Soft ferrite magnetic nanoparticles: a new strategy to improve the performance of high frequency electronic devices

The experimental work presented in this PhD thesis is focused on the synthesis of doped spinel ferrite-based, nanostructured materials with soft magnetic behaviour and on the translation of the nanostructure properties upon processing of the powders to high density final products. Micrometric soft magnetic materials found extensive use in the industry for the fabrication of electronic devices, such as inductors and transformers. These types of devices are composed by soft magnetic cores inserted in electric circuits. The effective use of these devices is strongly related to the efficiency in the process of conversion from electric to magnetic energy, reducing the dissipation of energy as heat and, in this way, the power losses associated to the conversion process itself. The present work moves within this contest, as it explores the design and development of nanostructured materials for soft electronic device applications, with the aim to reduce power losses at increased frequency respect to those currently used. This challenge has been tackled operating both on the intrinsic soft magnetic properties of the materials, improved thanks to the size reduction at the nanoscale, and on the propagation of eddy-currents, proportional to the square radius of the grains. Initial efforts were focused on the optimization of a synthetic strategy able to produce highly crystalline nanopowders, with controlled sizes and compositions, through a cheap, eco-friendly and facile methodology, suitable for an easier hypothetical industrial scale-up. Thus, co-precipitation method was chosen and, starting from aqueous solutions of transition metal chlorides, through a precipitation in basic media, highly crystalline nanoparticles with tuneable composition were produced. Chemical, structural, morphological and physical characterizations were carried out, confirming the obtaining of soft magnetic material. The determination of relative permeability and core losses on nanopowders with different magnetic behaviours required the development of a specific methodology and of the relative instrumentation. To this aim, the nanopowders were pressed into macroscopic toroidal shapes and characterized. The obtained results, i.e. low relative permeability and high core losses, showed how these magnetic properties are strictly related to the low density reached during the preparation of samples, compared to the bulk one. Based on these results, considerable efforts were focused on increasing the relative density of the final products. Since for relative permeability and core losses measurements the sample must have a toroidal shape, an improved die for the compaction of nanopowders in the desired shape was designed and fabricated. Once obtained the green toroids, i.e. not exposed to sintering, in order to enhance the final density, two strategies were explored: a classical heat treatment in a tubular furnace and a sintering technique driven by Joule effect (High Pressure Field Assisted Sintering Technique, HP-FAST) The first method, starting from pressed green toroid, allowed the attainment of micrometric structured toroids with a quasi-bulk density; the second one yielded nanostructured toroids with similar quasi-bulk density. In the classical sintering process, large efforts were devoted to the optimization of the sintering parameters. The investigation of the magnetic properties of classically sintered samples allowed to confirm the relationship between final density, relative permeability and core losses. Nevertheless, the final properties of the samples were not satisfactory, when compared to commercial products, suggesting that the simple approach of sintering, using as starting material highly crystalline nanopowders, is not a competitive route for the production of cores. Notwithstanding these results were not fully positive, a significant improvement of the properties of materials can still be envisaged, exploring several options, which in principle can lead to an enhancement of the characteristic of the materials (ferrite with different composition or mixing with additive). As mentioned above, as second method to prepare the nanostructured toroids, HP-FAST was investigated. Green toroids were sintered by exploiting the heat produced by Joule effect, applying a high intensity DC current to a non-conductive die containing the powders and simultaneously applying an uniaxial pressure. Variable sintering temperature and applied pressure were explored, to obtain nanostructured products. The as-obtained toroids presented lower relative permeability compared to classical sintered ones and, thus, higher core losses. However, comparing the nanostructured products with classically sintered ones with the same relative permeability, a decrease of the core losses with increasing frequency were observed, suggesting a reduction of the extra loss contribution arising from classical eddy currents and anomalous losses. Unfortunately, difficulties in obtaining toroidal geometry limited the investigation of HP-FAST to few attempts. However, the obtained results suggest that this is a very promising technique for the preparation of soft magnetic, highly compacted nanostructured materials but operative conditions need to be optimized. The choice of a toroidal shape is strictly related to the measuring technique, but, for many applications, this is not a mandatory requirement, as evidenced by the large variety of shape of commercial inductors and transformers. Considering that magnetic properties of some of the materials prepared in this work were interesting, we decided to test them as a constituent of an inductor prototype and compared to commercial products. Industrial assembling of inductors requires a the application of a pressure which is maintained in the final product. Thus, core losses were measured by applying a similar pressure. The obtained results showed that, on increasing the applied pressure, the products prepared in this Thesis present a lower variation of inductance and core losses and, most importantly, their power losses at high pressure are lower than those of commercial products. Indeed, the application of one compound prepared in this Thesis in an electronic device, will be patented soon, while the scale up of its synthesis to the industrial production is currently in progress, and hopefully the product will enter the market on a short time scale.