NANOPERMAG · HIGH PERFORMANCE NANOSTRUCTURE PERMANENT MAGNETS

High performance nanostructure permanent magnets

The development of a new generation of permanent magnets is more urgent than ever in view of the demand for more efficient engines in wind energy, all electric cars and space applications. The aim of this programme was to exploit the opportunities available to fabricate powders of magnetically hard rare earth intermetallic nanoparticles, study and understand their intrinsic magnetic properties and then use them in a 'bottom-up' approach to develop new classes of anisotropic nanocomposite magnets with previously unattainable high energy products, (BH)max. Our studies were focused on anisotropic high coercivity Sm-Co, Nd-Fe-B and Sm-Fe-N nanoparticles with sizes below 150 nm, and soft powders based on Fe(Co) nanoparticles with sizes in the range of 15-30 nm and with a high magnetisation (hard FePt was also used for modelling purposes). A number of different fabrication techniques were used to synthesise the particles including sputtering, chemical and mechanochemical synthesis and surfactant assisted milling. Fe-Co particles with a magnetisation of 145 emu/g and size below 540 nm have been synthesised by thermal decomposition of Fe(CO)5 and Co2(CO)8 in paraffin oil in the presence of oleic acid and oleylamine. High coercivity (7.8 kOe) fct FePt nanoparticles were synthesised at 300 degrees of Celsius by thermal decomposition of Fe(CO)5 in the presence of Pt(acac)2. SmCo5 and Pr(Co)5 nanoparicles (5-10 nm) and nanoflakes with a thickness below 100 nm and coercivity exceeding 15 kOe, were synthesised by surfactant assisted high energy ball milling in oleic acid / oleylamine. The Sm-Co flakes have a texture perpendicular to the flake. The same technique was also used to make Nd2Fe14B particles and flakes. Surprisingly, the texture in the Nd-Fe-B flakes is in the plane of the flakes. The coercivity of the Nd-Fe-B particles decreases substantially with decreasing size and for 25 nm it is only 4 kOe at room temperature. This behaviour has been attributed to an induced surface disorder during milling which decreases the particle's overall anisotropy and therefore the coercivity. Particles with a size about 120 nm showed a higher coercivity 7.4 kOe because of the smaller surface to volume ratio. Low temperature consolidation techniques including 'shear compaction' and hot compaction are currently being used to consolidate the hard / soft composites and make anisotropic hard / soft bulk magnets. Both of these techniques lead to magnets with density close to bulk but with substantially lower coercivity.

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