• Proceedings of the Materials Research Society of Korea Conference
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• 2012.05a
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• pp.107.1-107.1
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• 2012

Because of beautiful glossy and color, the value of gold leverage is very high in Europe. For improve the quality of white gold, we performed heat treatment on 14K white gold alloys at various age-hardening conditions. Age-hardening behavior and the related phase transformation changes were studied to elucidate the hardening mechanism of 14K white gold alloys. For solid solution treatment [ST], casted 14K white gold alloy specimens were treated at high temperature ($750^{\circ}C$) during 30 minute, and the specimens dropped to water for quenching immediately. For Age-hardening treatment [AT], the specimens were treated at various temperatures ($250^{\circ}C{\sim}300^{\circ}C$). After the heat treatment, we observed increased hardness from 144 Hv to 214 Hv by Vicker's hardness tester. Variation of the grain size measured by optical microscopy (OM) and scanning electron microscopy (SEM) images. By electron probe micro-analysis (EPMA) mapping analysis, we investigated that irregular particles were changed uniformly. After heat treatment, 14K white gold alloys showed improved hardness and became uniformity of grain size by age-hardening treatment.

• International Journal of Aeronautical and Space Sciences
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• v.7 no.1
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• pp.137-147
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• 2006

In this study, shape memory alloy damper with characteristics of pseudoelastic hysteresis for helicopter rotor blades are investigated. SMAs can be available in damping augmentation of vibrating structures. SMAs show large hysteresis in the process of pseudoelastic austenite-martensite phase transformation which takes place while subjected to loading above the austenite finish temperature. Since SMAs display pseudoelastic hysteresis behavior over large strain ranges, a significant amount of energy dissipation is possible. A damper can be designed with SMA wires prestressed to a baseline level somewhere in the middle of the pseudoelastic stress range. An experimental study of the effects of pre-strain and cyclic strain amplitude as well as frequency on the damping behavior of pseudoelastic shape memory alloy wires are performed. The effects of the shape memory alloy damper on aeroelastic and ground resonance stability of helicopter are studied. In aeroelastic stability, the dynamic characteristics of blades related to pitch angle and the amplitude of lag motion for the rotor equipped with SMA damper were examined. The performance of SMA damper on ground resonance instability are presented through the frequencies and modal damping with respect to rotating speed.

• Journal of the Semiconductor & Display Technology
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• v.6 no.3
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• pp.13-17
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• 2007

Effect of Co substitution on crystal structures of two nickel silicides, NiSi and $NiSi_2$, is investigated by using an ab initio calculation. Relaxed NiSi and $NiSi_2$ structures are calculated and the calculated lattice parameters are in good agreement with experimentally determined lattice parameters within about 2%. A Co atom substitutes a Ni and Si site, respectively, to evaluate the preferable site between them. Co prefers Ni site to Si site in both NiSi and $NiSi_2$. The calculated total energy also indicates that the Co substitution to Ni site stabilizes both the NiSi and $NiSi_2$ structures. Co also prefers Ni site in $NiSi_2$ to that in NiSi, indicating that $NiSi_2$ becomes more stable than NiSi with Co substitution. As Co addition to NiSi improves its thermal stability experimentally, this indicates that the energy barrier between the two phases is high enough to prevent the phase transformation from NiSi to $NiSi_2$ up to high temperature.

• Bulletin of the Korean Chemical Society
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• v.31 no.1
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• pp.59-63
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• 2010

Calculations are presented for the $\gamma$-aminobutyric acid-$(H_2O)_n$ (n = 0-5) clusters in both canonical and zwitterionic forms. We examine the effects of microsolvation on the structures and transformation between the canonical and zwitterionic forms. The canonical forms are predicted to be more stable for n = 0-4. With five microsolvating water molecules, the two forms of $\gamma$-aminobutyric acid become quasidegenerate, with the energies of zwitterionic forms slightly (by 1 - 3 kcal/mol) higher. The lowest energy zwitterionic conformer of $\gamma$-aminobutyric acid-$(H_2O)_5$ cluster is calculated to isomerize to canonical form through a barrier-less proton transfer process and is thus predicted to be kinetically unstable. Therefore, we predict that the canonical conformers of $\gamma$-aminobutyric acid should be observed predominantly in the gas phase at low temperature in presence of up to five water molecules.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.22 no.9
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• pp.776-780
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• 2009

In this study, the opto-electric properties of EL devices with PMN dielectric layer with variation of firing tempereature were investigated. For the PMN dielectric layer process, the paste was prepared by optimization of quantitative mixing of PMN powder, $BaTiO_3$, Glass Frit, $\alpha$-Terpineol and ethyl cellulose. The EL device stack consists of Alumina substrate ($Al_2O_3$), metallic electrode (Au), insulating layer (manufactured PMN paste), phosphor layer (ELPP- 030, ELK) and transparent electrode (ITO), which is well structure as a thick film EL device. The phase transformation properties of PMN dielectric with various firing temperatures of $150^{\circ}C$ to $850^{\circ}C$ was characterized by XRD. Also the opto-electric properties of EL devices with different firing temperature were investigated by LCR meter and spectrometer. We found the best opto-electric property was obtained at the condition of $550^{\circ}C$ firing which is 3432.96 $cd/m^2$ at 1948.3 pF Capacitance, 40 kHz Frequency, 40% Duty, Vth+330 V voltage.

• Journal of the Korean Ceramic Society
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• v.48 no.5
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• pp.379-383
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• 2011

This paper presents the synthesis of one-dimensional spinel $LiMn_2O_4$ nanostructures using a facile and scalable two-step process. $LiMn_2O_4$ nanorods with average diameter of 100 nm and length of 1.5 ${\mu}m$ have been prepared by solid-state lithiation of hydrothermally synthesized ${\beta}$-$MnO_2$ nanorods. $LiMn_2O_4$ nanowires with diameter of 10 nm and length of several micrometers have been fabricated via solid-state lithiation of ${\beta}$-$MnO_2$ nanowires. The precursors have been lithiated with LiOH and reaction temperature and pressure have been controlled. The complete structural transformation to cubic phase and the maintenance of 1-D nanostructure morphology have been evaluated by XRD, SEM, and TEM analysis. The size distribution of the spinel $LiMn_2O_4$ nanorods/wires has been similar to the $MnO_2$ precursors. By control of reaction pressure, cubic 1-D spinel $LiMn_2O_4$ nanostructures have been fabricated from tetragonal $MnO_2$ precursors even below $500^{\circ}C$.

• Journal of the Korean Society for Heat Treatment
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• v.31 no.5
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• pp.231-236
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• 2018

The purpose of this study was to investigate the microstructural characteristics and hardness distribution of AZ91 magnesium alloy furnace-cooled to room temperature after solution treatment, and to compare the results with those of as-cast condition. The as-cast alloy showed a partially divorced eutectic ${\beta}(Mg_{17}Al_{12})$ phase and discontinuous precipitates (DPs) with a lamellar morphology, while only DPs were observed in the furnace-cooled alloy. The DPs in the furnace-cooled AZ91 alloy had various apparent interlamellar spacings, which would be ascribed to the different transformation temperatures during the furnace cooling. The average hardness for the furnace-cooled alloy is similar to that for the as-cast alloy. It is interesting to note that the hardness values of the furnace-cooled alloy were distributed over a narrower range than those of the as-cast alloy. This is likely to be caused by the relatively more homogeneous microstructure of the furnace-cooled alloy in comparison with the ascast one.

• Journal of the Korean Wood Science and Technology
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• v.31 no.6
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• pp.1-7
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• 2003

The phase transformation from cellulose I into cellulose II in woods by way of Na-cellulose I was examined by x-ray diffraction analysis.The formation of Na-cellulose I in woods increased with the increase of treating time in alkali solution. When compression wood was treated with 20% NaOH solution at room temperature for 1 day, the x-ray diagram showed only Na-cellulose I. On the other hand, the x-ray diagram of tension wood showed a mixture of cellulose I and Na-cellulose I. Cellulose I of tension wood could not be transformed completely into Na-cellulose I even after 10-day treatment, but was transformed into Na-cellulose I after 30-day treatment. Na-cellulose I of compression and tension woods was converted to the cellulose I pattern and the mixture of cellulose I and cellulose II, respectively, after washing with water and drying at 20℃. Cellulose I regenerated from Na-cellulose I in wood could not be converted to cellulose II by delignification. Thus, it revealed that the delignification of the alkali-treated wood did not affect their cellulose structures. From the results, therefore, it can be concluded that lignin in woods prevents the formation of the stable Na-cellulose I and the conversion from cellulose I to cellulose II. This means that the conversion of chain polarity of wood cellulose hardly occurs during mercerization because cellulose microfibrils are fixed by lignin which not to be intermingled.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.6 no.2
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• pp.238-246
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• 1996

High-density SiC-AIN solid solutions were fabricated from powder mixtures of $\beta$-SiC and AIN by hot-pressing in the 1870 to $2030^{\circ}C$ temperature range. The reaction of AIN and $\beta$-SiC (3C) powder transformed to the 2 H (wurzite) structure appeared to depend on the temperature and SiC/A1N ratio and seeds present. The crystalline phases consisted of a SiC-rich solid-solution phase and an A1N-rich solid-solution phase. At $2030^{\circ}C$ for 1 h, for a composition of 50 % AIN/50 % SiC with a seeding of $\alpha$-SiC, the complete solid solution could be obtained and the microstructures are equiaxed with a relatively homogeneous grain size of 2 H phases. The variation of the seeding of $\alpha$-SiC in SIC-A1N solid solutions could be attributed to the transformation behaviour and differences in size and shape of the grains, as well as to other factors, such as grain size distributions, compositional inhomogeneity, and structural defects.

• Journal of the Mineralogical Society of Korea
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• v.25 no.4
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• pp.283-293
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• 2012

In order to have better insights into the chemical differentiation of Earth from its magma ocean phase to the current stratified structure, detailed information of crystallization kinetics of silicate melts consisting of the magma ocean is essential. The structural transitions in oxide glasses and melts upon crystallization provide improved prospects for a systematic and quantitative understanding of the crystallization processes. Here, we report the $^{27}Al$ 3QMAS NMR spectra for sol-gel synthesized $Al_2O_3$ glass with varying temperature and annealing time. The NMR spectra for the amorphous $Al_2O_3$ show well-resolved Al coordination environments, characterized with mostly $^{[4,5]}Al$ and a minor fraction of $^{[6]}Al$. The fraction of $^{[5]}Al$ in the alumina phase decreases with increasing annealing time at constant temperature. The NMR results of $Al_2O_3$ phases also imply that multiple processes (e.g., crystallization and/or changes in structural disorder within glasses) could involve upon its phase transition. The current results and method can be useful to understand crystallization kinetics of diverse natural and multi-component silicate glasses and melts. The potential result may yield atomic-level understanding of Earth's chemical evolution and differentiation from the magma ocean.