• Journal of the Korean Society for Heat Treatment
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• v.19 no.2
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• pp.83-89
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• 2006

The surface phase changes, the hardness variations, the nitrogen contents and the corrosion resistances of 17-4 PH stainless steel have been investigated after nitrogen permeation(solution nitriding) at a temperature ranges from $1050^{\circ}C$ to $1150^{\circ}C$ The phases appeared at the nitrogen-permeated surface layer were shown to martensite plus austenite and austenite, depending on the variation of nitrogen and chromium contents. And the surface hardness was also depended on the phases appeared at the surface layer from 370 Hv to 220 Hv. The precipitates exhibited at the nitrogen-permeated surface layer were niobium nitride, niobium chromium nitride and carbo-nitride in the austenite and martensite matrices. The surface nitrogen contents were followed by the Cr contents of the surface layers, representing 0.55% at the temperatures of $1050^{\circ}C$ and $1150^{\circ}C$ respectively, and 0.96% at $1100^{\circ}C$ at the distances of $60{\mu}m$ from the outmost surface. From the comparison of the corrosion resistances between nitrogen-permeated and solution-annealed steels, nitrogen permeation remarkably improved the corrosion resistance in the solution of 1 N $H_2SO_4$ due to the increase of nitrogen content in the surface austenite phase.

• Journal of the Korean Society of Visualization
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• v.9 no.3
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• pp.59-64
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• 2011

In this study the change of free surface vortex is expressed through the time volume fraction using multiphase unsteady condition in sump, because in previous studies of the pump sump did not represent the behavior of the free surface vortex exactly due to the reason it was calculated using single phase and steady condition. The reliability of the computational analysis is verified through comparing experimental results with that of present numerical analysis. Homogeneous free surface model is used to apply interactions of air and water. The results show that the free surface vortex can be identified on the isotropic surface at air volume fraction 1%~5%. The vortices make an air column from the free surface to the sump intake and are created and destroyed repeatedly. The behavior of free surface vortex at numerical analysis is quite similar to experimental test. The result of vortex motion according to time, works on a cycle.

• Transactions of the Korean Society of Mechanical Engineers
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• v.16 no.1
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• pp.142-155
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• 1992

Numerical analysis is performed for the unsteady axisymmetric two dimensional phase change problem of freezing of water around a vertical tube. Heat conduction in the tube wall and solid phase, natural convection in liquid phase and volume expansion caused by density difference between solid and liquid phases are included in the numerical analysis. Existing correlation is used for estimating density-temperature relation of water, and the effect of volume expansion is reflected as fluid velocity at the interface and the free surface. As pure water has maximum density at 4.deg. C, it is found that there exists an initial temperature at which the flow direction reverses near the interface and by this effect the slope of interface becomes reversed depending on the initial temperature of water. By considering natural convection and solid-liquid density difference in the calculation, their effects on phase change process are studied and the effects of various parameters are also studied quantitatively.

• 정보저장시스템학회:학술대회논문집
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• 2005.10a
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• pp.185-189
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• 2005

We reported here nano-scale electrical phase-change recording in amorphous $Ge_2Sb_2Te_5$ media using an atomic force microscope (AFM) having conducting probes. In recording process, a pulse voltage is applied to the conductive probe that touches the media surface to change locally the electrical resistivity of a film. However, in contact operation, tip/media wear and contamination could major obstacles, which degraded SNR, reproducibility, and lifetime. In order to overcome tip/media wear and contamination in contact mode operation, we adopted the W incorporated diamond-like carbon (W-DLC) films as a protective layer. Optimized mutilayer media were prepared by a hybrid deposition system of PECVD and RF magnetron sputtering. When suitable electrical pulses were applied to media through the conducting probe, it was observed that data bits as small as 25 nm in diameter have been written and read with good reproducibility, which corresponds to a data density of $1 Tbit/inch^2$. We concluded that stable electrical phase-change recording was possible mainly due to W-DLC layer, which played a role not only capping layer but also resistive layer.

• Journal of Powder Materials
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• v.19 no.3
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• pp.196-203
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• 2012

$TiO_2$ was successfully formed on a Ti specimen by MAO (Micro-Arc-Oxidation) method treated in $Na_3PO_4$ electrolyte. This study deals with the influence of voltage and working time on the change of surface microstructure and phase composition. Voltage affected the forming rate of the oxidized layer and surface microstructure where, a low voltage led to a high surface roughness, more holes and a thin oxidized layer. On the other hand, a high voltage led to more dense surface structure, wider surface holes, a thick layer and fewer holes. Higher voltage increases photocatalytic activity because of better crystallization of the oxidized layer and good phase composition with anatase and rutile $TiO_2$, which is able to effectively separate excited electrons and holes at the surface.

• Proceedings of the Korean Institute of Surface Engineering Conference
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• 1998.05a
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• pp.3-3
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• 1998

The atmospheric corrosIOn properties of Zinc (Zn) under wet/dry transition of $H_20$ film were investigated in this study. The atmospheric corrosion of metal is usually occurred as a result of repetitious thickness transition (so called wet/dry transition) of liquid phase which is covering the metal surface. Corrosion potential and the polarization behaviour of Zn during liquid film thickness transition were measured by Kelvin probe method which IS using vibrating reference electrode without touching the liquid film. The oxidized states of Zn as a result of successive wet/dry transition were also investigated by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The results show that the corrosion potential and the corrosIOn rate of Zn both are increasing during drying. However, the corrOSIon rate is decreasing again when the Zn surface is completely dried while the corrosion potential still remains high. This behaviour can be explained by the polarization behaviour change of Zn according to the $H_20$ film thickness change. The completely dried surface is consisted mostly with Zn and ZnO phases. After a number of cycles of wet/dry transition, however, the oxidized Zn phase of ${\varepsilon}-Zn(OH)_2$, which has rather voluminous and defected structure, were found.

• 한국전산유체공학회:학술대회논문집
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• 2009.11a
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• pp.218-222
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• 2009

A level-set (LS) method is presented for computation of boiling phenomena which involve liquid-vapor interfaces that evolve, merge and break up in time, the flow and temperature fields influenced by the interfacial motion, and the microlayer that forms between the solid and the vapor phase near the wall. The LS formulation for tracking the phase interfaces is modified to include the effects of phase change on the liquid-vapor interface and contact angle on the liquid-vapor-solid interline. The LS method can calculate an interface curvature accurately by using a smooth distance function. Also, it is straightforward to implement for two-phase flows in complex geometries. The numerical method is applied for analysis of nucleate boiling on a horizontal surface and film boiling on a horizontal cylinder.

• Journal of the Korean Ceramic Society
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• v.27 no.6
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• pp.735-740
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• 1990

The phase transformation of Y-TZP by low temperature aging treatments and its related behaviors of crack formation were investigated. The kinetics of phase transformation was greatly dependent on the amounts of Y2O3, grian size and microstructures of sintered body. The phase transformation happened to start at specimen surface and near the pore in the first place, where the change of strain energy during the phase transformation was small and the water vapor that accelerated phase transformation easily diffused.

• Restorative Dentistry and Endodontics
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• v.46 no.4
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• pp.54.1-54.10
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• 2021

Objectives: The purpose of this study was to quantify phase transformation after hydrofluoric acid (HF) etching at various concentrations on the surface of yttria-stabilized tetragonal zirconia polycrystal (Y-TZP), and to evaluate changes in bonding strength before and after thermal cycling. Materials and Methods: A group whose Y-TZP surface was treated with tribochemical silica abrasion (TS) was used as the control. Y-TZP specimens from each experimental group were etched with 5%, 10%, 20%, and 40% HF solutions at room temperature for 10 minutes. First, to quantify the phase transformation, Y-TZP specimens (n = 5) treated with TS, 5%, 10%, 20% and 40% HF solutions were subjected to X-ray diffraction. Second, to evaluate the change in bond strength before and after thermal cycling, zirconia primer and MDP-containing resin cement were sequentially applied to the Y-TZP specimen. After 5,000 thermal cycles for half of the Y-TZP specimens, shear bond strength was measured for all experimental groups (n = 10). Results: The monoclinic phase content in the 40% HF-treated group was higher than that of the 5%, 10%, and 20% HF-treated groups, but lower than that of TS-treated group (p < 0.05). The 40% HF-treated group showed significantly higher bonding strength than the TS, 5%, and 10% HF-treated groups, even after thermal cycling (p < 0.05). Conclusions: Through this experiment, the group treated with SiO₂ containing air-borne abrasion on the Y-TZP surface showed higher phase transformation and higher reduction in bonding strength after thermal cycling compared to the group treated with high concentration HF.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.1 no.2
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• pp.138-145
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• 1989

Experimental and numerical analysis were performed to investigate the heat transfer phenomena during phase change. N-octadecane were used as a phase change material and TRUMP computer code was used as a numerical tool. Also, two quarter segment of cylinder shape was chosen as a vessel to simulate to this research. The major contribution factor on the solidus surface movement was environmental temperature and the effect of roller gap and material initial temperature were insignificant. Experimental and numerical results were generally in good agreement and the effect of the mesh size ($22{\times}22$ and $33{\times}33$) was negligible.