Journal of Physics-Condensed Matter
The hydrogenation, disproportionation, desorption and recombination (HDDR) process has been employed to produce Nd2Fe14C-based permanent magnets. The master alloys for HDDR are Nd2Fe14C-based alloys which are obtained by annealing mechanically alloyed (MA) Nd16Fe75C9 powder at 850 degrees C. The disproportionation reaction at 850 degrees C results in the Formation of an intimate mixture of Nd-hydride, alpha-Fe and Nd2Fe2C3. On desorbing the hydrogen at the same temperature, different constituents recombine to form the thermodynamically stable Nd2Fe14C and the Nd-rich phase, depending sensitively on recombination time and slightly on hydrogenation time. The phases obtained by HDDR at 950 degrees C consist of Nd2Fe17Cx, alpha-Fe, the Nd-rich phase and Nd2C3, whereas those obtained at 700 degrees C are of Nd2Fe17Cx, the Nd-rich phase and a small amount of alpha-Fe. For comparison, the Nd2Fe17Cx-based alloys have also been processed by HDDR, in which more alpha-Fe and Nd-rich phases are found to coexist with Nd2Fe14C after recombination at the same temperature. The Nd2Fe14C-based alloy made by MA in conjunction with HDDR has better magnetic properties than that made by MA only.
关键词:
permanent-magnets
Journal of Magnetism and Magnetic Materials
Sm-Fe-N permanent magnets were prepared from the starting Sm2Fe17 alloy by hydrogenation and nitrogenation processes. The phase components, structures and magnetic properties at each reaction stage involved in the processes have been investigated systematically. It has been found that many microcracks are present in powdered particles during the hydrogenation process, which is helpful for the nitrogenation process as well as for the remilling process. When the hydrogenation temperature T-H is less than 550 degrees C, the main process is hydrogen decrepitation (HD) and the consequent nitrogenation results in the anisotropic Sm2Fe17Ndelta powders whose magnetic properties depend not only on the hydrogenation temperature, but also on the size of the particles after the re-milling process. The best values for the magnetic properties are obtained for a particle size of 2 mu m average diameter, hydrogenated at T-H = 300 degrees C. When T-H greater than or equal to 550 degrees C, the hydrogenation mainly shows a hydrogenation-disproportionation-desorption-recombination (HDDR) process. At first, the disproportionation reaction of hydrogen with Sm2Fe17 results in the hydride SmH2+x and alpha-Fe and then the desorption-recombination process leads to the formation of metastable SmFe7 phase with TbCu7 structure. With increasing the hydrogenation temperature, the metastable SmFe7 transforms into Sm2Fe17 and the transformation is thoroughly completed at T-H = 850 degrees C. Consequently, the Curie temperature of the metastable SmFe7 phase decreases from 188 degrees C at T-H = 650 degrees C to 129 degrees C at T-H = 850 degrees C of Sm2Fe17 phase. The optimum hydrogenation temperature is 800 degrees C for preparing the isotropic Sm-Fe-N magnets. The best results for the magnetic properties of the Sm-Fe-N powders achieved respectively in the above two processes are: (i) the HD process followed by nitrogenation: B-r = 1.19 T (11.9 kG), H-i(c) = 11.3 kA/cm (14.2 kOe), (BH)(max) = 199.0 kJ/m(3) (25.0 MG Oe); and (ii) the HDDR process followed by nitrogenation: B-r = 0.81 T (8.1 kG), H-i(c) = 16.7 kA/cm (21.0 kOe), (BH)(max) = 103.5 kT/m(3) (13.0 MG Oe).
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permanent-magnets
Journal of Applied Physics
The phase components, structures, and magnetic properties of Pr1-xGdxCo5 (x=0-1) powders synthesized by mechanical alloying and subsequent annealing have been investigated systematically. The optimal magnetic properties, with a coercivity of 12.5 kOe, a remanence ratio of 0.72, and a maximum energy product of 12.7 MGOe, have been obtained from PrCo5 powders milled for 5 h and annealed at 700 degreesC for 2 min. The remanence and maximum energy product were decreased monotonically with increasing Gd content, whereas the coercivity was increased, reaching a maximum of 23.7 kOe in Pr0.2Gd0.8Co5 powders. X-ray diffraction and transmission electron microscopy observations reveal that an uniform (Pr,Gd)Co-5/(Pr,Gd)(2)Co-17 nanostructure with an average grain size of 20-30 nm forms in the powders annealing at 700 degreesC. The obtained magnetic hardening apparently originates from the high anisotropy field of the hard (Pr,Gd)Co-5 phase and the uniform nanostructure developed by mechanical alloying and subsequent annealing. (C) 2003 American Institute of Physics.
关键词:
permanent-magnets
