The 271st MANA & the 115th IYCS Joint Seminar
Dr. Yoshihiro Tsujimoto & Dr. Hossein Sepehri Amin
| Date | June 15, Friday |
| Time | 15:30-16:30 |
| Place | Auditorium, 1F, WPI - MANA Bldg., NAMIKI Site, NIMS |
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15:30-16:00
Anion Chemistry in Solid State Materials: New routes to oxyfluoride materials
Electrochemical applications of graphene are of great interest to many researchers as they can potentially lead to crucial technological advancements in fabrication of electrochemical devices for energy production and storage, and highly sensitive sensors. There are many routes towards fabrication of bulk quantities of graphene based materials. Each of them yields different graphene materials with different functionalities and structural defects. Here, we discuss and compare the electrochemical properties of graphenes varying several parameters in their preparation, such as: i) various reduction methods; here we compare five different chemically modified graphenes: graphite oxide, graphene oxide, thermally reduced graphene oxide, chemically reduced graphene oxide, and electrochemically reduced graphene oxide. ii) various methods for oxidation of graphite to graphite oxide, such as Staudenmaier, Hofmann and Hummers methods. iii) graphenes with opened and closed edges. We always characterize these materials in detail using transmission electron microscopy, Raman spectroscopy, high-resolution X-ray photoelectron spectroscopy, electrochemical impedance spectroscopy, and cyclic voltammetry; this allows us to correlate the electrochemical properties with the structural and chemical features of the CMGs. Our findings have a profound impact for the applications of chemically modified graphenes in electrochemical devices.
Speaker
Dr. Yoshihiro Tsujimoto, ICYS-MANA Researcher, MANA, NIMS
Chair
Dr. Kazunari Yamaura, Principal Researcher, Superconducting Properties Unit, NIMS
16:00-16:30
Development of high coercivity and performance Dy-free Nd-Fe-B permanent magnets
Nd-Fe-B sintered magnets exhibit the highest maximum energy product among all types of permanent magnets. However, their relatively low coercivity of µ0Hcj~1.2 T puts a limit on certain applications such as traction motors of (hybrid) electric vehicles and wind power generators, for which a higher coercivity of 3.0 T is required. This level of coercivity is currently achieved by the partial substitution of Dy for Nd, with a typical composition of (Nd0.7, Dy0.3)14Fe80B6 at the expense of maximum energy product [1]. However, because of the limited natural resources of Dy, the sustainable use of Dy for Nd-Fe-B permanent magnets is becoming difficult leading to an intensive research on the development of high coercivity and performance permanent magnets that can substitute currently used (Nd,Dy)-Fe-B magnets. The coercivity is not an intrinsic property of materials, but is controlled by optimizing the microstructure such as grain size, grain shape, and chemistry at the interfaces and grain boundaries that can give rise to the highest coercivity using the Nd2Fe14B phase. One way to develop high coercivity Nd-Fe-B permanent magnets is grain size reduction. However, with the grain size reduction to the single domain size of N2Fe14B phase (250nm) and even smaller (20-50 nm), the coercivity is limited to ~2.2 T which is expected to be more than ~3.0 T [2,3]. In this work, the mechanism of the low coercivity in nano grain sized Nd-Fe-B magnets, processed using rapid solidification, will be discussed. Based on these results, the coercivity of rapidly solidified Nd-Fe-B powders with the grain size of 20-50nm will be enhanced to the level of 2.6 T by grain boundary chemistry/structure modification using Nd-Cu and Pr-Cu diffusion process to a non-magnetic phase. The mechanism of the coercivity enhancement will be explained based on detailed microstructure characterizations and micro-magnetic simulations. Thereafter, our efforts to develop high performance permanent magnets using the currently developed high coercivity powders will be discussed.
Speaker
Dr. Hossein Sepehri Amin, ICYS-Sengen Researcher, NIMS
Chair
Dr. Kazuhiro Hono, Unit Director, Magnetic Materials Unit, NIMS