• Korean Journal of Remote Sensing
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• v.34 no.6_2
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• pp.1261-1272
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• 2018

It is important to measure the Arctic surface air temperature because it plays a key-role in the exchange of energy between the ocean, sea ice, and the atmosphere. Although in-situ observations provide accurate measurements of air temperature, they are spatially limited to show the distribution of Arctic surface air temperature. In this study, we proposed machine learning-based models to estimate the Arctic surface air temperature in summer based on buoy data and Advanced Microwave Scanning Radiometer 2 (AMSR2)satellite data. Two machine learning approaches-random forest (RF) and support vector machine (SVM)-were used to estimate the air temperature twice a day according to AMSR2 observation time. Both RF and SVM showed $R^2$ of 0.84-0.88 and RMSE of $1.31-1.53^{\circ}C$. The results were compared to the surface air temperature and spatial distribution of the ERA-Interim reanalysis data from the European Center for Medium-Range Weather Forecasts (ECMWF). They tended to underestimate the Barents Sea, the Kara Sea, and the Baffin Bay region where no IABP buoy observations exist. This study showed both possibility and limitations of the empirical estimation of Arctic surface temperature using AMSR2 data.

• Clean Technology
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• v.19 no.3
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• pp.300-305
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• 2013

Zinc oxide (ZnO) and reduced graphite oxide (rGO) composites were synthesized and tested as adsorbents for the hydrogen sulfide ($H_2S$) adsorption at mid-to-high (300 to $500^{\circ}C$) temperatures. In order to investigate the critical roles of oxygen containing functional groups, such as hydroxyl, epoxy and carboxyl groups, attached on rGO surface for the $H_2S$ adsorption, various characterization methods (TGA, XRD, FT-IR, SEM and XPS) were conducted. For the reduction process for graphite oxide (GO) to rGO, a microwave irradiation method was used, and it provided a mild reduction environment which can remain substantial amount of oxygen functional groups on rGO surface. Those functional groups were anchoring and holding nano-sized ZnO onto the 2D rGO surface; and it prevented the aggregation effect on the ZnO particles even at high temperature ranges. Therefore, the $H_2S$ adsorption capacity had been increased about 3.5 times than the pure ZnO.

• Proceedings of the Materials Research Society of Korea Conference
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• 2012.05a
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• pp.68.1-68.1
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• 2012

The best part of graphene is - charge-carriers in it are mass less particles which move in near relativistic speeds. Comparing to other materials, electrons in graphene travel much faster - at speeds of $10^8cm/s$. A graphene sheet is pure enough to ensure that electrons can travel a fair distance before colliding. Electronic devices few nanometers long that would be able to transmit charge at breath taking speeds for a fraction of power compared to present day CMOS transistors. Many researches try to check a possibility to make it a perfect replacement for silicon based devices. Graphene has shown high potential to be used as interconnects in the field of high frequency electrical devices. With all those advantages of graphene, we demonstrate characteristics of electrical and optical properties of graphene such as the effect of graphene geometry on the microwave properties using the measurements of S-parameter in range of 500 MHz - 40 GHz at room temperature condition. We confirm that impedance and resistance decrease with increasing the number of graphene layer and w/L ratio. This result shows proper geometry of graphene to be used as high frequency interconnects. This study also presents the optical properties of graphene oxide (GO), which were deposited in different substrate, or influenced by oxygen plasma, were confirmed using different characterization techniques. 4-6 layers of the polycrystalline GO layers, which were confirmed by High resolution transmission electron microscopy (HRTEM) and electron diffraction analysis, were shown short range order of crystallization by the substrate as well as interlayer effect with an increase in interplanar spacing, which can be attributed to the presence of oxygen functional groups on its layers. X-ray photoelectron Spectroscopy (XPS) and Raman spectroscopy confirms the presence of the $sp^2$ and $sp^3$ hybridization due to the disordered crystal structures of the carbon atoms results from oxidation, and Fourier Transform Infrared spectroscopy (FTIR) and XPS analysis shows the changes in oxygen functional groups with nature of substrate. Moreover, the photoluminescent (PL) peak emission wavelength varies with substrate and the broad energy level distribution produces excitation dependent PL emission in a broad wavelength ranging from 400 to 650 nm. The structural and optical properties of oxygen plasma treated GO films for possible optoelectronic applications were also investigated using various characterization techniques. HRTEM and electron diffraction analysis confirmed that the oxygen plasma treatment results short range order crystallization in GO films with an increase in interplanar spacing, which can be attributed to the presence of oxygen functional groups. In addition, Electron energy loss spectroscopy (EELS) and Raman spectroscopy confirms the presence of the $sp^2$ and $sp^3$ hybridization due to the disordered crystal structures of the carbon atoms results from oxidation and XPS analysis shows that epoxy pairs convert to more stable C=O and O-C=O groups with oxygen plasma treatment. The broad energy level distribution resulting from the broad size distribution of the $sp^2$ clusters produces excitation dependent PL emission in a broad wavelength range from 400 to 650 nm. Our results suggest that substrate influenced, or oxygen treatment GO has higher potential for future optoelectronic devices by its various optical properties and visible PL emission.

• Resources Recycling
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• v.32 no.6
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• pp.3-9
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• 2023

Separation of LED packages from PCBs and analysis of the adhesive components was conducted to enhance the recycling potential of LED modules. LED package was separated from PCBs using heat treatment under optimal conditions: temperature of above 250 ℃ and time of 20 minutes. The separation equipment can be established using a hot air injector with controlling the rotational speed of the internal screw. The separation efficiency of each type of substrate (aluminum and glass fiber) was investigated with the thickness range of the adhesive materials (0.25-0.30 and 0.30-0.35 mm). Under the optimal conditions, the efficiency can reach to 97.5% for both types of substrates with adhesive materials of thickness 0.25~0.30mm. Characterization of the residual adhesive substances from the separated LED package and PCB using microwave digestion and ICP analysis showed that the residue contained of 95% of Sn, less than 5% of Cu and Ag.

• Journal of Magnetics
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• v.7 no.3
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• pp.63-71
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• 2002

Three fabrication techniques for forming thin barrier layer with uniform thickness and large barrier height in magnetic tunnel junction (MTJ) are discussed. First, the effect of immiscible element addition to Cu layer, a high conducting layer generally placed under the MTJ, is investigated in order to reduce the surface roughness of the bottom ferromagnetic layer, on which the barrier is formed. The Ag addition to the Cu layer successfully realizes the smooth surface of the ferromagnetic layer because of the suppression of the grain growth of Cu. Second, a new plasma source, characterized as low electron energy of 1 eV and high density of $10^{12}$ $cm^{-3}$, is introduced to the Al oxidation process in MTJ fabrication in order to reduce damages to the barrier layer by the ion-bombardment. The magnetotransport properties of the MTJs are investigated as a function of the annealing temperature. As a peculiar feature, the monotonous decrease of resistance area product (RA) is observed with increasing the annealing temperature. The decrease of the RA is due to the decrease of the effective barrier width. Third, the influence of the mixed inert gas species for plasma oxidization process of metallic Al layer on the tunnel magnetoresistance (TMR) was investigated. By the use of Kr-O$_2$ plasma for Al oxidation process, a 58.8 % of MR ratio was obtained at room temperature after annealing the junction at $300{^{\circ}C}$, while the achieved TMR ratio of the MTJ fabricated with usual Ar-$0_2$ plasma remained 48.4%. A faster oxidization rate of the Al layer by using Kr-O$_2$ plasma is a possible cause to prevent the over oxidization of Al layer and to realize a large magnetoresistance.

• Proceedings of the Korean Vacuum Society Conference
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• 2016.02a
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• pp.159-159
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• 2016

Vertically-aligned carbon nanotubes (VCNT) have attracted much attention due to their unique structural, mechanical and electronic properties, and possess many advantages for a wide range of multifunctional applications such as field emission displays, heat dissipation and potential energy conversion devices. Surface modification of the VCNT plays a fundamental role to meet specific demands for the applications and control their surface property. Recent studies have been focused on the improvement of the electron emission property and the structural modification of CNTs to enable the mass fabrication, since the VCNT considered as an ideal candidate for various field emission applications such as lamps and flat panel display devices, X-ray tubes, vacuum gauges, and microwave amplifiers. Here, we investigate the effect of surface morphology of the VCNT by water vapor exposure and coating materials on field emission property. VCNT with various height were prepared by thermal chemical vapor deposition: short-length around $200{\mu}m$, medium-length around $500{\mu}m$, and long-length around 1 mm. The surface morphology is modified by water vapor exposure by adjusting exposure time and temperature with ranges from 2 to 10 min and from 60 to 120oC, respectively. Thin films of SiO2 and W are coated on the structure-modified VCNT to confirm the effect of coated materials on field emission properties. As a result, the surface morphology of VCNT dramatically changes with increasing temperature and exposure time. Especially, the shorter VCNT change their surface morphology most rapidly. The difference of field emission property depending on the coating materials is discussed from the point of work function and field concentration factor based on Fowler-Nordheim tunneling.

• Journal of The Korean Astronomical Society
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• v.37 no.5
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• pp.387-392
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• 2004

X-ray observations of galaxy clusters have played an important role in cosmology, especially in determining the cosmological density parameter and the fluctuation amplitude. While they represent the bright side of the universe together with the other probes including the cosmic microwave background and the Type Ia supernovae, the resulting information clearly indicates that the universe is dominated by dark components. Even most of cosmic baryons turns out to be dark. In order to elucidate the nature of dark baryons, we propose a dedicated soft-X-ray mission, DIOS (Diffuse Intergalactic Oxygen Surveyor). Recent numerical simulations suggest that approximately 30 to 50 percent of total baryons at z = 0 take the form of the warm-hot intergalactic medium (WHIM) with $10^5K < T < 10^7K $which has evaded the direct detection so far. The unprecedented energy resolution (${\~} 2eV$) of the XSA (X-ray Spectrometer Array) on-board DIGS enables us to identify WHIM with gas temperature $T = 10^6 {\~} 10^7K$ and overdensity $\delta$ = 10 ${\~}$ 100 located at z < 0.3 through emission lines of OVII and OVIII. In addition, WHIMs surrounding nearby clusters are detectable with a typical exposure time of a day, and thus constitute realistic and promising targets for DIOS.

• Korean Journal of Materials Research
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• v.10 no.9
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• pp.593-600
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• 2000

This paper investigated the conduction path and conduction mechanism in undoped polycrystalline diamond thin films deposited by microwave chemical vapor deposition. The resistances measured by ac impedance spectroscopy with different directions can not be explained by the previously-known surface conduction model. The electrodeposition of Cu and electroetching of Ag experiments showed that the conduction path is the grain boundaries within the diamond films. The electodeposition of Cu with an insulating surface layer further proved that the main conduction path in polycrystalline films in the grain boundaries. The film with high electrical conductivity has low activation energy of 45meV and higher dangling bond density. By considering the results and surface C chemical bonds, the H-C-C-H bonds at surface and in grain boundaries might be the origin of high conductivity in undoped diamond films.

• Proceedings of the KIEE Conference
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• 1992.07b
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• pp.928-930
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• 1992

The design beam energy of PLS(Pohang Accelerator Laboratory) Linac is 2Gev. The linac employs total 11 units of modulators and klystrons. The maximum peak output powers of the modulators are 200MW (400kV, 500A, 4.4$\mu$S flat-top, 800$\Omega$ load) to drive the klystrons which have the peak microwave power of 80MW. Prior to the development of the 200MW modulators, a prototype 150MW modulator has been constructed and tested. We have achieved output pulses of 350kV, 420A and 3.5$\mu$S flat-top with 840$\Omega$ water load. In this article, the test results and computer simulations of charging, De-Q'ing, and discharging are presented.

• Proceedings of the Materials Research Society of Korea Conference
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• 2011.05a
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• pp.53.1-53.1
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• 2011

Bioactive glass powders were synthesized by hydrothermal chemical route by the use of ultrasonic energy irradiation. We used sodalime, calcium nitrate tetra hydrate and di ammonium hydrogen phosphate as the precursor material to synthesize $SiO_2$ rich bio-active glass materials. The $SiO_2$ content was varied in the precursor mixture to 60, 52 and 45 mole%. Dense compacts were obtained by microwave sintering at $1,100^{\circ}C$. Mechanical properties were characterized for the fabricated dense bioactive glasses and were found to be comparable with conventional CaO-$SiO_2$-$Na_2O$-$P_2O_5$ bioactive glass. Detailed biocompatibility evaluation of the glass composition was investigated by in-vitro culture of MG-63 cell and mesenchyme stem cell. Cell adhesion behavior was investigated for both of the cell by one cell morphology for 30, 60 and 90 minutes. Cell proliferation behavior was investigated by culturing both of the cells for 1, 3 and 7 days and was found to be excellent. Both SEM and confocal laser scanning microscopy were used for the investigation. Western blot analysis was performed to evaluate the bimolecular level interaction and extent and rate of specific protein expression. The ability to form biological apatite in physiological condition was observed with simulated body fluid (SBF). In-vivo bone formation behavior was investigated after implanting the materials inside rabbit femur for 1 and 3 month. The bone formation behavior was excellent in all the bioglass compositions, specially the composition with 60% $SiO_2$ content showed most promising trend.