Nd2Fe14B/CaF2 Composite Magnet Synthesized by Liquid-Phase Coating

The electrical resistance is one of the important properties for Nd₂Fe₁₄B magnets with low eddy-current loss. We have coated Nd-Fe-B melt-spun particles with CaF₂ by the liquid-phase coating technique and then performed hot-pressing and hot-deformation to make Nd₂Fe₁₄B/CaF₂ composite magnets. The morphology and distribution of CaF₂ in the magnets and its influence on the coercivity mechanism and the electrical properties of the Nd₂Fe₁₄B/CaF₂ magnets have been investigated by scanning electron microscope, hysteresisgraph, and four-probe resistivity meter. The morphology of CaF₂ depended on the liquid-phase synthesis parameters, especially the concentrations of the reactants. On the surface of Nd-Fe-B particles, there formed only a few CaF₂ nanoparticles and nanoflakes when 0.5 and 1 mol/l reactant aqueous solutions were used as precursors. With the increase of the concentrations of the reactants, there obtained a flake-shaped thin CaF₂ coating on the Nd-Fe-B particles. In addition, the CaF₂ coating became thicker, ~150-200 nm, with further increasing the concentrations of the reactants to 2 mol/l. After compaction and hot-deformation, the electrical resistances of the Nd₂Fe₁₄B/CaF₂ magnets fabricated using the concentrations of the reactants of 2 mol/l reached approximately 580 μΩ cm, which increased about 348% comparing with the corresponding pure Nd-Fe-B magnet. It is interesting to find that the coercivity of the Nd₂Fe₁₄B/CaF₂ composite magnets increased from 12.42 to 13.5 kOe when the concentrations of the reactants increased from 0.5 to 1.5 mol/l and then decreased to 10.2 kOe with further increasing the concentrations of the reactants to 2 mol/l. The reason for the increment of coercivity may be that CaF₂ nanoparticles - nd nanoflakes acted like lubricants that were beneficial to the rotation and the orientation of Nd₂Fe₁₄B particles during the hot-deformation process.