Role of Si in high B_s and low core-loss Fe_85.2B _10-XP₄Cu_0.8Si_X nano-crystalline alloys

The effects of Si element on structural and magnetic properties of Fe-rich Fe_85.2B_10-XP₄Cu_0.8Si_X (x 0-2.5 at. ) alloys were investigated. Our results show that addition of Si significantly reduces the activation energy for nucleation of α-Fe and increases the activation energy for grain growth. As a result, it is much easier to obtain a finer and uniform nanogranular structure (grain size ∼18 nm) made from densely packed α-Fe grains after annealing in the case of Si-containing alloys (Fe-B-P-Si-Cu) in comparison to Si-free alloys (Fe-B-P-Cu). However, addition of Si on the expense of B reduces the amorphous forming ability of the alloy, which results in lower reproducibility. The reproducibility improves significantly in Si-free alloy, but the structure of the alloy is relatively unstable on annealing, which means more strict annealing treatment is required. After optimum annealing treatment, Si-free alloys (Hc ∼ 6 A/m, Js ∼ 1.83 T) show superior soft magnetic properties than the Si-containing alloys (Hc ∼ 10 A/m, Js ∼ 1.78 T). Results show that the excellent soft magnetic cores can be obtained only if the extra heat generated on nano-crystallization of as-quenched amorphous phase can be released efficiently. The toroidal core of Si-free alloy (core-loss, W ∼ 0.58 W/kg at ∼1.7 T, 50 Hz) exhibits lower magnetic core loss than the Si containing alloys (core-loss, W ∼ 1.51 W/kg at ∼1.7 T, 50 Hz).