• Proceedings of the Korean Powder Metallurgy Institute Conference
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• 2006.09a
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• pp.60-61
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• 2006

In this contribution, we attempted a theoretical analysis on the validity of the widely-accepted idea that rough and singular surfaces can coexist in a crystal at equilibrium. By manipulating the Cahn and Hoffman capillarity vector, the conclusion that a crystal at equilibrium should be composed either of singular surfaces or of rough ones was reached.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.7 no.4
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• pp.548-554
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• 1997

The single crystal of $Mg_{0.16}Zn_{0.84}$Te:Co(Co:0.01 mole%) was grown by vertical Bridgman method. The crystal structure of $Mg_{0.16}Zn$_{0.84}$Te:Co and optical absorption properties of this compound were studied. The grown single crystal has a cubic structure and a lattice constant a=6.1422 $\AA$ were determined by X-ray diffraction. As a result of the optical absorption spectra of $Mg_{0.16}Zn_{0.84}$Te:Co, the intracenter transitions due to $Co^{2+}$ ions were detected for $A-band:^4A_2(^4F){\to}^4T_2(^4F),\; B-band:^4A_2(^4F){\to}^4T_1(^4F), C- band:^4A_2(^4F){\to}^4T_1(^4P)$.The charge transfer transition near the absorption edge was observed in the wavelength range of 550 to 770 nm. According to the crystal field theory, the crystal field parameter(Dq) and the Racah parameter(B) were determined.

• Journal of the Korean Magnetic Resonance Society
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• v.21 no.2
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• pp.63-66
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• 2017

The $^{14}N$ NMR spectra for $(NH_4)_2SO_4$ crystals were obtained near the phase transition temperature $T_C=223K$, and were found to precisely reflect the symmetry change in the crystal at this first-order phase transition. Changes in the resonance frequencies near $T_C$ were attributed to the structural phase transition. In the ferroelectric and paraelectric phases, two inequivalent NH4 groups were distinguished in the $^{14}N$ NMR spectra. The two types, $NH_4$(1) and $NH_4$(2), have slightly different local environments. Consequently, we conclude that the phase transition is caused by the change in the environment of the $^{14}N$ nuclei in the $NH_4$ groups, rather than by the $SO_4$ groups.

• Proceedings of the KIEE Conference
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• 2001.07c
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• pp.1496-1499
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• 2001

Optical and electrical properties of the $In_2Se_3$ single crystals grown by use of the Bridgman technique were examined in the transition temperature range between $\alpha$-phase and $\beta$-phase. $In_2Se_3$ single crystal has the rhombohedral structure and lattice constants are a=4.025 $\AA$, c=28.771 $\AA$ in C-axis. The transition temperatures of the stoichiometric $In_2Se_3$ single crystal is $10^{-2}{\Omega}cm^{-1}$ according to the specimens. However it varies rapidly in the transition region.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.6 no.4
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• pp.480-492
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• 1996

AZO (Aluminum doped Zinc Oxide) transparent conducting thin films were fabricated by reactive DC mangnetron sputtering method using zinc target containing 2 wt% of Al. Transition range with optimum transmittance and conductivity was obtained by contrlling partial pressure of reactive oxygen gas. Sputtering condition for this transition range could be kept stable by regulating the target voltage. According to XRD analysis, there was only one peak for (002) plane in AZO films and the films deposited in transition range.

• Proceedings of the Korean Vacuum Society Conference
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• 2012.02a
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• pp.253-253
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• 2012

The effects of oxidation on the order-disorder transition in NiPt bimetallic alloy crystal have been investigated using in-situ synchrotron x-ray scattering technique. The temperature dependence of the crystal structure and the order parameter were measured during in-situ heating and cooling under vacuum and oxygen environments. The order-disorder transition temperature of NiPt alloy crystals in vacuum was between $615^{\circ}C$ and $627^{\circ}C$. On the other hand under oxygen environment, the transition temperature decreases by about $31^{\circ}C$ after the oxidation. The change of the transition temperature can be explained by the formation of NiO crust on the surface of NiPt crystal, which alters the composition of the Ni and Pt atoms. Since the transition temperature depends sensitively on the Ni-Pt composition, the transition temperature changes as Ni atoms diffuse out to form NiO.

• Journal of Information Display
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• v.3 no.3
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• pp.17-23
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• 2002

In this work we report methods of formation of three-dimensional structures of particles in a liquid crystal host. We found that, under the appropriate conditions, the particles are captured and dragged by the moving isotropic/nematic front during the phase transition process. This movement of the particles can be enhanced significantly or suppressed drastically with the influence of an electric field and/or with changing the conditions of the phase transition, such as the rate of cooling. As a result, a wide variety of particle structures can be obtained ranging from a fine-grained cellular structure to stripes of varying periods to a course-grained "root" structures. Changing the properties of the materials, such as the size and density of the particles and the surface anchoring of the liquid crystal at the particle surface, can also be used to control the morphology of the three-dimensional particle network and adjust the physical properties of the resulting dispersions. These particle structures may be used to affect the performance of LCD's much as polymers have been used in the past.

• Proceedings of the Korea Association of Crystal Growth Conference
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• 1996.06a
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• pp.139-156
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• 1996

The phase field model is becoming the model of choice for the theoretical study of the morphologies of crystals growth from the melt. This model provides an alternative approach to the solution of the classical (sharp interface) model of solidification by introducing a new variable, the phase field, Ø, to identify the phase. The variable Ø takes on constant values in the bulk phases and makes a continuous transition between these values over a thin transition layer that plays the role of the classically sharp interface. This results in Ø being governed by a new partial differential equation(in addition to the PDE's that govern the classical fields, such as temperature and composition) that guarantees (in the asymptotic limit of a suitably thin transition layer) that the appropriate boundary conditions at the crystal-melt interface are satisfied. Thus, one can proceed to solve coupled PDE's without the necessity of explicitly tracking the interface (free boundary) that would be necessary to solve the classical (sharp interface) model. Recent advances in supercomputing and algorithms now enable generation of interesting and valuable results that display most of the fundamental solidification phenomena and processes that are observed experimentally. These include morphological instability, solute trapping, cellular growth, dendritic growth (with anisotropic sidebranching, tip splitting, and coupling to periodic forcing), coarsening, recalescence, eutectic growth, faceting, and texture development. This talk will focus on the fundamental basis of the phase field model in terms of irreversible thermodynamics as well as it computational limitations and prognosis for future improvement. This work is supported by the National Science Foundation under grant DMR 9211276

• Journal of Technologic Dentistry
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• v.31 no.4
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• pp.25-30
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• 2009

Zirconia($ZrO_2$) has attracted much attention in science and technology because of its high refractive index, high melting temperature, hardness, low thermal conductivity and corrosion barrier properties. And it is widely used as the dental restoration material because of its esthetic appearance. In this research, we analyzed the particle size and composition of the imported dental porcelain for zirconia. And the glass frit was produced. To decrease the glass transition temperature and softening temperature of the glass frit, $Li_2O$ was added into it and the effect of $Li_2O$ on the firing temperature was researched. Then the glass which contains leucite crystal with a high coefficient of thermal expansion(CTE) was manufactured and it was mixed with the glass frit to control the CTE. The phase composition were analyzed using the X-ray diffraction. The morphologies of the samples were observed by the scanning electron microscope. The 4wt% $Li_2O$-added glass frit has the optimal glass transition temperature and softening temperature. And 6 wt% leucite crystal was mixed with the glass frit to control the CTE. From the experimental results of crystallization, the crystal phase was found only leucite crystal.

• Journal of the Korean Ceramic Society
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• v.32 no.5
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• pp.582-586
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• 1995

The crystal structures of (Na0.3Sr0.7)(Ti0.7M0.3)O3 (M=Ta, Nb) compounds were determined using the Rietveld method. Due to the tilting of a oxygen octahedron, (Na0.3Sr0.7)(Ti0.7Nb0.3)O3 had a superlattice of doubled a, b and c of simple perovskite. The crystal structure of (Na0.3Sr0.7)(Ti0.7M0.3)O3 was tetragonal with a space group 14/mmm. The crystal structure of (Na0.3Sr0.7)(Ti0.7M0.3)O3 was a cubic with space group Pm3m, in which no tilting of oxygen octahedron was observed. The difference in the oxygen tilting of these two materials was due to the larger covalency of Nb-O bond than that of Ta-O bond, which induced a strong $\pi$Nb0 bonding in (Na0.3Sr0.7)(Ti0.7M0.3)O3. Therefore, the higher transition temperature of (Na0.3Sr0.7)(Ti0.7M0.3)O3 could be related to the larger tilting of oxygen octahedron.