• Proceedings of the Acoustical Society of Korea Conference
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• autumn
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• pp.223-226
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• 2001

Acoustic pressure transmission coefficient and phase velocity are measured as the functions of water porosity and air porosity in sand sediment slabs with water- and air-filled pores. Pores in the sand sediment slab we modeled as the structure of circular cylindrical tube shape filled with water and air. The first kind(fast) wave and second kind (slow) wave, identified by Biot, in the solid and fluid mixed medium are affected by the presence of water and air pores. Acoustic characteristics of such porous medium in water are also theoretically investigated in terms of the modified Biot-Attenborough (MBA) model, which uses the separate treatment of viscosity effect and thermal effect in non-rigid porous medium with water- and air-filed pores. The information on the fast waves introduces new concepts of the generalized tortuosity factor and dynamic shape factor.

• Journal of the Korean Ceramic Society
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• v.40 no.12
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• pp.1159-1164
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• 2003

Two processing routes (i.e., the gel casting and polymer preform routes) using polymer beads were studied to fabricate porous ceramics with a cellular structure. The gel casting route, comprising the gel casting of a ceramic slurry mixed with polymer beads, was found to be inadequate to produce porous ceramic bodies with a interconnected pore structure, due to complete coating of the slurry on the polymer beads, which left just isolated pores in the final sintered bodies. The polymer preform route, involving the infiltration of a polymer beads preform with the ceramic slurry, successfully produced porous ceramics with a highly interconnected network of spherical pores. The pore size of 250-300 $\mu\textrm{m}$ was demonstrated and the porosity ranged from 82 to 86%. This process is advantageous to control the pore size because it is determined by the sizes of polymer beads used. Another feature is the avoidance of hollow skeleton, giving a high strength.

• Computers and Concrete
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• v.11 no.4
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• pp.317-336
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• 2013

In this study, the pore structure of Portland cement paste is experimentally characterized by MIP (mercury intrusion porosimetry) and nitrogen adsorption, and simulated by a newly developed status-oriented computer model. Cement pastes with w/c=0.3, 0.4 and 0.5 at ages from 1 day to 120 days are comprehensively investigated. It is found that MIP cannot generate valid pore size distribution curves for cement paste. Nevertheless, nitrogen adsorption can give much more realistic pore size distribution curves of small capillary pores, and these curves follow the same distribution mode. While, large capillary pores can be effectively characterized by the newly developed computer model, and the validity of this model has been proved by BSE imaging plus image analysis. Based on the experimental findings and numerical simulation, a hypothesis is proposed to explain the formation mechanism of the capillary pore system, and the realistic representation of the pore structure of hydrated cement paste is established.

• Journal of Electrochemical Science and Technology
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• v.12 no.2
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• pp.257-265
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• 2021

In this study, we report niobium (Nb) with hierarchical porous structure produced by a one-pot, HF-free electrochemical etching process. It is proved experimentally that a well-defined hierarchical porous structure is produced from the combination of a limited repetition of pulse etching and high concentration of aggressive anion (i.e., SO₄^2-), which results in hierarchical pores with high order over 3. A formula is derived for the surface area of porous Nb as a function of the hierarchical order of pores while the experimental surface area is estimated on the basis of the electrochemical gas evolution rate on porous Nb. From the comparison of the theoretical and experimental surface areas, an in-depth understanding was gained about porous structure produced in this work in terms of the actual pore shape and hierarchical pore order.

• Korean Journal of Materials Research
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• v.23 no.12
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• pp.722-726
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• 2013

Freeze drying of a porous Cu-Sn alloy with unidirectionally aligned pore channels was accomplished by using a composite powder of CuO-$SnO_2$ and camphene. Camphene slurries with CuO-$SnO_2$ content of 3, 5 and 10 vol% were prepared by mixing with a small amount of dispersant at $50^{\circ}C$. Freezing of a slurry was done at $-25^{\circ}C$ while the growth direction of the camphene was unidirectionally controlled. Pores were generated subsequently by sublimation of the camphene during drying in air for 48 h. The green bodies were hydrogen-reduced at $650^{\circ}C$ and then were sintered at $650^{\circ}C$ and $750^{\circ}C$ for 1 h. XRD analysis revealed that the CuO-$SnO_2$ powder was completely converted to Cu-Sn alloy without any reaction phases. The sintered samples showed large pores with an average size of above $100{\mu}m$ which were aligned parallel to the camphene growth direction. Also, the internal walls of the large pores had relatively small pores. The size of the large pores decreased with increasing CuO-$SnO_2$ content due to the change of the degree of powder rearrangement in the slurry. The size of the small pores decreased with increase of the sintering temperature from $650^{\circ}C$ to $750^{\circ}C$, while that of the large pores was unchanged. These results suggest that a porous alloy body with aligned large pores can be fabricated by a freeze-drying and hydrogen reduction process using oxide powders.

• Korean Journal of Materials Research
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• v.28 no.2
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• pp.108-112
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• 2018

The present study demonstrates the effect of raw powder on the pore structure of porous W-Ni prepared by freeze drying of camphene-based slurries and sintering process. The reduction behavior of $WO_3$ and $WO_3-NiO$ powders is analyzed by a temperature programmed reduction method in Ar-10% H2 atmosphere. After heat treatment in hydrogen atmosphere, $WO_3-NiO$ powder mixture is completely converted to metallic W without any reaction phases. Camphene slurries with oxide powders are frozen at $-30^{\circ}C$, and pores in the frozen specimens are generated by sublimation of the camphene during drying in air. The green bodies are hydrogen-reduced at $800^{\circ}C$ and sintered at $1000^{\circ}C$ for 1 h. The sintered samples show large and aligned parallel pores to the camphene growth direction, and small pores in the internal wall of large pores. The strut between large pores, prepared from pure $WO_3$ powder, consists of very fine particles with partially necking between the particles. In contrast, the strut densification is clearly observed in the Ni-added W sample due to the enhanced mass transport in activation sintering.

• Journal of Powder Materials
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• v.21 no.1
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• pp.34-38
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• 2014

Cu-Ni alloys with unidirectionally aligned pores were prepared by freeze-drying process of CuO-NiO/camphene slurry. Camphene slurries with dispersion stability by the addition of oligomeric polyester were frozen at $-25^{\circ}C$, and pores in the frozen specimens were generated by sublimation of the camphene during drying in air. The green bodies were hydrogen-reduced at $300^{\circ}C$ and sintered at $850^{\circ}C$ for 1 h. X-ray diffraction analysis revealed that CuO-NiO composite powders were completely converted to Cu-Ni alloy without any reaction phases by hydrogen reduction. The sintered samples showed large and aligned parallel pores to the camphene growth direction, and small pores in the internal wall of large pores. The pore size and porosity decreased with increase in CuO-NiO content from 5 to 10 vol%. The change of pore characteristics was explained by the degree of powder rearrangement in slurry and the accumulation behavior of powders in the interdendritic spaces of solidified camphene.

• Journal of the Korean Ceramic Society
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• v.26 no.6
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• pp.729-736
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• 1989

This study aims to reinforce concrete structure by impregnation of molten sulfur. The improved properties of sulfur impregnated concerete were confirmed by compressive strength test and water proof effect. Following variables were adopted to evaluate impregnation parameters ; 1) the effect of water content in concrete structure (0-8%) 2) impregnation time of molten sulfur(0-22hr) 3) impregnation temprature of molten sulfur(13$0^{\circ}C$, 14$0^{\circ}C$). In partial ponding experiments, the concrete specimen of sulfur impregnated by 2wt% yields 1.5 times higher value of compressive strength than that of control one(non-impregnated concerte). In complete ponding experiments, the mortar specimen of slufur impregnated by 12-14wt% yields 2-3 times higher value of compressive strength than that of control one (non-impregnated mortar). From the examination of X-ray diffractions, $\alpha$-sulfur was found in concrete pores. Homogeneous impregation of molten sulfur into concrete pores was also identified with poresize analysis and micrographs of SEM.

• The Bulletin of The Korean Astronomical Society
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• v.39 no.2
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• pp.98-98
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• 2014

Due to the simple vertical structure of magnetic field, pores can be exploited to study the transport of mechanical energy by waves along the magnetic field to the chromosphere and corona. For a better understanding of physics of pores, we have investigated chromospheric traveling features running across two merged pores from their centers at the speed about 55 km s-1, in the active region AR 11828. The pores were observed on 2013 August 24 by using high time, spatial, and spectral resolution data from the Fast Imaging Solar Spectrograph (FISS) of the 1.6 meter New Solar Telescope (NST). We infer a LOS velocity by applying the bisector method to the Ca II $8542{\AA}$ band and $H{\alpha}$ band, and investigate intensity and the line-of-sight velocity changes at different wavelengths and different positions at the pores. We find that they have 3 minutes oscillations, and the intensity oscillation from the line center is preceded by that from the core ($-0.3{\AA}$) of the bands. There is no phase difference between the intensity and the LOS velocity oscillations at a given wavelength. The amplitude of LOS velocity from near the core spectra is greater than that from the far core spectra. These results support the interpretation of the observed wave as a slow magnetoacoustic wave propagating along the magnetic field lines in the pores. The apparent horizontal motion and a sudden decrease of its speed beyond the pores can be explained by the projection effect caused by inclination of the magnetic field with a canopy.

• Journal of Powder Materials
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• v.25 no.1
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• pp.25-29
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• 2018

Porous Cu with a dispersion of nanoscale $Al_2O_3$ particles is fabricated by freeze-drying $CuO-Al_2O_3$/camphene slurry and sintering. Camphene slurries with $CuO-Al_2O_3$ contents of 5 and 10 vol% are unidirectionally frozen at $-30^{\circ}C$, and pores are generated in the frozen specimens by camphene sublimation during air drying. The green bodies are sintered for 1 h at $700^{\circ}C$ and $800^{\circ}C$ in $H_2$ atmosphere. The sintered samples show large pores of $100{\mu}m$ in average size aligned parallel to the camphene growth direction. The internal walls of the large pores feature relatively small pores of ${\sim}10{\mu}m$ in size. The size of the large pores decreases with increasing $CuO-Al_2O_3$ content by the changing degree of powder rearrangement in the slurry. The size of the small pores decreases with increasing sintering temperature. Microstructural analysis reveals that 100-nm $Al_2O_3$ particles are homogeneously dispersed in the Cu matrix. These results suggest that a porous composite body with aligned large pores could be fabricated by a freeze-drying and $H_2$ reducing process.