• Nuclear Engineering and Technology
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• v.48 no.4
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• pp.1002-1008
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• 2016

An understanding of the coarsening process of the large fission gas pores in the high burn-up structure (HBS) of irradiated $UO_2$ fuel is very necessary for analyzing the safety and reliability of fuel rods in a reactor. A numerical model for the description of pore coarsening in the HBS based on the Ostwald ripening mechanism, which has successfully explained the coarsening process of precipitates in solids is developed. In this model, the fission gas atoms are treated as the special precipitates in the irradiated $UO_2$ fuel matrix. The calculated results indicate that the significant pore coarsening and mean pore density decrease in the HBS occur upon surpassing a local burn-up of 100 GWd/tM. The capability of this model is successfully validated against irradiation experiments of $UO_2$ fuel, in which the average pore radius, pore density, and porosity are directly measured as functions of local burn-up. Comparisons with experimental data show that, when the local burn-up exceeds 100 GWd/tM, the calculated results agree well with the measured data.

• Proceedings of the Membrane Society of Korea Conference
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• 1995.04a
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• pp.1-4
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• 1995

The microstructure of polymer membranes produced via thermally induced phase separation (TIPS) of polymer solutions is a strong function of both the early-stage (by spinodal decomposition or nucleation & growth) and the late-stage phase separation (referred to in general as coarsening). In the case of early stage effects, the membrane morphology resulting from a nucleation & growth mechanism is either a poorly interconnecsed, stringy, beady structure which is mechanically fragile or a well interconnected structure with highly nonuniform pore sizes. In contrast, spinodal decomposition results in a well interconnected, mechanically strong membrane with highly uniform pore sizes. Here I describe recent quantitative studies of the coarsening effects on the microstructure of membranes produced via TIPS process. The dependence of microstructure on coarsening time, quench depth, solution viscosity, and polymer molecular weight was investigated in order to distinguish among three possible coarsening mechanisms, Ostwald ripening, coalescence, and hydrodynamic flow, which may be responsible for structural evolution after the early-stage phase Separation (spinodal decomposition or nucleation & growth).

• Membrane Journal
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• v.5 no.3
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• pp.119-128
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• 1995

The effects of coarsening on microstructure formation in polystyrene-cyclohexane solutions and membranes made from them were studied by scanning electron miccoscopy(SEM). Thermal analysis of the polymer solutions was carried out with a differential scanning calorimeter and the binodal curve was determined from the onset temperature of the heat of demixing peak. Using thermally induced phase separation(TIPS) and a freeze drying technique, it was demonstrated that polymer membrane microstructure can be changed significantly by controlling coarsening time and quench route. For systems undergoing phase separation by spinodal decomposition, resulting in a well interconnecmd, microporous structure with nearly uniform pore sizes, it was found that extending the phase separation time prior m freezing and solvent removal can result in a significant increase in pore or cell size which is highly dependent on both quench depth and coarsening time. Also this study has revealed the important role of polymer concentration in dictating the material continuity of the membranes.

• Proceedings of the Korean Powder Metallurgy Institute Conference
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• 2006.09a
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• pp.557-558
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• 2006

In all conventional sintered PM products, the pores present are of two types, primary and secondary. Primary pores forming during compaction and latter during sintering, due to penetration of formed liquid through the matrix grain boundary. Effect of carbon addition on diffusion of Cu in SH737-2Cu system was investigated. After compaction and transient liquid phase sintering at $1120^{\circ}C$ and $1180^{\circ}C$, samples were characterized for densification, showing rise in sintering density and reduction in swelling on carbon addition. Quantitative microstructural characterization (shape factor and pore size) revealed bimodal distribution for 0% carbon, more rounded pores for 0.9% carbon and higher sintering temperature, and pore coarsening at higher sintering temperature.

• Journal of the Korean Ceramic Society
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• v.37 no.5
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• pp.479-490
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• 2000

The microstructure of Ni-YSZ composite as an anode of SOFC was investigated as a function of Ni content(10-70 vol%) in order to examine the correlation between microstructural-and electrical property. Image analysis based on quantitative microscopy theory was performed to quantify the microstructural property. We could get the informations about the size and distribution, contiguity and interfacial area of each phase or between the phases from the image analysis. According to the image analysis, contiguity between the same phae was mainly dependent on the amount of the phase while the contiguity between different phases was additionally influenced by the microstructural changes, especailly by the coarsening of the Ni phase. The whole length of pores perimeter was increased as Ni content increased, which indicated the overall microstructural evolution was mostly related with the coarsening of Ni phase. Ni-Ni interfacial area was also gradually increased as Ni content increased but controlled by pore phase at low Ni content region and by YSZ phase at intermediate Ni content region. These quantified microstructural properties were used to characterize the electrical properties of Ni-YSZ composite.

• Journal of the Korean Ceramic Society
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• v.47 no.2
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• pp.157-162
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• 2010

Effect of starting SiC particle size on nitridation rate and flexural strength of $Si_3N_4$-bonded-SiC (SNBSC) ceramics was investigated by using SiC particles of different size (${\sim}200\;{\mu}m$, ${\sim}100\;{\mu}m$ and ${\sim}45\;{\mu}m$). The specimen prepared from smaller SiC particles resulted in higher nitridation rate after nitridation at $1450^{\circ}C$, owing to the lower packing density in green body. The flexural strength showed maxima after 1-h nitridation for all specimens and then decreased with prolonged nitridation because of local densification-induced pore coarsening. The specimen prepared from smaller SiC particles showed better flexural strength because of smaller pore size and partly higher nitridation rate in the specimen. A maximal flexural strength of 29 MPa was obtained in the specimen with a density of $2.04\;g{\cdot}cm^3$, which was prepared from $45\;{\mu}m$-SiC particles.

• Macromolecular Research
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• v.17 no.9
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• pp.666-671
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• 2009

Water repellent elastomeric surfaces were fabricated successfully on SBS/MWNT nanocomposites films using the breath figure method and subsequent thermal treatment. The uniformly dispersed CNTs were found to play significant roles in tuning the size and ordering of the macroporous morphology at the nanocomposite surface as well as enhancing the mechanical properties of nanocomposites. In particular, the CNTs dispersed in a nanocomposite solution retarded the coarsening process of aqueous droplets during the breath figure process and decreased the pore size in the finally fabricated film. The water contact angle measurement showed that the double-scale structure comprised of self-organized macropores and surface the roughness induced by a thermal treatment produced a highly water-repellent nanocomposite surface.

• Journal of the Korean Ceramic Society
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• v.56 no.3
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• pp.211-232
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• 2019

Porous ceramics are promising materials for a number of functional and structural applications that include thermal insulation, filters, bio-scaffolds for tissue engineering, and preforms for composite fabrication. These applications take advantage of the special characteristics of porous ceramics, such as low thermal mass, low thermal conductivity, high surface area, controlled permeability, and low density. In this review, we emphasize the direct foaming method, a simple and versatile approach that allows the fabrication of porous ceramics with tailored microstructure, along with distinctive properties. The wet foam stability is achieved under the controlled addition of amphiphiles to the colloidal suspension, which induce in situ hydrophobization, allowing the wet foam to resist coarsening and Ostwald ripening upon drying and sintering. Different components, like contact angle, adsorption free energy, air content, bubble size, and Laplace pressure, play vital roles in the stabilization of the particle stabilized wet foam to the porous ceramics. The mechanical behavior of the load-displacements curves of sintered samples was investigated using Herzian indentations testes. From the collected results, we found that microporous structures with pore sizes from 30 ㎛ to 570 ㎛ and the porosity within the range from 70% to 85%.

• The Sea:JOURNAL OF THE KOREAN SOCIETY OF OCEANOGRAPHY
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• v.6 no.3
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• pp.126-134
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• 2001

Several angular sandstone fragments (about 7 cm in longest diameter) occur in two piston cores, obtained from the submarine trough in the northeastern part of Korea Strait. The origin of the sandstone fragments and the paleoenvironment of trough sediment could be suggested from sedimentary facies analysis of cores and identification of ostracod within sandstone fragments. Echo characteristics around two core sites in submarine trough represent the prolonged bottom echoes with diffuse or no subbottom reflectors. The cores consist of a lower bioturbated mud and an upper gravelly sand sediments with sandstone/shell fragments. The bioturbated mud sediments show low water contents (27-44%) and high shear strength (19.2->37 kPa) compared with those of Holocene sediments (60-219% and 1.0-2.7 kPa, respectively) in the inner shelf and continental slope. However, clay contents (48-56%) of the bioturbated mud sediments are similar to those of fluviatile Holocene sediments in the inner shelf. The mean grain size of gravelly sand sediments ranges from 2.3 to 3.0 ${\phi}$ and shows coarsening upward with sandstone/shell fragments. The Holocene palimpsest in the continental shelf are composed of muddy sand sediments or sandy mud sediments (mean grain size: 4.6-7.6 ${\phi}$). Those suggest that two core sediments might be formed from Paleofluvial and paleocoastal deposits during sea-level lowstand. However, sandstone fragments mainly consist of quartz grains and bioclasts, with carbonate matrix, hollow pore, and glauconite. Two extinct ostracod species, Normanicythere sp. and Kotoracythere sp., are recovered in the sand-stone fragments of core EP-7, and they continued to exist from late Pliocene to early Pleistocene in cold water environment of this area. Thus, the sandstone fragments are interpreted to be formed at the paleocoastal environment derived from the Plio-Pleistocene outcrops exposed around the submarine trough during the LGM (Last Glacial Maximum) period.