In a novel process to enhance the coercivity we have electrophoretically deposited DyF3 powder onto the surface of an as-sintered Nd–Fe–B magnet as the initial step in the grain-boundary diffusion process. After a conventional heat treatment at 850 and 500 °C the coercivities were higher than in the case of simple dipping after a typical 10 h, with Hci values exceeding 1600 kA/m for a 200 μm-thick deposited layer. The electrophoretic deposition (EPD) process is quick, easily controllable in terms of thickness and can be used to deposit the rare earth fluoride powder on the surface of complex and irregularly shaped magnets. Since the amount of deposited powder can be tailored to maximise the coercivity while minimising the quantity of expensive heavy rare earth there is no wasted powder, making the diffusion process, which takes place after the sintering process, more environmentally friendly and potentially cheaper than conventional dipping.
COBISS.SI-ID: 25708071
In this paper we have looked at the effects of a range of hydrogen pressures and temperatures on the magnetic and structural changes in Nd–Fe–Al alloys with compositions close to Nd60Fe30Al10, using vibrating-sample magnetometry, X-ray diffraction and transmission electron microscopy. Our results suggest that the material is resistant to hydrogen at low pressures and temperatures, but at approximately 30 bars and 100 °C the material absorbs about 0.60 ± 0.05 weight% of hydrogen. After this hydrogen absorption the coercive field decreases significantly, i.e., from 3750 Oe before to 120 Oe after the hydrogenation. We have considered the strong-domain-wall-pinning model to explain the coercive field and its drop as a result of the hydrogen absorption. This model can be used to describe the material in the temperature range 250–450 K before hydrogenation for a domain-wall width of 7 nm. After hydrogenation the material displays soft-magnetic behaviour and the possible origins of this are discussed. Our results have demonstrated the important role that hydrogen can play in modifying the structure and properties of rare-earth-transition-metal-based permanent-magnet materials.
COBISS.SI-ID: 26009127