Kyong-Soo Hong

Thermal conductivity of Fe nanofluids depending on the cluster size of nanoparticles

Nanofluids have been attractive for the last few years with the enormous potential to improve the efficiency of heat transfer fluids. This work focuses on the effect of the clustering of nanoparticles on the thermal conductivity of nanofluids. Large enhancement of the thermal conductivity is observed in Fe nanofluids sonicated with high powered pulses. The average size of the nanoclusters and thermal conductivity of sonicated nanofluids are measured as time passes after the sonication stopped. It is found from the variations of the nanocluster size and thermal conductivity that the reduction of the thermal conductivity of nanofluids is directly related to the agglomeration of nanoparticles. The thermal conductivity of Fe nanofluids increases nonlinearly as the volume fraction of nanoparticles increases. The nonlinearity is attributed to the rapid clustering of nanoparticles in condensed nanofluids. The thermal conductivities of Fe nanofluids with the three lowest concentrations are fitted to a linear function. The Fe nanofluids show a more rapid increase of the thermal conductivity than Cu nanofluids as the volume fraction of the nanoparticles increases.

Removal of Heavy Metal Ions by using Calcium Carbonate Extracted from Starfish Treated by Protease and Amylase

CaCO3 extracted from starfish by using the commercial protein lyase having α-amylase, β-amylase, and protease is applied to remove heavy metal ions. The extracted CaCO3 shows excellent characteristics in removing heavy metal ions such as Cu 2+ , Cd 2+ , Pb 2+ , and Cr 6+ compared with conventional materials such as crab shells, sawdust, and activated carbon except for removing Zn 2+ . SEM images reveal that the extracted CaCO3 has a good morphology and porosity. We characterize the removal efficiencies of the extracted CaCO3 for the heavy metal ions according to the concentrations, pH, temperatures, and conditions of empty bed contact times.

Synthesis and Photophysical Properties of Luminescent Erbium(III) Complexes Based on Coumarin Derivatives for Advanced Photonics Applications

The luminescent Er(III)-chelated complexes based on coumarin derivatives such as coumarin-3-carboxylic acid and coumarin 343 were prepared by ligand-exchange reaction in the presence of ErCl3and potassium coumarin derivative salts. The chemical structures of the present complexes were confirmed by elementary analysis, thermal gravimetric analysis. FT-IR, absorption and emission spectroscopies. The coumarin-based Er(III)complexes exhibited the near IR emission, corresponding to the characteristic4I13/2 → 4I15/2transition of trivalent erbium ions, taking place in the region of 1512 or 1530 nm. Their PL intensity also depends on the degree of the spectral overlap between the emission band of coumarin derivative and the absorption band of Er ion.

Studies of Thermal Conductivity of Gd2Zr2O7 and Diamond-Like Carbon Films and the Interfacial Effect

Gadolinium zirconium oxide (Gd2Zr2O7) films and diamond-like carbon (DLC) films were prepared by RF-magnetron sputtering and an ion gun method, respectively. To study the thermal conductivity of these films, we used the 3ω and time-domain thermoreflectance (TDTR) methods. The thermal conductivity showed a film thickness dependence for the 3ω method, and this dependence is described as an interfacial effect by considering heat penetration depth in the measurement. The obtained thermal conductivities of the Gd2Zr2O7 and DLC films, independent of the interfacial effect, are 1.6 and 2.5 W m-1 K-1, respectively. The thermal conductivity of the Gd2Zr2O7 films showed no significant thickness dependence for the TDTR method. The conductivities of Gd2Zr2O7 1.6–1.8 W m-1 K-1 obtained by the TDTR method were close to the thermal conductivity of the Gd2Zr2O7 films, independent of the interfacial thermal effect observed in the 3ω method.

Improvement of the Electrochemical Properties in Nano-Sized Al2O3 and AlF3-Coated LiFePO4 Cathode Materials

The surface conditions of LiFePO4 powder were modified by adding AlF3 and Al2O3 by using the sol-gel process to improve its electrochemical properties. The surface of LiFePO4 powders was partially covered with nano-sized AlF3 and Al2O3, which is confirmed by using a transmission electron microscope image. The states of coated Al materials were examined by using X-ray photoelectron spectrometer results. The nano-sized AlF3- and Al2O3-coated LiFePO4 powders showed no difference in the bulk structure compared with the pristine one. However, the AlF3- and Al2O3-coating on LiFePO4 powders improved the overall electrochemical properties such as the discharge capacity, the cyclability, and the rate capability compared with those of a pure LiFePO4. Such enhancements were attributed to the presence of a stable AlF3 and Al2O3 layer which acts as an interfacial stabilizer on the surface of LiFePO4 powders.

Synthesis and Photophysical Properties of Luminescent Lanthanide Complexes Based on Dansyl-N-Methyl-Aminobenzoic Acid for Advanced Photonics Applications

ABSTRACT Novel luminescent Ln(III)-chelated complexes based on dansyl-N-methyl-aminobenzoic acid exhibited the strong near IR emission bands of the characteristic 4I13/2 → 4I15/2 transition of Er(III) ion around 1530 nm and the characteristic 4F3/2 → 4F9/2, 4F3/2 → 4F11/2 and 4F3/2 → 4F13/2 transitions of Nd(III) ion around 880, 1060 and 1330 nm. Their photoluminescence intensity depends on the degree of the spectral overlap between the emission band of the ligand and the absorption band of lanthanide(III) ion. The radiative life times of Er(III) and Nd(III) ions in the lanthanide(III) complex films were estimated to be 1.6 and 1.2 μs, respectively.

FREQUENCY AND TEMPERATURE DEPENDENCE OF ELECTRICAL PROPERTIES IN PURE AND Nb-DOPED Bi4Ti3O12 CERAMICS

ABSTRACT Pure and Nb-doped Bi4Ti3O12 ceramics were prepared by using the conventional solid state reaction method. The dielectric permittivity and electrical modulus in pure and Nb-doped Bi4Ti3O12 ceramics were measured in the temperature range of 30 ∼ 700 °C and the frequency range of 1 Hz∼1 MHz through an impedance spectroscopy method. The obtained results showed well-defined relaxation peaks. The conductivity relaxation times are determined from the peaks of the modulus. The activation energies associated with hopping condition are 0.42 eV and 0.53 eV in pure and Nb-doped Bi4Ti3O12 ceramics, respectively.