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Because of their superior optical, and electrical properties, lead halide perovskites have emerged as potential materials for light-emitting-diodes. However, some obstacles have yet to be overcome for practical applications including low exciton binding energy, the difficulty of fabricating uniform perovskite films,, and their instability to heat, light, and moisture. In this review, we focus on strategies to improve the lifetime, and stability of perovskites such as core-shell structure, ligand exchange, and doping. (Received August 31, 2021; Accepted September 27, 2021)

keyword : halide perovskite, stability, core-shell, ligand exchange, doping

The metallic binder in WC-Co hard metals was effectively strengthened using the solid solution phases of Co and W. These metallic phases of Co and W (Co_1-xW_x, x<1), which consist of two kinds of structures (FCC and HCP), were successfully formed by hydrogen reduction of milled oxides mixtures (Co₃O₄ and WO₃) at over 1000 ℃. When hard metals are fabricated by pressureless sintering of mixed WC and Co_1-xW_x, the hard metals containing the WC₂ and M₆C phases (Co₂W₄C and Co₄W₂C) have brittleness, which degrades their mechanical properties, like hard metals fabricated from mixtures of WC, Co, and W. By rapidly sintering the WC-Co_1-xW_x hard metals for 5 min the W₂C and M₆C phases were eliminated, and a two-phase (WC and the metallic phase of Co and W) region was successfully obtained. The mechanical properties of the WC-Co_1-xW_x hard metals showed higher values for both hardness (max. 18.8 GPa) and fracture toughness (8.5 MPa·m^1/2) than conventional WC-Co hard metal (HV: 15.9 GPa, K_IC: 6.9 MPa·m^1/2). The enhancement in toughness was attributed to the solid solution strengthening of the metallic binder and the elimination of the WC₂ and M₆C phases. The suppression of grain growth due to the short duration of sintering also played a positive role in improving the hardness of the WC-Co_1-xW_x hard metals. The phase-controlled solid solution metallic binder could be the key material to enhance the hardness and toughness of hard metals. (Received August 31, 2021; Accepted October 13, 2021)

keyword : hard metal, solid solution, binder, spark plasma sintering, hardness, Toughness

This study was conducted to evaluate the corrosion resistance and optimize the heat-treatment process of AISI 439 ferrite stainless steel silicon and tin alloys with reduced chromium. The microstructure of the specimens and deposition under each condition were analyzed. The production of oxide films was compared based on the thickness of the film and the change in the contents of each element. In addition, electrochemical analyses of each heat-treatment condition was used to quantitatively compare corrosion resistance and passive film stability based on the relative chromium, silicon, and tin contents. It was found that the addition of silicon and tin compensated for the decrease in corrosion resistance induced by the chromium reduction. The addition of the two elements inhibited iron (Fe) oxide production in the surface oxide film, thereby improving the corrosion resistance of the material and improving the stability of the passive film. Moreover, the SiO₂ and SnO₂ layers inhibited the production of Fe oxide and contributed to the stability of the film along with Cr₂O₃, the main component of the passive film. However, when the heat treatment temperature increased above a specific temperature, the oxide inhibitory effect of the two elements was relatively offset. Nevertheless, further research to optimize the content of the three elements will help develop materials with superior mechanical properties and corrosion resistance. (Received June 26, 2021; Accepted October 18, 2021)

keyword : ferrite stainless steel, corrosion resistance, oxide layers, passive film, pitting corrosion

This study examined the effects of minor alloying elements (C, Ni, Cr, and Mo) on the long-term corrosion behaviors of ultrahigh-strength automotive steel sheets with a tensile strength of more than 1800 MPa. A range of experimental and analytical results showed that the addition of Ni, Cr, and Mo decreased the corrosion current density and weight loss in electrochemical and immersion tests, respectively, in a neutral aqueous condition. This suggests that the minor addition of elements to steel can result in improved corrosion resistance even for long-term immersion periods. This is closely associated with the formation of thin and stable corrosion scale on the surface, which was enriched with the alloying elements (Ni, Cr, and Mo). On the other hand, their beneficial effects did not persist during the prolonged immersion periods in steel with a higher C content, suggesting that the beneficial effects of the minor addition of Ni, Cr, and Mo were overridden by the detrimental effects of a higher C content as the immersion time was increased. Based on these results, lower C and the optimal use of Ni, Cr, and Mo are suggested as a desirable alloy design strategy for developing ultrahigh-strength steel sheets that can be exposed frequently to a neutral aqueous environment. (Received August 17, 2021; Accepted November 18, 2021)

keyword : ultrahigh-strength steel, carbon, nickel, chromium, molybdenum, corrosion

This study evaluated changes in delta-ferrite content depending on the preheating of AISI 316L stainless steel. We also determined the reasons for the variation in delta-ferrite content, which affects corrosion resistance. Changes in delta-ferrite content after preheating was confirmed using a Feritscope, and the microstructure was analyzed using optical microscopy (OM). We found that the delta-ferrite microstructure size decreased when preheating time was increased at 1295 ℃, and that the delta-ferrite content could be controlled through preheating. Potentiodynamic polarization test were carried out in NaCl (0.5 M) + H₂SO₄ (0.5 M) solution, and it was found that higher delta-ferrite content resulted in less corrosion potential and passive potential. To determine the cause, an analysis was conducted using energy-dispersive spectroscopy (EDS), which confirmed that higher delta-ferrite content led to weaker corrosion resistance, due to Cr degradation at the delta-ferrite and austenite boundaries. The degradation of Cr on the boundaries between austenite and delta-ferrite can be explained by the difference in the diffusion coefficient of Cr in the ferrite and austenite. A scanning electron microscopy (SEM) analysis of material used for actual semiconductor piping confirmed that corrosion begins at the delta-ferrite and austenite boundaries. These results confirm the need to control delta-ferrite content in AISI 316L stainless steel used for semiconductor piping. (Received August 26, 2021; Accepted October 5, 2021)

keyword : AISI316L, semiconductor piping, delta-ferrite, corrosion resistance

In this paper, we suggest a novel recycling process for hard metal sludge that does not use ammonium paratungstate. Ammonia, which in the conventional recycling process is essential for removing sodium and crystallized tungstate, was not used in the novel process. Instead of ammonia, acid was used to remove the sodium and crystallized tungstate resulting in the formation of tungstic acid (H₂WO₄). Tungsten powders were successfully synthesized by hydrogen reduction of the tungstic acid through H₂O decomposition, WO₃ to WO₂ reduction, and tungsten metal formation. The tungsten powders prepared from tungstic acid were spherical in shape and had a higher sintering density than the facet-shaped tungsten powders prepared from tungsten oxide. The spherical shape of the tungsten powders enhanced their sinterability and resulted in an increase in the size of grains. This is a result of the high diffusion rate of the atoms along the particle surfaces. Despite having a higher density, the hardness of the sintered tungsten was lower than that of tungsten from tungsten oxide. High energy milling effectively reduced grain size and improved hardness. The hardness of the tungsten prepared from milled tungstic acid was enhanced to a value (max. 471 HV) higher than the best previously reported value (389 HV). In sum, tungsten can be hardened, thereby improving its sinterability and reducing grain size, with tungstic acid prepared using the proposed recycling process. (Received August 31, 2021; Accepted September 29, 2021)

keyword : hard metal, sludge, recycling, tungsten, sintering, hardness

In the Fourth industrial age, there is increasing use of electronic devices with high frequency (GHz) operating circuits for radio wave transmission/reception. This can lead to electromagnetic noise, and malfunctions in nearby devices. Electromagnetic shielding technology has emerged as an important way of preventing device malfunctions due to noise, and interest in shielding materials for electromagnetic waves has also increased. To allow compact integration and light weight electronic devices, highly efficient, thin and multifunctional film materials are required. This study selected the Zn-Ni alloy, which has adequate corrosion resistance, to protect the metal parent material of electronic components. Various compositions of the alloy were deposited using magnetron sputtering. Phase formation and composition were confirmed through XRD and SEM and EDS. The surface resistance of the thin films was measured using the 4point probe method, to calculate the shielding rate of the thin films. The electromagnetic wave shielding/absorption rate then measured according to frequency and the results compared with the calculated values. Corrosion resistance was evaluated with a polarization test. The far field electromagnetic shielding/absorption rate increased as the Zn content increased, up to 52 dB in a film with 70 at% of Zn. Corrosion resistivity behavior was the opposite. However, the Ni₅Zn₂₁ phase formation, which occurred in films with lower Zn composition, helped to improve electromagnetic absorption in the near field as well as corrosion resistivity. Therefore, the optimum composition of the Zn-Ni films was provisionally determined to be around Ni: Zn = 43:57 for electronic devices using electromagnetic waves in the near field range. (Received August 25, 2021; Accepted October 15, 2021)

keyword : EMI shielding, Zn-Ni film, magnetron-sputtering, metallic material, GHz

A copper (Cu) metal-ceramic filler composite with high thermal conductivity and a suitable thermal expansion coefficient was designed for application as a high-performance heat dissipation material. The purpose of the designed material was to utilize the high thermal conductivity of Cu while lowering its high coefficient of thermal expansion by using a ceramic filler. In this study, a Cu-sol containing a certain amount of AlN or SiC ceramic filler was prepared using a non-aqueous solvent. A complex was produced by applying a PVB polymer to prepare a homogeneous precursor. The composite sintered without pressure in a reducing atmosphere showed low thermal conductivity due to residual pores, but the hot press sintered composite exhibited improved thermal conductivity. The Cu composite with 30 wt% AlN filler added exhibited a thermal conductivity of 290 W/m·K and a thermal expansion coefficient of 9.2 × 10^-6/℃. Due to the pores in the composite, the thermal conductivity showed some difference from the theoretical value calculated from the rule of mixture. However, the thermal expansion coefficient did not show any significant difference. (Received June 11, 2021; Accepted September 27, 2021)

keyword : metal-ceramic composite, thermal conductivity, thermal expansion coefficient, polymer solution method, hot press sintering

Smartphones were launched in the market as a product in the early 2000’s. Today, less than 20 years later, the smartphone has become one of the most frequently used devices for human contact. The market is saturated, and competition between products has intensified, and it is not difficult to find products that are culled. To be selected by consumers, product differentiation must be outstanding, which is not easy when the basic technology is standardized. Currently, competition in the fields of cameras and displays is intensifying, but it is expected that advanced technology will become commonplace in the near future. There are several ways to increase the competitiveness and differentiation of a product, for example, by considering the technical aspects, or making the personality of the product unique. Consumers communicate with products through materials. The process and selection of materials suitable for the design requirements of a product is a very important component in competitive product manufacturing. The final material selection is performed by dividing the design requirements into function, constraints, personality, objectives, and supporting information aspects, and the selecting materials suitable for each stage. By capturing the overall concept as a function, the processes of selecting materials that meet the constraints, determining the type of material that matches the personality, and using the objectives to obtain the top-ranked materials, the final material suitable for all conditions can be selected by referring to supporting information from the top-ranked materials. (Received August 23, 2021; Accepted October 18, 2021)

keyword : design requirements, material perception, material selection, product personality, smartphone rear panel

The effects of dissolution of the η′ phase by solution treatment on the mechanical properties of A7075-T6 alloy were investigated. Immediately after solution treatment of the T6 sheet at 450 ℃ or higher, elongation significantly increased and dissolution of the η′ phase occurred. η′ is the main hardening phase. After natural-aging, GPI, which is coherent with the aluminum matrix, was formed and strength increased. When bake hardening after natural-aging was performed, the yield strength slightly increased due to partial dissolution of the GPI and re-precipitation of the η′ phase. In contrast, after solution treatment at 400 ℃, there was less elongation increase due to the precipitation of the coarse η phase at grain boundaries and low dissolution of the η′ phase. In addition, when bake hardening after natural-aging was performed, the yield strength decreased due to insufficient GPI, which is the nucleation site of the η′ phase. To promote re-precipitation of the η′ phase, the solution treatment temperature was set to a level that would increase solubility. As a result, the yield strength was significantly increased through re-precipitation of a large number of fine and uniform η′ phase. In addition, to increase the effect of dissolution, a pre-aging treatment was introduced and the bake hardenability can be improved after dissolution. (Received August 6, 2021; Accepted October 1, 2021)

keyword : Al-Zn-Mg-Cu, 7075, dissolution, precipitation, mechanical properties