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

Core-shell structured amorphous FeSiBCr@phosphate/silica powders were prepared by phosphating and sodium silicate treatment. The soft magnetic composites (SMCs) were fabricated based on these powders. The effects of phosphoric acid (H3PO4) concentration and annealing temperature on their properties were investigated. During the phosphating process, the powder coated with a low concentration of H3PO4-ethanol solution leads to uneven phosphate coating, while the peeling of phosphate coating occurs for the high H3PO4 concentration. Using 0.5 wt.% phosphoric solution, a uniform and dense insulation layer can be formed on the surface of the powder, resulting in increased resistivity and the reduced eddy current loss of the amorphous soft magnetic composites (ASMCs). This insulation layer can increase the roughness of the powder surface, which is beneficial to the subsequent coating of sodium silicate. By optimizing sodium silicate treatment, a complete and uniform SiO2 layer can be formed on the phosphated powders well, leading to double layer core-shell structure and excellent soft magnetic properties. The magnetic properties of amorphous SMCs 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 773 K, but the deterioration of magnetic properties occurs as the annealing temperature over 798 K, mainly due to the increase of α-Fe(Si) and Fe3B phases, which hinder the domain wall displacement and magnetic moment rotation. The excellent soft magnetic properties with permeability μe = 35 and core loss Ps = 368 kW/m3 at 50 mT/200 kHz have been obtained when the SMCs prepared with the powders coated by 0.5 wt.% H3PO4 and 2 wt.% sodium silicate were annealed at 773 K.