발간논문

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Al-3wt%Mg/Al₂O_(3p) composite fabricated by pressureless infiltration method was investigated in terms of the infiltration behavior of molten metal, mechanical properties and interfacial reaction products. A direct addition of Mg powder to the bed of Al₂O₃ and Al powder mixture made the spontaneous infiltration of molten Al-3Mg alloy occur at a relatively lower temperature 700℃, compared with using the bed without Mg particulates. Tensile and yield strength and elongation tend to decrease with increasing infiltration temperature while the infiltration time had little effect on tensile properties. Both strength and ductility of the hot-rolled composite increased even in a relatively low reduction of 20% as compared to the as-fabricated composite. Also, the solution treatment greatly improved the ductility in both as-fabricated and hot -rolled conditions. MgAl₂O₄, was formed at interfaces as a reasult of interfacial reaction. In addition, AlN was formed in the matrix by in-situ reaction of molten Al alloy and nitrogen.

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Various surface properties of boride layer formed by plasma paste boronizing were examined. A boride layer of flat structure was formed on the STS 304 stainless steel substrate by plasma paste boronizing treatment at several different temperatures from 1023K to 1223K under gas mixture of Ar/H₂: 2/1(vol.%). The paste was composed of 30wt% boron and 70wt% borax. The properties of boride layer were evaluated by micro vickers hardness tester, SEM, EPMA, and corrosion tester. Metallographic cross section of the bonded specimen was consisted of the boride layer, Ni rich layer, Cr rich layer, and matrix structure. It was found to form boride layer in a short time as compared with the pack and molten salt boronizing. In XRD analysis of boride surface, various peaks such as FeB, Ni₃B and CrB were presented. Especially, the peak of CrB was more intensive than other peaks. Activation energy for the formation of boride layer was found to be 123kJ/㏖ which is lower than the pack and molten silt boronizing. The microhardness of these boride layers were in the range of HV 1800-2000. However, from the results of anodic polarization curve in 1M H₂SO₄ solution on the bonded specimens, it is found that the corrosion resistance is improved by boronizing treatment. And primary passive current density is increased with the boronizing temperature.

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The electrolytic boronizing on low carbon steel was conducted at 700-900℃ for 2 hours under the current density of 0.5A/㎠ using various contents of KCl-LiCl in the fused salt of Na₂B₄,O_γKCl-LiCl. The KCl·LiCl compound has the eutectic composition (54wt.%KCl and 46wt%LiCl). The effect of the additives such as Al or B₄C into the 4Na₂B₄O_7·KCl·LiCl(weight ratio) on the formation of boronized layer was investigated. The boronized layers were composed of two sublayers, i. e., FeB and Fe₂B, which have tooth structure. The electrolytic boronizing on medium carbon steel SM45C and stainless steel STS304 was also conducted at 800-950℃ for 1-6 hours under the current density of 0.5A/㎠ in the electrolyte of 4(Na₂B₄O_7·Al)·KCl LiCl. In this electrolyte, the average boronized layer thicknesses of the low carbon steel, SM45C and STS304 at 800℃ for 2 hours were 70, 47 and 15 ㎛, respectively. The thickness of the two sublayers was significantly increased with increasing boronizing temperature. To obtain a single Fe₂B layer without FeB sublayer, the boronized low carbon steels were homogenized at 900℃ for 2 hours and the boronized SM45C and STS304 were homogenized at 950℃ for 4 hours. It was found that the single layer with the microhardness of Hv=1300 was formed.

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In an effort to obtain soft magnetic materials with excellent high-frequency characteristics, effects of flash annealing on magnetic properties and microstructure in melt-spun Fe-B-Nb-Cu and Fe-Al-Nb-B-Cu alloy with ultrathin ribbon shape less than 10㎛ thickness were investigated. The Fe-B-Nb-Cu alloy subjected to optimum flash annealing treatment showed higher effective permeability and smaller magnetic core loss at 1 MHz than conventionally annealed alloy. The enhanced magnetic properties of the flash-annealed alloy were attributed to the reduced a-Fe phase grain size to 6㎚ and the lower remanence ratio of about 0.3 comparing to the conventionally annealed alloy with the values of 9㎚ and 0.7, respectively. These showed that the flash annealing technique was effective in improving the high-frequency soft magnetic properties of Fe-B-Nb-Cu nanocrystalline alloy. However, in flash-annealed Fe-Al-Nb-B-Cu alloy, improvement of the magnetic properties was not observec.

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Thin-film rechargeable lithium batteries with LiMn₂O₄ cathodes were fabricated. The lithium manganese oxide thin films were deposited by RF magnetron sputtering and spinel structure was formed by rapid thermal annealing (RTA) in an oxygen ambient. The as-deposited films turned out to be amorphous, and could be crystallized into a spinel structure by RTA above 650℃. LiMn₂O₄ spinel films prepared at 750℃ showed good rechargeability and fast lithium intercalation kinetics. All solid state thin-film batteries were fabricated with cell structure of Li/LiPON/LiMn₂O₄, by sequential deposition. Room temperature cycling of this cell showed a nealy constant cell potential of 4V and good rechargeability.

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