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There are many ongoing efforts to develop sustainable, clean, efficient, and economical pathways to produce renewable energy sources to satisfy worldwide energy demands. Electrochemical conversion processes, such as water splitting, CO₂ conversion and N2 electroreduction, have been considered as successful approaches to solve these energy issues. Over the past decade, combining of theory and experiment has proven to be an innovative strategy, providing a framework for the design of high-performance catalysts and to investigate their mechanisms. This review introduces recent progress in theoretical strategies for state-of-theart heterogeneous electrocatalysts. Theoretical approaches are essential for grasping the intrinsic nature of the catalytic materials. Various levels of model system, with corresponding descriptions to capture the realistic environment, are addressed. Meanwhile, machine learning using data obtained by high-throughput screening, exploited as a new scientific approach, is discussed. Based on this review, it is expected that theoretical approaches will shed light on the future design of electrocatalysts, allowing for the development of sustainable energy sources. (Received October 17, 2018; Accepted November 27, 2018)

keyword : electrocatalysts, density functional theory, first principle calculation, high-throughput screening, machine learning

Cu-Ti alloys were strengthened by Cu4Ti intermetallic compound precipitation in a Cu matrix during aging. The Cu-3.5 wt% Ti(Cu-4.6 at% Ti) alloys without deformation and with uniform and nonuniform deformation were aged at 450 ℃ for various times after solution treatment at 885 ℃. The uniformly and non-uniformly deformed alloys show slip and shear band formation in the matrix, respectively. The deformation bands, slip or shear bands, were still maintained even after 12 hours of aging at 450 ℃. The conductivity of all the specimens continuously increased, but the hardness reached the peak value and then decreased during aging. The hardness and conductivity of the specimen with shear bands had a higher value than the specimens without deformation and with slip bands. Additionally, the time to reach peak hardness of the specimen with shear bands was shortened to 30 minutes compared to 720 and 1440 minutes for specimens with slip bands and without deformation bands, respectively. The highest combination value of conductivity and hardness, 16.8% IACS and 302 Hv, was obtained in the specimen with shear bands after aging for 360 minutes. (Received September 27, 2018; Accepted ℃tober 19, 2018)

keyword : Cu-Ti alloy, aging, Cu4Ti intermetallic compound, precipitation hardening, electrical conductivity

The effect that forging routes (3 routes, A-C) exert on the development of deformation heterogeneity in AA1100 was theoretically performed using finite element analysis. Route A corresponded to a forging process that involved from 1 to 6 passes through multi-axial diagonal forging (MADF); Route B corresponded to a forging process that involved from 1 to 12 passes through the MADF; and, Route C corresponded to a conventional multi-axial forging process in which the operations from 1 to 6 passes of the MADF were simply repeated twice. From the aspect of strain uniformity developed on AA1100, Route B, which combined forging operations to the direction perpendicular to the face of the workpiece and forging operations to the diagonal direction of the face of the workpiece, was relatively advantageous compared with Routes A and C that only involved forging operations to the direction perpendicular to the face of the workpiece. Routes A and C showed a tendency toward regions where both the low and the high effective strains were restricted to specific regions of the workpiece. The low and high effective strains created by Route B, on the other hand, tended to be distributed over various regions of the workpiece. (Received October 1, 2018; Accpeted November 2, 2018)

keyword : AA1100, forging, effective strain, multi-axial diagonal, finite element analysis

The hot crack resistance and mechanical properties of flux cored arc (FCA) welds were investigated with three kinds of welding consumables having different boron (B) and manganese (Mn) contents for high strength carbon steel. The hot crack resistance, measured from self-restraint testing, strongly depended on the amount of B in the welding consumable. Welding consumables with higher B contents resulted in longer total crack length and an increased number of cracks. Boron was intensely detected near the grain boundary of the weld centerline by secondary ion mass spectrometry (SIMS) analysis, and precipitated with boron carbide (Fe₂₃(C,B)₆), as analyzed by transmission electron microscopy (TEM). This promoted hot crack propagation in the high strength carbon steel welds. However, removing B from the welding consumable decreased the low temperature toughness for root and face weld metals, due to the growth of ferrite side plate (FSP), compared with welding consumables having more B or Mn contents. The addition of Mn in the weld metal suppressed the formation of FSP and increased low temperature toughness. Therefore, the minimization of B and the supplement of Mn successfully achieved hot crack resistance and low temperature toughness for high strength carbon steel welds of 550 MPa tensile strength. (Received October 5, 2018 ; Accepted November 17, 2018)

keyword : hot crack resistance, low temperature toughness, self-restraint testing, FCA welds, boron, manganese

Many important material properties of ferroelectric ceramics such as piezoelectric coefficient and hysteresis curve are governed by domain structures and their evolution processes. In the present work, domain wall motions in a near-morphotropic Pb(Zr,Ti)O₃ under an external compressive stress were observed using in situ piezoresponse force microscopy (PFM). This specific PZT material was chosen due to its complex microstructure, which originates from the coexistence of tetragonal and rhombohedral phases. A micromechanical test rig was installed on the sample stage controller, enabling precise control of the sample position. For the real-time strain monitoring of the PZT sample, a strain gauge was attached to the sample surface on which the PFM scan was conducted. In addition, analysis of the ferroelectric domain structure was assisted by using the electron backscatter diffraction (EBSD) technique to identify the habit planes of non- 180° domain walls and grain boundaries in the PFM images. From a certain grain, a set of large non-180° needle-like domain patterns in the (101) plane were observed before applying a mechanical stress. With increasing compressive stress levels, another set of non-180° needle-like domain patterns grew in the (011) plane, overwriting the domain patterns in the (101) plane. This indicates that polarization switching processes in ferroelectric ceramics take place via a pattern evolution based on the pre-existing domain patterns. (Received October 22, 2018; Accepted November 19, 2018)

keyword : domain structures, ferroelectric ceramics, PZT, piezoresponse force microscopy, compressive stress

Compact Endless cast and rolling Mill (CEM) processes were developed and used to fabricate steel products such as steel slabs. However, the coiling furnace in this process was very expensive, so a new layout was suggested. As the coiling furnace was removed, the interval among the slab heaters had to be increased. This led to a temperature drop in the slab. The temperature distribution of the slab impacts quality, so new layout was developed. This paper presents a Finite Element Method (FEM) simulation of thermal behavior in the slab employing slab heater covers. All of the simulation results were verified by comparing them with experimental results. The slab moving distance at which the temperature was saturated during the process was determined to consider the steady-state and analyze the temperature distribution of the slab and slab heater. Those results revealed that the efficiency of heat conservation increased by more than 50% using the slab heater cover. Finally, a sensitivity analysis of the slab heater cover was conducted with respect to the cover design. The effects of insulator thickness, the gap distance between the slab and cover, and material parameters such as density, and specific heat were investigated to optimize the design of the slab heater cover to produce the best quality slab. (Received November 6, 2018; Accepted November 13, 2018)

keyword : compact endless cast and rolling mill (CEM), FEM simulation, sensitivity analysis

Three 304L austenitic stainless steel feedwater heater (FWH) tubes used in pressurized water reactors (PWRs) were investigated in this study. By comparing metallurgical differences, such as microstructural features, micro-hardness, and circumferential residual stress of the failed and non-failed FWH tubes by stress corrosion cracking (SCC), factors which could affect SCC susceptibility were assessed. Considering the impurity controlled and hydrogenated water chemistry, it is likely the mechanism of sudden SCC failures under PWR operation would be associated with material aspects rather than water chemistry. In tubes which experienced a large number of SCC failures, significant tensile circumferential residual stresses were measured. Such residual stress could be generated by work hardening during straightening of the rolling process, which directly applies stress on the surface of the tubes. The non-failed tube showed very low residual stress, by using straightening or stretching process. The rolling process induces a large amount of dislocations and an increasing tendency in micro-hardness values toward the outer region. Although the hardness values satisfied the material limitation requirement, the presence of large residual stress is thought to increase the SCC susceptibility of FWH tubes. Thus, this study indicates that straightening by stretching process has benefits in terms of low residual stress, assuring the long-term integrity of FWH tubes against SCC failure. Meanwhile, when the rolling process is used, additional heat treatment could be a practical method to reduce residual stress in FHW tubes. (Received September 3, 2018; Accepted November 28, 2018)

keyword : austenitic stainless steel, work hardening, micro-hardness, residual stress, split-ring test

Both the mechanical properties and probability distribution of liquid phase sintered SiC (LPS-SiC) materials were investigated, based on a detailed analysis of their microstructures. In particular, the effect of the starting sizes of SiC particles on the characterization of LPS-SiC materials was evaluated. LPS-SiC materials were fabricated with a constant applied pressure, using a complex powder of different sized SiC particles. Two types of initial SiC powders used for the fabrication of LPS-SiC materials were a unimodal SiC single powder with the average particle size of 0.3 μm, and a bimodal SiC powder mixture with average particle sizes of 0.3 μm and 30 nm, respectively. A complex powder of Al₂O₃ and Y₂O₃ particles was also utilized as an additive material for the consolidation of the LPS-SiC materials. The characterization of the LPS-SiC materials was performed using microstructural observation, three-point bending test and micro Vickers hardness test. The LPS-SiC materials containing the unimodal SiC powder exhibited a good sintered density of about 3.11 g/cm³ and a low porosity of about 5.4%. The bimodal mixture of different SiC particles did not significantly affect the sintered density of LPS-SiC materials. However, LPS-SiC materials containing the bimodal SiC powder exhibited an average flexural strength about 630 MPa higher than that of unimodal SiC powder, accompanied by the creation of fine SiC grains in the range of about 0.2~1.4 μm. The LPS-SiC materials exhibited a similar hardness level of about 2200 Hv regardless of the starting SiC powders. The bimodal SiC powder also produced a higher shape parameter and variability in hardness in the Weibull statistical analysis. (Received October 10, 2018; Accepted November 12, 2018)

keyword : silicon carbide, liquid phase sintering, initial SiC particle size, grain size, weibull distribution, mechanical property

Lack of understanding on the electrical properties of the solid electrolyte interface (SEI) has been one of the major hurdles for developing fast charging Li-ion batteries. Here, we report our preliminary experimental result on the measurement of the electrical resistivity of SEI using the direct-contact technique based on electron microscopy combined with 4-point-probe micro-electrical method. We observed that the SEI layer, which covers uniformly the surface of the graphite anode, exhibits a high resistivity (2.3 × 10⁵ Ω·m) comparable to those of typical insulators. In this study, using first principles calculations performed on the computational Li_xF_1-x (0.25 < x < 0.75) phases, we elucidate the structural origin responsible for the measured resistivity of SEI by analyzing the electronic structures. (Received September 4, 2018; Accepted September 19, 2018)

keyword : Li-ion battery, solid electrolyte interface, resistivity, 4-point-probe technique, ab-initio calculation