Papers by Author: Jung Ho Ahn

Abstract: In the present work, we have developed two ferritic stainless steels, Fe + 20%Cr + 0.8%Mn + 0.2%Ti + 0.5%Y2O3 and Fe + 17%Cr + 0.1%C + 1%Si + 1%Mn + 0.5%Y2O3 for oxide fuel cell (SOFC) interconnects. Nano-sized Y2O3 particles were dispersed in the metallic matrix by high-energy ball-milling. The alloys exhibited excellent electrical conductivity and thermal expansion coefficient suitable for SOFC applications. The finely dispersed nano-sized yttria is thought to be attributed to high conductivity.

Abstract: Mechanical alloying (MA) process has been examined to synthesize ferritic stainless steel powder dispersed with nano-sized Y2O3 particles. A pilot-scale horizontal mill was fabricated and compared with laboratory-scale ball mills and an attrition mill. Horizontal milling resulted in a much better distribution of particle size and dispersoids than other milling methods. Although horizontal milling is considered as a low-energy process requiring long times, processing time could be markedly reduced with increasing the diameter of horizontal mill with a proper control of milling parameters.

Abstract: Hypereutectic prealloyed Al-20wt.%Si powders were prepared by the gas atomization process. Characteristics of the atomized Al-Si powders were investigated. With increasing gas pressure of the atomization, the average powder size and oxygen content decreased. Primary Si particles in the as-atomized powders were about 8-10 ㎛. Sintering behavior of the atomized Al-Si powders was discussed with sintering temperature. As-atomized powders were hot-pressed and extruded into a cylindrical shape. 96-99% of theoretical density was achieved by the hot consolidation. Microstructure changes as well as hardness and tensile properties of the samples were evaluated in this paper.

Abstract: We have synthesized multi-wall carbon nanotube (MWCNT)-reinforced Al matrix composites. The Al/MWCNT composite powders were prepared by ball milling using pristine Al or ball-milled Al powders as starting materials. The composite powders were consolidated by a conventional cold-compaction, followed by sintering. Uniform dispersion of individual MWCNTs within the matrix was in particular a critical factor for obtaining high density and high quality Al/MWCNT composites. Compared to pristine Al powders as starting materials, the Al powders previously attrition-ball-milled with carbon-based PCA (process control agent) in an ammonia atmosphere resulted in a better distribution of carbon nanotubes within the Al matrix and a higher density after sintering.

Abstract: This study examines the ball-milling of CVD-processed multi-wall carbon nanotubes for cutting and opening their tips. Results show that dry milling causes nanotubes to rapidly collapse, hence transforming into graphite structure, while wet milling with some organic compounds effectively shortened and opened the nanotubes. Carbon-based milling agents are known to alter surface structure of carbon nanotubes, minimizing an excess algglomeration and consequently improving dispersion of nanotubes. The open tip structure was unstable and quite susceptible to re-close during milling and thermal treatments to minimize their surface energy.

Abstract: Oxide-dispersion strengthened (ODS) ferritic stainless steels have been considered as promising high-temperature materials such as interconnects for oxide-fuel cells and nuclear materials for Liquid Metal Fast Reactors or Super-Critical-Water-Cooled Reactors. In the present work, we have prepared Fe-14Cr-2Al-1Si-0.3Ta-1Y2O3 ferritic stainless steels which were dispersion-strengthened by nano-sized Y2O3 via mechanical alloying of elemental powder mixtures and subsequent hot consolidation. A comparison was made with MA 957 and DY-01 alloys. The mechanically alloying behaviour and consolidated mechanical properties of the Fe-14Cr-2Al-1Si- 0.3Ta-1Y2O3 ferritic steels were strongly influenced by processing parameters, especially milling atmosphere. The stability of yttrium oxides and oxidation resistance at high temperatures were examined. The preliminary result shows that the mechanically alloyed Fe-14Cr-2Al-1Si-0.3Ta- 1Y2O3 ferritic stainless steel exhibits interesting properties to be exploited as high temperature materials.

Abstract: Ag and SnO2 nanopowders were synthesized by reverse-micelle method. The reverse micelles were prepared to form tiny aqueous droplets dispersed in oil-based solvents. Two reverse micelles containing metallic salts and reductive agents were rigorously mixed to form nanoparticles inside aqueous droplets by a reductive reaction. The spherical powders of 20~50 nm were formed during the process. The resulting Ag and SnO2 nanopowders were examined as the anode electrode for lithium-ion cells. The reversible discharge capacity of the Ag and SnO2 nanopowders after 25 cycles were 315 and 380 mAh/g, respectively.