A two-step stress-annealing with stress-relaxation process for developing low-loss Fe-based nanocrystalline soft magnetic cores

Abstract

Fe72.5Nb2Mo2Cu1Si15.5B7 at.% nanocrystalline soft magnetic ribbon was stress-annealed at several temperatures around its primary crystallization temperature and magnetic properties were characterized in strips to determine conditions that led to minimal magnetic coercivity, Hc. This optimal stress-annealing (SA) temperature, though yielding minimal Hc in strips, also produced non-linear or rounded BH loops in wound cores. The rounded loops could be changed to linear or flat B-H loops following a secondary static annealing process. Through X-ray diffraction, transmission electron microscopy, and indirect magnetostriction measurements, we determined that the change in loop shape was driven by additional crystallization and stress-relief from the static annealing process. A change in mechanical properties including fracture toughness, loss modulus, and Young’s modulus after SA, were shown to limit the processability of the ribbon. Tape-wound inductors subjected to the optimal 640 °C / 85 MPa / 4 s SA process followed by a static annealing treatment of 500 °C for 1 h resulted in flat loops, an Hc of 0.3 A/m, low magnetostriction of ∼ 2 ppm, and core losses of only 7 W/kg at an induction of 0.4 T when excited at 20 kHz.