발간논문

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The fracture mechanism operative in Al-25Si-X composite was evaluated by FEM analysis. Two different types of Al-25Si-X composites were prepared for the investigation by varying matrix composition and heat treatment conditions; the hard/brittle sample revealed 0.8% elongation with 420 ㎫ U.T.S and the soft/ductile one revealed 16% elongation with only 165 ㎫ U.T.S. In the case of the hard/brittle sample, the crack was developed in Si particles under about 420 ㎫ by a stress concentration on it during elastic deformation. And then the sequential crack was developed in the Si particle in front of the cracked one, resulting in crack Propagation through matrix between both cracked Si particles. On the other hand, in the soft/ductile sample, the stress was also concentrated on the Si particles. However, cracks were not developed until the sample was deformed to about 7-9% elongation. Based on the calculations, a significant stress could be concentrated on the Si particles embedded in the matrix deformed up to 9% elongation, exceeding the fracture strength of Si particle even under low applied stress of 165 ㎫.

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Electrical insulation behavior of the plasma-sprayed ceramic coatings was investigated by measuring the electrical resistivity and the dielectric breakdown voltage. As a post-treatment of the coatings, sealing treatment was carried out. The penetration depth of the sealants to the ceramic coating was investigated by using fluorescent dye method and its effects on the electrical insulation properties. Electrical resistivity of the alumina based coatings was measured up to 10^(15)Ω·㎝, which was close to that (10^(16)Ω·㎝) of the sintered alumina, but decreased as the content of the TiO₂ is increased. It is considered that the mechanism of dielectric breakdown voltage of the plasma-sprayed ceramic castings is due to corona mechanism which can result in local erosion of the coating surface. Dielectric strength of the Al₂O₃-13%TiO₂ coating was approximately 9 ㎸/㎜ showing saturation as the thickness of the ceramic coating was increased. However, the addition of 40% TiO₂ resulted in near zero breakdown voltage. The electrical insulation properties of the coatings were improved by sealing treatment. However, the extent of improvement was different from the sealants used.

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Investigations were made to develop a new type of edge mask to give an adequate answer to the strip edge over-coating problem in electroplating process. Because of its inherent nature of the strip edge on which the electric field or current flux tends to be concentrated, many problems were caused, such as edge over-coating, powdering, burning and zinc tree. The conventional therapy for these problems adopted so far is, for example, like bringing either the current density or the coating weight lower. But with such a declining Control, the productivity was doomed to be lowered nor any appropriate answer was suggested for edge over-coating problem. Thus, a new type of edge mask and its optimal conditions are suggested in this paper. It was concluded that the edge mask should be positioned in between -1 to +1 ㎜ from the strip edge and kept from contacting with the strip surface. A new type edge mask satisfying fore-mentioned conditions and being compatible with the shape of electroplating cell was sketched as well.

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The removal of Cr and Zn impurities on Si substrates using the remote H₂ plasma was investigated. To determine the optimum process condition, the remote H₂ plasma cleaning was conducted for various rf-powers and plasma exposure time. The optimum process parameters for Cr and Zn removal were set at the rf-power of 20W and plasma exposure of 5 min. The concentrations of Cr and Zn impurities were reduced by more than a factor of 2 and the minority carrier lifetime increased after the remote plasma H₂ cleaning. Also RMS roughness decreased by more than 30%∼50% after the remote H₂ plasma cleaning. AFM analysis results also show that the remote H₂ plasma cleaning makes no damage to the Si surface. The removal mechanism of the Cr and Zn impurities in the remote H₂ plasma cleaning is proposed to be the lift-off mechanism during the removal of the underlying chemical oxides.

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Brass(60%Cu-40%Zn), brass-0.83%Cr, brass-0.81%Zr and brass-0.80%Sn alloys were prepared by induction melting followed by thermomechanical treatment. Oxidation tests were performed on the prepared alloys between 600 and 770℃ for up to 1320hr in air. It was found that the oxidation resistance increased in the order of brass, brass-0.81%Zr, brass-0.83%Cr and brass-0.80%Sn. The oxide layers formed on all the prepared specimens were primarily comported of ZnO. Throughout the existing ZnO layer, Cu-oxides in ease of brass, and (Cu-oxides + Sn-oxides) in case of brass-0.80%Sn were additionally present. ZrO₂ in case of brass-0.81%Zr, and CuCr₂O₄ in case of brass-0.83%Cr were found at the bottom part of the ZnO layer.

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Induction-melted Cu-(0, 10, 20, 30, 40)%Zn allays were oxidized between 600 and 770℃ in air. With an increase in Zn contents, the oxidation resistance increased and, correspondingly, thin scales were formed. However, the scale adherence decreased with an increase in Zn contents. The oxidation products formed on Cu-(0, 10)%Zn consisted of ZnO with a small amount of copper oxides, while those formed on Cu-(20, 30, 40)%Zn consisted of (Cu₂O + CuO) with a small amount of ZnO.

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This paper presents the results of a study that was undertaken to inhibit the corrosion, scale and microorganism problems for open recirculating cooling system. This study has been carried out on a new multicomponent inhibitor blend composed of HEDP(1-hydroxy-ethylidine-1,1-diphosphonic acid)/zinc/acrylate copolymer/isothiazolone. The effect of newly developed inhibitor was studied through weight-loss tests, potentiodynamic polarization tests, scale tests, microorganism tests and surface analyses. The obtained results showed that developed inhibitor revealed a very good corrosion, scale and microorganism inhibition simultaneously. Potentiodynamic polarization curves indicated that blended inhibitor was a cathodic inhibitor. The surface analyses result indicated that the inhibitive effects arose from formation of a coherent protective film which may contain calcium-HEDP, zinc hydroxide, zinc-HEDP and iron oxide complex.

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In this paper, we studied the plastic deformation behavior of specimens during equal channel multi-angular pressing(ECMAP) which gave a total strain of approximately 2 on a single pass through the die, using the two dimensional rigid-plastic finite element method. The ECMAP process was explained in terms of two stages based on the corner filling procedure and the calculated load curve. Numerical simulations an annealed pure copper show that the first corner gap which occurred during stage I doesn`t occur during stage II due to the back pressure effect developed by the second corner deformation. This back pressure effect results in the local severe shear at the bottom of the ECAP samples of the routes A and C. The strain distribution within the deformed samples using both the routes A and C of ECMAP becomes more homogeneous than that of the single pass ECAP.

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A numerical analysis was carried out to predict the defects caused by fluid flow and heat transfer in the casting process under centrifugal force. The VOF method was adopted to deal with the behavior of free surface during filling and the Temperature Recovery method was used to treat the release of the latent heat during solidification. Thermal stresses were calculated by the Finite Element method. For a cylindrical model, two different angular velocities were applied (900 rpm and 1,500 rpm), and the result showed that the intensity of circulating flow was weaker in the case of 1,500 rpm than that of 900 rpm. Solidification proceeded faster in the upper part of the casts due to the thinner thickness caused by the gravitational force. Tensile stresses were distributed along the outer surface of the casts, while compressive stresses were distributed in the inner part.

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Multilayer thin films are widely used in semiconductor devices and electronic packaging. In multilayer thin films, thermal stress, induced from mismatch in thermomechanical properties between constituents of each layer, causes micro mechanical failures such as interface delamination, cracking, yielding and creep. Thus, the evaluation of thermal stress evolution and its effects on the deformation behavior of multilayer system is very important. We presented an analytical model on thermal deformation of multilayer thin films on the basis of elastic/plastic beam bending theory concerning bending stress of the film layer. By using the model, we studied a common interconnection system in semiconductors. We deposited Al(1%Si) thin film and SiO₂ passivation layer of different thicknesses on Si wafers. In-situ laser scanning method was performed to measure the thermal stress induced curvature of the system. By applying the model to the measured curve of curvature vs. temperature, we could ascertain the validity of the model.

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