• Journal of the Korean Society of Safety
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• v.26 no.4
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• pp.41-46
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• 2011

The purpose of this paper is to analyze the causes of accidents related to the 3 phase 170 kV gas insulated switchgear of a power system collected from accident sites to secure data for the prevention of similar accidents and provide important points of view regarding diagnosis for the prevention of accidents involving gas insulated switchgears. The analysis results of the causes of accidents involving gas insulated switchgears showed deformation of the manipulation lever installed at the S-phase, disconnection of the insulation rod connection, melting of the upper conductor, a damaged tulip, damage to the lower spacer and the spacer at the breaker, etc. It is believed from this result that the potential for accidents has expanded due to accumulated energy as a result of repeated deterioration. The carbonization depth of a GIS was formed near the screw (T2, T3) used to secure the lower pole of the S-phase tulip. It is not known what has caused the screws to be extruded and melted. However, it is thought that an unbalanced electromagnetic force, micro-discharge, surface discharge, etc., have occurred at that point. In addition, even though 16 years have passed since its installation, there was no installation defect, act of arson, accidental fire, etc. General periodical inspection and diagnosis failed to find the factors causing the accidents. As a system contained in a closed metal container, it has a high risk factor. Therefore, it is necessary to design, install and operate a GIS in accordance with the standard operational procedure (SOP). In addition, it is necessary to apply conversion technology for periodical SF6 gas analysis and precision safety diagnosis. It is expected that tracking and managing these changes in characteristics by recording the results on the history card will provide a significant accident prevention effect.

• Polymer(Korea)
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• v.29 no.1
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• pp.96-101
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• 2005

Fluoropolymer of PTFE and PFA etc. is a heat resistance polymeric material that it is known as that maximum continuous use temperature reaches for 260 $^{\circ}C$. It was observed that these polymers had the enough thermal stability so enough that it was kept by thermal aging of 280 $^{\circ}C$/7 weeks too in this study. However, such thermal stability means that bulk material property is kept such as mechanical strength, melting point and initial pyrolysis temperature etc. If these polymers are evaluate by coating property such as surface contact angle, surface morphology, surface scratch, thing that heat resistance is not enough was confirmed in this study. Thermal aging of flouropolymer coating was achieved by gear aging oven that the exchange rate of air was controlled, and the analysis results were indicating serious damage of surface morphology and adhesive strength on metal substrate.

• Polymer(Korea)
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• v.24 no.3
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• pp.382-388
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• 2000

Polymer electrolyte films consisting of poly(vinylidenefluoride-hexafluoropropylene) (PVdF-HFP), LiClO$_3$ and a mixture of ethylene carbonate (EC) and ${\gamma}$-butyrolactone (GBL) were examined in order to obtain the best compromise between high ionic conductivity, homogeniety, dimensional and electrochemical stability. Measurements of ionic conductivity, differential scanning calorimetry and linear sweep voltammetry have been carried out for various compositions. The highest conductivity of 3.8$\times$10$^{-3}$ S$cm^{-1}$ / at 3$0^{\circ}C$ were obtained for a film of 30(PVdF-HFP)+7.8LiClO$_4$+62.2EC/GBL. From the DSC study, it has been found that the PVdF-HFP gels are stable up to 10$0^{\circ}C$, and the salt lowers the melting temperature of crystalline part of PVdF by interacting sensitively with polymer segments. When Lithium metal is in contact with the gel films, it tends to undergo corrosion and the reaction products accumulate resulting in the formation of a passive film on Li electrode. As the aging time progresses, the interfacial resistance increases continuously. Anodic stability is measured to extend up to about 4.5 V vs. Li.

• Journal of Welding and Joining
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• v.18 no.2
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• pp.86-94
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• 2000

This study was performed to investigate types and formation mechanism of cracks in two Al alloy welds, A5083 and A7N01 spot-welded by pulse Nd : YAG laser, using SEM, EPMA and Micro-XRD. In the weld zone, three types of crack were observed : center line crack({TEX}$C_{C}${/TEX}), diagonal crack({TEX}$C_{D}${/TEX}), and U shape crack({TEX}$C_{U}${/TEX}). Also, HAZ crack({TEX}$C_{H}${/TEX}) was observed in the HAZ region, furthermore, mixing crack({TEX}$C_{M}${/TEX}) consisting of diagonal crack and HAZ crack was observed. White film was formed at th hot crack region in the fractured surface after it was immersed to 10% NaOH water. In the case of A5083 alloy, white films in {TEX}$C_{C}${/TEX} crack and {TEX}$C_{D}${/TEX} crack region were composed of low melting phases, {TEX}$Fe_{2}SiAl_{8}${/TEX} and eutectic phases, $Mg_2$Al$_3$ and $Mg_2$Si. Such films observed $CuAl_2$, {TEX}$Mg_{32}(Al,Zn)_{3}${/TEX}, MgZn$_2$, $Al_2$CuMg and $Mg_2$Si were observed in the whitely etched films near {TEX}$C_{C}${/TEX} crack and {TEX}$C_{D}${/TEX} crack regions. The formation of liquid films was due to the segregation of Mg, Si, Fe in the case of A5083 alloy and Zn, Mg, Cu, Sim in the case of A7N01 alloy, respectively. The {TEX}$C_{C}${/TEX} and {TEX}$C_{D}${/TEX} cracks were regarded as a result of the occurrence of tensile strain during the welding process. The formation of {TEX}$C_{M}${/TEX} crack is likely to be due to the presence of liquid film at the grain boundary near the fusion line in the base metal as well as in the weld fusion zone during solidification. The {TEX}$C_{U}${/TEX} crack is considered a result of the collapsed keyhole through incomplete closure during rapid solidification.

• Korean Journal of Materials Research
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• v.15 no.1
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• pp.31-36
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• 2005

It is well known that PCB (Printed Circuit Board) is a complex mixture of various metals mixed with various types of plastics and ceramics. In this study, high temperature pyre-metallurgical process was investigated to extract valuable metallic components from the PCB scrap. For this purpose, PCB scrap was shredded and oxidized to remove plastic materials, and then, quantitative analyses were made. After the oxidation of the PCB scrap, $30.6wt\%SiO_2,\;19.3wt\%Al_2O_3\;and\;14wt{\%}CaO$ were analyzed as major oxides, and thereafter, a typical composition of $32wt\%SiO_2-20wt\%Al_2O_3-38wt{\%}CaO-10wt\%MgO$ was chosen as a basic slag system for the separation of metallic components. Moreover a size effect of crushed PCB scrap was also investigated. During experiments a high frequency induction furnace was used to melt and separate metallic components. As a result, it was found that the size of oxidized PCB scrap was needed to be less 0.9 m to make a homogeneous liquid slag and to recycle metallic components over $95\%$.

• Korean Journal of Materials Research
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• v.24 no.5
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• pp.255-258
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• 2014

Zirconium boride is an artificial or which is rarely found in the nature. $ZrB_2$ is popular in the hard material industry because it has a high melting point, excellent mechanical properties and chemical stability. There are two known methods to synthesize $ZrB_2$. The first involves direct reaction between Zr and B, and the second is by reduction of the metal halogen. However, these two methods are known to be unsuitable for mass production. SHS(Self-propagating High-temperature Synthesis) is an efficient and economic method for synthesizing hard materials because it uses exothermic reactions. In this study, $ZrB_2$ was successfully synthesized by subjecting $ZrO_2$, Mg and $B_2O_3$ to SHS. Because of the high combustion temperature and rapid combustion, in conjunction with the stoichiometric ratio of $ZrO_2$, Mg and $B_2O_3$; single phase $ZrB_2$ was not synthesized. In order to solve the temperature problem, Mg and NaCl additives were investigated as diluents. From the experiments it was found that both diluents effectively stabilized the reaction and combustion regime. The final product, made under optimum conditions, was single-phase $ZrB_2$ of $0.1-0.9{\mu}m$ particle size.

• Korean Journal of Metals and Materials
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• v.48 no.12
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• pp.1056-1063
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• 2010

We present a new joining method for Pd-Cu membrane foils used as permeation tubes to collect $CO_2$. Since foils have poor mechanical strength, joining should be done at low temperatures to reduce residual stresses and without joining pressure. This contradicts the well known conditions for good contact between base materials that determines joint qualities. We selected Sn-Ag-Cu alloys that are highly reactive with Pd and Cu as a filler metal. As the filler melts at joining temperatures as low as $220{\sim}280^{\circ}C$, Pd and Cu are dissolved into the melt and react with the filler elements, which raises the melting temperature of the filler based on eutectic structures among the elements. Then, isothermal solidification progresses for the rest of the joining time. Intermetallic compounds (IMC) in the joints, one of the main factors for brittle joints, are inevitably formed. However, by optimizing both joining time and temperature, we balanced the wettability with IMC. Sealing test results confirmed that the joints are mechanically reliable during operation.

• Korean Journal of Metals and Materials
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• v.48 no.7
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• pp.589-597
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• 2010

In the present study, the effect of variation in alloying elements on the carbide formation behavior during casting and homogenization treatment of M2 high speed steels was investigated. M2 high speed steels of various compositions were produced by vacuum induction melting. Contents of C, Cr, W, Mo, and V were varied from the basic composition of 0.8C, 0.3Si, 0.2Mn, 4.0Cr, 6.0W, 5.0Mo, and 2.0V in weight percent. Homogenization treatment at $1150^{\circ}C$ for 1.5 hr followed by furnace cooling was performed on the ingots. Area fraction and chemical compositions of eutectic carbide in as-cast and homogenized ingots were analyzed. Area fraction of eutectic carbide appeared to be higher in the ingots with higher contents of alloying elements the area fraction of eutectic carbide also appeared to be higher on the surface regions than in the center regions of ingots. As a result of the homogenization treatment, $M_2C$ carbide, which was the primary eutectic carbide in the as-cast ingots, decomposed into thermodynamically stable carbides, MC and $M_6C$. The latter carbide was found to be the main one after homogenization. Fine carbides uniformly distributed in the matrix was found to be MC type carbide and coarsened by homogenization.

• Resources Recycling
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• v.28 no.5
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• pp.30-41
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• 2019

Global production of zinc is about 13 million tons and zinc is the fourth-most widely used primary metal in the world following iron, aluminum and copper. When zinc is recycled to produce secondary zinc, it can save about 75 % of the total energy that is needed to produce the primary zinc from ore, and in therms of $CO_2$ emissions reduced by about 40 %. However, since zinc is mainly used for galvanizing of steel, the recycling rate of zinc is about 25 %, which is lower than other metals. The raw materials for recycling of zinc include dusts generated in the production of steel and brass, sludge in the production process of non-ferrous metals, dross in the melting of zinc ingots or hot dip galvanizing, waste batteries, and metallic scrap. Among them, steelmaking dust and waste batteries are most actively recycled up to now. Most of the recycling process uses pyrometallurgical methods. Recently, however, much attention has been given to a combined process of pyrometallurgical and hydrometallurgical processes.

• The Journal of Korean Academy of Prosthodontics
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• v.59 no.4
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• pp.442-450
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• 2021

The CAD-CAM technique is a rapidly developing field in the dental field and is applied to various fields of prosthetic treatment. Among them, the manufacturing of dentures using the milling technique classified as subtractive manufacturing is one of the commercialized digital full denture manufacturing methods. At the same time, it is possible to more efficiently manufacture a metal framework for implant overdenture by selective laser sintering or melting technique classified as an additive manufacturing method. The purpose of this article is to describe the fabrication of CAD-CAM maxillary complete denture and mandibular implant overdenture as well as its features.