Wooyeol Kim

Compressibility of hierarchic-architectured agglomerates of hydrogen-reduced copper nanopowders

AbstractPure copper nanopowders are hydrogen-reduced in order to eliminate surface oxides and produce hierarchic architectures having inner-sponge structures with partially bonded nano/ultrafine particles and outer irregular-agglomerate boundaries. Due to a decrease in surface area by the particle bonding, the newly designed agglomerates exhibit improved surface stability after reduction, resulting in enhanced oxidation-resistance in the air at room temperature. For a comparative analysis, we also prepare two conventional micropowders having spherical and irregular particles. The compressibility of these three types of powders is analyzed using mechanical compaction. Finite element analyses on the compaction behaviors of the spherical and irregular particles are performed. The mechanical properties and microstructures of the compacts are investigated using microhardness tests, X-ray diffraction, and electron backscatter diffraction technique. Dislocation density, crystallite size, and grain size are correlated with the mechanical and compaction behaviors. From the analyses, three advantages of the hydrogen-reduced copper nanopowder are noted: (1) suppression of oxidation while maintaining nano/ultrafine structure of particles, (2) lower pressure required for high-density compaction than for spherical powders with nano scale, and (3) favorable fabrication of bulk nano/ultrafine structures without cracks or fracture.

Planar Shock Wave Compaction of Oxidized Copper Nano Powders using High Speed Collision and Its Mechanical Properties

Bulk nanostructured copper was fabricated by a shock compaction method using the planar shock wave generated by a single gas gun system. Nano sized powders, average diameter of 100 nm, were compacted into the capsule and target die, which were designed to eliminate the effect of undesired shock wave, and then impacted with an aluminum alloy target at 400 m/s. Microstructure and mechanical properties of the shock compact specimen were analyzed using an optical microscope (OM), scanning electron microscope (SEM), and micro indentation. Hardness results showed low values (approximately 45~80 Hv) similar or slightly higher than those of conventional coarse grained commercial purity copper. This result indicates the poor quality of bonding between particles. Images from OM and SEM also confirmed that no strong bonding was achieved between them due to the insufficient energy and surface oxygen layer of the powders.

Effect of target-fixture geometry on shock-wave compacted copper powders

In shock compaction with a single gas gun system, a target fixture is used to safely recover a powder compact processed by shock-wave dynamic impact. However, no standard fixture geometry exists, and its effect on the processed compact is not well studied. In this study, two types of fixture are used for the dynamic compaction of hydrogen-reduced copper powders, and the mechanical properties and microstructures are investigated using the Vickers microhardness test and electron backscatter diffraction, respectively. With the assistance of finite element method simulations, we analyze several shock parameters that are experimentally hard to control. The results of the simulations indicate that the target geometry clearly affects the characteristics of incident and reflected shock waves. The hardness distribution and the microstructure of the compacts also show their dependence on the geometry. With the results of the simulations and the experiment, it is concluded that the target geometry affects the shock wave propagation and wave interaction in the specimen.

Manufacturing and Evaluation of Properties of Nanocrystalline Ni bulk by Dynamic Compaction of Nano Ni powders using a Gas-gun System

In this study, nanocrystalline nickel powders were cold compacted by a dynamic compaction method using a single-stage gas gun system. A bending test was conducted to measure the bonding strengths of the compacted regions and microstructures of the specimen were analyzed using a scanning electron microscopy. The specimen was separated into two parts by a horizontal crack after compaction. Density test shows that the powder compaction occurred only in the upper part of the specimen. Brittle fracture was occurred during the bending test of the compact sample. Dispersion of shock energy due to spalling highly affected the bonding status of the nanocrystalline nickel powder.

Manufacturing and Evaluation of the Properties of Hybrid Bulk Material by Shock-compaction of Nanocrystalline Cu-Ni Mixed Powder

Abstract In this study, nanocrystalline Cu-Ni bulk materials with various compositions were cold compacted by ashock compaction method using a single-stage gas gun system. Since the oxide layers on powder surface disturbs bond-ing between powder particles during the shock compaction process, each nanopowder was hydrogen-reduced to removethe oxide layers. X-ray peak analysis shows that hydrogen reduction successfully removed the oxide layers from thenano powders. For the shock compaction process, mixed powder samples with various compositions were preparedusing a roller mixer. After the shock compaction process, the density of specimens increased up to 95% of the relativedensity. Longitudinal cross-sections of the shock compacted specimen demonstrates that a boundary between two pow-ders are clearly distinguished and agglomerated powder particles remained in the compacted bulk. Internal crack tendedto decrease with an increase in volumetric ratio of nano Cu powders in compacted bulk, showing that nano Cu powdershas a higher coherency than nano Ni powders. On the other hand, hardness results are dominated by volume fraction ofthe nano Ni powder. The crystalline size of the shock compacted bulk materials was greatly reduced from the initialpowder crystalline size since the shock wave severely deformed the powders.Keywords: Nanocrystalline metallic powder, Gas gun system, Shock compaction

Thermoelectric Properties of Bi 2 Te 2.7 Se 0.3 Powder Synthesized by an Oxide-Reduction Process

The present study focused on the synthesis of Bi-Te-Se-based powder by an oxide-reduction process, and analysis of the thermoelectric properties of the synthesized powder. The phase structure, chemical composition, and morphology of the synthesized powder were analyzed by XRD, EPMA and SEM. The synthesized powder was sintered by spark plasma sintering. The thermoelectric properties of the sintered body were evaluated by measuring its Seebeck coefficient, electrical resistivity, and thermal conductivity. powder was synthesized from a mixture of , , and powders by mechanical milling, calcination, and reduction. The sintered body of the synthesized powder exhibited n-type thermoelectric characteristics. The thermoelectric properties of the sintered bodies depend on the reduction temperature. The Seebeck coefficient and electrical resistivity of the sintered body were increased with increasing reduction temperature. The sintered body of the powder synthesized at showed about 0.5 of the figure of merit (ZT) at room temperature.

Thermoelectric Properties of Bi 0.4 Sb 1.6 Te 3 Sintered Body Fabricated by Mechanical Grinding Process

The present study is to analyze the thermoelectric properties of thermoelectric materials fabricated by the mechanical grinding process. The powders were prepared by the combination of mechanical milling and reduction treating methods using simply crushed pre-alloyed powder. The mechanical milling was carried out using the tumbler-ball mill and planetary ball mill. The tumbler-ball milling had an effect on the carrier mobility rather than the carrier concentration, whereas, the latter on the carrier concentration. The specific electric resistivity and Seebeck coefficient decreased with increasing the reduction-heat-treatment time. The thermal conductivity continuously increased with increasing the reduction-heat-treatment time. The figure of merit of the sintered body prepared by the mechanical grinding process showed higher value than one of the sintered body of the simply crushed powder.