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An explosively formed penetrator (EFP) is a kind of kinetic energy warhead that contains a metal linear structure. After exploding, the linear structure in the EFP is deformed and hits the target as a form of aero-stable long-rod penetrator. To achieve high penetration performance, Ta is commonly used as the linear material due to its high density and formability. In this study, we investigated changes in the shape and microstructure of the Ta linear after explosion, especially for two cases; flying just before impact, and after impact. The computational simulation revealed that the disc shaped linear structure was elongated in the flying direction after the explosion, which was consistent with the results of the experiment. As the microstructure of the Ta linear could be changed during flight and after penetrating the target, samples of each stage were prepared. Microstructure analysis showed that severe deformation and dynamic recrystallization occurred repeatedly during flight after explosion, which seemed due to the increased temperature that results from the friction between the linear and air. After penetrating the target, the shape of the Ta linear was severely changed and grain size was drastically increased. At the moment when the EFP hit the target, the sharp increase in temperature would exert a critical impact on grain growth. (Received May 7, 2019; Accepted June 24, 2019)

keyword : explosively formed penetrator, linear, tantalum, microstructure

In this study, the effects of rolling temperature on the microstructural characteristics of a highspeed- rolled Mg alloy with a non-basal texture were investigated. To this end, commercial AZ31 alloy samples in which the basal poles of most grains were tilted at an angle of 45° from the normal direction (ND) to the transverse direction were hot-rolled with a rolling reduction of 80% at a rolling speed of 470 m/min at various temperatures (300 ℃, 350 ℃, and 400 ℃). With increasing rolling temperature, the area fraction of dynamically recrystallized (DRXed) grains increased and the internal strain energy accumulated in the rolled material decreased. This is attributed to the occurrence of homogeneous nucleation throughout the material at higher temperatures. When the rolling temperature increased from 300 ℃ to 350 ℃, the area fraction of coarse unDRXed grains decreased, but the size of the relatively fine DRXed grains increased. Consequently, the average grain size of the rolled material remained nearly unchanged. However, when the rolling temperature increased from 350 ℃ to 400 ℃, the size of the DRXed grains increased considerably owing to enhanced grain growth behavior. This increase led to an increase in the average grain size of the rolled material. The basal poles of the high-speed-rolled materials were mainly tilted 25° - 29° from the ND. This indicates that application of single-pass high-speed rolling causes the non-basal texture of the initial material to transform into a basal texture similar to the typical texture of rolled Mg alloys. (Received April 12, 2019; Accepted June 10, 2019)

keyword : Mg alloy, high-speed rolling, rolling temperature, texture, dynamic recrystallization, microstructure

Hydrogen-induced cracking (HIC) behavior was analyzed for gas metal arc (GMA) and laser beam (LB) welds on American Petroleum Institute (API) X65 steel. The GMA welds consisted of acicular ferrite (AF) with some widmanstatten ferrite (WF), while the LB welds had bainitic ferrite (BF) with some AF. The welds and heat affected zone (HAZ) of GMA exhibited a hardness of 220-250 HV, while those of the LB had a hardness of 230-290 HV. The LB welds and HAZ exceeded the hardness limit of 250 HV for pipeline steel, defined by the National Association of Corrosion Engineers (NACE) standard. Slow strain rate tests (SSRT) were performed in air and in-situ with hydrogen to observe HIC behavior. The ultimate tensile strength of the GMA welds decreased by 13%, while that of the LB welds decreased by 16% after hydrogen charging. Both welds showed a dimple fracture in the center and quasi-cleavage fracture along the edges. When austenite transforms to BF, it is known to grow along the directions of twins or the Kurdjumov-Sachs relation. These directions are strongly related to the coincidence site lattice, specifically Σ3 and Σ13b, and have good crack resistance. Because of this grain boundary characteristic, the LB welds with a BF microstructure showed good HIC behavior compared to the GMA welds. (Received May 7, 2019; Accepted June 12, 2019)

keyword : pipeline steel, hydrogen-induced cracking, coincidence site lattice, gas metal arc weld, laser beam weld

Terbium and dysprosium are rare earth elements which are employed in the manufacture of advanced materials. Separating Tb(III) and Dy(III) is very complicated because their chemical properties are similar. In order to develop a separation process based on solvent extraction, extraction and scrubbing experiments were conducted using weak hydrochloric solutions containing Tb(III) and Dy(III). In this work, three kinds of Cyanex 272, namely, single Cyanex 272, a mixture with Alamine 336 and an ionic liquid prepared with Aliquat 336 were employed. In the initial pH range between 1.5 and 5, the extraction behavior of both Tb(III) and Dy(III) was very similar using these extractants, resulting in low separation factor. In terms of extraction percentage and separation factor, the ionic liquid showed better performance than both single Cyanex 272 and the mixture. Scrubbing experiments were conducted using the loaded ionic liquid, employing a Dy(III) scrubbing solution. The optimum concentration of Dy(III) in the scrubbing solution was determined. A McCabe-Thiele diagram for the scrubbing of Tb(III) in the loaded ionic liquid with Dy(III) solution was constructed. In order to verify the separation of both metal ions, batch simulation cross-current scrubbing experiments were performed. After 6 cross-current scrubbing stages, the purity of Dy(III) in the loaded ionic liquid increased from 50% to 99%. By utilizing the data reported in this work, a process can be developed for the separation of Tb(III) and Dy(III) by solvent extraction. (Received April 5, 2019; Accepted June 17, 2019)

keyword : terbium(III), dysprosium(III), solvent extraction, scrubbing, separation

ZnO 50 nm/Ag 10 nm/SnO₂ 50 nm (ZAS) tri-layer films were deposited on a glass substrate by RF and DC magnetron sputtering and then underwent rapid thermal annealing in a low vacuum of 1×10^-3 Torr to investigate the effects of post-deposition annealing on the optical and electrical properties of the films. The peak intensity of the XRD pattern related to the ZnO (002) peak of the annealed films was higher than that of the as-deposited film and the full width at half-maximum of the ZnO (002) diffraction peak of the annealed films was smaller than that of the as-deposited film. Therefore, the crystallinity of ZnO was improved by rapid annealing. However, crystallization of the Ag interlayer and SnO₂ surface layer were not significantly affected by the annealing temperature, compared with the ZnO bottom layer. From the observed electrical properties and optical band gap, it was concluded that the blue shift in the optical band gap is related to the carrier density of the films. The band gap increased from 4.19 eV to 4.24 eV, with the carrier density increasing from 7.09 × 10²¹ cm^-3 to 7.77 × 10²¹ cm^-3. However, the film annealed at 450 ℃ showed a decreased band gap energy of 4.17 eV due to the decreased carrier density of 6.80 × 10²¹ cm^-3. The as-deposited ZAS films showed a sheet resistance of 11.0 Ω/□ and a visible transmittance of 80.8%, whereas the films annealed at 450 ℃ had a higher visible transmittance of 82.3% and a lower sheet resistance of 6.55 Ω/□. The results indicate that ZAS thin films may be possible substitutes for conventional Sn-doped In2O3 transparent electrodes in various optoelectronic devices. (Received May 13, 2019; Accepted June 20, 2019)

keyword : ZnO/Ag/SnO₂, magnetron sputtering, annealing, AFM, XRD

To decompose dye in aqueous solution, Pd nanoparticle-decorated TiO₂ nanotube photocatalysts were fabricated through electrochemical anodization and photoreduction processes. Morphological and surface characterization of the TiO₂/Pd heterojunction photocatalyst, as well as photoelectrochemical behaviors, were investigated by X-ray diffraction, UV/Vis diffuse reflectance and photocurrent response. It was found that the precipitated Pd particle size and distribution on TiO₂ surface were affected by the photoreduction time. As the photoreduction time increased from 10 min to 180 min, the corresponding average diameter of the precipitated Pd particles on TiO₂ photocatalyst increased from 4.35 nm to 9.29 nm. From the Aniline blue degradation results, the Pd decorated TiO₂ catalysts enhanced photocatalytic activity compared to bare TiO₂ catalyst, and the rate constant for dye degradation was dependent on the Pd particle size on the TiO₂ surface. For an average Pd particle size lower than 7.80 nm, the particles were uniformly precipitated on the surface of the TiO₂ nanotubes, and the dye degradation rate constants increased with increasing Pd particle size. However, for an average Pd particle size of more than 7.80 nm, the particles were agglomerated on the TiO₂ surface, and the dye degradation rate constants gradually decreased. With an optimum Pd particle size of 7.80 nm, the TiO₂/Pd photocatalyst showed higher photocatalytic activity and Aniline blue degradation rate, due to high photo absorbance response, and more efficient charge separation through electron transfer from the conduction band of the TiO₂ to the Pd particles. (Received May 9, 2019; Accepted June 11, 2019)

keyword : photoreduction, Pd nanoparticles, dye degradation, photocatalytic rate constant

To investigate the effect of anodic oxidation process parameters on TiO₂ nanotube formation in Ti-6Al-4V alloys, the composition of NH₄F and H₂O in the anodic oxidation processes were changed under the conditions of constant DC power ranging from 20 V to 60V and current density ranging from 10 mA to 30 mA. As the amount of NH₄F was increased, the surface reaction rate became faster and surface dissolution also became more active. This result can be explained by the fact that TiO₂ nanotubes were basically formed in the TiO₂ layer located in the front of the Ti matrix by selective dissolution of the oxide layer, due to F - ions existing in the electrolytes. In anodic oxidation in ethylene glycol + 0.2 wt% NH4F with different amounts of water ranging from 0 ~ 8 vol%, the increase in the amount of water resulted in an increase in both the diameter and the length of growing nanotubes. When the voltage increased, the diameter and the length of the nanotubes tended to increase. But no nanotubes were formed at the voltage and the current density lower than a certain critical value. An increase in electrolytic holding time resulted in the diameter of the nanotubes increasing and decreasing repeatedly, but eventually increasing. X-ray analysis indicated that the as-grown nanotubes were amorphous while the anatase phase and the rutile phase were detected after anodic oxidation treatment, followed by annealing at 650 ℃ under Ar atmosphere for 2 hours. (Received February 14, 2019; Accepted June 13, 2019)

keyword : Ti-6Al-4V alloy, anodic oxidation, TiO₂ nanotube, morphology

A Cu composite reinforced by a three-dimensionally interconnected graphene network was synthesized in situ by a simple two-step process utilizing compacted Cu powder (99% purity) as a template for the growth of graphene. Cu composites with different graphene contents were obtained by controlling the processing parameters. The composites were approximated to have two layers of graphene with curved shapes, high aspect ratios, and which were wrapped around Cu grains at the grain boundaries. The three-dimensionally interconnected graphene structure formed throughout the Cu matrix can act as a barrier to Cu diffusion and dislocation movement. Enhanced thermal conductivities (TCs) of 406 ± 5 W/mK and 385 ± 7 W/mK in the through- and in-plane directions, respectively, were obtained at room temperature for the Cu composite with a disc density of 8.16 g/cm³ and carbon content of ~ 73 ppm. The lower in-plane TC as compared to the through-plane TC could be explained by the increased number of defects as the measurement distance increased from 0.9 mm (through-plane) to 13 mm (in-plane). In conclusion, the spatial distribution of the three-dimensionally interconnected continuous graphene network throughout the Cu matrix must provide effective pathways for thermal conduction in the Cu composite, to thereby enable relatively high thermal conduction. (Received May 22, 2019; Accepted July 3, 2019)

keyword : graphene-Cu composite, 3D-interconnected graphene, Cu powder, chemical vapor deposition

We investigated the effects of materials and the film thickness of hole transport layers (HTLs) on inverted type perovskite solar cells using optics-charge transport coupled simulations. Power conversion efficiencies (PCEs), and the variations in efficiency induced by the film thickness dispersion, were intensively studied, to compare potential HTLs candidates like NiOx, PEDOT:PSS, CuSCN, and CuI. The optimum thickness of the solar cell layers differed based on the chosen combination of HTL and perovskite. It is suggested that the optoelectronic properties of HTLs like band gap, extinction coefficient, and refractive index can be used to determine the best ideal efficiencies, and sensitivity to process fluctuation. CuSCN showed the most promising behaviors, in that it can produce over-25% PCEs, and the lowest efficiency dispersion for various HTL thickness conditions. The best performance by CuSCN can be ascribed to its having a proper refractive index with the perovskite layer, and wide band gap characteristic. NiOx and CuI showed PCEs comparable to the CuSCN, but their efficiencies were sensitive to the varying thickness of the HTL. PEDOT:PSS exhibited the lowest simulated PCEs due to its small band gap. Our study suggests the best HTL candidates for inverted type perovskite solar cells, and demonstrates the importance of sophisticated numerical material studies, and device design, when developing highly efficient and robust perovskite solar cells. (Received April 30, 2019; Accepted June 10, 2019)

keyword : perovskite solar cells, hole transport layers, simulation, optimization, inverted type

The repairing parts were fabricated by PBF (powder bed fusion) using direct energy deposition (DED), interfacial cracks can occur at the interface between the substrate and the area to be repaired. Such interfacial defects are due to the thermal stress induced by the temperature gradient which results from repeated melting and solidification during the powder deposition by laser. These cracks degrade the mechanical properties of the repaired parts. Therefore, in this study, in-situ post heating is proposed to lower the cooling rate of deposited layers directly after repairing, in which several layers are additionally deposited onto the repair zone. To investigate the effect of the in-situ post heating, we have studied the microhardness, microstructure, and tensile properties according to the post heating, as well as the occurrence of cracks. The experimental results showed the formation of macro-scale cracks in the absence of post heating, whereas only micro-scale cracks (10 μm or less) were observed in the repaired sample with a low repair depth in the presence of post heating. Meanwhile, regardless of the use of in-situ post heating, complex dendritic structures (columnar and cellular shapes) were found on the deposited layer, which also appeared in the substrate built by PBF. Similar to the microstructure, no hardness changes were observed in the deposited layer; however, the hardness of the DED repaired zone tended to be slightly lower compared to that of the PBF substrate. For a specimen with a repair depth 1 mm, the tensile strength and elongation of the specimen repaired with the in-situ post heating increased by 8 and 13%, respectively, compared to the specimen repaired without the in-situ post heating. However, in the specimen with a large repair depth (2mm), macro-scale cracks occurred, which led to degradation of tensile properties. (Received March 28, 2019; Accepted June 11, 2019)

keyword : direct energy deposition (DED), crack, post heating, microhardness, tensile test