Young Jung Lee

Chemical vapour transport synthesis and optical characterization of MoO3 thin films

MoO3 thin films were successfully prepared through chemical vapour transport (CVT) deposition and post-annealing. These films showed significantly improved optical properties. It was found that the transmittance reaches 80% with low reflectivity due to improved crystallinity and removal of oxygen vacancy states. Optical analysis shows that the index of refraction is around 1.55 with a flat dispersion curve across the visible. Furthermore, the band-gap energy is estimated to be approximately 3.5eV. These properties suggest that CVT may be an effective thin film deposition technique for low-cost, large-area deposition of molybdenum oxides for chromogenic applications. (Some figures in this article are in colour only in the electronic version)

Phosphate filtering characteristics of a hybridized porous Al alloy prepared by surface modification

In this study, a porous Al alloy filter was designed for water purification systems. The combination of higher permeability for fluid flow and excellent filtering characteristics for removing pollutants is required for water purification. The filters macropore structure was controlled by a powder metallurgical process using granulated powders for high permeability and its micropore structure was generated by alkali surface modification on the macroporous sintered body for enhanced filtration efficiency. After surface modification, the specific surface area was increased by 10 times over the as-sintered specimen. Phosphate filtering characteristic was noticeably improved by a ligand exchange between phosphate and aluminum hydroxide formed by alkali surface modification.

Topological analysis of Au particles in Au/SiO2 nanocomposite films designed for molecular conduction measurement through Voronoi diagram

Monolayered Au/SiO2 nanocomposite films with a high Au particle number density and insulating property were prepared by radio frequency magnetron co-sputtering to develop a new substrate for molecular conduction measurement. The topologies of Au nanoparticles distributed in the SiO2 matrix were statistically evaluated by morphology observation using a field emission scanning electron microscope (FE-SEM) and the Voronoi diagram of a circle set by regarding the Au nanoparticles as a circle generator. The mean Au particle size and the interparticle distance between neighbours increased with deposition time. However, the fraction of the neighbouring Au nanoparticle combinations having interparticle distance shorter than a certain length increased as the deposition time increased. The results also demonstrated that many conducting paths several tens of nanometres long can be created by attaching conductive molecules 2.4 nm long between the Au nanoparticle combinations. Thus this suggests that the nanocomposite substrate can provide a facile way to measure conducting properties of molecules.

Effect of the W—W contiguity on conductivity of W—Cu composite using the Voronoi diagram

Abstract W-15wt.% Cu nanocomposite powder samples were fabricated by ball-milling and subsequent hydrogen-reduction. The powder samples were heated to 1200 °C at 5 and 20 °C min−1 and sintered for 1 h to obtain different sintered microstructures. The homogeneity of the sintered microstructures was evaluated by the standard deviation of the Vickers hardness. The W—W contiguity was calculated by using Voronoi diagrams. The sintered microstructure in case of the 20 °C min−1 sample was more homogeneous and had lower W—W contiguity than the 5 °C min−1 sample. The microstructural homogeneity was directly related to the W—W contiguity. Thermal conductivity of the 20 °C min−1 sample was about 5 % higher than the thermal conductivity of the 5 °C min−1 sample. This indicates that the thermal conductivity is affected by the W—W contiguity resulting from the homogeneity of the sintered microstructure.

Absorption Characteristics of Sound Proof Wall by Scrap Aluminum and Perforated Plate

Efforts to reduce noise in industrial application fields, such as automobiles, aircrafts, and plants have been gaining considerable attention while a sound proof wall to protect people from the noise has been intensively investigated by many researchers. In this study, our research group suggested creating a new sound proof wall composed of scrap aluminum chips and perforated plates in a commercial polyester sound proof wall, which was then successfully fabricated. This walls sound absorption characteristics were measured by an impedance tube method. The sound absorption property was evaluated by measuring the Noise Reduction Coefficient (NRC) to the standard, ASTM C 423-90a. The noise reduction coefficient of the sound proof wall composed of 3.5 vol.% and 7.5 vol.% of scrap aluminum chips relatively increased to 5% and 8% compared to the commercial polyester sound proof wall. The scrap aluminum perforated plate also relatively increased to 13% compared to the commercial polyester sound proof wall.

Effect of Al2O3 particle size on compressive behavior of al composites

Compressive behavior of 7xxx series Al metal matrix composite (MMC) powders with different ceramic contents and different particle size were investigated. As a starting powder of the experiments, ceramic contents of each starting powder were 5 and 10 wt.% and ceramic particle size of starting powder were 20 and 100 ㎛, respectively. And 7xxx Al blended powder was used for comparison. The powders were uniaxially cold-compacted using cylindrical die with a compacting pressure 250 MPa and sintered at 620oC in a dry N2 atmosphere for 60 min with heating rate of 20oC/min. In case of heat treatment condition, sintered parts were solution treated at 475oC and aged at 175oC. To reveal the effect of Al2O3 particle content and particle size on the mechanical properties of composites, compression test were conducted with constant strain rate of 1×10-3/s using sub-size cylindrical samples of 9 mm diameter. Compression test was performed 5 times and its average value was used. Then fractography analysis was conducted using scanning electron microscope.

Fabrication of Nanostructured Fe-Co Alloy Powders by Hydrogen Reduction and Its Magnetic Properties

Magnetic properties of nanostructured materials are affected by the microstructures such as grain size (or particle size), internal strain and crystal structure. Thus, it is necessary to study the synthesis of nanostructured materials to make significant improvements in their magnetic properties. In this study, nanostructured Fe-20at.%Co and Fe-50at.%Co alloy powders were prepared by hydrogen reduction from the two oxide powder mixtures, Fe2O3 and Co3O4. Furthermore, the effect of microstructure on the magnetic properties of hydrogen reduced Fe-Co alloy powders was examined using XRD, SEM, TEM, and VSM.

Effect of Grain Size on Nanostructured Fe-20 wt.%Si Alloy Powders Produced by High-energy ball milling

The structural and magnetic properties of nanostructued alloy powders were investigated. Commercial alloy powders (Hoeganaes Co., USA) with purities were used to fabricate the nanostructure Fe-Si alloy powders through a high-energy ball milling process. The alloy powders were fabricated at 400 rpm for 50 h, resulting in an average grain size of 16 nm. The nanostructured powder was characterized by fcc and hcp phases and exhibited a minimum coercivity of approximately 50 Oe.