Dae-Gun Kim

Microstructure and magnetic properties of nanosized Fe-Co alloy powders synthesized by mechanochemical and mechanical alloying process

Abstract An optimum route to synthesize nanosized Fe–Co alloy powder with enhanced magnetic properties was investigated. Two methods of mechanical alloying (MA) and mechanochemical alloying (MCA) for developing a nanosized alloy powder were compared on the basis of the resulting microstructural characteristics and magnetic properties. The alloy powder, synthesized by MCA process with ball milling and hydrogen reduction using Fe2O3 and Co3O4 powders, showed ordered BCC structure with the grain size of 40 nm. Also, this powder exhibited low coercivity of 43 Oe and good permeability compared with MA powder. Enhanced magnetic properties of the MCA powder were explained by the formation of ordered structure and relaxation of internal strain.

Hydrogen-reduction behavior and microstructural characteristics of WO3–CuO powder mixtures with various milling time

Abstract Microstructure and hydrogen reduction behavior of WO 3 –CuO with different milling time are discussed in terms of characteristic of hygrometry curve and W particle size of reduced W–Cu composite powders. With increased milling time, the peak temperatures for the reduction of CuO and WO 3 in hygrometry curve are shifted to low temperatures and the peaks are changed to sharp shape. Microstructural analysis revealed that the hydrogen-reduced powder with milling time of 20 h showed smaller particle size than that of 1 h. The change of reduction behavior and particle size is discussed based on the refinement of oxide powders and resultant increased surface area as well as reduction process of WO 2 by CVT.

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.

Ag/Al(OH)3 mesoporous nanocomposite film as antibacterial agent

In this study, Ag/Al(OH)(3) mesoporous nanocomposite film was fabricated for antibacterial agent. As a matrix, a mesoporous Al(OH)(3) layer was prepared by alkali surface modification of an Al substrate. Subsequently, mesoporous Al(OH)(3) matrix was immersed in a polyol solution for 4h at an elevated temperature to deposit silver nanoparticles (Ag NPs) by in situ dispersion. The Al(OH)(3) porous matrix had flaky crystals weaved and covered all substrate surface so that the specific surface area was considerably increased because of voids having several tens of nanometers between the flakes. After in situ dispersion of Ag NPs by polyol process, the Ag NPs were nucleated and grown at the surface of mesoporous Al(OH)(3) layer with tight binding. In antibacterial activity, Ag/Al(OH)(3) mesoporous nanocomposite film demonstrated an excellent bacterial growth inhibition property. It is believed that the Ag/Al(OH)(3) mesoporous nanocomposite film is suitable for the antibacterial agent in a wide variety of biomedical applications.

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.

Electrochromic Property of MoO3 Thin Films Deposited by Chemical Vapor Transport Synthesis

The transmittance of electrochromic MoO3 thin films by chemical vapor transport (CVT) deposition and post-annealing on indium tin oxide (ITO) glass reached 80% with low reflectivity. Optical analysis demonstrated a 3.60 eV band gap energy in MoO3 thin film. Transmittance changes of 50% between coloration and decoloration (~30 and ~80%) at 533 nm under the bias change frequency revealed reversible electrochromic properties and stability. A coloration efficiency of the annealed MoO3 thin film was 23.7 cm2/C. Coloration responsibility was predominant with reliable performances by bias change.

Integrated nitrogen removal biofilter system with ceramic membrane for advanced post-treatment of municipal wastewater

ABSTRACT The pre-denitrification biofilm process for nitrogen removal was combined with ceramic membrane with pore sizes of 0.05–0.1 µm as a system for advanced post-treatment of municipal wastewater. The system was operated under an empty bed hydraulic retention time of 7.8 h, recirculation ratio of 3, and transmembrane pressure of 0.47 bar. The system showed average removals of organics, total nitrogen, and solids as high as 93%, 80%, and 100%, respectively. Rapid nitrification could be achieved and denitrification was performed in the anoxic filter without external carbon supplements. The residual particulate organics and nitrogen in effluent from biofilm process could be also removed successfully through membrane filtration and the removal of total coliform was noticeably improved after membrane filtration. Thus, a system composed of the pre-denitrification biofilm process with ceramic membrane would be a compact and flexible option for advanced post-treatment of municipal wastewater.

Formation Mechanism of Mesoporous Aluminum Hydroxide Film by Alkali Surface Modification

In this study, a new, relatively simple fabrication method for forming a mesoporous film on Al substrates was demonstrated. This method, i.e., alkali surface modification, was simply comprised of dipping the substrate in a M NaOH solution at for one minute and then immersing it in boiling water for 30 minutes. After alkali surface modification, a mesoporous film was formed on the Al substrate, and its chemical state and crystal structure were confirmed by XPS and TEM. According to the results of the XPS analysis, the flake-like morphology after the alkali surface modification was mainly composed of , with a small amount of . The mesoporous layer was composed of three regions: an amorphousrich region, a region of mixed amorphous and crystal domains, and a crystalline-rich region near the layer surface. It was confirmed that the stabilization process in the alkali surface modification strongly influenced the crystallization of the mesoporous layer.

The Formation of Hybridized Porous Structure of Al Alloy by Alkali Surface Modification

To improve the filtration efficiency of porous materials used in filters, an extensive specific surface area is required to serve as a site for adsorption of impurities. In this paper, a method for creating a hybridized porous alloy using a powder metallurgical technique to build macropores in an Al-4 wt.% Cu alloy and subsequent surface modification for a microporous surface with a considerably increased specific surface area is suggested. The macropore structure was controlled by granulation, compacting pressure, and sintering; the micropore structure was obtained by a surface modification using a dilute NaOH solution. The specific surface area of surface-modified specimen increased about 10 times compare to as-sintered specimen that comprised of the macropore structure. Also, the surface-modified specimens showed a remarkable increase in micropores larger than 10 nm. Such a hybridized porous structure has potential for application in water and air purification filters, as well as membrane pre-treatment and catalysis.