• Korean Journal of Materials Research
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• v.23 no.7
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• pp.359-365
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• 2013

We investigated the nanostructural, chemical and optical properties of nc-Si:H films according to deposition conditions. Plasma enhanced chemical vapor deposition(PECVD) techniques were used to produce nc-Si:H thin films. The hydrogen dilution ratio in the precursors, [$SiH_4/H_2$], was fixed at 0.03; the substrate temperature was varied from room temperature to $600^{\circ}C$. By raising the substrates temperature up to $400^{\circ}C$, the nanocrystalite size was increased from ~2 to ~7 nm and the Si crystal volume fraction was varied from ~9 to ~45% to reach their maximum values. In high-resolution transmission electron microscopy(HRTEM) images, Si nanocrystallites were observed and the crystallite size appeared to correspond to the crystal size values obtained by X-ray diffraction(XRD) and Raman Spectroscopy. The intensity of high-resolution electron energy loss spectroscopy(EELS) peaks at ~99.9 eV(Si $L_{2,3}$ edge) was sensitively varied depending on the formation of Si nanocrystallites in the films. With increasing substrate temperatures, from room temperature to $600^{\circ}C$, the optical band gap of the nc-Si:H films was decreased from 2.4 to 1.9 eV, and the relative fraction of Si-H bonds in the films was increased from 19.9 to 32.9%. The variation in the nanostructural as well as chemical features of the films with substrate temperature appears to be well related to the results of the differential scanning calorimeter measurements, in which heat-absorption started at a substrate temperature of $180^{\circ}C$ and the maximum peak was observed at ${\sim}370^{\circ}C$.

• Journal of the Korean Chemical Society
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• v.39 no.1
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• pp.1-6
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• 1995

The surface of a sapphire substrate used for hetero-epitaxy was chemically polished in a mixture of $H_3PO_4\;and\;H_2SO_4$ solution. The extent of etching for various crystal orientations was found to be dependent on the etching time at $315{\pm}2^{\circ}C$ and at the composition of $H_2SO_4 : H_3PO_4$=3 : 1. In addition, the etching rates of the substrates were investigated in the mixture of $H_2SO_4 : H_3PO_4$=3 : 1 by volume and in the temperature range of 280~320$^{\circ}C$. From the plot of log R against 1/T, the activation penergy ($(E_a)$) was found to be in the order of $({\bar1}012) > (10{\bar1}0) > (11{\bar2}0) > (0001)$ plane. After removing the surface layers of the sapphire with (0001), $({\bar1}012),\;(10{\bar1}0)\;and\;(11{\bar2}0)$ plane by a thickness of 64.6, 46.5, 16.2 and 5.1 ${\mu}m$, respectively, the morphology of the resulting surface was observed by SEM.

• Microbiology and Biotechnology Letters
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• v.19 no.5
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• pp.444-449
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• 1991

Synthesis of maltosyl-$\beta$-cyclodextrin using maltose ($G_2$) and $\beta$-cyclodextrin ($\beta$-CD) as substrates through the reverse reaction of pullulanase was investigated. The optimal conditions for the condensation reaction were as below: mixing ratio of maltose to $\beta$-CD of 12.7, mixed substrate concentration of 70% (w/w, 70 g/100 ml $H_2O$), and amount of pullulanse of 350 units/100 ml. The concentration of synthesized maltosyl-P-CD concentration was reached up to 2.31 g/100 rnl at above reaction conditions, which corresponded the conversion yield of 43% (w/w, g of branched-CD/g of CD). The synthesis of maltosyl-$\alpha >\gamma >\beta$-CD was also attempted, and conversion yield was in the order of a>y>J3-CDs. Condensation reaction between various maltooligosaccharides ($G-1\sim G_6$ showed that maltose was the most effective oligorner for condensation reaction with $\beta$-CD. To increase the conversion yield various alcohols were added into the reaction mixture, amyl alcohol was found to be the most acceptable alcohol for increasement of convesion yield which increased from 43.0 to 83.0% upon addition of same volume of amyl alcohol into the reaction mixture.

• Journal of Microbiology and Biotechnology
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• v.24 no.11
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• pp.1559-1565
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• 2014

Cellulase and xylanase are main hydrolysis enzymes for the degradation of cellulosic and hemicellulosic biomass, respectively. In this study, our aim was to develop and test the efficacy of a rapid, high-throughput method to screen hydrolytic-enzyme-producing microbes. To accomplish this, we modified the 3,5-dinitrosalicylic acid (DNS) method for microwell plate-based screening. Targeted microbial samples were initially cultured on agar plates with both cellulose and xylan as substrates. Then, isolated colonies were subcultured in broth media containing yeast extract and either cellulose or xylan. The supernatants of the culture broth were tested with our modified DNS screening method in a 96-microwell plate, with a $200{\mu}l$ total reaction volume. In addition, the stability and reliability of glucose and xylose standards, which were used to determine the enzymatic activity, were studied at $100^{\circ}C$ for different time intervals in a dry oven. It was concluded that the minimum incubation time required for stable color development of the standard solution is 20 min. With this technique, we successfully screened 21 and 31 cellulase- and xylanase-producing strains, respectively, in a single experimental trial. Among the identified strains, 19 showed both cellulose and xylan hydrolyzing activities. These microbes can be applied to bioethanol production from cellulosic and hemicellulosic biomass.

• Journal of Convergence for Information Technology
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• v.7 no.5
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• pp.123-128
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• 2017

Nanostructured materials have received attention due to their unique electronic, optical, optoelectrical, and magnetic properties as a results of their large surface-to-volume ratio and quantum confinement effects. Thermal chemical vapor deposition process has attracted much attention due to the synthesis capability of various structured nanomaterials during the growth of nanostructures. In this study, silicon oxide nanowires were grown on Si\$SiO_2$(300 nm)\Pt(5~40 nm) substrates by two-zone thermal chemical vapor deposition with the source material $TiO_2$ powder via vapor-liquid-solid process. The morphology and crystallographic properties of the grown silicon oxide nanowires were characterized by field-emission scanning electron microscope and transmission electron microscope. As results of analysis, the morphology, diameter and length, of the grown silicon oxide nanowires are depend on the thickness of the catalyst films. The grown silicon oxide nanowires exhibit amorphous phase.

• Proceedings of the Korea Association of Crystal Growth Conference
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• 1996.06b
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• pp.285-309
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• 1996

Non oxide ceramics such as nitrides of transition metals have shown significant potential for future economic impact, in diverse applications in ceramic, aerospace and electronic industries, as refractory products, abrasives and cutting tools, aircraft components, and semi-conductor substrates amid others. Combustion synthesis has become an attractive alternative to the conventional furnace technology to produce these materials cheaply, faster and at a higher level of purity. However he process os highly exothermic and manifests complex dynamics due to its strongly non-linear nature. In order to develop an understanding of this process and to study the effect of operational parameters on the final outcome, numerical modeling is necessary, which would generated essential knowledge to help scale-up the process. the model is based on a system of parabolic-hyperbolic partial differential equations representing the heat, mass and momentum conservation relations. The model also takes into account structural change due to sintering and volumetric expansion, and their effect on the transport properties of the system. The solutions of these equations exhibit steep moving spatial gradients in the form of reaction fronts, propagating in space with variable velocity, which gives rise to varying time scales. To cope with the possibility of extremely abrupt changes in the values of the solution over very short distances, adaptive mesh techniques can be applied to resolve the high activity regions by ordering grid points in appropriate places. To avoid a control volume formulation of the solution of partial differential equations, a simple orthogonal, adaptive-mesh technique is employed. This involves separate adaptation in the x and y directions. Through simple analysis and numerical examples, the adaptive mesh is shown to give significant increase in accuracy in the computations.

• Korean Journal of Environmental Agriculture
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• v.22 no.4
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• pp.251-254
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• 2003

Phosphorous removal rate and emergent plant growth were examined of a surface-flow constructed treatment wetland system, whose dimensions were 31 meter in length and 12 meter in width. The system was established on floodplain in the down reach of the Kwangju Stream in Korea in one and half months from May to June 2001. Cattails(Typha angustiflora) were transplanted in the system. They were dug out of natural wetlands and stems were cut at about 40 cm height from their bottom ends. Water of the Kwangju Stream were funneled into it via a pipe by gravity flow and its effluent were discharged back into it. The stems of cattails grew from 45.2 cm in July 2001 up to 186 cm in September 2001 and the number of cattail stems per square meter increased from 22 in July 2001 to 53 in September 2001. The early establishment of cattails was good. Volume and water quality of inflow and outflow were analyzed from July 2001 through December 2001. Inflow averaged $40\;m^3/day$ and hydraulic retention time was about 1.5 days. The concentration of total phosphorous in influent and effluent was 0.85 mg/L, 0.41 mg/L, respectively. The average removal rate of total phosphorous in the system was about 52%. The retention efficiency was slightly lower, compared with that in surface-flow wetlands operating in North America, whose retention efficiency was reported to be about 57%. The lower abatement rate could result from the initial stage of the system and inclusion of two cold months into the six-month monitoring period. Root rhizosphere in wetland soils and litter-soil layers on bottoms were not properly developed. Increase of standing density of cattails within a few years will establish both root zones and substrates beneficial to the removal of phosphorous, which may lead to increase of the phosphorous retention rate. The system was submerged one time by heavy storm during the monitoring period. The inundation, however, scarcely disturb its environment.

• The Transactions of the Korean Institute of Electrical Engineers C
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• v.53 no.5
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• pp.237-241
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• 2004

Amorphous silicon (a-Si) films are used in a broad range of solar cell, flat panel display, and sensor. Because of the greater ease of deposition and lower processing temperature, thin films are widely used for thin film transistors (TFTs). However, they have lower stability under the exposure of visible light and because of their low field effect mobility ($\mu$$_{FE}$ ) , less than 1 c $m^2$/Vs, they require a driving IC in the external circuits. On the other hand, polycrystalline silicon (poly-Si) thin films have superiority in $\mu$$_{FE}$ and optical stability in comparison to a-Si film. Many researches have been done to obtain high performance poly-Si because conventional methods such as excimer laser annealing, solid phase crystallization and metal induced crystallization have several difficulties to crystallize. In this paper, a new crystallization process using a molybdenum substrate has been proposed. As we use a flexible substrate, high temperature treatment and roll-to-roll process are possible. We have used a high temperature process above 75$0^{\circ}C$ to obtain poly-Si films on molybdenum substrates by a rapid thermal annealing (RTA) of the amorphous silicon (a-Si) layers. The properties of high temperature crystallized poly-Si studied, and poly-Si has been used for the fabrication of TFT. By this method, we are able to achieve high crystal volume fraction as well as high field effect mobility.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 1992.05a
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• pp.110-113
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• 1992

$n^{+}$-p homojunction InP diodes were fabricated using thermal diffusion of Sulfur into p-type InP substrates(Zn doped, LEC grown, p=2.3${\times}$10$^{16}$c $m^{-3}$). The Sulfur diffusion was carried out at 550$^{\circ}C$, 600$^{\circ}C$, 700$^{\circ}C$ for 4 hours in a sealed quartz ampule(～2ml in volume) containing 5mg I $n_2$ $S_3$ and Img of red phosphorus. The formed junction depth was below 0.5$\mu\textrm{m}$. After the removal of diffused layer on the rear surface of the wafer, the beak ohmic contacts to the p-side were made with a vacuum evaporation of An-Zn(2%) followed by an annealing at 450$^{\circ}C$ for 5 minutes in flowing Ar gas. The front contacts were made with a vacuum evaporation of Au-Ge(12%) followed by an annealing at 500$^{\circ}C$ for 3 minutes in flowing Ar gas. The remarkable sprctral response of the cells obtained at the region of 6000-8000${\AA}$ region. The open circuit voltage $V_{oc}$ , short circuit current density $J_{sc}$ , fill factor and conversion efficiency η of the fabricated pattern solar cells(diffusion condition : at 700$^{\circ}C$ for 4 hours) were 0.660V, 14.04㎃/$\textrm{cm}^2$, 0.6536 and 10.09%, respectively.y.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.19 no.1
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• pp.699-704
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• 2018

One-dimensional nanostructures have attracted increasing attention because of their unique electronic, optical, optoelectrical, and electrochemical properties on account of their large surface-to-volume ratio and quantum confinement effect. Vertically grown nanowires have a large surface-to-volume ratio. The vapor-liquid-solid (VLS) process has attracted considerable attention for its self-alignment capability during the growth of nanostructures. In this study, vertically aligned silicon oxide nano-pillars were grown on Si\$SiO_2$(300 nm)\Pt substrates using two-zone thermal chemical vapor deposition system via the VLS process. The morphology and crystallographic properties of the grown silicon oxide nano-pillars were investigated by field emission scanning electron microscopy and transmission electron microscopy. The diameter and length of the grown silicon oxide nano-pillars were found to be dependent on the catalyst films. The body of the silicon oxide nano-pillars exhibited an amorphous phase, which is consisted with Si and O. The head of the silicon oxide nano-pillars was a crystalline phase, which is consisted with Si, O, Pt, and Ti. The vertical alignment of the silicon oxide nano-pillars was attributed to the preferred crystalline orientation of the catalyst Pt/Ti alloy. The vertically aligned silicon oxide nano-pillars are expected to be applied as a functional nano-material.