• Korean Journal of Environmental Biology
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• v.25 no.2
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• pp.81-87
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• 2007

The removal of Microcystis aeruginosa and Chlorella zofingiensis by ceramic powder was investigated on the basis of both the particle size (under 0.05, 0.1, 1 mm) and the dosage (0.1, 1, $10\;g\;L^{-1}$) of the ceramic powder. The removal efficiencies of M. aeruginosa and C. zofingiensis were highest with a particle size of 0.05 mm and a dosage of $1\;g\;L^{-1}$ of the ceramic powder in laboratory experiment. $chlorophyll-{\alpha}$ concentrations decreased in both field and enclosure samples with a particle size of 0.05 mm and a dosage of $1\;g\;L^{-1}$ of ceramic powder, resulting in the removal efficiencies of 67 and 69%, respectively. Consequently, it was concluded that the ceramic powder could be used to control algal bloom by removing $chlorophyll-{\alpha}$ in eutrophic waters.

• Bulletin of the Korean Chemical Society
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• v.26 no.11
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• pp.1723-1727
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• 2005

A generalized fractional diffusion equation (FDE) is presented, which describes the time-evolution of the spatial distribution of a particle performing continuous time random walk (CTRW) on a fractal lattice. For a case corresponding to the CTRW with waiting time distribution that behaves as $\psi(t) \sim (t) ^{-(\alpha+1)}$, the FDE is solved to give analytic expressions for the Green’s function and the mean squared displacement (MSD). In agreement with the previous work of Blumen et al. [Phys. Rev. Lett. 1984, 53, 1301], the time-dependence of MSD is found to be given as < $r^2(t)$ > ~ $t ^{2\alpha/dw}$, where $d_w$ is the walk dimension of the given fractal. A Monte-Carlo simulation is also performed to evaluate the range of applicability of the proposed FDE.

• Journal of the Korean Ceramic Society
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• v.25 no.4
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• pp.415-423
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• 1988

Mixtures of carbon and silica (about 0.46${\mu}{\textrm}{m}$) which was synthesized by the hydrolysis of ethyl silicate, the molar ratio of silica/carbon was fixed as 1/10(weight ratio : 1/2), were nitrided in the temperature range 135$0^{\circ}C$~150$0^{\circ}C$. The phse of the product Si3N4 was $\alpha$ phase and the morphology was hexagnoal prism and the nitridation reaction was completed in 5 hrs at 150$0^{\circ}C$ or 7hrs at 145$0^{\circ}C$. The reaction rate above 150$0^{\circ}C$ was diffusion-controlled, following Jander equation. Activation energy Q was derived from the Arrhenius plot and the value was about 101kcal/mol. Axis ratio of Lattice constants(c/a) was 0.726 and unit volume was $\AA$3, the larger the molar ratio of carbon/Alkoxide was, the smaller the particle size of $\alpha$Si3N4 was.

• Proceedings of the KSME Conference
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• 2004.11a
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• pp.1162-1165
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• 2004

Size and crystalline phase changes of $Fe_{2}O_{3}$ nanoparticles formed in a $H_{2}/O_{2}$ flame have been investigated. At flame temperatures below $1350^{\circ}C$, the mean particle size increased monotonously with the distance from the burner edge; but in high-temperature flames above $1650^{\circ}C$, it suddenly decreased from 20 nm to ${\sim}3$ nm with the distance from the burner edge. The results of X-ray diffraction and HRTEM showed that this sudden reduction of the size of nanoparticles was accompanied by a partial phase transformation from ${\gamma}$-$Fe_{2}O_{3}$ into ${\alpha}$-$Fe_{2}O_{3}$. We suggest the structural instability due to ${\gamma}-$ to ${\alpha}-phase$ transformation as a mechanism for a rapid fragmentation of 20 nm particles into 3 nm ones.

• Nuclear Engineering and Technology
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• v.38 no.5
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• pp.423-426
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• 2006

A developed neutron activation analysis(NAA) and gamma-spectrometry were applied to improve the analytical sensitivity and precision of impurities in electronic-circuit raw materials. It is well known that soft errors in high precision electronic circuits can be induced by alpha particles emitted from naturally occurring radioactive impurities such as U and Th. As electronic circuits have recently become smaller in dimension and higher in density, these alpha-particle emitting radioactive impurities must be strictly controlled. Therefore, new NAA methods have been established using a HTS(Hydraulic Transfer System) irradiation facility and a background reduction method. For eliminating or stabilizing fluctuated background caused by Rn-222 and its progeny nuclides in air, a nitrogen purging system is used. Using the developed NAA and gamma-spectrometry, ultra trace amounts of U(0.1ng/g) and Th(0.01ng/g) in an alumina ball and high purity silica used for an epoxy molding compound (EMC) could be determined.

• Journal of the Korean Ceramic Society
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• v.24 no.2
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• pp.147-154
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• 1987

Silicon nitride powders were prepared by the simultaneous reduction and nitridation from powder mixtures of Kim cheon quartzite and carbon (graphite or carbon black) at1400$^{\circ}C$ for 10 hours in nitrogen atmosphere. The effects of the reaction variables on the yield of products and on the ${\alpha}$/${\beta}$ ratio were examined. The average particle size, density, and the ${\alpha}$/${\beta}$ ratio of the obtained si3N4 were 1.0$\mu\textrm{m}$, 3.10g/㎤ and 90/10, respectively. It was found that the Si3N4 powders obtained in this work were comparable to the foreign commercial products.

• Nuclear Engineering and Technology
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• v.50 no.1
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• pp.159-164
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• 2018

An evaluation of various radiometric methods to analyze $^{226}Ra$ in water has been employed on a set of 10 standard solutions of different concentrations in the range of $1-10Bq/L^{-1}$. The analysis was carried out using well-established procedures by means of gamma-ray, alpha-particle and liquid scintillation spectrometry. The feasibility of the various methods has been quantified in terms of relative standard error and percentage error. Correlations between the various methods have been presented and discussed. In general, good agreement was found in the results of various methodologies, which assures the accuracy of the methods and allows for the validation of instrumentation and procedures. Of the different methods adopted here, a combined procedure for the determination of $^{226}Ra$ along with $^{228}Ra$ using Quantulus 1220 ultra-low level background liquid scintillation counting gave the most accurate results.

• Journal of Power System Engineering
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• v.6 no.1
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• pp.36-42
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• 2002

Two-dimensional natural convection in an inclined open cavity with bottom heated, two side insulated and the top open was investigated using PIV(Particle-Image-Velocimetry) measurement. Experimental results are presented for Prandtl number, Pr=6.62, cavity aspect ratio, A=1.0, Rayleigh number from $1.294{\times}10^6\;to\;3.8841{\times}10 ^6$, and inclination angles, ${\alpha}=0$, 30 and 60 deg to the horizontal. It was found that the fluid rises along both side walls in the boundary layer region at ${\alpha}=0\;deg$, and the inclination of the cavity induced flow entrainment. The experimental results are in good agreement with the numerical results.

• Journal of Nuclear Fuel Cycle and Waste Technology(JNFCWT)
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• v.18 no.2_spc
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• pp.227-235
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• 2020

In this study, we discussed the limitations of gross alpha measurements for the characterization of radioactive wastes produced in nuclear facilities through experimental tests and Monte Carlo N-particle transport simulations. The determination of gross alpha is essential for the disposal of radioactive waste produced in nuclear facilities in Korea. The measurements of gross alpha are easy to perform and yield rapid analytical results, but it cannot be used for quantitative analysis. The error of counting efficiency for gross alpha with various masses of the deposit on planchets using KCl and ²⁴¹Am was determined. The relative deviation of the counting efficiency in samples having the same mass was 20%. Uranium was extracted from the soil through acid leaching and extraction chromatography, and the concentration of U determined by inductively coupled plasma-mass spectrometry (ICP-MS) was compared with the results for gross alpha. The gross alpha was underestimated by 50% compared to the U concentration by ICP-MS. The counting efficiency depended on the energy from the alpha emitters, which differed by up to three times in determination of the counting efficiency depending on the kinds of alpha radionuclides of interest. Therefore, the gross alpha is not compatible with the sum of radioactivity for each alpha emitter and is suitable as a screening method.

• Journal of Korea Foundry Society
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• v.18 no.6
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• pp.541-549
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• 1998

Microstructural characteristics of hypereutectic Al-Si alloys during reheating at semi-solid temperature have been investigated. The size and morphology of primary Si particles in wedge-type mold-cast ingot has been compared with hot-rolled sheet and Si particulate reinforced Al composite. Effects of P and Sr addition on the morphological changes of primary Si particles have been also investigated. Observation of the solidification microstructures of the wedge-type mold-cast ingot at different cooling rates showed that alloying elements such as P and Sr affect the morphology of Si particles, especially in the area solidified at a slow cooling rate. Negligible change in the size of primary crystals was observed after reheating experiment, but ${\alpha}-halo$ formed around the Si particles and fine particles of Si precipitated in the surrounding area of the Si particles. In addition, there seemed to be no coarsening with increasing of holding time and the region of ${\alpha}-halo$ being decreased. Nucleation and recrystallization was accelerated with addition of alloying elements during hot rolling resulting in a decrease of primary Si particle size. In the case of extruded specimens, morphological change of primary Si particles was not observed after reheating. No ${\alpha}-halo$ formation was observed in Si reinforced Al composite because of the oxide film formed on the Si particles which acted as a diffusion barrier between substrate and the primary Si particles.