발간논문

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Equal channel angular pressing is a convenient forming procedure to extrude material without substantial change in its geometry and to make an ultrafine grained material. The properties of materials are strongly dependent on the plastic deformation behaviour during the pressing, which is controlled mainly by 1) die geometry, such as a channel angle and corner angles, 2) material properties, and 3) process variables, such as lubrication and deformation speed. In this study, the plastic deformation behaviour of materials during the equal channel angular pressing has been theoretically analysed by finite element method. The effect of the die geometry and the sample length on the plastic deformation behaviour during the pressing has been discussed.

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The effects of the zone melting temperature and growth rate on the thermoelectric properties for 0.15 wt% CdI₂ and 0.05 wt% CdCl₂ doped 90% Bi₂Te₃-10% Bi₂Se₃ single crystals were investigated. The Seebeck coefficient and electrical resistivity of the 0.15 wt% CdI₂ doped single crystals were initially decreased along the ingots due to the condensation of iodine in the melts. The sharp increase of the Seebeck coefficient and electrical resistivity at the last-to-freeze region were considered as results of the evaporation of iodine from the molten zone and the compensation of electrons due to the formation of the antistructure defects. The figure-of-merit was enhanced with increasing the zone melting temperature and lowering the growth rate. The single crystels grown at the zone melting temperature of 800℃ and the growth rate of 0.1 ㎜/min exhibited the maximum figure-of-merit of 2.8×10^(-3)/K.

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Selective nucleation and lateral growing method have been developed for high quality ferroelectric PZT(65/35) thin films using perovskite-phase PZT island seed. The PZT films on PZT seed island transformed into perovskite-phase at temperature as low as 540℃, which temperature is lower than 150℃ compared to that of the PZT thin films deposited on Pt films. The temperature difference enables the lateral growth without undesirable random nucleation. Maximum grain sizes of the perovskite-phase PZT films are determined by annealing temperature. The PZT thin films show leakage current density of 8×10^(-8) A/㎠, breakdown field of 1240 kV/㎝, saturation polarization of 42μC/㎠, and remanent polarization of 30μC/㎠, which values are maintained up to 2×10^11 cycles.

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Electrical and thermoelectric properties of the 2 wt% excess Te-doped (25% Bi₂Te₃-75% Sb₂Te₃)_(1-X) (Sb₂Se₃)_X single crystals have been investigated at temperatures ranging from 77K to 600K. With increasing the Sb₂Se₃ content, the Hall mobility decreased and electrical resistivity increased, which was attributed to the lattice distortion. The thermal conductivity decreased with increasing Sb₂Se₃ content mainly due to the decrease of the electronic thermal conductivity. However, the (κ-κ_(el)) value was not changed with the addition of Sb₂Se₃, and the most plausible explanation for such phenomena appeared to be the existence of the photon thermal conductivity in addition to the electronic and phonon thermal conductivity. The 2 wt% excess Te-doped (25% Bi₂Te₃-75% Sb₂Te₃)_(0.913)(Sb₂Se₃)_(0.087) single crystal exhibited the maximum figure-of-merit of 2.45×10^(-3)/K at 310K.

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The functional gradient material of p-type (Bi_(0.2)Sb_(0.8))₂Te₃/ (Pb_(0.7)Sn_(0.3))Te with the segment ratio of 1.3 : 1 was fabricated by hot pressing of the mechanically alloyed (Bi_(0.2)Sb_(0.8))₂Te₃ and 0.5 at% Na₂Te-doped(Pb_(0.7)Sn_(0.3))Te powders. The thermoelectric properties of the hot-pressed (Bi_(0.2)Sb_(0.8))₂Te₃/(Pb_(0.7)Sn_(0.3))Te were measured and compared with those of the (Bi_(0.2)Sb_(0.8))₂Te₃ and 0.5 at% Na₂Te-doped (Pb_(0.7)Sn_(0.3))Te alloys. Reaction layer of ∼300 ㎛ thickness was formed at the (Bi_(0.2)Sb_(0.8))₂Te₃/(Pb_(0.7)Sn_(0.3))Te interface. At temperatures ranging from 200℃ to 350℃, the (Bi_(0.2)Sb_(0.8))₂Te₃/(Pb_(0.7)Sn_(0.3))Te functional gradient material exhibited a superior figure-of-merit to those of the (Bi_(0.2)Sb_(0.8))₂Te₃ and the 0.5 at% Na₂Te-doped (Pb_(0.7)Sn_(0.3))Te alloys, implying that the efficiency of thermoelectric energy conversion can be improved by the segment bonding of different thermoelectric materials.

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The thermoelectric properties of the p-type 22.5% Bi₂Te₃-77.5% Sb₂Te₃ single crystals, grown by the zone melting method, have been studied with changing the amount of excess Te up to 6 wt% and the growth rate from 0.1 to 0.5 ㎜/min. The Seebeck coefficient and electrical resistivity along the ingot growth direction were significantly affected by the amount of excess Te and growth rate due to the retrograde solubility of Te and the segregation of free Te in the matrix. The 5 wt% excess Te-doped 22.5% Bi₂Te₃-77.5% Sb₂Te₃ single crystal, grown at 0.1 ㎜/min, exhibited a figure-of-merit higher than 3.2×10^(-3)/K. However, the figure-of-merit at the last frozen part of such ingot decreased significantly due to the segregation of free Te. With charging 5 wt% excess Te-doped 22.5% Bi₂Te₃-77.5% Sb₂Te₃ at the first-to-freeze part and 0.2 wt% Te-deficient 22.5% Bi₂Te₃-77.5% Sb₂Te₃ at the last-to-freeze part, p-type single crystals with the figure-of-merit higher than 3.2×10^(-3)/K throughout the whole ingot were obtained.

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The weldability of dissimilar materials between sintered materials which are consisted of Co, Co+Ni and carbon steel has been investigated using CO₂ laser. Autogeneous CO₂ laser welding were run along the butt between two alloys using sets of parameters variation-power and travel speed. In order to study weldability mechanical tests (bending strength test, microhardness test) and metallurgical analyses (microstructure, phase transformation, fracture mode) were carried out. From the results obtained, it was found that the porosity which exists in a weld metal greatly affects the soundness of the weld. The optimum energy input to have a proper strength over than the requirement by a specification, found to be around 0.3-0.35 kJ/ｍ. There are two kinds of fracture mode in the weld metal, depending upon alloy combination, brittle fracture in the case of Co-carbon steel and a ductile fracture in the case of Co+Ni-carbon steel. In general, Co+Ni sintered material showed a better weld properties as compared to the Co sintered material.

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We have investigated the effects of Cu additions to pure Sn and eutectic Sn-Pb solders on the morphology and growth kinetics of the intermetatllics (Cu_6Sn_5) and on the consumption rate of Cu substrate during isothermal reflow soldering. The consumption rate of Cu substrate could be reduced with increasing Cu content in both Sn and Sn-Pb solders up to the respective solubilities at reflow temperatures. The effect of Cu addition was more significant for the Sn solder which had a higher Cu solubility. The Cu addition also resulted in morphological changes of the intermetallic layer. With increasing Cu content, the interface between Cu_6Sn_5 layer and solder roughened for pure Sn solder whereas it flattened fur the eutectic Sn-Pb solder. When the time(t) dependence of the intermatallic layer thickness(X￣) was modelled as X￣ = a×t^n, the growth constant a appeared to increase with the Cu content whereas the growth exponent n decreased. It was shown that the growth kinetics of the intermetallic layer could be well interpreted by a model of grain boundary diffusion controlled growth.

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The leaching behavior of copper from copper/iron couples in ammonia solutions was investigated using a rotating disc. The effect of oxygen potential, rotation speed, temperature and lixiviant concentrations on the dissolution kinetics of copper has been studied. The dissolution rate of copper was reduced at relatively low oxygen potentials due to galvanic effect. On the other hand, when hydrogen peroxide was added in the ammonia solutions, the galvanic effect of the dissolution rate was diminished. The dissolution rate of copper under the conditions studied was found to be controlled by mass transfer of reactants and products through the diffusion boundary layer.

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The acid/chloride direct leaching of copper matte was experimentally studied for the optimization of the Korea Zinc copper recovery process. In leaching, the effects of key operational parameters such as particle size distribution, concentrations of chloride, sulfuric acid, and ferric ions, and leaching time were studied. The rate of copper dissolution in acid and ferric ions was found to be dependent upon both the leaching temperature and particle size distribution of Cu-matte. The copper recovery rate increased with the decrease in particle size, whereas it increases parabolically with the increase in the leaching temperature from 60 to 90℃. The activation energy of the copper leaching reaction was found to be 12.1 ㎉/mole. In the electrowinning of copper from the filtrate, the surface morphology of electrowon copper was strongly dependent upon the initial concentration of sulfuric acid. The efficiency was also adversely affected by the increasing concentrations of sulfuric acid, chloride ions, and total iron(Fe^(+2) / Fe^(+3)), within our experimental range. The current efficiency decreased with the concentrations of both chloride and total iron. The optimum concentration of sulfuric acid seemed to be 120-140 ｇ/ℓ, while that of total iron was less than 4 ｇ/ℓ.

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