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ZrB₂ is considered a candidate material for ultra-high temperature ceramics because of its high thermal conductivity, high melting point, and low coefficient of thermal expansion. Despite these attractive properties, applications of ZrB₂ are limited by its low fracture toughness below the brittle-ductile transition temperature. To improve its ductile properties, the approach universally utilized has been to add a second material to form composites and fabricate nanostructured materials. One example of this is the adding of SiC to ZrB₂ to improve fracture toughness. SiC has low density, excellent resistance to oxidation in air, and a high melting point. Therefore, SiC may be a promising additive as a reinforcing material for ZrB₂-based composites. A dense nanostructured ZrB₂-SiC composite was rapidly synthesized and sintered by high-frequency induction heating (HFIH) within 4 min in one step, from mechanically activated powders of ZrC, 2B and Si. Simultaneous combustion synthesis and consolidation were accomplished using the combination of current and mechanical pressure. A highly dense ZrB₂-SiC composite with a relative density of up to 98.4% was fabricated using the simultaneous application of 70 MPa pressure and an induced current. The mechanical properties (toughness and hardness) and the average grain size of the composite were investigated. (Received April 2, 2021; Accepted April 8, 2021)

keyword : composite, synthesis, nanomaterials, mechanical properties, ZrB_2-SiC

The metallurgical aspects of weld cracking in Ni-based superalloys remain relatively unexplored in existing research. The present study performed comprehensive metallurgical and manufactural investigations into the weldability of an Ni-based superalloy, CM247LC, from the viewpoint of the liquation cracking behavior and its susceptibility. Metallurgical solutions to suppress the liquation-cracking susceptibility were derived via the visualization-based Varestraint test, and the possibility of liquation crack-free welding was explored by employing pre-weld heat treatments and laser beam welding. The alloy that was subjected to aging treatment exhibited the lowest liquation-cracking susceptibility (liquation cracking temperature range: 66 K), while the as-cast alloy specimen exhibited the highest liquation-cracking susceptibility (liquation cracking temperature range: 620 K). The metallurgical mechanisms of the liquation cracking susceptibility of as-cast CM247LC weld were elucidated via microstructural analyses and thermodynamic calculations. The suppressed liquation cracking susceptibility of the aged CM247LC can be attributed to the MC-type carbide fraction and homogenized matrix phase, as compared with those of as-cast CM247LC. The aged CM247LC specimen was subjected to gas tungsten arc welding to validate its minimal liquation-cracking susceptibility. The results confirmed the suppression of liquation cracking, due to the low susceptibility of the specimen. However, crackfree welds could not be obtained. Finally, metallurgically sound welds without liquation cracks were successfully obtained via laser beam welding. The outcomes of the present study will facilitate the generation of electric power from fossil fuels via a clean and efficient gas turbine-based power generation cycle. (Received March 22, 2021; Accepted April 9, 2021)

keyword : CM247LC superalloy, heat treatment, liquation cracking susceptibility, varestraint test, visualization, microstructure

This study was carried out to obtain data to design a process to recover rare earth elements, specifically Y(Yttrium), La(Lanthanum), Ce(Cerium), Eu(Europium), Tb(Terbium) from waste phosphor powder. For this purpose, we investigated the effect of temperature, concentration, time and acids on leaching of the rare earth elements. The effect of roasting temperature, roasting time, roasting agent and its dosage on the leaching of rare earth elements were also investigated. 92% of the Yttrium, 70% of the Europium and 8% of the Cerium contained in the waste phosphor powder was leached at the condition of 50 ℃and 0.3N HCl solution for 3hours. However, Terbium and Lanthanum were never leached at this condition. The leaching ratio increased to 100% of Yttrium and Europium, 98% of Cerium, 92% of Terbium and 89% of Lanthanum by leaching after soda ash roasting. In the leaching experiment with unroasted phosphor at 80 ℃, the initial leaching reaction rate of Yttrium was 0.035 mol/L·s in 0.3N sulfuric acid solution, 0.033 mol/L·s in nitric acid solution and 0.028 mol/L·s in 0.3N hydrochloric acid solution. And the initial leaching reaction rate of Europium was 0.0017 mol/L·s in 0.3N sulfuric acid solution, 0.00114 mol/L·s in nitric acid solution and 0.00113 mol/L·s in 0.3N hydrochloric acid solution. For Cerium, the initial leaching reaction rate was 0.00019 mol/L·s in 0.3N sulfuric acid solution, 0.00025 mol/L·s in nitric acid solution and 0.00014 mol/L·s in 0.3N hydrochloric acid solution. (Received February 16, 2021; Accepted April 26, 2021)

keyword : leaching, recover, rare earth, waste phosphor, soda ash roasting

Spent electroplating solutions contain small amounts of Pd(II). Cementation of Pd(II) with zinc metal powder is practiced to concentrate the Pd. Dissolution of the cemented Pd metal is necessary to recover pure Pd metal or compounds. In this work, the leaching behavior of Pd metal in inorganic acid solutions (hydrochloric and sulfuric acid) in the presence of an oxidizing agent such as H₂O₂, NaClO₃, or NaClO was investigated. To determine the optimum conditions for Pd leaching, experiments were conducted by adjusting the concentration of the acids and oxidizing agents, reaction temperature and time, and pulp density. Complete leaching of Pd was possible using a hydrochloric acid solution with 3 kinds of oxidizing agents, whereas only NaClO was effective in the leaching with sulfuric acid solution. The chloride ions of HCl and NaClO₃/NaClO play an important role in enhancing the leaching of Pd, due to the formation of Pd chlorocomplexes and by decreasing the reduction potential of Pd(II). Compared to H₂O₂, NaClO₃ and NaClO showed some merits as oxidizing agents in terms of reaction time, temperature and low acid concentration. Based on the optimum leaching conditions of single Pd metal, the conditions for the complete leaching of Pd and Zn from the metallic mixtures were obtained. (Received March 24, 2021; Accepted April 26, 2021)

keyword : leaching, inorganic acid, oxidizing agent, palladium, zinc

We report highly efficient and robust quantum dot light-emitting diodes (QLEDs) with Li-doped TiO₂ nanoparticles (NPs) as an electron transport layer (ETL). As core materials, ZnO-based inorganic NPs can enhance the performance of QLEDs due to their suitable energy level and solution processability. However, their fast electron mobility and instability in organic solvents are two main obstacles to practical display applications. The colloidal stability of TiO₂ NPs in ethanol was confirmed after three day-storage, while ZnO NPs showed severe agglomeration. Inverted structure QLEDs using 3% Li-doped TiO₂ NP were successfully fabricated and their optical/electrical properties were investigated. With 3% Li-doped TiO₂ NPs, the charge balance in the emitting layer of the QLEDs was improved, which resulted in a maximum luminance of 159,840 cd/㎥ and external quantum efficiency (EQE) of 9.12%. These results were comparable to the performance of QLEDs with commonly used ZnO NPs. Moreover, the QLEDs with the Li-doped TiO₂ NPs showed more stable characteristics than those with ZnO NPs after 7 days in ambient conditions. The EQE of the QLEDs with Li-doped TiO₂ NPs was reduced by only 4.9%. These results indicate that Li-doped TiO₂ NPs show great promise for use as a solution based inorganic ETL for QLEDs. (Received March 23, 2021; Accepted April 16, 2021)

keyword : quantum dot, TiO_{2, nanoparticles, QLED}

We attempted to maximize the transmittance of 2D NiTe₂ thin film using the liquid-phase exfoliation (LPE) process to confirm the applicability of NiTe₂ as a transparent electrode. The LPE process, using a co-solvent of organic solvent and water, is a stable and efficient method of increasing transmittance at low cost. In this report, the effect of 12 different co-solvents, mixtures of acetone, ethanol, isopropyl alcohol, and water, on exfoliation rate was studied. NiTe₂ thin film with a thickness of 6.3 nm prepared by sputtering, and exhibited a highest transmittance of 68% and a lowest resistivity of 291 μΩ·cm after 12 hrs sonication in ethanol/water co-solvent (ethanol : water = 60 : 40 vol. %). Three physical properties, polarization and dispersion ratio (p/d ratio), boiling point, and water contents, were compared to determine which property was the main control factor for the LPE process. Unlike previous LPE processes for powders of 2D materials, it was revealed that the improvement in the transmittance of the NiTe₂ thin film was more strongly dependent on both of vol.% of water and boiling point of the solvents. This was because the transmittance improved after removing the NiTe₂ thin film from the substrate, rather than layer by layer exfoliation. We believe that NiTe₂ thin film prepared by sputtering followed by exfoliation process can be one of the potential candidates for transparent electrode. (Received April 7, 2021; Accepted April 24, 2021)

keyword : nickel ditelluride, 2D materials, sputtering, co-solvent, exfoliation

Transition-metal-based layered double hydroxides (LDHs) have attracted substantial attention as highly efficient oxygen evolution reaction (OER) catalysts because they are earth-abundant, low-cost, and environmentally friendly materials with favorable adsorption/desorption energies for intermittent reactants. However, the application of these LDHs as high-performance electrocatalysts is often hindered by their relatively sluggish electronic transport kinetics resulting from their intrinsically low conductivity. Here, we report the effects of incorporating a metalloid into transition metal LDHs on their electrocatalytic activity. In this study, Te-incorporated NiCo LDH (χTe-NiCo LDH) was grown on a three-dimensional porous nickel foam (NF) using a facile solvothermal method with χ = 0.2, 0.4, 0.6 and 0.8. The crystal structure and surface nanostructure were investigated by X-ray diffraction and field-emission scanning electron microscopy. A homogeneous nanosheet structure on the NF was clearly observed for the NiCo LDH and χTe-NiCo (χ = 0.2, 0.4, 0.6) LDHs. However, irregular and collapsed nanostructures were found on the surface of the NF when the Te precursor ratio (χ) exceeded 0.6. The electrocatalytic OER properties were analyzed by linear sweep voltammetry and electrochemical impedance spectroscopy. The amount of Te used in the electrocatalytic reaction was found to play a crucial role in improving the catalytic activity. The optimum Te amount (χ) introduced into the NiCo LDH is discussed with respect to the OER performance. (Received March 22, 2021; Accepted April 12, 2021)

keyword : electrocatalyst, oxygen evolution reaction, tellurium, layered double hydroxide, nickel foam

Solid solutions and composites of a mixture of (K_0.5Na_0.5)NbO₃ (KNN) and SrTiO₃ (ST) were respectively synthesized by different process routes using a conventional solid state reaction. All starting materials were mixed simultaneously and calcined at 800 ℃ for 4 h to obtain the solid solutions of KNN-ST. Meanwhile, the composites of KNN-ST were obtained by mixing the synthesized KNN and synthesized ST, respectively. KNN-ST calcined powders were sintered at 1150, 1250 and 1300℃for 2 h. The calcined powders, in which all starting materials were mixed at once, showed a single-phase pseudo-cubic structure after sintering. On the other hand, the powders with the mixture of KNN and ST had two phases of cubic ST and orthorhombic KNN as composites. The dielectric loss values of all samples decreased as the sintering temperature increased because of increased densification. The dielectric constants of the solid solutions were increased as the sintering temperature increased, however, the dielectric constants of the composites decreased with increasing sintering temperature. It was concluded that, for enhanced dielectric properties, the dielectric materials should be synthesized as solid solutions. (Received April 12, 2021; Accepted April 29, 2021)

keyword : sodium potassium niobate, microstructure, solid solution, crystal structure, dielectric property

A three-dimensionally interconnected hexagonal boron nitride (3Di-hBN) networked Cu-Ni (3DihBN- Cu-Ni) composite was successfully synthesized in situ using a simple two-step process which involved the compaction of mixed Cu-Ni powders (70 wt.% Cu and 30 wt.% Ni) into a disc followed by metal-organic chemical vapor deposition (MOCVD) at 1000 ℃. During MOCVD, the Cu-Ni alloy grains acted as a template for the growth of hexagonal boron nitride (hBN) while decaborane and ammonia were used as precursors for boron and nitrogen, respectively. Boron and nitrogen atoms diffused into the Cu-Ni solution during the MOCVD process and precipitated out along the Cu-Ni interfaces upon cooling, resulting in the formation of the 3Di hBN-Cu-Ni composite. Energy-dispersive spectroscopic analysis confirmed the presence of boron and nitrogen atoms at the interfaces of Cu-Ni alloy grains. Optical microscopy examination indicated that there was a minimum amount of bulk hBN at a certain compaction pressure (280 MPa) and sintering time (30 min). Scanning electron microscopy and transmission electron microscopy revealed that an interconnected network of hBN layers surrounding the Cu-Ni grains developed in the 3Di-hBN-Cu-Ni composite. This 3Di-hBN network is expected to enhance the mechanical, thermal, and chemical properties of the 3Di-hBN-Cu-Ni composite. Moreover, the foam-like 3Di-hBN extracted from 3Di-hBN-Cu-Ni composite could have further applications in the fields of biomedicine and energy storage. (Received March 15, 2021; Accepted April 7, 2021)

keyword : hexagonal boron nitride, reinforcement, 3Di-hBN-Cu-Ni composite, MOCVD