Preparation of soft magnetic composite from Fe-6.9wt%Si by different heat treatment strategies.

Present study investigated the effect of isothermal heat treatment strategies between 800 degrees C and 1150 degrees C on the magnetic properties of toroidal samples made from Fe-6.9wt%Si powder. The samples were prepared by classical powder metallurgy method since the classical sheet forming methods no longer work with the high silicon content. Our results presented here are part of a series of comparative experiments where we study the effectiveness of the insulating layers created during and before the compacting of soft magnetic composites (SMCs). Our goal was to create a soft magnetic composite made of ferromagnetic and inorganic insulating material with a frequency limit already in the megahertz range and a Snoek limit of few gigahertz. In the case of samples made from Fe-6.9wt%Si powder, the computed tomography results showed that significant porosity is to be expected after pressing. Its positive effect occurred during the heat treatment in the atmospheric agent, where silicon is precipitated and deposited on the surface of the particle. This coating is an electrically insulating layer at the grain boundaries. Depending on the heat treatment strategy, 1 or 2 ferromagnetic phases were observed. The frequency limit approached the target values, but due to the low value of static permeability, the Snoek limit did not reach the gigahertz range. However, there is a significant improvement in magnetic properties compared to the heat-treated samples in a protective gas.