발간논문

Home > KJMM 논문 > 발간논문

In the present study, Cu₅₄Ni₆Zr₂₂Ti₁₈ bulk metallic glass composites were developed by spark plasma sintering(SPS) using gas atomized Cu₅₄Ni₆Zr₂₂Ti₁₈ metallic glass powders and Ta powders. Metallic glass composites with Ta phase were fabricated by SPS. The successful consolidation of Cu₅₄Ni₆Zr₂₂Ti₁₈ metallic glass matrix composites with the Ta phase was achieved through the strong bonding due to the plastic deformation of the Ta powder and the super-plastic behavior of the metallic glass powder in the supercooled liquid state during SPS. The deformed Ta phases were well distributed in the Cu₅₄Ni₆Zr₂₂Ti₁₈ metallic glass matrix. The compressive fracture strength and total strain were 1770 Mpa and 10.2%, respectively, for the Cu₅₄Ni₆Zr₂₂Ti₁₈ bulk metallic glass composite with 40 wt% Ta phases. The uniformly dispersed deformed Ta phase in the Cu₅₄Ni₆Zr₂₂Ti₁₈ metallic glass matrix effectively impedes the propagation of the first shear band and generates a second shear band, causing a crossing of the shear bands, resulting in an improvement in plastic strain. This increase in plastic deformation is related to the fact that the deformed Ta phase, uniformly distributed in the Cu₅₄Ni₆Zr₂₂Ti₁₈ metallic glass matrix, acts as a source of shear bands and at the same time effectively suppresses the movement of the shear bands, dispersing the stress and causing wide plastic deformation. (Received February 24, 2021; Accepted February 25, 2021)

keyword : powder, spark plasma sintering, bulk metallic glass, composite, shear band

The powder microstructure and sintering behavior of W prepared by ultrasonic spray pyrolysis and spark plasma sintering were investigated. Fine-grained W powders were synthesized by ultrasonic spray pyrolysis using an ammonium metatungstate hydrate solution and hydrogen reduction. The XRD analysis of the powder, pyrolyzed below 600 ℃, showed tungsten oxide hydrate and WO₃ peaks, while the powder pyrolyzed at 700 ℃ was composed of only the WO₃ phase. As the precursor concentration increased, the particle size of the WO₃ powder increased, which was interpreted to be due to an increase in the amount of solute in the droplet. The hydrogen-reduced powder showed a spherical shape with fine pores inside. XRD and XPS analysis revealed that the WO₃ powder was completely reduced to metallic W by hydrogen reduction, and some oxide layers existed on the powder surface. The consolidated specimen prepared by spark plasma sintering of hydrogen-reduced W powder exhibited a relative density of 94.1% and a Vickers hardness value of 3.89 GPa. The relative density and hardness of the specimens prepared by ultrasonic spray pyrolysis showed relatively lower values than when commercial W powder, with an average particle size of 1.22 μm, was sintered under the same conditions. These results were explained by the formation of agglomerates in the W powder prepared by the ultrasonic spray pyrolysis method. (Received February 23, 2021; Accepted March 15, 2021)

keyword : tungsten, ultrasonic spray pyrolysis, hydrogen reduction, spark plasma sintering, microstructure

Blast furnace sludge, which comes from the iron making process, contains many valuable materials including iron, carbon, and zinc, etc. Because a cohesive agent is added during filtration, fine sludge particles are agglomerated together. Therefore, This makes it necessary to disperse the sludge in solution before separating or recovering valuable materials. In this study, the effects of solid/liquid (g/L) ratio, ultrasonic dispersion conditions, the pH of solvent, and the concentration of dispersant on the dispersion of sludge were investigated by measuring the interfacial properties (zeta potential and hydrodynamic size) of sludge particles. High absolute value of zeta potential and small hydrodynamic size suggests that the sludge particles in the solution presents good dispersion. The absolute value of zeta potential increased gradually at high solid/liquid ratio and ultrasonic dispersion intensity. But when the sludge in solution was dispersed for more than 30 minutes, the absolute value of the zeta potential decreased due to increasing contact and interaction between the particles. Optimal dispersion operations were conducted and when the pH of the solution was adjusted to 11, the zeta potential value was measured to be -44.8 mV. This means that the sludge formed the most stable dispersed phase. The lowest zeta potential was measured to be -46.4 mV with the addition of sodium hexametaphosphate (NaPO₃)₆ in the solution. It is thought that the sodium hexametaphosphate reduced ionic strength by removing alkali metal ions from the solution of sludge. (Received January 21, 2021; Accepted March 9, 2021)

keyword : blast furnace, sludge, recycling, zeta potential, pH, dispersant

In this study, we studied the surface and mechanical properties of multiple additives as well as the initial plating properties of individual additives. With the individual additive groups, copper crystals tended to converge at a stage above the critical amount of additive. When chloride ions were added, large crystals formed at several places on the surface. Thereafter, small crystals were attached to and grew on the surface of the large crystals. When collagen and JGB (Janes Green B) were added individually, the crystals were more uniformly distributed on the surface as compared with the group with added chloride ions. In addition, starlike crystals were grown depending on the amount of addition. It was necessary to use multiple additives, which is why it is difficult to make a uniform surface layer with individual additives. Large crystals of more than 10 μm formed unevenly on the surface treated with multiple additives of chloride ions and MPSA (3-mercapto-1-propane sulfonic acid). Large crystals disappeared on the surface treated with additional collagen along with multiple additives. However, valley like shapes were observed on this surface, due to the large crystals. But, addition of JGB additives to this reduced the valley and formed a uniform plating layer. MPSA and 20 ppm of collagen were added as multiple additives, grain sizes increased by 93.5% (220 peak) and 172.3% (311 peak) compared with the non-additive group. As a result, tensile strength decreased by 24.5% and elongation increased by 17.8%. The crystal size was reduced 25.0% on average by the addition of 10 ppm JGB, which contributed to a 5.4% increase in tensile strength and a 16.0% decrease in elongation. The grain size and surface properties could be controlled by adding multiple combinations of additives. As a result, It was confirmed that mechanical properties could be controlled by the proper amount and the optimum combination of additives. (Received November 3, 2020; Accepted November 18, 2020)

keyword : mechanical properties, copper foil, organic additive, surface roughness, electroplating

Inkjet printing is a very attractive technology for printed electronics and a potential alternative to current high cost and multi-chemical lithography processes, for display and other applications in the electronics field. Inkjet technology can be employed to fabricate organic light emitting diodes (OLED), quantum dots displays, and thin-film transistors (TFTs). Among potential applications, metal oxide TFTs, which have good properties and moderate processing methods, could be prepared using inkjet printing in the display industry. One effective method of improving their electrical properties is via doping. Lithium doping an oxide TFT is a very delicate process, and difficult to get good results. In this study, lithium was added to indium-zinc oxide (IZO) for inkjet printing to make oxide TFTs. Electrical properties, transfer and output curves, were achieved using inkjet printing even at the relatively low annealing temperature of 200 ℃. After optimizing the inkjet process parameters, a 0.01 M Li-doped IZO TFT at 400 ℃ showed a mobility of 9.08 ± 0.7 cm²/V s, a sub-threshold slope of 0.62 V/dec, a threshold voltage of 2.66 V, and an on-to-off current ratio of 2.83 × 10⁸. Improved bias stability and hysteresis behavior of the inkjet-printed IZO TFT were also achieved by lithium doping. (Received January 11, 2021; Accepted February 24, 2021)

keyword : inkjet printing, oxide TFT, doping effect, low temperature

Halide perovskite solar cells have been attracting tremendous attention as next-generation solar cell materials because of their excellent optical and electrical properties. Formamidinium lead tri-iodide (FAPbI₃) exhibits the narrowest band gap among lead iodide perovskites and shows excellent thermal and chemical stability, also. However, the large-area coating of FAPbI3 needed for commercialization has not been successful because of the instability of the black phase of FAPbI₃ at ambient temperature. This study presents a compositional engineering direction to control the polymorph of the FAPbI₃ thin film for the shear coating processes, without halide mixing. By adopting a hot substrate above 100 ℃, our shear coating process can produce the black phase FA-based halide perovskites without halide mixing. We carefully investigate the Cs- FA and MA-FA mixed lead iodide perovskites’ phase stability by combining the study with thin-film fabrication and ab initio calculations. Cs-FA mixing shows promising behaviors for stabilizing α-FAPbI₃ (black phase) compared with MA-FA. Stable FA-rich perovskite films cannot be achieved via shear coating processes with MA-FA mixing. Ab initio calculations revealed that Cs-FA mixing is excellent for inhibiting phase decomposition and water incorporation. This study is the first report that FA-based halide perovskite thin films can be made with the shear coating process without MA-Br mixing. We reveal the origin of the stable film formation with Cs-FA mixing, and present future research directions for fabricating FA-based perovskite thin films using shear coating. (Received January 14, 2021; Accepted March 4, 2021)

keyword : perovskite, shear-coating, solar-cell, large-area, FAPbI₃

Mn-doped tetrahedrites Cu_12-xMn_xSb₄S₁₃ (0.1 ≤ x ≤ 0.4) were synthesized by mechanical alloying (MA) and sintered by hot pressing (HP). A single tetrahedrite phase was synthesized by MA without post-annealing, and it was stable without any phase changes after HP. The hot-pressed specimens had a relative density higher than 98.6%. The lattice constant of the Mn-doped samples increased compared to that of undoped Cu₁₂Sb₄S₁₃, but no significant change in the lattice constant was observed with a change in Mn content. All Mn-doped tetrahedrites acted as p-type semiconductors, as confirmed from positive Hall and Seebeck coefficient values. The Seebeck coefficient increased with increasing temperature but decreased with increasing Mn content; maximum Seebeck coefficient values of 200-219 μVK^-1 were obtained at 323-723 K for x = 0.1. Electrical conductivity increased with increasing temperature and Mn content; the highest electrical conductivity values of (1.76-2.45) × 10⁴ Sm^-1 were obtained at 323-723 K for x = 0.4. As a result, Cu_11.6Mn_0.4Sb₄S₁₃ exhibited a maximum power factor of 0.80 mWm^-1K^-2 at 723 K. As the Mn content increased, both the electronic and lattice thermal conductivities increased, and thus, the total thermal conductivity was the lowest at 0.48-0.63Wm^-1K^-1 at 323-723 K for x = 0.1. A maximum dimensionless figure of merit of 0.75 was obtained at 723 K for Cu_11.7Mn_0.3Sb₄S₁₃. The MA-HP process is suitable for preparing doped tetrahedrites exhibiting excellent thermoelectric performance. (Received February 18, 2021; Accepted March 2, 2021)

keyword : thermoelectric, tetrahedrite, charge transport, mechanical alloying, hot pressing

Porous carbon has found commercial applications as a filter material based on the sorption ability of its pores. The pore size and surface properties of the carbon can be varied depending on the type of particles to be filtered. Here, mesoporous carbon was induced through the pyrolysis of polyvinylidene fluoride (PVDF) to fabricate a porous material for microparticle filtration. Because removal of the constituent fluorine at elevated temperature leaves small-sized micropores, the PVDF precursor mainly generates micropores during pyrolysis. To suppress the micropore evolution mechanism, the PVDF precursor was defluorinated before the heat treatment using 1,8-Diazabicyclo[5.4.0]undec-7-ene(DBU) and then pyrolyzed. The suppressed evolution of the micropores during carbon synthesis leads to a lower specific surface area, suggesting low adsorption capacity. The polytetrafluoroethylene (PTFE) was mixed with the PVDF precursor to induce mesoporosity. The PVDF precursor mixed with the PTFE enhanced the surface area since the PTFE could be removed, leaving mesopores after pyrolysis. The effect of the defluorination process on the porosity was investigated by varying the ratio of DBU to vinylidene fluoride unit (1, 5, 10, 20) in the precursor solution. With higher DBU content in the precursor, the micropore evolution was reduced with a lower specific surface area. The porous carbons synthesized from the precursor with a high DBU amount (DBU/vinylidene fluoride unit = 5, 10, 20) were almost entirely composed of mesopores. In addition, the higher DBU content reduced the hydrophilicity of the synthesized carbon. In summary, to separate and absorb relatively large impurities, the mesoporous carbon should be synthesized using a mixture of PVDF and PTFE precursor with an appropriate amount of DBU for a higher specific surface area. (Received February 17, 2021; Accepted March 18, 2021)

keyword : polyvinylidene Fluoride, PVDF, polytetrafluoroethylene, PTFE, 1, 8-Diazabicyclo, 5, 4, 0, undec-7-ene, DBU, micropore, mesopore

A photo-rheological fluid (PRF) is a smart fluid which exhibits different viscosity under UV irradiation. A PRF is comprehensively presented in this work, with particular focus on its responses under UV off/on conditions. The isomeric conversion from SP to MC and vice versa under UV off and on, respectively, showed unequal rates of transformation. As a result, a complex non-linear hysteretic response was observed. To be used indifferent types of sensors and actuators which can exploit its rheological properties, it is essential the PRF have linearized hysteresis behavior. To minimize the asymmetric non-linear hysteresis characteristics under UV on and off conditions, the well-known long-lasting phosphor SAO (SrAl₂O₄:Eu²⁺, Dy³⁺) was incorporated. The incorporation of SAO in the PRF improved the linearity of the PRF response, although the conversion rate was not identical under UV off/on conditions. The SAO particles were observed to settle over time due to phase splitting, undermining the usefulness of the SAO-PRF composite. Instead of improving the PRF response by further adjusting the PRF composite, a software approach based on Long Short-Term Memory Recurrent Neural Networks (LSTM RNN) was employed to model and compensate the asymmetric non-linear hysteresis response, ensuring the realization of sensors and actuators that exploit PRF as hardware. (Received March 2, 2021; Accepted March 5, 2021)

keyword : smart fluid, photorheological fluid, phosphorescence, linearization, LSTM RNN

keyword :