Abstract
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.
Highlights
The best performing anisotropic magnetically hard MnBi powders which are of interest for the manufacturing of rare-earth-free permanent magnets – including nanocomposites – are being currently prepared by milling
Both the mean particle size and its variance decrease with the τbm, and the former tend to saturate at around 1 μm after some 15 min of milling [see Fig. 2(a)]. This size is an order of magnitude smaller than what could be expected for metallic alloys milled under the similar conditions without the CaO “matrix.”[8] This difference indicates that cold welding of the MnBi particles is suppressed by the CaO spacer
Since the washing relied on magnetic separation, the declining alloy yield may reflect irreversible decomposition of the magnetic α-MnBi phase; the X-ray diffraction (XRD) data indicate that the fraction of this phase in the collected alloy declines with the τbm
Summary
The best performing anisotropic magnetically hard MnBi powders which are of interest for the manufacturing of rare-earth-free permanent magnets – including nanocomposites – are being currently prepared by milling. Higher values of the coercivity Hc in these powders require longer and/or more intensive milling which may result in agglomeration of the particles[1,2,3] and gradually destroys the essential α-MnBi phase[3,4,5,6] generating in the process a low-melting-temperature Bi phase. The α-MnBi can be at least partially recovered by annealing at ≈300 ◦C,3 the presence of the liquid Bi at this temperature leads to unwanted sintering.[1,7] In this work, we attempted to address the problems associated with the milling and annealing treatments by embedding the MnBi particles into a removable CaO matrix and inhibiting their agglomeration and sintering The effect of such CaO-matrix processing was studied for both the MnBi powders and the warm-compacted MnBi magnets
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