Enhanced soft magnetic properties with high frequency stability of pure iron powder cores via high-pressure compaction – An environment and cost saving solution as a prospective alternative to soft magnetic composites

The paper presents the analysis of the magnetic behaviour of soft magnetic powder compacts vs. the increasing compacting pressure. An unexpectedly positive result was obtained at a pressure of 1500 MPa, as the pure iron compact without coating of powder particles and without subsequent heat treatment showed very good magnetic properties compared to the class of soft magnetic composites (SMCs). In particular, the effective relative permeability of ${\mu }_{eff}$ ∼120, stable up to a frequency $f$ ∼200 kHz, the maximum total relative permeability of ${\mu }_{tot}^{\max }$ ∼700, and the specific electrical resistivity of ${\rho }_R$ ∼10⁻⁵ Ω m. This phenomenon was explained on the basis of analyses of the samples microstructure, the magnetic and electrical properties, magnetization processes, inner demagnetizing fields, Barkhausen noise and thermal diffusivity. It was found that the grain size refinement inside iron particles occurs at certain elevated compaction pressure because the deformation bands gradually rise and break up with compaction pressure, leading to a higher resistivity of the compact thus to its SMC-like behaviour, despite the counteracting effect of increasing number of iron-iron bridges among neighbouring particles. The grain size refinement causes also the refinement of magnetic domain structure, which facilitates the magnetization reversal, although, the increased internal stresses and microstructural defects affect domain wall mobility negatively. The most favourable combination of the mentioned factors influences, finally resulting in the soft magnetic properties enhancement, appeared at 1500 MPa. Due to high-pressure compaction, the high density (above ∼95 % of iron density) of a compact was achieved, ensuring sufficient mechanical properties. The presented material can serve as a potential supplanter of SMCs in many applications as it provides evident advantages, such as its easy production with minimum chemical waste (because any additional chemical processes and substances needed for particle coatings in conventional SMCs are completely omitted), as well as easy recycling process, which makes it eco-friendly and cost-effective, nevertheless, maintaining the advantages of SMCs.