Gil-Geun Lee

Effect of Powder Synthesis Atmosphere on the Characteristics of Iron Nanopowder in a Plasma Arc Discharge Process

To investigate the effect of the powder synthesis atmosphere on the characteristics of iron nano powder in a plasma arc discharge process, the hydrogen volume fraction in the powder synthesis atmosphere was changed from 10 to 50%. The particle size, phase structure, and magnetic property of the synthesized iron powder were studied using FE-TEM, XRD, XPS, and a vibration sample magnetometer at room temperature. The particle size increased simultaneously with the increase in the hydrogen volume fraction, and the particle size ranged from about 20 to 100 nm with the change in the hydrogen volume fraction from 10 to 50%. The synthesized iron powder particles had a two-layered shell-core structure composed of α-Fe in the core, Fe3O4 in the inner shell, and FeO(OH) in the outer shell. The thickness of the oxide shells decreased with increasing hydrogen volume fraction in the powder synthesis atmosphere.

Effects of mechanical milling on the carbothermal reduction of oxide of WC/Co hardmetal scrap

The effects of mechanical milling on the carbothermal reduction of oxidized WC/Co hardmetal scrap with solid carbon were examined. Mixed powders were manufactured by milling the WC/Co hard metal scrap oxide and carbon powder in either a tumbler-ball mill or a planetary-ball mill. The milling type affected the carbothermal reduction of the oxide owing to the differing collision energies (mechanical milling energies) in the mills. The hardmetal scrap oxide powder (WO3, CoWO4) milled at high energy was more greatly reduced and at a lower temperature than that milled at lower mechanical energy. The formation of WC by the carburization reaction with solid carbon reached completion at a lower temperature after higher-energy milling than after lower-energy milling. The WC/Co composite particles synthesized by the combined oxidationmechanical milling-carbothermal reduction process were smaller when the initial powder was milled at higher mechanical energy.

The effect of compaction temperature and pressure on the pores in nanostructured metal compacts prepared by magnetic pulsed compaction

The pore behavior of nano Al compacts prepared by magnetic pulsed compaction in a temperature range from 20°C to 300°C and under pressure of 0.7 and 1.6 GPa was studied by small angle neutron scattering (SANS) analysis. The size distribution and volume fraction of pores in the compact were determined by corrected scattering curves using a direct model fitting, under the assumption that the pores were spherical in shape. The densities obtained from the SANS analysis were well matched with those of direct measurement in the range of 76–95% of theoretical density. As the compaction temperature increased from 20°C to 300°C, the size of pores increased from 1.69 to 2.31 nm; however, the number of pores decreased from 169 to 71 (1/cmÅ2) in the case of 1.6 GPa compaction pressure. As a result, the density increased as the compaction temperature increased.

Functionally Graded Dual-Nanoparticulate-Reinforced Aluminum Matrix Composite Materials

Functionally graded carbon nanotubes (CNT) and nano Silicon carbide (nSiC) reinforced aluminum (Al) matrix composite materials were fully densified by a simple ball milling and hot-pressing processes. The nSiC was used as a physical mixing agent to increase dispersity of the CNT in the Al particles. It was observed that the CNT was better dispersed in the Al particles with a nSiC mixing agent compared to without it used. SEM micrograph showed that the interface of the each layers had very tightly adhesion without any serious pores and micro-cracks. This functionally graded dual-nanoparticulate-reinforced Al matrix composite by powder metallurgical approach could also be applied to comples matrix materials.

Effect of Mechanical Milling Parameters on the Particle Size of Silver Flake

Abstract This study is focused on investigating the relation between the particle size of silver flake powder andmechanical milling parameters. Mechanical milling parameters such as ball size, impeller rotation speed and millingtime of the attrition ball-mill were controlled to produce silver flake powder. The particle size of the silver flake powderincreased with increasing ball size and impeller rotation speed. The change of the particle size of the silver flake pow-der with mechanical milling parameters was analyzed based on balls motion in the mill container of the attrition ball-mill. The silver flake particles were formed at the elastic deformation area of the ball due to the collision between balls.The change of the particle size of the silver flake powder with mechanical milling parameters well consists with thechange of the collision energy of ball with parameters mentioned above.Keywords: Flake powder, Mechanical milling, Ball motion, Collision energy ······························································································································· ·································································································

Dynamic Compaction of Mechanochemically Alloyed Fe-Si Nano Powders by Magnetic Pulsed Pressure

Nano Fe-6.5wt%Si powders have been synthesized by mechano-chemical process (MCP) for an application of soft magnetic core. Owing to hard and brittle characteristics of Fe-6.5Si nano powders having large surface area, it is very difficult to reach high density more than 70% of theoretical density (~7.4 g/) by cold compaction. To overcome such problem a magnetic pulsed compaction (MPC), which is one of dynamic compaction techniques, was applied. The green density was achieved about 78% (~5.8 g/) by MPC at room temperature.

Carbothermal Reduction of Oxide of WC/Co Hardmetal Scrap

This study conducted an analysis of the carbothermal reduction of oxidized WC/Co hardmetal scrap with solid carbon under an argon and hydrogen gas atmosphere. The WC/Co hardmetal scrap oxide consisted of WO3 and CoWO4. WO3 was reduced before CoWO4 by the solid carbon regardless of gas atmosphere. The simple oxide WO3 transformed to a tungsten carbide phase without the formation of tungsten under the argon atmosphere. However, WO3 reduced to tungsten and then tungsten carbide was formed under the hydrogen atmosphere. Further, the complex oxide CoWO4 transformed to tungsten carbide and cobalt phases via a Co-W-C system complex carbide phase under the argon atmosphere. However, in contrast to the reaction under the argon atmosphere, under the hydrogen atmosphere an additional phase transformation route was observed, i.e., the formation of tungsten carbide and cobalt phases from CoWO4 through the formation of tungsten oxide and cobalt-tungsten alloy phases. The tungsten carbide formation under argon and hydrogen atmospheres occurred at about 950 °C and 850 °C, respectively. †(Received April 18, 2016; Accepted May 10, 2016)

Fabrication of Silver Flake Powder by the Mechanical Milling Process

This study focuses on fabricating silver flake powder by a mechanical milling process and investigating the formation of flake-shaped particles during milling. The silver flake powder is fabricated by varying the mechanical milling parameters such as the amount of powder, ball size, impeller rotation speed, and milling time of the attrition ballmill. The particle size of the silver flake powder decreases with increasing amount of powder; however, it increases with increasing impeller rotation speed. The change in the particle size of the silver flake powder is analyzed based on elastic collision between the balls, taking energy loss of the balls due to the powder into consideration. The change in the particle size of the silver flake powder with mechanical milling parameters is consistent with the change in the diameter of the elastic deformation contact area of the ball, due to the collision between the balls, with milling parameters. The flake-shaped silver particles are formed at the elastic deformation contact area of the ball due to the collision.

Effect of Added B4C on the Mechanical Properties of WC/Ni-Si Hardmetal

Abstract The effects of B 4 C on the mechanical properties of WC/Ni-Si hardmetal were analyzed using sintered bod-ies comprising WC(70-x wt.%), Ni (28.5 wt.%), Si (1.5 wt.%), and B 4 C (x wt.%), where 0 x 1.2 wt.%. Sampleswere prepared by a combination of mechanical milling and liquid-phase sintering. Phase and microstructure character-izations were conducted using X-ray diffractometry, scanning electron microscopy, and electron probe X-ray micro anal-ysis. The mechanical properties of the sintered bodies were evaluated by measuring their hardness and transverse rupturestrength. The addition of B 4 C improved the sinterability of the hardmetals. With increasing B 4 C content, their hardnessincreased, but their transverse rupture strength decreased. The changes of sinterability and mechanical properties wereattributed to the alloying reaction between B 4 C and the binder metal (Ni, Si).Keywords: Nickel base hardmetal, Boron carbide, Sintering, Mechanical property ······························································································································· ·································································································

The Electric and Thermal Properties of Spark Plasma Sintered Bi 0.5 Sb 1.5 Te 3

The present study was focused on the analysis of the electric and thermal properties of spark plasma sintered thermoelectric material. The crystal structure, microstructure, electric and thermal properties of the sintered body were evaluated by measuring XRD, SEM, electric resistivity, Hall effect and thermal conductivity. The sintered body showed anisotropic crystal structure. The c-axis of the crystal aligned in a parallel direction with applied pressure during spark plasma sintering. The degree of the crystal alignment increased with increasing sintering temperature and sintering time. The electric resistivity and thermal conductivity of the sintered body showed anisotropic characteristics result from crystal alignment.