Papers by Author: Ji Hun Yu

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: The nano-sized Fe powders were prepared by plasma arc discharge (PAD) process using pure Fe rod. The microstructure of the prepared nanopowders was evaluated and the effect of hydrogen gas in the chamber atmosphere was investigated. In addition, the sintering behavior of nanosized Fe powders was analyzed and compared with those of conventional micron powders. The prepared Fe nanopowders had nearly spherical shapes and consisted of metallic core and oxide shell structures. In PAD nanopowder sintering, the higher volume shrinkage at low sintering temperature was observed due to the reduction of surface oxide. The PAD nanopowders showed 6 times higher densification rate and more significant isotropic shrinkage behavior than those of micron sized Fe powders.

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: Bi-Te-Sb compound material was developed by spark plasma sintering process. The electrical, thermal, thermoelectric and mechanical properties also obtained by the experiments. This paper discusses performance and strength of the thermoelectric cooler made of Bi-Te-Sb compound alloy. The effect of the current has been investigated. The coefficient of performance (COP) is the primary factor to evaluate the performance of the coolers. Using the FEMLAB software package finite element (FE) analysis was carried out with properties of Bi-Te-Sb compound alloy from experiments. The temperature distributions, the stress distributions and the COP in the thermoelectric cooler were obtained.

Abstract: Nanosized Fe based powders have been synthesized by both chemical vapor condensation (CVC) and plasma arc discharge (PAD) processes. Fe, Fe-C and Fe-N nanopowders were successfully synthesized. The influence of experimental parameters on microstructures and phase composition of nanopowders was investigated. The prepared powders were nearly spherical in shape and core-shell type in structure. Various kinds of Fe-C and Fe-N composite nanopowders could be synthesized by controlling the carrier gas and precursor decomposition temperature, etc.