• 천문학회보
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• 제38권1호
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• pp.77.2-77.2
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• 2013

xIn the present work, we explored the effect of the radiation on the effective impact parameter for capture in a fully general relativistic manner. To summarize our results, evidently due to the radiation drag (the Poynting-Robertson effect), critical impact parameter of point particle gets larger by the factor of two, thus, the effective cross section of the luminous relativistic star becomes 4 times larger than that of the star without radiation emission. In addition, the finite size effect of the star adds up to this growth of the effective cross section.

• Molecular & Cellular Toxicology
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• 제4권1호
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• pp.52-60
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• 2008

In order to investigate whether the particles reduced to almost nano grade might affect the chemical and physical properties of organic materials, whole Korean Black-Red Ginseng was pulverized into almost nano size and then ginsenosides, minerals, carbohydrates, lipids and proteins in the ultrafine particles were compared with those in the regular particles as control. The mean size of the ultrafine particles was in the 350 nm range, while that of the regular particles was $127{\mu}m$. More ginsenosides, minerals, carbohydrates, lipids and proteins were detected in the ultrafine particles than in the regular particles. Interestingly, more lipids from the ultrafine particles dissolved in the water than those from the regular particles in the ethanol. Absorption and transport of carbohydrate, lipid or antioxidant activity across the intestinal wall using everted intestine sacks of mice was also enhanced by particle size reduction at the almost nano scale. More cytotoxic effect against hepatoma cell growth by ultrafine particles was also found. These results could be used as the basic data for the understanding and evaluation of the effects of organic nanomaterials on the human health.

• 한국세라믹학회지
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• 제30권4호
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• pp.279-288
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• 1993

A study on calcination and reduction of AUC(ammonium uranyl carbonate, (NH4)4UO2(CO3)3) has been carried out by using TG-DTA in N2, air and H2 atmospheres, respectively. Phases of various intermediate obtained during thermal analysis of AUC in different atmospheres were confirmed by XRD. Powder characteristics of each intermediate were investigated by measuring particle size and specific surface area, and also observed by SEM. As a results, regardless of applied atmosphere AUC was calcined into amorphous UO3, which was converted to $\alpha$-U3O8 Via $\alpha$-UO3 in both H2 and N2 atmosphere, but directly into $\alpha$-UO3 in air atmosphere. Further reduction of U3O8 was only detectable in hydrogen atmosphere. During calcination and reduction, average particle size was reduced to less than 30% of original value without morphology change. Specific surface area was dramatically increased with release of NH3, CO2 and H2O from AUC powder and reached maximum value around 25$0^{\circ}C$, and then gradually decreased with the increase of temperature due to sintering effect of uranium oxides such as UO3 and U3O8. It was also found that the change of average crystallite size and pore size were closely related to the changes of specific surface area of uranium oxides.

• Advances in nano research
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• 제3권3호
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• pp.169-176
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• 2015

Iron nanoparticles were made by using the modified coprecipitation technique. Usually the characteristics of synthesised particles depend upon the process parameters such as the ratio of the iron ions, the pH of the solution, the molar concentration of base used, type of reactants and temperature. A modified coprecipitation method was adopted in this study. A magnetic stirrer was used for mixing and the morphology and nature of particles were observed after synthesis. Nanoparticles were characterised through XRD. Obtained nanoparticles showed the formation of magnetite and maghemite under citric acid and oxalic acid as stabilisers respectively. The size of nanoparticle was greatly affected by the use of different types of stabilisers. Results show that citric acid greatly reduced the obtained particle size. Particle size as small as 13 nm was obtained in this study. The effects of different kinds of nucleating agents were also observed and two different types of nucleating agents were used i.e. potassium hydroxide (KOH) and copper chloride ($CuCl_2$). Results show that the use of nucleating agent in general pushes the growth phase of nanoparticles towards the end of coprecipitation reaction. The particles obtained after addition of nucleating agent were greater in size than particles obtained by not utilising any nucleating agent. These particles have found widespread use in medical sciences, energy conservation and electronic sensing technology.

• 한국진공학회:학술대회논문집
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• 한국진공학회 2011년도 제41회 하계 정기 학술대회 초록집
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• pp.213-213
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• 2011

Gold catalysts supported on TiO2 have shown a unique catalytic behavior on CO oxidation, depending on surface effects. Particle size has an influence on the surface activity. To make monodisperse Au nanoparticles, organic capping ligands, such as alkylthiols, were used by a "greener" synthesis method [1,2] and Au nanoparticles were deposited on TiO2. However, organic capping ligands must be removed for high catalytic activities by the Au nanoparticles without changing the Au size [3]. We used UV ozone treatment to decompose thiol ligands. The samples have been characterized by X-ray photoelectron spectroscopy to examine the surface modification by UV ozone treatment. We show the size distribution of the gold nanoparticles by light scattering analysis and transmission electron microscopy. Au/TiO2 have been prepared using the wetness impregnation method. The catalytic performance of CO oxidation over Au supported on TiO2 under oxidizing reaction conditions (40 Torr CO and 100 Torr O2) were tested. The results show that the catalytic activity depends on particle size and the time of UV ozone exposure, which suggests the role of sulfur bonding in determining the catalytic activity of Au/TiO2 catalysts.

• Macromolecular Research
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• 제17권7호
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• pp.483-490
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• 2009

Stable poly(styrene-co-divinylbenzene) [P(St-co-DVB)] micro spheres with narrow size distribution were synthesized in the presence of 2,2'-azobis(2,4-dimethyl valeronitrile) (V-65) and co-initiator in an acetone/water mixture in the precipitation polymerization at $53^{\circ}C$ for 24 h. Potassium peroxodisulfate (KPS), ammonium peroxodisulfate (APS) and sodium peroxodisulfate (NaPS) were used as co-initiators. The optimum ratio of acetone to water for the formation of a narrow distribution of P(St-co-DVB) particles was 49:11 (g/g). The optimum co-initiator compositions for narrow distribution were 9:1 (g/g) for V-65 to KPS, 11:1 for V-65 to APS and 6:1 for V-65 to NaPS. The yield for these compositions was $54{\sim}57%$ and the largest particle size was obtained with the lowest zeta-potential and CV values. From the XPS measurements, the charge density was increased but the zeta potential decreased with increasing sulfur content, implying that the sulfate group provides the electrostatic stabilization on the particle surface. This suggested that the self-crosslinking between styrene and DVB, the electrostatic stabilization of initiators, and the balanced hydrophobic and hydrophilic properties of the solvents are responsible for the formation of stable P(St-co-DVB) spherical particles with narrow size distribution.

• 한국재료학회지
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• 제16권12호
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• pp.739-742
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• 2006

The ${\gamma}-Fe_2O_3$ nano particles in the size range of $5{\sim}30$ nm were prepared by a chemical coprecipitation method. The nano particles were coated with 2nd surfactants for obtaining the water-based. The size effect of ${\gamma}-Fe_2O_3$ nano particles for the ultraviolet protection was investigated. The variation of the UV-Vis transmittance spectra as a function of wavelength for a ${\gamma}-Fe_2O_3$ nano particles were showed red-shifted increase with the particle size. The protective effects of UVA onset at near 469, 494, 591 nm for a particle size of 8.7, 9.1 and 12 nm. It is shown that the ${\gamma}-Fe_2O_3$ nano particles was good materials for protect of UV.

• Nuclear Engineering and Technology
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• 제53권12호
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• pp.4142-4149
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• 2021

In the radiation protection application, the metal-polymer composites have been developed for their radiation shielding properties. In this research, the elastomer composites doped by 10 ㎛ and 100nm size of lead, bismuth and tungsten particles as filler with 30 and 60 wt percentages were prepared. To survey the shielding properties of the polymer composites using gamma-ray emitted from 152Eu and 137Cs sources, the gamma flux was measured by using NaI(Tl) detector, then the linear attenuation coefficient was calculated. Also, the Monte Carlo simulation (MCs) method was used. The results showed a direct relationship between the linear attenuation coefficients of the absorbent and filler ratio. Also, the decrease in the particle size of the shielding material in each weight percentage improved the radiation shielding features. When the dimension of the particles was in the order of nano-size, more attenuation was achieved. At low energies used for medical diagnostic X-ray applications due to the predominance of the photoelectric effect, bismuth and lead were suitable selection as filler.

• 한국분말재료학회지
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• 제28권2호
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• pp.120-126
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• 2021

In this study, AlSi10Mg alloy powders are synthesized using gas atomization and sieving processes for powder bed fusion (PBF) additive manufacturing. The effect of nozzle diameter (ø = 4.0, 4.5, 5.0 and 8.0 mm) on the gas atomization and sieving size on the properties of the prepared powder are investigated. As the nozzle diameter decreases, the size of the manufactured powder decreases, and the uniformity of the particle size distribution improves. Therefore, the ø 4.0 mm nozzle diameter yields powder with superior properties. Spherically shaped powders can be prepared at a scale suitable for the PBF process with a particle size distribution of 10-45 ㎛. The Hausner ratio value of the powder is measured to be 1.24. In addition, the yield fraction of the powder prepared in this study is 26.6%, which is higher than the previously reported value of 10-15%. These results indicate that the nozzle diameter and the post-sieve process simultaneously influence the shape of the prepared powder as well as the satellite powder on its surface.

• 한국콘크리트학회:학술대회논문집
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• 한국콘크리트학회 1993년도 봄 학술발표회 논문집
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• pp.34-39
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• 1993

Recently inappropriate aggregates are used as a raw material for ready-mixed concretes, due to the shortage of natural aggregate resources and the prohibition of their extraction for the environmental protection. We, therefore, have conducted experiments to obtain some knowledge of properties of domestic aggregates and to investigate subsequent changes in the properties of the fresh and hardened concretes. To this end, aggregates currently used in 13 domestic ready-mixed concrete plants were collected. Most of aggregate used in this experiment satisfied the KS in density and unit weight. But some of the aggregates have the particle size distribution that can effect bad influence on concrete. In this experiment the aggregates are found to have a bad particle size distribution, resulting in high amount of a unit water content and a unit cement content in concrete.