발간논문

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Abstract: (Cr,Ti)2AlC compounds were synthesized by hot pressing, and oxidized at 700, 850, and 1000℃ in air for up to one year. They consisted of Ti-incorporated Cr2AlC phase, Ti-rich phase, and Cr-rich phase. The formed scales divided into a uniform oxide layer and oxide nodules. The uniform oxide layer consisted primarily of a thin Al2O3 scale. The oxide nodules that originated from the Ti-rich phase consisted of TiO2, which grew by both the outward diffusion of Ti4+ ions and inward diffusion of O2- ions. Chromium did not appreciably enter the oxide scale. Instead, it existed in the Ti-incorporated Cr7C3 sublayer after oxidation. (Cr,Ti)2AlC compounds had good oxidation resistance, because of formation of the Al2O3 barrier layer. (Received January 10, 2013)

keyword : compounds, sintering, oxidation, scanning electron microscopy, alumina

The microstuructural properties and electrical characteristics of sputtering films deposited with Mo targets prepared by commercial hot isostatic pressing (HIP) and spark plasma sintering (SPS) were compared and analyzed. The analysis results revealed that the Mo films obtained from the SPS Mo targets had a quite similar resistivity and microstructure as the commercial HIP ones. Further, characteristics of the Mo films according to the sputtering conditions were investigated by XRD, SIMS and TEM techniques. Such films are expected to be used as electrode materials for display/solar cell devices. (Received October 29, 2012)

keyword : metals, sputtering, microstructure, transmission electron microscopy, spark palsma sintering

Nanocrystalline materials have received much attention as advanced engineering materials with improved physical and mechanical properties. As nanomaterials possess high strength, high hardness, excellent ductility and toughness, undoubtedly, more attention has been paid for the application of nanomaterials. Nanopowders of Al2O3, MgO and TiO2 were made by high energy ball milling. The simultaneous synthesis and sintering of a nanostructured MgTiO3-MgTi2O5-MgAl2O4 composite was investigated by the pulsed current heated sintering process. A highly dense nanostructured MgTiO3-MgTi2O5-MgAl2O4 composite was produced with simultaneous application of 80 MPa pressure and pulsed current within one min. The sintering behavior, gain size and mechanical properties of MgTiO3-MgTi2O5-MgAl2O4 composite were investigated. (Received December 11, 2012)

keyword : compounds, nanostructured materials, sintering, mechanical properties

Abstract: In this study, the effect of sintering parameters such as sintering temperature, pressure, and degassing treatment on the microstructural evolution and properties of a Mg-6%Al alloy consolidated by spark plasma sintering (SPS) was investigated. Experimental results revealed that degassing treatment prior to sintering was a critical step in the spark plasma sintering process. We concluded that the impurity concentration on the surface of the sintered material was remarkably reduced by degassing treatment and consequently bonding strength among particles was enhanced. A magnesium alloy of high ductility and strength was obtained when a sintering temperature and pressure of 0.82 Tm and 130 MPa, respectively, was employed combined with degassing treatment. (Received January 11, 2013)

keyword : oxides, mechanical alloying, spark plasma sintering, microstructure, scanning electron microscope

It is extremely important that the junction temperature of a high-power, multi-chip LED package be managed so as not to exceed the allowable temperature range (-40-100℃) due to the large heat dissipated from the package. In this study, chip configuration was modified in a 4-chip LED package, and the number of chips was changed to 12-chip and 16-chip packages to investigate their effects on the thermal characteristics of the packages. Thermal resistance was estimated using thermal transient analysis, and the ambient temperature of the package was measured by thermo-couple. Ultimately, the junction temperatures of the packages were obtained using these data. The results showed that the maximum allowable input power of the 4-chip package was 3.4 W, which was enlarged up to 5.5 W using 12-chip and 16-chip LED packages. (Received July 10, 2012)

keyword : optoelectronic materials, thermomechanical processing, thermodynamic properties, thermal analysis, LED Package

In order to improve the energy conversion efficiency by applying the Al/Ru bilayer on a flat glass substrate of a counter electrode (CE), a 0.45㎠ dye-sensitized solar cell (DSSC) device with glass/FTO/ blocking layer/TiO2/N719 (dye)/electrolyte/(50 nm Ru-50 nm Al)/glass was prepared. For a comparison, a Ru CE of 100 nm-thickness on a flat glass substrate was also prepared using the same method. The photovoltaic properties, such as the short circuit current density (Jsc), open circuit voltage (Voc), fill factor (FF), energy conversion efficiency (ECE), and impedance, were characterized using a solar simulator and potentiostat. By Jsc, Voc, FF and ECE measurements, the ECE of the device with Al/Ru bilayer CE was 2.36%. The increase in efficiency was 1.46 times that of the device with a 100 nm Ru CE. The impedance at the interface between CE and the electrolyte decreased with Al/Ru bilayer films. The results imply that the application of an Al/Ru bilayer film improves the efficiency of DSSC devices. †(Received January 15, 2013)

keyword : solar cells, sputtering, optical properties, solar simulator, bilayer

In this work, MgH2 was employed as a starting material rather than Mg. Samples with compositions of 94 wt% MgH2-6 wt% Ni and 85 wt% MgH2-15 wt% Ni were prepared by reactive mechanical grinding. They were named MgH2-6Ni and MgH2-15Ni, respectively. Their hydriding and dehydriding properties were then examined and compared. The activation of MgH2-6Ni and MgH2-15Ni was completed after just the first hydriding (under 12 bar H2)-dehydriding (in vacuum) cycle at 573 K. MgH2-6Ni after reactive mechanical grinding has a larger specific area and a slightly smaller mean particle size (37㎡/g, 1.22 μm) than MgH2-15Ni after reactive mechanical grinding (30㎡/g, 1.24 μm). The activated MgH2-15Ni has slightly lower hydriding rates and a smaller quantity of hydrogen absorbed for 60 min at 573 K under 12 bar H2 than MgH2-6Ni. At 573 K under 1.0 bar H2, the activated MgH2-6Ni at first had a higher dehydriding rate than the activated MgH2-15Ni; however, the activated MgH2-15Ni had higher dehydriding rates after approximately 10 min and a larger quantity of hydrogen desorbed for 60 min than the activated MgH2-6Ni. The decrease in particle size and creation of defects due to Ni addition via reactive mechanical grinding are considered to increase the hydriding and dehydriding rates of MgH2. (Received November 2, 2012)

keyword : hydrogen absorbing materials, mechanical alloying, milling, microstructure, X-ray diffraction, starting materials MgH2 and Ni

In order to increase the hydrogen storage performance of Mg or MgH2-based materials, Li3N, LiBH4, Fe, and/or Ti were added. Mixtures with compositions of 50MgH2-50Li3N, 40Mg-10MgH2-50Li3N, 68MgH2-17LiBH4-15Fe, and 70MgH2-17LiBH4-13Ti were prepared by reactive mechanical grinding, and their hydrogen-storage properties were examined. 68MgH2-17LiBH4-15Fe and 70MgH2-17LiBH4-13Ti had higher hydriding rates than the Li3N-containing samples, 50MgH2-50Li3N and 40Mg-10MgH2-50Li3N, at 573 K under 12 bar H2. However, the dehydriding rates of 68MgH2-17LiBH4-15Fe and 70MgH2-17LiBH4-13Ti were as low as those of the Li3N-containing samples at 573 K under 1.0 bar H2. (Received December 18, 2012)

keyword : hydrogen absorbing materials, mechanical alloying, milling, microstructure, X-ray diffraction, Li3N and LiBH4 addition

The nanostructure of a Cu-15wt%Ag composite processed by Equal Channel Angular Pressing (ECAP) was observed by transmission electron microscopy (TEM). ECAP was found to greatly improve the yield strength while maintaining good electrical conductivity. The nanostructured Cu-15 wt%Ag composite contained a high density of {111}/<112> nanotwins and the migration of the twin boundary and detwinning process of those nanotwins was also observed. Nanotwins can be nucleated by bowing out of twinning dislocations at the Cu/Ag interface and propagated by the sequential emission of twinning dislocations on every adjacent twinning plane. Steps having the height of one or three atomic layers on the {111} plane were observed on the coherent twin boundary (CTB). The stair rod dislocation was formed by a cross-slip of 30° partial at one layer high step on the twin boundary, while a cross-slip of 90° partial transformed the incident dissociated dislocation into dissociated dislocation in the twin plane without leaving a stair-rod dislocation behind. The three layer high step was formed by migration of the (111) twin boundary in the direction normal to the twin plane, and it produced a {112} incoherent twin boundary (ITB). The advance or retreat of {112} ITB can lead to the extension or retraction of (111) on the CTB. The detwinning process involves the collective glide of a group of three Shockley partial with different Burgers vectors and the dynamical overlap of stacking faults (SFs) of intrinsically and/or extrinsically dissociated dislocations. †(Received January 7, 2013)

keyword : Cu-Ag nanocomposite, deformation twins, equal channel angular pressing, transmission electron microscopy