Substantial thinning of melt-spun ribbons by an optimised and high-yield ball-milling process

Abstract

Melt-spun Fe-based ribbons are widely used as the core of transformers and inductors due to their high flux density and low coercivity. However due to their high thickness (∼19 µm) these ribbons are prone to large eddy current losses at MHz frequencies. Despite low yield, ball milling has been widely used to break such ribbons down to thinner flakes to suppress the eddy current losses at high frequency. In this work, we demonstrated an optimized ball milling process with increased yield for flakes in the desired size range (2–4 µm). We have demonstrated that reducing pre milling annealing temperature from 450 to 350 °C increases the yield in desired size range from 2% to 5% and further increasing batch size from 10 to 20 g increases the yield to 21%. The coercivity of the milled flakes increases from 139 to 1352 A/m due to the ball milling process. A post-mill annealing at 350 °C in Ar atmosphere decreases the coercivity to 341 A/m. X-ray diffraction analysis showed no sign of crystallization during ball milling. The result presented here demonstrates an efficient approach to fabricate ultra-thin flakes out of soft magnetic ribbons for high-frequency applications.