발간논문

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To lower the annealing temperature and the deviation of the mechanical properties of bake hardening steels, high purity steels were investigated. The steels were characterized by treating at low recrystallization temperature. It was confirmed that the strengthening originated from the solid solution of carbon and the ferrite grain refinement by fine MnS precipitates as carbon and sulfur contents increased in high purity steels. However, it was observed that there was no more increase of strength in steels containing over 40 ppm of carbon. It was considered that the excess carbon formed either the carbon cluster or the low temperature unstable carbides which had the negligible effect on the strengthening because they were reported to be highly coherent with the matrix. The carbon cluster and unstable carbides could be transformed to the stable cementite during bake hardening treatment. MnS was not observed in the high purity steel containing 5 ppm S, resulting in very coarse recrystallized grains and good ductility. As sulfur content increased, the recrystallized grain size decreased due to the formation of the fine MnS precipitates. (Received January 7, 2009)

keyword : high purity steel, manganese sulfides, drawability, recrystallization temperature

The purpose of this study is to make the tantalum powder for solid electrolyte capacitor with SHS (self-propagating high-temperature synthesis) process. Raw materials for manufacturing Ta powder were used Ta2O5, Mg and NaCl. While progressing SHS process, Ta2O5 powder was reduced by Mg powder. The combustion temperature and velocity were easily controled by the varying mole ratio of NaCl, Mg and initial reaction pressure. In the case of only using NaCl as an inorganic agent, the shape is unagglomerated and has high surface area. whereas we were given the powder which has good net structure by the addition of excessive Mg as a diluent. (Received January 13, 2009)

keyword : tantalum powder, combustion, SHS, capacitor, magnesium

Dense 4.25 Co0.53Fe0.47-Al2O3 composite was simultaneously synthesized and consolidated by pulsed current activated combustion method within 2 min from mechanically activated powders. Consolidation was accomplished under the combined effects of a pulsed current and mechanical pressure. Dense 4.25 Co0.53Fe0.47-Al2O3 with relative density of up to 96% was produced under simultaneous application of 80 MPa pressure and the pulsed current. Fracture toughness and hardness of the composite are 6 MPa·m1/2 and 570 kg/mm2 respectively. (Received March 3, 2009)

keyword : combustion synthesis, composite materials, nanopowder, Fe-Al2O3, Co-Al2O3

In the solvent extraction of Ru(IV) with Alamine336, it was found that Ru took part in the reaction as RuCl6(2-) in the HCl concentration range of 1 to 5 M. Interaction parameter between hydrogen ion and RuCl6(2-) was estimated by applying Bromley equation to the extraction data. From the mixed solutions of Pd(II) and Ru(IV), the distribution coefficients of Pd were found to be higher than those of Ru in the experimental ranges. Separation factor between Pd and Ru rapidly increased with the decrease of Alamine336 concentration. About 60% of the Ru from the mixed solutions was extracted by TBP at 8.3 M HCl, while Pd was not extracted in the HCl concentration range of 1.6 to 8.3 M. (Received April 8, 2009)

keyword : RuCl4, PdCl2, HCl, Alamine336, TBP

In order to vary the primary recrystallization textures, AA 1050 sheets were cold rolled in two different manners. Differences in cold rolling schedule gave rise to the formation of different cold rolling textures also leading to the formation of different primary recrystallization textures. Upon annealing for grain growth, changes in microstructure and texture hardly occurred in the sample depicting Cube recrystallization texture, while grain growth was accompanied with the development {001}<100> Cube texture in the sample displaying a recrystallization texture comprising of weak rolling texture components. The selective growth of Cube oriented grains is attributed to the high mobility of their grain boundaries. (Received January 6, 2009)

keyword : symmetrical rolling, asymmetrical rolling, cold rolling textures, recrystallization texture, grain growth

The effect of lubrication on the evolution of strain states during shape rolling in AA5052 sheet was studied by the finite element method (FEM) simulation. The strain states calculated by FEM were verified by texture analysis. Shape rolling with and without lubrication produces shape-rolled samples in fairly similar outer shapes, since the distribution of normal strain components is nearly independent of the lubrication condition. In contrast, the distribution of shear strain components strongly depends on the lubrication condition. Shape rolling without lubrication gives rise to the development of strong shear strain gradients leading to the formation of highly inhomogenous textures and microstructures. The {011}//ND fiber develops during rolling with the operation of plane strain plus ε22. (Received January 14, 2009)

keyword : shape rolling, strain state, texture, microstructure, finite element method

We report on hysteresis behavior in the electrical resistance-hydrogen concentration of Pd thin films. The variation of the electrical resistance has been investigated during the process of absorption and desorption of hydrogen gas (H2) as a function of thickness of Pd thin films. The hysteresis behavior in the electrical resistance with H2 concentration was found for Pd thin films and consists of α phase, α+β phase, and β phase regions. The sensitivity of Pd thin films with H2 concentration was found to follow Sieverts` law in the α phase region. However, the sensitivity was observed to increase abruptly with H2 concentration in the α+β phase co-exist region. This is because Pd-H interaction is stronger in the β phase than in the α phase and needs a higher concentration gradient as a driving force to desorb. The formation of the β phase also was observed to cause the structural change because of the lattice expansion during absorption. The hysteresis height and the trace of structural change were affected by the thickness of the Pd film. As the film becomes thinner, the hysteresis height becomes lower and the amount of delamination on the surface becomes smaller. For films thinner than 20 nm in thickness, the delamination was not found but electrical resistance hysteresis was still observed. (Received December 22, 2008)

keyword : Palladium, Pd, hydrogen (H2), sensor, resistance-pressure hysteresis, absorption, desorption, structural change

We investigated the effects of deposition conditions on the fabrication of Fe3O4 thin films using a reactive DC magnetron sputtering at room temperature. The structural, electrical, and magnetic properties of Fe oxide films dependence on the film thickness, oxygen flow rate, and the substrate crystallinity were also studied. We have successfully fabricated Fe3O4 film with thickness of about 10 nm under optimal reactive sputtering conditions. The saturation magnetization, resistivity, and Verwey transition of the Fe3O4 film were 298 emu/cc, 4.0×10(-2) Ωcm, and 125 K, respectively. (Received February 3, 2009)

keyword : Fe3O4 film, half metallic reactive sputtering

The effects that processing atmospheres and alloy composition have on the wetting characteristics of Al-Mg alloys on the SiO2 substrate were investigated by measuring the wetting angles. It was observed that the wetting behavior of the Al-Mg alloys on the SiO2 substrate vary depending on the Mg content of the alloys and atmospheres. The results showed that the contact angle decreases with increasing Mg content, angle is generally larger in the N2 atmosphere than in the Ar atmosphere. We discussed the validity of the results obtained from the wetting test on the basis of recent theories and the results observed from the actual penetration tests. (Received January 9, 2009)

keyword : wetting, contact angle, atmosphere, sessile drop, Al alloys