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Compound layers developed at 520 ℃ during gaseous nitriding of AISI 1045 steels were investigated. The nitriding potential (Kn) of a gas atmosphere of dissociating ammonia gas was checked by observation of the partial pressure of hydrogen gas. The microstructural evolution of the compound layer was studied by using optical microscopy and scanning electron microscopy. It was observed that the surface hardness was about 600 Hv after the nitriding treatment and the case depth was about 0.5mm after a nitriding treatment of 1440 min. The compound layer was composed of  ′(Fe4N) and ε(Fe2-3N). The thickness of the compound layer was simultaneously increased to 16 μm during the nitriding treatment of 1440 min. The kinetics of the compound layer growth were discussed with a modified Lehrer diagram of AISI 1045 steels, which was obtained from the thermodynamic calculation. †(Received January 15, 2016; Accepted March 2, 2016)

keyword : nitriding, microstructure, compound layer, AISI1045, hardness test

The effect of age hardening and bake hardening treatment on the mechanical properties of 7075 aluminum alloy sheets fabricated by twin roll strip casting (TRC) was studied. The hardness of the TRC 7075 sheets was continuously increased by natural aging up to 96 h and the highest yield strength at that condition was about 308 MPa. When the natural aged sample was bake-hardened at 180℃ for 30 min, the yield strength was 304 MPa, which indicated that the bake-hardening treatment did not increase the strength. The result suggests that the temperature was too high to produce the optimum amount of fine precipitates which result from coarsening of the Π-phase. To improve the paint-bake hardening properties without the loss of elongation, pre-aging treatment before natural aging was additionally applied at 120 ℃ for 30 min, and then the yield strength after the bake-hardening treatment was increased to 432 MPa. †(Received December 18, 2015; Accepted February 16, 2016)

keyword : metals, aging, mechanical properties, hardness test, precipitation

The corrosion behavior of low alloy steel containing Cu, Sb and 409L stainless steel was investigated for application in the low-temperature section of a flue gas desulfurization (FGD) system. The electrochemical properties were evaluated by potentiodynamic polarization testing and electrochemical impedance spectroscopy (EIS) in 16.9 vol% H2SO4 + 0.35 vol% HCl at 60 ℃. The inclusions in these steels were identified by electron probe microanalyzer (EPMA). The corrosion products of the steels were analyzed using scanning electron microscope (SEM) with energy dispersive spectroscopy (EDS) and transmission electron microscopy (TEM). The corrosion rate of the low alloy steel containing Cu, Sb was about 100 times lower than that of 409L stainless steel. For stainless steel without passivation, active corrosion behavior was shown. In contrast, in the low alloy steel, the Cu, Sb compounds accumulated on the surface improved the corrosion resistance by suppressing the anodic dissolution reaction. †(Received July 30, 2015; Accepted February 22, 2016)

keyword : low alloy steel, copper, precipitation, corrosion, flue gas desulfurization (FGD) system

A metal hydro-borate Zn(BH4)2 was prepared by milling ZnCl2 and NaBH4 in a planetary ball mill in an Ar atmosphere. This sample contained NaCl. 95 wt% MgH2-2.5 wt% Zn(BH4)2-2.5 wt% Ni samples [named MgH2-2.5Zn(BH4)2-2.5Ni] were then prepared by milling in a planetary ball mill in a hydrogen atmosphere. The hydrogen absorption and release properties of the prepared samples were investigated. In particular, variations in the initial hydriding and dehydriding rates with temperature were examined. MgH2-2.5Zn(BH4)2-2.5Ni dehydrided at the fourth cycle contained Mg, MgO, and small amounts of -MgH2 and Mg2Ni. The sample after hydriding-dehydriding cycling had a slightly smaller average particle size and a larger BET specific surface area than the sample after milling. Increasing the temperature from 573 K to 623 K led to a decrease in the initial hydriding rate. The initial dehydriding rate increased as the temperature increased from 573 K to 643 K. At 573 K under 12 bar H2, the sample absorbed 3.85 wt% H for 2.5 min, 4.60 wt% H for 5 min, 4.64 wt% H for 10 min, and 4.80 wt% H for 60 min. The MgH2-2.5Zn(BH4)2-2.5Ni had an effective hydrogen storage capacity (the quantity of hydrogen absorbed for 60 min) of near 5 wt% (4.96 wt% at 593 K). †(Received October 7, 2015; Accepted March 7, 2016)

keyword : hydrogen absorbing materials, mechanical alloying/milling, scanning electron microscopy (SEM), X-ray diffraction, Zn(BH4)2-added MgH2-based alloy

90 wt% MgH2+5 wt% Ni+2.5 wt% Fe+2.5 wt% Ti (called MgH2+Ni+Fe+Ti), a hydrogen storage and release material, was fabricated by grinding in a hydrogen atmosphere, and then its quantities of stored and released hydrogen as a function of time were examined. A nanocrystalline MgH2+Ni+Fe+Ti specimen was made by grinding in a hydrogen atmosphere and subsequent hydrogen storage-release cycling. The crystallite size of Mg and the strain of the Mg crystallite after ten hydrogen storage-release cycles, which were obtained using the Williamson-Hall method, were 38.6 (±1.4) nm and 0.025 (±0.0081) %, respectively. The MgH2+Ni+Fe+Ti sample after the process of grinding in a hydrogen atmosphere was highly reactive with hydrogen. The sample exhibited an available storage capacity of hydrogen (the amount of hydrogen stored during 60 minutes) of about 5.7 wt%. At the first cycle, the MgH2+Ni+Fe+Ti sample stored hydrogen of 5.53 wt% in 5 minutes, 5.66 wt% in 10 minutes and 5.73 wt% in 60 minutes at 573 K and 12 bar of hydrogen. The MgH2+Ni+Fe+Ti after activation released hydrogen of 0.56 wt% in 5 minutes, 1.26 wt% in 10 minutes, 2.64 wt% in 20 minutes, 3.82 wt% in 30 minutes, and 5.03 wt% in 60 minutes. †(Received November 6, 2015; Accepted February 13, 2016)

keyword : hydrogen absorbing materials, mechanical alloying/milling, microstructure, X-ray diffraction, addition of transition metals to magnesium hydride

We prepared working electrodes with blocking layers containing 0.0∼0.5 wt% Au nano powder to improve the energy conversion efficiencies (ECEs) of a dye sensitized solar cell (DSSC). TEM, FE-SEM, and AFM were used to characterize microstructure. XRD and micro-Raman were used to determine the phase and localized surface plasmon resonance (LSPR) effect of the blocking layer with Au nano powder. A solar simulator and a potentiostat were used to confirm the photovoltaic properties of the DSSC with the Au nano powder. From the results of the microstructure analysis, we confirmed that the Au nano powder had particle sizes of less than 70 nm, dispersed uniformly on the blocking layer. Based on the phase and composition analysis, we identified the presence of Au, and the Raman intensity increased as the amount of Au was increased. The photovoltaic results showed that the ECE reached 5.52% with the Au addition, compared to an ECE of 5.00% without the Au addition. This enhancement was due to the increased LSPR of the blocking layer with the Au addition. Our results suggest that we might improve the efficiency of a DSSC by the proper addition of Au nano powder on the blocking layer. †(Received November 24, 2015; Accepted February 22, 2016)

keyword : solar cell, coating, electrical properties, atomic force microscopy, localized surface plasmon resonance

Au-Pd bimetallic nanoparticles (NPs) have received a lot of attention in the fields of catalysts and hydrogen sensors. In this study, Au-Pd core-shell NP arrays were successfully fabricated using two steps: formation of the ordered array of Au NPs cores via solid-state dewetting of a Au thin film on a topographic silica substrate, and Pd shell formation via chemical synthesis using two different surfactants (CTAB and CTAC). Using the CTAB surfactant in particular, a 2-D composite structure comprised of an ordered array of Au-Pd NPs, with smaller Pd NPs on the nanoscopic gaps between the Au-Pd NPs, could be formed. This structure is expected to have potential application in resistance-base hydrogen sensors. †(Received October 23, 2015; Accepted February 13, 2016)

keyword : nanostructured materials, chemical synthesis, surface, X-ray diffraction, gas sensors

The microstructure of nano-sized oxide particles formed in Fe-5Y2O3 alloys uniaxial-hot-pressed at 1150 ℃ for 1 hour after a mechanical alloying of 95 wt%Fe and 5 wt%Y2O3 powders was examined. Several kinds of nano-sized oxide particles were found in the uniaxial hot pressed Fe-5Y2O3 alloy, and their crystal structures seemed to be Y2O3, Y3Fe5O12 and YFeO3. The oxide particles with diameters of about 20 nm showed a semi-coherent relationship with the matrix while those more than 200 nm in diameter exhibited a non-coherent relationship with the matrix, and this tendency appeared to be independent of the kind of oxide particles. †(Received September 11, 2015; Accepted January 9, 2016)

keyword : ODS, mechanical alloying, nano-sized oxide particle

Grain-oriented electrical steel is used as a core material in transformers and motors. To obtain improved magnetic properties from the grain-oriented electrical steel, the steel should have a strong {110}<001> Goss texture. Recently, controlled manufacturing processes have been employed for developing electrical steels with a strong Goss texture. It is important to carry out research on the {411}<148> and {111}<112> oriented grains in relation to coincidence site lattice (CSL) boundaries, as they have an effect on the easy growth of the Goss grains upon secondary recrystallization. In this study, the behavior of the {411}<148> and {111}<112> oriented grains, which are neighbored with Goss grains after recrystallization with rapid and typical heating rates, and after decarburization, was examined by using x-ray diffraction (XRD) and electron back-scattered diffraction (EBSD) measurements. In the decarburized specimen, the Goss grains encroached the {411}<148> and {111}<112> oriented grains to a greater extent with a rapid heating rate than with a typical heating rate, and larger Goss grains were observed with the rapid heating rate. The {111}<112> oriented grains especially affect the easy growth of the Goss grains, as they are located near the Goss grains. Therefore, larger Goss grains can be produced at rapid heating rates, and the product is estimated to exhibit improved magnetic properties after secondary recrystallization. †(Received December 2, 2015; Accepted March 7, 2016)

keyword : electrical materials, annealing, grain growth, EBSD, texture

Extreme ultraviolet lithography (EUVL) runs in reflective optics and the shadowing effect causes several new issues limiting the performance of EUV mask. To get over this shadowing effect, many studies are made by reducing the absorber thickness with high extinction coefficient materials. Nickel is a promising absorber material candidate which can replace tantalum compounds due to its high extinction coefficient. However, nickel is vulnerable to conventional mask cleaning process. In this paper, novel Ta-Ni compound material is fabricated by co-sputtering process and its improved chemical durability and shadowing effect is presented. †(Received April 25, 2016; Accepted May 4, 2016)

keyword : extreme ultraviolet lithography, semiconductor, sputtering, optical properties, computer simulation