발간논문

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Mechanism of dynamic strain aging (DSA) and its effect on the low-cycle fatigue (LCF) behavior in type 316L stainless steel were investigated by performing LCF tests in air in a wide temperature range from 20 to 650℃ with strain rates of 3.2x10^(-5)-1x10^(-2)/s. The regime of DSA was evaluated using the anomalies associated with DSA and was in the temperature range of 250-550℃ at a strain rate of 1x10/s, in 250-600℃ at 1x10^(-4)/s, and in 250-650℃ at 1x10^(-2)/s. The activation energies for each type of serration were about 0.57-0.74 times those for lattice diffusion indicating that a mechanism other than lattice diffusion is involved. It seems to be reasonable to infer that DSA is caused by the pipe diffusion of solute atoms through the dislocation core. Dynamic strain aging reduced the crack initiation and propagation life by way of multiple crack initiation, which comes from the DSA-induced inhomogeneity of deformation, and rapid crack propagation due to the DSA-induced hardening, respectively. (Received October 29, 2004)

keyword : Dynamic strain aging, DSA, Planar slip, Pipe diffusion, Fatigue resistance, Low-cycle fatigue, LCF, 316L stainless steel

Generally, as objects impact each other, coating layer is regarded as little affecting transmission and reflection of shock wave. However, we thought that material properties and thickness of coating layer would actually affect objects. So this paper was performed by finite element analysis (FEA) for transmission and reflection of stress wave propagation at two bars having different material properties. Also, as coating layer having different material properties was inserted between bars, we looked into the behavior of stress wave propagation and compared the result of FEA and theoretical result. As using them, FEA for actual piezo electric ceramic was performed. The damaged cause of piezo electric ceramic was confirmed by the effect of reflected wave. To decrease the effect of reflected wave, we analyzed it as changing thickness and material of coating layer and the shape of piezo electric ceramic. Afterwards, we inquired thickness and material of coating layer and the shape of piezo electric ceramic being able to minimize the effect of reflected wave. (Receivad September 30 2004)

keyword : Stress wave propagation, Piezo electric ceramic

In this study, Taguchi method, one of the design tools of experiments, was applied to optimize casting process parameters of the high silicon corrosion-resistant iron. Three casting process parameters such as melting temperature, rare earth metal (mischmetal) addition, and pouring temperature were chosen. Using SN (signal-to-noise) ratio calculated from ultimate tensile strength of each experimental casting run, the relationship between the casting parameters and mechanical properties was statistically evaluated. The casting condition of a melting temperature of 1,650℃, 0.2% rare earth metal addition, and a pouring temperature of 1,350℃ led to an excellent ultimate tensile strength above 11 kgf/mm², which is beyond the industrial criterion. The effects of the casting parameters on mechanical properties and corrosion resistance were further confirmed by combined analysis of fractography, hydrogen content determination, microscopic test, and acid resistance test. (Received August 23, 2004)

keyword : High silicon corrosion-resistant iron, Casting process parameter, Tensile strength, Taguchi method, Design of experiment, Rare earth metal

The presence of porosity in the coatings can lead to the electrochemical dissolution of the substrate. Thus, the characterization of porosity in a quantitative manner is of great interest to researchers. Recent technological advances in porosity measurement using electrochemical methods have resulted in overcoming the traditional porosity measurement limitations. It is possible to establish the method of quantitative and reliable porosity measurement using electrochemical impedance spectroscopy. The accurate porosity measurement should be calculated from the initial immersion time to critical immersion time. The induction time to obtain the accurate porosity value was based on the correlation between the porosity enlarged ratio and the charge transfer resistance. The porosity values are summed up to the critical time, and then the total porosity values were divided by the number of measurements taken. (Received September 8, 2004)

keyword : Porosity, Porosity enlarged ratio, Electrochemical impedance spectroscopy, Induction time

This study was performed to investigate whether the bioactivity of the anodized and hydrothermally treated Ti-6A1-7Nb alloy were affected by the process conditions of anodic oxidation. Anodizing was performed at a current density of 5-50 mA/㎠ in up to 240~300 V in electrolyte solutions containing DL-α-glycerophosphate(DL-α-GP) and calcium acetate (CA). Hydrothermal treatment was done at 300℃ for 2 hours to produce hydroxyapatite (HA) for bioactivity. Bioactivity was evaluated from surface activation layer formed on the surfaces in a Hanks` solution with pH 7.4 at 36.5℃ for 30 days. The size of micropores and the thickness of oxide film were increased by increasing the spark forming voltage, and more complex oxide layers were formed. HA crystals were observed on the anodic oxide film after hydrothermal treatment at 300℃ for 2 hours. The anodic oxide films were composed of strong anatase peak and rutile peak, and showed the increase in intensity of anatase peak after hydrothermal treatment. When electrolyte composition, voltage and current density were increased, the deposited HA crystals showed a fine shapes. Bioactivity in Hanks` solution was accelerated when the oxide films were composed of strong anatase peak with rutile peak. Oxide layers selectively showed surface activity at the pertinent electrolyte composition. When voltage and current density were increased, the surface activity was increased. The amount of Ca and P was increased in surface having bioactivity in Hanks` solution. (Received December 1, 2004)

keyword : Bioactivity, Anodizing, Hydrothermal treatment, Hydroxyapatite, Ti-6Al-7Nb

Electrical properties and time dependent dielectric breakdown (TDDB) characteristics of Cu (0.7 at.% Mg) alloy films are investigated to improve the reliability of Cu interconnects used in microelectronic devices. After Cu(Mg) films are annealed in vacuum at 400℃ for 30 min, resistivity decreases from 3.4 to 2.0 μΩcm and, thin and continuous MgO layers form both on Cu(Mg) film surface and on Cu(Mg)/SiO₂ interface. Metal-Oxide-Silicon (MOS) capacitors composed of Cu(Mg) film deposited on thermal oxide grown on Si wafer are subjected to bias temperature stressing at 180 through 220℃ under electric fields of 1.5 through 2.6 MV/cm. Median time to failure (MTTF) and activation energy of Cu(Mg)/SiO₂ are much larger than those of pure Cu/SiO₂. Cu drifts into dielectric are inhibited by the continuous Mg oxide formed at Cu(Mg)/SiO₂ interface which improve interfacial adhesion and finally lead to better interconnect reliabilities. (Received October 29, 2004)

keyword : Cu(Mg), Reliability, Time dependence dielectric breakdown, Time to failure, Cu drift

The effect of surface treatment of n-type SiGe using the inductively coupled plasma (ICP) was studied by current-voltage and x-ray photoemission spectroscopy measurements. The ICP treatment produced surface oxides and point defects at the surface of SiGe. The x-ray photoemission spectroscopy measurements showed that atomic ratio of Ge/Si was increased after the etching treatment. These results provide the evidence that Si vacancies were produced at the etched surface. Si vacancies acting as donor for electrons resulted in shift of Fermi level to near the conduction band. As a result, Fermi level could be pinned at such Si vacancies, leading to the remarkable reduction of Schottky barrier height and the reduced dependence of Schottky barrier height on metal work function. (Received September 7, 2004)

keyword : SiGe, Inductively coupled plasma, Vacancy, Schottky barrier height, Fermi Level

The silicide layer used as a diffusion barrier in microelectronics is typically required to be below 500.E thick, and the silicides also need to have low contact resistance at high processing temperatures. We fabricated 150ÅNi/150A-Co/p-Si(100) samples with a thermal evaporator, and annealed the samples for 40 seconds at temperatures ranging from 700℃ to 1100℃ using rapid thermal annealing. Sheet resistance of the annealed sample stack was measured with a four point probe. In addition, we investigated microstructural and compositional changes during annealing using transmission electron microscopy and auger electron spectroscopy. The sheet resistance measurements for our proposed Ni/Co composite silicide was below 5 Ω/Sq.. Moreover our newly proposed silicides were stable with the additional elevated annealing at 900℃ for 30 min. Microstructures and Auger depth profiling showed that the silicides in our samples were consisted of Ni-rich and Co-rich ternary compound. Our result implies that Ni/Co composite silicide may have excellent high temperature stability which can be employed in sub-0.1 μm CMOS process. (Received September 24, 2004)

keyword : Silicidation, Ni/Co silicide, RTA, Ternary compound, Composite silicide

Introducing the general MEMS (Micro Electro Mechanical System) technology to BioMEMS (Biological MEMS) field, PCR (Polymerase Chain Reaction) micro-device, which is for DNA multiplication and analysis, is designed and fabricated in this study. PCR micro-device is characteristic of the accurate reaction and temperature control, and the thermal uniformity. Silicon wafer suitable for these requirements was utilized for 20x20 mm²PCR micro-device fabrication. Gold film of 50 μm line width was developed to optimize the power of 200 Ω resistance heater. Reaction chamber of 8x8 mm²size was fabricated by chemical wet etch, together with a cooling line of 0.35 mm width which circulates the reaction chamber periphery twice. PDMS (Polydimethylsioxane) film was utilized to cover the reaction chamber and the cooling line. Computer simulation was carried out to predict the behavior of temperature changes in the fabricated PCR micro-device. Heating 60 seconds resulted in 84-88℃ temperature profile in the reaction chamber, and then 30 seconds cooling was enough to reach to 55℃. It was recognized that this condition was acceptable to PCR multiplication operation. (Received March 29, 2004)

keyword : BioMEMS, PCR device, Reaction chamber, Computer simulation

A thermodynamic equilibrium between aluminum and oxygen along with the inclusion morphology in Fe-16%Cr stainless steel was investigated to understand the fundamentals of aluminum deoxidation technology for ferritic stainless steels. Further, the effects of calcium addition on the changes in chemistry and morphology of inclusions were discussed. The measured results for aluminum-oxygen equilibria exhibit relatively good agreement with the calculated values, indicating that the introduction of the first- and second-order interaction parameters, recently reported, is reasonable to numerically express aluminum deoxidation equilibrium in a ferritic stainless steel. In the composition of dissolved aluminum content greater than about 60 ppm, pure alumina particles were observed, while the alumino-manganese silicates containing Cr₂O₃ were appeared at less than 20 mass ppm of dissolved aluminum. The formation of calcium aluminate inclusions after Ca treatment could be discussed based on the thermodynamic equilibrium with calcium, aluminum, and oxygen in the steel melts. In the composition of steel melt with relatively high content of calcium and low aluminum, the log(X_(cao)/X_(Al_(2)o_(3)) of inclusions linearly increases with increasing the log[a_(ca)/a？(Al)？？(o)] of with the slope close to unity. However, the slope of the line is significantly lower than the expected value in the composition of steel melt with relatively low calcium and high aluminum contents. (Received September 30, 2004)

keyword : Inclusion morphology, Fe-16%Cr stainless steel, Aluminum deoxidation, Calcium treatment, Interaction parameter, Alumina, Alumino-manganese silicates, Calcium aluminates