• 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 Powder Materials
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• v.22 no.3
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• pp.181-186
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• 2015

In this study, the effect of the friction stir welding (FSW) was compared with that of the gas tungsten arc welding (GTAW) on the microstructure and microhardness of Cu-Ni alloy weldment. The weldment of 10 mm thickness was fabricated by FSW and GTAW, respectively. Both weldments were compared with each other by optical microstructure, microhardness test and grain size measurement. Results of this study suggest that the microhardness decreased from the base metal (BM) to the heat affected zone (HAZ) and increased at fusion zone (FZ) of GTAW and stir zone (SZ) of FSW. the minimum Hv value of both weldment was obtained at HAZ, respectively, which represents the softening zone, whereas Hv value of FSW weldment was little higher than that of GTAW weldment. These phenomena can be explained by the grain size difference between HAZs of each weldment. Grain size was increased at the HAZ during FSW and GTAW. Because FSW is a solid-state joining process obtaining the lower heat-input generated by rotating shoulder than heat generated in the arc of GTAW.

• Journal of Welding and Joining
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• v.33 no.6
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• pp.6-12
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• 2015

Thermal fatigue life of the automobile exhaust manifold is directly affected by the restraint force according to the structure of exhaust system and bead shape of the welded joints. In the present study, the microstructural changes and precipitation behavior during thermal fatigue cycle of the 18wt% Cr ferritic stainless steel weld heat affected zone (HAZ) considering restraint stress were investigated. The simulation of weld HAZ and thermal fatigue test were carried out using a metal thermal cycle simulator under complete constraint force in the static jig. The change of the restraint stress on the weld HAZ was simulated by changing the shape of notch in the specimen considering the stress concentration factor. Thermal fatigue properties of the weld HAZ were deteriorated during cyclic heating and cooling in the temperature range of $200^{\circ}C$ to $900^{\circ}C$ due to the decrease of Nb content in solid solution and coarsening of MX type precipitates, laves phase, $M_6C$ with coarsening of grain and softening of the matrix. As the restraint stress on the specimen increased, the thermal fatigue life was decreased by dynamic precipitation and rapid coarsening of the precipitates.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.24 no.7 s.178
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• pp.1817-1825
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• 2000

The purpose of this study is to examine the fracture toughness of the welded pipe from the viewpoint of FATT for the S38 and S42 steels used widely as the pipe material. Post weld heat treatment(PW HT) was carried out like following conditions: temperature of 67$0^{\circ}C$, I hour of holding time and cooling in furnace. Fracture toughness was obtained by measuring the crack opening displacement(COD) of the notched specimens over the range of temperature from -14$0^{\circ}C$ to -$25^{\circ}C$. Hardness values at fusion line near around were the highest and the microstructures at welded zone were coarsened. Regardless of the pipe materials, COD and temperature curves of the as-welds were moved toward higher temperature compared with those of the parents. However, COD and temperature curves of the PWHT specimens were positioned at lower temperature compared with those of the as-welds. The more heat input causes to decrease the COD values at the constant temperature. It was verified through the recrystallization treatment that PWHT was attributed to move toward lower temperature region considerably due to the improved plastic deformation at the same applied COD value of 0.3mm and softening effect. In case of the weldment of S38 steel, cleavage fracture was observed at -105$^{\circ}C$ unlike the structural steels, in which brittle fracture mode was generally shown at - 196$^{\circ}C$.

• Journal of the Korean Society for Heat Treatment
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• v.23 no.4
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• pp.198-204
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• 2010

Modified 9Cr-1Mo steels possess excellent high-temperature mechanical properties and are widely used in energy conversion industries. However, in-service materials degradation, such as softening, carbide-induced embrittlement, temper embrittlement, etc., can take place during long-term operation. Evolution of microstructure due to service exposure to high temperature has a strong effect on the performance of heat resistant steels. In case of modified 9Cr-1Mo steels, precipitation of $Fe_2Mo$-type laves phases and coarsening of $M_{23}C_6$-type carbides are the primary cause of degradation of mechanical properties such as toughness, hardness, tensile strength and creep resistance. This study was aimed at finding reliable parameter for assessing the integrity of modified 9Cr-1Mo steels. Characteristic parameters were attained between mechanical and ultrasonic properties.

• The Korean Journal of Food And Nutrition
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• v.17 no.3
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• pp.246-253
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• 2004

This study was carried out to examine softening stability, exponent of Avrami equation, color change, sensory characteristcs during storage when hydrocolloid was added to the sea tangle paste treated with acetic acid and heat treatment. Rate constant of solidification showed the least value of 0.05 in Avrami equation. In addition hardness of the softened sea tangle paste was not changed after two days of storage in case of carrageenan. Rate of hardness in the softened sea tangle paste formulated with carrageenan exhibited the lowest value of 0.28 kg/mm/day. Heat melting spreadability of the softened sea tangle paste showed the highest value in case of carrageenan and its fluid behavior was rheopectic. Viscosity change in the sea tangle paste formulated with carrageenan was the least during storage and its significant difference at the level of p < 0.05 was exhibited. Change of L, a and b value of softened sea tangle formulated with carrageenan during storage was significantly different at level of p<0.05. Color preference, odor, cohesiveness, softerness, process compatibility and overall acceptance of softened sea tangle were revealed to be in best when carrageenan was added. When hydrocolloid was added to the softened sea tangle paste, it showed the positive result in quality and storage stability of softened sea tangle paste. It was extremely effective on softening stability when carrageenan was added to the softened sea tangle paste.

• Food Science and Preservation
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• v.16 no.5
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• pp.623-628
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• 2009

Fresh ginger rhizomes were heat-treated for 10 min, 30 min, or 60 min using hot air ($40^{\circ}C$, $50^{\circ}C$, or $60^{\circ}C$), and stored in low-density polyethylene (0.04 mm thickness) bags for 2 months at $12{\pm}1^{\circ}C$. We studied the effects of heat pretreatment on changes in gas levels after packaging, and quality characteristics of the rhizomes. Oxygen and carbon dioxide levels progressively fell and rose, respectively, as the temperature of heat treatment rose and the duration of such treatment was extended. The sprouting rate of ginger rhizomes treated at $40^{\circ}C$ was higher than that of other samples. Rotting, softening, and increasing pH of rhizomes were accelerated by treatment at higher temperature for a longer time. Weight loss and soluble solid levels were not affected by heat treatment. Sensory qualities such as appearance, odor, and overall acceptability of rhizomes fell with treatment at a higher temperature for a longer time. These results suggest that heat pretreatment has a detrimental effect on the quality of fresh ginger rhizomes.

• Food Science and Preservation
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• v.23 no.1
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• pp.27-33
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• 2016

This study investigated the effect of heat treatments on the quality characteristics of fresh-cut 'Romaine' lettuce by treating in hot water (45, 50, and $55^{\circ}C$) for 2 minutes. Sensory properties, respiration rate, ethylene production, microbial growth, browning index, total phenolics (TP), vitamin C, and antioxidant properties (DPPH, ABTS, and FRAP assays) of samples were evaluated after 5 days at $5^{\circ}C$. All heat treatment conditions tested in this study did not affect the change in TP after storage. Treatment at $45^{\circ}C$ enhanced respiration rate and ethylene production wheres decreased vitamin C content and antioxidant activities. There was no significant difference in browning index at $45^{\circ}C$ treatment. The rapid tissue softening occurred when treated with $55^{\circ}C$ hot water for 2 minutes. The $50^{\circ}C$ heat treatments exhibited the best quality index including texture and color, and inhibited microbial growth and browning after storage. In addition, the $50^{\circ}C$ heat treatment showed the highest vitamin C content and antioxidant activities (DPPH, ABTS, FRAP assay) after storage. Therefore, the $50^{\circ}C$ heat treatment can be used to maintain quality and antioxidant property of fresh-cut 'Romain' lettuce.

• Journal of the Korean Society of Marine Environment & Safety
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• v.18 no.3
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• pp.273-278
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• 2012

The use of Al alloys instead of fiber-reinforced plastic(FRP) in ship construction has increased because of the advantages of Al-alloy ships, including high speed, increased load capacity, and ease of recycling. This paper describes the effects of probe diameter on the optimum friction stir welding conditions of 5456-H116 alloy for leisure ship, measured by a tensile test. In friction stir welding using a probe diameter of 5 mm under various travel and rotation speed conditions, the best performance was achieved with a travel speed of 61 mm/min. Using a probe diameter of 6 mm, rotation speeds of 170-210 rpm, and a travel speed of 15 mm/min produced a rough surface and voids because of insufficient heat input produced by the low rotation speed. At 500-800 rpm, chips were observed, although there were no voids, and the weld surface was excellent. However, at 1100-2500 rpm, many chips were produced due to excessive heat input. Heat effects were very evident on the bottom. For a travel speed of 15 mm/min, heat input caused by friction increased as the rotation speed increased. The mechanical characteristics were degraded by accelerated softening due to increasing heat input.

• Journal of the Korean Society for Heat Treatment
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• v.29 no.3
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• pp.113-123
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• 2016

In the present work, we investigated the effects of the carbon potential on the formation of carbide at the carburized surface and anti-pitting fatigue strength in the supercarburized steels. Two low carbon steels with different Cr concentrations were adopted and the repeated supercarburizing treatment carried out with the different carbon potential conditions. The microstructure and carbides at the supercarburized surface were observed by using optical microscope and scanning electron microscope. The microhardness test was performed and the hardness distribution and the effective case depth at the supercarburized surface were discussed. The roller pitting fatigue test was carried out and the fatigue strength was evaluated with different the carbon potential conditions. The microstructure of the fatigue specimen surface was observed by means of scanning electron microscope and scanning transmission electron microscope. Depending on the chemical composition of the steels and the carbon potential condition, the resistance of temper softening and pitting failure was influenced due to the carbide distribution and the formation of coarse network carbide. Thus, it was confirmed that the control of the carbide formation is a key factor to improve the anti-pitting fatigue strength in the supercarburized steels.