Hee-Ryoung Cha

Effect of the dehydrogenation speed and Nd content on the microstructure and magnetic properties of HDDR processed Nd-Fe-B magnets

The effect of Nd content and dehydrogenation speed on the microstructure and magnetic properties of hydrogenation-disproportionation-desorption-recombination (HDDR) processed Nd-Fe-B magnetic powders was studied. The NdxB6.4Ga0.3Nb0.2Febal (x=12.5–13.5, at.%) mold casting alloys were subjected to HDDR process after homogenization heat treatment. During desorption-recombination stage, dehydrogenation speed and time were systematically changed to control the speed of the desorption-recombination reaction. The higher Nd content resulted in better magnetic properties of the HDDR powder, and this was attributed to the thicker and more uniform Nd-rich phase at grain boundaries. It was also confirmed that the slow dehydrogenation speed could maximize the effect of high Nd content on the magnetic properties of HDDR powder. At the optimized dehydrogenation speed, the coercivity and remanence was 15.3 kOe and 13.0 kG, respectively, at 12.9 at.% Nd content, which resulted in a (BH)max of 36.8 MGOe.

The Influence of Gelatin Additives on the Mechanical Properties of Electrodeposited Cu Thin Films

To modify the physical properties of Cu thin films, gelatin is generally used as an additive. In this study, we assessed the effect of gelatin on the mechanical properties of electrodeposited Cu films. For this purpose, Cu/gelatin composite films were fabricated by adding 100 ppm of gelatin to an electrolyte, and tension and indentation tests were then performed. Additional tests based on pure Cu films were also performed for comparison. The Cu films containing gelatin presented a smaller grain size compared to that of pure Cu films. This increased the hardness of the Cu films, but addition of gelatin did not significantly affect the elastic modulus of the films. Cu films prepared at room temperature showed no significant change in the yield strength and tensile strength with an addition of gelatin, but we observed a dramatic decrease in the elongation. In contrast, Cu films prepared at 40°C with gelatin presented a significant increase in the yield strength and tensile strength after the addition of gelatin. Elongation was not affected by adding gelatin. Presumably, the results would be closely related to the preferred orientation of the Cu thin film with the addition of gelatin and at temperatures that lead to a change in the microstructure of the Cu thin films. (Received June 8, 2010)

Enhancement of coercivity in sintered Nd-Fe-B magnets by grain-boundary diffusion of electrodeposited Cu-Nd Alloys

We report an enhancement in the coercivity of sintered Dy free Nd-Fe-B magnets from 11.84 to 14.26 kOe by the grain-boundary diffusion of electrochemically deposited Cu-Nd. In the optimized electrochemical deposition and heat treatment conditions, a distinct Nd-rich grain-boundary phase was observed after the diffusion process; distributions of each element was carefully mapped by scanning electron microscopy equipped with backscattered electron detector. X-ray diffraction patterns indicated that Nd2Fe14B was oxidized by the inward diffusion of oxygen, which might be formed during the electrodeposition of Cu-Nd, forming antiferromagnetic Fe2O3 that might degrade the overall coercivity. A mechanism underlying the enhancement of coercivity is basically the same as the well-known proposed mechanism, distribution of a thin Nd-rich phase by grain-boundary diffusion process. In this study, electrochemical deposition process has been extensively investigated, and then the process was demonstrated to be successful and economically useful method to improve coercivity of the magnet.

The Influence of Dehydrogenation Speed on the Microstructure and Magnetic Properties of Nd-Fe-B Magnets Prepared by HDDR Process

The influence regarding the dehydrogenation speed, at the desorption-recombination state during the hydrogenation-disproportionation-desorption-recombination (HDDR) process, on the microstructure and magnetic properties of Nd-Fe-B magnetic powders has been studied. Strip cast Nd-Fe-B-based alloys were subjected to the HDDR process after the homogenization heat treatment. During the desorption-recombination stage, both the pumping speed and time of hydrogen were systematically changed in order to control the speed of the desorption-recombination reaction. The magnetic properties of HDDR powders were improved as the pumping speed of hydrogen at the desorption-recombination stage was decreased. The lower pumping speed resulted in a smaller grain size and higher DoA. The coercivity and the remanence of the 200-300 μm sized HDDR powder increased from 12.7 to 14.6 kOe and from 8.9 to 10.0 kG, respectively. In addition, the remanence was further increased to 11.8 kG by milling the powders down to about 25-90 μm, resulting in (BH) max of 28.8 MGOe.

Microfabrication and optical properties of highly ordered silver nanostructures

Using thermal evaporation, we fabricated five uniform and regular arrays of Ag nanostructures with different shapes that were based on an anodized aluminum oxide template and analyzed their optical properties. Round-top-shaped structures are obtained readily, whereas to obtain needle-on-round-top-shaped and needle-shaped structures, control of the directionality of evaporation, pore size, length, temperature of the substrate, etc., was required. We then observed optical sensitivity of the nanostructures by using surface-enhanced Raman scattering, and we preliminarily investigated the dependency of Raman signal to the roughness and shape of the nanostructures.

Effects of Strain and Stain Rate on Microstructure and Magnetic Properties of Nd–Fe–B Magnets During Die-Upsetting Process

The effect of strain and stain rate on microstructure and the magnetic properties of Nd-Fe-B magnets during hot-deformation process have been studied with commercial melt-spun flakes with a composition of Nd13.6Fe73.6Co6.6Ga0.6B5.6. The compacts produced by hot pressing at 700 °C under 100 MPa in vacuum were subjected to die upsetting at 700 °C with different deformation conditions of strain rate 0.1-0.001 s-1 and height reduction 40%-75%. The remanence and coercivity tended to increase and decrease with increasing strain, respectively. At below the strain of 0.5, the remanence increased more quickly with decreasing the strain rate, but the tendency was gradually reversed with increasing the strain. This was because the prolonged deformation induced the squeeze-out of Nd-rich phase from grain boundaries and the formation of coarse Nd-rich phase. In addition, the sample subjected to the prolonged deformation had quite thin and discontinuous grain boundary phase, which resulted in the decrease of the coercivity.

Anisotropic Consolidation Behavior of Isotropic Nd–Fe–B HDDR Powders During Hot-Deformation

In this paper, the anisotropic consolidation behavior of isotropic Nd–Fe–B HDDR powders has been studied as a function of the strain, strain rate, and processing temperature. The compacts, produced by hot-pressing at 700 °C under 400 MPa in vacuum, were subjected to die-upsetting at 700 °C and 800 °C with different deformation conditions of strain rate 0.01–0.001 s⁻¹ and strain 0.5–1.5. After die-upset, the coercivity and remanence decreased and increased with increasing strain, respectively, with the change of grain morphology from the sphere to platelet. And the high deformation temperature and slow deformation rate reduce the coercivity further. The coercivity of die-upset magnet, produced at 700 °C with a strain of 1.4 and a strain rate of 0.001 s⁻¹, was largely increased about 2 kOe after post-annealing at 800 °C. The maximum coercivity, remanence, and (BH)_max were 10.4 kOe, 12.4 kG, and 35.6 MGOe, respectively.

Effects of strain ratio and stain rate on microstructure and magnetic properties of Nd-Fe-B nanocrystalline magnets during hot-deformation process

This study investigates the effects of strain ratio and strain rate on microstructure and magnetic properties of Nd-Fe-B nanocrystalline magnets during hot-deformation process. The consolidated Nd-Fe-Ga-Co-B isotropic magnet is prepared by hot-pressing under 100 MPa in a vacuum. Field emission scanning electron microscopy, transmission electron microscopy, and vibrating sample magnetometry are used for characterization. Observations show that the remanence and coercivity tended to increase and decrease with increasing strain ratio, respectively. Moreover, remanence increases while coercivity decreases as strain rate decreases. These results indicate that slow strain rate is favorable for improvement of remanence with well-aligned grains. However, a change in grain boundary phase as thinned Nd-rich phase is a major drawback, caused by increasing strain ratio including extended deformation time in slow strain-rate hot-deformation process.