Sodium Nitrate Passivation as a Novel Insulation Technology for Soft Magnetic Composites

Sodium nitrate passivation has been developed as a new insulation technology for the production of FeSiAl soft magnetic composites (SMCs). In this work, the evolution of coating layers grown at different pH values is investigated involving analyses on their composition and microstructure. An insulation coating obtained using an acidic NaNO3 solution is found to contain Fe2O3, SiO2, Al2O3, and AlO(OH). The Fe2O3 transforms into Fe3O4 with weakened oxidizability of the NO3– at an elevated pH, whereas an alkaline NaNO3 solution leads to the production of Al2O3, AlO(OH), and SiO2. Such growth is explained from both thermodynamic and kinetic perspectives and is correlated to the soft magnetic properties of the FeSiAl SMCs. Under tuned passivation conditions, optimal performance with an effective permeability of 97.2 and a core loss of 296.4 mW∙cm−3 is achieved at 50 kHz and 100 mT.