• Journal of Powder Materials
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• v.30 no.4
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• pp.297-304
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• 2023

Iron oxide (Fe₂O₃) nanoclusters exhibit significant potential in the biomedical and pharmaceutical fields due to their strong magnetic properties, stability in solutions, and compatibility with living systems. They excel in magnetic separation processes, displaying high responsiveness to external magnetic fields. In contrast to conventional Fe2O3 nanoparticles that can aggregate in aqueous solutions due to their ferrimagnetic properties, these nanoclusters, composed of multiple nanoparticles, maintain their magnetic traits even when scaled to hundreds of nanometers. In this study, we develop a simple method using solvothermal synthesis to precisely control the size of nanoclusters. By adjusting precursor materials and reducing agents, we successfully control the particle sizes within the range of 90 to 420 nm. Our study not only enhances the understanding of nanocluster creation but also offers ways to improve their properties for applications such as magnetic separation. This is supported by our experimental results highlighting their size-dependent magnetic response in water. This study has the potential to advance both the knowledge and practical utilization of Fe₂O₃ nanoclusters in various applications.

• Journal of Powder Materials
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• v.30 no.3
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• pp.268-275
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• 2023

The demand for energy is steadily rising because of rapid population growth and improvements in living standards. Consequently, extensive research is being conducted worldwide to enhance the energy supply. Transpiration power generation technology utilizes the vast availability of water, which encompasses more than 70% of the Earth's surface, offering the unique advantage of minimal temporal and spatial constraints over other forms of power generation. Various principles are involved in water-based energy harvesting. In this study, we focused on explaining the generation of energy through the streaming potential within the generator component. The generator was fabricated using sugar cubes, PDMS, carbon black, CTAB, and DI water. In addition, a straightforward and rapid manufacturing method for the generator was proposed. The PDMS generator developed in this study exhibits high performance with a voltage of 29.6 mV and a current of 8.29 µA and can generate power for over 40h. This study contributes to the future development of generators that can achieve high performance and long-term power generation.

• 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.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.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.

• Journal of Powder Materials
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• v.30 no.5
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• pp.431-435
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• 2023

An irradiation hardening of Inconel 718 produced by selective laser melting (SLM) was studied based on the microstructural observation and mechanical behavior. Ion irradiation for emulating neutron irradiation has been proposed owing to advantages such as low radiation emission and short experimental periods. To prevent softening caused by the dissolution of γ' and γ" precipitates due to irradiation, only solution annealing (SA) was performed. SLM SA Inconel 718 specimen was ion irradiated to demonstrate the difference in microstructure and mechanical properties between the irradiated and non-irradiated specimens. After exposing specimens to Fe³⁺ ions irradiation up to 100 dpa (displacement per atom) at an ambient temperature, the hardness of irradiated specimens was measured by nano-indentation as a function of depth. The depth distribution profile of Fe³⁺ and dpa were calculated by the Monte Carlo SRIM (Stopping and Range of Ions in Matter)-2013 code under the assumption of the displacement threshold energy of 40 eV. A transmission electron microscope was utilized to observe the formation of irradiation defects such as dislocation loops. This study reveals that the Frank partial dislocation loops induce irradiation hardening of SLM SA Inconel 718 specimens.