Spomenka Kobe

Abstract:

Permanent magnets based on Nd-Fe-B are the highest energy magnets for more than 30 years and are vital components in the rapidly-developing renewable energy sector, where the motors for electric vehicles and the generators in wind turbines require strong magnets with the ability to operate at temperatures well over 100�C. In 2014 EU published a new list of critical raw materials (CRM). Regarding supply risk, the rare earth was considered to be by far the most problematic and especially the heavy rare earth (HRE) are far above all light rare earth. Since HRE such as dysprosium or terbium is required to assure the high-temperature performance of the magnets, our research has focused on drastic decrease the amount of this elements and to achieve the same quality in sintered magnets as well as in the basic powers used for bonded magnets. Our research was focused on HRE free Nd-Fe-B melt spun ribbons doped with small amounts of DyF3 to improve the coercivity and to minimize the need for HRE. We will report on the correlation between magnetic properties, the amount of the additive, and the processing parameters. The explanation of the HRE influence on the improved properties will be based on microstructural analysis using high-resolution electron microscopy and chemical analyses. We will show that the addition of DyF3 up to 2.2 wt.% to the melt-spun powder showed a positive effect on the Hci of the heat-treated samples. The maximum coercivity (Hci) achieved represents a 25 % increase over the untreated samples. The interphase between the grains was thoroughly studied. The EEL results, which confirmed EDX investigations showed in the case of annealed sample the (Dy, Nd)-Fe-B phase formation at the shell around the pure Nd-Fe-B core grains. The high increase in Hci is strongly linked with the heat-treatment process where Dy diffused along grain boundaries into the outer parts of Nd-Fe-B grains and partial substitute Nd by Dy forming core-shell-like grains. We will document that fluorine does not penetrate into the grains but it accumulates in the grain boundary regions. Furthermore, the thermal gravimetric reaction coupled with mass spectroscopy revealed that none of the toxic fluorine-based compounds evaporate or form during heat-treatment to 1200 �C. Therefore, we can conclude that our technique for boosting the Nd-Fe-B magnets with small amount of DyF3 is one of the most efficient and environment safe processes.