• 한국초전도학회:학술대회논문집
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• v.9
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• pp.189-195
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• 1999

We have studied the vortex dynamics in YBa$_2Cu_3O_7$ single crystals with columnar defects using micro Hall-probe array. The Hall-probe array technique allowed a simultaneous measurement of the time and spatial dependence of the vortex density so that more detailed information on flux dynamics could be obtained. We found that field profiles inside sample were similar to the Bean's critical state model from the magnetic hysteresis measurement. Normalized relaxation rates were maximum near the center and decreased toward the edge if applied field H$_{app}$ is greater than the penetration field H. But applied magnetic field H$_{app}$ is less than H, relaxation rates were minimum near the center and increased toward edge. We found that glassy exponent ${\mu}$ has the value of ${\sim}$ 1 whose corresponding vortex motion is half-loop excitation. However, single vortex creep, ${\mu}$ ${\sim}$ 1/7, was also found at 30 K and H$_{app}$ ${\cong}$ H'. Calculation of activation energy, U, was possible from direct analysis of the local relaxation data using the basic diffusion equation. From these results, we found that U increases logarithmically with time and U around center was lower than that at the edge.

• Journal of the Korean Vacuum Society
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• v.15 no.3
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• pp.308-313
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• 2006

We report a photoluminescence (PL) study on the growth process of self-assembled InAs quantum dots (QDs) under the various growth conditions. Distinctive double-peak feature was observed in the PL spectra of the QD samples grown at the relatively high substrate temperature. From the excitation power-dependent PL and the temperature-dependent PL measurements, the double-peak feature is associated with the ground state transitions from InAs QDs with two different size branches. In addition, the variation in the bimodal size distribution of the QD ensembles with different InAs coverage is demonstrated.

• Journal of the Korean Chemical Society
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• v.39 no.5
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• pp.384-392
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• 1995

Steady-state sublimation vapour pressures of anhydrous bismuth trichloride have been measured by the continuous gravimetric Knudsen effusion method from 370.6 to 488.8 K. Additional effusion measurements have also been made from 438.2 to 495.7 K by the torsion effusion method. Based on a correlation of ${\Delta}_H0298$.15 and ${\Delta}_H0298$.15, a recommended p(T) equation has been obtained for BiCl3(s); log(p/Pa)=－C/(T/K)－2.838log(T/K)－8.097${\times}$10-2(K/T)2＋22.588, where p is in Pa, T in Kelvin and ${\Delta}_H0298$.15 in kJmo1-1 and C=(${\Delta}_H0298$.15＋5.9988)/1.9146${\times}$10-2. Condensation coefficients and their temperature dependence have been derived from the effusion measurements.

• Journal of Magnetics
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• v.13 no.1
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• pp.23-29
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• 2008

The magnetic and other physical characteristics of $Mn_xSi_{1-x}Te$ have been investigated by electron magnetic resonance (EMR), X-ray diffraction (XRD) and other experiments. $Mn_xSi_{1-x}Te$ is found to have corundum structure for manganese contents up to 10% and also to be ferromagnetic for temperatures below 80 K. While ferromagnetic resonance signal coexists with the usual paramagnetic resonance signal, invariance of the g-factor inferred from the electron paramagnetic resonance signals throughout all temperature ranges clearly confirms that the manganese ions are in the electronic 3d5 state. The temperature dependence of EMR line-width is the same as other diluted magnetic semiconductors. From the EMR signals relaxation times $T_2$ and $T_1$ of $Mn_xSi_{1-x}Te$ compounds are estimated to be about $4.4{\times}10^{-10}s$ and $9.3{\times}10^{-8}s$ respectively and are found to vary slightly with temperature or composition change. Exchange narrowing of the EMR line-width becomes dominant for the sample in which the substitution ratio, x = 30%. For one sample, in which x = 0.5%, spin glass-like behavior is indicated by EMR signals for temperatures lower than 60 K. This behavior may authentic for samples within a certain range of x.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2006.11a
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• pp.367-370
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• 2006

The ballistic range has long been employed in a variety of engineering fields such as high-velocity impact engineering, projectile aerodynamics, creation of new materials, etc, since it can create an extremely high-pressure state in very short time. Two-stage light gas gun is being employed most extensively. The present experimental study has been conducted to develop a new type of ballistic range which can easily perform a projectile simulation. The ballistic range consists of a high-pressure tube, piston, pump tube, shock tube and launch tube. The experiment is conducted to find out the dependence of various parameters on the projectile velocity. The pressure in high-pressure tube, pressure of diaphragm rupture and projectile mass are varied to obtain various projectile velocities. This study also addresses the effect of the presence of a shock tube located between the pump tube and launch tube on system study. The experimental results are compared with those obtained through an author's theoretical study.

• Journal of the Korean Ceramic Society
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• v.44 no.12
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• pp.655-659
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• 2007

Zirconia polycrystals co-doped with x mol% CaO and (10-x) mol% $Y_2O_3$ were prepared by solid state reaction method. The compositions were chosen for nominally the same oxygen vacancy concentration of 5 mol%. X-ray diffraction patterns indicated the formation of cubic zirconia by heat treatment at $1600^{\circ}C$. Impedance spectroscopy was applied to deconvolute the bulk and grain boundary response. Electrical conductivity was measured using the complex impedance technique from 516 to 874 K in air. Maximum conductivity was exhibited by the composition with equal amounts of CaO and $Y_2O_3$, which may be ascribed to the smaller degree of defect-interactions in that composition due to the competition of different ordering schemes between the two systems. When compared to the composition containing $Y_2O_3$ only, co-doping of CaO increases the grain boundary resistance considerably. The activation energy of grain and grain boundary conductivity was 1.1 eV and 1.2 eV, respectively, with no appreciable dependence on dopant compositions.

• Journal of the Korean Ceramic Society
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• v.52 no.1
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• pp.77-82
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• 2015

The effect of the electrode type on the dielectric and piezoelectric properties of $Pb(Mg_{1/3}Nb_{2/3})O_3-PbZrO_3-PbTiO_3$ (PMN-PZT) single crystals was investigated in an effort to improve their properties for various piezoelectric applications. First, three different types of PMN-PZT single crystals [PMN-PZT-A (piezoelectrically soft type; dielectric constant ~ 10,000), PMN-PZT-B (piezoelectrically soft type; phase-transition temperature between the rhombohedral and tetragonal phases ($T_{RT}$) ~ $145^{\circ}C$), PMN-PZT-C (piezoelectrically hard type; high mechanical quality factor ($Q_m$) ~ 1,000)] were fabricated using the solid-state single crystal growth (SSCG) method. Then, four different types of electrodes [sputtered Au, sputtered Cr/Au, sputtered Ti/Au, and fired Ag] were formed on the single crystals, and their dielectric and piezoelectric properties were measured. The single crystals with a sputtered Ti/Au electrode showed the highest dielectric and piezoelectric constants but the lowest coercive electric field ($E_C$). The single crystals with a fired Ag electrode showed the lowest dielectric and piezoelectric constants but the highest coercive electric field ($E_C$). This dependence on the type of electrode was most significant in the piezoelectrically hard PMN-PZT-C single crystals. However, the effects of the electrode type on the phase transition temperatures ($T_C$, $T_{RT}$) and dielectric loss were negligible. These results clearly demonstrate that it is important to select an appropriate electrode so as to maximize the dielectric and piezoelectric properties of single crystals in each type of piezoelectric application.

• Progress in Superconductivity and Cryogenics
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• v.17 no.2
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• pp.25-30
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• 2015

In order to realize economical applications, it is important to reduce the ac loss of 2G high-temperature superconductor coated conductors. It seems to be reasonable that a multi-filamentary wire can decrease the magnetization loss. In this study, we prepared two samples of YBCO coated conductors with striations. We measured local superconducting properties of both samples by using Low Temperature Scanning Laser and Hall Probe Microscopy (LTSLHPM). The distribution of the local critical temperature of samples was analyzed from experimental results of Low Temperature Scanning Laser Microscopy (LTSLM) near the superconducting transition temperature. According to LTSLM results, spatial distributions of the local critical temperature of both samples are homogeneous. The local current density and the local magnetization in samples were explored from measuring stray fields by using Scanning Hall Probe Microscopy (SHPM). From SHPM results, the remanent field pattern of the one bridge sample in an external magnetic field confirms the Bean's critical state model and the three bridge sample has similar remanent field pattern of the one bridge sample. The local magnetization curve in the three bridge sample was measured from external fields from -500 Oe to 500 Oe. We visualized that the distribution of local hysteresis loss are related in the distribution of the remanent field of the three bridge sample. Although the field dependence of the critical current density must be taken into account, the relation of the local hysteresis loss and the remanent field from Bean's model was useful.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.21 no.9
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• pp.1165-1173
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• 1997

The steady state close-contact melting phenomenon occurring between a phase change material and an isothermally heated flat plate with relative motion is investigated analytically, in which the effects of vertical convection in the liquid film and solid-liquid density difference are incorporated simultaneously. Not only the scale analysis is conducted to estimate a priori qualitative dependence of system variables on characteristic parameters, but also an analytical solution to a set of simplified model equations is obtained to specify the effects under consideration. These two results are consistent with each other, in that the vertical convection affects both the solid descending velocity and the film thickness, and that the density difference alters only the solid descending velocity. While the effect of vertical convection can be characterized conveniently by a newly introduced temperature gradient factor which asymptotically approaches the unity/zero with decreasing/increasing the Stefan number, that of density difference is represented by the liquid-to-solid density ratio. It is shown that the solid descending velocity depends linearly on the density ratio, and that the ratios of solid descending velocity, film thickness and friction coefficient to the conduction solution are proportional to 3/4, 1/4 and -1/4 powers of the temperature gradient factor, respectively. Also, established is the fact that the effect of convection can be legitimately neglected in the analysis for the range of the Stefan number less than 0.1.

• BMB Reports
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• v.36 no.2
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• pp.196-200
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• 2003

Previously, we reported the biochemical properties of RGA1 that is expressed in Escherichia coli (Seo et al., 1997). The activities of RGA1 that hydrolyzes and binds guanine nucleotide were dependent on the $MgCl_2$ concentration. The steady state rate constant ($k_{cat}$) for GTP hydrolysis of RGA1 at 2 mM $MgCl_2$ was $0.0075{\pm}0.0001\;min^{-1}$. Here, we examined the effects of pH and cations on the GTPase activity. The optimum pH at 2 mM $MgCl_2$ was approximately 6.0; whereas, the pH at 2 mM $NH_4Cl$ was approximately 4.0. The result from the cation dependence on the GTPase (guanosine 5'-triphosphatase) activity of RGA1 under the same condition showed that the GTP hydrolysis rate ($k_{cat}=0.0353\;min^{-1}$) under the condition of 2mM $NH_4Cl$ at pH 4.0 was the highest. It corresponded to about 3.24-fold of the $k_{cat}$ value of $0.0109\;min^{-1}$ in the presence of 2 mM $MgCl_2$ at pH 6.0.