Presentation Information
[O12-1]Do we really need a gap magnet?
*J. M. D. Coey1 (1. Trinity College Dublin, Ireland (Ireland))
Keywords:
gap magnet,energy product,supply chains
90% of the global market for permanent magnets is now divided between hexagonal ferrites and variants of tetragonal Nd-Fe-B; the rare earth magnets have a two-thirds share. A 2012 Viewpoint Paper in Scripta Materialia [1] discussed the prospects of realizing new magnets at appropriate price points with an energy product of 100–200 kJ m−3, in the gap between ferrite (<38 kJ m−3) and Nd–Fe–B (>200 kJ m−3). Most of the candidate materials have been known for many years, and waves of research in the wake of the 1982 discovery of Nd2Fe14B and the 2007 rare-earth crisis have failed to plug the gap, or even propose new candidate materials. An empirical rule is that materials that can remain permanently magnetized whatever their shape be should be made of a material with hardness parameter κ > 1, equivalent to an intrinsic uniaxial anisotropy K1 > µ0Ms3, where the spontaneous magnetization Ms is big enough to create a useful stray field. The energy product cannot exceed ¼ µ0Ms3, so a gap magnet minimally requires Ms > 560 kAm-1 and K1 > 400 kJm-3. Decent intrinsic magnetic properties are only the first step. The challenges of discovering an economic process to develop the microstructure needed for thermodynamically metastable coercivity and square hysteresis loops in a new material remain formidable. No rare-earth free material has succeeded. Progress in recent years has been limited to minor improvements in ferrites, and a dramatic expansion of the range of properties and price of variants of Nd-Fe-B with little or no heavy rare earth and increasing substitution of Ce, La and Y for Nd or Pr at the low end.
The answer to the question posed in the title is less scientific than political, perhaps depending on the ability to establish independent supply chains covering all five steps from mining to manufacturing products that exploit permanent magnetic fields.
[1] J. M. D. Coey, Permanent Magnets: Plugging the gap. Scripta Materialia 67 S24 (2012)
The answer to the question posed in the title is less scientific than political, perhaps depending on the ability to establish independent supply chains covering all five steps from mining to manufacturing products that exploit permanent magnetic fields.
[1] J. M. D. Coey, Permanent Magnets: Plugging the gap. Scripta Materialia 67 S24 (2012)