Invited paperNanocrystalline high performance permanent magnets
Introduction
Nanocrystalline materials, including those of magnetic materials, have been at the centre of numerous R&D activities during the last decade because of their particular scientific and technological properties. In the case of hard magnetic rare earth–transition metal (R–T) compounds, it is the grain size and the presence or absence of intergranular phases which give rise to unusual magnetic properties because of surface/interface effects different from those of bulk or microcrystalline materials. Large coercivities can be obtained once the grain size is below a certain threshold where the crystallites become single domain. In most of the R–T-compounds discussed here, the critical single-domain particle size dc is a fraction of a micron.
Assuming idealized microstructures, three prototypes of NdFeB-type magnets can be distinguished on the basis of the ternary phase diagram [1]: Type (I) is rare earth rich and the individual crystallites are separated by a thin paramagnetic layer, the rare earth-rich intergranular phase. This structure leads to a decoupling of the hard magnetic grains resulting in high coercivities. Type (II) is obtained using the stoichiometric R2Fe14B composition and the hard magnetic grains are in direct contact with each other (‘single-phase exchange coupled magnets’) [2]. Type (III) nanocomposite magnets are R deficient (i.e., R concentrations <11.76 at%) and the coupling occurs between the R2Fe14B grains (to provide high coercivity) and soft magnetic Fe3B or Fe rich grains (to provide high magnetisation; e.g. Js(α–Fe)=2.16 T). The exchange interaction between the grains of the different phases leads to single-phase demagnetisation curves despite a multi-phase microstructure provided grain sizes are below a certain threshold and paramagnetic intergranular phases are absent [3], [4], [5]. Enhanced remanences of the isotropic hard magnetic materials, larger than those predicted by the Stoner-Wohlfarth theory [6] for systems of isotropically oriented, magnetically uniaxial, non-interacting single domain particles where Mr/MS⩽0.5, are the consequence.
The development of melt-spun or rapidly quenched Nd–Fe–B magnets by Croat and Herbst [7] coincided with that of sintered magnets by Sagawa [8]. Nanocrystalline structures can also be synthesised by mechanical alloying [9], intensive milling or hydrogenation disproportionation desorption and recombination (HDDR) processing [10], [11]. These nanostructures, provide energy barriers preserving the metastable, permanently magnetised state. The resulting isotropic powders are most commonly used for the production of bonded magnets, where they are usually mixed with polymer resin and are then injection or compression moulded. Bonded magnets have the advantage of easily accomplished near net-shape processing, the avoidance of eddy-currents and good mechanical properties. The disadvantage being the dilution of magnetic properties due to the polymer binder.
The randomly oriented grain structure results in magnetically isotropic magnets, with the remanent polarisation, Jr, and (BH)max limited to 0.5 and 0.25, respectively, of the values obtainable for ideal microstructures consisting of single domain grains and with full crystallographic alignment. Therefore various concepts have to be developed in order to increase remanence as shown in Fig. 1. The three most relevant ways of maximising the energy density (BH)max are hot deformation [12], [13], inducement of texture via ‘anisotropic’ HDDR [14] or thirdly, remanence enhancement via exchange coupling [3], [4].
In summary, the task of transferring good intrinsic properties such as high values of Curie temperature (TC>500 K), high saturation magnetisation (Ms>1 T) and high anisotropy field, HA into useful extrinsic properties of nanocrystalline magnets such as coercive field HC, remanent magnetisation Br and maximum energy density (BH)max by appropriate processing is described in this paper.
Section snippets
High energy ball milling
As a non-equilibrium processing technique, mechanical alloying circumvents, like rapid quenching, many limitations of conventional alloying and thus can be used for the preparation of metastable alloys. The mixing of the elements is achieved by an interdiffusional reaction, enabled by the formation of ultrafine layered composite particles during high energy ball milling. Depending on the thermodynamics of the alloy system, energy input and the mechanical workability of the starting powders, the
Conclusions
Nowadays about 85% of the limit for the energy density (BH)max (based on the Nd2Fe14B phase) can be achieved in commercially produced sintered Nd–Fe–B grades [44], [45]. Coercivity values however, rarely exceed 20–30% of the anisotropy field HA. Recent exciting developments include excellent anisotropic HDDR powders for polymer bonded magnets and SmCo-type magnets for application temperatures as high as 550°C. In terms of maximised energy densities, there is still a lot of scope for improvement
Acknowledgements
The support of parts of this work by the Deutsche Forschungsgemeinschaft (SFB 463), SfP (Science for Peace, Nato) and the EU (HITEMAG) is gratefully acknowledged.
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