발간논문

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In the present study, monodisperse silica nanospheres were synthesized using a tubular reaction system in a continuous way. Screw-type blades were inserted inside a T-mixer for static mixing of the reactant streams which consisted of TEOS and NH₄OH/H₂O diluted with ethanol, for the continuous synthesis of the silica suspension. The diameter of the silica powder was monitored as a function of production time using dynamic light scattering to determine the optimum retention time and tube length, which were found to be 125 minutes and 7.5 m, respectively. The effects of reactant compositions on particle size were investigated by adjusting the amount of ammonia and water in the sol-gel reaction, which were then compared with the results from a batch reactor. Both the particle size and polydispersity index (PDI) of the silica suspension were measured to be comparable to nanospheres synthesized using a batch reactor. This implies that the tubular reaction system is more beneficial for potential industrial production applications in the size range from 115 to 310 nm, due to its continuous powder synthesis. The effect of the reaction medium was also studied, by replacing ethanol with methanol or propanol, indicating that the deviation in particle size with production time was not a serious issue in alcohols with lower molecular weight, such as methanol and ethanol. (Received 7 January, 2022; Accepted 10 March, 2022)

keyword : colloids, sol-gel, particle size distribution, continuous synthesis

The hot deformation behavior of medium-carbon Cr-Mo alloy, which has been developed for high strength cold-heading quality wire rod, was investigated to evaluate its hot workability. A flow curve was derived using the hot torsion test, under conditions with temperatures of 1173-1273 K and strain rates of 0.1-1.0 s^-1. At lower deformation temperature and higher strain rate, the overall stress of the flow curve increased, and the flow curve showed a three-stage variation related to the offset of dynamic recrystallization and dynamic recovery. First, as the strain increased, the stress also increased due to work hardening, and reached peak stress. After that, the stress decreased due to softening of the dynamic recrystallization. And when the effect of the dynamic recrystallization and the dynamic recovery reached equilibrium, the stress became steady state. In this paper, the constitutive equation of the peak stress was established using a form of a hyperbolic sine function, and here the thermal activation energy for deformation of the specimen was 244.90 kJ/mol. The peak stresses calculated from the constitutive equation were in good agreement with the experimental results. The dynamic-recrystallized grains were observed using electron backscatter diffraction (EBSD). It showed that the volume fraction of dynamic recrystallization increased as the strain increased under hot deformation. Based on the Avrami kinetic equation, a dynamic recrystallization kinetic model was established. The volume fraction of dynamic recrystallization was predicted from the kinetic model, and can be applied at arbitrary deformation temperatures and strain rates. (Received 3 January, 2022; Accepted 22 February, 2022)

keyword : hot torsion test, flow behavior, constitutive equation, dynamic recrystallization

A dual laser beam joining process is introduced for joining dissimilar plastics. A customized laser head was built, and two different diode lasers with central wavelengths of 980 and 1940 nm were simultaneously applied to obtain PMMA/PC-1 and PC-2/ABS joints. Various experimental conditions were employed for the dual laser beam joining, which were performed at different laser powers for the 1940 nm laser beam and a fixed laser power for the 980 nm laser beam. The tensile shear force of the PMMA/PC-1 and PC-2/ABS joints was measured, and compared with those using the 980 nm single laser beam joining process. For both joints, a decrease in tensile shear force was observed when the laser power of the 1940 nm laser beam increased. Only the dual laser beam joining using the 1940 nm laser beam at the smallest laser power showed relatively higher tensile shear force than those of the 980 nm single laser beam joining. The transmission values of the PMMA and PC-2 at the wavelength of 980 nm were measured with different powers of 1940 nm laser beam, and a correlation between the joining properties and transmission values was found. (Received 1 November, 2021; Accepted 9 February, 2022)

keyword : Laser joining, Dual laser beam, Dissimilar plastics, Joining properties, Transmission

Si-doped Cu₁₂Sb_4-ySi_yS₁₃ (y = 0.1-0.4) compounds were prepared using solid-state synthesis. Each specimen consisted of a single tetrahedrite phase with a densely sintered body whose relative density exceeded 98.9%. The lattice constant decreased from 1.0357 nm to 1.0336 nm as the Si content increased. When the Si doping content (y) exceeded 0.3, the decrease in the lattice constant was reduced and residual Si appeared. This established the solubility limit of Si at the Sb sites was y = 0.3. The Seebeck coefficient increased with the temperature and Si content, achieving a maximum value of 178 μVK^-1 at 723 K for y = 0.3. For the specimens with y ≤ 0.2, the electrical conductivity increased with temperature, and then slightly decreased at temperatures higher than 623 K, while it gradually increased with temperature for the specimens with y ≥ 0.3. The electrical conductivity decreased as the Si content increased at a constant temperature and the highest electrical conductivity of (2.8-3.4) × 10⁴ Sm^-1 was obtained at 323-723 K for Cu₁₂Sb_3.9Si_0.1S₁₃. When y = 0.4, the electrical conductivity did not decrease further, which is related to the solubility limit of Si. The power factor reached a maximum value of 0.86 mWm^-1K^-2 at 723 K for Cu₁₂Sb_3.9Si_0.1S₁₃. As the Si content increased, the thermal conductivity tended to decrease, and Cu₁₂Sb_3.7Si_0.3S₁₃ exhibited the lowest thermal conductivity of 0.85 Wm^-1K^-1 at 723 K. Hence, the highest dimensionless figure of merit, ZT = 0.63 was achieved at 723 K for Cu₁₂Sb_3.8Si_0.2S₁₃. (Received 19 January, 2022; Accepted 11 February, 2022)

keyword : thermoelectric, tetrahedrite, mechanical alloying, hot pressing

Due to its high theoretical capacity in the conversion reaction, Co₃O₄, a transition metal oxide, has been attracting attention as an anode material for lithium-ion batteries. Comparing conventional slurry method with the electrophoretic deposition (EPD) method without additives (conductive agents and binders), we investigated the effect of the Co₃O₄/additive interface and thermal annealing-induced Co₃O₄ crystallite size on Li-ion battery capacity and cycle stability. The EPD deposition system based on Co₃O₄ active material without additives was not significantly affected by thermal annealing-induced crystallite size. However, the slurry deposition system in which Co₃O₄/binder and Co₃O₄/conductive agent interfaces are embedded showed significant differences in capacity and cycle stability. This result reveals that the Co₃O₄/additive interface in the slurry system works as a limiting step, depending on the Co₃O₄ crystallite size, for reversible electrochemical reactions associated with Li-ion battery charging/discharging processes. On the other hand, for the EPD system, the capacity was higher than that in the slurry system with superior cycle stability, indicating that the limiting step was eliminated by removing the Co₃O₄/additive interface. Moreover, the current collector/active material interface was demonstrated to be crucial in determining the intrinsic electrochemical properties of Co₃O₄ in the EPD system. Our findings contribute further understanding of the relationship between the battery electrode/additive interface and the electrochemical reaction resulting from the conversion reaction in transition metal oxide electrode materials. (Received 30 January, 2022; Accepted 3 March, 2022)

keyword : Li-ion battery, transition metal oxide, electrophoretic deposition, interfaces, crystallite size

Skinnerite Cu₃SbS₃, which consists of non-toxic and low-cost elements, is a potential thermoelectric material, however, studies on its thermoelectric properties are limited. In this study, the optimal conditions of mechanical alloying (MA) and hot pressing (HP) for the synthesis of the Cu₃SbS₃ phase were investigated, and its thermoelectric properties were evaluated. The MA powder had a theoretically predicted cubic skinnerite phase, which remained even after HP. Thermal analyses revealed that the MA powders and HP specimens exhibited large endothermic peaks at approximately 880 K, corresponding to the melting point of Cu₃SbS₃. Compacts with high relative densities above 99% without cracks and pores were obtained at an HP temperature of 623 K and above. The lattice constants of the MA powder and HP specimens were in the range of 1.0343-1.0394 nm. As the HP temperature increased, the electrical conductivities of the specimens decreased because of changes in their carrier concentrations; however, their Seebeck coefficients increased. As a result, the power factor values of all the specimens were in the range of 0.58-0.61 mW m^-1 K^-2 at 623 K. The thermal conductivities were lower than 0.78 Wm^-1K^-1. The maximum dimensionless figure of merit, ZT_max of 0.57 was obtained at 623 K when the skinnerite was prepared under the optimal solid-state synthesis conditions (HP at 673 K for 2 h using MA powder at 350 rpm for 18 h). (Received 22 February, 2022; Accepted 17 March, 2022)

keyword : thermoelectric, skinnerite, mechanical alloying, hot pressing

We present a study on the synthesis of Bi₂Te_2.7Se_0.3 powders through an oxide reduction process and also on the effect of the oxide reduction process on the thermoelectric transport properties of the resulting Bi₂Te_2.7Se_0.3 compounds. Starting materials of Bi₂O₃, TeO₂ and SeO₂ were mechanical milled for homogeneous mixing, and then oxidized to prepare complex oxide powders. The complex oxide powders were reduced at different reduction temperatures under hydrogen atmosphere, and a single phase of Bi₂Te_2.7Se_0.3 could be achieved in the reduced powder, with shorter reduction times as the reduction temperature was increased. The shape of the synthesized powder particles changed from granules to flakes with increasing reduction temperature and longer reduction time. Bi₂Te_2.7Se_0.3 powders with different shapes were consolidated by spark plasma sintering (SPS). The SPSed Bi₂Te_2.7Se_0.3 compounds exhibited anisotropic n-type thermoelectric transport properties along the vertical and parallel planes with respect to the pressing direction of the SPS. The degree of anisotropy in the thermoelectric properties was quite consistent with the degree of anisotropy in the microstructure, which originated from the anisotropic shapes in the Bi₂Te_2.7Se_0.3 powders depending on the reduction conditions. Detailed thermoelectric transport properties of the n-type Bi₂Te_2.7Se_0.3 compounds were interpreted in terms of anti-site defect formation and their structural anisotropy. (Received 4 March, 2022; Accepted 29 March, 2022)

keyword : thermoelectric, Bi_{2Te3, oxide reduction process, spark plasma sintering}

Nodular graphite cast iron is used in the railway and automobile industries because of its excellent workability, abrasion resistance, mechanical strength, and hardenability. To use nodular graphite cast iron in harsh environments, it is essential to examine their friction-wear properties in conjunction with the surface hardness of the materials. Therefore, in this study, a pin-on-disk type method was used to investigate specimens whose surface hardness was improved by the addition of Sn alloying element, followed by high-frequency heat treatment. The results revealed that Sn inhibited the growth of nodular graphite by acting as a barrier to the movement of carbon and promoted the transformation of some matrix from ferrite to pearlite. In contrast, most of the matrix of the high-frequency heat treated specimen was transformed into pearlite. Consequently, the specimens prepared using both methods achieved higher surface hardness than the as-cast state. With regards to the friction-wear properties, the Sn added alloy exhibited a relatively weak matrix, and therefore, delamination of the surface and spalling on the nodular graphite occurred, with significant wear width and depth. In contrast, surface delamination was not observed in the specimen after high-frequency heat treatment. (Received 7 February, 2022; Accepted 3 March, 2022)

keyword : GCD600, Modification, high-frequency heat treatment, Microstructure, Friction-wear properties

Sodium silicate solutions are used as an eco-friendly binder in the casting process. However, the strength of sand molds using the sodium silicate solutions is reduced because of their low humidity resistance. In this study, silica nanoparticles were added to a sodium silicate solution with a constant SiO₂/Na₂O ratio to improve the humidity resistance of the inorganic binder. The effects of the dissolution times and particle size of the silica on the flexural strength of the molds was also investigated. The physiochemical properties of the inorganic binders were characterized by viscometer, Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA) and scanning electron microscope (SEM). Sand molds for the flexural strength test were prepared by mixing artificial sand and inorganic binders. The addition of silica nanoparticles reduced the viscosity and water evaporation rate of the inorganic binders and improved the humidity resistance of the molds due to an increase in the SiO₂/Na₂O ratio. The flexural strength of the molds increased as the size of the silica nanoparticles increased, due to formation of discrete silica aggregates with high surface area. However, as the dissolution times of the silica nanoparticles increased, the strength of the molds decreased due to an enhanced Q₂/Q₃ ratio. Therefore, we suggest that the addition of silica nanoparticles will improve the humidity resistance of molds, and that varying the particle size and dissolution time of the silica nanoparticles affects the strength of the molds. (Received 29 September, 2021; Accepted 10 February, 2022)

keyword : dissolution time, flexural strength, humidity resistance, particle size, silica nanoparticle dispersion, sodium silicate solution