• Bulletin of the Korean Chemical Society
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• v.16 no.3
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• pp.238-243
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• 1995

A pulsed molecular beam source having short pulse duration (typically 70 ${\mu}s)$ and narrow velocity distribution (${\Delta}$v/v=8% for helium) has been costructed utilizing a commercial fuel injector. Beam characteristics of helium and ammonia seeded in helium expansions are accomplished by the use of an electron impact time-or-flight mass spectrometer. The comparisons between experimental data and theoretical calculations show that the proper beam speed is important to predict the evolution of stream temperature and valve shutter function. The decreasing tendency of pulse duration with increasing cluster size leads to the conclusion that the cluster beam property is described as a function of cluster mass and disinct cluster temperature.

• Journal of the Institute of Electronics Engineers of Korea SD
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• v.44 no.12
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• pp.1-6
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• 2007

In this study, we have performed the channel modification of the conventional MHEMT (metamorphic high electron mobility transistor) to improve the breakdown characteristics. The Modified channel consists of the InxGal-xAs channel and the InP sub channel instead of the InxGa1-xAs channel. Since InP has the lower impact ionization coefficient in comparison with In0.53Ga0.47As, we have adopted the InP-composite channel in the modified MHEMT. We have investigated the breakdown mechanism and the RF characteristics for the conventional and the InP- composite channel MHEMTs. From the measurement results, we have obtained the enhanced on and off-state breakdown voltages of 2.4 and 5.7 V, respectively. Also, the increased RF characteristics have brought about the decreased output conductance for the InP-composite channel MHEMT. The cut-off frequency (fT) and the maximum oscillation frequency (fmax) for the InP-composite Channel MHEMT were 160 GHz and 230 GHz, respectively. It has been shown that the InP-composite channel MHEMT has the potential applications for the millimeter wave power device.

• Journal of the Microelectronics and Packaging Society
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• v.15 no.4
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• pp.65-70
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• 2008

The mechanical shear strength of BGA(Ball Grid Array) solder joints under high impact loading was investigated. The Sn-37Pb solder balls with a diameter of $500{\mu}m$ were placed on the pads of FR-4 substrates with ENIG(Electroless Nickel Immersion Gold) surface treatment and reflowed. For the High Temperature Storage(HTS) test, the samples were aged a constant testing temperature of $120^{\circ}C$ for up to 250h. After the HTS test, high speed shear tests with various shear speed of 0.01, 0.1, 1, 3 m/s were conducted. $Ni_3Sn_4$ intermetallic compound(IMC) layer was observed at the solder/Ni-P interface and thickness of IMC was increased with aging process. The shear strength increased with increasing shear speed. The fracture surfaces of solder joints showed various fracture modes dependent on shear speed and aging time. Fracture mode was changed from ductile fracture to brittle fracture with increasing shear speed.

• Clean Technology
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• v.19 no.4
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• pp.401-409
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• 2013

BBiodegradable polymers have attracted great attention because of the increased environmental pollution by waste plastics. In this study, PLA (polylactic acid)/Clay-20 (Cloisite 20) and PLA (polylactic acid)/PBS (poly(butylene succinate)/Clay-20 (Cloisite 20) nanocomposites were manufactured in a twin-screw extruder. Specimens for mechanical properties of PLA/Clay-20 and PLA/PBS (90/10)/Clay-20 nanocomposites were prepared by injection molding. Thermal, mechanical, morphological and raman spectral properties of two nanocomposites were investigated by differential scanning calorimetry (DSC), tensile tester, scanning electron microscopy (SEM) and raman-microscope spectrophotometer, respectively. In addition, hydrolytic degradation properties of two nanocomposites were investigated by hydrolytic degradation test. It was confirmed that the crystallinity of PLA/Clay-20 and PLA/PBS/Clay-20 nanocomposite was increased with increasing Clay-20 content and the Clay-20 is miscible with PLA and PLA/PBS resin from DSC and SEM results. Tensile strength of two nanocomposites was decreased, but thier elongation, impact strength, tensile modulus and flexural modulus were increased with an increase of Clay-20 content. The impact strength of PLA/Clay-20 and PLA/PBS/Clay-20 nanocomposites with 5 wt% of Clay-20 content was increased above twice than that of pure PLA and PLA/PBS (90/10). The hydrolytic degradation rate of PLA/Clay-20 nanocomposite with 3 wt% of Clay-20 content was accelerated about twice than that of pure PLA. The reason is that degradation may occur in the PLA and Clay-20 interface easily because of hydrophilic property of organic Clay-20. It was confirmed that a proper amount of Clay-20 can improve the mechanical properties of PLA and can control biodegradable property of PLA.

• Analytical Science and Technology
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• v.17 no.4
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• pp.322-336
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• 2004

The improved derivatization technique of tamoxifen metabolite in human urine is described for the acylation method that they are substituted by derivatization reagent like acyl anhydride for use of gas chromatography/mass spectrometry. The hydroxyl group of tamoxifen metabolite was derivatized by trifluoroacetic anhydride (TFAA), pentafluoroacetic anhydride (PFPA) and heptaflorobutylic anhydride (HFBA). It was investigated to the gas chromatography/mass spectrometry (GC/MS) technique use negative ion chemical ionization (NCI), positive ion chemical ionization (PCI) and electron impact (EI). In acylation of the metabolites of tamoxifen, the effective reaction temperature and time were shown to be at $50^{\circ}C$ for 30 min. The 4-hydroxytamoxifen, which is known to major metabolite of tamoxifen, was not detected in human urine, whileas the hydroxymethoxytamoxifen was detected. We thought that this result was from the single dose of tamoxifen.

• Korean Journal of Materials Research
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• v.24 no.10
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• pp.520-525
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• 2014

Effects of Cu and B on effective grain size and low-temperature toughness of thermo-mechanically processed high-strength bainitic steels were investigated in this study. The microstructure of the steel specimens was analyzed using optical, scanning, and transmission electron microscopy; their effective grain size was also characterized by electron back-scattered diffraction. To evaluate the strength and low-temperature toughness, tensile and Charpy impact tests were carried out. The specimens were composed of various low-temperature transformation products such as granular bainite (GB), degenerated upper bainite (DUB), lower bainite (LB), and lath marteniste (LM), dependent on the addition of Cu and B. The addition of Cu slightly increased the yield and tensile strength, but substantially deteriorated the low-temperature toughness because of the higher volume fraction of DUB with a large effective grain size. The specimen containing both Cu and B had the highest strength, but showed worse low-temperature toughness of higher ductile-brittle transition temperature (DBTT) and lower absorbed energy because it mostly consisted of LB and LM. In the B-added specimen, on the other hand, it was possible to obtain the best combination of high strength and good low-temperature toughness by decreasing the overall effective grain size via the appropriate formation of different low-temperature transformation products containing GB, DUB, and LB/LM.

• Journal of Sensor Science and Technology
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• v.29 no.5
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• pp.324-327
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• 2020

The impact of output conductance (g_o) on the short-circuit current-gain cut-off frequency (f_T) in In_0.8Ga_0.2As high-electron-mobility transistors (HEMTs) on an InP substrate was investigated. An attempted was made to extract the values of f_T in a simplified small-signal model (SSM) of the HEMTs, derive an analytical formula for f_T in terms of the extrinsic model parameters of the simplified SSM, which are related to the intrinsic model parameters of a general SSM, and verify its validity for devices with L_g from 260 to 25 nm. In long-channel devices, the effect of the intrinsic output conductance (g_oi) on f_T was negligible. This was because, from the simplified SSM perspective, three model parameters, such as g_{m_ext}, C_{gs_ext} and C_{gd_ext}, were weakly dependent on g_oi. However, in short-channel devices, g_oi was found to play a significant role in degrading f_T as L_g was scaled down. The increase in g_oi in short-channel devices caused a considerable reduction in g_{m_ext} and an overall increase in the total extrinsic gate capacitance, yielding a decrease in f_T with g_oi. Finally, the results were used to infer how f_T is influenced by g_oi in HEMTs, emphasizing that improving electrostatic integrity is also critical importance to benefit fully from scaling down L_g.

• Proceedings of the Korean Vacuum Society Conference
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• 2013.02a
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• pp.164-164
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• 2013

Among multicomponent nanostructures, hybrid nanocatalysts consisting of metal nanoparticle-semiconductor junctions offer an interesting platform to study the role of metal-oxide interfaces and hot electron flows in heterogeneous catalysis. In this study, we report that hot carriers generated upon photon absorption significantly impact the catalytic activity of CO oxidation. We found that Pt-CdSe-Pt nanodumbbells exhibited a higher turnover frequency by a factor of two during irradiation by light with energy higher than the bandgap of CdSe, while the turnover rate on bare Pt nanoparticles didn't depend on light irradiation. We also found that Pt nanoparticles deposited on a GaN substrate under light irradiation exhibit changes in catalytic activity of CO oxidation that depends on the type of doping of the GaN. We suppose that hot electrons are generated upon the absorption of photons by the semiconducting nanorods or substrates, whereafter the hot electrons are injected into the Pt nanoparticles, resulting in the change in catalytic activity. We discuss the possible mechanism for how hot carrier flows generated during light irradiation affect the catalytic activity of CO oxidation.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2008.11a
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• pp.76-76
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• 2008

갈륨-질화물(GaN) 기반의 고속전자이동도 트랜지스터(high electron mobility transistor, HEMT)는 최근 마이크로파 또는 밀리미터파 등의 고주파 대역의 통신시스템에 널리 사용되는 전자소자이자, 차세대 고주파용 전력 소자로 각광받고 있다. AlGaN/GaN HEMT에서 AlGaN층과 GaN층의 이종접합 구조(heterostructure)는 두 물질 간의 큰 전도대의 불연속성으로 인해 발생하는 이차원 전자가스(two-dimensional electron gas, 2DEG) 채널을 이용하여 높은 전자이동도, 높은 항복전압 및 우수한 고출력 특성을 얻는 것이 가능하다. 그러나 이린 이론적인 우수한 특성에도 불구하고 실제 AlGaN/GaN HEMT 소자에서는 AlGaN 표면과 AlGaN과 GaN 층 사이 접합면, AlGaN과 GaN 벌크층에 존재하는 트랩의 영향으로 이론보다 낮은 DC 출력 특성을 갖는다. 본 논문에서는 표면, 접합면, 벌크 층에 존재하는 트랩들을 각각의 존재 유무에 따라 시뮬레이션 함으로써 각각의 트랩이 DC 특성에 미치는 영향에 대해서 알아본다. 또한 소스와 게이트, 드레인과 게이트간의 거리에 따라 표면 트랩에 따른 영향과 AlGaN층과 GaN 층의 두께를 변화시켜가면서 각 층의 두께에 따라 벌크 트랩이 DC 특성에 미치는 영향을 알아보았다. 본 논문에서 트랩에 따른 특성의 파악을 위해서 $ATLAS^{TM}$를 이용하여 전산모사 하였다.

• Proceedings of the KWS Conference
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• 2010.05a
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• pp.70-70
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• 2010

In the present study, to estimate the mechanical properties of 800 MPa grade weld metal, welding was carried out using 800 and 600 MPa grade flux cored arc welding (FCAW) consumable and characteristics of the weld metals were investigated. The chemical composition of weld metals was investigated by an optical emission spectroscopy (OES) method. The microstructure of weld metals was analyzed by optical microscopy (OM) and secondary electron microscopy (SEM). The compositions and sizes of inclusions which are the dominant factors for the nuclei of acicular ferrite were analyzed by an transmission electron microscopy (TEM). In addition, mechanical properties of the weld metals were evaluated through tensile tests and charpy impact tests. Mostly the acicular ferrite phase which has high strength and toughness was observed. The 600 MPa grade weld metal was consisted of 75% acicular ferrite and 25% ferrite which was formed at high temperature (grain boundary ferrite, widmanstatten ferrite, polygonal ferrite). However, the 800 MPa grade weld metal was composed of about 73% acicular ferrite and 27% low temperature phase (bainite, martensite). Toughness was considerably decreased due to the increase of tensile strength (from 600 MPa to 800 MPa). The sizes of inclusions which were observed in both weld metal were $0.4{\sim}0.8\;{\mu}m$, it is effective size to form acicular ferrite.