발간논문

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STD 61 steel was boronized in Boronizing paste mainly consisted of B₄C and Na₂B₄O_7 at various temperatures and times. Microhardness and thickness of boride layers were measured and distributions of B, Si, Cr, and V on the cross section of specimens were observed by EPMA line analysis. Microscopic examination and results of EPMA showed that the boride layer consisted of two layers outer layer of FeB and inner layer of Fe₂B. Microhardness of these boride layers was in the range of Hv 1800∼2300. Thickness of the boride layer increased with a square root of time for treatment at constant temperature. Activation energy for diffusion of B in the specimen obtained from the slope of Arrhenius plots was 260kJ/㏖. Si-rich α layer was formed between boride layer and matrix. Element such as Cr concentrated as Cr_(23)(B, C)_6 beneath the boride layer.

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In this study, Al-Fe alloy layers were made by Al powder spray on the steel sheets heated at 700℃, 800℃, 900℃, 1000℃ and 1100℃ for 30minutes, respectively. Microstructure observations and phase analyses of these alloy layers were carried out by XRD and EPMA, and mechanical and chemical properties of these alloy layers were examined. The experimental results were as follows. For alloy layers formed at 700, 800 and 900℃, main phases were (FeAl₃) and (Fe₂Al_5) phases, hardness was very high (Hv=700∼800) and corrosion resistance was good, but wear resistance was bad. For alloy layers formed at 1000℃ and 1100℃, main phase was Fe₃Al phase, hardness was low (Hv=300∼400), but corrosion and wear resistance were good. It was concluded that the alloy layers formed at 1000℃ for 30minutes had excellent characteristics compared to the layers formed at the other temperatures.

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A kinetic study on the electrodeposition of Ni-P alloys onto a steel has been performed using a rotating disk geometry. The effect of potential and temperature on the deposition rate of Ni and P was higher than the stirring speed of the solution. The calculated activation energies of Ni and P in the higher temperature ranges between 66 ℃ and 86℃ were about 8 ㎉/mole and 9 ㎉/mole respectively, however, in the lower temperature ranges between 46℃ and 60℃ were about 29 ㎉/mole and 30 ㎉/mole, respectively. Accordingly, the reaction rate of Ni-P codeposition seems to be controlled partly by electrochemical reaction and partly by mass transport, namely mixed controlled. As the applied potential increased, the ratio of P in the deposit was decreased, however, the stirring speed and temperature had little effect on the ratio of P in the deposit The ratio of Ni in the deposit was increased, however, that of P was decreased as the increase of difference between bulk concentration and surface concentration(C_h-C_s) of the electrolytic solution in the experimental range of potential, temperature and stirring speed. The as-deposited Ni-P was amorphous but the deposit was crystallized to Ni₃P after heat treatment for 4 hours at 400℃.

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The vacuum brazing of alumina(99.9% α-Al₂O₃) to Ni-Cr steel, Cu with Ag-Cu-Zr active braze alloy has been performed to investigate the interfacial reaction of Ag-Cu-Zr brazing alloy on Al₂O₃. The effect of additional element in the brazing alloy was also examined for the interfacial reaction and joint strength. EDS results showed that reaction product was composed of Zr, O and a little amount of Ag. XRD analysis indicated that the reaction product was one substance of monoclinic ZrO₂(a=5.1463, b=5.2135, c=5.3110Å) and the element of bonded metal did not participate in the formation of reaction product, in spite of the presence of silver in the reaction product. So the reaction product doesn`t contain any of dilute ceramic phase and intermetallics which are commonly seen in other active metal brazing system. Detected Ag should be from the trapped Ag grains or cluster in the reaction product during the interfacial reaction. The 3rd additional elements(Sn, Al) in the brazing alloy changed the thickness of the reaction product layer and the morphology of the brazement. But fracture strength was strongly influenced by the morphology- of the brazement which was closed to ductility of brazement. Maximum shear strength of 140.5MPa was obtained for the Al₂O₃/steel joint brazed with Sn added brazing alloy due to the improvement of brazement ductility. Al₂O₃/Cu joint had relatively low strength due to the decreased brazement gap and distinct brazed region was not observable because the excessive inter-diffusion and the reaction between Cu, Zr of brazing alloy and bonded copper.

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In-situ Ni and ex-situ NiO(Li) cathodes were fabricated by cold pressing using Ni powder and Li₂CO₃(in case of ex-situ cathode) with paraffin wax as a binder. The characteristics of in-situ and ex-situ cathodes according to the sintering temperature were examined. Also the behavior of the in-situ and ex-situ cathodes under the MCFC operation condition were investigated in this study. The optimum sintering temperature was 800℃ of neck growth step leading to the porosity of about 70% and strength of about 350MPa. To obtain reasonably high electric conductivity in ex-situ cathode, the sintering temperature was chosen to be below 950℃ to maintain the cation fraction of L^+ in the range of 0.02∼0.06. In terms of the Ni-dissolution rate, ex-situ cathode was more efficient than in-situ cathode in the long-term operation except the initial 24hr. Corrosion product was Li₂NiO_(10) at the in-situ and ex-situ cathodes.

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Experiments were performed by thermogravimetric(TG) technique for A356.2(Al-7Si-0.4Mg) aluminum alloy containing 0.4wt%Mg to examine the effect of magnesium on the oxidation kinetics in dry and moist air over the temperature range of 200∼550℃. The results of TG analysis showed that oxidation in moist air was much faster than in dry air at high temperatures above 525℃ while oxidation in both dry and moist air below 200℃ was stagnant. For the surface analysis of oxidation, the results of Auger electron spectroscopy(AES), X-ray diffraction patterns(XRD), analysis showed that only aluminum oxide was formed at relatively low temperature of 300℃ while the duplex film structure of Mg oxide/Al oxide/matrix was formed at high temperature of 550℃. MgO was a major constituent on the surface of oxide film, but the relative concentration of MgO decreased with increasing depth, while that of spinel, MgAl₂O₃ increased.

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