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The surface roughness and reflection characteristics of electroless-plated Sn-3.5 wt%Ag on a LED (light emitting diode) lead frame were investigated. Cu electroplating was carried out prior to electroless plating of Sn-3.5Ag to improve the reflectivity of the Sn-3.5Ag deposit. A thermal shock test was performed to evaluate the stability of the deposits, namely the surface roughness and the reflectivity of the Sn-3.5Ag layer. The experimental results revealed that the surface roughness of the Sn-3.5Ag deposit was 0.135μm (arithmetical average roughness, Ra), 1.77 μm (maximum height roughness, Rmax) and the reflectivity was 1.86 GAM. After 500 thermal shock cycles of the Sn-3.5Ag deposit between 85℃ and -55℃, the surface roughness became 0.145 μm (Ra) and 1.93 μm (Rmax), and the reflectivity was 1.76 GAM, which indicates that the Sn-3.5Ag deposit was stabilized by 500 thermal cycles.

keyword : alloys, plating, surface, thermal analysis, reflectivity

The effect of high-energy ball milling on the sintering, microstructure and mechanical properties of Al2O3 were investigated. The temperature at the beginning of consolidation decreased with high-energy ball milling because the driving force for sintering and the contact points of the powders for atomic diffusion increased. The mechanical properties (hardness and fracture toughness) and the relative density of Al2O3 sintered from high-energy ball milled powder were higher than those of Al2O3 sintered from unmilled powder. 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 to the application of nanomaterials. A highly dense nanostructured Al2O3 was produced using high-energy ball milled powder with simultaneous application of 80 MPa pressure and a pulsed output current of 2800 A within 2 min.

keyword : compound, nanostructure, sintering, mechanical properties, pulsed current activated combustion

An optimum route to synthesize a Fe-Co alloy-dispersed Al2O3 nanocomposite powders and their magnetic properties were investigated. Nanocomposite powders of Al2O3 and Fe-Co alloys were prepared by the solution chemistry method using Al2O3 and metal nitrates powders. Al2O3/Fe-Co alloy powders were sythesized by calcination at 350℃ and hydrogen reduction at 700℃ of Al2O3, Fe- and Ni-nitrate powders. Microstructural analysis for the nanocomposite powders synthesized by ball milling and a hydrogen reduction process showed that the Fe-Co alloy particles with an average size of 20 nm were well distributed on the surface of the Al2O3 powders. The ferromagnetism of the nano-sized Fe-Co alloy contributed to the magnetic properties of the nanocomposite powders. The increase in coercive force was explained as being due to nano-sized particles of the Fe-Co alloy.

keyword : nanostructured materials, powder processing, microstructure, X-ray diffraction, magnetic properties

The present work demonstrated the influence of sintering temperature on the microstructure and magnetic properties of Fe- 6.5%Si alloys fabricated via metal injection molding. The samples were sintered in an Ar atmosphere at three different temperatures, 1100, 1200, and 1300℃. Microstructural observations of the sintered sample revealed that, with increasing sintering temperature, the average grain size of the ferrite phase increased, while the fraction of the micro pore decreased. These results were attributed to the increasing mobility of the grain boundaries. The hysteresis curves showed that the magnetic properties of the sintered samples as a soft magnet were enhanced with increasing sintering temperature since the saturation magnetization increased and the coercivity decreased, reaching the values of 1.22 tesla and 1.84 kA/m at 1300℃. Moreover, the values of the magnetic hysteresis loss in the sintered samples gradually decreased with increasing sintering temperature due to both the fairly high densification and coarse grain size of the sintered sample.

keyword : metals, sintering, metal injection molding, magnetic properties, scanning electron microscopy

Titanium is added to ferritic stainless steels in the range of 0.2-0.3 wt% to improve corrosion resistance and mechanical properties. However, titanium is very reactive with oxygen in liquid steel, and it can cause an unstable Ti yield. Therefore, titanium is generally added after the aluminum deoxidation process in the ladle. However, the inclusions formed by Al-Ti deoxidation can cause nozzle clogging and various defects in final products. Calcium injection can be carried out to resolve these problems. In this study, two different deoxidation patterns of Al→Ti and Al→Ti→Ca additions were carried out in Fe-18%Cr ferritic stainless steel melt at 1873 K. The melt composition and the inclusion morphology changes during the deoxidation process were investigated. With Al→Ti addition, the Al2O3 inclusions changed to dual phase Al2O3-TiOX inclusions with time by the reaction with Ti in the melt. The morphology of the inclusions in the melt finally changed to a polygonal type indicating that the inclusions were sol d phase. The size and number of inclusions in the melt did not change with time after Ti addition. With of Al→Ti→Ca addition, Ca reacted with Al2O3-TiOX inclusions to form liquid CaO-Al2O3 inclusions embedded with solid CaTiO3 particles. The morphology of the inclusions in the melt were observed to be spherical and polygonal. The size of inclusions in the melt increased and the number of inclusions decreased by the coalescence of liquid inclusions.

keyword : ferritic stainless steel, deoxidation, aluminum, titanium, calcium

In order to investigate the formation of a high pressure form of magnesium hydride, γ-MgH2, X-ray diffraction patterns before and after the hydriding reaction were obtained for samples of MgH2, MgH2 after reactive mechanical grinding (RMG), and 94 wt% MgH2-6 wt% Ni after reactive mechanical grinding. In addition, absorbed hydrogen quantity versus time curves at the first cycle were examined. The MgH2 sample did not contain γ-MgH2. However, the MgH2 after RMG and the 94 wt% MgH2-6 wt% Ni after RMG contained γ-MgH2. All these results prove that after reactive mechanical grinding, γ-MgH2, one of the high pressure forms of MgH2, was formed in the MgH2 and 94 wt% MgH2-6 wt% Ni samples under low hydrogen pressures.

keyword : hydrogen absorbing materials, mechanical alloying/milling, phase transformation, X-ray diffraction, magnesium hydride high pressure form

In this study, we fabricated anode-supported solid oxide fuel cells by the decalcomania method. To investigate the effect of the anode functional layer (AFL) in a cell, AFL decalcomania paper was coated between the electrolyte and the anode support. The results showed that the surface roughness of the electrolyte coated on the anode support was not uniform (Ra = 7.86 nm), but that of electrolyte coated on AFL was uniform. A thin electrolyte layer with very low surface roughness was obtained. Then Ra was 2.65 nm. In this case, while the output power of the cell with the AFL (Cell 2) had an open circuit voltage (OCV) of 1.12 V and a maximum power density (MPD) of 650 mW cm-2 at 800℃, the cell without the AFL layer (Cell 1) had an OCV of 0.9 V and an MPD of 635 mW cm-

keyword : Fuel Cells, coating, electrical properties, scanning electron microscopy, Surface roughness

The thermo-mechanical degradation of NiCr films during thermal cycling was investigated in terms of microstructural and chemical changes to the surface. Thermal cycling was performed between room temperature and 200℃ in thermally evaporated NiCr films on an Mn-Ni-Co-O substrate. Isothermal exposures were also performed as a control experiment. The sheet resistance of the NiCr films increased linearly (from 35 to 52 Ω/sq.) with a temperature difference of the thermal cycling, whereas the films subjected to isothermal aging increased only slightly, from 35 to 38 Ω/sq. After 300 thermal cycles at ΔT=175℃, cracks formed and proliferated on the coating surface, and local delamination occurred at the interfaces. XPS results showed that the intensity of a metallic peak at 574.0 eV decreased and that of the Cr oxide peak at ~577 eV increased with an increase of the thermal cycles. Consequently, the degradation of the NiCr films by accelerated thermal cycling stress was attributed to a proliferation of lateral surface cracks from thermally induced tensile stress compounded by surface oxidation, causing an increase in sheet resistance.

keyword : NiCr film, thermal cycling, accelerated test, degradation, failure mechanism

This study evaluated the purity of single-walled carbon nanotubes (SWCNTs) in the arc-synthesized SWCNT samples by using thermogravimetric analysis (TGA). The as-produced SWCNT samples were heat-treated in air for 20 h at 275-475℃ and characterized by scanning and transmission electron microscopes and TGA to establish oxidation temperature ranges of SWCNTs and carbonaceous impurities comprising the samples. Based on these oxidation temperature ranges, derivative thermogravimetric curves were deconvoluted, and differentiated peaks were assigned to SWCNTs and carbonaceous impurities. The compositions and the SWCNT purities of the samples were obtained simply by calculating the areal ratios under the deconvoluted curves. TGA studies on purity evaluation and thermal stabilities of SWCNTs and carbonaceous impurities are likely to provide us with a simple route of thermal oxidation purification to acquire high-purity SWCNT samples.

keyword : carbon nanotubes, purity evaluation, oxidation, purification, thermogravimetric analysis

The degradation behavior of AZO thin films against thermal and humidity stress was examined by a damp heat test. The resistivity of all films tested under various conditions increased due to the decreased carrier concentration and mobility with elapsed damp heat test time. The resistivity of the films also increased with increasing humidity, and these results indicated that the resistivity of AZO films had a stronger dependence on humidity than on temperature. Among the tested samples, the AZO films at 85℃/85% relative humidity conditions highly degraded, indicating that additional -OH bonds may deteriorate the properties of ZnO thin films. The degradation of the AZO films under the damp heat test is attributed to chemisorption and diffusion of water vapors, and the AZO reacts with the water molecules and is then transformed into insulating Zn(OH)2.

keyword : AZO(Al doped ZnO) thin film, degradation, damp heat test, chemisorption, electrical property