Papers by Author: Tae Suk Jang

Abstract: FePt binary alloy nanopowder has been synthesized by a chemical vapor condensation process using a mixture of iron acetylacetonate and platinum acetylacetonate. Particle size of the synthesized powder was less than 10 nm and the powder had very narrow size distribution with relatively high dispersivity. FePt nanopowder possessing L10 ordered phase was synthesized at the condition of well controlled precursor mixing ratio and reaction temperature with some disordered cubic phase.

Abstract: Nanoparticles of TiC and TiCN were synthesized by a magnesium thermal reduction process. The process was performed in the sequence of (1) the transfer of TiCl4+C2Cl4 solutions into a liquefied magnesium protected with Ar, (2) the magnesium reduction of metal chlorides, (3) the formation of a titanium carbide by the reaction of Ti and C atoms released by the magnesium reduction, and finally (4) the vacuum separation of the residual liquid Mg and MgCl2. Titanium carbonitride could be synthesized by maintaining the reaction chamber in nitrogen atmosphere. The average particle size of both TiC and TiCN synthesized was about 80 nm each. The composition of the TiC nanoparticles was approximately TiC0.95 with 0.9 wt % free carbon whereas that of the TiCN nanoparticles was TiC0.46~0.53N0.46~0.51 with 0.1 wt % free carbon.

Abstract: A preplated frame (PPF) consisting of Au/Pd/Cu-Sn/Cu substrate, i.e., preplated with a Cu-Sn alloy instead of Ni commercially used, was fabricated by electroplating and then the feasibility of the frame as an alternative PPF was investigated. The wettability of the Cu-Sn alloy was better than that of Ni, resulting in excellent contact with the substrate and smoother surface on the upper Au/Pd protective layer. By XPS analyses, it was confirmed that Cu atoms in the Cu-Sn alloy layer did not diffuse through the thin protective layer to the surface at temperatures used in IC assembly. The wire pull-strength and solderability of the Cu-Sn alloy PPF were almost equivalent to those of the Ni PPF. However, the former showed much better corrosion resistance than the latter.

Abstract: Nanostructured CeO2 powder was synthesized by a thermochemical process and its applicability as an oxygen gas sensor was investigated. An amorphous precursor was prepared by spray drying a cerium-nitrate solution and then heat-treated in air to induce evaporation of volatile components from the precursor powder, which then forms nanostructured ceria. The powder produced possessed a loose agglomerated structure with extremely fine CeO2 particles about 15 nm in size, resulting in a very high specific surface area (110 m2/g). The average grain size of a specimen sintered from the ceria powder at 1000 oC was about 40 nm, and its oxygen sensitivity n (logσ
log(PO2 /Po)-n) was about 1/4 at 600 oC. The response time t90, time for responding to a change of oxygen partial pressure, of the specimens sintered at 800 ~ 1100 oC was very short, i.e., 3 ~ 5 seconds: The signal output was almost identical and uniformly repeated in a stable manner.

Abstract: The hypereutectic prealloyed Al-Si powders were prepared by gas atomization process. The microstructure and compacting ability of the atomized Al-Si powders were investigated. With increasing the gas pressure of the atomization, the average powder size and oxygen content were decreased. The size of primary eutectic Si particles in the as-atomized powders was about 8~10. As-atomized powders were hot-pressed into a cylindrical shape. Relative density of the hot-pressed samples was achieved about 96~99 % of theoretical density. Some properties of the samples were evaluated in this paper.

Abstract: Iron and its nitride (e-Fe3N) nanoparticles were fabricated by the CVC using Fe(CO)5 precursor without the aid of LN2 chiller. The iron particles synthesized at 400 oC were a mixture of amorphous and crystalline a-Fe. Fully crystallized iron particles were then obtained above 600 oC. Iron-nitride particles that were easily formed at 500 oC at 1 atm., however, were not fully developed in vacuum unless the reaction temperature reached 850 oC. Nevertheless, the work chamber needed to be maintained in vacuum to obtain finer iron-nitride particles. The synthesized particles possessing the core-shell type structure were all nearly spherical and enclosed with Fe3O4 or Fe3O4-related amorphous layer. The iron nanoparticles (~20 nm) synthesized at 600 oC at 760 torr exhibited iHc ~ 1.0 kOe and Ms ~ 170 emu/g, whereas the iron-nitride particles (~20 nm) synthesized at 850 oC at 0.01 torr exhibited iHc ~ 0.45 kOe and Ms ~ 115 emu/g.

Abstract: Effect of washing step in R-D process on the structure and magnetic properties of Nd-Fe-B powder fabricated by the process using a spray-dried precursor was investigated. Although the powder washed in water contained much more residual CaO than that washed in dilute acetic acid, magnetic properties of the former were much better than those of the latter due to less removal of Nd-rich phase. In the powder washed in water Nd2Fe14B particles were enclosed with thin layer of Nd-rich phase, and each particle consisted of one or more subgrains of the size 50 ~ 100 nm or more. Milling the powder before water washing was effective to remove more CaO, thereby increased the remanence of the powder.

Abstract: A new axial pressing method using pulse magnetizing field was studied to improve the remanence of Nd-Fe-B sintered magnets. In pulsed axial die pressing(PADP) process , tapping density of powders, method of pulse supply and strength of pulse field were optimized to increase magnetic alignment and remanence. As a results, we could find that an axial pressing under pulse magnetizing field was very effective to increase the degree of magnetic alignment of powder, yielding remanences even higher than those obtained by the transverse die pressing. The remanence of 14.1 kG was easily obtained by PADP process from 31R-68TM-1B (in wt.%) sintered magnet