Great reduction in pressure by particle grading for Fe-Si-Al SMCs with good low-frequency magnetic properties

Abstract

Gas-atomized Fe-Si-Al powder cores with optimized particle grading are successfully prepared at various pressures much lower than those of conventional ungraded powder cores (UGCs). The influences of the particle grading and the pressure on the frequency dependence of μ’, μ’’ and tanδ and DC-bias performance are studied in detail. The optimized discontinuous particle size distribution based on the powder’s tape density is found to improve the core’s density effectively and thus reduce the demagnetizing field. This helps to realize high μ' with good frequency stability, low μ'' and small tanδ in low-frequency range below 10 kHz, close to those of conventional UGCs, without adopting high-pressure pressing. Besides, good anti-saturation ability, similar to that of conventional UGCs, is also achieved in the graded cores (GCs) because no annealing treatment retains internal stress introduced by the pressing process. This suggests that the utilization of particle grading makes it possible to realize good low-frequency magnetic performance in powder cores pressed at greatly reduced pressures. For such a particle grading, the downside that needs further improvement is the great decrease of the core’s volume resistivity, which leads to a high eddy current within the GCs and thus deteriorates the frequency stability of the permeability and the core loss in high-frequency range above 20 kHz.