발간논문

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Vacuum kinetic spraying (VKS) is a relatively advanced process, by which a dense film can be fabricated at room temperature. Various materials including ceramic, metal, polymer and composite can be used as the substrate and coating materials. In this research, red and blue color films were successfully fabricated on glass substrates using Co(Al, Cr)2O4 and C9H13CdN2O4SSe ceramic pigment powders in a VKS system. Although the deposition behavior of each powder differed according to particle size and/or materials, it was confirmed that the brightness and transmittance of the color film could be easily changed by controlling the process parameters. Furthermore, using masking technology, design color patterns were fabricated. As a result, it is cautiously expected that the VKS process can be utilized to produce decorative coating/patterning for product design. (Received January 6, 2016; Accepted March 17, 2016)

keyword : vacuum kinetic spray (aerosol deposition), decorative coating, optical property, ceramics, patterning system

A tungsten coating was deposited onto a graphite substrate using the atmospheric plasma spraying (APS) technique. In order to increase the adhesion strength between the metallic tungsten(W) and graphite, a molybdenum (Mo) interlayer was pre-deposited onto the graphite surface by utilizing the APS technique. Also, after deposition of a APS-W coating, a zirconia (ZrO2) was deposited onto the W coating layer. For the APS process, argon and helium were used as the plasma-forming gases, and argon was used as the shield gas to protect the plasma from oxidation. After the APS coating process, heat exposure treatment was performed at 2100 ℃ for 360 h within a sapphire single crystal-growing furnace in order to evaluate the thermal stability of the coatings. After heat treatment, the ZrO2/W/Mo coating layers were bound with the graphite without any peeling off. The microvickers hardness of the APS-W coating layer was increased after heat treatment due to the formation of carbides. Also, carbide phases such as Mo2C, WC, ZrC and Mo3C2 were identified by XRD diffraction and EDS analysis, by analyzing the depths below the coating surface. It was considered that the Mo interlayer served as a good buffer layer between the APS-W coating and the graphite after the heat exposure treatment because the lattice structure of the molybdenum carbide was similar to that of the graphite. (Received September 16, 2015; Accepted Mach 11, 2016)

keyword : metals, microstructure, scanning electron microscopy, Mo interlayer, plasma deposition spray

TiC-Co or TiC-Ni hard materials have been used for cutting tools. However, the high cost and the low hardness of Ni or Co, and the low corrosion resistance of the TiC-Ni and TiC-Co cermets have generated interest in recent years in using them as alternative binder phases. In this study, TiAl was used as a novel binder and consolidated by the pulsed current activated sintering(PCAS) method. Nanopowders of TiC and TiAl were fabricated using high energy ball milling. Highly dense TiC-TiAl hard materials with a relative density of up to 99.5% were sintered within three min by PCAS. Not only the hardness but also the fracture toughness of the TiC-10 vol%TiAl were better than those of TiC-10 vol%Ni or TiC-10 vol%Co. (Received December 14, 2015; Accepted March 20, 2016)

keyword : composites, nanomaterials, sintering, hardness, fracture toughness

Macroporous titania crystallites were synthesized using an emulsion-assisted self-assembly method to prepare a photocatalyst for the decomposition of organic dyes such as methylene blue. The porous structure was prepared using polymeric particles as sacrificial templates, which were synthesized by large-scale dispersion polymerization. An in-depth study was performed to characterize the photocatalytic performance of the porous spherical titania particles by evaluating the rate constants of the decomposition reactions. Factors affecting the reaction rates were studied by controlling the size of macropores, the average diameter of the porous particles, the amount of photocatalysts and methylene blue in reaction mixture, and the power of the UV light sources. Enhanced photocatalytic performances were also observed for porous particles with hollow microstructure obtained using a rotating cylinder system, or bimodal porous particles. (Received February 2, 2016; Accepted March 31, 2016)

keyword : porous materials, powder processing, microstructure, scanning electron microscopy, self-assembly

Overlay welding technique is one of methods used to improve metal mechanical properties such as strength, toughness and corrosion resistance. Generally, Inconel 625 alloy is used for overlay welding layer on low carbon steels for economic consideration. However, the method produces some problems in the microstructure of the cast structure and some defects, caused by the elevated temperatures of the overlay welding process. To resolve these problems, heat treatments are required. In this study, Inconel 625 alloy was welded on a low carbon steel by the overlay welding process to investigate the resulting microstructure and mechanical properties. A double heat treatment was performed to improve the mechanical properties of the welding and substrate layers. It was found that Inconel 625 alloy had an austenite microstructure after the first heat treatment, but the low carbon steel had a ferrite-pearlite microstructure after the second heat treatment. After the double heat treatment, the sample showed the optimum hardness because of grain refinement and homogenization of the microstructure. (Received October 23, 2015; Accepted February 29, 2016)

keyword : metals, welding, microstructure, hardness test, double heat treatment

Mixtures of acidic extractants and LIX 63 were employed to improve the extraction efficiency of Pr and Nd from chloride solutions. The effect of the composition of the extractant mixtures has been studied. The order of metal extraction by single acidic extractant was D2EHPA > PC88A > Cyanex 272 > Cyanex 301. The addition of LIX 63 to the acidic extractants resulted in a synergistic effect, and the strength of the effect was the reverse order of that found for extraction by single extractants. Moreover, mixing of saponified Cyanex 272 and LIX 63 enhanced the extraction, while the addition of Alamine 336 to the mixtures of Cyanex 272 and LIX 63 depressed the extraction. (Received November 17, 2015; Accepted March 24, 2016)

keyword : Pr, Nd, organophosphorus acid, LIX 63, synergistic solvent extraction

This work details how the sawing velocity influences the chipping damage of semiconductor wafers with different thicknesses during wafer separation. The experimental result shows that at a sawing velocity higher than 60000 rpm, in thinner wafers the magnitude of chipping damage more strongly depends on the revolving velocity of the saw blade. It is also shown that the aspect ratio of the sawing-induced groove formed in a wafer is a key factor in determining the dependence of the chipping damage magnitude on wafer thickness. That is, at a sawing velocity higher than 60000 rpm, the saw blade thickness should be thin enough to allow the sawing-induced scribe (in the shape of a trench) in the wafer to have an aspect ratio larger than 2. Consequently, in order to prevent degradation in device reliability due to the mechanical dicing of a semiconductor wafer thinner than 3T (approximately 45 um), the sawing velocity or saw blade thickness should be reduced. (Received November 16, 2015; Accepted March 8, 2016)

keyword : semiconductor, silicon, chip, fracture, reliability

In and Ga doped ZnO (IGZO, 100-nm thick) thin films were deposited by radio frequency magnetron sputtering without intentional substrate heating on a bare glass substrate and a TiO2-deposited glass substrate to determine the effect of the thickness of a thin TiO2 buffer layer on the structural, optical, and electrical properties of the films. The thicknesses of the TiO2 buffer layers were 5, 10 and 15 nm, respectively. As-deposited IGZO films with a 10 nm-thick TiO2 buffer layer had an average optical transmittance of 85.0% with lower resistivity (1.83×10(-2) Ω cm) than that of IGZO single layer films. The figure of merit (FOM) reached a maximum of 1.44×10(-4) Ω-1 for IGZO/10 nm-thick TiO2 bi-layered films, which is higher than the FOM of 6.85×10(-5) Ω-1 for IGZO single layer films. Because a higher FOM value indicates better quality transparent conducting oxide (TCO) films, the IGZO/10 nm-thick TiO2 bi-layered films are likely to perform better in TCO applications than IGZO single layer films. (Received January 19, 2016; Accepted March 20, 2016)

keyword : thin films, sputtering, optical properties, electrical properties, AFM

The vapor phase transport (VPT) process is usually used for the growth of one-dimensional nanostructures rather than for film deposition. In this study, for the first time, we report on the fabrication and optical propeties of ZnO film produced by VPT combined with rapid heating. The X-ray diffraction results showed that the intensity of the ZnO (002) peak was the largest, which indicated that the c-axis orientation of the ZnO grains was perpendicular to the substrate. In the photoluminescence (PL) spectra of the ZnO film at 12 K, PL peaks were observed at 3.374, 3.362, 3.319, and 3.242 eV, which were attributed to free excitons (FXs), excitons bound to neutral donors, donor-acceptor pairs (DAPs), and first-order longitudinal optical phonon replicas of DAPs (DAPs-1LO), respectively. The energy interval between DAPs and DAPs-1LO was almost the same as the longitudinal optical (LO) phonon energy of bulk ZnO of ~76 meV. The PL spectra in the temperature range from 12 to 300 K shifted to lower energy with an increasing temperature. The activation energy of FX was estimated to be 61 meV. This value is similar to the exciton binding energy of 60 meV. (Received September 4, 2015; Accepted December 11, 2015)

keyword : thin films, vapor deposition, optical properties, X-ray diffraction, photoluminescence

Nickel silicide was used as a counter electrode to replace the Pt catalytic layer of a dye-sensitized solar cell (DSSC) device. 50 nm Si/ 50 nm Ni was formed on glass or quartz by sputtering, and nickel silicides were formed by vacuum heat treatments at 450 and 800 ℃ for 30 min. X-ray diffraction (XRD) and Auger electron spectroscopy (AES) depth profiling analysis was used to confirm the formation of nickel silicides. Also, cyclic voltammetry (CV) analysis was employed to confirm the catalytic activity, and photovoltaic properties were confirmed using a simulator and potentiostat. The XRD and AES results reveated that NiSi and NiSi2 were formed by annealing at 450 and 800 ℃, respectively. The results of the CV analysis showed that both NiSi and NiSi2 exhibited catalytic activity. The energy conversion efficiencies (ECE) of DSSCs with NiSi and NiSi2 catalysts were 1.31% and 3.86%, respectively, while the device employing the Pt catalyst showed an ECE of 5.49%. This result implies that NiSi2 can replace Pt when the processing condition is optimized. (Received January 14, 2016; Accepted February 28, 2016)

keyword : solar cells, annealing, phase transformation, X-ray diffraction, catalytic activity