Effects Of Tuning Magnetic Anisotropy On The Soft Magnetic Properties Of FeSiBNbCu(Al,P) Alloy

With the rapid development of the consumer electronics industry,wireless charging is increasingly demanded due to its convenient and safe features.Magnetic shielding sheet is one of the key components in wireless charging equipment,which can prevent magnetic leakage,reduce electromagnetic eddy current,and significantly improve charging efficiency.Fe-based nanocrystalline alloys have become the first choice for wireless charging devices due to their high saturation magnetic flux density（B_s）,high permeability（μ）,low coercivity（H_c）and extremely low core loss at 100-200 kHz.Due to its low B_s,the FeSiBNbCu alloy（named as Finemet alloy）commercially available on the market currently cannot meet the requirements of high power and thinness.Therefore,there is an urgent need to develop nanocrystalline alloys with higher B_s and excellent soft magnetic properties.Based on the previous researcher’s work,this thesis manipulates magnetic anisotropy by designing alloy composition and optimizing annealing process,which provides a method to develop nanocrystalline alloys with higher B_s and excellent soft magnetic properties.The main contents are as follows:（1）Systematically studied the effects of in Si/B ratio and annealing temperature on the magnetic properties and the microstructure of Fe₇₇Si_10+xB_9-xNb₂Cu₁Al₁（x=0,1,2 at%）alloys.Successfully developed an Fe₇₇Si₁₂B₇Nb₂Cu₁Al₁ alloy with excellent soft magnetic properties and high thermal stability,i.e.,B_s=1.49 T,H_c=1.9 A/m,μ=19700（1 kHz）,andΔT=197℃.It was found that the substitution of 2%Si for B significantly increases the activation energy ofα-Fe（Si）grain growth and Fe-boride compounds precipitation,resulting the formation of uniform,fine,and stableα-Fe（Si）grains.This structure is beneficial to reduce the anisotropy of the alloy,and form wide and smooth-moving magnetic domains.Thus,the soft magnetic properties could be improved significantly by increasing proper Si/B ratio in this alloy system.（2）On the basis of the above research work,we successfully developed an Fe_77.8Si_11.6B₇Nb₂Cu_0.6P₁ nanocrystalline alloy by microalloying P elements and changing other elements.In this alloy,uniform,tiny and stable nanocrystallineα-Fe（Si）grains are easy to form.Under the optimized annealing condition,the soft magnetic properties with B_s=1.46 T,H_c=0.8 A/m,μ=21000（1 kHz）andμ=13700（100 kHz）meets the requirement for nanocrystalline soft magnetic materials in future wireless charging equipment application.（3）The Fe_77.8Si_11.6B₇Nb₂Cu_0.6P₁ amorphous ribbons with a width of 60 mm were prepared by industrial raw materials.The melt-spun ribbons were wound into toroidal cores beneficial to normal annealing（NA）and transverse magnetic annealing（TFA）.Comparing the NA cores,the soft magnetic properties of the TFA cores were improved significantly,achieving B_s=1.5 T,μ=18400（100 kHz,0.06 A/m）,H_c=1.7 A/m,and P_s=209 k W/m³（100 kHz,0.2 T）.（4）The effects of tuning magnetic anisotropy on high-frequency magnetic properties under TFA condition has been studied.Optimizing the<K₁>and K_u to small and ratio of～1 of TFA cores under optimum annealing condition is beneficial for obtaining excellent high-frequency soft magnetic properties.Comparing the NA cores,the dynamic magnetic process of TFA cores is dominated primarily by domain rotation with fewer wall displacements.With the further increase of the TFA temperature,the texture of theα-Fe（Si）grains appearing,leading to an increase in<K₁>.Thus,<K₁>and K_u with larger values and mismatch deteriorates soft magnetic properties.