발간논문

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Aluminum conductors are increasingly replacing copper because Al has lower cost and lighter weight, and consequently the demand for Al-Cu joints for industrial applications is growing. However, an intermetallic compound is known to grow at the Al-Cu interface, and will result in fracture if it grows above a critical thickness. It has been reported that the thickness of this intermetallic compound layer significantly increases when alternating current flows through the Al-Cu interface. For long distance power transmission, direct current is preferred because it reduces energy losses. In the present study, the formation and growth of the intermetallic compound under direct current was investigated using FE-SEM and EPMA. The thickness of the intermetallic compound was measured and compared based on the direction of the electron flows. It was thus confirmed that under commercial conditions below 10A/㎟ the electric current accelerates the growth of the intermetallic compound, with distinct effect from electromigration. In addition, an empirical equation to predict the thickness of the intermetallic compound was obtained, to estimate the life time of the Al-Cu joint. (Received December 8, 2016; Accepted January 3, 2017)

keyword : conductors, intermetallics, joining, welding, diffusion, direct current

The resistance of hydrogen induced cracking (HIC) was investigated with different microstructures of API X70MS steel. Ferrite/acicular ferrite (F/AF), deformed ferrite/acicular ferrite, ferrite/bainite (F/B) and single acicular ferrite (AF) were obtained by thermo-mechanical controlled process (TMCP) with changing rolling and cooling conditions. HIC resistance was found to be affected by the type as well as morphology of the microstructure, and thus the behavior of crack initiation and propagation could be analyzed. It was found that single AF and deformed F/AF with uniformly distributed dislocation reduced HIC initiation. Banded microstructure with a hardness value below 250 HV, such as AF, showed good HIC propagation resistance due to high toughness. AF generally exhibited excellent crack initiation and propagation resistance, namely the best HIC susceptibility performance. AF might redeem the HIC resistance for the banded microstructure also. (Received January 24, 2017; Accepted March 7, 2017)

keyword : linepipe steel, microstructure, hydrogen induced cracking (HIC), acicular Ferrite, banded microstructure

An as-received EMC/S (electromagnetic casting and stirring)-processed Al-12Si based alloy billet was homogenized to examine its microstructure and high temperature plastic deformation behavior, using compressive tests over the temperature range from 623 to 743 K and a strain rate range from 1.0×10^-3 to 1.0×10⁰s^-1. The results were compared with samples processed by the direct chill casting (DC) method. The fraction of equiaxed structure for the as-received EMC/S billet(41%) was much higher than that of the as-received DC billet(6%). All true stress - true strain curves acquired from the compressive tests exhibited a peak stress at the initial stage of plastic deformation. Flow stress showed a steady state region after the appearance of peak stress with increasing strain. The peak stress decreased with increasing temperature at a given strain rate and a decreasing strain rate at a given temperature. A constitutive equation was made for each alloy, which could be used to predict the peak stress. A recrystallized grain structure was observed in all the deformed specimens, indicating that dynamic recrystallization is the predominant mechanism during high temperature plastic deformation of both the homogenized EMC/S and DC-processed Al-12Si based alloys. (Received September 28, 2016; Accepted December 9, 2016)

keyword : Al alloy, deformation, compression test, transmission electron microscopy (TEM), recrystallization

The influence of ultrasonic melt treatment (UST) combined with a change in cooling rates on the microstructure and elevated temperature mechanical properties of a hypereutectic Al-18Si-4Cu-3Ni piston alloy was investigated. Microstructural observation confirmed that UST effectively refined the sizes of primary Si and intermetallic compounds (e.g. ε-Al3Ni) while promoting their homogeneous distribution. Besides the refinement of the constituent phases, the size of the dendrite arm spacing (DAS), which was hardly affected by UST, significantly deceased with increasing cooling rates. The refinement of the solidification structure in the alloy achieved through both UST and increased cooling rates resulted in an improvement in tensile properties, ultimate tensile strength and elongation in particular, after T5 heat treatment followed by overaging at 350 ℃. However, the elevated temperature yield strength of the alloy was not associated with the refinement, but was rather correlated with the 3-D interconnectivity, morphology and volume fraction of the primary Si. (Received October 21, 2016; Accepted December 7, 2016)

keyword : hypereutectic Al-18Si casting alloy, ultrasonic melt treatment, microstructure, interconnectivity, elevated temperature tensile properties

The solvent extraction of tungsten(VI) from hydrochloric acid solutions using 5,8-diethyl- 7-hydroxydodecan-6-one oxime (LIX 63) was analyzed in solutions having an initial pH range from 2 to 5, by varying the concentration of metal and extractant. In our experimental range, the cationic exchange reaction as well as the solvation reaction occurred simultaneously. The cation exchange reaction was identified by applying a slope analysis method to the extraction data. The existence of cationic tungsten(VI) species was confirmed by ion exchange experiments with Diphonix resin at pH 3. Further study is needed to identify the nature of this tungsten cationic species. (Received November 23, 2016; Accepted January 10, 2017)

keyword : solvent extraction, ion exchange, tungsten, hydrochloric acid, LIX 63

The square loop (SL) is the typical pattern of a frequency selective surface (FSS) with high inductance (L) and capacitance (C). In this study, modified patterns including split square loop FSS (SSL-FSS) and I-type FSS (I-FSS) are proposed, and their L and C values were estimated using the equivalent circuit model. The insertion of an air gap in the SL conductor stripes decreased both L and C, due to the reduction in stripe length. Meanwhile, when I-FSS was polarized in parallel with an electric field, it had the effect of reducing stripe width and, thereby, exhibited a higher value of L. Using the retrieved L and C values, microwave absorbance can be determined in the three-layer structure composed of FSS on a grounded dielectric substrate. The simulation results obtained with computational tool provided almost the same reflection loss result, which indicates that the equivalent circuit approach is reliable and efficient for determining the L, C values of various FSS. (Received September 13, 2016; Accepted January 10, 2017)

keyword : electric properties, magnetic properties, computer simulation, frequency selective surface, microwave absorber

In this study, neodymium (Nd) nuclides in LiCl-KCl eutectic salts were captured and solidified using a synthetic inorganic composite (Li₂O-SiO₂-Al₂O₃-B₂O₃), a process that allows the selective capture of Nd and fabrication of a composite with Nd captured from waste, without additional additives or mixing. The Nd nuclides in the LiCl-KCl eutectic salt were mainly captured in the form of LiNdSiO₄, and it was confirmed that NdSiO₃ can be formed in the composite with captured Nd when the content of Nd in the composite is increased. The capture efficiency was higher than about 98 wt%. It was thought that the salt recovered from the Nd capture test was a renewable form could be reused in the pyroprocessing of used nuclear fuel, because the composite has high chemical durability in a LiCl-KCl eutectic salt at 900 ℃. The composite captured Nd was fabricated into a homogeneous glass form and a stable ceramic form. (Received August 22, 2016; Accepted December 1, 2016)

keyword : oxides, chemical synthesis, crystal structure, X-ray diffraction, capture and solidification of neodymium

We report the effects of incorporated Cu on the thermoelectric transport properties of Bi_0.42Sb_1.58Te₃ compounds prepared by the hot pressing method. The hole concentration in the compound increased significantly with the increase in the Cu content. The acceptor role of Cu was manifested by Hall measurements and also by the temperature-dependent Seebeck coefficient. Mobility in the compound was mainly governed by phonon scattering, and was additionally affected by ionized impurity scattering. Density-of-state effective mass increased with the Cu content and its effects on the power factor were discussed. (Received October 4, 2016; Accepted December 22, 2016)

keyword : thermoelectric, bismuth telluride, hot pressing, charge transport, power factor

Hybrid organic/inorganic composites have recently been investigated in the thermoelectric field in an effort to interdependently manipulate the charge carriers, which are interconnected with electrical and thermal transport. The decoupling of a charge carrier`s roles in the hybrid structures can result in enhanced thermoelectric performance, by only exploiting the high seebeck coefficient of the inorganic part and the high electrical conductivity of the organic part. In this work, we demonstrated the enhanced thermoelectric performance of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) by employing the nanoimprinted Ge₂Sb₂Te₅ nanowire arrays to form a vast network of PEDOT:PSS/ Ge₂Sb₂Te₅ nanowire composites. These structures showed an approximately ten times higher Seebeck coefficient than the PEDOT:PSS itself without damaging electrical conductivity. In addition, ethylene glycol (EG) was used to selectively de-dope PSS, to enhance the electrical conductivity of the composites. Different EG concentrations were needed to optimize the de-doping effect, and the best thermoelectric power factor (P.F.) of ~1.6 × 10³ μW/mK² was achieved with an EG level of 6 vol% in our configuration. This result is 5 times and 22 times higher than that of GST thin film (320 μW/mK²) and PEDOT:PSS/Te nanorods composites (70.9 μW/mK²). Electrical and thermoelectric measurements were conducted in both parallel and perpendicular directions to observe the dependence of the transport property on the charge carrier pathway under the influence of the interface, to achieve a higher thermoelectric P.F.. The parallel direction was 1.6 times higher than the perpendicular direction. (Received October 4, 2016; Accepted December 1, 2016)

keyword : thermoelectric, composite, PEDOT, PSS, Ge₂Sb₂Te₅

We have investigated the properties of nickel silicides as catalysts for dye-sensitized solar cells (DSSCs) by preparing several nickel silicide layers with a 100 nm-Ni/Si (100) structure using silicidation annealing for 30 minutes at 300 ℃ - 900 ℃. X-ray diffraction (XRD), four-point probe, field emission scanning electron microscope (FE-SEM), and cyclic-voltammetry (CV) analyses were carried out to measure the phase, sheet resistance, microstructure, and catalytic activity, respectively. The photovoltaic properties of DSSCs employing nickel silicides were characterized using a solar simulator and a potentiostat. The XRD, four-point probe, and FE-SEM results showed that island-like Ni2Si, uniform NiSi, and NiSi₂ were successfully formed under the given silicidation conditions. CV analysis determined that the catalytic activity of the Ni₂Si, NiSi, and NiSi₂, which exhibited values of 66.58%, 89.69% and 66.07%, respectively, compared to that of Pt catalyst. In particular, the energy conversion efficiency of the DSSC using NiSi was 3.29%. (Received November 23, 2016; Accepted January 10, 2017)

keyword : reduction catalyst, nickel silicide, annealing, catalytic activity, cyclic voltammetry