• Korean Journal of Materials Research
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• v.28 no.3
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• pp.174-181
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• 2018

Three kinds of STS304-Zr alloys were fabricated by varying the Zr content, and their microstructure and fracture properties were analyzed. Moreover, we performed heat treatment to improve their properties and studied their microstructure and fracture properties. The microstructure of the STS304-Zr alloys before and after the heat treatment process consisted of ${\alpha}-Fe$ and intermetallics: Zr(Cr, Ni, Fe)2 and Zr6Fe23. The volume fraction of the intermetallics increased with an increasing Zr content. The 11Zr specimen exhibited the lowest hardness and fine dimples and cleavage facets in a fractured surface. The 15Zr specimen had high hardness and fine cleavage facets. The 19Zr specimen had the highest hardness and large cleavage facets. After the heat treatment process, the intermetallics were spheroidized and their volume fraction increased. In addition, the specimens after the heat treatment process, the Laves phase (Zr(Cr, Ni, Fe) 2) decreased, the Zr6Fe23 phase increased and the Ni concentration in the intermetallics decreased. The hardness of all the specimens after the heat treatment process decreased because of the dislocations and residual stresses in ${\alpha}-Fe$, and the fine lamellar shaped eutectic microstructures changed into large ${\alpha}-Fe$ and spheroidized intermetallics. The cleavage facet size increased because of the decomposition of the fine lamellar-shaped eutectic microstructures and the increase in spheroidized intermetallics.

• 한국해양학회지
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• v.5 no.1
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• pp.30-36
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• 1970

The water quality of the laver bed at Yongwon Ri, Changwon Gun was investigated during the spring and neap tide in March 1970. The effect of tide on the contents of various chemical constituents was irregular and the variation ranges of the contents were narrow. The pH value during flood and ebb tide was a constant of 8.2. The chlorosity range varied from 19.15 to 19.33g/l, the difference of 0.18g/l being comparatively small for coastal waters. The nutrient salts contents varied irregulary with the change in tide, but nitrite and soluble iron were not detected. In local distribution, chlorosity and silicate- silicon contents were found to be more at Sts. 1, 2 and 3 in the eastern area than at Sts. 4, 5 and 6 in the western area, wheras nitrate and ammonia contents were found to be more at Sts. 4, 5 and 6. The nitrate content was especially high, being twice as much as that at Sts. 1, 2 and 3 in the eastern area. In the spring tide, chlorosity was found, on the average, to be as much as 0.06g/l higher than in the neap tide, but the contents of nutrient salts were higher in the neap tide, especially the nitrate content was twice as much. When compared with other selected local laver beds, i.e., the tidal flats of the Nackdong and Somjin rivers, and of Wan Do Gun, the chlorosity level was highest but the nutrient salts contents level was, in general, slightly lower and the variation ranges narrow in the laver bed at Yongwon ri. The nitrate content, in particular was one tenth smaller than the others.

• Geomechanics and Engineering
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• v.12 no.1
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• pp.85-109
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• 2017

The paper presents the laboratory study of clayey soil stabilized with Pond ash (PA), Rice husk ash (RHA), cement and their combination used as stabilizers to develop and evaluate the performance of clayey soil. The effect of stabilizer types and dosage on fresh and mechanical properties is evaluated through compaction tests, unconfined compressive strength tests (UCS) and Split tensile strength tests (STS) performed on raw and stabilized soil. In addition SEM (scanning electron microscopy) and XRD (X-ray diffraction) tests were carried out on certain samples in order to study the surface morphological characteristics and hydraulic compounds, which were formed. Specimens were cured for 7, 14 and 28 days after which they were tested for unconfined compression tests and split tensile strength tests. The moisture and density curves indicate that addition of RHA and pond ash results in an increase in optimum moisture content (OMC) and decrease in maximum dry density (MDD). The replacement of clay with 40% PA, 10% RHA and 4% cement increased the strength (UCS and STS) of overall mix in comparison to the mixes where PA and RHA were used individually with cement. The improvement of 336% and 303% in UCS and STS respectively has been achieved with reference to clay only. Developed stabilized soil mixtures have shown satisfactory strength and can be used for low-cost construction to build road infrastructures.

• Horticultural Science & Technology
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• v.33 no.3
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• pp.356-363
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• 2015

This study examined the effects of different day/night temperature regimes or silver ion on growth and flowering of passion fruit 'Tai-nung No.1'. Low temperature treatment ($20/15^{\circ}C$) caused passion fruit cultivar 'Tai-nung No.1' to fail to flower. Flowering induction occurred within a temperature range of $20-30^{\circ}C$, with no significant difference in the days to first flower bud and the total number of flower buds between plants grown at $30/25^{\circ}C$ and $25/20^{\circ}C$. However, plants grown at $30/25^{\circ}C$ exhibited their first flower buds set on the higher nodes and had higher abortion rates of flower buds than those at $25/20^{\circ}C$. Plants grown at $30/25^{\circ}C$ had the most rapid growth and the shortest plastochron. We also evaluated the effect of the ethylene response inhibitors silver nitrate ($AgNO_3$) and silver thiosulfate (STS) on growth and flowering of potted passion fruit 'Tai-nung No.1', when they were exposed to low temperature conditions ($20/15^{\circ}C$) following chemical treatments ($AgNO_3$ or STS, at 0.5 or 1.0 mM). $AgNO_3$ and STS treatments induced flower formation and initial flower bud formation within approximately two weeks at $20/15^{\circ}C$ whereas non-treated control plants exhibited no flower formation. ACC content and activity of ACC oxidase in the leaves of passion fruit 'Tai-nung No.1'exposed to low temperature conditions ($20/15^{\circ}C$) were significantly inhibited by the ethylene inhibitor treatments. These results indicate that ethylene, which is produced under low temperature conditions, plays an important role in inhibiting flower formation in passion fruit.

• Journal of Korean Elementary Science Education
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• v.33 no.2
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• pp.207-216
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• 2014

Understanding the nature of science (NOS) has been a major component of scientific literacy and an important learning goal in science education (American Association for the Advancement of Science, 1990, 1993; Millar & Osborne, 1998; National Research Council, 1996, 2013). To achieve this goal, it is necessary to include appropriate views of the nature of science in science curriculum such as a textbook. The aim of this study was to conduct a content analysis of the elementary science textbooks to examine the presentation of the nature of science (NOS) using the four themes of scientific literacy, which are (1) nature of scientific knowledge, (2) nature of scientific inquiry, (3) nature of scientific thinking, and (4) nature of interactions among science, technology, and society (STS). Findings show while $3^{rd}$ and $4^{th}$ grade levels of science textbooks heavily emphasize on the Theme I, nature of scientific knowledge and the Theme II, the nature of scientific inquiry, the upper grade levels such as $5^{th}$ and $6^{th}$ grades of science textbooks are well balanced with the four themes of the nature of science. In addition, most of elementary science textbooks little focus on the Theme IV, nature of interactions among science, technology, and society (STS) among the four themes of the nature of science. It might be a shortcoming because the understanding the nature of interacions among science, technology and society (STS) is one of the key components in order for students to be a scientific literate person.

• Journal of the Korean earth science society
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• v.23 no.2
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• pp.194-206
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• 2002

This article describes the major themes to change in historical and philosophical perspectives of science education that lead the US and Korean science curriculum reform movements since 1957. Inquiry teaching and criticism of teaching science as inquiry in the late 1950s and the 1960s, Science-Technology-Society (STS) Curricula, and Science Literacy and the 1980s science literacy crisis are discussed. In the US, three major curricular projects as responses to the scientific literacy crisis are exemplary such as the Project 2061 sponsored by the American Association for the Advancement of Science, the Project on Scope, Sequence, and Coordination (SS&C) initiated by the National Science Teachers Association (NSTA), and the National Science Education Standards (NSES) published by the National Research Council. To identify how each set of national content standards differ, we compared specific content standards related to the theory of plate tectonics in Earth and Space science in grades 9-12 over the three national standards: Benchmarks of AAAS, NSES of the NRC, and SS&C of the NSTA. Against this historical background of the US science education reform movements, the curriculum reform movements in Korea is briefly discussed. In general, Korean science curriculum reform movements have reflected and resembled the recommendations of the US reform movements. In addition, it is important to note that throughout the history of curriculum revision in Korea, there have been continuing pendulum swings between a theoretical, discipline-centered curriculum and a liberal, humanistic, and student-centered curriculum, which pays more attention to students in terms of their interest and psychological preparedness. In conclusion, the sixth and seventh national science curriculum revisions reflect rather a student-centered movement by reducing technical and sophisticated topics, taking constructivism learning theory into consideration, and adding more STS related topics.

• Journal of Welding and Joining
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• v.18 no.1
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• pp.59-69
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• 2000

The purpose of the present study was to investigate weld metallurgical phenomena such as primary solidification mode, microstructural evolution and hot cracking susceptibility in nitrogen-bearing austenitic stainless steel GTA welds. Eight experimental heats varying nitrogen content from 0.007 to 0.23 wt.% were used in this study. Autogenous GTA welding was performed on weld coupons and the primary solidification mode and their microstructural characteristics were investigated from the fusion welds. Varestraint test was employed to evaluate the solidification cracking susceptibility of the heats and TCL(Total Crack Length) was used as cracking susceptibility index. The solidification mode shifted from primary ferrite to primary austenite with an increase in nitrogen content. Retained delta ferrite exhibited a variety of morphology as nitrogen content varied. The weld fusion zone exhibited duplex structure(austenite+ferrite) at nitrogen contents less than 0.10 wt.% but fully austenitic structure at nitrogen contents more than 0.20 wt.%. The weld fusion zone in alloys with about 0.15 wt.% nitrogen experienced primary austenite + primary ferrite solidification (mode AF) and contained delta ferrite less than 1% at room temperature. Regarding to solidification cracking susceptibility, the welds with fully austenitic structure exhibited high cracking susceptibility while those with duplex structure low susceptibility. The cracking susceptibility increased slowly with an increase in nitrogen content up to 0.20 wt.% but sharply as nitrogen content exceeded 0.20 wt.%, which was attributed to solidification mode shift fro primary ferrite to primary austenite single phase solidification.

• Korean Journal of Metals and Materials
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• v.46 no.12
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• pp.817-822
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• 2008

Metal matrix composite (MMC) materials having low electrical contact resistance based on 316L stainless steel (STS) matrix alloy with $ZrB_2$ particles were fabricated for PEMFC (Polymer Electrolyte Membrane Fuel Cell) separator by powder metallurgy (PM). The effects of the boride particle addition into the matrix alloy on microstructure, surface morphology, and interfacial contact resistance (ICR) between the samples and gas diffusion layer (GDL) were investigated. Both conventional and PM 316L STS samples showed high ICR due to the existence of non-conductive passive film on the alloy surface. The addition of the boride particles, however, remarkably reduced ICR of the samples. SEM observation revealed that the boride particles were protruded out of the matrix surface and particle density existing on the surface increased with increasing the boride content, causing increase of the total contact area between the conductive particles and GDL. ICR of the samples also decreased with increasing the boride content resulted from the increased contact area.

• Journal of the Korean Society for Heat Treatment
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• v.21 no.5
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• pp.244-250
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• 2008

This study has investigated the surface phase change, hardness variation, surface precipitates, nitrogen content and corrosion resistance in STS 431 (17Cr-2Ni-0.2C-0.01Nb) martensitic stainless steel after high temperature gas nitriding (HTGN) treatment at the temperature range between $1050^{\circ}C$ and $1150^{\circ}C$. The HTGN-treated surface layer appeared $Cr_2N$ of rod type, carbo-nitride of round type and fine precipitates in the austenite matrix. On the other hand the interior region where the nitrogen was not permeated, exhibited martensite phase. The surface hardness showed 250~590 HV, depending on the HTGN treatment conditions, while the interior martensitic phase represented 520 HV. The permeation depth of nitrogen increased with increasing the HTGN-treated temperature. The nitrogen concentration of the surface layer appeared approximately ~0.17% at $1100^{\circ}C$. On comparing the corrosion resistance between solution-annealed and HTGN-treated steels, the corrosion resistance of HTGN-treated steel was superior to that of solution-annealed specimens.

• Corrosion Science and Technology
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• v.9 no.3
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• pp.122-128
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• 2010

The corrosion of the flexible tube in the automobile exhaust system is caused by the ambient water and chloride ions. Since welding is one of the key processes for the flexible tube manufacturing, it is required to select a proper welding method to prevent the flexible tube corrosion and to increase its lifetime. There are many studies about the efficiency of the welding method, but no systematic study is performed for the effect of welding method on the corrosion property of the austenitic stainless weldment. The aim of the present study is to provide information on the effect of two different welding methods of TIGW (tungsten inert gas welding) and PAW (plasma arc welding) on the corrosion property of austenitic stainless steel weldment. Materials used in this study were two types of the commercial austenitic stainless steel, STS321 and XM15J1, which were used for flexible tube material for the automotive exhaust system. Microstructure was observed by using optical microscopy (OM) and scanning electron microscopy (SEM). To evaluate the corrosion behavior, potentiodynamic and potentiostatic tests were performed. The chemical state of the passive film was analyzed in terms of XPS depth profile. Metallurgical analysis show that the ferrite content in fusion zone of both STS321 and XM15J1 is higher when welded by PAW than by TIGW. The potentiodynamic and potentiostatic test results show that both STS321 and XM15J1 have higher transpassive potential and lower passive current density when welded by PAW than by TIGW. XPS analysis indicates that the stable $Cr_2O_3$ layer at the outermost layer of the passive film is formed when welded by PAW. The result recommends that PAW is more desirable than TIGW to secure corrosion resistance of the flex tube which is usually made of austenitic stainless steel.