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To measure the mechanical properties of Sb₂S₃, a two-component compound semiconductor used in the light absorption layer of a solar cell, Sb₂S₃ thin films were formed on FTO glass using the spin coating method. The spin-coated Sb₂S₃ thin films were heat-treated at 200 ℃ in an Ar atmosphere for up to 1 hour to form a thin film with continuous crystalline structures. A nanoindentation system was used to measure the mechanical properties of the spin-coated Sb₂S₃ thin films, and the phenomena appearing during indentation were analyzed. We used the continuous stiffness measurement (CSM) technique, and Young's modulus and hardness measured with the indentation depth of 250 nm were about 53.1 GPa and 1.43 GPa, respectively. The results were analyzed and compared with literature values, which varied from 40 GPa for the nanowire forms of Sb₂S₃ to 117 GPa, based upon simulation results. Since there are few studies on the mechanical properties of spin-coated Sb₂S₃ thin films, the results of this study are worthwhile. Besides, we observed that the Sb₂S₃ thin film had a little brittleness in the indentation test at higher load, and the microstructure was pushed around the indenter depending on the degree of bonding to the FTO glass substrate. This is a matter to be considered when making flexible devices in the future. (Received September 23, 2020; Accepted October 20, 2020)

keyword : binary compounds, Sb_{2S3, mechanical behavior, nanoindentation}

The behavior of dynamic strain aging (DSA) in a Zr-1.5Nb-0.4Sn-0.2Fe-0.1Cr alloy strip was investigated at temperature ranges of 25-600 °C via a tensile test. The tensile test was performed at two different strain rates 8.33 × 10^-5 and 1.67 × 10^-2 s^-1. The shear stress of the alloy strip revealed a linear dependency on the test temperature when the specimens were tested under a higher strain rate (1.67 × 10^-2 s^-1). However, the linear relationship was broken due to DSA when the samples were deformed under a lower strain rate (8.33 × 10^-5 s^-1). The discrepancy was most significant at 400 °C. The trend in DSA behavior was similar irrespective of the orientation of the samples, i.e., rolling direction (RD) or transverse direction (TD). However, the effect of DSA was larger in the TD samples than the RD samples. The phenomena were interpreted to the variation in work hardening exponents and strain rate sensitivity. The value of the exponent decreased from 0.14 to 0.08 along a RD and from 0.1 to 0.07 along a TD, respectively. However, the smallest value was observed at 400-500 °C irrespective of the specimen orientation, which is consistent with the DSA behavior. It is suggested that the DSA was caused by an interaction of moving dislocations with solute atoms typically oxygen. (Received October 19, 2020; Accepted November 12, 2020)

keyword : Zr alloy strip, dynamic strain aging, texture, mechanical strength

In ironmaking, the optimal size of the iron ore charged into a blast furnace is generally 10-30 mm. Oversized ores, which have a smaller reaction surface area, are crushed, while undersized ores, which decrease permeability, undergo a sizing process before being charged into the blast furnace. Recently, however, iron ore has been micronized, and there is less high-quality iron ore. Also, in accordance with the Paris climate change accord, the Republic of Korea must reduce CO2 gas emissions by about 39% before 2030 to conserve the environment. In response, steelmakers have researched a sinter-briquette complex firing process which employs a method of charging where the raw materials are sintered together with briquettes made of ultra-fine ore. Extra heat is needed to sinter the briquettes. If the briquettes are broken during transporting and charging, the sinter bed permeability decreases, which decreases productivity. In this study, briquettes were made by changing manufacturing conditions such as moisture content, feeding speed, and size, and were simulated by changing the pocket depth in a numerical analysis. Consequently, it was determined that the compressive strength of the briquette was highest when moisture in the briquette was 6 wt%, in proportion to feeding speed and large particle size. Briquette density was in inverse proportion to pocket depth, and when the depth was over 15 mm, the briquette was broken in the pocket. (Received December 16, 2019; Accepted November 9, 2020)

keyword : powder processing, iron ore briquette, sintering process, discrete element method

Hydrogen flowing into a weld causes hydrogen embrittlement (HE), so to prevent cold cracks in the weld, HE must be suppressed. In this study, single-pass fillet weldments were fabricated using two different welding fillers with the same strength level as the American Welding Society standard. The specimen F-HNi, which had a high, reversible hydrogen content, exhibited cold cracking in the fillet weld. The HE sensitivity index (HE index) was calculated using the in-situ slow strain rate test (SSRT). The reversible hydrogen trap concentration (C_rev) and hydrogen diffusion coefficient (D_eff) were calculated using the permeation test. The formation of low-temperature transformation phases such as bainite and martensite increased the C_rev and decreased the D_eff, thereby increasing the HE index. In addition, it was determined that reversible hydrogen was most effectively trapped in the low-temperature transformation phase, as confirmed by silver decoration. We concluded that the cold cracks in the F-HNi specimen were associated with the large C_rev, small D_eff, and large HE index. HE sensitivity can be controlled by optimizing the microstructure, even when welding fillers with the same level of strength are used. To reduce HE sensitivity, it is important to reduce the number of reversible hydrogen trap sites, by reducing the fraction of the low-temperature transformation phase where the reversible hydrogen trapping most occurs. (Received November 23, 2020; Accepted December 4, 2020)

keyword : weld metal, hydrogen embrittlement, hydrogen diffusion, hydrogen trapping

Cu-Pb and Cu-Sb alloys were prepared at various ratios, from 10:90 to 90:10, and leaching tests with sulfuric acid were conducted to investigate the effect of Pb and Sb on the leaching of Cu from speiss, which is obtained from the top submerged lance furnace process. The Cu leaching efficiency increased as the amount of Cu increased in both alloys, but the leaching efficiencies were lower in the Cu-Sb alloy than in the Cu-Pb alloy. For example, in alloys with 70% Pb and Sb ratio, the leaching efficiency of Cu from the Cu-Pb alloy increased to 95%. The leaching efficiency of the Cu-Sb alloy was 67% in 2 mol/L sulfuric acid solution with 1% pulp density and 1000 cc/min O₂ at 90 °C, 400 rpm, and 6 hours. When the leaching residues were examined with SEM (scanning electron microscopy)-EDS (Energy-dispersive X-ray spectroscopy), it was found that in all Cu-Pb alloys, Cu and Pb exist as independent metal phases, while, in Cu-Sb alloys, Cu formed intermetallic compounds with Sb such as Cu2Sb, because the Cu-Sb alloy has a lower melting point than the Cu-Pb alloy. These results suggest that Sb may retard the leaching rate of Cu from the alloy. When the leaching residue of speiss obtained from a top submerged lance furnace, intermetallic alloys of Cu-Sb were also observed, having a net structure. The net structure contains Cu metal in the center of the speiss particle, while the intermetallic alloys of Cu-Sb were present in the outer layer of the particle, in good agreement with the results using the alloys in this study. This suggests the intermetallic alloys of Cu-Sb can prevent copper from leaching. (Received August 21, 2020; Accepted November 9, 2020)

keyword : top submerged lance, speiss, copper leaching, antimony, lead

This study analyzed the current state of technical development of the BF-based process, to determine ways to reduce carbon consumption. The technical features of the hydrogen reduction ironmaking process were also examined as a decarbonized ironmaking method, and related issues that should be considered when converting to hydrogen reduction are discussed. The coal rate consumed by the reduction reaction in the coal-based BF process should be less than 50%. The heat requirement for indirect reduction in hydrogen reduction is higher than that of CO reduction, since hydrogen reduction is endothermic. The BF-based integrated steel mill is an energy independent process, since coal is used for the reduction of iron ore and melting, and the by-product gases evolved from the BF process are utilized for reheating the furnace, the power plant, and steam production. For hydrogen reduction, only green hydrogen should be used for the reduction of iron ore, and the power required to melt the iron and for the downstream rolling process will have to be provided from the external grid. Therefore, to convert to hydrogen reduction, green power should be supplied from an external infrastructure system of the steel industry. It will be necessary to discuss an optimized pathway for the step-by-step replacement of current coal-based facilities, and to reach agreement on the socio-economic industrial transition to hydrogen reduction steel. (Received September 14, 2020; Accepted November 16, 2020)

keyword : ironmaking, reduction, CO_{2, H2}

Using thermoelectric refrigerators can address climate change because they do not utilize harmful greenhouse gases as refrigerants. To compete with current vapor compression cycle refrigerators, the thermoelectric performance of materials needs to be improved. However, improving thermoelectric performance is challenging because of the trade-off relationship between the Seebeck coefficient and electrical conductivity. Here, we demonstrate that decreasing conductivity effective mass by engineering the shape of the Fermi surface pocket (non-parabolicity factor) can decouple electrical conductivity from the Seebeck coefficient. The effect of engineering the non-parabolicity factor was shown by calculating the electronic transport properties of a state-of-the-art Bi-Sb-Te ingot via two-band model with varying non-parabolicity. The power factor (the product of the Seebeck coefficient squared and electrical conductivity) was calculated to be improved because of enhanced electrical conductivity, with an approximately constant Seebeck coefficient, using a non-parabolicity factor other than unity. Engineering the non-parabolicity factor to achieve lighter conductivity effective mass can improve the electronic transport properties of thermoelectric materials because it only improves electrical conductivity without decreasing the Seebeck coefficient (which is directly proportional to the band mass of a single Fermi surface pocket and not to the conductivity effective mass). Theoretically, it is demonstrated that a thermoelectric figure-of-merit zT higher than 1.3 can be achieved with a Bi-Sb-Te ingot if the non-parabolicity factor is engineered to be 0.2. Engineering the non-parabolicity factor is another effective band engineering approach, similar to band convergence, to achieve an effective improvement in power factor. (Received October 7, 2020; Accepted November 14, 2020)

keyword : thermoelectric, non-parabolicity factor, power factor, conductivity effective mass, band mass of a single Fermi pocket

The Charpy impact test is used to identify the transition between ductility and brittleness. The percentages of ductile and brittle fractures in steel can be evaluated based on each fracture area, which is presently determined by an analyzer with the naked eye. This method may lead to subjective judgement, and difficulty accurately quantifying the percentage. To resolve this problem, a new analysis method based on image processing is proposed in this study. A program that can automatically calculate the percentage of the ductile and brittle fractures has been developed. The analysis is performed after converting an RGB fracture image into a binary image using image processing techniques. The final binary image consists of 0 and 1 pixels. The parts with the pixel values of 1 correspond to the brittle fracture areas, and the pixel values of 0 represent the ductile fracture areas. As a result, by counting the number of 0 pixels in the entire area, it is possible to automatically calculate the percentage of ductile fracture. Using the proposed automatic fracture analysis program, it is possible to selectively distinguish only the brittle fracture from the entire fracture area, and to accurately and quantitatively calculate the percentages of ductile and brittle fractures. (Received June 22, 2020; Accepted October 12, 2020)

keyword : image processing, texture segmentation, charpy impact test, fracture, ductile, brittle

The properties of 6 mM urea added perovskite solar cells (PSCs) according to ZrO₂ electron transport layer (ETL) thicknesses of 204, 221, 254, and 281 nm were examined. A solar simulator was used to characterize the photovoltaic properties of the cell. Optical microscopy and field emission scanning electron microscope were used for the microstructure analysis, and a 3D profiler was used to analyze surface roughness. UV-VIS-NIR was used to analyze transmittance. From the photovoltaic analysis result, an energy conversion efficiency (ECE) of 14.93% was exhibited by the cell with a 221 nm-ZrO₂ layer and added urea. From the analysis result of microstructure and surface roughness, 384 nm grain size was obtained through appropriate surface roughness of base layer for perovskite growth and the grain size coarsening by the urea under the 221 nm-ZrO₂ condition. For this reason, ECE increased as the resistance of the grain boundary decreased. When the thickness of the ETL was increased above 250 nm-ZrO₂, the ECE decreased due to the reduction in light transmittance, and light reaching the perovskite layer. Therefore, the ECE of PCS could be enhanced by selecting a ZrO₂ layer with the appropriate thickness and the addition of urea. (Received October 12, 2020; Accepted December 5, 2020)

keyword : perovskite grain, perovskite solar cells, ZrO_{2 layer, transmittance}