발간논문

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Samples of pure Mg, 76.5 wt%Mg-23.5 wt%Ni, and 71.5 wt%Mg-23.5 wt%Ni-5 wt%Fe2O3 were prepared by reactive mechanical grinding and their hydriding and dehydriding properties were then investigated. The reactive mechanical grinding of Mg with Ni is considered to facilitate nucleation and to shorten diffusion distances of hydrogen atoms. After hydriding-dehydriding cycling, the 76.5 wt%Mg- 23.5 wt%Ni and 71.5 wt%Mg-23.5 wt%Ni-5 wt%Fe2O3 samples contained Mg2Ni phase. In addition to the effects of the creation of defects and the decrease in particle size, the addition of Ni increases the hydriding and dehydriding rates by the formation of Mg2Ni. Expansion and contraction of the hydride-forming materials (Mg and Mg2Ni) with the hydriding and dehydriding reactions are also considered to increase the hydriding and dehydriding rates of the mixture by forming defects and cracks leading to the fragmentation of particles. The reactive mechanical grinding of Mg-Ni alloy with Fe2O3 is considered to decrease the particle size.

keyword : hydrogen absorbing materials, mechanical alloying/milling, microstructure, X-ray diffraction, role of additives

We studied the effect of heat treatment on the microstructures and mechanical strength of the solder joints in the Light Emitting Diode (LED) packages. The commercial LED packages were mounted on the a flame resistance-4 (FR4) Printed Circuit Board (PCB) in the reflow process, and then the joints were aged at 125℃ for 100, 200, 300, 500 and 1000 hours, respectively. After the heat treatment, we measured the shear strength of the solder joints between the PCB and the LED packages to evaluate their mechanical property. We used Pb-free Sn-3.0Ag-0.5Cu solder to bond between the LED packages and the PCBs using two different surface finishes, Electroless Nickel-Immersion Gold (ENIG) and Electroless Nickel-Electroless Palladium-Immersion Gold (ENEPIG). The microstructure of the solder joints was observed by a scanning electron microscope (SEM). (Cu,Ni)6Sn5 intermetallic compounds (IMCs) formed between the solder and the PCB, and the thickness of the IMCs was increased with increasing aging time. The shear strength for the ENIG finished LED package increased until aging for 300 h and then decreased with increasing aging time. On the other hand, in the case of an ENEPIG finished LED package, the shear strength decreased after aging for 500 h.

keyword : intermetallics, soldering, mechanical property, scanning electron microscopy, LED package

Aluminum nitride(AlN) is a compound (III-V group) of hexagonal system with a crystal structure. Its Wurzite phase is a very wide band gap semiconductor material. It has not only a high thermal conductivity, a high electrical resistance, a high electrical insulating constant, a high breakdown voltage and an excellent mechanical strength but also stable thermal and chemical characteristics. This study is on the preferred orientation characteristics of AlN thin films by reactive evaporation using NH3. We have manufactured an AlN thin film and then have checked the crystal structure and the preferred orientation by using an X-ray diffractometer and have also observed the microstructure with TEM and AlN chemical structure with FT-IR. We can manufacture an excellent AlN thin film by reactive evaporation using NH3 under 873 K of substrate temperature. The AlN thin film growth is dependent on Al supplying and NH3 has been found to be effective as a source of N2. However, the nuclear structure of AlN did not occur randomly around the substrate a particle of the a-axis orientation in fast growth speed becomes an earlier crystal structure and is shown to have an a-axis preferred orientation. Therefore, reactive evaporation using NH3 is not affected by provided H2 amount and this can be an easy a-axis orientation method.

keyword : magnetic materials, deposition, crystallization, X-ray diffraction, preferential orientation

Ti-6Al-4V extra low interstitial (ELI) alloy has been widely used as an orthopedic implant material because of its excellent biocompatibility, corrosion resistance and mechanical properties. However, V-free titanium alloys such as Ti-6%Al-7%Nb and Ti-5%Al-2.5%Fe have recently been developed because of the toxicity of V. Hydroxyapatite (HA) is used as a coating material on Ti or Ti biomaterials due to its good biocompatibility. However, HA coated on Ti alloy causes a problem for tissue by peeling off during usage. Therefore, such peeling off during long time usage can be suppressed by adding HA in Ti or Ti alloy composites. The aim of this study was to manufacture an ultra fine grained (UFG) Ti-Nb-HA bulk alloy, which is usually difficult to fabricate using melting and casting technology, by rapid sintering process using high energy mechanical milled (HEMM) powder.

keyword : biomaterials, sintering, corrosion resistance, SEM, Ti-Nb-HA composite