Complex Magnetic Fluids for Advanced Energy Applications

Magnetic nanoparticles hold extraordinary potential for applications in various fields, such as microfabrication, thermoelectricity, biomedicine and catalysis. The iron-cobalt alloy (FeCo) exhibits remarkable properties among the magnetic phases, such as the highest saturation magnetization and low coercivity. In addition, controlled oxidation of the alloy can increase coercivity, widening the application window. However, these promising properties are counterbalanced by well-known challenges, such as chemical stabilization and potential toxicity. Furthermore, the typical synthetic strategies have poor energy efficiency or are not environmentally friendly. A promising approach combines the sol-gel combustion (SCS) method and reactive annealing under hydrogen (H2) atmosphere. SCS produces cobalt-iron or iron oxide (CoFe2O4, Fe2O3) nanoparticles embedded in a protective silica (SiO2) matrix. Then, it is possible to reduce the nanocrystals to iron cobalt (FeCo) or iron (Fe) by reactive annealing at an intermediate temperature (773K) under H2 flux (SCS-H2). In this framework, Section 2.1 initially investigated the impact of the SiO2 textural properties on the reduction process efficiency (FeCo-SiO2), registering a magnetization increase of 70%. Afterward, the effect of the Fe2O3 wt.% on the average size of the Fe nanocrystals was investigated, measuring an average diameter of 23 nm (30 wt.%) and 15 nm (15 wt.%). A coercivity of 40 mT (FeCo) and 50 - 20 mT (Fe) showed that the matrix provides good interparticle separation, providing a robust platform for fundamental studies as a further opportunity. However, the physical state of dried powder narrows the application range of such materials in comparison with that of a magnetic nanofluid. Therefore, a thermal decomposition (TD) method under H2 flux (TD-H2) was also investigated for obtaining magnetic nanocrystals dispersed in a fluid and the product was compared to the FeCo-SiO2 phase obtained by SCS-H2 (Section 2.2). The experimental observations highlighted that the pre-nuclei, typically formed in the TD reaction environment by the precursors, require milder conditions to reduce to FeCo nanocrystals compared to the conventional 11 reduction of CoFe2O4 nanocrystals. A gas mixture at 5%mol of H2 leads to a FeCo/(Fe,Co)O/CoFe2O4 nanocomposite with a magnetization of ~80 A·m2·kg-1, normalized by the total mass of powder, and a coercivity of 80 mT. Furthermore, this study highlights both the method’s flexibility in tuning the obtained phase (hence the magnetic properties) as a function of the molar H2%, and the potential of improving the method’s energy-efficiency. The technological application of magnetic fluids relies on different characteristics, such as particle response to the magnetic field and reciprocal interactions. From a microscopic perspective, Section 2.3.2 investigated the Brown relaxation of a FeCo nanoparticle in different magnetic fields (0.25, 0.50, 1.00, 2.00 mT) through different simulations. However, the manipulation of fluids by remote was investigated also from a macroscopic perspective (Section 2.3.3), analyzing the macroscopic flow of a magnetic colloid using different magnetic fields (4, 8, 12 mT). The recirculation induced by the macroscopic flow was applied to increase the dissolution of an alginate bead, providing an example of magnetic fluid’s application in the technology field.