발간논문

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The kerosene-based magnetic fluids were prepared with the synthesized ultra-fine magnetite and the sodium oleate as a surfactant. The properties of magnetic fluids were investigated with relation to magnetite particle size, added amount of sodium oleate, solid content and washing process. The optimum conditions to synthesize ultra fine magnetite for magnetic fluids were reaction temperature = 80℃, (OH)^-/(Fe^(2+)Fe^(3+))=8/3, reaction time 5min and O₂ flow rate=0ℓ/min.. In cases of F^(2+)/Fe^(3+)-0.5 and Fe^(2+)/Fe^(3+)=1.0, the synthesized particles showed all spherical shapes having mean size of 63.3Å and 94.2Å, respectively. The limits of adsorbed amount of sodium oleate on the magnetite surface in the conditions Fe^(2+)/Fe^(3+)=0.5 and 1.0 were appoximately 30 persents and 20 percents of the magnetite weight, respectively. The kerosene-based magnetic fluids showed the maximum dispersion when sodium oleate was added about 1.6 times of the amount corresponding to monomolecular adsorption. To get this result, washing process with distilled water and methanol was surely required. The viscosity change of kerosene-based magnetic fluids according to added amount of sodium oleate showed almost the same tendency to the dispersion change. The effects of sodium oleate on the magnetization change of kerosene-based magnetic fluids showed almost the same tendency to the dispersion change. Its magnetization increased with increasing solid content and had the maximum value of 3. 9 emu/g at solid content=0.4g/㏄.

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The oxidation behaviors of Co-Ni hydroxide coprecipitates were investigated with relation to the conditions of the solution such as R value(=2〔OH^-〕 / 〔SO^(2-)₄〕), temperature and initial 〔Ni(Ⅱ)〕/〔Co(Ⅱ)〕 molar ratio. The characteristics of reaction products were examined by composition analysis, XRD, TEM and magnetic property measurement. When increasing R value, the reaction time and particle sizes were increased. And the reaction products were Co-Ni ferrite. The amount of Co incorporated into the spinel structure increased and that of Ni decreased with increasing R value and temperature. In reaction products represented by Co, Ni_yFe_(3-(x+y))O₄, the amount of (x+y) shows the ranges between 0.6 and 0.7, regardless of R value and temperature. In the conditions of R=1.5, 70℃, 〔Co(Ⅱ)+Ni(Ⅱ)〕/〔Fe(Ⅱ)〕 ratio 0.5, 〔Ni(Ⅱ)〕/〔Co(Ⅱ)〕 ratio =2.0 in the initial solution, Co and Ni incorporated with equal amounts into spinel structures and the reaction product of Co_(0.29)Ni_(0.29)Fe_(2.41)O₄ was obtained.

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