Nanostructural formation and magnetic properties of Fe-B-Nd and Mn-Bi permanent magnets
thesisposted on 23.02.2017 by Ly, Vanalysa
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Permanent magnets are key components of electromagnetic devices, finding widespread use in domestic appliances, computers, automobiles and multitude of modern technologies. The demands for miniaturising, increasing the efficiency and lowering the cost of such devices drives a search for stronger magnets, that is, to improve the maximum energy product value. This thesis investigates two magnetic systems: Fe-B-Nd and Mn-Bi. To lower the cost of the first magnetic system: Fe-B-Nd, an approach to simplify and understand the manufacturing process was investigated by developing a physically based precipitation model coupling the competing nucleation and growth of six phases (bcc-Fe, Fe₃B, Nd₂Fe1₁₄B, Nd₂Fe₂₃B₃, NdFe₄B₄ and Fe₂B). The model monitored the nucleation, growth and dissolution of the precipitates at all times and temperatures. It explained the change in the crystallisation sequence as a function of heating-rates observed for the composition Fe₇₇.₅B₁₈Nd₄.₅ as a consequence of the heterogeneous nucleation of the Nd₂Fe₁₄B phase at the Fe₃B interface. In addition, the model indicated that the metastable Nd₂Fe₂₃B₃ phase (magnetically unfavourable), once formed, does not dissolve back into the amorphous matrix. This is consistent with the experimental in-situ crystallisation sequences which were found to be the same as the reported experimental ex-situ crystallisation sequences. In the second magnetic system, Mn-Bi, the effect of mechanical milling on the magnetic properties of Low Temperature (LT) MnBi was investigated in order to increase the efficiency of the magnet. First, mechanical milling of LT MnBi powder from 0.6 ks to 43.2 ks showed an increase in coercivity and suppression in the spin reorientation temperature. The current investigation suggests that the magnetic hardening as a result of mechanical milling is due primarily to the reduction in grain size and a minor contribution from the suppression in spin reorientation temperature. Secondly, nanocomposite LT MnBi/Fe magnets were developed using mechanical milling through the addition of Fe by fragmentation of the stainless steel vials and Fe-rich milling medium to LT MnBi. The microstructure (i.e. MnBi particles surrounded by Fe particles at the nanoscale) was confirmed by scanning transmission electron microscopy images. These magnets exhibit a saturation magnetisation higher than undoped LT MnBi magnets due to the presence of Fe. Thus, this is another avenue to enhance maximum energy product. Lastly, the effect of B, C and Bi additions on the magnetic properties of LT MnBi was investigated by mechanical milling these elements at different concentrations. Mechanical milling of C or Bi to LT MnBi resulted in a similar observation, an increase in coercivity and a decrease in saturation magnetisation. An increase in coercivity is probably due to (i) the segregation of C or Bi at the grain boundaries and (ii) grain refinement. In contrast, mechanical milling B to LT MnBi resulted in no obvious change in coercivity and a decrease in saturation magnetisation. The reduction in saturation magnetisation is possibly due to the formation of new phases.