발간논문

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In this study, we researched changes in the properties of a Thermal Barrier Coating depending on the powder structure. For this purpose, we used YSZ (Yttria Stabilized Zirconia), a commercial Thermal Barrier Coating material, to produce a powder with a Core-Shell structure. Bulk samples were prepared by hot pressing to analyze their properties according to the powder structure, and Thermal Barrier Coating samples were prepared by APS (Atmospheric Plasma Spray) to compare differences in properties according to the powder structure. The results of the bulk sample analysis showed that the thermal conductivity of YSZ was 3~4.2 W/m*K, the CeO₂ mixed structure was 2.2~3.3 W/m*K, and the Core-Shell Composite was 2.2~2.9 W/m*K. The thermal Barrier Coating sample analysis showed that the TGO growth behavior was different depending on the powder structure. The YSZ coating sample was 7.24 ㎛, the YSZ+CeO₂ coating sample was 6.68 ㎛, and the Core-Shell coating sample was 4.79 ㎛. In the case of high-temperature thermal conductivity, YSZ and YSZ+CeO₂ showed similar results, but the Core-Shell coating sample had 79.07% thermal conductivity, compared to YSZ at 1000oC. These results indicate that the core-shell composite has improved thermal insulation performance and mechanical properties compared to YSZ, and it is expected that the core-shell composite will exhibit improved thermal properties compared to YSZ when applied to Thermal Barrier Coating.

keyword : Core-Shell, Thermal Barrier Coating, APS, YSZ, CeO₂

This study investigated the effect of a Ni-P coating layer on the solidification cracking behavior of Cu-mild steel dissimilar welds during the manufacturing of cylindrical Li-ion battery packs for electric vehicles. Four Cu plates were prepared and characterized: uncoated Cu and three levels (12, 50, and 100 μm) of Ni-P-coated Cu. The welding experiments used a single-mode fiber laser (2 kW) at extremely low heat input (1.82 J/mm) and high welding speed (1100 mm/s). Three laser beam patterns were used: linear, spiral, and wobble+spiral. Solidification cracking was detected for the Cu-Steel dissimilar welds for all the laser beam patterns on the uncoated Cu and the 50 and 100 μm Ni-P-coated Cu materials. Conversely, the dissimilar welds using 12 μm of Ni-P-coated Cu considerably suppressed solidification cracking behavior. Similarly, the welds with suppressed solidification cracking (using 12 μm of Ni-P-coated Cu) exhibited superior mechanical properties under the laser beam pattern. The weakest mechanical properties were confirmed for the welds using 100 μm of Ni-P-coated Cu. The solidification cracking and mechanical properties were highly dependent on the weld solidification of Ni and P. The suppression of solidification cracking in the welds using 12 μm of Ni-P coated Cu was attributed to the reduction in the weld mushy zone temperature range, due to the mixing of Ni, which reduced the solidification segregation of Cu. In contrast, the severe solidification cracking for the welds using 50 and 100 μm of Ni-P-coated Cu was estimated to result from the increased amount of incorporated P, which expands the weld mushy zone range.

keyword : Lithium-ion battery pack, Cu-Steel dissimilar welding, Ni-P coated layer, Solidification cracking, Singlemode fiber laser

To form excellent solder joints in both thermal cycling and drop tests, Sn-2.5Ag-0.8Cu-0.05Ni-1Bi and Sn-0.75Cu-0.065Ni-1.5Bi composition solder balls were developed. In this study, undercooling and solidification characteristics of the alloys, resulting microstructural changes, the solid solution effect of Bi, physical properties, and interfacial reaction properties were investigated and compared with existing solder compositions of SAC305 and SAC1205N. The Sn-2.5Ag-0.8Cu-0.05Ni-1Bi and Sn-0.75Cu-0.065Ni-1.5Bi solders were found to have large undercooling of 38.36 ℃ and 33.38 ℃, respectively. As a result, the Sn-2.5Ag- 0.8Cu-0.05Ni-1Bi solder ball had the smallest average size of Sn grains, and the eutectic structures between Sn grains formed relatively small areas and were observed to solidify into fine and uniform structures. Consequently, the total area of the β-Sn phase decreased, while the total area of the eutectic structure relatively increased. Using XRD and STEM analysis, we observed that the addition of a small amount of Bi resulted in a solid solution of the β-Sn phase, which increased the interplanar spacing of certain crystal planes, and contributed to the improvement in mechanical properties such as the hardness of the β-Sn phase. When using the Sn-2.5Ag-0.8Cu-0.05Ni-1Bi solder ball, the intermetallic compound (IMC) layer at the bottom Cu pad interface of the solder joint was relatively thin from right after reflow soldering and maintained a thin thickness throughout the thermal cycling test. The growth suppression property of the IMC layer by Sn-2.5Ag- 0.8Cu-0.05Ni-1Bi composition was also confirmed in cases where the paste of this composition was applied to the existing solder ball.

keyword : solder alloy, solidification, undercooling, solid solution, microstructure

The development and design of metal materials have been carried out through experimental method and simulation based on theoretic. Recently, with the widespread application of artificial intelligence (AI) in various fields, many studies have been actively incorporating artificial intelligence into the field of metal material design. Especially, many studies have been reported on adding rare-earth elements to aluminum alloys to improve corrosion resistance and mechanical properties using AI. However, the performance evaluation of artificial intelligence through experimental verification has not yet been reported related to metal material. In this study, we investigated the artificial intelligence algorithm capable of predicting the hardness based on the composition ratio of aluminum alloy with added Lanthanum (La) using experimental data and conducted a comparative analysis of the predicted hardness values. The machine learning models employed Adaptive Boosting Regressor (ADA), Gradient Boosting Regressor (GBR), Random Forest Regressor (RF), and Extra Trees Regressor (ET). The dataset comprised 1,210 encompassing 9 composition elements constituting the alloy. In the result, the findings revealed that the ET model demonstrated the most effective performance in predicting hardness. In addition, the microstructure became fine and showed the highest hardness at 0.5 wt.% La and hardness tended to decrease as the amount of La increased. The ET model showed excellent performance in predicting this tendency through experimental verification.

keyword : Machine learning, Experimental Verification, Aluminum alloy, Rare-earth element, Hardness

Metal foam materials are used for various purposes including electrode materials, catalyst filters, and gas diffusion filters due to their porous structure. Increasing demand for metal foams has generated research to increase porosity as well as produce different pore sizes. The present paper illustrates a comparison of open-cell aluminum foams prepared using the space holder technique. The Al foams were fabricated by two different methods: spark plasma sintering (SPS) and the compression molding (CM) method. The effect of the content of sodium chloride particles, used as the space holder, as well as manufacturing technologies on the Al foam structure and their mechanical properties were investigated. The morphology and structure of the obtained Al foams were analyzed by scanning electron microscopy (SEM) and micro-computed tomography (CT). Compressive testing was performed to investigate mechanical properties. The porosity of the SPS Al foam sample was 61-74%, and was 60-72% for the CM sample. The compressive strength and Young’s modulus were 1.40 MPa, 1.41×10_-2 GPa for the SPS sample and 0.9 MPa, 1.33×10_-2 GPa for the CM sample, respectively. The space holder technique is a promising technique for fabricating metal foam materials for cathode current collectors in lithium-ion batteries applications.

keyword : Al foam, Spark plasma sintering, Compression molding, Space holder technique, Powder metallurgy

Skutterudite compounds have excellent thermoelectric performance in the intermediate-to high temperature range. Their lattice thermal conductivity can be reduced by intensifying phonon scattering through independent vibrations of the guest atoms, by filling the voids within the lattice. Furthermore, the thermoelectric figure of merit (ZT) can be enhanced by optimizing the carrier concentration through charge compensation between transition elements. In this study, we compared the thermoelectric properties of p-type filled skutterudite materials, R_yFe_4-xCo_xSb₁₂, where R represents rare-earth elements (La/Ce/Pr/Nd/Yb), which were filled in the voids, and Co was charge-compensated at the Fe site. In the case of La_yFe_4-xCo_xSb₁₂, the introduction of La filling and Co doping led La_0.9Fe₃CoSb₁₂ to exhibit a high power factor and low thermal conductivity (ZT = 0.67 at 723 K). In the case of Ce_yFe_4-xCo_xSb₁₂, in addition to Ce filling, the substitution of Co for Fe resulted in additional lattice scattering, leading to a decrease in thermal conductivity. However, CeFe₄Sb₁₂ exhibited a maximum performance of ZT = 0.70 at 823 K. In the case of Pr_yFe4_-xCo_xSb₁₂, the thermal conductivity was reduced through phonon scattering induced by Pr filling and additional lattice scattering caused by Co substitution; as a result, Pr_0.8Fe₃CoSb₁₂ exhibited ZT = 0.89 at 723 K. In the case of Nd_yFe_4-xCo_xSb₁₂, the phonon scattering was enhanced by adjusting the filling of Nd and substitution of Co, resulting in a lower thermal conductivity; Nd_0.9Fe_3.5Co_0.5Sb₁₂ exhibited ZT = 0.91 at 723 K. For Yb_yFe_4-xCo_xSb₁₂, Yb_0.9Fe₃CoSb₁₂ exhibited a thermoelectric performance of ZT = 0.56 at 823 K. In addition, in this study, for the fabrication (application) of thermoelectric modules, the p-type Nd_0.9Fe_3.5Co_0.5Sb₁₂ skutterudite, which exhibited the best thermoelectric performance, was prepared in bulky compacts to verify the uniformity and reproducibility of its thermoelectric performance.

keyword : thermoelectric, skutterudite, rare-earth filling, charge compensation, reproducibility, uniformity

Bi₂Te₃ shows high thermoelectric performance near room temperature, making it the most widely used material in thermoelectric cooling applications. Cu doping has been found to be effective in improving the thermoelectric performance of Bi₂Te₃. However, due to the problem of easy migration of Cu ions, the stability of Cu-doped Bi₂Te₃ is always an issue, and therefore worth exploring. This study utilizes the Single Parabolic Band (SPB) model to analyze the electronic transport properties of Cu_xBi_0.3Sb_1.7-xTe₃. We investigate how electronic band parameters (effective mass, non-degenerate mobility, weighted mobility, and B-factor) evolve with increasing Cu content (x). Additionally, the influence of electric current pulse (ECP) treatment is examined. Experimentally, the zT of x = 0.001 was higher than x = 0.0025 samples near room temperature. However, the SPB model predicts that due the higher B-factor of the x = 0.0025 sample, its theoretical maximum zT can be as high as ~1.48 at 350 K. Based on literature data on thermoelectric transport properties in the x = 0.001 sample after the ECP treatment, the impact of the ECP treatment on the electronic band parameters and the lattice thermal conductivity of the x = 0.0025 sample is estimated. ECP treatment slightly reduces electrical performance below 350 K, but it significantly suppresses the lattice thermal conductivity, ultimately leading to an enhanced zT. The predicted maximum zT reaches ~1.54 at 300 K.

keyword : Bi_{2Te3, Single Parabolic Band model, density-of-states effective mass, non-degenerate mobility, weighted mobility, Electric current pulse treatment}

SnO₂ nano/micro-crystals with different morphologies were fabricated by the thermal evaporation of SnO₂ powders mixed with Ga2O₃ powder. The synthesis process was performed at 1300oC in air. X-ray diffraction (XRD) analysis, energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) were used to examine the morphology, microstructure, elemental composition and chemical property of the as-synthesized products. X-ray diffraction analysis revealed that the products were SnO₂ with a tetragonal rutile crystal structure. From the Fourier transform infrared spectra of the products, Sn-O stretching mode was observed, which confirmed the formation of SnO₂. Scanning electron microscopic analysis clearly showed that the morphology of the SnO₂ crystals was significantly affected by the addition of Ga2O₃ to SnO₂ source powder. SnO₂ crystals with a belt-like morphology were grown when the source powder without Ga2O₃ powder was used. Rod-like SnO₂ crystals were grown by using SnO₂ powder mixed with Ga2O₃ powder as the source powder. When the amount of Ga2O₃ mixed in the source powder was increased, the morphology of the SnO₂ crystals changed from rod to tube. Energy dispersive X-ray analysis indicated that the inner core of the tube-like crystals was composed of Snrich metastable phase. No catalytic particles were observed at the tips of the SnO₂ nano/micro-crystals, suggesting that the growth process occurred by vapor-solid growth mechanism.

keyword : tin oxide, nano, micro-crystals, gallium oxide, morphological change, thermal evaporation

The stacking fault energy (SFE) of face-centered cubic (FCC) alloys is a critical parameter that controls microstructural and crystallographic texture evolution during deformation and annealing treatments. This review focuses on several FCC alloys, aluminum (Al), copper (Cu), austenitic stainless steels (ASSs), and high entropy alloys (HEAs), all of which exhibit varying SFEs. These alloys are often subjected to thermo-mechanical processing (TMP) to enhance their mechanical properties. TMP leads to the evolution of deformation-induced products, such as shear bands (SBs), strain-induced martensite (SIM), and mechanical/deformation twins (DTs) during plastic deformation, while also influencing crystallographic texture. High-medium SFE materials, such as Al and Cu, typically exhibit the evolution of Copper-type texture during room temperature rolling (RTR), while low SFE materials, such as ASSs and HEAs, display Brass-type texture at high reduction ratios. Moreover, the presence of second-phase particles/precipitates can also impact the microstructure and texture evolution in Al and Cu alloys. Particle-stimulated nucleation (PSN) during the annealing treatment has been reported for Al, Cu, ASSs, and HEAs, which causes texture weakening. Another interesting observation in severely deformed Cu alloys is the room-temperature softening phenomenon, which is discussed in the reviewed work. Additionally, plastic deformation and heat treatment of ASSs result in phase transformation, which was not observed in Al, Cu, or HEAs. Furthermore, the dependence of special boundaries in HEAs on plastic deformation temperature, strain rate, and annealing temperature is also discussed. Thus, this review comprehensively reports on the impact of TMP on microstructural and crystallographic texture evolution during plastic deformation and the annealing treatment of Al, Cu, ASSs, and HEAs FCC materials, using results obtained from electron microscopy.

keyword : FCC, Plastic deformation, Texture, EBSD, Recrystallization