발간논문

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In this study, low voltage Plasma Electrolytic Oxidation (PEO) was utilized to eliminate the drawbacks of high voltage PEO such as high cost, dimensional deformation, and porosity. Low voltage PEO produces a thin coating, which leads to low corrosion resistance. In order to solve this problem, 0.1~0.6 M pyrophosphates were added to a bath containing 1.4 M NaOH and 0.35 M Na2SiO3.PEO at 70Vwas conducted at 25℃ for 3 minutes. The chemical composition, morphology, and corrosion resistance of the anodized coating were analyzed. The anodized film was composed of MgO, Mg2SiO4, and Mg2O7P2. Themorphology of the film showed a inappropriately dense structure and low porosity in the anodized layers. It is found that low voltage Plasma Electroly tic Oxidation in cooperation with phosphating treatment can provide good corrosion protection for the AZ31B magnesium alloy.

keyword : magnesium, AZ31B, plasma electrolytic oxidation, pyrophosphate, low voltage

The oxidation behavior of amorphous boron carbide thin film, deposited using the unbalanced magnetron sputtering method, was investigated. Weight change was measured using thermo gravity analysis under the condition of dry flowing air (100 mL/min), with increasing temperature of up to 1200 ℃ as well as at constant temperature. For the isothermal oxidation experiment, dry Ar gas was used until the oxidation temperature was stabilized; then, the Ar gas was replaced with dry air. The weight gain was negligibly small under 600 ℃ and then increased rapidly with increasing temperature. The isothermal oxidation curves show decreasing weight gain rate with oxidation time. At 1000 ℃, the weight gain was shown to become negative after a certain period. This isothermal weight change behavior can be ascribed to diffusional oxidation and weight loss due to the evaporation of liquid boron oxide. The possibility of the transition of boron oxide to boric acid at room temperature under an ambient atmosphere was also discussed.

keyword : thin films, sputtering, oxidation, raman spectroscopy, thermal gravimetric analysis

Glass materials have been applied in various industrial fields. Research on improvement of mechanical properties has consistently been performed due to this material’s intrinsically vulnerable characteristics. Surface modification is considered as the promising method to supplement the weakness; especially, ceramics are thought of as appropriate materials for film materials because of their outstanding mechanical properties and their chemical stability. However, unfortunately, the optical transmittance of ceramic materials is poor, and can rather restrict this material’s utilization in glass. In this regard, the optical transmittance mechanism of vacuum kinetic sprayed Al_{2}O_{3} film was investigated. To control and analyze the film’s optical property, the gas flow rate, as a main process variable, was controlled; then, post heat-treatment was performed. Consequently, the film’s optical property was determined by three microstructural factors. That is, crystallite size, defects (including lattice distortion), and amorphous phase, generated during particle deposition, were found to strongly affect the transmittance.

keyword : glass, coating, optical property, UV-vis spectrometer, vacuum kinetic spray

Pure Cu compacts were fabricated by spark plasma sintering (SPS) process for sputtering target application. For the fabrication of the Cu compacts, conditions such as temperature, pulse ratio, pressure, and heating rate were optimized during the sintering process. The final sintering temperature required to fabricate the target materials with high density, and the heating rate up to the final temperature, were 800 °C and 80 ℃/min, respectively. The heating directly progressed to 800 °C without any holding time. Sputtering target materials with high relative density of 99.7% were fabricated in a uniaxial pressure range of 60 MPa at a sintering temperature of 800 °C, without any significant change in the grain size. Also, the shrinkage displacement of the Cu target materials was found to considerably increase with increasing pressure at sintering temperatures up to 800 °C.

keyword : metals, sintering, microstructure, scanning electron microscopy, SEM, spark plasma sintering

Effects of post-weld heat treatment (PWHT) on the microstructure evolution and the mechanical properties at -196 ℃ have been investigated with electron beam (EB) welds of cast Inconel 718. Coarse Laves particles in EB fusion zones after conventional PWHTs provided a preferential site for crack initiation, leading to a significant decrease in cryogenic Charpy impact toughness. A modified PWHT, which was highlighted as higher-temperature solution treatment followed by slow cooling to 980 ℃, was designed to ensure a sufficient micro-plastic region ahead of crack tip by a significant reduction of the Laves size and fraction. The EB weld after the modified PWHT exhibited no blocky Laves particles. Instead NbC with a size of 200~300 nm were observed. This modified PWHT led to 1.7 times higher CVN impact toughness than base metal. The cryogenic mechanical properties of EB welds are discussed in terms of dendrite arm spacing, the Laves size and fraction.

keyword : alloys, welding, microstructure, transmission electron microscopy, TEM, cryogenic mechanical properties

During laser welding of Al-10 wt%Si coated boron steel, which is used in the automotive industry, Al and Si, which are elements of the coated layer, are diluted in the fusion zone; then, the concentration of Al and Si is distributed randomly. The segregation can be roughly classified into two types. The first forms along the fusion boundary in a long comet shape in the depth direction of the fusion zone. The Al concentration of this comet shape is Fe3(Al, Si)and the material is composed of a single phase. This segregation phase is formed at the same time as the melting of the base metal and the diluting of the coated layer of Fe(Al, Si), without reaction of the molten metal. Then, a static dissolution reaction of the diluted coated layer occurs, scarcely stirring the molten metal; the concentration of Al and Si is reduced to 1/2. The second type of segregation is formed by martensite and bainte in the fusion zone of the segregated zone; the composition of Al is 1.28~0.48 wt%. Considering the results of the analysis of the Fe(Si,C, Mn,Cr)-xwt%Al quasi binary phase diagram, performed using Thermo-Calc, segregated zones are solidified in the form “L→ L+α→γ”. Also, and as a result, it was found that the phase transformation from γ under-cooling results in the solid phase, which undergoes bainite transformation and is transformed to martensite.(Received April 4,2015)

keyword : boron steel, laser welding, segregation, microstructure, phase transformation

The anisotropic etching properties of single crystal silicon with the Tetramethyl ammonium hydroxide (TMAH) solution were investigated. The variations in Si etching rate and surface morphology at different etching temperatures and TMAH concentrations were evaluated. The effects of different additives and solution agitation were also discussed. As the THAM concentration (10∼25 wt%) decreased, the etching rate increased from 10 μm/h to 70 μm/h in the range of 70∼90 ℃. However, the etched surface roughness was degraded as the hillock density and corner undercut ratio increased. To solve these problems, the agitation and wafer holder direction were changed and two additives (Pyrazine and Ammonium Persulfate (AP)) dissolved in the TMAH solution. Experimental results showed that additives played an important role to increase the etching rate significantly up to 20% and the change of agitation led to smooth etched surface. Hillock formation was also suppressed by adding these additives and changing agitation direction.

keyword : semiconductor, surface modification, microstructure, SEM

We prepared a TiO2 blocking layer containing 0~0.5 wt% nano diamonds (NDs) to increase the effective surface area of working electrodes in a dye sensitized solar cell (DSSC). The result was a DSSC with a 0.45 cm2 active area of a glass/FTO/blocking layer (TiO2 with NDs)/TiO2/dye/electrolyte/100 nm Pt/glass structure. The microstructure of the blocking layer was examined by optical microscope, FESEM, and AFM. UV-VIS-NIR was used to determine the optical absorbance of the working electrodes containing NDs. The photovoltaic properties for the ND added DSSC, such as short circuit current density, open circuit voltage, fill factor, energy conversion efficiency, and impedance, were checked using a solar simulator and potentiostat. Microstructure characterization showed that the NDs were evenly dispersed in the blocking layer. The absorbance in the visible light regime increased as the ND content increased. The photovoltaic properties indicated that energy conversion efficiency increased from 3.53% to 4.96% with 0 wt% and 0.4 wt% ND addition. This was due to the decreased interface resistance of the working electrode and the increased surface area and shunt resistance of the blocking layer resulting from the addition of the NDs. The DSSC with 0.5 wt% NDs was less efficient due to a reduction of the effective electron transport area caused by excess NDs. Our results suggest that we can improve the efficiency of a DSSC by proper addition of NDs into the blocking layer.

keyword : dye sensitized solar cells, spin coating, energy conversion efficiency, nano diamond, blocking layer

Radiation damage and its effects on Reactor Pressure Vessel (RPV) steel, an Iron (Fe) based alloy, have been important issues to assess the lifetime of nuclear power plants since nuclear reactor operation changed from an annual cycle to a high burn-up and extended cycle mode in the late 1990s. In this study, using an MC-50 accelerator, 99.99 % iron was irradiated with 3.5 MeV protons with doses in a range from 1x1010 to 1x1012 protons/cm2. First, the proton ion range was estimated at 45 μm using SRIM simulation; Optical Microscope (OM) observation results were in good agreement with the simulation. X-Ray Diffraction (XRD) analysis showed that the diffraction peak on the (110) plane, which is the most compact plane of the BCC structure, was intensified, probably due to the stress concentration stemming from the implanted proton atoms on the plane. Moreover, SEM results revealed that sputtered and recoiled iron atoms from the surface were deposited on the surface and that cracks developed and propagated at the sputtered spot.

keyword : metals, implantation, microstructure, x-ray diffraction, irradiation