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摘要
Nd-Fe-B powders were coated with CaF2 by three different chemical synthesis methods, named as A: One-step direct precipitation, B: One-step slow dropping, and C: Two-step process. The CaF2-coated Nd-Fe-B powders were hot-pressed and then hot-deformed to fabricate composite magnets. The microstructures, electrical resistivities, and magnetic properties of the Nd-Fe-B composite magnets obtained with different coating methods and parameters were investigated. The results showed that the thickness and continuity of CaF2 coating depended on the coating methods with different Ca(NO3)(2) concentrations and coating time. When the Ca(NO3)(2) concentration was 2mol/l, the CaF2 coating synthesized by one-step direct precipitation was a loose and discontinuous film, while the CaF2 coating synthesized by one-step dropping for 30 min was a continuous and dense film, and its thickness reached to 410-450 nm. If the Ca(NO3)(2) concentration was further increased to 5.5mol/l, the thickness of CaF2 reached to 710-900nm. The electrical resistivities of the composite magnets prepared by one-step slow dropping for 30 min with the Ca(NO3)(2) concentrations of 2 and 5.5mol/l were approximately 680 and 890 mu Omega cm, which was a 195% and 287% increase, respectively, compared to that of the corresponding magnet prepared with uncoated Nd-Fe-B powders. The coercivities of the composite magnets decreased with increasing thickness of CaF2, while the remanence had only a slight reduction. The composite magnet fabricated by a two-step process achieved both higher maximum energy product ((BH)(m) =47.2MGOe) and electrical resistivity (847 mu Omega cm). (C( 2014 AIP Publishing LLC.
