Preparation,Structure And Magnetic Properties Of Fe-based Nanocrystalline Alloy With High-Frequency Permeability

Fe-Si-B-Cu-Nb nanocrystalline soft magnetic alloys（Finemet）have been extensively applied as core materials in transformers,sensors and current mutual-inductor due to their low coercivity（H_c）,high permeability（μ_e）,low core loss,and low magnetostriction.With the development of modern power electronic devices towards high frequency,the core materials possessing enhanced high-frequency magnetic properties are highly required.In this work,to meet the requirement of the power electronic devices,the composition and annealing process of the Finemet alloys were optimized in order to develop the nanocrystalline alloys with excellent high-frequencyμ_e and the corresponding annealing process.The effects of changing Si and Nb contents on the structure,thermal properties,crystallized structure,magnetic properties,and resistivity（ρ）of a melt-spun Fe_73.5Si_13.5B₉Nb₃Cu₁ alloy were systematically investigated,and the alloy compositions were optimized.Then,the influences of C addition on the structure and properties of the nanocrystalline alloys were studied.In addition,the effects of the annealing process parameters including the cooling method,isothermal duration,and multi-step annealing on the structure and magnetic properties of nanocrystalline alloys were investigated.The mechanisms of the influence of the alloy composition and annealing process on the structure and magnetic properties were discussed as well.The main research results are as follows:1.The melt-spun Fe_73.5Si_xB_22.5-xCu₁Nb₃（x=13.5-16.5）alloys all can form a fully amorphous structure.After annealing,the nano-orderα-Fe（Si）phase is precipitated in the amorphous matrix.An appropriate increase in the Si content can refine the grains and improve the high-frequencyμ_e of the nanocrystalline alloys.The alloy with x=14.5 exhibits a low average grain size（D）of 11.9 nm and highμ_e of 30800 and 19200 at 10 and 100 k Hz,respectively,which are superior to those of 13.3 nm,28700 and 18700,respectively,for the alloy with x=13.5.2.Increasing the Nb content in the Fe_73.5Si_14.5B₈Nb₃Cu₁ alloy improves the thermal stability of the melt-spun amorphous alloy,and refines the nanostructure and enhances the high-frequencyμ_e of the annealed alloys.The D,H_c andμ_e at 100 k Hz of the nanocrystalline alloy with 3.5 at.%Nb are 10.7 nm,0.7 A/m and 22400,respectively.3.Adding an appropriate amount of C into the Fe_73.5Si_14.5B₈Nb₃Cu₁ nanocrystalline alloy improves the saturation magnetic flux density（B_s）and high-frequencyμ_e.After addition of0.5 at.%C,the B_s of the alloy rises from 1.25 T to 1.26 T,and theμ_e at 10 and 100 k Hz increase to 32500 and 24600,respectively.The enhancement of the high-frequencyμ_e is mainly due to the increase of theρcaused by C alloying.4.Compared with the air-and furnace-cooling methods,the water-cooling makes the Fe_73.5Si_14.5B₈Cu₁Nb₃ nanocrystalline alloy possess a finer nanostructure and higher high-frequencyμ_e.Extending the isothermal duration can also improve the high-frequencyμ_eof the nanocrystalline alloy.In addition,the multi-step annealing with a high-temperature pre-annealing and low-temperature annealing further increases theμ_e at 100 k Hz to 20300.