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The effects of the electrogalvanizing conditions (a combination of plating current and time) on hydrogen infusion, embrittlement, and corrosion-induced hydrogen embrittlement (HE) behaviors of ultra-high strength steel were examined. A range of experimental and analytical methods, including electrochemical impedance spectroscopy, hydrogen permeation, polarization, and slow strain rate test, were employed. The results showed that the applied cathodic current density during electrogalvanizing had an inverse relationship with the Zn crystalline size. A smaller cathodic current density and longer plating time led to a larger crystalline size, resulting in a higher infusion rate of hydrogen atoms, and HE-sensitivity (i.e., mechanical degradation with larger density of secondary crack in fracture surface). On the other hand, a larger cathodic current density and shorter plating time caused a higher anodic dissolution rate and smaller polarization resistance of the coating when exposed to neutral aqueous environments. Hence, a higher rate of galvanic corrosion between the coating and exposed steel substrate (e.g., locally damaged areas around coating layer) resulted in a higher infusion rate of hydrogen atoms and HE-sensitivity. This study provides insight into the desirable plating conditions for electro-Zn plating on ultra-high strength steels with enhanced resistance to hydrogen infusion and embrittlement, induced by aqueous corrosion. (Received 27 September, 2022; Accepted 28 December, 2022)

keyword : ultra-high strength steel, electro-galvanizing, hydrogen absorption, hydrogen embrittlement, corrosion

Carbon-based thin film tool coatings, such as diamond-like carbon (DLC), have excellent low-friction and anti-sticking properties. These thin films are widely used for the cutting and machining of increasingly widely-used lightweight non-metallic and non-ferrous metal materials, as a part of countermeasures against global warming. However, non-metallic and non-ferrous metal materials are significantly inferior in strength and heat resistance compared to iron-based metals. Therefore, they are primarily employed in high-content fiber reinforced composite materials, which significantly improves their mechanical and thermal properties. Tetrahedral amorphous carbon (TAC) coating has a hardness level similar to diamond coating. However, when TAC is deposited as a thick film, delamination of the coating layer may occur because of the high internal compressive stress between the carbide-based substrate and coating layer, thereby restricting its scalability to other applications. Other factors to be controlled for thick film TAC deposition include minimizing droplets generated during the coating process, and improving interfacial properties like hardness and fatigue resistance. Here, C in the form of CH4, which has high solubility over Cr and forms various compounds, was added during the interfacial deposition process, between the carbide and TAC, to improve interfacial strength and adhesion by precipitation of carbide at the interface. This eventually led to thick TAC film with the thickness and adhesion of commercially viable thick film. (Received 5 December, 2022; Accepted 20 December, 2022)

keyword : TAC, tetrabond, CrC, carbide, interface, thick film

The effects of fabrication method on the microstructure and sinterability of W-1 wt% Y₂O₃ were analyzed. W composite powders dispersed with Y₂O₃ particles were synthesized by the ultrasonic spray pyrolysis process or the ultrasonic spray pyrolysis/polymer solution process. A dense composite was fabricated by a combination of spark plasma sintering and final hot isostatic pressing. The powder synthesized by the ultrasonic spray pyrolysis had fine dispersed particles on the surface of the cubic powder and was composed of W, Y₂O₃ and W-oxides. On the other hand, in the case of the ultrasonic spray pyrolysis/polymer solution process, the nano-sized particles formed agglomerates and existed only as pure W and Y₂O₃ phases. All the sintered compacts treated with HIP showed an increase in relative density, and the sintered compacts of the powder synthesized by the ultrasonic spray pyrolysis/polymer solution process showed a maximum relative density of 97~99% and a fine grain size. The change in microstructure with different powder processing was explained by the presence of W-oxide and the size and distribution of Y₂O₃ particles. The Vickers hardness of the sintered compact reached the largest value of about 5 GPa in the powder synthesized by the ultrasonic spray pyrolysis/polymer solution process, which was interpreted to be a result of the relatively high density and decreased grain size. (Received 21 October, 2022; Accepted 8 December, 2022)

keyword : W-Y₂O₃, ultrasonic spray pyrolysis, spark plasma sintering, hot isostatic pressing, properties

We demonstrated a bonding technology using polyethyleneimine (PEI) as a bonding layer between printed Ag electrodes and an emission material layer (EML) to fabricate all-solution-processed Organic Light-Emitting Diodes (OLEDs). We manufactured Ag electrodes on a glass substrate by the screen printing method, which was bonded to another substrate with ITO anodes and an EML layer using a PEI layer. Since the bonding layer needs to have both good bonding characteristics and electrical conductivity, we investigated how both characteristics were affected by some experimental factors, namely, PEI concentration, the thickness of the layer and additives. The bonding strength and the electrical current density were investigated by tensile tests and electron only device (EOD) experiments, respectively. The results showed that at higher PEI concentration the bonding strength reached a higher value, but the electrical current through the PEI layer decreased rapidly with increased PEI layer thickness. When sorbitol was added into the PEI solution, both bonding strength and electrical conductivity were improved, and when 10 wt% of sorbitol was added into 0.1 wt% PEI solution, the device showed an electrical current density of 1,000 mA/㎠ and a good bonding strength also. Finally, we manufactured a two-substrate OLED device using the bonding layer and measured the luminance, which revealed that the device turned on at 4 V and the maximum luminance was 7,000 cd/㎡. These data, both electrical and optical, demonstrate the potential to fabricate all-solution-processed OLEDs using the two-substrate bonding technology. (Received 29 November, 2022; Accepted 19 December, 2022)

keyword : OLED, solution process, PEI, bonding property, Ag, screen printing

Perovskite manganese La_0.7Sr_0.3MnO₃ (LSMO) powders were prepared by sol-gel synthesis and calcination in the temperature (T) range of 500~1200 ℃ and their structures and electromagnetic (EM) wave absorption properties were investigated. X-ray diffraction (XRD) analysis revealed that the crystalline perovskite phase can be formed at T ≥ 600 ℃. The average grain size was ~15 nm at the calcination temperature (T_cal) = 600 ℃ and it increased to ~1 μm when T_cal increased up to 1200 ℃. MH curves of the samples showed soft magnetic behaviors for all the crystallized samples, and the value of saturation magnetization increased with increasing T_cal. The real and imaginary parts of permittivities and permeabilities were measured on LSMO powder-epoxy (10 wt%) composites using a network vector analyzer in the frequency range of 100 MHz ≤ f ≤ 18GHz. The complex permittivities (ε', ε") increased significantly in samples calcined above 800 ℃ because the concentration of free electrons increased, due to the LSMO's unique magnetotransport effect, as the crystallinity and the M_S value increased significantly. As the T_cal increased, the height of the μ' and μ" spectra also gradually increased. The large values of ε', ε" of the LSMO-epoxy are dominant factors in the EM wave absorption characteristics, and it showed good absorption characteristics when it had a thickness of 1.5 mm or less at a frequency of 12 GHz or higher. At the same time, it also exhibited EM wave shielding ability by reflection in the several GHz band. (Received 12 October, 2022; Accepted 28 November, 2022)

keyword : LSMO, sol-gel method, permittivity, permeability, reflection loss, EM absorption

Green hydrogen has attracted significant attention as one of the future energy sources because no greenhouse gases are emitted during production and its energy density is much higher than fossil fuels. Precious metals such as platinum (Pt) and iridium (Ir)-based catalysts are commonly used for water splitting catalysts. However, because of high cost of these precious metals, the mass production of green hydrogen is restricted. In this study, water splitting catalysts based on relatively inexpensive ruthenium (Ru), cobalt (Co), and iron (Fe) were synthesized. The metal nanoparticles were anchored on reduced graphene oxide (rGO) by a microwave-assisted process. The nanoparticles were uniformly distributed on the rGO supports with sizes of about 1.5 and 2 nm in Ru/rGO and RuCoFe/rGO, respectively. This promoted the reaction by further increasing the specific surface area of the catalysts. In addition, it was confirmed by EDS mapping results that the nanoparticles were made of RuCoFe alloy. Among the prepared catalysts, Ru/rGO was excellent toward the hydrogen evolution reaction (HER), which required an overpotential of 50 mV to reach a current density of -10 mA cm^-2. In addition, RuCoFe/rGO, which contained the RuCoFe alloy, was the best for the oxygen evolution reaction (OER), and it required 362 mV at the current density of 10 mA cm^-2. (Received 7 November, 2022; Accepted 7 December, 2022)

keyword : microwave-assisted process, ruthenium, transition metal, nanoparticles, water splitting

YbCd₂Sb₂ is a promising Zintl compound for waste heat recovery applications due to its low thermal conductivity, originating from its complex crystal structure. Many strategies such as alloying or doping have been suggested to further reduce the thermal conductivity of YbCd₂Sb₂ to improve its thermoelectric performance. However, the effects of alloying or doping on the electronic transport properties of YbCd₂Sb₂ have not been evaluated in detail. Here, previously reported thermoelectric properties of YbCd_2-xMg_xSb₂ (x = 0, 0.2, 0.4) with drastic thermal conductivity suppression were evaluated using the Single Parabolic Band (SPB) model and Callaway von Bayer (CvB) model. The SPB and CvB models evaluate any changes in electronic band parameters and phonon scattering strength, respectively, due to Mg alloying. Based on the SPB model, Mg alloying deteriorated the weighted mobility, mostly due to non-degenerate mobility reduction. However, the magnitude of point-defect phonon scattering significantly increased with Mg alloying, as evaluated by the CvB model. As a result, the maximum zT is achieved when x = 0.4 at 700 K despite the decreased electronic transport properties from Mg alloying. Our work suggests that carefully designed alloying can improve the thermoelectric performance of the Zintl compound even when it changes its electronic and thermal transport properties in opposite directions. (Received 21 November, 2022; Accepted 6 December, 2022)

keyword : zintl phase, YbCd₂Sb₂, single parabolic band model, callaway von bayer model

Bi₂Te₃-based alloys have been extensively studied as thermoelectric materials near room temperature. In this study, the electrical, thermal, and thermoelectric transport properties of a series of Co-doped n-type Cu_0.008Bi₂Te_2.6Se_0.4 polycrystalline alloys (Cu_0.008Bi_2-xCo_xTe_2.6Se_0.4, x = 0, 0.03, 0.06, 0.09 and 0.12) are investigated. The electrical conductivity of the Cu_0.008Bi_1.97Co_0.03Te_2.6Se_0.4 (x = 0.03) sample was significantly enhanced, by 34%, to 1199 S/cm compared to 793 S/cm of the pristine Cu_0.008Bi₂Te_2.6Se_0.4 (x = 0) sample at 300 K, and gradually decreased to 906 S/cm for x = 0.12 upon further doping. Power factors of the Co-doped samples decreased compared to the 3.26 mW/mK² of the pristine Cu_0.008Bi₂Te_2.6Se_0.4 sample at 300 K. Meanwhile, the power factor of the Cu_0.008Bi_1.97Co_0.03Te_2.6Se_0.4 (x = 0.03) sample became higher at 520 K. The lattice thermal conductivities of the Co-doped samples decreased due to additional point defect phonon scattering by the Co dopant. Consequently, the zT for the Cu_0.008Bi_1.97Co_0.03Te_2.6Se_0.4 alloy at 520 K was 0.83, which is approximately 15% larger than that of pristine Cu_0.008Bi₂Te_2.6Se_0.4, while the zT of the Cu doped samples at 300 K was smaller than that of the pristine Cu_0.008Bi₂Te_2.6Se_0.4 sample. Electrical transport properties of the Co-doped Cu_0.008Bi_2-xCo_xTe_2.6Se_0.4 samples were analyzed by experimental phenomenological parameters, including the density-of-state, effective mass, weighted mobility, and quality factor. (Received 5 December, 2022; Accepted 20 December, 2022)

keyword : thermoelectric, Bi₂Te₃, doping

Herein we report the optimized processing conditions of hot extrusion for fabricating an n-type Bi₂Te_2.7Se_0.3 thermoelectric compound, with high electronic transport properties as well as improved mechanical reliability. We fabricated a Bi₂Te_2.7Se_0.3 extrudate that was 3.8 mm in diameter and 700 mm in length by controlling the processing parameters of temperature and pressure. A 3-point bending strength of over 70 MPa, which is 7 times higher that of the commercial zone melting ingot, was obtained in the samples prepared at 460 ℃ temperature under 6-6.5 MPa pressure. The samples benefitted from the formation of a highly-dense microstructure (relative density > 98%). It is noted that the electronic transport properties (electrical conductivity and Seebeck coefficient) could be manipulated by controlling the applied pressure of hot extrusion at 460 ℃, mainly due to the change in the characteristics of the 00l crystal orientation, which originated from grain rotation and rearrangement. Power factor values of ~2.9 mW/mK² at 300 K and ~2.95 mW/mK² at 320 K, similar to those of sintered bulks, were obtained in the hot extrudate fabricated under processing parameters of 460 ℃ and 6 MPa. Moreover, a high power factor value of 2.25 mW/mK² was observed even at the high temperature of 480 K, which is 70% higher than that of Bi₂Te_2.7Se_0.3 bulk fabricated by hot pressing. (Received 6 December, 2022; Accepted 20 December, 2022)

keyword : thermoelectric, Bi₂Te_2.7Se_0.3, electronic transport properties, hot extrusion, processing parameters

Technologies to reduce CO₂ emissions in the steel industry have been actively developed since the early 1990s in order to deal with global climate change. In particular, the utilization of various types of biomass including wood, bamboo, grass, food and agricultural by-product are being attempted as carbon neutral fuels in the blast furnace process. In this study, an alternative reducing agent for coke and pulverized coal using biomass was developed and the effect on blast furnace performance and CO₂ reduction was evaluated. The first investigation was whether a burden material made of torrefied biomass mixed with iron ore and coal (Iron-bearing Biomass Coke, IBC) could be used as a substitute for coke in the form of a carbonized briquette. In addition, a method of mixing torrefied biomass with pulverized coal (Bio-PC) to inject through tuyere in a blast furnace was examined. As a result, when coke was replaced with 10% of IBC, the reducibility of the sintered ore was improved in association with an increased CO gas utilization ratio, hence CO₂ emissions decreased by 2.3%. Furthermore, the combustion efficiency of Bio-PC mixed with 10% of torrefied biomass was improved by 13.2%, thus, the amount of CO₂ emissions was calculated to decrease by 4%. These results suggest that the possibility of using IBC and Bio-PC in blast furnaces, as they could effectively reduce CO₂ emissions in the ironmaking process. (Received 18 October, 2022; Accepted 27 December, 2022)

keyword : ironmaking process, blast furnace, CO₂ emissions, biomass