발간논문

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Hematite particles were obtained by hydrothermal reaction from ferric hydroxide in the presence of citric acid which is acted as crystal growth controller. Citric acid was adsorbed with citrate anion on the ferric hydroxide. Adsorbed citrate anion influenced crystallization temperature and morphology of α-Fe₂O₃in the solid reaction. Adsorbed citrate anion inhibited crystallization of α-Fe₂O₃from ferric hydroxide at pH 9.0-10.5 and induced to needle-like α-Fe₂O₃particles at pH 10.75-11.75 in the hydrothermal reaction.

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A Thermodynamic model which considered chemical and phase equilibria as well as mass and charge balance equations and activity coefficients of all solutes simultaneously was developed for the separation of cobalt and nickel from buffered sulfate solutions with D₂EHPA dissolved in kerosene. By using the phase equilibrium constants of extracted species and interaction parameters obtained from the respective extraction experiments of individual metal ions, we could predict the distribution coefficients of cobalt and nickel and separation factors from initial extraction conditions of mixed solution. The separation factors increased with the concentration of D₂EHPA in the range of more than 0.05M due to the increase of the activity coefficients of extracted nickel species. In applying the model to the effect of extraction temperatures, the predicted values of distribution coefficients and separation factors were in good agreement with those obtained experimentally at 25℃ and 15℃.

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Microhardness change and phase transformation behaviors were investigated in the cold rolled C300 and T300 maraging steels after electron beam welding and heat treatments. Although the hardness variations around weldments were similar in both steels, the hardness well of aged specimen of T300 maraging steel was wider and deeper than that of the C300 steel. The as-welded and solution treated and aged C300 maraging steel had fully martensitic structure, but welded and aged one had austenitic phase. On the other hand, the T300 maraging steel always had austenitic phase in martensitic matrix structure. The phase instability on welded zone in T300 maraging steel was severer than that in C300 maraging steel mainly due to no Co element and severer segregation of Ni, Mo, Ti elements, which resulted in wider and deeper hardness well.

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Corrosion resistance of various surface treated steel sheets through A.C. impedance technique was examined and also compared with that obtained from salt spray test. A.C. impedance spectra was measured from 10 m ㎐ to 10000 ㎐ with applied amplitude of ± 10 mV at the open circuit potentual in 0.2% NaCl solution. Several equivalent circuits for surface treated steel sheets could be suggested from A.C. impedance spectra and the value of Rp(polarisation resistance) in equivalent circuits was used to evaluate corrosion resistance in this study. Thus, it could be determined that A.C. impedance technique is the simple and exact evaluation method for surface treated steel sheet in comparison with results of salt spray test.

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The turbulent combustion phenomena in the gas phase occurring at the post combustion stage in a bath-type smelting reduction process were simulated by use of a mathematical model, which combines the SCRS combustion model with instantaneous reaction up to the chemical equlibrium state allowing the concentration fluctuation, the k-ε turbulent model and flux method. The finite difference method based on the control volume approach was used to solve the transport equations in a converter of 100 ton capacity. The effects of injecting method on the oxidation were mainly studied. the combustion flame is spread more widely with the decrease of inlet velocity of injecting oxygen. The efficiency of the post combustion operation was evaluated by the post combustion ratio at the outlet of furnace. The post combustion ratio increases with the decrease of inlet velocity of injecting oxygen and with the increase of lance height from the iron bath surface.

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The temperature profile of Czochralski single crystal growth was simulated by considering the fluid flow and internal radiation. Internal radiation was considered by using diffusion approximation, which consequently enhanced the thermal conductivity. The crucible was included in the calculation of temperature and latent heat was acounted by iterative heat evolution method. Effects of absorption coefficient of the solid and liquid state of the semi-transparent materials(LiNbO₃, in this study) together with the variation of the crystal radius and the rotation of the crucible and the crystal on the temperature profile were simulated.

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