Properties Optimization of Amorphous Soft Magnetic Composites Based on the Powders with Double Layer Inorganic Coating by Phosphating and Sodium Silicate Treatment

Abstract

Based on the core-shell structured FeSiBCr@phosphate/silica powders prepared by phosphating and sodium silicate treatment, the amorphous soft magnetic composites (ASMCs) were fabricated. The effects of phosphoric acid concentration and annealing temperature on their properties were investigated. The results show that the powder coated with a low concentration of phosphoric acid-ethanol solution leads to uneven coating, while the peeling of phosphate coating occurs for the increased phosphoric acid concentration. It is not conducive to reducing the core loss of the ASMCs. However, when the phosphoric acid content is 0.5%, a uniform and dense insulation layer can be formed on the surface of the magnetic powder, resulting in the increased resistivity and the reduced eddy current loss of the ASMCs. Meanwhile, this insulation layer increases the roughness of the surface of the magnetic powder, which favors the subsequent coating of sodium silicate. A complete and uniform SiO2 layer can be formed on the well phosphated powders by optimized sodium silicate treatment. The soft magnetic properties can be further improved by post annealing due to the effectively released residual stress. The enhanced permeability and greatly reduced core loss can be achieved by annealing at 500 ◦C. The deterioration of magnetic propertiesoccurs as the annealing temperature over 525 ◦C, mainly due to the increase of α-Fe(Si) and Fe3B phases, so that magnetic anisotropy and grain boundaries hinder the domain wall displacement and magnetic moment rotation. The excellent magnetic properties with permeability μe = 35 and core loss Ps = 368 kW/m3 at 50 mT/ 200 kHz have been obtained when the ASMCs prepared with the powders coated by 0.5 wt.% H3PO4 and 2 wt.% sodium silicate, then annealed at 500 ◦C .

Keywords: Amorphous soft magnetic composites, Phosphoric acid, Sodium silicate, Annealing temperature, Magnetic properties