• 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.

• Korean Journal of Materials Research
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• v.29 no.12
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• pp.753-756
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• 2019

Orthorhombic dysprosium manganite DyMnO₃ with single phase is synthesized using solid-state reaction technique and the crystal structure and dielectric properties as functions of temperature and frequency are investigated. Thermally activated dielectric relaxations are shown in the temperature dependence of the complex permittivity, and the respective peaks are found to be shifted to higher temperatures as the measuring frequency increases. In Arrhenius plots, activation energies of 0.32 and 0.24 eV for the high- and low-temperature relaxations are observed, respectively. Analysis of the relationship between the real and imaginary parts of the permittivity and the frequencies allows us to explain the dielectric behavior of DyMnO₃ ceramics by the universal dielectric response model. A separation of the intrinsic grain and grain boundary properties is achieved using an equivalent circuit model. The dielectric responses of this circuit are discerned by impedance spectroscopy study. The determined grain and grain boundary effects in the orthorhombic DyMnO₃ ceramics are responsible for the observed high- and low-temperature relaxations in the dielectric properties.

• Journal of the Korean Ceramic Society
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• v.28 no.7
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• pp.525-530
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• 1991

The effects of 0.15mol% Y2O3 doped semiconducting (Ba1-xPbx)TiO3 ceramics with 0.5 mol% Pb5Ge3O11 as sintering additives have been investigated as function of Pb contents (from 0.05 mol to 0.3 mol) and sintering temperatures (from 1050$^{\circ}C$ to 1200$^{\circ}C$). As the Pb content increases in the (Ba1-xPbx)TiO3 system, the size and resistance of the grain increase but the capacitance of the grain boundary decreases due to the formation of liquid phase during the sintering. And with increasing the sintering temperatures, the resistance of the grain decreases but the capacitance of the grain boundary increases. The PTCR effects decrease with increasing the Pb content and the sintering temperature.

• Proceedings of the KWS Conference
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• 2002.10a
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• pp.798-802
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• 2002

Metallurgical studies on the bonded interlayer of directionally solidified Ni-base superalloy GTD111 joints were carried out during transient liquid phase bonding. The formation mechanism of solid during solidification process was also investigated. Microstructures at the bonded interlayer of joints were characterized with bonding temperature. In the bonding process held at 1403K, liquid insert metal was eliminated by well known mechanism of isothermal solidification process and formation of the solid from the liquid at the bonded interlayer were achieved by epitaxial growth. In addition, grain boundary formed at bonded interlayer is consistent with those of base metal. However, in the bonding process held at 1453K, extensive formation of the liquid phase was found to have taken place along dendrite boundaries and grain boundaries adjacent to bonded interlayer. Liquid phases were also observed at grain boundaries far from the bonding interface. This phenomenon results in liquation of grain boundaries. With prolonged holding, liquid phases decreased gradually and changed to isolated granules, but did not disappeared after holding for 7.2ks at 1473K. This isothermal solidification occurs by diffusion of Ti to be result in liquation. In addition, grain boundaries formed at bonded interlayer were corresponded with those of base metal. In the GTD-ll1 alloy, bonding mechanism differs with bonding temperature.

• Macromolecular Research
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• v.17 no.6
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• pp.365-377
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• 2009

In this feature article, we briefly review the new methods we have utilized recently in the investigation of morphology and phase behavior of block copolymers. We first describe the chromatographic fractionation method to purify block copolymers from their side products of mainly homopolymers or block copolymer precursors inadvertently terminated upon addition of the next monomer in the sequential anionic polymerization. The chromatographic method is extended to the fractionation of the individual block of diblock copolymers which can yield the diblock copolymer fractions of different composition and molecular weight, which also have narrower distributions in both molecular weight and composition. A more detailed phase diagram could be constructed from the set of block copolymer fractions without the need of acquiring many block copolymers each prepared by anionic polymerization. The fractions with narrow distribution in both molecular weight and composition exhibit better long-range ordering and sharper phase transition. Next, epitaxial relationships between two ordered structures in block copolymer thin film is discussed. We employed the direct visualization method, transmission electron microtomography(TEMT) to scrutinize the grain boundary structure.

• Journal of Welding and Joining
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• v.21 no.2
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• pp.89-96
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• 2003

The change of microstructures in the base metal during transient liquid phase bonding process of directionally Ni base superalloy, GTD-111 was investigated. Bonds were fabricated using a series of holding times(0-7.2ks) at three different temperatures(1403, 1418 and 1453K) under a vacuum of 13.3mPa. In raw material, ${\gamma}$- ${\gamma}$' eutectic phases, platelet η phases, MC carbide and PFZ were seen in interdendritic regions or near grain boundary and size of primary ${\gamma}$' precipitates near interdendritic regions were bigger than core region. The primary ${\gamma}$' precipitates in dendrite core were dissolved early in bonding process, but ${\gamma}$' precipitates near interdendritic regions were dissolved partially and shape changed. The dissolution rate increased with increasing temperature. Phases in interdendritic regions or near pain boundary continually changed with time at the bonding temperature. In the bonding temperature of 1403K, eutectic phases had not significantly changed, but η phases had transformed from platelet shape to needle morphology and PFZ region had widened with time. The interdendritic region and near pain boundary were liquated partially at 1423k and fully at 1453k by reaction of η phases and PFZ. In the bonding temperature of 1453K, interdendritic region and near pain boundary were liquated and then new phases which mixed with η phases, PFZ and MC carbide crystallized during cooling. Crystallized η phases transformed from rod shape to platelet shape with increasing holding time.

• Journal of the Korean Ceramic Society
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• v.25 no.5
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• pp.523-531
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• 1988

The dielectric and piezoelectric properties with compositions in Pb(Ni1/3Nb2/3)O3-PbTiO3-PbZrO3-(PNN-PT-PZ)solid solution ceramics were investigated. In this study, the compsition ranges were 30 PNN 45mole%, 20 PT 50mole% and 50 PZ5mole%. As PT fraction were increased the grain size was increased and the fired density was decreased, but the changes of PNN fraction had no effect on the grain size. The Curie temperature was increased when PT and PNN fraction were increased. The displacement was increased but had a great hysteresis loss when PT fraction was increased. In morphotropic phase boundary, the maximum piezoelectric and electromechanical coupling factor were indicated. Morphotropic phase boundary(MPB) was 34 PT 36mole% in chang of compositions.

• Structural Engineering and Mechanics
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• v.65 no.4
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• pp.465-476
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• 2018

This article investigates buckling behavior of a multi-phase nanocrystalline nanobeam resting on Winkler-Pasternak foundation in the framework of nonlocal couple stress elasticity and a higher order refined beam model. In this model, the essential measures to describe the real material structure of nanocrystalline nanobeams and the size effects were incorporated. This non-classical nanobeam model contains couple stress effect to capture grains micro-rotations. Moreover, the nonlocal elasticity theory is employed to study the nonlocal and long-range interactions between the particles. The present model can degenerate into the classical model if the nonlocal parameter, and couple stress effects are omitted. Hamilton's principle is employed to derive the governing equations and the related boundary conditions which are solved applying an analytical approach. The buckling loads are compared with those of nonlocal couple stress-based beams. It is showed that buckling loads of a nanocrystalline nanobeam depend on the grain size, grain rotations, porosities, interface, elastic foundation, shear deformation, surface effect, nonlocality and boundary conditions.

• Journal of the Korean Magnetics Society
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• v.5 no.5
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• pp.416-420
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• 1995

An attempt has been made to investigate the mechanism of magnetic alignment in the magnets produced by upset forging the $Pr_{20}Fe_{74}B_{4}Cu_{2}$ cast bulk alloy. Upset forging of the cast alloy was carried out for 20 sec to an 80 % thickness reduction (strain rate : $4{$\times}10^{-2}s^{-1}$) in an open die configuration at varying temperatures in the range $600^{\circ}-900^{\circ}C$. It has been found that the upset forging process at temperatures above $800^{\circ}C$ can achieve a magnetic alignment to a great extent from copper-containing Pr-Fe- B-type cast ingot. The growth manner of the ferromagnetic $Pr_{2}Fe_{14}B$ matrix grain in Pr-Fe-B-type alloys was studied by examining the morphology change of the matrix grain in sintered body, and it was found that the matrix grains grew in anisotropic manner such that the grain grew more rapidly along the a- or b-axis than along the c-axis. This anisotropic grain growth led to the plate-like shape of the matrix grain. The magnetic alignment during the upset forging was attributed to grain boundary gliding of the plate-like grains, and the geometry of the grains in the cast ingot and the presence of a large amount of the praseodymium-rich grain boundary phase were thought to play a key role in the achievement of magnetic alignment.

• International Journal of Korean Welding Society
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• v.3 no.2
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• pp.46-52
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• 2003

The refinement of microstructure and phase transformation near the interface of pure copper/carbon steel dissimilar metals joints with various friction welding parameters have been studied in this paper. The microstructure of copper and carbon steel joints were changed to be a finer grain compared to those of the base metals due to the frictional heat and plastic deformation. The microstructure of copper side experienced wide range of deformed region from the weld interface and divided into very fine equaxied grains and elongated grains. Especially, the microstructures near the interface on carbon steel were transformed from ferrite and pearlite dual structure to fine ferrite, grain boundary pearlite and martensite due to the welding thermal cycle and rapid cooling rate after welding. These microstructures were varied with each friction welding parameters. The recrystallization on copper side is reason for softening in copper side and martensite transformation could explain the remarkable hardening region in carbon steel side.