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In this study, the effects of nickel and copper on the mechanical and thermal properties of Al-Si alloys were investigated, for different alloy compositions. Thermodynamic calculations were carried out to predict the alloys’ solidification behavior (solidification temperature, solid fraction and physical properties (density, thermal conductivity, elastic modulus, bulk modulus)). The aluminum alloys were produced by high pressure die-casting process with a 125 ton die-casting machine. Microstructures in the Al-Si-Ni alloys were characterized by optical microscopy and scanning electron microscopy. The mechanical properties were evaluated by tensile tests. Thermal conductivity was measured using a laser flash method. The addition of Ni and Cu improved the mechanical properties of the alloys by different strengthening mechanisms. Solution strengthening was the more effective method of improving mechanical properties, compared with strengthening by intermetallic compounds. The addition of an appropriate concentration of soluble alloying elements consistently improved alloy strength. However, addition of alloying elements decreased the thermal conductivity of the primary alpha aluminum phase. EDS analysis confirmed that soluble alloying elements like Cu not only can form intermetallic compounds, but can also change other kinds of compounds. Accordingly, careful consideration should be given to the chemical composition of soluble/insoluble alloying elements when attempting to improve mechanical properties and thermal conductivity at the same time. (Received July 3, 2019; Accepted February 26, 2020)

keyword : aluminum, high pressure die casting, thermal conductivity, thermodynamic calculation, alloy design

Lattice structures are multi-functional materials with various advantages such as high specific stiffness, high energy absorption capacity and good thermal management capability. Recently, the development of manufacturing technologies using metal powders has facilitated fabrication of complex products; consequently, interest in lattice structures has grown. In this work, two kinds of lattice structures, pyramidal and tetrahedral, were designed and fabricated via a selective laser melting (SLM) process using stainless steel 316L powder. Scanning electron microscope (SEM) and optical microscope (OM) results revealed that lattice structures with various unit cell sizes and angles of inclination can be manufactured using SLM without the need for additional support structures. However, many unmelted and partially melted particles were observed on the surface of the lattice structures, which caused dimensional errors related to the struts. This research examined the effects of topology and unit cell design parameters on the macroscopic compressive behavior of lattice structures. Compressive characteristics, including elastic modulus, initial peak stress, strain energy absorption and mean stress, were evaluated through uniaxial compression tests. Lattice structures with the same relative density exhibited excellent elastic modulus, initial peak stress, energy absorption and mean stress results at inclination angles of 45-50°. These characteristics showed a tendency to increase with increasing relative density at the same inclination angle. The experimental results suggested these design parameters are the main factors influencing the mechanical characteristics of lattice structures. (Received November 18, 2019; Accepted February 4, 2020)

keyword : additive manufacturing, selective laser melting, lattice structures, 3D printing, stainless steel

Three kinds of Ni-based single crystals with the compositions of 63.8Ni-7.5Cr-5.1Co-4.8Al-1.9Mo-0.9Ti-3Re-11.8Ta-1.2W, 61.4Ni-7.4Cr-5Co-4.8Al-1.8Mo-0.9Ti-3.1Re-11.6Ta-4W, and 60.9Ni-7.5Cr-5Co-4.8Al-2Mo-1Ti-2.9Re-10.9Ta-1.2W-3.8Ru, in wt%, were cast in a Bridgman furnace. In the cast alloys, Cr, Co, Re, Mo, W, and Ru became microsegregated in dendrites consisting of γ-Ni, while Ni, Ta, and Al microsegregated in interdendrites consisting of eutectic γ/γ’. The cast alloys were oxidized at 1000 ℃ up to 275 h in air to study the effect of alloying elements on high-temperature oxidation. The oxide scales consisted primarily of CrTaO₄, with some NiCr₂O₄, NiO, and α-Al₂O₃. The oxidation resistance was dependent on the formation and continuity of the α-Al₂O₃ scale. Ta and W were beneficial, while Ru was harmful in improving the oxidation resistance. The selective oxidation of Al in dendrites led to the formation of thin, uniform α-Al₂O₃ scales, i.e., uniform oxidation. The competitive oxidation of active elements such as Al, Ti, and Ta in interdendrites led to the formation of porous, crack-susceptible oxide nodules, i.e., nodular oxidation. Less active elements such as Ru, Re, Ni, Co, Mo, W, and Cr tended to enrich in the vicinity of the oxide nodules. The oxidation progressed through the outward diffusion of cations and the inward diffusion of oxygen. This inward diffusion formed internal alumina islands, beneath the oxide scale. (Received June 12, 2019; accepted March 03, 2020)

keyword : nickel-based single crystal, oxidation, refractory element

For surface hardening of a continuous casting mold component, a thermal spray coating of NiCrBSi (Metco-16C) and CoCrWC (Stellite-1) was performed followed by laser heat treatment of the coatings. To support selective modification of the thermal spray coating, a metallurgically determined surface temperature was maintained during the laser heat treatment, by real-time control of the laser power. In other words, nonhomogeneities in the macrosegregation of certain alloying elements, and voids in the as-sprayed state, could be improved. The main microstructural features of the Metco-16C coating laser-heat-treated at 1423 K were nanosized (100-150 nm) Cr₅B₃, M₇C₃, and M₂₃C₆ precipitates with a lamellar structure of Ni (FCC) and Ni₃Si as the matrix phase. Those of the laser heat-treated Stellite- 1 coating at 1473 K were fine (30-250 nm) precipitates of WC, M₇C₃, and M₂₃C₆ based on a Co (FCC) matrix. The results show that laser heat treatment at 1423 K increased the hardness of the Mecto-16C coating to 1115 HV from the as-sprayed state (754 HV), while treatment at 1473 K increased the hardness of the Stellite-1 coating from 680 HV to 860 HV. (Received December 6, 2019; Accepted February 4, 2020)

keyword : thermal spray coating, laser heat treatment, carbides, borides

Lithium-doped zinc oxide (Zn_0.99Li_0.01O) film was manufactured by the sol-gel method using uniform and stable solution of zinc acetate dehydrate and lithium acetate dehydrate in methanol. Films were deposited by spin-coating using spinner between 4500 and 5000 rpm on silicon substrates. The prepared samples were sintered at various temperatures (600 ℃ ~ 1000 ℃) in the air. The structural, morphological, and optical properties of the prepared lithium-doped ZnO films were investigated. The XRD pattern of Zn_0.99Li_0.01O film demonstrated the hexagonal wurzite structure. However, new crystal phase was discovered at a sintering temperature of 800 ℃. New peak was found near 2θ = 22.6˚ in XRD patterns. The peak is thought to be the (101) plan in SiO₂ cristobalite structure. Moreover, another new crystal phase related to Li₂SiO₅ was occurred at a sintering temperature of 900 ℃. From XRD analysis, it was confirmed that C axis was decreased and the stress was increased, as sintering temperatures was increased from 600 ℃ to 900 ℃. In cathodo luminescence (CL) data, zinc oxide usually appears ultra violet and green emission. However, the green emission did not appear in all these samples used in this study. The ultra violet emission showed red shift from 600 ℃ to 800 ℃ in the CL spectrum as the sintering temperature was increased. The phenomenon of the red shift can be explained in Burstein-Moss effect. At sintering temperature of 700 ℃, the intensity of ultra-violet emission was the largest and full width at half maximum (FWHM) was the smallest. (Received January 6, 2020; Accepted March 9, 2020)

keyword : ZnO, cathodoluminescence, burstein-moss effect

In this study, Fe₂O₃ photoelectrode thin films were grown on fluorine-doped tin oxide substrates at various temperatures ranging from 145 to 220 ℃ using modified chemical bath deposition. The morphological, structural, electrical, and photoelectrochemical properties of the resulting Fe₂O₃ photoelectrode were analyzed using field emission scanning electron microscopy, X-ray diffraction, electrochemical impedance spectroscopy, and a three-electrode potentiostat/galvanostat, respectively. Growth temperature and hydrochloric acid etching both affected the growth of the Fe₂O₃ photoelectrode, with Fe₂O₃ thin film thickness first increasing and then decreasing as growth temperature increased. The pH value of the precursor solution varied according to growth temperature, which in turn affected film thickness. The highest photocurrent density (0.53 mA/cm² at 0.5 V vs. saturated calomel electrode) was obtained from the Fe₂O₃ photoelectrode grown at 190 ℃, which yielded the thickest thin film, smallest full width at half maximum and largest grain size for the (104) and (110) plane, and highest flat-band potential value. Based on these findings, the photoelectrochemical properties of Fe₂O₃ photoelectrodes grown at various temperatures are strongly affected by their morphological, structural, and electrical properties. (Received August 12, 2019; Accepted December 11, 2019)

keyword : Fe₂O₃, modified chemical bath deposition (M-CBD), growth temperature, HCl etching, photoelectrochemical (PEC)

Tetrahedrite has low lattice thermal conductivity because of the lone-pair electrons of Sb, which cause the Cu atoms to vibrate at a low frequency and high amplitude. The synthesis of tetrahedrite compounds by conventional melting methods requires a long-time reaction and annealing. However, a homogeneous and solid-state synthesis can be conducted in a short time using mechanical alloying (MA) because the volatilization of the constituent elements is inhibited and a subsequent heat treatment is not necessary. In this study, Bi-doped tetrahedrites Cu₁₂Sb_4-yBiyS₁₃ (y = 0-0.4) were prepared by MA and hot pressing. X-ray diffraction analyses revealed that all specimens consisted of single-phase tetrahedrite. However, with increasing Bi content, skinnerite Cu₃SbS₃ was detected. The electrical conductivity increased and the Seebeck coefficient deceased with increasing Bi content as result of the substitution of Bi at Sb sites. In addition, the thermal conductivity increased as the Bi content increased because of the increase in electronic thermal conductivity. A high dimensionless figure of merit of 0.88 was obtained at 723 K for Cu₁₂Sb_3.9Bi_0.1S₁₃. (Received January 6, 2020; Accepted February 3, 2020)

keyword : thermoelectric, tetrahedrite, mechanical alloying, hot pressing

Binder jetting additive manufacturing is one of the 3D printing technologies currently used to manufacture 3D geometries. In this process, a liquid binder agent is ejected to a desired position of a substrate. The binder’s properties and the jetting condition used for form droplets can affect the formability of the geometries. Herein, we optimized the solid content and jetting condition of a sodium silicate-based inorganic binder, for 3D printing. To observe the range of single droplet formation, the behavior of the discharged droplets was analyzed by Z value, which is the inverse of the Ohnesorge number. As the solid content increased, a higher driving voltage was required to form the droplets to overcome viscous dissipation. For 40S(Z = 4.33) with a content of 40 wt%, the droplet tail from the nozzle was stretched further. The droplets of 25S(Z = 15.09) with a content of 25 wt% were accompanied by satellite droplets. The jetting condition was optimized for 25S, which was capable of ejection at various driving voltages. Stable single droplets were formed at a driving voltage of 20 V and a dwell time of 4 μs. In addition, when ethylene glycol and glycerol were added into 25S as a humectant, stable droplets were formed under the optimum jetting condition, and each droplets was in the range of 2.70 < Z < 15.09. (Received October 29, 2019; Accepted February 25, 2020)

keyword : 3D printing, binder jetting, inorganic binder, jettability, nozzle clogging, ohnesorge number

In this work, LiOH was synthesized using highly soluble Li₂SO₄. To enhance efficiency, this synthesis was performed using the precipitation method, and the correlation between each experimental condition and the synthesis of LiOH was investigated. The particle size and crystalline properties were tailored by controlling various experimental conditions, including the mole ratio of [Li]/[OH](Li: lithium sulfate, OH: barium hydroxide), reaction temperature, and reaction time. First, precursors with a ratio of 1:0.5 were reacted for 60 min at a solution temperature of 40 ℃ and filtered to remove precipitates. For the double reaction, half the hydroxyl precursor was added to the filtered solution and reacted under the same conditions. Using two-step precipitation, we were able to synthesize powder with a pure LiOH phase, a particle mean size of 100 nm, and purity over 99%. (Received November 18, 2019; Accepted February 14, 2020)

keyword : LIBs, lithium hydroxide, lithium sulfate, precipitation, nanoparticles