Young Rang Uhm

Magnetic nanoparticles of Fe2O3 synthesized by the pulsed wire evaporation method

Nanoparticles of Fe2O3 with a mean particle size of 4–50 nm have been prepared by the pulsed wire evaporation method, and its structural and magnetic properties were studied. From the main peak intensity of x-ray diffraction the amount of v-Fe2O3 and ɑ-Fe2O3 in sample is composed about 70% and 30%, respectively. The coercivity (53 Oe) and the saturation magnetization (14 emu/g) are about 20% of those of the bulk v-Fe2O3. A quadrupole line on the center of Mossbauer spectrum represents the superparamagnetic phase of v-Fe2O3 with a mean particle size of 7 nm or below.

Magnetic properties of NiZnCu ferrite powders and thin films prepared by a sol-gel method

Ultrafine Ni/sub 0.63/Zn/sub 0.17/Cu/sub 0.2/Fe/sub 2/O/sub 4/ powders and thin films were fabricated by a sol-gel method and their magnetic and structural properties were investigated with thermogravimetric and differential thermal analysis (TG-DTA), X-ray diffractometer (XRD), transmission electron microscope (TEM), Mossbauer spectrometer, atomic force microscope (AFM), and vibrating sample magnetometer (VSM). TG-DTA measurements showed an exothermic reaction peak at 306/spl deg/C with weight loss of 49%. NiZnCu ferrite powders which were fired at and above 450/spl deg/C had only a single phase spinel structure and behaved ferrimagnetically. Powders annealed at 250 and 350/spl deg/C had a typical spinel structure and were simultaneously paramagnetic and ferrimagnetic in nature. The magnetic behavior of NiZnCu ferrite powders fired at and above 550/spl deg/C showed that an increase of the annealing temperature yielded a decrease in the coercivity and an increase in the saturation magnetization. The maximum coercivity and the saturation magnetization of NiZnCu ferrite powders were H/sub c/=160 Oe and M/sub s/=64 emu/g, respectively. NiZnCu ferrite thin films annealed at 650/spl deg/C had a single phase spinel structure and there was no significance difference of their magnetic properties for external fields applied parallel and perpendicular to their planes. The microstructure of thin films annealed at 650/spl deg/C consisted of spherical grains with the average size of 120 nm and 5 nm in surface roughness (rms).

Synthesis of nano cryatalline Ni and Fe by levitational gas condensation (LGC) method

In this article, nickel and iron nanoparticles were synthesized by the LGC method and phase evolution and magnetic properties were investigated in details.

Crystallization and Mössbauer studies of melt-spun NdFe10.7TiB0.3Nδ alloys

Magnetic properties of melt-spun NdFe10.7TiB0.3Nδ ribbons have been investigated as functions of quenching rate and nitriding period. NdFe10.7TiB0.3 were prepared with substrate velocity vs⩽18 m/s and were nitrogenated at 500 °C for 15 min. The NdFe10.7TiB0.3Nδ retains the ThMn12-type tetragonal structure with lattice constants a0=8.640 A and c0=4.811 A, but with an increase in the unit cell volume. The NdFe10.7TiB0.3Nδ was confirmed to have uniaxial anisotropy by x-ray diffraction. Mossbauer spectra were taken at various temperatures ranging from 13 to 855 K. The Curie and Debye temperatures are determined to be Tc=833 K and Θ=390 K, respectively. Each spectrum below Tc was fitted with six subspectra of Fe sites (8i1, 8i2, 8j2, 8j1, 8f, and α-Fe). The area fraction of the subspectra at 13 K are 10.2%, 8.2%, 16.5%, 17.5%, 44.3%, and 3.3%, respectively. The magnetic hyperfine fields for the Fe sites decrease in the order, Hhf(8i)>Hhf(8j)>Hhf(8f).

Structure and Magnetic Properties of Carbon-Encapsulated Ni Nanoparticles

Carbon encapsulated Ni (Ni@C) nanoparticles with core/shell structure have been synthesized by levitational gas condensation (LGC) method. The methane (CH4) gas was used for the coating on the surface of Ni nanoparticles. The as-prepared Ni@Cs were characterized by X-ray diffraction (XRD), vibrating samples magnetometer (VSM), and transmission electron microscopy (TEM). The particles had core structure of diameter at and below 10 nm and were covered by carbon 2-3 nm thin layers. The hysteresis loop of the as-prepared Ni@Cs materials with unsaturated magnetization shows a superparamagnetic state at room temperature.

Novel synthesis of nanorod ZnO and Fe-doped ZnO by the hydrolysis of metal powders.

Fe-doped ZnO nanorods have been synthesized by a novel process employing a hydrolysis of metal powders. Zn and Fe nano-powders were used as starting materials and incorporated into distilled water. The solution was refluxed at 60 degrees C for 24 h to obtain the precipitates from the hydrolysis of Zn and Fe. X-ray diffraction patterns for all the samples showed a pure wurtzite single phase, without any segregation of the Fe into the particulates within the instrumental resolution limit. The TEM results for ZnO with and without an Fe-doping showed that the produced powders had a rod-like shape. The rod shape was attributable to the zinc oxide from the hydrolysis of Zn. With an increasing Fe content, the UV-vis spectra were shifted to a long wave length and this result indicates that the band gap was changed by an Fe-doping.

Mossbauer studies of nanocrystalline Fe/sub 83/B/sub 9/Nb/sub 7/Cu/sub 1/ alloy by flash annealing

Melt-spun Fe/sub 83/B/sub 9/Nb/sub 7/Cu/sub 1/ alloy with ultrathin ribbon has been studied with Mossbauer spectroscopy and X-ray diffraction. The enhanced magnetic property of the flash-annealed alloy was attributed to the reduced /spl alpha/-Fe phase grain size to 6 nm and the higher effective permeability and smaller magnetic core loss at 1 MHz than conventional annealed alloys. The occupied area of the nanocrystalline phase at the optimum 773 K is about 73% whereas that for conventional annealing temperature at 893 K is about 71%. The flash annealing technique was effective in improving the high-frequency soft magnetic property of nanocrystalline Fe/sub 83/B/sub 9/Nb/sub 7/Cu/sub 1/ alloy.

Effect of particle size, dispersion, and particle–matrix adhesion on W reinforced polymer composites

W dispersed mixed polymers of ethylene propylene monomer and high density polyethylene were prepared by means of a twin-screw extruder by the conventional technique using a co-rotated two-roll mill. The W nanoparticles used as filler were prepared by pulse wire evaporation then coated with low-density polyethylene (LDPE) as polymeric surfactant. Surface treatment of the nanoparticles with LDPE was conducted to enhance the wettability and lubrication of the fillers in the polymer matrix. According to SEM images and mechanical properties, dispersion of W/LDPE nanoparticles in the polymer matrix was homogeneous, and adhesion of the nanoparticles to the matrix was strong. The polymer nanocomposites had better mechanical properties than those containing dispersed micro-W powder. The γ-ray attenuation factor of nanofiller-reinforced composites was substantially enhanced compared with that containing micro filler.

Synthesis and Magnetic Properties of Ni and Carbon Coated Ni by Levitational Gas Condensation (LGC)

The nickel (Ni), and carbon coated nickel (Ni@C) nanoparticles were synthesized by levitaional gas condensation (LGC) methods using a micron powder feeding (MPF) system. Both metal and carbon coated metal nano powders include a magnetic ordered phase. The synthesis by LGC yields spherical particles with a large coercivity. The abnormal initial magnetization curve for Ni indicates a non-collinear magnetic structure between the core and surface layer of the particles. The carbon coated particles had a core structure diameter at and below 10 nm and were covered by 2-3 nm thin carbon layers. The hysteresis loop of the as-prepared Ni@Cs materials with unsaturated magnetization shows a superparamagnetic state at room temperature.

Magnetic Properties of R-YIG (R = La, Nd, and Gd) Derived by a Sol-gel Method

Y 3-x R x Fe₅O 12 (R = La, Nd, and Gd) powder were fabricated using a sol-gel pyrolysis method. Their magnetic properties and crystalline structures were investigated using x-ray diffraction (XRD), a vibrating sample magnetometer (VSM), and Mossbauer Spectrometer. The Mossbauer spectra for the powders were taken at various temperatures ranging from 12 K to Curie temperature (Tc). The isomer shifts indicated that the valence states of Fe ions for the 16(a) and 24(d) sites have a ferric character. The saturation magnetization (Ms) increases from 32 to 34 (emu/g) for the YIG, and Nd-YIG, respectively. However, Ms decreases to 27 (emu/g) at Gd-YIG.