Nanoengineered high-performance hexaferrite magnets by morphology-induced alignment of tailored nanoplatelets
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Date
2018-11-15
Journal Title
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Publisher
American Chemical Society
Abstract
Magnetic materials are ubiquitous in electric devices and motors making them indispensable for modern-day society. The hexaferrites currently constitute the most widely used permanent magnets (PMs), accounting for 85% (by weight) of the global sales of PMs. This work presents a complete bottom-up nanostructuring protocol for preparation of magnetically aligned, high-performance hexaferrite PMs with a record-high (BH)max for dry-processed ferrites. The procedure includes the supercritical hydrothermal flow synthesis of anisotropic magnetic-single-domain strontium hexaferrite (SrFe12O19) nanocrystallites of various sizes, and their subsequent compaction into bulk magnets by spark plasma sintering (SPS). Interestingly, Rietveld modeling of neutron powder diffraction data reveals a significant difference between the magnetic structure of the thinnest nanoplatelets and the bulk compound, indicating the Sr-containing atomic layer to be the termination layer. Subsequently, high-density SrFe12O19 magnets (>95% of the theoretical density) are produced by SPS of the flow-synthesized nanoplatelets. Texture analysis by X-ray pole figure measurements demonstrates how the anisotropic shape of the nanoplatelets causes a self-induced alignment during SPS, without application of an external magnetic field. The self-induced texture is accompanied by crystallite growth along the magnetic easy-axis, i.e., the thickness of the platelets, resulting in high-performance PMs with square hysteresis curves and (BH)max of 30 kJ/m3. The (BH)max is further enhanced by annealing, reaching 36 kJ/m3 after 4 h at 850 °C, which exceeds the (BH)max of the highest grade of dry-processed commercial ferrites worldwide. © 2018 American Chemical Society
Description
Keywords
Neutron diffraction, Nanoparticles, Nanostructures, Texture, Permanent magnets, Plasma, Sintering, Strontium, Ferrite, Magnetic materials
Citation
Saura-Múzquiz, M., Granados-Miralles, C., Andersen, H. L., Stingaciu, M., Avdeev, M., & Christensen, M. (2018). Nanoengineered high-performance hexaferrite magnets by morphology-induced alignment of tailored nanoplatelets. ACS Applied Nano Materials, 1(12), 6938–6949. doi:10.1021/acsanm.8b01748