FeCo Nanowire–Strontium Ferrite Powder Composites for Permanent Magnets with High-Energy Products

Abstract

Due to the issues associated with rare-earth elements, there arises a strong need for magnets with properties between those of ferrites and rare-earth magnets that could substitute the latter in selected applications. Here, we produce a high remanent magnetization composite bonded magnet by mixing FeCo nanowire powders with hexaferrite particles. In the first step, metallic nanowires with diameters between 30 and 100 nm and length of at least 2 μm are fabricated by electrodeposition. The oriented as-synthesized nanowires show remanence ratios above 0.76 and coercivities above 199 kA/m and resist core oxidation up to 300 °C due to the existence of a >8 nm thin oxide passivating shell. In the second step, a composite powder is fabricated by mixing the nanowires with hexaferrite particles. After the optimal nanowire diameter and composite composition are selected, a bonded magnet is produced. The resulting magnet presents a 20% increase in remanence and an enhancement of the energy product of 48% with respect to a pure hexaferrite (strontium ferrite) magnet. These results put nanowire–ferrite composites at the forefront as candidate materials for alternative magnets for substitution of rare earths in applications that operate with moderate magnet performance.

Scaling Up the Production of Electrodeposited Nanowires: A Roadmap towards Applications

“…Choosing the proper anodization process, it is possible to reduce the production time and, more importantly for industrial applications, to reduce the production costs by more than three orders of magnitude, just by the selection of the appropriate template. In spite of the larger dispersion in the diameter of the AAO templates using recycled Al as the starting material, the energy product of all magnetic nanocomposites is the same: 44% with respect to the counterpart bonded magnet made using only SFO nanoparticles [ 23 , 47 ]. Therefore, it is worth noting that the procedure described here allowed us to increase the production rate of CoFe NWs suitable for applications in magnetic nanocomposites, from g/week to mg/week, while decreasing fabrication costs.…”

“…Using this technique, the properties of the NWs can be easily tailored [ 17 , 18 ] to adapt them to specific applications. In particular, taking advantage of their elongated shape, the magnetic properties of metallic NWs can be controlled just by tailoring their shape and composition [ 19 , 20 ]; this has been, for example, recently exploited in the fabrication of corrosion resistant composites [ 21 ], magnetoelectric nanocomposites [ 22 ] and bonded magnets [ 23 ]. In spite of having some promising performance, there are few NW-based magnetic applications on the market, due to the lack of a growth protocol allowing the fabrication of the large amounts of NWs needed to prepare a composite, preventing the transition from the laboratory proof of concept toward the industrial scale to take place.…”

Aluminum Doping and Nanostructuring Enabled Designing of Magnetically Recoverable Hexaferrite Catalysts

“… 1 , 3 , 7 , 11 , 12 Due to its superior magnetic parameters, high permeability, and low conductivity loss, SFO has been widely used in numerous technological applications, which include permanent magnet designs, microwave absorbers, magnetic recording media and sensors, high-frequency electromagnetic (EM) devices, EM shielding devices, and so forth. 1 , 3 − 7 , 10 …”

“… 2 − 15 Also, composites of SFO with soft magnetic phases and improved magnetic properties achieved through the exchange-spin mechanism have been reported in the literature. 3 , 10 , 16 , 17 For instance, using a conducting polymer and a SFO content for tunability, polypyrrole/SFO composites were designed and tested for efficient EM-shielding purposes. 10 It was demonstrated that the electrical and magnetic properties of such composites can be controlled simply by tuning the SFO content.…”

“… 2 Similarly, nanostructuring of SFO is shown to enhance the property and performance effectively. 3 − 7 SFO nanomaterials with highly aligned nanocrystallites combined with crystal growth during spark plasma sintering (SPS) exhibit enormous enhancement of the magnetic properties compared to the as-synthesized powders, leading to high-performance bulk magnets with high-energy products (26 kJ m –3 ). 4 , 5 Based on the extensive work, Christensen and team proposed a complete bottomup nanostructuring protocol for preparation of magnetically aligned, high-performance hexaferrite permanent magnets with a record-high (BH) max for dry-processed ferrites.…”