Effects Of Acidizing Process And Powder Characteristics On The Magnetic Properties Of Nanocrystalline Soft Magnetic Composites And Its Mechanism

Fe-based nanocrystalline alloys exhibit high potential to work as the soft magnetic composites with high permeability and low core loss,since they have high permeability,high resistivity and low coercivity.However,it is difficult to obtain excellent comprehensive performances for nanocrystalline soft magnetic composites（NSMCs）,due to the poor insulation and molding ability of FeSiBNbCu nanocrystalline powders.Therefore,their development and appilication have been limited.To solve these problems,the nitric acid oxidation and the addition of phosphatized carbonyl iron powders（CIP）have been presented in this work.In addition,the effects of demagnetizing field on the magnetic properties of NSMCs and its influencing factors were also discussed.These studies have a great guidance to develop the high permeability advantages of FeSiBNbCu nanocrystalline alloy,which is also beneficial to the performance optimization of NSMC.The main results are as follows:After 30 wt.%nitric acid oxidation,the sharp edges of the FeSiBNbCu nanocrystalline powders disappeared.Then a complete and uniform distributed Fe₃O₄ layer formed on the powder surface,and the core-shell structured FeSiBNbCu@Fe₃O₄ powders were obtained.The formed Fe₃O₄ layer shows good heat resistance,which can resist the high annealing temperature up to 600^oC.The excellent magnetic properties with effective permeability of 80and core loss of 269 k W/m³（measured at 50 m T and 100 k Hz）have been achieved in the NSMC prepared with 30 wt.%nitric acid oxidized powders and annealed at 600^oC.Compared to the NSMC prepared with phosphorous acid treated powders with same effective permeability,the core loss of the NSMC prepared with nitric acid oxidized powders has been greatly improved.After adding phosphatized CIP,the enhenced saturation magnetic flux density and effective permeability have been obtained,and the improved core loss and high-frequency performance also have been achieved.With increasing of phosphatized CIP content,the number of pores decreases gradually,which would help to decrease the demagnetizing field of NSMCs,resulting in the increase of effective permeability.In addition,the electrically isolated microstructure formed by Fe₃O₄ layers and Fe PO₄ layers was prepared,which is beneficial to restrict inter-particle eddy current.The optimized magnetic properties with B_s of1.30 T,μ_e of 83 and P_s of 330 k W/m³（measured at 50 m T and 100 k Hz）have been achieved in the NSMC with the addition of 15 wt.%phosphatized CIP.A 38%decrease of core loss was obtained due to its lower inter-particle eddy loss,compared with the NSMC with the addition of unphosphatized CIP.The effects of powder size and binder content on the magnetic properties of NSMCs have were investigated.It is found that the demagnetizing field has siginificant effects on the the effective permeability,leading to a significant difference of effective permeability between the NSMCs and nanocrystalline ribbon.Moreover,with the decreasing of powder size and the increasing of binder content,the number and the thickness of gap increase gradually,resulting in a larger demagnetizing field,which contributing to the reduction of effective permeability and increase of hysteresis loss.It is one of the possible methods to prepare the NSMC with high permeability and low hysteresis loss by increasing the size of FeSiBNbCu nanocrystalline powder.