科研之友
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摘要
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, similar to 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 mu Omega 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 and nanoflakes acted like lubricants that were beneficial to the rotation and the orientation of Nd2Fe14B particles during the hot-deformation process.
