Myung-Jin Suk

Fabrication of a porous material with a porosity gradient by a pulsed electric current sintering process

A porous structure with a porosity gradient can be applied to the preparation of continuous FGM, where liquid or chemical vapor of the second phase is infiltrated into the graded pores. It also has applications in skeletal implant materials and ultrafiltration media. An attempt was made to fabricate a porous material with a porosity gradient by means of a pulsed electric current sintering (PECS) process. The present work describes not only the measured value of the temperature difference between the upper and lower part of the specimen, which brings about a gradual change in pore distribution, but also the sintering characteristics of the porous structure obtained by the pressureless PECS process.

Microstructure of porous Cu fabricated by freeze-drying process of CuO/camphene slurry

Abstract Porous Cu with macroscopically aligned channels was synthesized using a freeze-drying process. Camphene-based CuO slurry was prepared by milling at 60 °C with a small amount of dispersant. Freezing of a slurry was done at −25 °C while unidirectionally controlling the growth direction of the camphene. Pores were generated subsequently by sublimation of the camphene during drying. The green body was hydrogen-reduced at 300 °C for 30 min, and sintered in the furnace at 700 °C for 1 h under a hydrogen atmosphere. Microstructural observation reveals that all of the sintered samples are composed of only Cu phase and show macroscopic open pores with an average size of 100 μm which are aligned along its macroscopic growth direction. The internal wall of the macroscopic aligned pore shows relatively small pores due to the traces of the camphene left between the concentrated Cu particles on the internal wall. Increase in the porosity and pore size with increasing camphene content was explained by the change of the growth behavior of the camphene crystals.

Micropatterns of W-Cu Composites Fabricated by Metal Powder Injection Molding

The metal powder injection molding (MIM) process has been applied to fabricate micropatterns of W-Cu composites. A 150μm×150μm×300 μm column array patterned lost plastic mold was used as the mold insert. Several parameters were examined to overcome limitations of lost plastic molding such as low plastic strength, unvented blind hole structure and parting line. Molding temperature was a more dominant factor than molding pressure for the complete filling of feedstock into the micro patterns. The intrinsic defects originating from the lost plastic mold could be eliminated by the re-injection molding of the disc-shaped green part in a vacuum. The final micropatterns of W-Cu composite were fabricated by sintering at 1100°C and 1300°C for 1 h.

Structural size effects of intermetallic compounds on the mechanical properties of Mo-Si-B alloy: An experimental investigation

In general, size, shape and dispersion of phases in alloys significantly affect mechanical properties. In this study, the mechanical properties of Mo-Si-B alloys were experimentally investigated with regards to the refinement of intermetallic compound. To confirm the size effect of the intermetallic compound phases on mechanical properties, two differently sized intermetallic compound powders consisting Mo5SiB2 and Mo3Si were fabricated by mechano-chemical process and high-energy ball milling. A modified powder metallurgy method was used with core-shell intermetallic powders where the intermetallic compound particles were the core and nano-sized Mo particles which formed by the hydrogen reduction of Mo oxide were the shells, leading to the microstructures with uniformly distributed intermetallic compound phases within a continuous α-Mo matrix phase. Vickers hardness and fracture toughness were measured to examine the mechanical properties of sintered bodies. Vickers hardness was 472 Hv for the fine intermetallic compound powder and 415 Hv for the coarse intermetallic compound powder. The fracture toughness was 12.4 MPa·√m for the fine IMC powders and 13.5 MPa·√m for the coarse intermetallic compound powder.

W-Particle shape in liquid Cu during heat treatment of Cu-W composite powder prepared by mechanical alloying

The equilibrium morphology of W-particles in liquid Cu was observed to be spherical (nonfaceted). When W and Cu were subjected to mechanical alloying treatment, the morphology of the W-particle in contact with liquid Cu was faceted. With prolonged heat treatment, the W-particle gradually became nonfaceted in morphology. The time needed to attain the equilibrium nonfaceted morphology depends on the duration of the mechanical alloying process. The present paper describes the morphological variation in the W-particle in liquid Cu that occurs during the heat treatment of Cu-5wt.%W composite powder prepared by a mechanical alloying process. The effect of mechanical alloying treatment on the morphology of the W-particle in contact with liquid Cu is discussed.

Effect of Ball-milling Time on Reduction Behavior in Mechanochemical Process for Preparation of W-Cu Composite Powders

W-Cu composite powders can be prepared by mechanochemical process, where the -CuO composite powders were mechanically synthesized from the elemental oxide powders and subsequently reduced to W-Cu composite powders. In the present work, reduction behavior of-CuO composite powders that were synthesized at different milling time was examined in terms of hygrometric analysis. In case of -CuO ball-milled for 20 h, the reaction temperature of CuO\longrightarrowCu became lower than in case of 1 h. Also, the reaction of \longrightarrow and \longrightarrowwere shifted to lower temperatures and the peaks were changed to much sharper shape. While the reaction of \longrightarrowW in case of ball-milling for 20 h started at lower temperature, the peak temperature was the same as in 1 h ball-milling. The reduced W particle size was somewhat finer fer 20 h ball-milling. It was considered that the refinement of oxide particles caused by ball-milling process leads to such a change in the reduction behavior.