• Geomechanics and Engineering
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• v.18 no.3
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• pp.299-313
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• 2019

This research investigated the nonlinear compressibility, permeability, the yielding due to structural degradation and their effects on consolidation behavior of structured soft soils. Based on oedometer and hydraulic conductivity test results of natural and reconstituted soft clays, linear log (1+e) ~ $log\;{\sigma}^{\prime}$ and log (1+e) ~ $log\;k_v$ relationships were developed to capture the variations in compressibility and permeability, and the yield stress ratio (YSR) was introduced to characterize the soil structure of natural soft clay. Semi-analytical solutions for one-dimensional consolidation of soft clay under time-dependent loading incorporating the effects of soil nonlinearity and soil structure were proposed. The semi-analytical solutions were verified against field measurements of a well-documented test embankment and they can give better accuracy in prediction of excess pore pressure compared to the predictions using the existing analytical solutions. Additionally, parametric studies were conducted to analyze the effects of YSR, compression index (${\lambda}_r$ and ${\lambda}_c$), and permeability index (${\eta}_k$) on the consolidation behavior of structured soft clays. The magnitude of the difference between degree of consolidation based on excess pore pressure ($U_p$) and that based on strain ($U_s$) depends on YSR. The parameter ${\lambda}_c/{\eta}_k$ plays a significant role in predicting consolidation behavior.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.35 no.1
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• pp.86-92
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• 2022

Despite otherwise advantageous properties, the performance and reliability of devices manufactured in β-Ga₂O₃ on semi-insulating Ga₂O₃ substrates may degrade because of poorly mitigated self-heating, which results from the low thermal conductivity of Ga₂O₃ substrates. In this work, we investigate and compare self-heating and device performance of β-Ga₂O₃ MESFETs on substrates of semi-insulating Ga₂O₃ and 4H-SiC. Electron mobility in β-Ga₂O₃ is negatively affected by increasing lattice temperature, which consequently also negatively influences device conductance. The superior thermal conductivity of 4H-SiC substrates resulted in reduced β-Ga₂O₃ lattice temperatures and, thus, mitigates MESFET drain current degradation. This, in turn, allows practically reduced device dimensions without deteriorating the performance and improved device reliability.

• Applied Microscopy
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• v.47 no.3
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• pp.105-109
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• 2017

The effect of temperature and vanadium metal concentration on the electronic and thermoelectric properties of Si in the diamond cubic structure has been investigated using a combination of density functional theory simulations and the semi classical Boltzmann's theory. The BotzTrap code within the constant relaxation time approximation has been used to obtain the Seebeck coefficient and other transport properties of interest for alloys of the structure $Si_{1-x}V_x$, where x is 0, 0.125, 0.25, 0.375, and 0.5. The thermoelectric properties have been extracted for a temperature range of 300 K to 1,000 K. The general trend with V atom substitution for Si causes the Seeback coefficient to increase and the thermal conductivity to decrease for the various alloys. The optimum values are for $Si_5V_3$ and $Si_4V_4$ alloys for charge carrier concentrations of $10^{21}cm^{-3}$ in the mid temperature range of 500~800 K. This is a very desirable effect for a promising thermoelectric and the figure of merit ZT approaches 0.2 at 600 K for the p-type $Si_5V_3$ alloy.

• Proceedings of the Korean Society of Soil and Groundwater Environment Conference
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• 2005.04a
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• pp.284-287
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• 2005

The flow of groundwater in fractured medium is related to the geometric characteristics of the fracture system. And a fracture aperture and a fracture density are considered as important factor concerning the permeability. Data acquisition of the properties of fracture such as aperture and density is so difficult and has uncertainty. We also cannot know the fracture characteristics through the in-situ tests. We usually obtain the fracture information from a ultrasonic scan logging or borehole television indirectly. Using the deduced results, we can make the fracture system and simulate the groundwater flow and solute transport in the crystalline rock. This study aimed to analyze the correlation between the properties of fracture and hydraulic conductivities obtained at the same interval. The properties of fracture are examined by acoustic televiwer and hydraulic conductivities are obtained by constant Pressure injection test. The distributioin of fracture width and fracture frequency shows the log-normal probability plot. And, Results of correlation analysis explain that opened type fractures have proper relation with hydraulic conductivity. But, as though there are semi-opened type fractures or closed type fractures, those have the permeable structure.

• Journal of the Korean Society for Heat Treatment
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• v.26 no.4
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• pp.165-172
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• 2013

In the present study, the microstructure and solution treatment response of Al-Zn-Mg alloys bars by thixo-extrusion was investigated. The alloy bars were solution treated at 400, 430, 460 and $490^{\circ}C$ for various times. In order to examine the microstructures and phase analysis of the samples after solution treatment, it was performed by optical and scanning electron microscopy. And, Vickers hardness and electrical conductivity was measured on the solution treated samples for each condition to investigate the solution treatment response of extruded bars during solution treatment. The results show that the optimum solution heat treatment conditions of thixo-extruded Al-Zn-Mg alloy for minimization of the grain growth and degradation promotion of the second phase is a temperature of $460^{\circ}C$ and holding time of 0.5 to 2 h.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2009.04b
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• pp.17-17
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• 2009

We report the invention of a direct growth method termed On-Film Formation of Nanowire (OFF-ON) for making high-quality single-crystal nanowires, i.e. Bi and $Bi_2Te_3$, without the use of conventional templates, catalysts, or starting materials. We have used the OFF-ON technique to grow single crystal semi-metallic Bi and compound semiconductor $Bi_2Te_3$ nanowires from sputtered Bi and BiTe films after thermal annealing, respectively. The mechanism for nanowire growth is stress-induced mass flow along grain boundaries in the polycrystalline films. OFF-ON is a simple but powerful method for growing perfect single-crystal semi-metallic and compound semiconductor nanowires of high aspect ratio with high crystallinity that distinguishes it from other competitive growth approaches that have been developed to date. Our results suggest that Bi and $Bi_2Te_3$ nanowires grown by OFF-ON can be an ideal material system for exploring their unique thermoelectric properties due to their high-quality single crystalline and high conductivity, which have consequence and relevance for high-efficiency thermoelectric devices.

• Journal of Korean Society for Atmospheric Environment
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• v.34 no.1
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• pp.177-186
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• 2018

It is a well-known fact that precipitation plays an important role in capturing ambient particles, however, the details of particle scavenging properties have not been fully proved. To clarify the particle scavenging properties, a complementary study was carried out with the bulk and semi-bulk rain samples collected in an urban city of Japan. pH showed a continued downturn for a little bit at the beginning rainfall and then a turn-up in the following rainfall. The recorded pH values of rainwater (ranged from 3.5-4.6) demonstrated that the strong acid rain was observed during our field measurements. Compared to the subsequent rainfall, electrical conductivity in the beginning rainfall had about 1.3 times higher level. Sulfur showed an overwhelmingly high concentration compared to other elements in both ambient total suspended particles (TSP) and rain samples. On the contrary to ambient TSP, every element including Ca and Zn in rain showed a continued rise in concentration accompanied by increasing of rainfall amount. During the first period of the rainfall there was no meaningful change in elemental carbon concentration, however, it was largely increased (up to $0.2mg\;L^{-1}$) in the sequential rainfall (4.0-4.5 mm rainfall amount). The theoretically calculated number concentration of particles scavenged by raindrops showed a strong decrease of with the increasing droplet diameter regardless of particle type.

• Composites Research
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• v.34 no.1
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• pp.16-22
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• 2021

Polyurethane foam (PUF) can be manufactured in soft, semi-rigid, and hard forms, so it is used in various fields industrially. Among them, rigid PUF has excellent mechanical properties and low thermal conductivity, and is used as a thermal insulation material for buildings and as a cold insulation material in the natural gas transportation field. In this field, there is a steady demand on higher mechanical strength and lower thermal conductivity. In this study, a rigid PUF was manufactured, and the microstructure and physical properties were studied according to the impeller type (propeller, dispersed turbine) of the agitator. Through FE-SEM and Micro-CT analysis, it was confirmed that the average pore size of the foam manufactured with the dispersed turbine was 21.5% smaller than that of the pore made by the propeller. The compressive strength was improved by 15.4%, and the thermal conductivity decreased by 3.1% in the foam with small pores. This result can be utilized for fabricating PUF composites.

• Tunnel and Underground Space
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• v.9 no.1
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• pp.27-35
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• 1999

The characteristics of heat conduction for the heat source boundary like an arch shape cavern are different from those for the semi-infinite or circular boundary which can be driven theoretically. A new form of transient heat conduction equation in rock mass around the arch shape cavern is evaluated with analyzing the pattern of the rock temperature distribution measured at the cold storage pilot plant. The new equation, which is driven by adopting a shape function, $SF=\sqrt{logx_0/log(x_0+x)}$ to the solution for a semi-infinite boundary, has the semi-radial form of temperature variation with distance. And, thermal properties of rock mass are estimated by comparing the rock temperature distributions by this equation with those by measurement. Thermal conductivity and specific heat of rock mass are estimated as giving the difference of 20~25% compared to those of laboratory scale. This difference seems to be caused by discontinuity like joint and water content in rock mass.

• Journal of the Korean Geotechnical Society
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• v.27 no.10
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• pp.105-116
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• 2011

No analytical solution exists for evaluating in-situ hydraulic conductivity of vertical cutoff walls by analyzing slug test results. Recently, an analytical solution to interpret slug tests has been proposed for a partially penetrated well in an aquifer. However, this analytical solution cannot be directly applied to the cutoff wall because the solution has been developed exclusively for an infinite aquifer instead of a narrow cutoff wall. To consider the cutoff wall boundary conditions, the analytical solution has been modified in this study to take into account the narrow boundaries by introducing the imaginary well theory. Two boundary conditions are considered according to the existence of filter cakes: constant head boundary and no flux boundary. Generalized steady-state shape factors are presented for each geometric condition, which can be used for evaluating the in-situ hydraulic conductivity of cutoff walls. The constant head boundary condition provides higher shape factors and no flux boundary condition provides lower shape factors than the infinite aquifer, which enables to adjust the in-situ hydraulic conductivity of the cutoff wall. The hydraulic conductivities calculated from the analytical solution in this paper give about 1.2~1.7 times higher than those from the Bouwer and Rice method, one of the semi-empirical formulas. Considering the compressibility of the backfill material, the analytical solution developed in this study was proved to correspond to the case of incompressible backfill materials.