발간논문

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Thin-film rechargeable lithium batteries with amorphous vanadium tungsten oxide cathodes were fabricated. The vanadium tungsten oxide thin films were formed by RF magnetron sputtering and the effects of tungsten addition on the crystallinity and on the electrochemical behavior were investigated. According to X-ray analysis, it turned out that vanadium oxide films became amorphized by tungsten addition. When the tungsten was added to the V₂O_5 films, cycling reversibility was considerably improved. Electrochemical test showed that the cell capacity was about 70μ Ah/㎠· ㎛ when the amount of tungsten added in the film reached 30 atomic percent. No appreciable degradation of the cell capacity could be observed after one hundred cycles. All solid state thin-film batteries were fabricated with cell structure of Li/LiPON/V-W-O by sequential deposition.

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Ultrathin Fe_(78)Al₄B_(12)Nb_5Cu_t nanocrystalline ribbon alloys with 8㎛ in thickness were annealed in longitudinal or transverse magnetic field, and then the high-frequency soft magnetic properties were investigated. High remanence ratio(B₁/B_10) of 0.83 was obtained when annealed in longitudinal magnetic field at 450℃. In case of transverse magnetic field annealing, very high effective permeability and excellent magnetic core loss characteristics in ㎒ range were obtained by transeverse magnetic field annealing. Moreover, for the ultrathin ribbon alloy with 5㎛ in thickness annealed in transverse field, much improved soft magnetic properties in the high frequency range were revealed. The effective permeability at 1㎒ and core loss at 1kG and 1㎒ were 9,000, 1.3W/cc, respectively. Thus, the reduction in ribbon thickness was very effective in improving the high-frequency magnetic properties upon transverse magnetic field annealing

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The low cycle fatigue properties and microstructures of Nimonic80A/SNCRW and Nimonic80P,/X45CrSi93 friction weldments were characterized to know whether friction welding techniques could be applied to reduce the cost of MTU diesel engine valves. Fatigue crack which initiated from the interface between SNCRW steel and the HAZ advanced towards SNCRW. It is dued to the weakening of SNCRW steel occurred by grain growth during postweld heat treatment (700℃, 16hrs) which is purposed to re-precipitate the resolved γ phase in the HAZ during welding to recover strength. The fatigue cracking of Nimonic80A/X45CrSi93 friction weldments initiated from the X45CrSi93 steel side. Superior fatigue resistance were obtained in Nimonic80A/X45CrSi93 weldment than Nimonic80A/SNCRW weldment. The reason was due to, in the HAZ of Nimonic80A/X45CrSi93, the fine spherical carbides (M_7C₃) precipitated at grain boundary and within grain. However, in the HAZ of Nimonic80A/SNCRW, the carbides (M_(23)C_6) precipitated along coarse grain boundaries during friction welding and post weld heat treatment could cause to decreased fatigue strength.

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The variation of carbide morphologies in ASP30 and T15 high speed steels produced at different melt temperatures by the spray forming process has been investigated. The solidification behavior of a high speed steel during spray forming was greatly influenced by melt superheat that governed the thermal condition of deposited droplets. The M₂C or M_6C carbides were observed to form a carbide cell, whereas fine spherical MC carbides were found to be uniformly distributed. In case of the higher melt superheat, the cell carbides were plate-shaped and discontinuous with one another, which were identified as the M₂C type. In contrast, at the lower melt superheat, the cell carbides were the M_6C-type, whose shape were somewhat coarser and more continuous. It was observed that the size and distribution of MC carbide were insensitive to the melt superheat. It can be concluded that a higher melt superheat favors the formation of M₂C carbides rather than M_6C carbides in spray formed high speed steels.

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