Pyungwoo Jang

Effects of annealing on the magnetic properties of Fe–6.5%Si alloy powder cores

Powder compressed cores can overcome poor workability of Fe–6.5%Si alloys. In this study, effects of annealing temperature on the losses of Fe–6.5%Si powder compressed cores were investigated. The hysteresis loss was two to three times larger than the eddy current loss. With the increasing annealing temperature, the hysteresis loss decreased due to a residual stress relief. With increasing the temperature, on the contrary, the eddy current loss increased even though the resistivity increased abruptly. An increase of the eddy current loss with the temperature can be understood by coarsening of DO3 phase. It was thought that controlling the microstructure of the powders is more important than that of resistivity. It is also very effective to anneal the powders at a temperature higher than 1000°C in order to reduce the total loss further.

Effects of Insulator Resistivity on Eddy Current Loss of Compressed Powder Cores Studied by FEM

Effects of the resistivity of the insulation layer on the eddy current loss of compressed Fe and Fe-6.5% Si powder cores were simulated by using 3-D finite element method. Applied field was abruptly reduced and then kept a constant value with the resistivity of the insulation layer of the compressed cores. Permeability of the Fe-6.5% Si compressed cores was increased with the resistivity of the isolation layer. Eddy current losses of the cores were not reduced continuously with the resistivity of the layer. Instead eddy current loss kept a constant loss when the resistivity is about 105 times higher than that of the particle. Eddy current flows all over the cores at low resistivity while the current was confined inside particles if the resistivity is higher than a critical value. To reduce eddy current loss of the compressed cores with high resistivity, it seems that other factors such as microstructure should be considered.

Acoustic Noise Characteristics and Magnetostriction of Fe-Si Powder Cores

Fe-Si powder compressed cores have been widely used as reactors in the powder conditioning systems (PCS) of photovoltaic applications and electrical vehicles. Reduction of acoustic noise from the reactor is important when the PCS is installed indoors. In this paper, magnetostriction of the Fe-Si powder cores was investigated in connection with acoustic noise from the reactors assembled with Fe-Si cores. A dummy gage in the coil, wound noninductively, is useful in eliminating the contribution of thermal expansion of cores under strain. Magnetostriction varied with the mechanical strength of the Fe-Si core, which is dependent on the conditions of impregnation. Magnetostriction of the cores was several ppm under the maximum field of 18 kA/m. The Fe-Si cores with lower mechanical strength show lower magnetostriction and thus lower acoustic noise. Strain due to Maxwell force is much larger than that of magnetostriction in the rectangular-type reactor.

Role of Ag seed layer for CoCrPt/Ti perpendicular recording media

2 nm metal seed layers (M=Al, Cu, Ni, Cr, Ag, Mg, Fe, Co, Pd, Au, Pt, Mo, and Hf) were sputtered to increase coercivity (Hc) and anisotropy (Ku) of CoCrPt/Ti perpendicular recording media. Among them 2 nm Ag seed layer was very effective to increase Hc of (Co78Cr22)100−xPtx/Ti (x=14,20). However, the effect was more pronounced when (Co78Cr22)100−xPtx films became thinner. In addition α [=4π(dM/dH)Hc] decreased when the Ag layer was used. The film thickness below which the seed Ag layer was effective was reduced with decreasing Pt content. However, x-ray diffraction data showed that the Ag seed layer did not promote (002) texture of Ti and CoCrPt layers. In magnetic force microscope observations, domain size was reduced when the Ag seed layer was used. The reason for the higher coercivity of CoCrPt films with the Ag seed layer is thought to be due to change of exchange constant of the grains, for which the grain boundary area plays an important role. Effects of film thickness and Pt content can also be exp...

Effects of Ordering on the Magnetic Properties of Gas-Atomized Fe–Si–Cr Powders

Fe–9%Si–2%Cr powder was gas-atomized and then annealed in order to investigate the effects of DO3 and B2 ordered phases on high-frequency permeability of the powder. The B2 phase is formed during the atomization process. The DO3 phase began to form at 450 °C and grew rapidly when annealed at higher temperature as confirmed by X-ray diffractometer, whereas the existence of the DO3 phase in the as-atomized powder could be confirmed by electron transmission microscope. With the increase in the temperature, the average lattice parameter and the coercivity decreased. There was an abrupt increment of the coercivity in the powder annealed at 450 °C. Highest permeability could be obtained in the powder annealed at a relative low temperature of 150 °C and then the permeability real decreased with the temperature. The behavior of the coercivity and the permeability with the temperature could be successfully explained by the formation of the DO3 ordered phase and the change of the electrical resistivity.

Formation of particulate Fe-Al films by selective oxidation of aluminum

Fe-5wt%Al films were RF-sputtered and annealed in an atmosphere of hydrogen and water vapor mixture at 1173 K for up to 200 min in order to selectively oxidize aluminum. As the annealing time increased, the morphology of the films changed from the continuous to the discontinuous type; thus, particulate Fe-Al films formed after 100 min. Thermodynamics simulation was performed to determine the ideal conditions for this process. Temperatures exceeding 1073 K are necessary to prevent iron from oxidation confirmed by both the depth profile in XPS and magnetic moment increment in VSM. Annealing the films in an atmosphere with a very low dew point of 77 K did not make the films become particulate. New findings are expected to be applied to the thin film inductors for GHz application as well as to manufacturing process of nanoparticles.

Fabrication of Fe-Al nanoparticles by selective oxidation of Fe-Al thin films

The possibility of a new technique for fabricating nanoparticles from thin films using selective oxidation in an atmosphere mixture of water vapor and hydrogen was investigated. Fe-5wt.%Al films were RF-sputtered and annealed in the atmosphere mixture at 900°C for up to 200 min, in order to oxidize aluminum selectively. Thermodynamics simulation showed that temperatures exceeding 800°C are necessary to prevent iron from being oxidized, as confirmed by the depth profile of XPS. As the annealing time increased, the morphology of the 200-nm Fe-Al films changed from the continuous to the discontinuous type; thus, particulate Fe-Al films formed after 100 min. The particulate 10- to 100-nm Fe-Al films showed super-paramagnetic behavior after the oxidation. Thus, a new technique for fabricating nanoparticles was successfully introduced using selective oxidation.

New manufacturing method for Fe-Si magnetic powders using modified pack-cementation process

This paper describes a new method for making Fe-Si magnetic powders using a pack-cementation process. It was found that Fe-Si alloy powders were formed by a reaction of the pack mixture of Fe, Si, NaF, and Al2O3 powders at 900 °C for 24 h under a hydrogen atmosphere. Separation of the Fe-Si alloy powders was dependent on the particle size of the Fe powders in the pack. For small Fe powders, magnetic separation in a medium of strong alkali solution was recommended. But, for relatively larger Fe powders, the Fe-Si alloy powders were easily separated from Al2O3 powders using a magnet in air atmosphere. The Si content in the Fe-Si magnetic powders were easily controlled by changing the weight ratio of Si to (Si+Fe) in the pack.

Properties of Fe-Al Cores Made From Fe-Al Powders Annealed in a Damp Hydrogen Atmosphere

Gas-atomized Fe-4, 6wt.%Al powders were annealed in a damp hydrogen atmosphere with a dew point of 0^°C and -17^°C in order to cover the powder surface with a high electrical resistivity layer of Al₂O₃ and then compressed into cores. The powders should be oxidized at a temperature higher than 800 ^°C in order to oxidize the aluminum alone on the surface of powder, as confirmed by simulation using the STANJAN program. An Al-rich layer, i.e., an Al₂O₃ layer, was formed on the surface of the Fe-Al powder, as confirmed by SEM. Loss of the cores made from the selectively oxidized powders was much lower than those of the cores with phosphate coats, so the selectively oxidized powder can be used to fabricate electrical actuators with high levels of performance.

Fabrication and properties of FePt thick films for alternative local field micromagnet

Growth of FePt films thicker than several tens of a micrometer was attempted on Fe substrates for the use of micromagnets. Several underlayer or intermediate layers were found to be totally ineffective to success. However, 20 μm thick FePt films on the Fe substrates were successfully grown by sputtering after HF surface treatment. (BH)max of the non-annealed film was about 10.7 MG Oe under a maximum applied field of 10 kOe. An excellent adhesion between the Fe substrate and the FePt thick films seems to be due to both improvement of surface cleanliness and roughening which enhanced the chemical and physical bonding strengths.