• Journal of Korea Water Resources Association
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• v.37 no.7
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• pp.553-567
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• 2004

This study proposes a modified standardized precipitation index (MSPI) which was developed to make up for the weakness of the SPI. Both MSPI and SPI are applied to the monthly rainfall at the Seoul station for the drought analysis. The MSPI proposed is nothing but the SPI for the normalized monthly rainfall, that is, an extra step for normalizing the monthly rainfall is included before driving the SPI. Thus, the MSPI has a structure to transfer the relative amount of rainfall to the next months, but the SPI the absolute amount of rainfall. The monthly rainfall data at the Seoul station used in this study are those collected from 1777 to 1996. The rainfall data collected before and after the long dry period around 1900 were also analyzed separately for the comparison. The results derived are as follows. (1) The MSPI was found to be more practical compared to the SPI. This was assured by comparing the analysis results of the data including and excluding the long dry period around 1900. (2) The MSPI is found to be less sensitive than the SPI to the extreme rainfall events. For the MSPI, the occurrence probabilities of moderate drought before and after the long dry period are similar, but those for the extreme drought becomes slightly decreased after the long dry period (from about 18 years of return period before the long dry period to the 16 years after the long dry period). However, the duration becomes longer after the long dry period (the duration for the extreme drought has been increased from 2 to 2.5 months after the long dry period). This results can also be compared with a rather unreasonable result derived by applying the SPI (for the extreme drought the return period has been decreased to be from 25 to 10 years after the long dry period, on the other hand the duration has been increased from 1.5 months to 3.5 months). So, we man conclude that the MSPI is more practical for the drought analysis that the SPI.

• Journal of the Korean Ceramic Society
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• v.39 no.11
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• pp.1035-1041
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• 2002

The adhesion characteristics of adsorbent during impregnation of Y-type and ZSM-5type zeolites into ceramic paper were analyzed, as the amount of silica sol in slurry for impregnation was varied. 31 wt% of zeolite particle, which is useful for VOC adsorption, was evenly dispersed and adhered on ceramic paper and original crystal structure of the zeolite remained unchanged even after binder application and heat treatment. Surface area of the impregnated ceramic paper was decreased compared with that of zeolite powder. And it was found to be attributed to the reduction of volume of mesopore while the volume of micropore under $20{\AA}$ was unchanged. Zeolite-impregnated honeycomb cylinder, whose diameter and length were 10 cm and 40 cm, respectively, was subjected to adsorption/desorption test with respect to toluene, MEK, cyclohexanone. All of the VOC's were removed by adsorption with efficiency higher than 97% and from the static adsorption test, $42 Nm^3/h$ of 300 ppmv-VOC-laden air was calculated be treated continuously, when the honeycomb was used in an adsorptive rotor system.

• Journal of the Korean Ceramic Society
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• v.39 no.11
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• pp.1090-1096
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• 2002

Six different composition of water-swellable bentonite rheology modifiers(WSB-1~WSB-6) were prepared by the compounding of peptizers and anionic surfactants as an additives with Bentonite(BEN) of montmorillonite group. Average particle size, particle morphology and water-swellability of WSB and the viscosity with additives were measured, respectively. And the rheological behavior of WSB were investigated using the rheometer. The viscosity of WSB-1 increased with decreasing both pH and average particle size of BEN, WSB-2 treated $Na_2CO_3$ as a peptizer showed the maximum viscosity. These results can be interpretated cause for rearrangment as the edge-to-face structure of BEN particles containing WSB. Also, WSB-4∼WSB-6 containing both peptizer and anionic surfactant was sol phase that their viscosity was not nearly with the shear rate, however, WSB-3 containing Tetrasodium Pyrophosphate(TSPP) as an anionic surfactant showed the thixotropy by the viscosity difference of 1000 times with the shear rate. From this result, the anions of TSPP can be explained to arrange in edge of BEN particles containing WSB-3.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.22 no.4
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• pp.194-199
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• 2012

The $LiMn_{1.5}Ni_{0.5}O_4$, substituting a part of Mn with Ni in the $LiMn_2O_4$, the spinel structure has good charge-discharge cycle stability and high discharge capacity at 4.7 V. In this study $LiMn_{1.5}Ni_{0.5}O_4$ powders were synthesized by polymerization complex method. The effect on the characteristics of synthesized $LiMn_{1.5}Ni_{0.5}O_4$ powders was studied with citric acid (CA) : metal ion (ME) molar ratio (5 : 1, 10 : 1, 15 : 1, 30 : 1) and calcination temperature ($500{\sim}900^{\circ}C$). Single phase of $LiMn_{1.5}Ni_{0.5}O_4$ was observed from XRD analysis on the powders calcined at low ($500^{\circ}C$) and high temperatures ($900^{\circ}C$). The crystalline size and crystallinity increased with calcination temperature. At low calcination temperature the particle size decreased and specific surface area increased as the CA molar ratio increased. On the other hand, high particle growth rate at high calcination temperature interfered the particle size reduction and specific surface area increase induced by the increase of CA molar ratio.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.21 no.6
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• pp.259-265
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• 2011

This research was performed to stabilize heavy metals in mine tailing using fly ash and clay. Fly ash-clay-mine tailing system were investigated using XRD (X-ray diffractometer), XRF (X-ray fluorescence spectrometer), TG-DTA, SEM (Scanning Electron Microscope), Dilatometer and UTM with various mine tailing contents (~15 wt%). The fly ash used in this research was mainly composed of $SiO_2$ (33.01 wt%), $Al_2O_3$ (28.54 wt%), $K_2O$ (3.32 wt%), $Fe_2O_3$ (1.47 wt%), CaO(9.97 wt%). $SiO_2$ and $Al_2O_3$ composition of the clay was over 61 wt%. And the mine tailing have high composition of $SiO_2$ (26.91 wt%), CaO (24.25 wt%), $Fe_2O_3$ (22.97 wt%). Therefore, it was estimated that fly ash-clay-mine tailing have enough sintering characteristics. The shrinkage of specimens started at around $850^{\circ}C$ and changed little up to $1100^{\circ}C$, but increased markedly at above $1100^{\circ}C$. The shrinkage rate is strongly related to the decarbonization amount of coal fly ash. As the result of SEM, structure of the specimens with mine tailing addition showed more close than the one without mine tailing. Compressive strength of the specimens with mine tailing was highly increased to approximately 200~420 kgf/$cm^2$, it satisfied the first grade criterion for clay brick by KS L 4201. The specification of leaching characteristics of the sintered specimens were within the Korean regulation standard.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.21 no.3
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• pp.114-118
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• 2011

We report on the growth and characterization of nano and micro scale GaN structures selectively grown on the vertex of hexagonal GaN pyramids. $SiO_2$ near the vertex of hexagonal GaN pyramids was removed by optimized photolithgraphy process and followed by a selective growth of nano and micro scale GaN structures by metal organic vapor phase epitaxy (MOVPE). The pyramidal GaN nano and micro structures which have crystal facets of semi-polar {1-101} facets were formed only on the vertex of GaN pyramids and the size of the selectively grown nano and micro GaN structures was easily controlled by growth time. As a result of TEM measurement, Reduction of threading dislocation density was conformed by transmission electron microscopy (TEM) in the selectively grown nano and micro GaN structures. However, stacking faults were newly developed near the edge of $SiO_2$ film because of the roughness and nonuniformity in thickness of the $SiO_2$ film.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.21 no.3
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• pp.110-113
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• 2011

In this paper, we propose a new method for the fabrication of GaN microstructures formed only on the vertex of GaN pyramid by using of metal catalysts. GaN pyramidal structures were selectively grown on 3 ${\mu}m$ $SiO_2$ dot patterns followed by thin film deposition of Au and Cr only on the vertex area of the GaN pyramids with precisely controlled photolithography. After the metal deposition, the samples were loaded in the MOVPE reactor for the growth of GaN microstructures for 10 minutes. Temperature for the growth of the GaN microstructures was changed from $650^{\circ}C$ to $750^{\circ}C$. Rod type GaN microstructures were grown in the direction of vertical to the six {1-101} facets and the shape of the GaN microstructures was changed depend on the type of metal.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.21 no.5
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• pp.187-192
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• 2011

Tilt angle of r-plane wafer is a one of the important factors related with the quality of the GaN epi, so the fine control of the tilt angle is important for the growing of high quality non-polar a-GaN epi. We prepared the R-plane sapphire wafers with slight tilt angles for nonpolar a-plane GaN. The target tilt angles of ${\alpha}$ and ${\beta}$ were 0, -0.1, -0.15, -0.2, -0.4, $-0.6^{\circ}$ and -0.1, 0, $0.1^{\circ}$, respectively. The tilt angles of sliced R-plane sapphire wafers were measured by x-ray and the statistical evaluation of reliability of tilt angles of wafers were performed. The tolerance of the tilt angle was ${\pm}0.03^{\circ}$. R-plane sapphire wafers have relatively large distributions of BOW and TTV data than c-plane sapphire wafers due to the large anisotropy of R-plane. As the tilt angle ${\alpha}$ was increased from -0.1 to $-0.6^{\circ}$, the step widths and heights were decreased from 156 nm to 26 nm and 0.4 nm to 0.2 nm, respectively. The growth and qualities of GaN epi seems to be largely affected by the change of step structure of R-plane sapphire wafers with tilt angle.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.19 no.2
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• pp.61-65
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• 2009

In this paper, we report on the growth and optical characteristics of white-LED without fluorescent material. The growth of DH(double heterostructure) with AlGaN active layer was performed on a n-GaN/(0001) $Al_{2}O_{3}$ by the mixed-source HVPE and multi-sliding boat. The CRI(color rendering index) of packaging device charged in the range 72-93 with CIE chromaticity coordinates(x=$0.26{\sim}0.34$, y=$0.31{\sim}0.40$). And CCT(correlated color temperature) values was measured $5126{\sim}10406K$ with increasing injection current. The CIE point of conventional phosphor white LED shifts blue region, but cm point of non-phosphor white LED shifts opposite direction. These results show the mixed-source HVPE can be possible to newly fabricate method of phosphor free white LED with high CRI value.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.19 no.2
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• pp.79-83
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• 2009

The glass-ceramic of the $Li_{2}O-Al_{2}O_{3}-SiO_{2}$ system was investigated to develop the low thermal expansion materials. The glass of this system was heat treated at $775^{\circ}C$ for 2 h for nucleation and subsequently at $825{\sim}900^{\circ}C$ for 2 h for crystallization. The crystal structure of the glass-ceramic of this system was a single phase of $\beta$-quartz solid solution($Li_{x}Al_{x}Si_{1-x}O_{2}$). The thermal expansion of the glass-ceramic showed $4.40{\times}10^{-7}{\sim}1.33{\times}10^{-6}K^{-1}$ between $25{\sim}300^{\circ}C$ and $1.56{\times}10^{-6}{\sim}2.53{\times}10^{-6}K^{-1}$ between $25{\sim}800^{\circ}C$, higher than lower temperature range. The mechanical strength remained almost same at around high 110 MPa with heating temperature changes.