Masashi Matsuura

Preparation of ultrafine jet-milled powders for Nd-Fe-B sintered magnets using hydrogenation–disproportionation–desorption–recombination and hydrogen decrepitation processes

Dy addition is used to increase the coercivity of Nd-Fe-B sintered magnets. Given that Dy is rare and expensive, a method is needed for reducing the Dy content in such magnets without decreasing their coercivity. Refining Nd2Fe14B grains is a prospective method for increasing the coercivity of Nd-Fe-B magnets. Conventional jet milling, however, cannot crush strip-casted Nd-Fe-B alloys into powders less than 1u2009μm in size. We report a process for preparing ultrafine jet-milled powders with an average size of 0.33u2009μm for Nd-Fe-B sintered magnets: a combination of hydrogenation–disproportionation–desorption–recombination, hydrogen decrepitation, and He jet milling.

Microstructure evaluation for Dy-free Nd-Fe-B sintered magnets with high coercivity

Nd-Fe-B sintered magnets are used for motors of hybrid or electric vehicles due to their high energy products. Dy is added to Nd-Fe-B sintered magnets to work in a high temperature environment. Although the addition of Dy decreases the magnetization of Nd-Fe-B magnets, it increases coercivity; a decrease in the amount of Dy is strongly required. Recently, Nd-Fe-B sintered magnets with a grain size of 1u2009μm achieved high coercivity of ∼20 kOe without the addition of Dy or other heavy rare earth elements. In this paper, the microstructure of their magnets was observed and compared to magnets with a grain size of ∼3u2009μm. The coercivity of magnets consisting of larger particles was 17 kOe. Microstructures were observed by the scanning electron microscope and the shapes of grains and the distribution of the Nd-rich phase were evaluated. The observation was promoted in two directions. One direction is the plane perpendicular to the magnetically aligned direction (c plane side) and the other is the side parallel t...

Effect of Cu addition on coercivity and interfacial state of Nd-Fe-B/Nd-rich thin films

This study provides the effect of Cu addition on coercivity (HcJ) and interfacial microstructure in Nd-Fe-B/Nd-rich thin films. All films were deposited by using ultra high vacuum (UHV) magnetron sputtering, and the Nd-Fe-B layer was oxidized under several atmospheres with different oxygen content. Then, the films were annealed at 250–550 °C under UHV. The films oxidized in low vacuum (10−2−10−5 Pa) (under low oxygen state) exhibited the recovery of HcJ by the annealing at 450 °C. On the contrary, the HcJ of the films oxidized in Ar (under high oxygen state) decreased with increasing annealing temperature. However, the HcJ increased drastically at the temperatures above 550 °C. In addition, the Cu added films, which were annealed at temperatures above 350 °C, showed higher coercivities than the films without Cu addition. The XRD analysis suggested the existence of C-Nd2O3 phase in the Cu added films annealed at 550 °C. It can be considered that the Cu addition decreases the eutectic temperature of Nd-rich phase and influences the interfacial state between Nd2Fe14B and Nd-rich phase.

Influence of microstructural change of the interface between Nd2Fe14B and Nd-O phases on coercivity of Nd-Fe-B films by oxidation and subsequent low-temperature annealing

This study provides the influence of microstructural change of the interface between Nd2Fe14B and Nd-O phases on coercivity of Nd-Fe-B thin films during annealing at low temperature (~350 °C). All films were prepared by using ultra high vacuum (UHV) magnetron sputtering, and the Nd-Fe-B layer was oxidized under Ar gas atmosphere (O2 content; ~2 Vol.ppm). Then, the films were annealed at 250–350 °C under UHV condition. After oxidation, the coercivity of Nd-Fe-B film decreased to around 40% of the coercivity of as-deposited Nd-Fe-B film. The Nd-rich phase changed from α-Nd to amorphous Nd(-O), and the interface of Nd2Fe14B/Nd(−O) became rough. In the Nd-Fe-B films oxidized and subsequent annealed at 350 °C, the coercivity decreased to around 20%. In the films, poly crystalline hcp Nd2O3 phase crystallized in Nd-rich phase, and there were some steps at the surface of Nd2Fe14B phase contacting with hcp Nd2O3 phase. Regardless of crystal orientation of Nd2Fe14B, the microstructural changes of the interface described above were observed.

Nd-Fe-B sintered magnets fabrication by using atomized powders

Nd-Fe-B sintered magnets are required to achieve high coercivity for improvement of their thermal stability. Dy is added to increase coercivity, however, this element decrease magnetization and energy products. Therefore, Dy-lean Nd-Fe-B sintered magnets with high coercivity are strongly demanded. To increase coercivity, it is necessary that microstructure of sintered magnets is consisted of both fine main phase particles and homogeneously distributed Nd-rich phases around the main phase. To meet those requirements, Nd-Fe-B atomized powders were applied to the fabrication process of sintered magnets. Comparing with the case of using strip casting (SC) alloys, jet-milled powders from atomized powders show homogeneous distribution of Nd-rich phase. After optimized thermal treatment, coercivities of sintered magnets from atomized powders and SC alloys reach 1050 kAm−1 and 1220 kAm−1, respectively. This difference in coercivity was due to initial oxygen concentration of starting materials. Consequently, Nd-rich phases became oxides with high melting points, and did not melt and spread during sintering and annealing.