Study On The Insulation Coating And Magnetic Properties Of FeSiAl Soft Magnetic Composites

Soft magnetic composites with high electrical resistivity and saturation magnezation have been widely used in electrommunication, power switching, etc. With the recent development of high-end power and electronic devices, the demand and requirements for soft magnetic composites have grown significantly. They are usually prepared by powder metallurgic procedures including insulation coating, addition of binding agents, compaction and annealing. For high-frequency applications, insulation coating of the FeSiAl powders is critical to increase the electrical resistivity and achieve low loss. Various materials, both organic and inorganic have been used as the coating layer in the preparation of the SMCs. Although the organic coatings provide good adhesion between the magnetic powders, they tend to decompose during the heat treatment for stress relaxation after compaction. Inorganic coatings such as oxides have been considered as excellent alternatives with high resistivity and thermal stability.In this paper, Oxide coating layers mainly consisting of Al2O3 have been prepared by Al(NO3)3 hydrolyzation in the fabrication of FeSiAl soft magnetic composites (SMCs). The coating layer exhibits good uniformity, stability and high temperature resistance, which will not be destroyed in the process of compaction and heat treatment. The effects of processing parameters, including Al(NO3)3 concentration, reaction temperature and pH on the properties of FeSiAl soft magnetic powder cores were investigated. It has been found that the preparation parameters are optimized at 0.6mol/L and 75℃. With increased pH, two inflection points appear for the core loss. Different composition of the coating layer and two coating mechanisms have been identified depending on the pH value during the hydrolysis. Appropriate heat treatment temperature is beneficial to improve the performance of FeSiAl magnetic powder core, and the best performance is obtained at 750℃. Increased annealing temperuature give rise to decreased the electrical resistivity and tensile strength. Due to the adhesion provided by the oxide coating, the process of insulation coating and binding can be achieved in one step.