• Transactions of the Korean Society of Mechanical Engineers B
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• v.27 no.8
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• pp.1165-1173
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• 2003

The effects of (1) phonon dispersion on thermal conductivity model and (2) differentiation of group velocity and phase velocity are examined for germanium. The results show drastic change of thermal conductivity regardless of the same relaxation time model. Also the contribution of transverse acoustic (TA) phonon and longitudinal acoustic (LA) phonon on the thermal conductivity at high temperatures is reassessed by considering more rigorous dispersion model. Holland model, which is commonly used for modeling thermal conductivity, underestimates the scattering rate for TA phonon at high frequency. This leads the conclusion that TA is dominant heat transfer mode at high temperatures. But according to the rigorous consideration of phonon dispersion, the reduction of thermal conductivity is much larger than the estimation of Holland model, thus the TA at high frequency is expected to be no more dominant heat transfer mode. Another heat transfer mechanism may exist at high temperatures. Two possible explanations we the roles of (1) Umklapp scattering of LA phonon at high frequency and (2) optical phonon.

• Journal of the Korean Ceramic Society
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• v.49 no.4
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• pp.380-384
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• 2012

This paper deals with the recent developments of high thermal conductivity silicon nitride ceramics. First, the factors that reduce the thermal conductivity of silicon nitride are clarified and the potential approaches to realize high thermal conductivity are described. Then, the recent achievements on the silicon nitride fabricated through the reaction bonding and post sintering technique are presented. Because of a smaller amount of impurity oxygen, the obtained thermal conductivity is substantially higher, compared to that of the conventional gas-pressure sintered silicon nitride, while the microstructures and bending strengths are similar to each other between these two samples. Moreover, further improvement of the thermal conductivity is possible by increasing ${\beta}/{\alpha}$ phase ratio of the nitrided sample, resulting in a very high thermal conductivity of 177 W/($m{\cdot}K$) as well as a high fracture toughness of 11.2 $MPa{\cdot}m^{1/2}$.

• Journal of the Korean Society for Heat Treatment
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• v.20 no.6
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• pp.318-322
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• 2007

The high electrical conductivity Cu-0.15% Sn alloys containing various P contents, and the high conductivity and high strength Cu-0.1% Sn-0.1%Ag alloys with various Mg/P additions were fabricated and their mechanical properties and electrical conductivity were investigated. The electrical conductivity was generally decreased as the P content was increased where as the hardness and strength was shown to increase. When Mg was added to P-containing Cu alloys, the detrimental effect of P on the conductivity was significantly reduced, and TEM observations indicated that the formation of $Mg_3P_2$ phase is responsible for this result.

• Analytical Science and Technology
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• v.13 no.6
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• pp.775-780
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• 2000

The electrical conductivity of glass melter at high temperature has been measured. The conductivity is an important physical property for the research and the manufacturing process of glass. Because high temperature is an inconvenient situation to measure the conductivity of glass melter, we have made a platinum crucible and electrode and have measured the conductivity at high temperature. KCl solution, of which concentration is adjusted to the conductivity of glass melter, is used to get parameters of the conductivity cell. A measuring circuit is composed with an AC 1 kHz sine wave generator and an operational amplifier. The cell constants are determined from the measured voltages and the equivalent conductances of KCl solution. Various cells are tested to find a suitable shape for high temperature experiment. The results are compared by cell size, electrode depth, and cell configuration. The conductivity of the borosilicate melter is $0.053{\Omega}^{-1}cm^{-1}$ at $1,450^{\circ}C$.

• Proceedings of the KSME Conference
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• 2003.04a
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• pp.1627-1632
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• 2003

The effect of (1) phonon dispersion in thermal conductivity model and (2) the differentiation of group velocity and phase velocity for Ge is examined. The results show drastic change of thermal conductivity regardless of using same relaxation time model. Also the contribution of transverse acoustic (TA) phonon and longitudinal acoustic (LA) phonon is changed by considering more rigorous dispersion model. Holland model underestimates the scattering rate for high frequency TA, so misleading conclusion, i.e. TA is dominant heat transfer mode at high temperature. But the actual reduction of thermal conductivity is much larger than the estimation by Holland model and high frequency TA is no more dominant heat transfer mode. Another heat transfer mechanism may exist for high temperature. Two possible explanations are (1) high frequency LA by Umklapp scattering and (2) optical phonon.

• Geomechanics and Engineering
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• v.25 no.2
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• pp.159-170
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• 2021

Particle size of tailings in different areas of dams varies due to sedimentation and separation. Saturated hydraulic conductivity of high-stacked talings materials are seriously affected by void ratio and particle breakage. Conjoined consolidation permeability tests were carried out using a self-developed high-stress permeability and consolidation apparatus. The hydraulic conductivity decreases nonlinearly with the increase of consolidation pressure. The seepage pattern of coarse-particle tailings is channel flow, and the seepage pattern of fine-particle tailings is scattered flow. The change rate of hydraulic conductivity of tailings with different particle sizes under high consolidation pressure tends to be identical. A hydraulic conductivity hysteresis is found in coarse-particle tailings. The hydraulic conductivity hysteresis is more obvious when the water head is lower. A new hydraulic conductivity-void ratio equation was derived by introducing the concept of effective void ratio and breakage index. The equation integrated the hydraulic conductivity equation with different particle sizes over a wide range of consolidation pressures.

• Nuclear Engineering and Technology
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• v.32 no.5
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• pp.495-503
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• 2000

The hydraulic conductivities in the bentonite-sand mixtures with high density were measured, and the effects of sand content and dry density on the hydraulic conductivity were investigated. The hydraulic conductivities of the bentonite-sand mixtures with a dry density of 1.6 Mg/㎥ and 1.8 Mg/㎥ are less than 10$^{-11}$ m/s when the sand content is not higher than 70 wt%. However at the sand content of 90 wt%, the hydraulic conductivity increases rapidly At the same dry density, the logarithm of hydraulic conductivity increases linearly with increasing sand content. The hydraulic conductivity of the bentonite-sand mixture can be explained by the concept of effective clay dry density, and using this concept, the hydraulic conductivities for the mixtures with various sand contents and dry densities can be estimated.

• Proceedings of the Korean Society For Composite Materials Conference
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• 2005.04a
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• pp.257-260
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• 2005

Graphite reinforced conductive polymer composites were fabricated by the compression molding technique. Graphite powder was mixed with an phenol resin to impart electrical property in composites. The ratio and particle size of graphite powder were varied to investigate electrical conductivity of cured composites. In this study, graphite reinforced conductive polymer composites with high filler loadings(>66wt.%) were manufactured to accomplish high electrical conductivity. With increasing the loading ratio of graphite powder, the electrical conductivity and flexural strength increased. However. above 80wt.% filler loadings, flexural strength decreased due to lack of resin. Regardless of graphite particle size, electrical conductivity wasn’t varied. On the other hand, with decreasing particle size, flexural strength increased due to high specific surface area.

• Korean Journal of Materials Research
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• v.32 no.11
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• pp.474-480
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• 2022

Lithium-ion batteries (LIBs) are powerful energy storage devices with several advantages, including high energy density, large voltage window, high cycling stability, and eco-friendliness. However, demand for ultrafast charge/discharge performance is increasing, and many improvements are needed in the electrode which contains the carbon-based active material. Among LIB electrode components, the conductive additive plays an important role, connecting the active materials and enhancing charge transfer within the electrode. This impacts electrical and ionic conductivity, electrical resistance, and the density of the electrode. Therefore, to increase ultrafast cycling performance by enhancing the electrical conductivity and density of the electrode, we complexed Ketjen black and graphene and applied conductive agents. This electrode, with the composite conductive additives, exhibited high electrical conductivity (12.11 S/cm), excellent high-rate performance (28.6 mAh/g at current density of 3,000 mA/g), and great long-term cycling stability at high current density (88.7 % after 500 cycles at current density of 3,000 mA/g). This excellent high-rate performance with cycling stability is attributed to the increased electrical conductivity, due to the increased amount of graphene, which has high intrinsic electrical conductivity, and the high density of the electrode.

• Journal of Korea Foundry Society
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• v.34 no.5
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• pp.151-155
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• 2014

The relatively low conductivity of conventional Mg-Al alloys often limits their areas of application. Therefore, several attempts to develop new high-conductivity magnesium alloys have been made recently. In this research, A Ce-rich rare-earth (RE)material and zinc were added to magnesium which contained no aluminum. As the RE and Zn content were increased, both the hardness and tensile strength were gradually increased, despite the fact that the electrical conductivity decreased slightly. The effects of an aging treatment on the conductivity and mechanical properties of Mg-RE-Zn alloys were also investigated. The electrical conductivity did not change according to the heat treatment conditions; however, the mechanical properties could be enhanced by proper aging heat treatments.