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1

Effect Of Heat-Treatment On Microstructure And Magnetic Properties Of Nanocrystallized Mn-Zn Ferrite Powders

100%

Lee W. H. , Hong C. S. , Chang S. Y.

Archives of Metallurgy and Materials

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2015

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tom Vol. 60, iss. 2B

1347--1350

EN

The initial ferrite powders were subjected to high energy ball milling at 300rpm for 3h, and subsequently heat-treated at 573-1273K for 1h. Based on the observation of microstructure and measurement of magnetic properties, the heat-treatment effect was investigated. The size of initial powders was approximately 70μm. After milling, the powders with approximately 230nm in size were obtained, which were composed of the nano-sized particles of approximately 15nm in size. The milled powders became larger to approximately 550nm after heat-treatment at 973K. In addition, the size of particles increased to approximately 120nm with increasing temperature up to 973K. The coercivity of initial powders was almost unchanged after milling, whereas the saturation magnetization increased. As the heat-treatment temperature increased, the saturation magnetization gradually increased and the maximum coercivity was obtained at 773K.

2

Binary Coalescence of a Strange Star with a Black Hole: Newtonian Results

100%

Lee W. H. , Kluźniak W. , Nix J.

Acta Astronomica

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2001

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tom Vol. 51, No. 4

331--346

EN

We present Newtonian three-dimensional hydrodynamical simulations of the merger of quark stars with black holes. The initial conditions correspond to non-spinning stars in Keplerian orbits, the code includes gravitational radiation reaction in the quadrupole approximation for point masses. We find that the quark star is disrupted, forming transient accretion structures around the black hole, but 0.03 of the original stellar mass survives the initial encounter and remains in an elongated orbit as a rapidly rotating quark starlet, in all cases. No resolvable amount of mass is dynamically ejected during the encounters - the black hole eventually accretes 99.99%±0.01% of the quark matter initially present.

3

Spontaneous Formation of Titanium Nitride on the Surface of a Ti Rod Induced by Electro-Discharge-Heat-Treatment in an N2 Atmosphere

75%

Lee W. H. , Yoon Y. H. , Kim Y. H. , Lee Y. K. , Kim J. Y. , Chang S. Y.

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tom Vol. 62, iss. 2B

1281--1285

EN

A single pulse of 2.0 to 3.5 kJ of input energy from a 450 mF capacitor was applied to a commercially pure Ti rod in a N2 atmosphere. The surface of the Ti rod transformed from TiO2 into titanium nitride in times as short as 159 msec, providing a bimodal morphology of the cross-section. A much higher value of hardness that was observed at the edge of the cross-section was attributed to nitrogen-induced solid-solution hardening that occurred during the electrical discharge process. The activation energy (Ea) for the diffusion process was estimated to be approximately 86.9 kJ/mol. Results show that the electrical discharge process is a possible potential method for the nitriding of Ti; advantages include a short processing time and control of the nitrided layer without dimensional changes.

4

Self-Consolidation Mechanism Of Porous Ti-6Al-4V Implant Prototypes Produced By Electro-Discharge-Sintering Of Spherical Ti-6Al-4V Powders

75%

Lee W. H. , Jo Y. J. , Kim Y. H. , Jo Y. H. , Seong J. G. , Van Tyne C. J. , Chang S. Y.

Archives of Metallurgy and Materials

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2015

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tom Vol. 60, iss. 2B

1185--1189

EN

Electro-Discharge-Sintering (EDS) was employed to fabricate Ti-6Al-4V porous implant prototypes from atomized powders (100 – 150 μm), that were subjected to discharges of 0.75 to 2.0 kJ/0.7g-powder from 150, 300, and 450 μF capacitors. Both fully porous and porous-surfaced Ti-6Al-4V compacts with various solid core sizes were self-consolidated in less than 86 – 155 μsec. It is known that EDS can simultaneously produce the pinch pressure to squeeze and deform powder particles and the heat to weld them together. The formation of a solid core in these prototypes depends on the amounts of both the pinch pressure and heat generated during a discharge. The size of the solid core and the thickness of the porous layer can be successfully controlled by manipulating the discharge conditions such as input energy and capacitance.

5

Self-Consolidation Mechanism of Ti5Si3 Compact Obtained by Electro-Discharge-Sintering Directly from Physically Blended Ti-37.5 At.% Si Powder Mixture

75%

Chang S. Y. , Cheon Y. W. , Yoon Y. H. , Kim Y. H. , Kim J. Y. , Lee Y. K. , Lee W. H.

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tom Vol. 62, iss. 2B

1299--1302

EN

Characteristics of electro-discharge-sintering of the Ti-37.5at.% Si powder mixture was investigated as a function of the input energy, capacitance, and discharge time without applying any external pressure. A solid bulk of Ti5Si3 was obtained only after in less than 129 μsec by the EDS process. During a discharge, the heat is generated to liquefy and alloy the particles, and which enhances the pinch pressure can condensate them without allowing a formation of pores. Three step processes for the self-consolidation mechanism during EDS are proposed; (a) a physical breakdown of oxide film on elemental as-received powder particles, (b) alloying and densifying the consolidation of powder particles by the pinch pressure, and (c) diffusion of impurities into the consolidated surface.

6

Self-Consolidation Mechanism Of Porous Ti-6Al-4V Implant Prototypes Produced By Electro-Discharge-Sintering Of Spherical Ti-6Al-4V Powders

75%

Lee W. H. , Jo Y. J. , Kim Y. H. , Jo Y. H. , Seong J. G. , Van Tyne C. J. , Chang S. Y.

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nr 2

7

Self-Consolidation Mechanism of Ti5Si3 Compact Obtained by Electro-Discharge-Sintering Directly from Physically Blended Ti-37.5 at.% Si Powder Mixture

75%

Chang S. Y. , Cheon Y. W. , Yoon Y. H. , Kim Y. H. , Kim J. Y. , Lee Y. K. , Lee W. H.

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8

Compositional Dependence Of Hardness Of Ge-Sb-Se Glass For Molded Lens Applications

63%

Park J. K. , Lee J. H. , Shin S. Y. , Yi J. H. , Lee W. H. , Park B. J. , Choi J. H. , Kim N. Y. , Choi Y. G.

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nr 2

9

Surface Modification of Self-Consolidated Microporous Ti Implant Compacts Fabricated by Electro-Discharge-Sintering in Air

63%

Jo Y. J. , Yoon Y. H. , Kim Y. H. , Chang S. Y. , Kim J. Y. , Lee Y. K. , Van Tyne C. J. , Lee W. H.

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tom Vol. 62, iss. 2B

1287--1291

EN

A single pulse of 0.75-2.0 kJ/0.7g of atomized spherical Ti powders from 300 mF capacitor was applied to produce a microporous Ti implant compact by electro-discharge-sintering (EDS). A solid core in the middle of the compact surrounded by a microporous layer was found. X-ray photoelectron spectroscopy was employed to study the surface characteristics of the EDS Ti compact and it revealed that Ti, C and O were the main constituents on the surface with a smaller amount of N. The surface was lightly oxidized and was primarily in the form of TiO2 resulting from the air oxidation during EDS processing. The lightly oxidized surface of the EDS compact also exhibited Ti nitrides such as TiN and TiON, which revealed that the reaction between air constituents and the Ti powders even in times as short as 128 msec.

10

Compositional Dependence of Hardness of Ge-Sb-Se Glass for Molded Lens Applications

63%

Park J. K. , Lee J. H. , Shin S. Y. , Yi J. H. , Lee W. H. , Park B. J. , Choi J. H. , Kim N. Y. , Choi Y. G.

Archives of Metallurgy and Materials

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2015

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tom Vol. 60, iss. 2B

1205--1208

EN

Chalcogenide glass in the ternary Ge-Sb-Se system is inherently moldable, thus being considered as a strong candidate material for use in infrared-transmitting lens applications from the viewpoint of thermal and mechanical stability. In an effort to experimentally determine compositional region suitable for the molded lens applications, we evaluate its compositional dependence of hardness. Among the constituent atoms, Ge content turns out to exert a most conspicuous correlation with hardness. This phenomenological behavior is then explained in connection with the structural evolution that Ge brings about.