Min Ku Lee

Advanced Use of Nanobismuth/Nafion Electrode for Trace Analyses of Zinc, Cadmium, and Lead

Trace analyses of zinc, cadmium, and lead at a surface-modified thick-film graphite electrode with bismuth nanopowder have been carried out using the square wave anodic stripping voltammetry (SWASV) technique. For strong adhesion of the bismuth nanopowder onto the screen-printed carbon paste electrode, a Nafion solution was added to the bismuth-containing suspension. From the analyses of the zeta potential and the scanning electron microscopy image of the electrode surface, it is suggested that Nafion plays an important role in the homogeneous distribution of bismuth nanoparticles as well as in creating a strong chemical bond between them. To maximize the electrochemical characteristics of the nanobismuth/Nafion electrode, the optimum conditions of SWASV and the nanobismuth labeling amount were suggested. The sensitivity and detection limit of the nanobismuth/Nafion electrode were quantitatively estimated from the analyses of stripping voltammograms. The proposed mercury-free carbon strip electrode, modified with bismuth nanopowder, is conveniently usable and directly applicable to a trace metal analysis without predeposition of bismuth and complicated surface polishing steps.

Effect of phase stability degradation of bismuth on sensor characteristics of nano-bismuth fixed electrode

Effect of phase stability degradation of bismuth on sensor characteristics of nano-bismuth fixed electrode has been investigated using square-wave anodic stripping voltammetry technique, scanning electron microscopy (SEM) and X-ray diffraction (XRD) spectroscopy. From the analyses of square-wave anodic stripping voltammograms (SWASV) repetitively measured on the nano-bismuth fixed electrode, it was found that the oxidation peak currents dropped by 81%, 68% and 59% for zinc, cadmium and lead, respectively, after the 100th measurement (about 400 min of operation time). The sphere bismuth nanoparticles gradually changed to the agglomerates with petal shape as the operation time increased. From the analyses of SEM images and XRD patterns, it is confirmed that the oxidation of Bi into BiOCl/Bi(2)O(2)CO(3) and the agglomeration of bismuth nanoparticles caused by the phase change decrease a reproducibility of the stripping voltammetric response. Moreover, most of the bismuth becomes BiOCl at pH 3.0 and bismuth hydroxide, Bi(OH)(3) at pH 7.0, which results in a significant decrease in sensitivity of the nano-bismuth fixed electrode.

Nanogenerators consisting of direct-grown piezoelectrics on multi-walled carbon nanotubes using flexoelectric effects

We report the first attempt to prepare a flexoelectric nanogenerator consisting of direct-grown piezoelectrics on multi-walled carbon nanotubes (mwCNT). Direct-grown piezoelectrics on mwCNTs are formed by a stirring and heating method using a Pb(Zr0.52Ti0.48)O3 (PZT)-mwCNT precursor solution. We studied the unit cell mismatch and strain distribution of epitaxial PZT nanoparticles, and found that lattice strain is relaxed along the growth direction. A PZT-mwCNT nanogenerator was found to produce a peak output voltage of 8.6u2009V and an output current of 47u2009nA when a force of 20u2009N is applied. Direct-grown piezoelectric nanogenerators generate a higher voltage and current than simple mixtures of PZT and CNTs resulting from the stronger connection between PZT crystals and mwCNTs and an enhanced flexoelectric effect caused by the strain gradient. These experiments represent a significant step toward the application of nanogenerators using piezoelectric nanocomposite materials.

Formation of interfacial brittle phases sigma phase and IMC in hybrid titanium-to-stainless steel joint

The microstructures of the brazed joints for commercially pure Ti and stainless steel were investigated by the applications of various filler alloys including Ag-, Ti-, Zr- and Ni-based alloys. Generally, the dissimilar joints between Ti and stainless steel were dominated by the Ti-based intermetallic compounds (IMCs), e.g. (Ti, Zr)2(Fe, Ni), TiFe, TiCu, and Ti2(Fe, Ni), due to a significant dissolution of Ti from the base metal. The (Fe-Cr) σ phase was also observed near the stainless steel due to a segregation of Cr into the interface region. This research demonstrates empirically that the brittleness of the Ti and stainless steel joint can not be avoided only by applying single braze alloy or single insert metal, and thus an introduction of additional suitable interlayer between the filler alloy and the base metal is necessary to prevent the brittleness of the joint.

Synthesis Of Ti50Ni50 Alloy Nanopowders Synthesized by Modified Levitational Gas Condensation Method

The stoichiometric Ti50Ni50 alloy nanopowders were synthesized by levitational gas condensation (LGC) using micron powder feeding system and their particulate properties were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM) and Brunauer- -Emmett-Teller (BET) method. The starting Ti and Ni micron powders ∼ 150μm were incorporated into the micron powder feeding system. The ingot type of Ti-Ni alloy was used as a seed material for levitation and evaporation reactions. The collected powders were finally passivated by oxidation. The x-ray diffraction experiments have shown that the synthesized powders were completely alloyed with 50Ti and 50Ni in at.% and comprised of two different cubic and monoclinic crystalline phases. The TEM results showed that the produced powders were a very fine and uniform with the spherical particle size of 18 to 32 nm. The typical thickness of passivated oxide layer on the particle surface was about 2 to 3 nm. The specific surface area of the Ti-Ni alloy nanopowders was 54.8 m2/g based on a BET method.

High functional adsorbent alumina nanofibers prepared by a hydrolysis of Al nanopowder

The alumina (Al 2 O 3 ) nanofibers with high specific surface area, and chemical stability were prepared by a simple hydrolysis and a subsequent dehydration of Al nanopowder. The performance of these nanofibers as an adsorbent of radio isotope (RI) and a filter for bacteria and heavy metal was examined using Nal scintillation counter and UV-spectrophotometer. The observed filtering ability of the alumina nanofibers was remarkably high, which was 197 milligram per a gram of alumina (mg/g) for the RI of Mo-99 under pH 5.1 and 20 °C, and 21, 8, and 37 mg/g for Zn, Cr and Cu, respectively. According to the experiment of removal of bacteria, strain of Bacillus subtilis, the aluminium hydroxide nanofiber coated on the micron-sized glass fibers removed the bacteria completely with an effective removal rate of 99.999%. The prominent filtering effect of the alumina nanofibers is attributed to a very high surface reaction area induced by ultrafine nanofibrous structure with the typical diameters of 2 nm to 3 nm, and an intrinsic electropositive surface charge character in aqueous solution.

Fabrication of High Density Y2O3 Ceramics by Magnetic Pulsed Compaction

Highly dense Y2O3 ceramics have been fabricated by a magnetic pulsed compaction (MPC) which is capable of reaching a sufficiently high pressure (~1GPa) in a very short duration (a few microseconds), and a subsequent pressureless sintering at 1600°C. The Y2O3 green bodies with a relative density of about 68% were achieved by the application of the MPC process due to the effect of an enhanced rearrangement and a high speed movement of the particles, without the help of ceramic binder. Those compacts showed densities greater than 95%, which is very close to the theoretical density, after the subsequent pressureless sintering process at 1600 oC. The shrinkage rates of the diameter for the samples compacted by the MPC process were markedly reduced, when compared to those for the ones by the conventional compaction (CC) process.

The Magnetic and Photo-Catalytic Properties of Fe Doped TiO2 Nanocrystalline Powder Synthesized by Mechanical Alloying

Fe-doped TiO2 nanocrystalline powders were prepared by mechanical alloying (MA) by varying Fe contents up to 8.0 wt.%. The TEM analyses were carried out to clarify morphologies and position of Fe within the mechanically alloyed powders. The Fe-doped powder consisted of spherical particles, and the average grain size was less than 10 nm. For the Fe-doped TiO2, the color of the powders changed from white to bright yellow with increasing concentration of Fe. The UVvis absorption showed that the UV absorption for the Fe-doped powder shifted to a longer wavelength (red shift) and the photo-efficiency was enhanced. The absorption threshold depends on the concentration of nano-sized Fe dopant. Mössbauer spectrum for 4 wt.% Fe showed the ferromagnetic phase (sextet) and paramagnetic phase (doublet). However, the only paramagnetic phase (doublet) was seen for 8 wt.% Fe. As the Fe concentration increased up to 4 wt.%, the UV-vis absorption and the magnetization were increased. The beneficial effect of Fe doping for photocatalysis and ferromagnetism was observed at the critical dopant concentration of 4 wt.%. Based on the UV absorption and magnetization, the dopant level was localized to the valence band of TiO2.

Highly sensitive electrochemical sensor based on bismuth nanopowders for detecting heavy metals and uranium

This work introduced a novel electrochemical sensor based on bismuth (Bi) nanopowders for trace analyses of heavy metals and uranium. High purity Bi nanopowders with an average size of 35 nm were synthesized by gas condensation method. The prepared Bi nanopowders were strongly fixed on the screen-printed carbon electrode using a Nafion. From the quantitative analyses of sensitivity and detection limit for Zn, Cd, Pb ions, the nano-Bi electrode has been proven to be more sensitive and reliable by increasing the electrochemical active surface area and by decreasing the electrical resistivity of the carbon electrode. It is also proposed that the nano-Bi can be applied to the uranium trace analysis.

Brazing of Ti Using a Zr-Based Amorphous Filler

The microstructure and mechanical properties of Ti joints brazed with a Zr41.2Ti13.8Ni10.0Cu12.5Be22.5 (at.%) amorphous filler were investigated. With a Zr-based amorphous filler, in this study, Ti joints with a homogeneous composition could be obtained by heating to well below the α-β transformation temperature for a short time, so that the undesirable effects of the high temperature heating are considerably diminished. The joints brazed at 790 °C for 10 min consisted of the coarse acicular structure rather than the fine Widmanstätten structure which generally deteriorates the ductility of the joints. The joints with the homogenous coarse acicular structure, i.e. without a residual liquid region, show almost the same mechanical properties as those required for base metals without heating. Although the residual liquid region in the joints deteriorates the ductility of the joints, this region could be successfully removed by a diminution in the quantity of the filler.