• Journal of the Korean Society of Safety
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• v.27 no.3
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• pp.15-19
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• 2012

The fatigue behavior of AM60 magnesium alloy produced by equal channel angular pressing(ECAP) process was investigated through fatigue lifetime and fatigue crack propagation rate tests. The grain structure of the material was refined from 19.2 ${\mu}m$ to 2.3 ${\mu}m$ after 6 passes of ECAP at 493 K. The yield strength(YS) and ultimate tensile strength (UTS) increase after two passes but decrease with further pressing, although the grain size becomes finer with increasing pass number. The softening effect due to texture anisotropy overwhelmed the strengthening effect due to grain refinement after 2 passes. A large enhancement in fatigue strength was achieved after two ECAP passes. The current finding suggests that two passed material is better than the multi-passed material in view of the static strength and fatigue performance.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2006.06a
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• pp.220-221
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• 2006

We investigated the electrical properties of polymer-carbon composite materials for temperature sensitive resistor applications. Cu/polymer/Cu sheets were fabricated by laminating low density polyethylene(LDPE) containing carbon powders. Weight ratio of carbon powder to LDPE was varied in a range of 0.9~2. With increasing the carbon concentration, the electrical resistance of the composite material was decreased from 0.75 to $0.08\;{\Omega}cm$. The composite layer showed a abrupt increase in the electrical resistance at $115^{\circ}C$ because of the softening of the polymer.

• International Journal of Naval Architecture and Ocean Engineering
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• v.12 no.1
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• pp.868-880
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• 2020

One of the main concerns in the structural integrity of offshore pipelines is mechanical damage from external loads. Pipelines are exposed to fatigue failure in welded joints due to geometric discontinuity. In addition, fatigue loads such as currents, waves, and platform motions may cause significant plastic deformation and fracture or leakage within a relatively low-cycle regime. The 2007 ASME Div. 2 Code adopts the master S―N curve for the fatigue evaluation of welded joints based on the mesh-insensitive structural stress. An extension to the master S―N curve was introduced to evaluate the low-cycle fatigue strength. This structural strain method uses the tensile properties of the material. However, the monotonic tensile properties have limitations in describing the material behavior above the elastic range because most engineering materials exhibit hardening or softening behavior under cyclic loads. The goal of this study is to extend the cyclic stress-strain behavior to the structural strain method. To this end, structural strain-based procedure was established while considering the cyclic stress-strain behavior and compared to the structural strain method with monotonic tensile properties. Finally, the improved prediction method was validated using fatigue test data from full-scale girth-welded pipes.

• Journal of Korea Technical Association of The Pulp and Paper Industry
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• v.47 no.1
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• pp.35-44
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• 2015

Thermomechanical pulping process uses large amounts of energy, mostly electricity to run electrical facilities. Thermomechanical pulp (TMP) made from Pinus densiflora also has a big drawback that refining consumes 90 per cent of the total energy used in TMP process. This study explored to draw up a way to save refining energy through different thermal treatment at the stages of presteaming and refining. Presteaming temperature was $80^{\circ}C$, $100^{\circ}C$, and $120^{\circ}C$. After presteaming at each temperature, refining was carried out at $100^{\circ}C$, $120^{\circ}C$, and $140^{\circ}C$ respectively. In a presteaming stage, steaming temperature over $120^{\circ}C$ greatly contributed to the decrease of refining energy leading to earlier attainment of a target freeness, irrespective of refining temperature. In addition, NaOH treatment with presteaming enhanced better development of fiber properties during refining than presteaming without NaOH. High temperature refining at $140^{\circ}C$ produced a high strength paper, and wood chips treated by alkali responded better to refining than at over $120^{\circ}C$. Improved softening effect on wood chips led to the decrease in shives contents but it gave no effect on pitch contents of TMP.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.25 no.6
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• pp.263-271
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• 2015

Commercial AR(Alkali Resistant)-glass fiber has a good chemical resistant property, but also has a problem of difficulty in fiberizing process because of high viscosity in melted glass compare with E-glass fiber which is the most widely used for reinforced fiber of composite materials. In this study, we fabricated AR-glass fiber with low zirconia contents compare with commercial AR-glass fiber relatively, and measured properties against E-glass fiber. We obtained transparent clear glass with zirconia contents of 0.5~16 wt% by melting at $1600^{\circ}C$ for 2 hours. These AR-glass samples had high visible transmittance of 89~90 %, softening temperature of $703{\sim}887^{\circ}C$. And softening temperatures of them were increased according to the increasing zirconia contents. Compare with E-glass, AR-glass contains 4 wt% zirconia has different value of $-94^{\circ}C$ in softening temperature, $+68^{\circ}C$ at Log3 temperature and $-13^{\circ}C$ at Log5 temperature in viscosity. We could verify good alkali resistant property of the AR-glass fiber with SEM after dipping in alkali solution for 48~72 hours, and also high tensile strength, 1.7 times compare with E-glass fiber at 48 hours and 2.2 times at 72 hours. We conclude that this AR-glass fiber can be widely used as general alkali resistant glass fiber because of easy manufacturing condition and good properties even though it has low zirconia contents.

• Journal of the Korean Ceramic Society
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• v.52 no.4
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• pp.229-233
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• 2015

Currently, the majority of commercial white LEDs are phosphor converted LEDs made of a blue-emitting chip and YAG yellow phosphor dispersed in organic silicone. However, silicone in high-power devices results in long-term performance problems such as reacting with water, color transition, and shrinkage by heat. Additionally, yellow phosphor is not applicable to warm white LEDs that require a low CCT and high CRI. To solve these problems, mixing of green phosphor, red phosphor and glass, which are stable in high temperatures, is common a production method for high-power warm white LEDs. In this study, we fabricated conversion lenses with LUAG green phosphor, SCASN red phosphor and low-softening point glass for high-power warm white LEDs. Conversion lenses can be well controlled through the phosphor content and heat treatment temperature. Therefore, when the green phosphor content was increased, the CRI and luminance efficiency gradually intensified. Moreover, using high heat treatment temperatures, the fabricated conversion lenses had a high CRI and low luminance efficiency. Thus, the fabricated conversion lenses with green and red phosphor below 90 wt% and 10 wt% with a sintering temperature of $500^{\circ}C$ had the best optical properties. The measured values for the CCT, CRI and luminance efficiency were 3200 K, 80, and 85 lm/w.

• Journal of the Korean Ceramic Society
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• v.50 no.6
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• pp.470-475
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• 2013

In this study, we report that effects of a filler with a high coefficient of thermal expansion on a sealant for high-temperature ($750{\sim}850^{\circ}C$) SOFC. We designed a $SiO_2-BaO-ZnO-B_2O_3-Al_2O_3$ glass system with a softening temperature higher than $750^{\circ}C$. The properties of the glass system show not only low volumetric shrinking but also low swelling. The glass system did not create a crystal phase during along-term heat treatment. We fabricated a seal gasket with 0, 10, 15, and 20 wt% cristobalite added as filler materials with glass powder. The coefficient of thermal expansion of the seal gasket increased according to cristobalite content. During along-term heat treatment, the leak rate decreased by about 5% after a heat treatment in an oxidizing atmosphere at $750^{\circ}C$ for 2000 h, also decreasing by about 6% after a heat treatment in a reducing atmosphere at $750^{\circ}C$ for 1000 h.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.22 no.1
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• pp.177-189
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• 1998

The contents of this paper include a non-destructive technique for evaluating the degradation of the boiler superheater tube in a fossil power plant through an electrochemical polarization test. Correlation between Ip of polarization parameter and SP-DBTT for the superheater tubes in long-term use was obtained. 1.0Cr-0.5Mo steel was degraded by softening, and the degree of degradation was dependent upon carbides with Cr and Mo elements. Since brittle fracture at low temperature and ductile fracture mode at high temperature were shown, similarity between standard Charpy and small punch tests could be found. In addition, SP-DBTT showing the degree of degradation was higher, as the time-in use of the materials got longer. Electrolyte including picric acid of 1.3 g in distilled water of 100ml at 25.deg. C temperature and sodium tridecylbenzene sulfonate with 1g could be applied to evaluate the degradation of 1.0Cr-0.5Mo steel by means of the electrochemical polarization test. Ip and Ipa values measured through the electrochemical test are the appropriate parameters for representing the degradation of the superheater tube(1.0Cr-0.5Mo steel) for the fossil power plant. It is poassible to evaluate the degradation of materials with different time histories electrochemically, by Ip value only, at field test.

• Journal of the Microelectronics and Packaging Society
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• v.10 no.4
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• pp.47-52
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• 2003

A new type of Bi based glass frit was developed for Ag paste in PDP applications and its properties are compared with the commercially used Pb based glass frit. After optimization of the properties of Bi based frits for PDP application such as the softening temperature and the coefficient of thermal expansion (C.T.E), the screen printed electrodes prepared with the Bi based fit contained Ag paste were characterized. In $Bi_2O_3-B_2O_3-Al_2O_3$ glass system with the more than 50% of $Bi_2O_3$, the softening temperature, the thermal expansion coefficient and the line resistivity was 400∼$480^{\circ}C$, 7.31∼$10.02\times 10^{-6}/^{\circ}C$> and 4.1∼4.8$\Omega$ respectively. Properties of the Bi based frits are comparable with the Pb based frits. A printability and an uniformity of the Bi based frits were excellent in screen printed Ag eletrode. The Bi based frit system is an excellent candidate material for Pb free and Alkali free frit in PDP applications.

• Journal of the Korean Society for Nondestructive Testing
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• v.36 no.5
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• pp.377-383
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• 2016

The effect of long-term aging degradation on magnetic properties of ferritic 11Cr low-carbon steel was investigated. Coercivity and hysteresis loss measured from the hysteresis loops decreased with long-term aging time and showed that the relation was well fitted by a second order exponential function. Vickers hardness also decreased with aging time and resulted in mechanical softening. In addition, the microstructural evolution was observed by the scanning electron microscopy, backscattered electron image and X-ray diffraction. The $Cr_{23}C_6$ precipitates along grain boundary grew fast and Laves ($Fe_2W$) phase on martensitic lath boundaries in interior grains was developed. The solid solution atoms depleted in matrix and lath subgrains recovered owing to precipitate coarsening with long-term aging degradation. There was a close relation with softening of magnetic and mechanical properties.