발간논문

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The purpose of this paper is to review the state of laser processing technology using metal powders. In recent years, a series of research and development efforts have been undertaken worldwide to develop laser processing technologies to fabricate metal-based parts. Layered manufacturing by the laser melting process is gaining ground for use in manufacturing rapid prototypes (RP), tools (RT) and functional end products. Selective laser sintering / melting (SLS/SLM) is one of the most rapidly growing rapid prototyping techniques. This is mainly due to the processes`s suitability for almost any materials, including polymers, metals, ceramics and many types of composites. The interaction between the laser beam and the powder material used in the laser melting process is one of the dominant phenomena defining feasibility and quality. In the case of SLS, the powder is not fully melted during laser scanning, therefore the SLS-processed parts are not fully dense and have relatively low strength. To overcome this disadvantage, SLM and laser cladding (LC) processes have been used to enable full melting of the powder. Further studies on the laser processing technology will be continued due to the many potential applications that the technology offers.

keyword : Laser processing technology, selective Laser sintering, SLS, selective Laser melting, SLM, Laser cladding, LC, metal powder

The effects of various manganese precursors for the low-temperature selective catalytic reduction (SCR) of NOx were investigated in terms of structural, morphological, and physico-chemical analyses. MnOx/ TiO2 catalysts were prepared from three different precursors, manganese nitrate, manganese acetate(II), and manganese acetate(III), by the sol-gel method. The manganese acetate(III)-MnOx/TiO2 catalyst tended to suppress the phase transition from the anatase structure to the rutile or the brookite after calcination at 500℃ for 2 h. It also had a high specific surface area, which was caused by a smaller particle size and more uniform distribution than the others. The change of catalytic acid sites was confirmed by Raman and FT-IR spectroscopy and the manganese acetate(III)-MnOx/TiO2 had the strongest Lewis acid sites among them. The highest de- NOx efficiency and structural stability were achieved by using the manganese acetate(III) as a precursor, because of its high specific surface area, a large amount of anatase TiO2, and the strong catalytic acidity.

keyword : porous materials, powder processing, grain growth, raman spectroscopy, catalytic acid-sites

Graphene has recently received high attention as a promising material for various applications, and many related studies have been undertaken to reveal its basic mechanical properties. However, the tribological properties of graphene film fabricated by the chemical vapor deposition (CVD) method are barely known. In this study, the contact angle and frictional wear characteristics of graphene coated copper film were investigated under room temperature, normal air pressure, and no lubrication condition. The contact angle was measured by sessile drop method and the wear test was carried out under normal loads of 660 mN and 2940 mN, respectively. The tribological behaviors of a graphene coating layer were also examined. Compared to heat treated bare copper foil, the graphene coated one shows a higher contact angle and lower friction coefficient.

keyword : nanostructured materials, chemical synthesis, wear, SEM, graphene

This study was carried out to investigate the effect of the hot forging ratio on the microstructure and mechanical properties of ultra high carbon steel. The microstructure of ultra high carbon steel with 1.5%wt.C consisted of a proeutectoid cementite network and acicular microstructure in pearlite matrix. With increasing hot forging ratio, the volume and thickness of the network and acicular proeutectoid cementite decreased. Lamella spacing and the thickness of eutectoid cementite decreased with increasing hot forging raito, and were broken up into particle shapes, which then became spheroidized. When the forging ratio was over 65%, the network and acicula shape of the as-cast state disappeared. With increasing hot forging ratio, hardness, tensile strength, elongation and impact value were not changed up to 50%, and then rapidly increased with the increase of the forging ratio.

keyword : ultra high carbon steel, forging ratio, proeutectoid and eutectoide cementite, network and acicular cementite, spheroidizing of cementite

Fe-(8.5, 18.5, 28.3, 36.9) wt% Cr alloys were corroded between 600 and 800℃ for up to 70 h in a 1 atm gas mixture that consisted of 0.0242 atm of H2S, 0.031 atm of water vapor, and 0.9448 atm of nitrogen gas. Their corrosion resistance increased with an increment in the Cr content. The Fe-8.5%Cr alloy corroded fast, forming thick, fragile, nonadherent scales that consisted primarily of an outer FeS layer and an inner (Fe, Cr, O, S)-mixed layer. The outer FeS layer grew into the air by the outward diffusion of Fe2+ ions, whereas the inner mixed layer grew by the inward diffusion of oxygen and sulfur ions. At the interface of the outer and inner scales, voids developed and cracking occurred. The Fe-(18.5, 28.3, 36.9)% Cr alloys displayed much better corrosion resistance than the Fe-8.5Cr alloy, because thin Cr2O3 or Cr2S3 scales formed.

keyword : alloys, casting, corrosion, scanning electron microscopy, SEM, H2S gas

In this study, the effect of hot-stamping heat treatment on the microstructure and hardness of TWB(Tailor Welded Blank) laser joints in Al-Si-coated boron steel and Zn-coated DP(Dual Phase)590 steel was investigated. In the TWB joints without heat treatment, hardness profiles showed local hardness deviation near the fusion zone. However, there was no hardness deviation in the heat treated specimen and its hardness was higher than that of the one without the heat treatment, due to a fully martensite microstructure. In the TWB joints of both the boron and DP steels, the maximum hardnesses were observed at the HAZ(Heat Affected Zone) near the base metal, and the hardness decreased gradually to the base metal. In the heat treated joints, the hardnesses of the HAZ and the base metal of the boron steel side were similar to the maximum hardness of the weld, while those of the HAZ and the base metal of the DP steel side were higher than the maximum hardness.

keyword : metals, joining, microstructure, scanning electron microscopy (SEM), transmission electron microscopy, TEM

ZnO nanowires were fabricated through thermal evaporation of Zn or ZnS powder using solar energy. The Zn or ZnS powder was heated and evaporated by sunlight. The sunlight was concentrated on the Zn or ZnS powder by a converging lens and then the Zn or ZnS powder was evaporated and oxidized in air. After oxidation, ZnO nanowires were fabricated in the focal point. Strong ultraviolet emission, which corresponds to the near band-edge emission, was observed from the ZnO nanowires synthesized using Zn powder as a source material. Meanwhile, green emission, related to intrinsic defects such as oxygen vacancies, prevailed for the ZnO nanowires fabricated using ZnS powder. No catalysts were used in the fabrication of the ZnO nanowires, which suggested the ZnO nanowires were grown by a vapor-solid mechanism.

keyword : nanostructured materials, crystal growth, optical properties, scanning electron microscopy, green technology

The objective of this study is to investigate the effect of additives on the mechanical and electrical characteristics of electrodeposited copper foils. Additives A(leveler) and B(brightener) were used in this study and Cl ions were used as an accelerator. In case of using these additives A and B, it showed a disadvantage that decreased the elongation of electrodeposited layer due to decreased grain sizes and increased tensile strength. On the other hand, the Cl ions decreased the specific resistance of the copper layer and increased elongation owing to increasing grain sizes. The highest elongation and lowest resistivity were measured in the group added only Cl ions, whose values were 21.9% and 3.11 μΩ-cm, respectively.

keyword : electrodeposite, copper foil, additive, elongation, surface morphology

A ruthenium (Ru) catalytic layer and a conventional Pt layer were assessed as counter electrodes (CE) for dye sensitized solar cells (DSSCs). Ru films with different thicknesses of 34, 46, and 90 nm were deposited by atomic layer deposition (ALD). Pt layers with the same thicknesses were prepared by sputtering. 0.45 cm2 DSSCs were prepared and their properties were characterized by FE-SEM, cyclic voltammetry (CV), impedance spectroscopy (EIS), and current-voltage (I-V). FE-SEM revealed that the crystallized Ru films and Pt films had been deposited quite conformally. CV showed that the catalytic activity of Pt was much greater than that of Ru. In addition, although the catalytic activity of Pt did not depend on the thickness, that of Ru showed an increase with increasing thickness. Impedance analysis revealed high charge transfer resistance at the Ru interface and a decrease with increasing Ru thickness, whereas Pt showed low resistance with no thickness dependence. Despite the relatively small catalytic activity of Ru, the I-V result revealed the average energy conversion efficiency of Ru and Pt to be 2.98% and 6.57%, respectively. These results suggest that Ru can be used as counter electrodes in DSSCs due to its extremely low temperature process compatibility.

keyword : solar cells, sputter, optical properties, nano electrode

This study investigated the hydrogen storage properties of Zr-Based AB2-xMx metal hybride made by HCS (Hydriding Combustion Synthesis). The materials were prepared by HCS 80 wt% AB2-15 wt% Mg- 5 wt% Mm, HCS 80 wt% AB2-20 wt% Mg and pure Zr-Based AB2, These materials were activated at 298 K under 20 bar. Both HCS 80 wt% AB2-20 wt% Mg and HCS 80 wt% AB2-15 wt% Mg-5 wt% Mm were absorbed within 1 minute. In the case of the AB2, it was perfectly absorbed within 6 minutes. Then, the materials were evaluated to obtain P-C-T (Pressure-Composition-Temperature) curves at 298K. As a result, the hydrogen storage capacity of HCS 80 wt% AB2-20 wt% Mg, HCS 80 wt% AB2-15 wt% Mg-5 wt% Mm and pure Zr-Based AB2 were determined to be 1.2, 1.6 and 1.74 wt%, respectively. The activation energy and rate controlling step were calculated by the Johnson-Mehl Avrami equation. The activation energies of HCS 80 wt% AB2-20 wt% Mg, HCS 80 wt% AB2-15 wt% Mg-5 wt% Mm and pure Zr-Based AB2 were 26.91, 20.45, and 60.41 kJ/mol, respectively. Also, the values of η in the Johnson-Mehl Avrami equation for HCS 80 wt% AB2-20 wt% Mg, HCS 80 wt% AB2-15 wt% Mg-5 wt% Mm and pure Zr-Based AB2 are 0.60, 0.51, and 0.44. So, the rate controlling steps which indicate hydrogen storage mechanism are an one dimensional diffusion process.

keyword : energy storage materials, powder processing, hydrogen, activation analysis, hydriding combustion