1Jožef Stefan Institute, Department for Nanostructured Materials, Ljubljana, Slovenia
2Vacuumschmelze, Hanau, Germany
Abstract:
In this study, our aim was to substantially improve the coercivity of Nd-Fe-B magnets, based on the grain-boundary diffusion process (GBDP) initiated with the electrophoretic deposition (EPD) of TbF3 powder.
In order to retain the remanence as high as possible Nd-Fe-B magnets with a rather low amount of heavy rare earths (HRE) were used as starting material. The overall amount of HRE was composed of 0.1 wt. % of Dy and 1.5 wt. % of Tb. The magnets with the dimensions 12.5 mm × 8 mm × 3.5 mm were evenly coated with 30 mg of TbF3 powder using the electrophoretic deposition at a constant applied voltage of 60 V. After the magnets were coated with TbF3 powder, they were exposed to the heat treatment in vacuum at 875°C for 10 h followed by aging at 500°C for 1 h. The magnetic properties were measured with a Steingroever permeameter. Since the magnets could not be demagnetized at room temperature, the remanence and the coercivity were determined to be Br = 1.25 T and HcJ = 1580 kA/m at 70°C, respectively. By applying the temperature coefficients TK(Br) = -0.11 %/K and TK(HcJ) = -0,63%/K, the remanence and coercivity are calculated to be 1,32 T and 2080 kA/m at room temperature, respectively. Using the electrophoretic deposition of TbF3, followed by the grain-boundary diffusion process, we have successfully enhanced the coercivity at room temperature to more than 2000 kA/m, while the remanence exceeded the ambitious value of 1.3 T. Taking into account mass of the TbF3 powder, the overall HRE content in the magnet after the GBDP is 2.4 wt. %, which means that we added only 0.8 wt. % of HRE and gained a large improvement of the coercivity.
The financial support from the European funding scheme FP7 NMP 2012 SMALL 6 (ROMEO) is acknowledged.
In order to retain the remanence as high as possible Nd-Fe-B magnets with a rather low amount of heavy rare earths (HRE) were used as starting material. The overall amount of HRE was composed of 0.1 wt. % of Dy and 1.5 wt. % of Tb. The magnets with the dimensions 12.5 mm × 8 mm × 3.5 mm were evenly coated with 30 mg of TbF3 powder using the electrophoretic deposition at a constant applied voltage of 60 V. After the magnets were coated with TbF3 powder, they were exposed to the heat treatment in vacuum at 875°C for 10 h followed by aging at 500°C for 1 h. The magnetic properties were measured with a Steingroever permeameter. Since the magnets could not be demagnetized at room temperature, the remanence and the coercivity were determined to be Br = 1.25 T and HcJ = 1580 kA/m at 70°C, respectively. By applying the temperature coefficients TK(Br) = -0.11 %/K and TK(HcJ) = -0,63%/K, the remanence and coercivity are calculated to be 1,32 T and 2080 kA/m at room temperature, respectively. Using the electrophoretic deposition of TbF3, followed by the grain-boundary diffusion process, we have successfully enhanced the coercivity at room temperature to more than 2000 kA/m, while the remanence exceeded the ambitious value of 1.3 T. Taking into account mass of the TbF3 powder, the overall HRE content in the magnet after the GBDP is 2.4 wt. %, which means that we added only 0.8 wt. % of HRE and gained a large improvement of the coercivity.
The financial support from the European funding scheme FP7 NMP 2012 SMALL 6 (ROMEO) is acknowledged.
Figure 1: The demagnetization curves before and
after the grain-boundary diffusion process,
initiated by electrophoretic deposition of Tb-
fluoride powder. The measurements were done
at 70 °C.
after the grain-boundary diffusion process,
initiated by electrophoretic deposition of Tb-
fluoride powder. The measurements were done
at 70 °C.