정하였다. 실리콘 멜트와 유사한 프란틀(Pr) 수를 갖는 저 융점의 Woods metal을 작동유체로 채택하였다. 전기 전도성을 갖는 유체에서 속도 측정이 가능한 일체형 자석 프로우브(Incorporated magnet probe)를 제작하여 멜트 내부의 여러 지점에서 유속을 3차원저긍로 측정하였다. 측정 결과 관찰된 속도장은 자연대류가 지배적이며 비축대칭적인 유동양상을 나타내었다. 또한 멜트의 두 지점에서 동시에 측정된 온도 데이터로부터 상관계수 및 도가니 회전에 의한 온도 wave의전파를 분석한 결과 상관계수의 크기는 기존의 소형 실리콘 멜트의 연구에서 구한 값보다 작게 나타났으며 이러한 현상은 규모가 큰 멜트의 유동은 난류인 거동이 더 강해지기 때문에 발생하는 것으로 파악되었다.

• 한국에너지공학회:학술대회논문집
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• 한국에너지공학회 2002년도 춘계 학술발표회 논문집
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• pp.227-234
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• 2002

With the increasing environmental consideration and stricter regulations, gasification of waste is considered to be more attractive technology than conventional incineration for energy recovery as well as material recycling. The experiment for combustible waste mixed with plastic and cellulosic materials was performed in the fixed bed gasifier to investigate the gasification behavior with the operating conditions. Waste pelletized with a diameter of 2~3cm and 5cm of length was gasified at the temperature range of 1100~145$0^{\circ}C$. It was shown that the composition of H$_2$ was in the range of 30~40% and CO 15~30% depending upon oxygen/waste ratio. Casification of waste due to thermoplastic property from mixed plastic melting and thermal cracking shows a prominent difference from that of coal or coke. It was desirable to maintain the top temperature up to foot to ensure the mass transfer and uniform reaction through the packed bed. As the bed height was increased, the formation of H$_2$ and CO was increased whilst $CO_2$ decreased by the char-$CO_2$ reaction and plastic cracking. From the experimental results, the cold gas efficiency was around 61% and heating values of product gases were in the range of 2800~3200㎉/Nm3.