Synthesis and characterization of L10-FeNi for novel permanent magnet materials
- Autores: Alonso José Campos Hernández
- Directores de la Tesis: Alberto Bollero Real (dir. tes.), Ester María Palmero Rodríguez (dir. tes.)
- Lectura: En la Universidad Autónoma de Madrid ( España ) en 2024
- Idioma: inglés
- Número de páginas: 159
- Títulos paralelos:
- Síntesis y caracterización de L10-FeNi para nuevos materiales de imán permanente
- Tribunal Calificador de la Tesis: Daniel Salazar Jaramillo (presid.), Nuria Gordillo García (secret.), Gorka Salas Hernández (voc.), Peter Svec (voc.), Siân Elizabeth Dutton (voc.)
- Programa de doctorado: Programa de Doctorado en Física de la Materia Condensada, Nanociencia y Biofísica por la Universidad Autónoma de Madrid; la Universidad de Murcia y la Universidad de Oviedo
- Materias:
- Enlaces
- Tesis en acceso abierto en: Biblos-e Archivo
- Resumen
Permanent magnet materials are at the core of our technological development and are key drivers of our electrification to make possible the achievement of a greener future. They are used in multiple applications, such as electric motors, generators and actuators, and in different technological sectors such as transport, energy or electronics.
Since the emergence of modern materials science in the second half of the 20th Century, researchers have discovered and developed multiple new permanent magnet materials, to achieve an improved performance. Since the discovery of Sm-based magnets in the 60s and Nd-based magnets in the 80s, rare-earth magnets have dominated the permanent magnet sector, with sintered NdFeB magnets being unmatched in their room-temperature performance. However, in recent years the complex geopolitical situation and the expected rise in demand for permanent magnets due to the necessities related to the European Green Transition have raised fears on the stability of the rare-earths supply. Therefore, researchers are devoting strong efforts to finding different alternatives that will relieve the situation.
One such approach is the development of new rare earth-free permanent magnet materials that can fill the performance gap between current commercial rare earth-free magnets (Alnico, Sr-ferrite) and rare earth-based magnets (namely, SmCo and NdFeB), to mitigate the demand for rare-earth magnets in specific applications. This thesis focuses on the artificial synthesis and establishment of characterization protocols of one of the most promising long-term rare earth-free permanent magnet alternatives: the ordered L10-FeNi phase. This phase can be found naturally only in some meteorites. Experimental measurements on meteorite samples and theoretical predictions have shown the potential of this material to compete with the strongest permanent magnets (NdFeB).
In Chapter 1 of this thesis, the basics of permanent magnets materials science are presented, the importance and history of permanent magnets is discussed, and the problems associated with rare earth-based permanent magnets are explained. Then, the ordered L10-FeNi phase is presented as a viable long-term rare earth-free alternative material. The main challenges for the development of this ordered phase as a technologically useful material are twofold: firstly, the extremely low atomic mobility of Fe and Ni means that extremely long timescales (millions of years) are required in nature to produce the phase, and secondly, the almost-equivalent cross section of Fe and Ni under X-rays makes the detection of the presence of the ordered phase very difficult when using traditional laboratory methods (i.e. standard X-ray diffraction).
Hence, the work on this thesis has focused on the study of different synthesis techniques with a potential to enable the formation of L10-FeNi, by making use of different thermodynamic drivers to increase the atomic mobility of Fe and Ni, combined with advanced experimental techniques for the detection of the phase. In this regard, the experimental techniques used for both synthesis and characterization throughout this thesis are presented and discussed in Chapter 2.
Chapter 3 is focused on the use of melt-spun amorphous ribbons (based on FeNiPC) as a platform for the formation of the L10-FeNi phase. During the crystallization of the amorphous ribbons, atomic mobility increases greatly, and this should promote the beginning of formation of L10-FeNi. Thus, temperature-ramp X-ray diffraction (XRD) was used in combination with differential scanning calorimetry (DSC) measurements to study the different crystallization steps of the alloy as the annealing temperature increases. Following these results, annealing of the as-quenched ribbons at different temperatures was carried out, and the evolution of the coercivity and the saturation magnetization with the annealing temperature was analyzed. The possible presence of the L10-FeNi phase in the samples was investigated using both synchrotron XRD and transmission electron microscopy (TEM). In the case of the first method, the L10-FeNi superlattice peaks were found to overlap with peaks from a Fe3C phase (previously detected by standard XRD). As for the TEM measurements, possible L10-FeNi diffraction spots were detected, but they were found to be mathematical artefacts of the fast Fourier transform (FFT) TEM procedure. Therefore, the presence of the L10-FeNi phase in the samples could not be unequivocally proven, which served as the basis for a critical discussion on some findings reported in literature on the synthesis and identification of the L10-FeNi phase. It was found that multiple studies concluded the presence of L10-FeNi in their samples without sufficient scientific rigor (shortcomings in the critical evaluation of the results or application of a deficient experimental protocol).
In Chapter 4, a new synthesis method for L10-FeNi is presented: the simultaneous crystallization and reduction (SCR) method applied to an amorphous nickel ferrite powder synthesized by co-precipitation. With this method the atomic mobility increases and therefore encourages the formation of the L10-FeNi phase. In this manner, high coercivity samples (compared to that of soft magnetic FeNi) were produced, consisting of a combination of Ni ferrite, disordered A1-FeNi and ordered L10-FeNi phases. Synchrotron XRD in combination with superconducting quantum interference device (SQUID) magnetometry was used to determine the presence of L10-FeNi in the samples, and a critical discussion and analysis of these results was carried out to showcase their reliability. Temperature-ramp XRD was then used to demonstrate that SCR does induce crystallization and reduction in a single step, while SQUID magnetometry gave an estimation of the amount of the coercivity that is provided by L10-FeNi. Finally, the samples were soaked in hydrochloric acid solution, which dissolves A1-FeNi and Ni ferrite faster than the L10-FeNi (based on the analysis of geological samples reported in literature). These acid purification experiments were successful and demonstrated a significant improvement in coercivity and remanence ratio in the samples.
Chapter 5 summarizes the conclusions of this thesis. The materials synthesis, processing and characterization methods used and studied in this work open multiple promising paths for the future development and identification of L10-FeNi as a rare earth-free permanent magnet material. In particular, a critical analysis of the results obtained on amorphous FeNiPC ribbons after crystallization was carried out to avoid a mistaken identification of the ordered phase, a simultaneous crystallization and reduction process was presented as a novel approach used for the synthesis of L10-FeNi starting from a co-precipitated Ni-ferrite precursor, and an acid treatment was validated as a useful technique to purify the powders synthesized through the co-precipitation and crystallization-reduction process.
An Appendix has been added at the end of the manuscript, which is not directly focused on the formation of the L10-FeNi phase, but on the synthesis of a system (FeNi nanowires) that could be used in the future to study the first stages of the formation of this phase. The FeNi nanowires were grown by electrodeposition as a model system to analyse the anomalous co-deposition effect of the alloy and the influence of the composition on the magnetic properties of the nanowires.
While much work remains to transform L10-FeNi into a technologically useful permanent magnet material, the work here presented contributes to setting the basis for future advances in this field, and the general advancement of rare earth-free permanent magnets.
