Comparative investigation of insulation techniques based on APTES for different structures and magnetic properties of soft magnetic composites

FeNi powders were insulated with APTES by solution method and solidification method, respectively. Different coating mechanisms have been systematically studied according to analysis on microstructure, surface charge state, thermal stability of magnetic powders and magnetic properties of corresponding soft magnetic composites (SMCs). It was found that a slight amount of polysiloxane and amorphous silica would be deposited on FeNi powders in solution method, resulting in an ultra-thin insulation layer. In contrast, continuously increasing concentration of polysiloxane in reaction zone gives rise to a thick and smooth SiO2 layer in solidification method. Owing to unique growth mechanisms of solidification method, it is easy to control the thickness of SiO2 layer by adjusting the concentration of APTES, which is in favor of improving insulation quality. Accordingly, FeNi SMCs prepared in solidification method own much higher resistivity, lower μ″ and lower tanδ (particularly at high frequency), compared with samples obtained by solution method with the same volume of APTES. Besides, increasing volume of APTES in solidification method leads to the decrease of both μ′ and μ″. This work may not only provide novel insights into APTES-based insulation process, but also offer powerful technique for fabricating high-frequency SMCs.