Preparation and performance of ultra-low core loss FeSiAl soft magnetic composites

Currently, electronic components applied in household electric appliances, cars and mobile phones are increasingly presenting a trend of miniaturization and intelligence. Thus, it is urgent to reduce the core loss of soft magnetic composites (SMCs) applied at high frequency and high power. Organic HPO solution is widely used in industry to coat magnetic powder for reducing the core loss of SMCs. However, residual HPO poses a risk of deteriorating the magnetic properties of SMCs. In addition, the massive use of volatile organic solvents can cause serious pollution.Therefore, in this work, aqueous solution of HPO was used to coat magnetic powder to grow a phosphate insulation layer, followed by heat treating to remove the residual HPO for further reducing the core loss of the obtained SMCs .

A proper amount of atomized Fe-9.6 wt%Si-5.4 wt%Al (FeSiAl) powder was weighed, and a certain amount of HPO (whose dosage was designed to be 0.1 wt%,0.2 wt%,0.3 wt%, 0.4 wt%,and 0.5 wt% of FeSiAl powder) was weighed and dissolved in 10 mL water(10 mL), and then FeSiAl powder was mixed with the prepared HPO solution, followed by stirring and heating the mixture until dry to get five types of phosphated magnetic powder samples. The phosphated FeSiAl powder was heated at 680℃ in N atmosphere for 1 h, then it was mixed with a binder of silicone (0.5 wt%) and a mold release agent of zinc stearate (0.3 wt%). The treated FeSiAl powder was molded under a pressure of 2090 MPa, and shaped into a ring-shaped core with an outer diameter of 26.92 mm, an inner diameter of 14.73 mm, and a height of 11.10 mm. Finally, the ring-shaped core was heated at 680 ℃ for 1 h in N atmosphere to get ultra-low core loss FeSiAl SMCs. Also the SMCs without 680 ℃ heat-treatment was preprared as the control group. According to the content of HPO, the samples without heat-treatment were denoted as 0.1~0.5wt%HP/FeSiAl and the samples with heat-treatment were denoted as 0.1~0.5wt%HPT/FeSiAl, respectively.

① Insulation layer can be formed on the surface of FeSiAl powder in aqueous HPO solution. and the insulation layer is mainly composed of phosphate and transformed into the mixture mainly composed of AlO, FeSiO and PO based on the XRD,XPS and FTIR results. ② Compared with the 0.5wt%HP/FeSiAl core, the 0.5wt%HPT/FeSiAl core shows lower density, due to the heat-treated FeSiAl particles being less sliding and movable to fill the gap between the particles. Relative to the 0.5wt%HP/FeSiAl core, the 0.5wt%HPT/FeSiAl core shows a decrease in permeability from 74.52 to 52.10 (100 kHz) due to the lower content of magnetic component, and a decrease in core loss from 79.44mW·cm to 58.56mW·cm(50 mT, 100 kHz)。③The reduction extent of hysteresis loss for the HPT/FeSiAl is close to its core loss, and its eddy current loss is slightly reduced, based on the fitting the core loss results of HPT/FeSiAl and HP/FeSiAl.④The XRD pattern was fitted using Gaussian function and the average grain size of the magnetic particles was obtained based on Debye-Scherrer formula. Calculation results showed that the grain size of the magnetic particles increased after heating. It was reported that the grains grew significantly and the grain boundaries decreased in the magnetic particles after high-temperature heat treatment, thereby weakening the pinning effect on the magnetic domain wall and reducing hysteresis loss. ⑤ Compared with the 0.5wt%HP/FeSiAl, the HPT/FeSiAl showed a decrease of resistivity from 25.23×10 Ω·cm to 2.48×10 Ω·cm, which was due to the reduction of its grain boundaries and the formation of elemental Fe during heating process.. The reduction of high-resistivity grain boundaries in the SMCs after heat treatment caused an increase tend in classical eddy current losses, but the anomalous eddy current losses decreased. Since the extremely small proportion of classical eddy current losses, a significant proportion of the anomalous eddy current losses reduced the eddy current losses slightly as a result.Conclusions: Ultra-low core loss HPT/FeSiAl SMCs can be obtained by phosphating FeSiAl powder in the aqueous HPO solution and the following heating treatment.. The HPO residues on the surface of the magnetic powder can be removed by heat treatment. And the effective permeability of the HPT/FeSiAl SMCs can be remarkably reduced, which allows it to be suitable for the applications in the circumstances of lower permeability. Core loss is also significantly reduced, which meets the standard of atomized FeSiAl SMCs.