CaO-matrix processing of MnBi alloys for permanent magnets

• Published in: AIP advances 2017-05, Vol.7 (5), p.056217-056217-5
• Main Authors: Gabay, A. M., Hadjipanayis, G. C.
• Format: Article
• Language: English
• Subjects: Annealing; Coercivity; Crystal defects; Defect annealing; Embedding; Permanent magnets; Sintering (powder metallurgy)
• ISSN: 2158-3226; 2158-3226
• DOI: 10.1063/1.4974974

The possibility to suppress agglomeration of MnBi alloy particles during milling and their unwanted sintering during subsequent annealing was explored by embedding the particles in CaO through co-milling. A 15 h annealing of the micron-sized MnBi particles embedded in the CaO matrix at 300 °C is not accompanied by sintering or growth of the particles while it significantly increases their coercivity – presumably by healing the milling-induced crystal defects. After separation from the CaO matrix, the annealed MnBi powder combines a calculated energy product of 10 MGOe with a room-temperature coercivity of 14.4 kOe. At the same time, the partial loss and degradation of the MnBi low-temperature phase during warm compaction of the powders makes the effect of the CaO-matrix annealing less pronounced in the case of fully dense magnets; the residue from the solvents employed for the removal of the CaO might have contributed to the decline of the properties. Still, a relatively high room-temperature coercivity of 8.5 kOe was obtained for the fuslly-dense MnBi magnet exhibiting an energy product of 5.3 MGOe.