The Preparation, Microstruture, And Properties For MnBi Permanent Magnetic Alloys

The huge demands of rare earth permanent magnetic materials lead to the excessive consumption of rare earth resources. Low rare earth and free-rare earth permanent magnetic materials have turned into a hot topic. As novel free-rare earth permanent magnetic materials, MnBi magnets have received much attention due to high coercivity, moderate saturation magnetization, and positive coercivity temperature coefficient.It is hard to obtain Mn Bi with low temperature phase(LTP) content because of the peritectic reaction between Mn and Bi, where Mn phases starts to separate during solidification process. In this work, arc-meling, melt-spinning technique, high energy ball milling and followed annealing were adopted to prepare Mn Bi alloys with high content LTP MnBi. The microstructure and magnetic properties were investigated in details.DSC tests indicated that the LTP MnBi was formed in the temperature range of 533K-633 K. The excess Mn was added to incease the content of LTP Mn Bi. During the initial temperature range from LTP MnBi to HTP MnBi, the decomposition is expected to be small, and Mn and Bi precipitates to react at higher temperature is advantageous. The coerivity of 604kA/m can be obtained for ingot with Mn55Bi45+5wt. % composition under the heat treatment of 535 K/8h. The magnet with high reamnence of 38emu/g was prepared under the two-step annealing of 535K/6h and 613K/5h. For MnBi ribbons, the remanence of 20 emu/g and coercivity of 165kA/m were achieved using annealing condition of 543K/30 min. For the composition with Mn55Bi45+5wt. %, the optimized annealing processes of ingots and ribbons were acquired. For the ingot prepared by the annealing process of 613K/8h, it possesses the remanence of 20 emu/g and the coercivity of 165 kA/m. Magnetic properties with coercivity of 79 kA/m and remanence of 29 emu/g can be obtained for MnBi ribbons annealed under the condition of 613K/40 min. It is proved that melt-spinning techqiue and the annealing are useful in increasing the content of LTP MnBi.MnBi/?-Fe nanocomposite materials were made by the nanocrystalline MnBi flakes prepared by melt-spinning technique, and soft magnetic phase ?-Fe produced by coprecipitation technique. Amorphous soft magnetic phase was produced when the molar ratio of Fe ion and boron hydrogen ion was 1:2.5.The MnBi/?-Fe nanocomposite materials with demagnetization characteristics of single hard magnetic phase, indicate the existence of effective exchange-coupling between MnBi phase and ?-Fe phase. It is showed that MnBi/?-Fe nanocomposite materials were fabricated successfully.