Institute of Fundamental Technological Research

Residues resulting from the manufacture of strontium ferrite magnets have been recycled for further use in magnet fabrication instead of disposal as waste. The quality of the recycled ferrite powder has been tested and compared to that of the new starting ferrite material. The magnetic properties of the recycled powder not only match those of the starting material acquired by the company for the production of magnets but exceed them. A coercivity value 3.5 times larger than that of the new starting ferrite powder, accompanied by a 25% increase in remanence, makes this material a new and improved ferrite product to re-enter the production chain in the factory with an extended applications range. This improvement is proven to be due to tuning of the morphology and microstructure through processing and subsequent heat treatment. The use of processing conditions in the same range as those typically used in the preparation of ferrite powders and magnets, in combination with the superior magnetic quality of the resulting powders, makes this method a suitable path to guarantee sustainability and an efficient use of resources in permanent magnet companies.

Ferrites, Permanent magnets, Recovery, Recycling, Sustainability

The impact of ultra-short milling at liquid nitrogen temperatures on structural and magnetic properties of cobalt ferrite (CoFe2O4) powders has been explored for the first time. Cryomilling for only up to 9 min increases the coercivity of the isotropic powder from 139 to 306 kA/m (1.74–3.85 kOe) and results in its modifications comparable with milling for hours at room temperature. A thermal treatment of processed CoFe2O4 enables further optimization of powder magnetic properties and leads to a high value of energy product (13.5 kJ/m3) for the sample treated at 600 °C. Systematic studies, comprising analysis of structural and microstructural properties, based on synchrotron powder X-ray diffraction, scanning and transmission electron microscopy demonstrate the high efficiency of cryomilling in reduction of crystallite sizes and formation of lattice strain in the processed cobalt ferrite samples.

Cryomilling, Cobalt ferrite, Magnetic properties

We report on a comparative study of high performance isotropic cobalt ferrite (CoFe2O4) powder processed by dry and surfactant assisted (wet) ball milling. Milling times as short as 1.5 min (dry) and 6 min (wet) have resulted in a 4-fold increase in coercivity, with a maximum achieved value above 318 kA/m (4 kOe). The use of surfactant is shown to be advantageous in the formation of a more homogeneous structure constituted by non-agglomerated and strained nanoparticles. A record (BH) max value of 18.6 kJ m −3 (2.34 MGOe) has been obtained for isotropic powder after post-processing annealing. This magnetic performance combined with the required short processing times and the unnecessary requirement of oxygen avoidance in the milling process, makes this CoFe2O4 powder a good candidate for permanent magnet applications.

Milling, Nanopowders, Powders, Coercive force, Surfactants

Synthesis of nanocrystalline Co-ferrite powders with tunable magnetic properties is demonstrated by using co-precipitation and a novel ultrafast milling route. Milling times as short as a few minutes are re- ported for the first time to be sufficient to refine microstructure and to induce microstrain, and act efficiently, providing a 5-fold increase in coercivity. This is achieved with no compositional change during processing, but exclusively through microstructural modification. The efficiency of this process and its feasible scalability pave the way for development of Co-ferrite powders for permanent magnet applications.