High permeability and low core loss nanocrystalline soft magnetic composites based on FeSiBNbCu@Fe3O4 powders prepared by HNO3 oxidation

Abstract To develop nanocrystalline soft magnetic composites (NSMCs) with good combination of soft magnetic properties, the core-shell structured FeSiBNbCu@Fe3O4 powders were prepared by nitric acid (HNO3) oxidation of the nanocrystalline FeSiBNbCu alloy. The NSMCs were then fabricated by binding and pressing with resin followed by annealing treatment. The formation mechanism of the Fe3O4 layer on the FeSiBNbCu powders was discussed and the effects of HNO3 concentration and annealing temperature on the magnetic properties of the NSMCs were investigated. It was found that the sharp edges of the initial powders disappeared after the powder was oxidized with 30 wt% HNO3. A complete and uniformly distributed Fe3O4 layer formed on the powder surface. For the NSMCs annealed at 500 °C, the formation of Fe3O4 layer by HNO3 oxidation treatment significantly reduced the core loss but also decreased the permeability. After annealing at 550 °C and 600 °C, the permeability of the NSMCs can be greatly improved. The formed Fe3O4 layer can resist the high annealing temperature up to 600 °C, which is beneficial to effectively releasing the residual stress. The core loss shows a significant increase as the annealing temperature over 600 °C, which is mainly attributed to the decomposition of resin binders. The excellent magnetic properties with permeability μe = 80 and core loss Ps = 269 kW/m3 at 50 mT and 100 kHz have been achieved in the present work.