발간논문

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A Zn/KOH fluidized bed electrode was mathematically modelled to investigate the performance and characteristics of the electrode in the case of recharging period for the possible use of energy storage. The model is based on electrodeposition reactions between particulate bed and electrolytes flooded the bed. The potential distribution through the bed was studied and it was found that the shape of the distribution was affected both by the exchange current density of the secondary reaction and the effective resistivity ratio of particulate bed and electrolytes.

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The stress rupture properties of 0.2 wt% Ni activated sintered W were studied using direct load tester at 1000℃, 1100℃ and 1200℃ in H₂. The stress rupture strength of 4 hr sintered specimen was 60% higher than that of 1 hr sintered specimen because of the larger grain size and the higher relative density. As the temperature increased from 1000℃ to 1100℃, rupture time was decreased but the slope of stress vs rupture time was decreased due to the grain growth at the higher temperature. The activation energy for the stress rupture was determined as 75 Kcal/mole. That was about the same energy for the diffusion of W in Ni. Therefore it was postulated that the deformation was controlled by the diffusion of W in the Ni rich area between the grains. All of the specimens showed intergranular fracture owing to the grain boundary sliding.

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Characteristics of ultra fine Al₂O₃ powders prepared by Sol-Gel process was investigated with hydrolysis condition and calcination temperature. 1) The transition process of Al₂O₃ took place through the series, Boemite Gel(AlooH)-δ-Alumina-θ-Alumina-α-Alumina. 2) The grain size of the calcined products was increased and the surface area was decreased linearly with increasing calcination temperature. 3) The size of first forming product and the agglomerated was less than 10 ㎚ and about 0.1-0.3 ㎛, respectively. 4) The recovery of Al₂O₃ was increased with decreasing PH of solution and increasing both the ratio of Alkoxide : H₂O and agitation temperature.

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The effect of replacing grain refining element Mn by Zr on fracture toughness of 2048 aluminum alloy was investigated. Fracture toughness changed by Zr replacement was analyzed as follows. 1. Quantity of large inclusions gave a primary effect on fracture toughness when grain size and material strength level were kept constant. 2. Dispersoids of Mn compounds were rod shape and 1.0㎛ in length, while those of Zr compounds, Al₃Zr, were spherical and 0.1㎛ in diameter. Dispersoids of Zr compounds, Al₃Zr, were better than those of Mn compounds for improving fracture toughness. 3. Experimentally proved the effects of dispersoids change on fracture toughness of aluminum alloy 2048 when other metallurgical factors-grain size, inclusion, strength level-were kept constant. Zr bearing modified 2048 aluminum alloy showed 5 % improvement in yield strength and 50 % in toughness compared with those of Mn bearing conventional 2048 aluminum alloy.

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A new fabrication technique, $quot;Continuous Compo-Casting Process(CCC Process)$quot; has been developed to produce the FRM(Fiber Reinforced Metals) composites. In the present study, the stainless steel fiber and aluminum matrix were selected because of their mutual passible wettability and good handling conditions. The aims of this studiy are to establish a new process and to optimize the main fabrication parameters. Microstructural observations and mechanical tests were conducted on the composites produced by the CCC process. Continuous composite materials can be produced with a simple experimental apparatus and at low cost. The geometry of the composites can be modified easily by changing the mold geometry. It may be concluded that the CCC process will have a wide application in producing metal matrix composites for structural purposes.

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A laboratory investigation was carried out to study the effect of hot rolling conditions on the mechanical properties of cold rolled and continuously annealed ultra-low carbon steels stabilized with Titanium. The drawability(r￣-values) of cold rolled and annealed steels was found to be very sensitive to the microstructure of hot bands. Higher r￣-values were observed when the grain structure of the hot bands were more uniform and finer. The as-hot rolled grain structure was fine and uniform when FT was higher than Ar₃. If FT was lower than Ar₃, the grain structure was pancaked or abnormally coarse when the amount of deformation was small. However, when the amount of deformation was sufficiently large and CT was relatively high, the grain structure was uniform and fine by the recrystallization of ferrite. This led to the development of a new hot rolling process where the reheating temperature was adjusted to a lower level so that strips were finish rolled in the ferrite region.

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The machinability of Al alloys can be improved by addition of selected elements such as Pb, Bi, Sn, Cd and In. However, these elements have not only different melting points and densities but also form miscibility gap with Aluminium. This kind of phenomenon induces severe gravity segregation. The aim of this study is to observe the effect of several additives such as Pb, Bi on the machinability of Al-Cu alloys. Pb, Bi particles were finer with increasing of cooling rate and Ar bubbling pressure, and were coarsened with longer holding time after stirring. Due to the coarseness of particles with descending by gravitation, the more addition of Pb and Bi, the particle size becomes coarser. As the contents of additional elements increased, cutting properties were remarkably enhanced such as smaller cutting force and lower bite temperature without degradation of tensile properties. It is also found that the Al alloy is the optimum for free-cutting when 0.4% of Pb and Bi are added to Al-Cu alloys.

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The magnetic properties and their degradation kinetics of thermally coevaporated Gd-Fe and Tb-Fe amorphous alloy films during aging under atmospheric pressure and vacuum were studied using the torque magnetometry and polar Kerr rotation hysteresisgraphy. The perpendicular magnetic anisotropy existed in the range of 20-30 at%Gd and 15-35 at%Tb. The degradation of Ku during aging was increased with RE content and this was largely due to the rate and amount of the structureal relaxation and the oxidation with RE content. The activation energies for degradation was in the range of 0.61-1 eV and 0.47-0.91 eV for Gd-Fe and Tb-Fe respectively and the n values were increased and the activation energy were decreased with RE content. Al coated films have lower n values and higher activation energy than those of SiO coated films. Al was found to be more effective for protective coating than SiO at high temperature. Thickness of more than 500Å for SiO and 200Å for Al was found to be needed to protect the RE-TM alloy films from the oxidation.

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In order to analyze the texture variation of Zircaloy-4 sheet the Roe`s method of calculating the crystallite orientation distribution function(CODF) for hcp system was computer programmed. The coefficients W_(lmn) of CODF were calculated from plane-normal distribution pole figures obtained by X-ray diffraction, and the CODF was computed from a series expansion of spherical harmonics. The Legendre function, which is the basis of the harmonics, was computed up to ℓ=16 to account for the symmetry systems of specimen and hcp crystal. A cross-rolling followed by beta-phase heat treatment and furnace cooling increased the density of basal poles along the sheet normal direction and rotated prism poles around the c axis.

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Solvent extraction of Ga(III) with D₂EHPA in sulfuric acid solutions was studied. The distribution coefficient was increased with an increase of pH and the slope of log D vs. pH was 3. The distribution coefficient was increased with decreasing the concentration of gallium ions in aqueous phase and increasing the concentration of D₂EHPA and temperature. The coefficient was higher with D₂EHPA dissolved in kerosene than in benzene. The McCabe-Thiele diagram showed that two extraction stages with the A/O ratio of 1 would be needed to extract gallium ions effectively.

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