Microstructure evolution and soft magnetic properties of Fe-based nanocrystalline soft magnetic composites coated with lubricant

Fig. 1. (a-c) Bubble-like spheres on the surface of the NSMCs and (d-i)cross-sectional TEM images of the FeSiBCuNb NSMC powders with corresponding EDS mapping exhibiting the distribution. [Display omitted] •A continuous, multi-layer, uniform and ultra-thin insulating coating were successfully fabricated.•Excessive addition of zinc stearate decomposes to CO2 with some small bubble-like spheres on the surface of NSMCs.•After annealing, the NSMCs with 2.0 wt% zinc stearate exhibit high μe and low Pcv. Here, the influence of zinc stearate on the microstructure and soft magnetic properties of the Finemet nanocrystalline soft magnetic composites (NSMCs) was systematically investigated. The results demonstrate that the core loss (Pcv) decreases and the effective permeability (μe) increases of the NSMCs with increasing zinc stearate from 0 wt% to 2.0 wt%, which can be attributed to the reduction of residual stress during compaction. Further increasing zinc stearate up to 3.0 wt%, the Pcv increases sharply, while the μe shows a decreasing trend. It can be considered that the addition of excess lubricant decomposes to CO2 during annealing, which deteriorates the magnetic properties. After annealing at 560 °C, a thin hybrid layer of only about 53.6 nm containing iron phosphate, ZnO, and SiO2 is formed in the NSMCs with 2.0 wt% zinc stearate, which exhibits excellent soft magnetic properties such as low Pcv of 174 kW/m3 and high μe of 66.7 at Bm = 0.1 T for 50 kHz. In addition, loss separation has been carried out and suggests that the addition of lubricant mainly affects hysteresis loss in the total energy loss of NSMCs.