발간논문

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The electrical charging characteristics of metal oxides dispersed in liquid Sn were experimentally determined by the differential potential analysis method. Also, effects of dissolved oxygen on the surface charge of the oxides in liquid Sn were investigated in vacuum and in hydrogen atmosphere, respectively. Experimental results showed that metal oxides carried a positive charge by providing electrons from the oxides to liquid Sn. The potential differences measured were in the range of 0.008 to 0.08 ㎷ and was increased with increasing amount of oxide added to liquid Sn, and also with increasing temperature. Under the vacuum and the reducing atmosphere, potentials were considerably increased.

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A Pt/PZT/ZT/Si structure for transistor gate materials in metal/ferroelectric/insulator/semiconductor field-effect transistors(MFISFET) was fabricated and the electrical properties of the MFIS structure with various ZT thin film thickness were investigated. The ZT thin film was annealed at 800℃ for 1 hour under an oxygen atmosphere for crystallization. The perovskite PZT thin film was able to be crystallized on the ZT/Si substrate. The memory window widths of the C-V curves for the MFIS structure decreased with increasing ZT thin film thickness. The remanent polarization values of the MFIS structure were -0.1 μC/㎠ which were sufficient charge density required to induce the inversion layer at the Si surface.

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A kinectics of hot metal desulfurization by CaO based slags has been studied through the interpretation of experimental results by a reaction model based on the mass transfer theory. The rate determining step of desulfurization reaction rate is the mass transfer of sulfur in slag rather than that in metal. Si or Al in hot metal increases the driving force for desulfurization by decreasing the equilibrium oxygen potential at the stag/metal interface, however, they could not change the rate determining step. Kinetic factors affecting the mass transfer coefficient of sulfur in slag are proven to be the slag viscosity and the stirring intensity.

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Reduction rate of chromium oxide in slag phase was studied under an operating condition of EAF process. Synthetic CaO-SiO₂-Cr₂O₃ slag containing Al₂O₃, Fe₂O₃, or MgO were respectively prepared by sintering and pre-melting. STS 304 steel was melted in a graphite crucible under Ar-atmosphere by the high frequency induction furnace. The synthetic slag was placed on the steel melt and was reduced by carbon saturated melt. Reduction rates were measured by analyzing both steel and reacted slag sample. Microscopic analysis showed that chromium oxide in EAF slag existed largely in spinel form of FeO and MgO. Reduction yield was significantly increased by increasing temperature, and also decreasing basicity 1.5 to 1.0. The optimum temperature and basicity were found to be 1650℃ and 1.33, respectively. And the addition of Fe₂O₃ promoted the reduction rate considerably. Experimental results indicated that the reduction rate was controlled by the diffusion of chromium ion through the liquid slag phase at the initial stage of reduction and the activation energy for diffusion was found to be 129.79 kJ/㏖. It is worthy to note that reduction rate of chromium oxide increased by slag foaming at the later stage of reaction. It was also observed that SiC powder was far more effective than carbon powder as reductant at the initial stage of reaction.

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In order to investigate the formation of inhomogeneous cold rolling textures in extra low carbon steel sheets, rolling was performed by different deformation parameters. The center layer textures mainly consisted of the {hkl}$lt;110$gt; α-fiber and the {111}$lt;uvw$gt; γ-fiber. It was found that the evolution of the center layer textures was insensitive to rolling conditions. However, variations in rolling conditions gave rise to the pronounced through-thickness texture gradients. Dry cold rolling with a slow rolling speed led to the formation of the {011}$lt;100$gt; Goss-orientation at the surface layer. Applying a lower ratio of the sheet thickness to the contact length between roll and sample resulted in the strong {001}$lt;110$gt; in the α-fiber. The evolution of preferred orientations in the inhomogeneously cold rolled low carbon steel sheets was interpreted by simulations based on the full constraint and the relaxed constraint model. The through-thickness texture gradients in the inhomogeneously deformed samples can be explained in terms of strain states during cold rolling.

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Effects of air plasma spraying parameters on the formation of the icosahedral quasicrystalline coatings onto a mild steel plate were investigated using gas atomized Al_(62)Cu_(26)Fe_(12) powders. The as-atomized powders were appropriately sieved to have a size range of-200∼+400 mesh. Heat treatment at 750℃ for 3 hours was carried out to investigate the phase stability of the powders. Main process parameters such as arc current, spraying distance, powder feeding rate, preheating of the substrate were correlated to the coating layer characteristics. The volume fraction of the quasicrystalline phase in the coating layer was found to be a function of the chemistry and solidification history of the splats. Due to excessive evaporation of Al atoms during spraying, an initial powder composition more or less changed to form the β-AlFe(Cu) phase. As the arc current increased from 300 A to 500 A, both surface hardness and cross sectional hardness increased. The coating thickness was found to be a function of arc current, powder feeding rate, but less influenced by the spraying distance. The porosity level increased with an increase of arc current, spraying distance, and powder feeding rate. It is suggested that the atmospheric plasma coating process has a potential possibility of obtaining quasicrystalline coating layer, provided that the processing parameters are effectively optimized for obtaining high quantity of quasicrystalline phase.

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