• Electrical & Electronic Materials
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• v.9 no.4
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• pp.396-403
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• 1996

In this paper, the (S $r_{0.85}$.C $a_{0.15}$)Ti $O_{3}$ of paraelectric grain boundary layer (GBL) ceramics were fabricated, and the analysis of microstructuye and the thermally stimulated current(TSC) were investigated for understanding effects of GBL's interfacial phenomenon on variations of electrical properties. As a result, the three peaks of .alpha., .alpha. and .betha. were obtained at the temperature of -20 [.deg. C], 20[.deg. C] and 80[.deg. C], respectively. The origins of these peaks are that the .alpha. peak observed at -20[.deg. C] looks like to be ascribed to the ionization excitation from donor level in the grain, and the .alpha.' peak observed at 20[.deg. C] appears to show up by detrap of the trapped carrier of border between the oxidation layer and the grain, and the .betha. peak observed at 80[.deg. C] seems to be resulted from hopping conduction of existing carrier in the trap site of the border between the oxidation and second phase. and second phase.

• Journal of the Korean Chemical Society
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• v.34 no.3
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• pp.248-254
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• 1990

Theoretical studies on the electrophilic substitution reactions of five-membered heteroaromatic compounds, furan, pyrrole and thiophene, with isopropyl cation were carried out using the MNDO method. The results indicated that site selectivities of ${\alpha},{\beta}$ and hetero-atoms are not controlled by electrostatic interactions but are determined mainly by charge trasfer stabilization between the HOMO of heteroaromatics and the LUMO of the electrophile. The reactivity order for ${\alpha}\;and\;{\beta}$ positions was pyrrole > furan > thiophene, in agreement with the solution-phase as well as the gas-phase experimental results.

• Journal of the Korean Ceramic Society
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• v.29 no.5
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• pp.367-376
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• 1992

The powder characteristics of seeded alumina prepared from alkoxide by sol-gel method were studied. When ${\alpha}$-Al2O3 seeded powders used, these ${\alpha}$ phase transformation temperatures decreased than those of unseeded powders by 110$^{\circ}C$ and fine powders under 0.1 $\mu\textrm{m}$ could be obtained. When Fe-nitrate added powders used, fast transformation rate resulted from ionic effects of Fe3+, but hard aggregated morphology exhibited. When ${\alpha}$-Al2O3 and Fe nitrate simultaneously added, these powders represented lower transformation temperature but resulted in microstructure with aggregated particles.

• Bulletin of the Korean Chemical Society
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• v.14 no.4
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• pp.461-465
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• 1993

Reductive transformations of gem-bromonitro compounds and ${\alpha}$-nitro ketones were carried out conveniently with sodium dithionite by using dioctyl viologen as an electron-transfer catalyst in dichloromethane-water two-phase system:the bromine atom in gem-bromonitro compounds and the nitro group in ${\alpha}$-nitro ketones are replaced by hydrogen.

• Journal of Integrative Natural Science
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• v.8 no.2
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• pp.111-116
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• 2015

Liquid chromatographic enantiomer separation of ${\alpha}$-amino acid esters as nitrobenzoxadiazole (NBD) derivatives was performed using several chiral stationary phases (CSPs) based on polysaccharide derivatives under fluorescence detection. For enantiomer separation by normal HPLC, the non-aqueous derivatization method of ${\alpha}$-amino acid esters for NBD analytes was introduced. Among the six CSPs used in this study, the performance of Chiralpak IA was superior for enantiomer resolution of NBD derivatives of several ${\alpha}$-amino acid methyl esters. Also the convenient analytical method using polysaccharide-derived CSPs developed in this study was applied to determine the optical purity of ${\alpha}$-amino acids esters. It was investigated that the enantiomeric impurity levels of 0.02-1.73% were found after determination of enantiomeric purities of several commercially available L-amino acid methyl esters. It is expected to be quite useful for enantiomer separation of other ${\alpha}$-amino acid esters as NBD derivatives by normal HPLC.

• Bulletin of the Korean Chemical Society
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• v.32 no.7
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• pp.2453-2456
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• 2011

• Bulletin of the Korean Chemical Society
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• v.25 no.3
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• pp.347-348
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• 2004

• Journal of the Korean Ceramic Society
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• v.35 no.4
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• pp.413-419
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• 1998

${\beta}$-silicon carbides with yttrium aluminum garnet of 2,5,10 mol% were prepared by a liquid--phase sint-ering and the microstructural evolution and phase transformation were investigated during sintering as functions of liquid-phase amount and sintering time. The rate of grain growth decreases with the addition of the amount of yttrium aluminum garnet (YAG) in the SiC starting powder however the apparent density and the aspect ratio of grains in sintered body increase. The phase transformation from ${\beta}$-SiC to ${\alpha}$-SiC were dependent on the liquid-phase amount and sintering time.

• Journal of Korea Foundry Society
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• v.33 no.3
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• pp.103-112
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• 2013

Effect of Fe and Mn contents on the tensile properties of Al-4 wt%Mg-0.9 wt%Si alloy system has been studied. Common phases of Al-4 wt%Mg-0.9 wt%Si alloy system were ${\alpha}$-Al, $Mg_2Si$, ${\alpha}-Al_{12}(Fe,Mn)_3Si$ and ${\beta}-Al_5FeSi$. As Fe content of Al-4 wt%Mg-0.9 wt%Si alloy system increased from 0.15 wt% to above 0.3 wt%, ${\beta}-Al_5FeSi$ compound appeared. When Mn content of the alloy increased from 0.3 wt% to 0.5 wt%, morphology of plate shaped ${\beta}-Al_5FeSi$ compound changed to chinese script ${\alpha}-Al_{12}(Fe,Mn)_3Si$. As Fe content of Al-4 wt%Mg-0.9 wt%Si-0.3 wt%Mn alloy increased from 0.15 wt% to 0.4 wt%, tensile strength of the as-cast alloy decreased from 191 MPa to 183 MPa and, elongation of the alloy also decreased from 8.0% to 6.2%. Decrease of these properties can be explained as the formation of plate shape, ${\beta}-Al_5FeSi$ phase with low Mn/Fe ratio of the alloy. However, when Mn content of Al-4 wt%Mg-0.9 wt%Si-0.3 wt%Fe alloy increased from 0.3 wt% to 0.5 wt%, tensile strength of as-cast alloy increased from 181 MPa to 194 MPa and, elongation of the alloy increased from 6.8% to 7.0%. These improvements attribute to the morphology change from ${\beta}-Al_5FeSi$ phase to chinese script, ${\alpha}-Al_{15}(Fe,Mn)_3Si_2$ phase shape-modified from with high Mn/Fe ratio of the alloy.

• Transactions of the Korean hydrogen and new energy society
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• v.9 no.1
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• pp.25-30
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• 1998

4-probe resistivity measurement technique was used to study kinetics of hydrogen absorption on Pd film ($180{\AA}$ thick) in the ${\alpha}$ phase. Hydrogen gas was introduced to the activated Pd film. For very low hydrogen concentration the following rate law is valid in ${\alpha}$ phase very thin Pd film $$v=k\frac{1}{1+KX{_H}}PH{_2}-k^{\prime}\frac{KX{_H}{^2}}{1+KX{_H}}$$ which is similar to that of bulk. The activation energy of the forward reaction is 4.6kcal/mol H and of the backward reaction 8.4kcal/mol H, which yields the reaction enthalpy -3.8kcal/mol H in the temperature range between 25 and $40^{\circ}C$. The values of activation and enthalpy of thin film are rather smaller than that of bulk sample. This may be due to surface area difference between bulk and film.