발간논문

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A lithium thionyl battery has been developed by using a compacted lithium powder electrode. Lithium powder of 10∼20 ㎛ mean diameter was fabricated safely by droplet emulsion technique. The charge capacity of a compacted powder electrode cell was longer than that of a conventional foil electrode cell. The TMV of a powder cell was also larger than that of a foil cell. The improved electrochemical properties of the powder cell might be indebted to the increased reactive areas and characters of the SEI of the powder electrode.

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A fluxless reflow process for solder bump flip chip package was attempted to protect environment using Ar+10%H_2 plasma. The 100 ㎛-diameter Sn-3.5 mass%Ag solder balls were bonded to 250 ㎛ pitch Cu/Ni under bump metallurgy (UBM) pads by laser ball bonding method. Then, Sn-3.5Ag solder balls were reflowed in Ar+H_2 plasma. The plasma reflow time and power conditions to show the optimum bump shear strength and shape were 40s at 100 W and 20s at 200 W. During plasma reflow, the solder bump reshaped and the crater on the top of bump disappeared. The bump shear strength increased as the Ni_3Sn_4 intetmetallic compounds formed in the initial reflow stage but began to degrease as coarse Cu-Ni-Sn ternary intermetallic compounds formed at the solder/UBM interface. As the plasma reflow time increased, the fracture mode changed from ductile fracture within the solder to the fracture at the solder/UBM interface. The off-centered bumps self-aligned to patterned UBM pads during plasma reflow.

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Hardfacing layers of Stellite 6 with different molybdenum content are deposited on AISI 1045-carbon steel using Plasma Transferred Arc welding(PTA) machine. The microstructures, wear and corrosion resistances of the hardfacing layer are investigated in order to clarify the effect of molybdenum in Stellite 6 alloys. The microstructural characterizations of the hardfacing layers are performed using X-ray diffraction, back scattered electron images, and scanning electron microscopy. With an increase of Mo contents, M_6C type carbide are formed instead of Cr-rich type carbides, and so this microstructural change enhanced the mechanical properties such as wear and corrosion resistance of a Mo-modified Stellite 6 alloy.

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The viscosity of CaO-SiO_2(-10MgO)-CaF_2 slags was measured to clarity the effect of CaF_2 on the viscous flow of molten slags at high temperatures and the solidification behavior of slags. The theoretical analysis for the relationship between solid fraction and viscosity of slags was also conducted. CaF_2 generally decreases the critical temperature (T_CR) of slags. The T_CR of CaO-SiO_2-CaF_2 (B=1.0) slags decreased about 150-200 K by addition of 10 mass% MgO. On the other hand, the T_CR of the slags with higher basicity (B=1.3) increased about 100 K by MgO addition. The viscosity of CaO-SiO2-CaF_2 slags (B=1.0 and 1.3) decreased with increasing CaF_2 content up to 15 mass% at 1823 K. The effect of CaO/SiO_2 ratio on slag viscosity was not significant in CaO-SiO_2-CaF_2 slags. The effect of CaF_2 addition on the viscosity of CaO-SiO_2-10 (mass%)MgO-CaF_2 (B=1.0) slags was negligible. However, the effect of CaF_2 on the viscosity of the slags with higher basicity (B=1.3) was significant. A theoretical calculation indicated that the apparent viscosity of heterogeneous slags (solid/liquid mixture) exponentially decreased with increasing CaF_2 content.

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Powders of Al-Ni-Co-Si alloys were produced by gas-atomization under protective N_2 atmosphere. The gas-atomized powders were sieved to select them having a size range of -200∼+400 mesh. Quasicrystalline coatings were deposited onto the 304 stainless steel substrates using two thermal spraying techniques: high velocity oxygen fuel (HVOF) and air plasma spraying(APS). The friction and wear behaviors of the coatings were evaluated under the various conditions. The wear amount of both the coating and the chromium coated cast iron disc during the testing was determined by measuring the weight loss after the test. Analysis of the wear debris and of the wear track on both the coating and Cr coated cast iron disk was observed by using scanning electron microscopy. Friction coefficient of the plasma coating was about 0.36 and of the HVOF coating was about 0.43. The APS-deposited coatings, which had a higher level of porosity, exhibited a higher friction coefficient and lower wear resistance than the HVOF processed coatings. However, as the temperature during wear testing increased, the wear resistance of APS was improved and tended to be similar to that of HVOF coatings. Higher wear resistance of HVOF was attributed to the densification of the coating sub-surface with lower porosity and agglomeration of shall wear debris into large cylindrical particles and foramtion of intermediate layer.

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