발간논문

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In present study, the chunk breakage of precipitate and the negative difference effect which is a typical electrochemical behavior of Mg-alloy was investigated. In order to estimate the effect of precipitate, the calibration curves were obtained by XRD analyses(internal standard method). The chunk breakage of precipitate was observed under open circuit potential and maintained its original morphology. And, it revealed that the fraction of precipitate in corrosion products was approximately same as the fraction of precipitate in as-cast alloy. This is because of the corrosion behavior which the Mg-rich matrix is preferentially dissolved against the precipitate. Therefore, it was considered that the fraction of precipitate has a role of fixed factor on the corrosion rate of as-cast alloy. Also, the negative difference effect is arisen because the corrosion behavior of Mg-rich matrix is varied by the Al content, e. g. corrosion behavior of surface protective film. And, it reveals that the chunk breakage of precipitate operates only as a factor for the variation of the degree of difference effect(Δ).

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In microelectromechanical system(MEMS) such as sensors and actuators, thin films have been widely used as microstructural materials. As the mechanical structures used in microsystems are miniaturized, the use of macroinstruments for testing of mechanical properties limits the minimum scale of the test structure and poses difficulties in aligning and attaching it to the instruments. The electrostatically actuated test device was proposed to measure the micromechanical characteristics of micromaterials forming the microsystems. Then, the test device can be used to evaluate elastic modulus, yield strength and deflection of micromaterials. The designed test device consists of comb drives for loading and a suspending beam for testing. The device is fabricated by the surface micromachining processes and laterally driven by the electrostatic force. A methodology to evaluate elastic modulus and yield strength is proposed on the basis of the electrostatic comb test device and linear elastic mechanics. From the results, elastic modulus and yield strength are measured by analyzing the variations of load and displacement of the free standing beam in the test device. Finally, we can predict the driving characteristics and the mechanical reliability of Al layer for MEMS applications from the deflection curve.

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Giant magnetoresistance(GMR) has been investigated for nano-granular Co_xCu_(100-x)(X = 10, 20, 30) sputtered films. Solid solutions were obtained in a wide range of concentration and the microstructure associated with the spinodal decomposition was observed by after-aging. Guinier radii, which were evaluated by small angle neutron scattering, of Co magnetic particles in the Co_xCu_(100-x)(X = 10, 20, 30) films aged at 773K for 1h were 1.3-1.5 ㎚ and the surface-to-volume ratio of Co magnetic particles increased with increasing Co concentration. The MR ratio of Co-Cu films aged at 773K for 1h has the largest value, 7.6%, in 20 at.% Co concentration and decreases in 30 at.% Co concentration. However, the field dependence in low field increased with increasing Co concentration. These behaviours were investigated in terms of the Co particle size, the surface-to-volume ratio and the inter-distance between the Co particles.

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The equilibrium phosphorus distribution ratios, L_P between CaO-CaF₂ and CaO-BaO-CaF₂ slags and Fe-C_(sat·)-P melt were measured at 1400℃. The effect of BaO addition on L_P was determined at various CaO/CaF₂ ratios in slag. The addition of BaO up to 30 wt% in CaO-80%CaF₂ slag decreases L_P since BaO lowers the activity of CaO in liquid slag. BaO lowers the melting point of CaO-CaF₂ slags. As the CaO/CaF₂ratio increases in slag, where the slag is saturated with CaO on BaO addition, the decrease in L_P is not observed. At a BaO content higher than 30 wt%, however, L_P increases drastically due to the increase in BaO activity. The SEM-EDS analysis on a quenched CaO-40%BaO-CaF₂ slag containing 4% P indicates that the phosphate ions are dissolved in liquid phase and primarily associated with BaO in slag. Correlation between the phosphate capacities and the optical basicities of slags was also examined. The theoretical optical basicity of slags containing various components is not an universal standard of refining capability of slags. The thermodynamic activity of basic oxide in slag is one of main factors in determining the dephosphorization capability of slags.

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KR desulphurization facilities has been adopted to Steelmaking Plant I in Pohang Works, POSCO since 1976. However, it has been more important than ever to improve desulphurization efficiency with increasing demand of high-quality steel. Therefore, water model and plant scale experiments were conducted to improve desulphurization efficiency in KR equipment by inserting a certain baffle on ladle bottom or wall. By the application of a baffle on ladle bottom or wall, the perfect mixing time was decreased to 20-42% and higher disperse and penetrating degree of particles were observed by giving turbulence to helical flow of fluid compared with that of the conventional condition. The plant scale equipment experiments show that using a certain baffle on ladle bottom or wall increase desulphurization efficiency in KR.

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A linear annealing method was utilized to make a direct wafer bonding of 10 ㎝ Si wafers for a SOI application. In the linear annealing method, a halogen lamp moves with a constant velocity above silicon wafer pair bonded with hydrogen bonding. In order to optimize the processing parameters such as initial heat treatment time and moving velocity, bonding strengths were measured when the moving velocity varies in the range of 0.05-0.5 ㎜/s and these results were compared with numerical calculation using finite difference method. The bonding strength from the high speed anneal (0.5 ㎜/s) showed a similar strength as that from the slow speed (0.05 ㎜/s) process and this result agrees with the numerical calculation. Based on the present study, it is expected that the process speed can be increased as high as 200 sec/wafer without decreasing the bonding strength.

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The wetting balance method was used to estimate the fluxless wetting properties of UBM-coated Si-wafer to the binary lead-free solders(SnAg, SnSb, SnIn, SnBi, SnZn). The wetting property estimation of UBM-coated Si-wafer was possible with the new wettability indices from the wetting curves of one side coated specimen; F_(min), F_s, and t_s . For UBM of Si-chip, Au/Cu/Cr UBM is better than Au/Ni/Ti in the paint of wetting time. At general reflow process temperature, the wettability of high melting point solders(SnSb, SnAg) is better than that of low melting point ones(SnBi, SnIn). The nitrogen atmosphere made more significant improvement in fluxless wettability than in fluxed wettability. The contact angle of the one side-coated Si-plate to the solder could be calculated from the force balance equation by measuring the static state force and the tilt angle.

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Mg -1, 5, 10 mass%Ni hydrogen absorbing alloys were fabricated by Rotation-Cylinder Method(RCM) and their hydrogenation properties were evaluated by PCI(Pressure-Composition Isotherm) measurement. The microstructure of as-cast Mg-10mass%Ni alloy is consisted of an island-like hydride forming α-Mg phase and the eutectic phase. The composition of the eutectic phase is measured about 80 mass%Mg and 20 mass%Ni by EPMA. Nickel has a catalytic effect on hydrogen dissociated reaction and the eutectic phase is rich in nickel composition compared with a phase. So the eutectic phase may act as a hydrogen dissociation site and a channel of diffusion of protium. This microstructure may promote the hydrogenation behaviors. The results of the maximum hydrogen storage capacity and reversible capacity of Mg-10 mass%Ni alloy were measured about 6.3 mass% and 4 mass% at 623K, respectively.

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