Browsing by Author "Granados-Miralles, C"

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    Elucidating the relationship between nanoparticle morphology, nuclear/magnetic texture and magnetic performance of sintered SrFe12O19 magnets
    (Royal Society of Chemistry, 2020-04-22) Saura-Múzquiz, M; Eikeland, AZ; Stingaciu, M; Andersen, HL; Granados-Miralles, C; Avdeev, M; Luzin, V; Christensen, M
    Several M-type SrFe12O19 nanoparticle samples with different morphologies have been synthesized by different hydrothermal and sol–gel synthesis methods. Combined Rietveld refinements of neutron and X-ray powder diffraction data with a constrained structural model reveal a clear correlation between crystallite size and long-range magnetic order, which influences the macroscopic magnetic properties of the sample. The tailor-made powder samples were compacted into dense bulk magnets (>90% of the theoretical density) by spark plasma sintering (SPS). Powder diffraction as well as X-ray and neutron pole figure measurements and analyses have been carried out on the compacted specimens in order to characterize the nuclear (structural) and magnetic alignment of the crystallites within the dense magnets. The obtained results, combined with macroscopic magnetic measurements, reveal a direct influence of the nanoparticle morphology on the self-induced texture, crystallite growth during compaction and macroscopic magnetic performance. An increasing diameter-to-thickness aspect ratio of the platelet-like nanoparticles leads to increasing degree of crystallite alignment achieved by SPS. Consequently, magnetically aligned, highly dense magnets with excellent magnetic performance (30(3) kJ m−3) are obtained solely by nanostructuring means, without application of an external magnetic field before or during compaction. The demonstrated control over nanoparticle morphology and, in turn, crystal and magnetic texture is a key step on the way to designing nanostructured hexaferrite magnets with optimized performance. © Royal Society of Chemistry 2020
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    Nanoengineered high-performance hexaferrite magnets by morphology-induced alignment of tailored nanoplatelets
    (American Chemical Society, 2018-11-15) Saura-Múzquiz, M; Granados-Miralles, C; Andersen, HL; Stingaciu, M; Avdeev, M; Christensen, M
    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
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    Neutron diffraction studies of nanostructured SrFe12O19 magnets
    (International Conference on Neutron Scattering, 2017-07-12) Saura-Múzquiz, M; Stingaciu, M; Eikeland, AZ; Andersen, HL; Granados-Miralles, C; Lucin, V; Avdeev, M; Christensen, M
    Phase pure, highly crystalline SrFe12 19 nanoparticles have been synthesized by hydrothermal and sol-gel synthesis methods. By varying synthesis parameters and method, SrFe12 19 nanoplatelets of various sizes and morphologies can be obtained. The nuclear and magnetic structure of the samples have been studied by neutron and X-ray diffraction, revealing a clear size dependency on the long range magnetic order. Subsequent compaction of the tailor-made powder samples into bulk magnets is carried out by Spark Plasma Sintering. Powder diffraction as well as X-ray and neutron pole figure analyses were performed on the compacted magnets. The obtained results, together with macroscopic magnetic measurements, reveal a direct influence between nanoparticle morphology, texture and magnetic performance. The platelet-like morphology of the nanoparticles leads to highly aligned magnets without the need of an externally applied magnetic field. Therefore, by varying the morphology of the platelets prior to compaction, the final magnetic properties of the sample can be tuned. Meticulous characterization based on neutron and X-ray diffraction techniques reveals the relationship between synthesis conditions, crystal-, nano- and magnetic structure, and macroscopic magnetic performance. Extensive control over each step of the nanostructuring process is essential in the design of materials with tailored physical properties.