Institut für Materialwissenschaft, Technische Universität Darmstadt, D-64287 Darmstadt, Germany
Abstract:
Nd-Fe-B permanent magnets exhibit excellent magnetic properties and are widely used in industry as essential components of energy applications [1]. However, the temperature stability of the magnets is rather poor. Heavy rare earths are usually added into the starting alloy to enable high temperature applications,. As a drawback, the saturation magnetization Ms as well as the energy product (BH)max are decreasing. To overcome these problems, the grain boundary diffusion process (GBDP) was proposed by Park et al. [2], where the Dy is first deposited on to the magnet´s surface and then diffused into the magnet during a post-sintering annealing treatment. As a result, same performance with a significantly reduced amount of Dy is realized which allows the fabrication of more resource efficient high coercivity magnets.
We carried out investigations of different GBDP routes for the optimization of the heavy RE focusing on two main routes: novel coating techniques and novel coating materials. For the former one the electrophoretic deposition (EPD) of different Dy and Tb compounds on sintered magnets has been applied successfully. With this technique uniform rare earth containing films can be deposited. Subsequent annealing enables the GBDP and leads to an enhancement in coercivity [3]. We apply now EPD to hot compacted and hot deformed magnets. Several binary and ternary low melting eutectics based on either Dy, Tb, Nd or a mix of them have been explored.
We also investigate an alternative approach of introducing the heavy RE into the Nd-Fe-B magnets. In contrast to the conventional GBDP, we coated the particles prior to the hot compaction [4]. To do so powders of heavy RE eutectics were mixed with melt-spun Nd-Fe-B powder, ball-milled together and subsequently compacted at high temperatures. The influence of the milling and consolidation parameters on the magnetic properties is reported.
Finally, the temperature dependent diffusion behavior of pure Dy in sintered Nd-Fe-B magnets and its effect on magnetic properties and microstructure has been studied [5]. The results show a coercivity gain of over 400 kA/m (0.5 T) for optimally processed magnets and a diffusion depth of 3-4 mm. With scanning electron microscopy (SEM) wavelength dispersive X-ray (WDX) and scanning transmission electron microscopy (STEM) - energy dispersive x-ray (EDX) an exponentially decreasing Dy concentration within the magnet has been found, from about 6 at.% at the immediate surface to about 1 at.% in a depth of 1500 µm.
[1] O. Gutfleisch et al., Magnetic Materials and Devices for the 21st Century: Stronger, Lighter, and More Energy Efficient, Adv. Mat. 23 (2011) 821.
[2] K.T. Park, et al., in: Proc. 16th Int. Workshop on Rare-Earth Magnets and Their Applications, Sendai, 2000, p. 257.
[3] M. Soderznik et al., Intermetallics 23 (2012) 158-162
[4] S. Sawatzki et al., J. Appl. Phys. 115 (2014)
[5] K. Loewe et al. Acta Mat 83 (2015), 248-255.
We carried out investigations of different GBDP routes for the optimization of the heavy RE focusing on two main routes: novel coating techniques and novel coating materials. For the former one the electrophoretic deposition (EPD) of different Dy and Tb compounds on sintered magnets has been applied successfully. With this technique uniform rare earth containing films can be deposited. Subsequent annealing enables the GBDP and leads to an enhancement in coercivity [3]. We apply now EPD to hot compacted and hot deformed magnets. Several binary and ternary low melting eutectics based on either Dy, Tb, Nd or a mix of them have been explored.
We also investigate an alternative approach of introducing the heavy RE into the Nd-Fe-B magnets. In contrast to the conventional GBDP, we coated the particles prior to the hot compaction [4]. To do so powders of heavy RE eutectics were mixed with melt-spun Nd-Fe-B powder, ball-milled together and subsequently compacted at high temperatures. The influence of the milling and consolidation parameters on the magnetic properties is reported.
Finally, the temperature dependent diffusion behavior of pure Dy in sintered Nd-Fe-B magnets and its effect on magnetic properties and microstructure has been studied [5]. The results show a coercivity gain of over 400 kA/m (0.5 T) for optimally processed magnets and a diffusion depth of 3-4 mm. With scanning electron microscopy (SEM) wavelength dispersive X-ray (WDX) and scanning transmission electron microscopy (STEM) - energy dispersive x-ray (EDX) an exponentially decreasing Dy concentration within the magnet has been found, from about 6 at.% at the immediate surface to about 1 at.% in a depth of 1500 µm.
[1] O. Gutfleisch et al., Magnetic Materials and Devices for the 21st Century: Stronger, Lighter, and More Energy Efficient, Adv. Mat. 23 (2011) 821.
[2] K.T. Park, et al., in: Proc. 16th Int. Workshop on Rare-Earth Magnets and Their Applications, Sendai, 2000, p. 257.
[3] M. Soderznik et al., Intermetallics 23 (2012) 158-162
[4] S. Sawatzki et al., J. Appl. Phys. 115 (2014)
[5] K. Loewe et al. Acta Mat 83 (2015), 248-255.