Sung-Tag Oh

Fabrication of porous Al2O3 by microwave sintering and its properties

Abstract Porous composites of Al2O3/ZrO2 with improved mechanical properties were fabricated by microwave sintering. The microstructure of the composite was characterized by the enhanced neck growth between the initially touching particles. This neck growth was ascribed to the selective heating of ZrO2 between Al2O3 particles in the microwave heating, because of the high dielectric loss factor and good microwave-energy absorption ability of ZrO2. The composite prepared by the microwave sintering exhibited higher elastic modulus and fracture strength than that prepared by conventional sintering. The improved mechanical properties of the microwave-sintered composites were attributable to the enhanced neck growth.

Broadband RF Noise Suppression by Magnetic Nanowire-Filled Composite Films

The characteristics of the electromagnetic noise suppression have evaluated using magnetic nanowire-filled composite film on a microstrip line in the broadband radio-frequency range from 0.5 to 20 GHz. The ferromagnetic resonance frequency and the relative complex permeability with the change of aspect ratio were calculated by Kittels and Landau-Lifshitz-Gilbert equations. The FMR frequency is gradually increased with the increment of aspect ratio (up to 50) of magnetic nanowire. When the magnetic nanowires were mixed with various aspect ratio in composite, the FMR frequency exhibited the broadband frequency regions with 10-28 GHz, 5-14 GHz, 2.5-7 GHz, and 1-2.7 GHz for the change of magnetizations (4piMs: 20, 10, 5, and 2 kG), respectively. Using the calculated FMR frequency profile, the power losses were obtained in broadband frequency region. The transmission power absorption of the composite film on microstrip line was simulated using a 3-D FEM HFSS simulation program. The microstrip line model was composed of Cu conductor line and grounded dielectric substrates, which was designed by IEC standard (IEC 62333-2). The conduction electromagnetic noise was suppressed by the various aspect ratios of the magnetic nanowire-filled composite films in broadband frequency region.

Synthesis of Fe(Ni) nanoparticles by calcination and hydrogen reduction of metal nitrate powders

The calcination and hydrogen reduction behavior of Fe- and Ni-nitrate powders has been investigated by using thermal analysis and microstructure observation. Fe-oxide/NiO composite powder was prepared by calcination at 350??C for 2?h of Fe- and Ni-nitrate. Microstructural observation revealed that FeNi3 particles with an average size of 40?nm were formed by hydrogen reduction at 320?? C of calcined powders. Changes of the DSC curve and the XRD pattern with the increase in reduction temperature were explained as presumably due to the phase transformation of Fe(Ni) powders.

Ultraviolet photoelectron spectroscopy study of colossal magnetoresistive La0.7−xPrxCa0.3MnO3

We have investigated the electronic structure of colossal magnetoresistive perovskite manganites La0.72xPrxCa0.3MnO3 using ultraviolet photoelectron spectroscopy as a function of temperature and Pr concentration x (x ¼ 0:13; 0.3, and 0.4). Unusual temperature-dependent and x-dependent spectral changes are observed in the entire valence band region. Since Pr doping does not introduce carriers into the system but brings about strong lattice distortion effects which exert significant influence on the p ‐ d hybridization, the degree of mixing between Mn 3d and O2p levels is considered to be the major factor responsible for the spectral weight transfers with temperature and Pr doping level. The observed changes are not consistent with the results of local-density-approximation (LDA) band calculation, but can be understood by the density-of-states from the LDA þ U calculation or configuration-interaction cluster calculations. This implies that a strong correlation effect between d electrons plays an important role in these manganites. High resolution spectra near the Fermi level shows a clear metallic edge at low temperature for low Pr concentration sample ðx ¼ 0:13Þ; but the metallic Fermi edge disappears at higher Pr doping levels (x ¼ 0:3 and 0.4) even at the temperature below Tc where the bulk metal ‐ insulator transition occurs. This may be due to the low effective carrier density resulting from the reduced ferromagnetic ordering and the lattice distortion, or the pseudo-gap phenomena and the surface effect. q 2002 Elsevier Science Ltd. All rights reserved.