In Situ TEM Observation of Low-Temperature-Phase Formation in Mn-Bi Nanopowders Synthesized by Low-Oxygen Induction Thermal Plasma

The low-temperature-phase MnBi (LTP-MnBi) powders were successfully synthesized via an in-house developed low-oxygen induction thermal plasma system, followed by post-annealing treatment. However, achieving high-purity LTP-MnBi alloy remains a challenge due to persistent undesired Mn, Bi and Bi-rich phases post-peritectic reaction during a heat treatment. Here, we employed the in-situ transmission electron microscopy to investigate phase transformation processes dynamically. It is found that micro-sized Bi clusters formed after the heat treatment which can significantly shift the desired Mn:Bi composition and reduce the area fraction of Mn/Bi interface which is essential for an efficient peritectic phase transformation. It sequentially reduces the volume fraction of the desired LTP-MnBi phase in the final product. The in-situ kinetics analysis in this work elucidates the structural evolution and phase transformation mechanisms in Mn-Bi nanopowders, offering useful insights for developing high-performance rare-earth-free permanent magnet.