Ho-Soon Yang

Study of the enhanced thermal conductivity of Fe nanofluids

Nanofluids, a mixture of nanoparticles and fluid, have enormous potential to improve the efficiency of heat transfer fluids. Fe nanofluids are prepared with ethylene glycol and Fe nanocrystalline powder synthesized by a chemical vapor condensation process. Sonication with high-powered pulses is used to improve the dispersion of nanoparticles in the preparation of nanofluids. Nanofluids exhibit an enhancement of thermal conductivity after sonication. Thermal conductivity of a Fe nanofluid is increased nonlinearly up to 18% as the volume fraction of particles is increased up to 0.55 vol. %. Comparing Fe nanofluids with Cu nanofluids, we find that the suspension of highly thermally conductive materials is not always effective to improve thermal transport property of nanofluids.

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.

Auger recombination and state filling of resonantly excited ground state in CdSe quantum dots

Dynamics and state filling of the resonantly excited ground state (1se‐1S3∕2h) in a strongly confined CdSe quantum dot were investigated by degenerate pump-probe measurements. With increasing the electron-hole (e-h) pairs per dot the state filling and bleaching were observed with ∼2.7 e-h pairs. While radiative recombination was dominant with a small number of the e-h pairs (⪡1), the Auger-type recombination became significant near the bleaching number of e-h pairs (∼2.7). This result suggests that, for resonant excitation, either cold electron or hole of the ground state is scattered into the higher-energy states of a ZnS shell via Auger recombination process (∼28ps).

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.

Modal gain enhancement by cylindrical waveguide and gain saturation in CdSe nanocrystal quantum dots

Optical modal gain enhancement of CdSe nanocrystal quantum dots in a cylindrical glass-waveguide has been observed by using a variable stripe length method. While poor modal gain was observed near the threshold excitation energy (3.5 mJ) for a thin film structure, significant modal gain (~ 500 cm−1) was seen in the cylindrical structure due to the waveguide effect. Modal gain saturation for a stripe length was also investigated with a new gain analysis (Kyhm et al 2001 Appl. Phys. Lett. 79 3434). The stripe length dependence of gain saturation is also suppressed in the waveguide structure above the threshold excitation energy (5.5 mJ). We suggest the modal gain enhancement is involved with the intrinsic two electron–hole pair excited states (anti-bonding biexcitons) and the extrinsic waveguide effect.

Electron-Phonon Interactions and Vibrational Modes in Insulating Nanocrystals

The temperature and particle size dependence of the holewidths for the 7F0 → 5D0 transition of Eu3+ ions in Eu2O3 are calculated and compared with experiment. The calculation is able to describe the effect of nanocrystal vibrational modes on the dephasing of the electronic states of probe ions. The nanocrystal was treated as an elastically isotropic sphere from which the acoustic phonon displacements and phonon spectrum were calculated. Then we applied this to the temperature and size dependencies of the electron-phonon interaction. Also the vibrational modes in nanocrystals are discussed.

Dynamics of optical nonlinearities in CdSe/ZnS nanocrystals

We performed time-resolved luminescence and polarization-resolved pump-probe experiments in CdSe/ZnS nanocrystals (NCs) under low and high excitations at room temperatures. At high excitations, the PL spectra shows a red shift (0.54 meV) and two-step decay. In pump-probe experiments, we observed very fast spin-flip process (~1 ps) and population oscillation, which is attributed to the Rabi oscillation.

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.