발간논문

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In this paper, the deformation characteristics of polycrystalline solid with arbitrarily shaped grains are analyzed by FEM simulation for a plane strain tension. The FE mesh of the polycrystalline solid is modelled by using 2D Voronoi tessellation and each grain is assigned to have a randomly oriented slip-direction. The comparison of the deformation characteristics of polycrystalline solid between regularly and arbitrarily shaped grains have been performed. Also the effect of differently oriented slip direction for the same model has been clarified.

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The effect of MgO and Fe_tO on the reoxidation and sulfide capacity of CaO-SiO₂ Al₂O₃ MgO_(sat)-Fe_t O( <10 wt% )slag system was investigated through the slag/metal equilibrium technique using MgO crucible at 1873 K for clean steel technology in ladle refining process. The MgO added into a slag to prevent the dissolution of refractory gives the high possibility to cause reoxidation of aluminum in liquid iron and deteriorates sulfide capacity of slag system. The CaO-SiO₂ Al₂O₃ MgO_(sat) slag system containing around 5 wt% Fe_tO is superior to that containing Fe_t O less than 2 wt% from the standpoint of both reoxidation of aluminum in liquid iron and sulfide capacity. The sulfide capacity as a function of slag composition could be obtained at 1873 K. logC_(S^(2-))=X_(CaO)-5.59X_(SiO₂)-5.30X_(Al₂O₃)+0.29X_(Mgo)+7.42X_(Fe₃O)-1.33.

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Thiele`s modulus for non-spherical particle was assumed by introducing non-spherical radius. Based on this assumed Thiele`s modulus, the intermediate (Ishida-Wen`s) model considering the particle shape effect was proposed. Using this model, gaseous reduction of iron oxide pellets of various shapes was analyzed. According to the analysis result, Thiele`s modulus for non-spherical particle was proportional to non-spherical radius as assumed while the rate parameters included in this model were the same as those of the spherical particle obtained under the same experimental condition. Therefore, it was concluded that the assumption of Thiele`s modulus for non-spherical particle was reasonable and the proposed model was practicable.

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High temperature deformation behavior of Al_(85)Ni_(10)Y_5 alloy extrudates were investigated by torsion tests at temperature ranges from 300 to 550℃ in steps of 50℃ under a strain rate of 0.5/sec. The extrudates were fabricated by the powder metallurgy processing using amorphous ribbons. which were obtained from the melt spinning technique. Crystallization behavior of the ribbons as a function of aging temperature was studied by the thin film X-ray diffraction and Differential Scanning Calorimetry (DSC). As a results, the Al phase crystallite firstly formed in the amorphous ribbons and its crystallization temperature was ∼210℃. During the processings of consolidation and extrusion, nano-grained structure (∼100㎚) was formed in the Al_(85)Ni_(10)Y_5 alloy extrudates. Also, it was found that from the flow curves and hot deformed microstructures of the Al_(85)Ni_(10)Y_5 alloy extrudates, the dynamic recovery (DRV) was responsible for the hot restoration mechanism at 400-550℃ under a strain rate 0.5/sec.

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The design of a laboratory cold crucible for melting titanium and its alloys has been optimized using both the numerical simulation and the physical model experiment with tin. The electromagnetic field and magnetic pressure distribution in the cold crucible were calculated using a three dimensional finite element method. Temperature of the molten tin and titanium was measured under various electric power input and with different melt weight. The shape of the melt free surface, total melt surface area, soft-contacting area, and the Joule heating rate were also calculated. With the help of the heat transfer equation, overall heat transfer coefficient at the interface between the melt and the cold crucible was determined. With changing electric power input, stable and/or unstable melting states have occurred sequentially. Only in the stable melting state, superheating of tin and titanium, about. 70-100 K above the their melting temperatures was achieved. Overall heat transfer coefficient for tin and titanium were estimated as 3500 and 800 V/㎡K, respectively.

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Cu-TiB₂ composite alloy has been fabricated by spray forming process. The spray-deposited and the extruded billet have been characterized by optical microscopy, scanning electron microscopy, X-ray diffractometry and tensile testing. Coarse TiB₂ dispersoids were observed from the Cu matrix. TiB₂ particles were formed by chemical reaction between Ti and B in the Cu melt at 1500℃. The volume fraction of TiB₂ particle varies continuously with the position of spray deposited billet due to the density difference between the TiB₂ particle and the Cu melt. The volume fraction of TiB₂ particles, in the extruded Cu-TiB₂ alloy, was also varied with increasing length of extruded material. The strengthening mechanism of the Cu-TiB₂ composite was studied based on continum mechanics, Orowan mechanism, and dislocation pileup model.

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The n-type thermoelectric material of 95% Bi₂Te₃+5% Bi₂Se₃ doped with CuBr were prepared by hot pressing method assited A. C current. Property variations of materials were investigated as a function of variables, such as hot pressing time and powder size. By this method, it is possible to minimize the defects arising from the vaporization of Te due to the very short processing time. At the conditions of hot pressing at 400℃ for 2 min. under 150Kgf/㎠, and the particle size of 70 to 125㎛ range of powder, the maximum Z value (figure of merit) was 2.2 x 10^(-3)K^(-1).

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Single crystals of CuBr doped n-type 85% Bi₂Te₃-15% Bi₂Se₃ with various doping concentration have been prepared and the temperature dependences of the electrical resistivity, carrier mobility and Hall coefficient have been characterized at temperatures ranging from 77K to 600K. Undoped 85% Bi₂Te₃-15% Bi₂Se₃ single crystal exhibited a transition from p-type to n-type at 430K. With doping 0.03-0.1 wt% CuBr to 85% Bi₂Te₃ -15% Bi₂Se₃, n-type conduction occurred at all temperatures. With addition of CuBr, the electron concentration of 85% Bi₂Te₃ -15% Bi₂Se₃ increased with the relationship as n_i=10.2×10^(20) CuBr(wt%). The scattering parameter and the ratio of the electron and hole mobility (e/ h) of 85% Bi₂Te₃-15% Bi₂Se₃ were determined as 0.1 and 1.45, respectively. The bandgap energy at OK was found as 0.245 eV.

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Bi-doped n-type PbTe thermoelectric materials were fabricated by mechanical alloying and hat pressing, and the sintering characteristics and thermoelectric properties of the hot-pressed PbTe were characterized. Even without Bi doping, the hot-pressed PbTe exhibited a negative Seebeck coefficient and thus n-type conduction. With increasing the hot pressing temperature from 650℃ to 800℃, the temperature at which a maximum figure-of-merit was obtained shifted to higher temperature. When hot pressed at 650℃, 0.3 wt% Bi-doped PbTe exhibited a maximum figure-of-merit of 1.33×10³/K at 200℃ With hot pressing at 750℃ and 800℃, 0.1 wt% Bi-doped PbTe exhibited maximum figure-of-merits of 1.27×10^(-3)/K at 250℃ and 1.3×10^(-3)/K at 400℃ respectively.

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0.3 wt% Bi-doped (Pb_(1-x)Ge_x)Te (0 ≤ x ≤ 0.15) alloy powders were fabricated by melting/grinding and mechanical alloying processes, and then hot pressed at 650℃. The (Pb_(1-x)Ge_x)Te alloys exhibited more negative Seebeck coefficient, higher electrical resistivity and lower thermal conductivity with increasing the GeTe content. The (Pb_(1-x)Ge_x)Te alloys fabricated by mechanical alloying exhibited much higher electrical resistivities, compared to the specimens processed by melting/grinding. Maximum figure-of-merits of 1.07×10^(-3)/K at 400℃ and 1.25×10^(-3)/K at 300℃ were obtained for PbTe and (Pb_(0.9)Ge_(0.1))Te processed by melting/grinding. A maximum figure-of-merit of 1.33×10^(-3)/K was obtained at 200℃ for PbTe fabricated by mechanical alloying. The figure-of-merit of (Pb_(1-x)Ge_x)Te fabricated by mechanical alloying decreased substantially with increasing the GeTe content due to the remarkable increase of the electrical resistivity.

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