발간논문

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In the present study, we systematically investigated the effect of entropy of mixing (ΔS_mix) on the mechanical and thermal properties of single phase FCC solid solutions with multi-principal elements. An increase in ΔS_mix led to an increase in micro-hardness and a decrease in thermal conductivity (κ) due to severe lattice distortion. Furthermore, NiCoFeCrMn high-entropy alloy (HEA) exhibited the highest κ₁/κ_e ratio of ~1.13, because with increasing ΔS_mix the reduction in lattice thermal conductivity (κ₁) was less than that of electron thermal conductivity (κe). In particular, a crossover in the properties` variation was noticeable at a specific ΔS_mix of ~8.3±0.3 J/mol·K, which means the properties of single phase FCC solid solutions can be optimized by modulating ΔS_mix. The results can provide a guideline for manipulating the properties of single solid solutions using the HEA design concept, and help develop novel materials with optimal properties that are usable in extreme environments. (Received June 7, 2016; Accepted July 20, 2016)

keyword : alloy design, high-entropy alloy, microstructure, mechanical property, thermal property

Fe-TiB₂ powder was synthesized in-situ by the planetary ball milling and subsequent heat-treatment of an iron boride (FeB) and titanium hydride (TiH₂) powder mixture. Mechanical activation of the (FeB+TiH₂) powder mixtures was observed after a milling time of 3 hours at 700 rpm of rotation speed, but activation was not the same after 1 hour milling time. The particle size of the (FeB+ TiH₂) powder mixture was reduced to the nanometer scale, and each constituent was homogeneously distributed. A sharp exothermic peak was observed at a lower temperature (749 ℃) on the DSC curves for the (FeB+TiH₂) powder mixture milled for 3 hours, compared to the one milled for 1 hour (774 ℃). These peaks were confirmed to have resulted from the formation reaction of the TiB2 phase, from Ti and B elements in the FeB. The Fe-TiB₂ composite powder fabricated in situ exhibited only two phases of Fe and TiB₂ with homogeneous distribution. The size of the TiB2 particulates in the Fe matrix was less than 5 nm. (Received July 22, 2016; Accepted July 25, 2016)

keyword : Fe-TiB₂, nanocomposite, mechanical alloying/milling, in situ fabrication, powder processing, iron boride, titanium hydride

Laser surface carburizing can be achieved by a laser alloying mechanism, which includes melting the graphite coated substrate and dissolution of the graphite in the liquid state. The drawback of laser carburizing is that process-induced cracks frequently form on the carburized surface as a result of carbon enrichment and the rapid re-solidification rates. Hence, in this study laser surface carburizing of a substrate coated with a mixed slurry of graphite and Fe powders was implemented to increase the surface hardness without surface cracking. The cracking susceptibility, carburized depth and the hardness of the carburized layer were investigated for various laser processing conditions. The results showed that a lower relative content of graphite powder in mixed slurry reduces the cracking susceptibility on the carburized surface without significantly decreasing hardness. From the analysis results, operational guides for laser surface carburizing processing are provided for achieving a crack-free carburized surface with desired hardness. (Received February 11, 2016 ; Accepted May 23, 2016)

keyword : metals, surface hardening, mechanical properties, hardness, laser carburizing

The carbon composite iron oxide briquette (CCB) is considered a potential solution to the upcoming use of low grade iron resources in the ironmaking process. CCB is able to reduce raw material cost by enabling the use of low grade powdered iron ores and coal. Additionally, the fast reduction of iron oxides by direct contact with coal can be utilized. In this study, the reduction behaviors of CCB were investigated in the temperature range of 200-1200 ℃ under oxidizing atmosphere. Briquettes were prepared by mixing iron ore and coal in a weight ratio of 8:2. Then reduction experiments were carried out in a mixed gas atmosphere of N₂, O₂, and CO₂. Compressive strength tests and quantitative analysis were performed by taking samples at each target temperature. In addition, the reduction degree depending on the reaction time was evaluated by off-gas analysis during the reduction test. It was found that the compressive strength and the metallization degree of the reduced briquettes increased with increases in the reaction temperature and holding time. However, it tended to decrease when the re-oxidation phenomenon was caused by injected oxygen. The degree of reduction reached a maximum value in 26 minutes. Therefore, the re-oxidation phenomenon becomes dominant after 26 minutes. (Received April 12, 2016; Accepted August 9, 2016)

keyword : iron oxide, reduction degree, SEM, strength, off-gas analysis

Solvent extraction experiments were performed with Re(VII) and V(V) from concentrated HCl solutions. In this work, Alamine 336 and LIX 63 were employed as the basic and cationic extractants. At an HCl concentration range from 1 to 5 M, Re(VII) was selectively extracted by Alamine 336, while V(V) was selectively extracted by LIX 63. A low HCl concentration was favorable for separating the two metals by solvent extraction. Although an alkaline solution can completely strip both Re(VII) and V(V) from loaded Alamine 336 and LIX 63, it was difficult to separate the two metals by stripping. However, it was found that an HCl solution could selectively strip the metal with a low extraction percentage from the loaded Alamine 336 and LIX 63, and thus separation of the two metals was achieved. Our data can be utilized to develop a process for separating Re(VII) and V(V) from concentrated HCl solutions. (Received July 18, 2016; Accepted August 7, 2016)

keyword : Rhenium, Vanadium, solvent extraction, Alamine336, LIX63

Cobalt silicide was used as a counter electrode to replace the Pt catalytic layer of a dye-sensitized solar cell (DSSC) device. 100 nm Si/ 100 nm Co on quartz were formed by sputtering, and cobalt-silicides were formed by vacuum heat treatment at 500 ℃ and 700 ℃ for 30 min, respectively. Field emission scanning electron microscopy (FE-SEM) was used to analyze the surface microstructure. X-ray diffraction (XRD) and Auger electron spectroscopy (AES) depth profiling analysis were used to confirm the phases. Also, cyclic-voltammetry (CV) analysis was employed to confirm the catalytic activity, and photovoltaic properties were confirmed using a simulator and potentiostat. The microstructure analysis indicated that the 500 ℃ and 700 ℃ silicidation led to a uniform planar layer, and island-like agglomerates, respectively. In the XRD and AES results, those phases were structures of quartz/CoSi/Co and quartz/dot-(CoSi₂/Co). CV analysis showed that Si/Co and CoSi/Co exhibited catalytic activity, while dot-(CoSi₂/Co) did not show catalytic activity due to the isolated dot structure. The energy conversion efficiencies of DSSCs with CoSi/Co and dot-(CoSi₂/Co) were 3.75% and 0%, respectively, while that of Pt employed DSSC was 5.13%. Our result implies that using the nano-thick CoSi as a reduction catalytic layer may be an effective replacement for Pt. (Received March 4, 2016; Acceted July 22, 2016)

keyword : solar cells, annealing, phase transformation, auger electron spectroscopy, catalytic activity

We investigated the effects of growth duration on the morphological, structural, and photoelectrochemical properties of TiO₂ nanorods grown on a TiO₂-buffered fluorine-doped tin oxide (FTO) substrate using the hydrothermal method. From the study, we found that the photoelectrochemical properties were mainly dependent on the nanorod length and (002) XRD peak intensity, which indicates vertical growth of the TiO₂. The morphological and structural properties of the TiO₂ nanorods largely affect their photoelectrochemical properties. The photocurrent density was increased when the growth duration was increased from 1 hour to 4 hours, and then changed little above 4 hours. Consequently, the highest photocurrent density, 0.897 mA/cm² (at 1.0 V vs. SCE), was obtained from the 8 hour-sample which had good morphological and structural properties. We used field emission scanning electron microscopy (FE-SEM) for analysis of the morphological properties, x-ray diffraction (XRD) for structural properties, and three-electrode potentiostat for photoelectrochemical properties. (Received April 7, 2016; Accepted July 7, 2016)

keyword : titanium dioxide, TiO₂, hydrothermal, nanostructure, photoelectrochemical (PEC), photocurrent density, growth duration

The objective of this study was to obtain basic data for trapping gaseous technetium (Tc) oxide generated from the voloxidation process in spent nuclear fuel pyroprocessing. Rhenium (Re) and Ca(OH)₂ were used as surrogates for the technetium and a trapping material, respectively. The trapping characteristics of rhenium oxide were investigated with changing temperatures and molar ratios of calcium (Ca) over rhenium, and the thermal behaviors of the trapping products were observed. The products following after the trapping test were identified as Ca(ReO₄)₂ and Ca₅Re₂O₁₂. The conversion to Ca5Re₂O₁₂ was preferred with increasing temperatures, and the trapping products were completely converted into Ca₅Re₂O₁₂ under conditions exceeding 800 ℃, or when maintained at 750 ℃ for 4 hr. The trapping efficiency at a molar ratio of 2.5 (Ca:Re=5:2) was significantly superior to that at the molar ratio of 2. (Received July 6, 2016; Accepted July 29, 2016)

keyword : oxides, chemical synthesis, crystal structure, X-ray diffraction, rhenium trapping

Yttrium Aluminum Garnet (YAG) single crystal has received much attention as the high power solid-state laser`s key component in industrial and medical applications. Various growth methods have been proposed, and currently the induction-heating Czochralski (IHCZ) growth method is mainly used to grow YAG single crystal. Due to the intrinsic properties of the IHCZ method, however, the solid/liquid interface has a downward convex shape and a sharp tip at the center, which causes a core defect and reduces productivity. To produce YAG single crystals with both excellent quality and higher yield, it is essential to control the core defects. In this study, using computer simulations we demonstrate that the resistance-heating CZ (RHCZ) method may avoid a downward convex interface and produce core defect free YAG single crystal. We studied the effects of various design parameters on the interface shape and found that there was an optimum combination of design parameter and operating conditions that produced a flat solid-liquid interface. (Received April 22, 2016; Accepted June 29, 2016)

keyword : implant, titanium alloy, low elastic modulus, direct aging, microstructure, mechanical properties

ZnO buffer layers were deposited on mica substrates using a sol-gel spin coating method. Then, a thin film of metallic Zn was deposited onto the ZnO buffer layer/mica substrate using a thermal evaporator, and the deposited Zn thin films were then thermally oxidized in a furnace at 500 ℃ for 2 h in air. Finally, In-doped ZnO (IZO) thin films with different In concentrations were grown on the oxidized ZnO film/ZnO buffer layer/mica substrates using the sol-gel spin-coating method. All the IZO films showed ZnO peaks with similar intensities. The full width at half maximum values of the ZnO (002) peak for the IZO thin films decreased with an increase in the In concentration to 1 at%, because the crystallinity of the films was enhanced. However, a further increase in the In concentration caused the crystal quality to degrade. This might be attributed to the fact that the higher In doping resulted in an increase in the number of ionized impurities. The Urbach energy (E_U) values of the IZO thin film decreased with an increase in the In concentration to 1 at% because of the enhanced crystal quality of the films. The EU values for the IZO thin films increased with the In concentration from 1 at% to 3 at%, reflecting the broadening of localized band tail state near the conduction band edge of the films. (Received March 4, 2016; Accepted July 22, 2016)

keyword : thin films, sol-gel, optical properties, x-ray diffraction, Zn-deposited mica substrate