• Journal of Powder Materials
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• v.30 no.3
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• pp.203-209
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• 2023

Aluminum alloy-based additive manufacturing (AM) has emerged as a popular manufacturing process for the fabrication of complex parts in the automotive and aerospace industries. The addition of an inoculant to aluminum alloy powder has been demonstrated to effectively reduce cracking by promoting the formation of equiaxed grains. However, the optimization of the AM process parameters remains challenging owing to their variability. In this study, the response surface methodology (RSM) was used to predict the crack density of AM-processed Al alloy samples. RSM was performed by setting the process parameters and equiaxed grain ratio, which influence crack propagation, as independent variables and designating crack density as a response variable. The RSM-based quadratic polynomial models for crack-density prediction were found to be highly accurate. The relationship among the process parameters, crack density, and equiaxed grain fraction was also investigated using RSM. The findings of this study highlight the efficacy of RSM as a reliable approach for optimizing the properties of AM-processed parts with limited experimental data. These results can contribute to the development of robust AM processing strategies for the fabrication of high-quality Al alloy components for various applications.

• Journal of Powder Materials
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• v.30 no.3
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• pp.217-222
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• 2023

Machine learning-based data analysis approaches have been employed to overcome the limitations in accurately analyzing data and to predict the results of the design of Nb-based superalloys. In this study, a database containing the composition of the alloying elements and their room-temperature tensile strengths was prepared based on a previous study. After computing the correlation between the tensile strength at room temperature and the composition, a material science analysis was conducted on the elements with high correlation coefficients. These alloying elements were found to have a significant effect on the variation in the tensile strength of Nb-based alloys at room temperature. Through this process, a model was derived to predict the properties using four machine learning algorithms. The Bayesian ridge regression algorithm proved to be the optimal model when Y, Sc, W, Cr, Mo, Sn, and Ti were used as input features. This study demonstrates the successful application of machine learning techniques to effectively analyze data and predict outcomes, thereby providing valuable insights into the design of Nb-based superalloys.

• Journal of Powder Materials
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• v.30 no.3
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• pp.249-254
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• 2023

The influence of MgO addition on the densification and microstructure of alumina (Al₂O₃) was studied. Compacted alumina specimens were manufactured using ball-milling and one-directional pressing followed by sintering at temperatures below 1700℃. Relative density, shrinkage, hardness, and microstructure were investigated using analytical tools such as FE-SEM, EDS, and XRD. When the MgO was added up to 5.0 wt% and sintered at 1500℃ and 1600℃, the relative density exhibited an average value of 97% or more at both temperatures. The maximum density of 99.2% was with the addition of 0.5 wt% MgO at 1500℃. Meanwhile, the specimens showed significantly lower density values when sintered at 1400℃ than at 1500℃ and 1600℃ owing to the relatively low sintering temperature. The hardness and shrinkage data also showed a similar trend in the change in density, implying that the addition of approximately 0.5 wt% MgO can promote the densification of Al₂O₃. Studying the microstructure confirmed the uniformity of the sintered alumina. These results can be used as basic compositional data for the development of MgO-containing alumina as high-dielectric insulators.

• Journal of Powder Materials
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• v.30 no.3
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• pp.242-248
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• 2023

The Ag/WC electrical contacts were prepared via powder metallurgy using 60 wt% Ag, 40 wt% WC, and small amounts of Co₃O₄ with varying WC particle sizes. After the fabrication of the contact materials, microstructure observations confirmed that WC-1 had an average grain size (AGS) of 0.27 ㎛, and WC-2 had an AGS of 0.35 ㎛. The Ag matrix in WC-1 formed fine grains, whereas a significantly larger and continuous growth of the Ag matrix was observed in WC-2. This indicates the different flow behaviors of liquid Ag during the sintering process owing to the different WC sizes. The electrical conductivities of WC-1 and WC-2 were 47.8% and 60.4%, respectively, and had a significant influence on the Ag matrix. In particular, WC-2 exhibited extremely high electrical conductivity owing to its large and continuous Ag-grain matrix. The yield strengths of WC-1 and WC-2 after compression tests were 349.9 MPa and 280.7 MPa, respectively. The high yield strength of WC-1 can be attributed to the Hall-Petch effect, whereas the low yield strength of WC-2 can be explained by the high fraction of high-angle boundaries (HAB) between the WC grains. Furthermore, the relationships between the microstructure, electrical/mechanical properties, and deformation mechanisms were evaluated.

• Journal of Powder Materials
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• v.30 no.6
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• pp.463-469
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• 2023

Aluminum alloys, known for their high strength-to-weight ratios and impressive electrical and thermal conductivities, are extensively used in numerous engineering sectors, such as aerospace, automotive, and construction. Recently, significant efforts have been made to develop novel aluminum alloys specifically tailored for additive manufacturing. These new alloys aim to provide an optimal balance between mechanical properties and thermal/electrical conductivities. In this study, nine combinatorial samples with various alloy compositions were fabricated using direct energy deposition (DED) additive manufacturing by adjusting the feeding speeds of Al6061 alloy and Al-12Si alloy powders. The effects of the alloying elements on the microstructure, electrical conductivity, and hardness were investigated. Generally, as the Si and Cu contents decreased, electrical conductivity increased and hardness decreased, exhibiting trade-off characteristics. However, electrical conductivity and hardness showed an optimal combination when the Si content was adjusted to below 4.5 wt%, which can sufficiently suppress the grain boundary segregation of the α-Si precipitates, and the Cu content was controlled to induce the formation of Al₂Cu precipitates.

• Journal of Powder Materials
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• v.30 no.6
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• pp.509-515
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• 2023

Lead-free perovskite ceramics, which have excellent energy storage capabilities, are attracting attention owing to their high power density and rapid charge-discharge speed. Given that the energy-storage properties of perovskite ceramic capacitors are significantly improved by doping with various elements, modifying their chemical compositions is a fundamental strategy. This study investigated the effect of Zn doping on the microstructure and energy storage performance of potassium sodium niobate (KNN)-based ceramics. Two types of powders and their corresponding ceramics with compositions of (1-x)(K,Na)NbO₃-xBi(Ni_2/3Ta_1/3)O₃ (KNN-BNT) and (1-x)(K,Na)NbO₃-xBi(Ni_1/3Zn_1/3Ta_1/3)O₃ (KNN-BNZT) were prepared via solid-state reactions. The results indicate that Zn doping retards grain growth, resulting in smaller grain sizes in Zn-doped KNN-BNZT than in KNN-BNT ceramics. Moreover, the Zn-doped KNN-BNZT ceramics exhibited superior energy storage density and efficiency across all x values. Notably, 0.9KNN-0.1BNZT ceramics demonstrate an energy storage density and efficiency of 0.24 J/cm³ and 96%, respectively. These ceramics also exhibited excellent temperature and frequency stability. This study provides valuable insights into the design of KNN-based ceramic capacitors with enhanced energy storage capabilities through doping strategies.

• Journal of Powder Materials
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• v.30 no.6
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• pp.521-527
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• 2023

Piezoelectric technology, which converts mechanical energy into electrical energy, has recently attracted drawn considerable attention in the industry. Among the many kinds of piezoelectric materials, BaTiO₃ nanotube arrays, which have outstanding uniformity and anisotropic orientation compared to nanowire-based arrays, can be fabricated using a simple synthesis process. In this study, we developed a flexible piezoelectric energy harvester (f-PEH) based on a composite film with PVDF-coated BaTiO₃ nanotube arrays through sequential anodization and hydrothermal synthesis processes. The f-PEH fabricated using the piezoelectric composite film exhibited excellent piezoelectric performance and high flexibility compared to the previously reported BaTiO₃ nanotube array-based energy harvester. These results demonstrate the possibility for widely application with high performance by our advanced f-PEH technique based on BaTiO₃ nanotube arrays.

• Journal of Powder Materials
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• v.30 no.6
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• pp.470-477
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• 2023

The effects of annealing on the microstructure and mechanical properties of Al-Zn-Mg-Cu-Si alloys fabricated by high-energy ball milling (HEBM) and spark plasma sintering (SPS) were investigated. The HEBM-free sintered alloy primarily contained Mg₂Si, Q-AlCuMgSi, and Si phases. Meanwhile, the HEBM-sintered alloy contains Mg-free Si and θ-Al₂Cu phases due to the formation of MgO, which causes Mg depletion in the Al matrix. Annealing without and with HEBM at 500℃ causes partial dissolution and coarsening of the Q-AlCuMgSi and Mg₂Si phases in the alloy and dissolution of the θ-Al₂Cu phase in the alloy, respectively. In both alloys, a thermally stable α-AlFeSi phase was formed after long-term heat treatment. The grain size of the sintered alloys with and without HEBM increased from 0.5 to 1.0 ㎛ and from 2.9 to 6.3 ㎛, respectively. The hardness of the sintered alloy increases after annealing for 1 h but decreases significantly after 24 h of annealing. Extending the annealing time to 168 h improved the hardness of the alloy without HEBM but had little effect on the alloy with HEBM. The relationship between the microstructural factors and the hardness of the sintered and annealed alloys is discussed.

• Journal of Powder Materials
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• v.30 no.6
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• pp.484-492
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• 2023

In this study, Ni-Y₂O₃ powder was prepared by alloying recomposition oxidation sintering (AROS), solution combustion synthesis (SCS), and conventional mechanical alloying (MA). The microstructure and mechanical properties of the alloys were investigated by spark plasma sintering (SPS). Among the Ni-Y₂O₃ powders synthesized by the three methods, the AROS powder had approximately 5 nm of Y₂O₃ crystals uniformly distributed within the Ni particles, whereas the SCS powder contained a mixture of Ni and Y₂O₃ nanoparticles, and the MA powder formed small Y₂O₃ crystals on the surface of large Ni particles by milling the mixture of Ni and Y₂O₃. The average grain size of Y₂O₃ in the sintered alloys was approximately 15 nm, with the AROS sinter having the smallest, followed by the SCS sinter at 18 nm, and the MA sinter at 22 nm. The yield strength (YS) of the SCS- and MA-sintered alloys were 1511 and 1688 MPa, respectively, which are lower than the YS value of 1697 MPa for the AROS-sintered alloys. The AROS alloy exhibited improved strength compared to the alloys fabricated by SCS and conventional MA methods, primarily because of the increased strengthening from the finer Y₂O₃ particles and Ni grains.

• Journal of Powder Materials
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• v.30 no.6
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• pp.516-520
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• 2023

Highly safe lithium-ion batteries (LIBs) are required for large-scale applications such as electrical vehicles and energy storage systems. A highly stable cathode is essential for the development of safe LIBs. LiFePO₄ is one of the most stable cathodes because of its stable structure and strong bonding between P and O. However, it has a lower energy density than lithium transition metal oxides. To investigate the high energy density of phosphate materials, vanadium phosphates were investigated. Vanadium enables multiple redox reactions as well as high redox potentials. LiVPO₄O has two redox reactions (V⁵⁺/V⁴⁺/V³⁺) but low electrochemical activity. In this study, LiVPO₄O is doped with fluorine to improve its electrochemical activity and increase its operational redox potential. With increasing fluorine content in LiVPO₄O_1-xF_x, the local vanadium structure changed as the vanadium oxidation state changed. In addition, the operating potential increased with increasing fluorine content. Thus, it was confirmed that fluorine doping leads to a strong inductive effect and high operating voltage, which helps improve the energy density of the cathode materials.