• Journal of Korea Foundry Society
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• v.36 no.5
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• pp.159-166
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• 2016

The microstructural evolution of a cast Ni base superalloy, IN738LC, has been investigated after long term exposure at several temperatures. Most of the fine secondary ${\gamma}^{\prime}$ particles resolved after 2000 hour exposure at $816^{\circ}C$. At higher temperatures of $871^{\circ}C$ and $927^{\circ}C$, secondary ${\gamma}^{\prime}$ resolved after 1000 hours of exposure, and cuboidal primary ${\gamma}^{\prime}$ grew with exposure time. During the thermal exposure, ${\sigma}$ phase formed at all tested temperatures, and ${\eta}$ phase was observed around interdendritic regions due to carbide degeneration. The influence of microstructural evolution during thermal exposure on the mechanical properties has been analyzed. The effects of ${\gamma}^{\prime}$ particle growth are more pronounced on the high temperature creep properties than on the room temperature tensile properties.

• Journal of the Korean Ceramic Society
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• v.56 no.3
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• pp.233-255
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• 2019

Thermal batteries are reserve batteries with molten salts as an electrolyte, which activates at high temperature. Due to their excellent reliability, long shelf life, and mechanical robustness, thermal batteries are used in military applications. A high-performance cathode for thermal batteries should be considered in terms of its high capacity, high voltage, and high thermal stability. Research progress on cathode materials from the recent decade is reviewed in this article. The major directions of research were surface modification, compounding of existing materials, fabrication of thin film cathode, and development of new materials. In order to develop a high-performance cathode, a proper combination of these research directions is required while considering mass production and cost.

• Journal of Korea Foundry Society
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• v.27 no.2
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• pp.56-64
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• 2007

• Carbon letters
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• v.4 no.1
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• pp.36-39
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• 2003

Carbon and carbon-based materials are used in nuclear reactors and there has recently been growing interest to develop graphite and carbon based materials for high temperature nuclear and fusion reactors. Efforts are underway to develop high density carbon materials as well as amorphous isotropic carbon for the application in thermal reactors. There has been research on coated nuclear fuel for high temperature reactor and research and development on coated fuels are now focused on fuel particles with high endurance during normal lifetime of the reactor. Since graphite as a moderator as well as structural material in high temperature reactors is one of the most favored choices, it is now felt to develop high density isotropic graphite with suitable coating for safe application of carbon based materials even in oxidizing or water vapor environment. Carboncarbon composite materials compared to conventional graphite materials are now being looked into as the promising materials for the fusion reactor due their ability to have high thermal conductivity and high thermal shock resistance. This paper deals with the application of carbon materials on various nuclear reactors related issues and addresses the current need for focused research on novel carbon materials for future new generation nuclear reactors.

• Journal of Korea Foundry Society
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• v.40 no.4
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• pp.118-127
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• 2020

Microstructural evolution during a heat treatment and high-temperature tensile properties have been investigated in conventionally cast CM247LC. In as-cast specimens, MC carbides with high amounts of Ta, Ti, Hf, and W were found to exist in the interdendritic regions, and γ' was observed in the form of cubes and octocubes prior to decomposition into cubes. In the heat-treated condition, some portion of eutectic γ-γ' remained, and uniform cubic γ' was observed in both interdendritic regions and dendrite core. Three types of carbides with different stoichiometries and compositions were found at the grain boundaries. MC carbides with high Hf contents were observed in the vicinity of eutectic γ-γ'. The highest tensile strength value was found at 750℃, whereas the greatest ductility appeared at 649℃. The effect of the temperature on the tensile properties was closely related to the dislocation structure. With increase in the test temperature, the density of dislocations inside γ' decreased, whereas that in the γ matrix increased. Stacking faults generated in γ' at 750℃ had a strengthening effect, whereas thermally activated dislocation motion at a high temperature was considered to have the opposite effect.

• Journal of Korea Foundry Society
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• v.35 no.6
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• pp.170-177
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• 2015

Segregation during solidification and homogenization during thermal exposure in GTD 111 were investigated. The microstructures of as-cast, standard heat-treated, and thermally exposed specimens were observed by SEM. A compositional analysis of each specimen was conducted by EDS. The dendrite core was enriched in W and Co, though lower levels of Ti and Ta were observed. An unexpected phase, in this case like the ${\eta}$ phase, was observed due to segregation near the ${\gamma}-{\gamma}^{\prime}$ eutectic in the standard heat-treated specimen. Segregation also induced microstructural evolution near the ${\gamma}-{\gamma}^{\prime}$ eutectic during the standard heat treatment. A quantitative analysis and microstructural observations showed that the thermal exposure at a high temperature enhanced the chemical homogeneity of the alloy.

• Journal of Korea Foundry Society
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• v.36 no.5
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• pp.167-173
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• 2016

The effect of various types of heat-treatment on the mechanical properties of cast Alloy 718 has been investigated. Cast Alloy 718 bars were subjected to 'standard heat-treatment'_(SHT), 'HIP (Hot Isostatic Pressing) heat-treatment'_(HHT), and 'HIP-simulated heat-treatment'_(HS). In the absence of long time high temperature heat-treatment, a small amount of Laves phase remained in the 'SHT' specimen, and needle shaped ${\delta}$ precipitated in the vicinity of the Laves phase. Due to the formation of the Laves and ${\delta}$ phases in the 'SHT' specimen, it exhibited lower tensile properties than those of the others_specimens. On the other hand, the Laves phase was completely dissolved into the matrix after 'HHT' and 'HS' treatments. It is known that isostatic pressure reduces the self-diffusion coefficient, because of the lower self-diffusivity under HIP conditions in the interdendritic region, Nb segregation and the high amount of ${\gamma}^{{\prime}{\prime}}$ precipitation that occurs. Due to the higher fraction of coarse ${\gamma}^{{\prime}{\prime}}$ phases, the 'HHT' treated Alloy 718 showed excellent tensile strength.

• Journal of the Korean Ceramic Society
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• v.53 no.5
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• pp.568-573
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• 2016

We explore the crystalline structure and phase transition of lithium thiophosphate ($Li_7P_3S_{11}$) solid electrolyte using electron microscopy and X-ray diffraction. The glass-like $Li_7P_3S_{11}$ powder is prepared by the high-energy mechanical milling process. According to the energy dispersive X-ray spectroscopy (EDS) and selected area diffraction (SAD) analysis, the glass powder shows chemical homogeneity without noticeable contrast variation at any specific spot in the specimen and amorphous SAD ring patterns. Upon heating up to $260^{\circ}C$ the glass $Li_7P_3S_{11}$ powder becomes crystallized, clearly representing crystal plane diffraction contrast in the high-resolution transmission electron microscopy image. We further confirm that each diffraction spot precisely corresponds to the diffraction from a particular $Li_7P_3S_{11}$ crystallographic structure, which is also in good agreement with the previous X-ray diffraction results. We expect that the microscopic analysis with EDS and SAD patterns would permit a new approach to study in the atomic scale of other lithium ion conducting sulfides.

• Proceedings of the Korean Magnestics Society Conference
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• 2015.05a
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• pp.187-188
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• 2015

• Ceramist
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• v.22 no.2
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• pp.110-121
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• 2019

Triboelectric nanogenerators and piezoelectric nanogenerators are a spotlighted energy harvesting method that converts the wasted mechanical energy from the environment into usable electrical energy. In the case of triboelectric nanogenerators, researches have been mainly focused on high permittivity and flexible polymer materials, and in the case of piezoelectric nanogenerators, researches have been focused on ceramic materials exhibiting high polarization characteristics. Recently, many researches have been conducted to improve durability and power in various environments by using composite materials which have flexible properties of polymer, high permittivity, thermal resistance and high polarization properties of ceramics. This article reviews the energy harvesting studies reported about composites materials using ceramics and polymers.