Study On The Insulation Coating And Binding Technique Of Iron Based Soft Magnetic Composites

Soft magnetic composites (SMCs) are technologically important for applications in electronic communications, switching power supply and so on due to their unique properties including high saturation induction density,high electrical resistivity,very low eddy current loss, relatively low total core loss and three-dimensional (3D)isotropic ferromagnetic behavior. As the electronic equipment is designed for high frequency used, high performance and energy saving, SMCs are to be required for low core loss and high frequency stability.The SMCs' performance behavior greatly depends on its preparation process,among which insulation coating and binding techniques are the key factors. This paper focuses on the phopsphating process for Fe and FeSiAl SMCs, and the effects of high-temperature annealing on the phosphate coating and the magnetic properties of the SMCs have also been studied. In addition, a high-temperature resistant resin,namely epoxy-modified silicone resin (ESR), was chosen as the binder for the iron SMCs, and the resin's thermal properties was investigated. To further increase the ESR's thermal stability, two methods were employed to introduce the organic silica nanoparticles into the ESR to prepare inorganic-organic hybrid resin. The influence of two different fabrication methods for hybrid resins on the properties of the iron SMCs were studied, and effects of annealing temperature on the inorganic-organic hybrid coating and the magnetic properties of the Fe SMCs have also been revealed. The obtained conclusions are as follows:For Fe SMCs, iron phosphate is mainly obtained after the phosphating process for Fe powders. After the high-temperature annealing (600?), the iron phosphate decomposes into iron phosphides and iron oxides, resulting in decreased electrical resistivity and deteriorated magnetic properties of the Fe SMCs. For FeSiAl SMCs,the coating layer for the FeSiAl powders is mainly Al(PO3)3 after the phosphating process. The Al(PO3)3 coating layer obtained for the FeSiAl powders degrades into oxides including Al2O3 and P2O5 during annealing, such oxides with higher electrical resistivity than the phosphate coating continues to serve as the insulating material for the FeSiAl composites. Consequently, it is beneficial to annealing the FeSiAl SMCs with phosphate coating at relatively high temperatures (above 600?) for enhanced magnetic properties.The maximum processing temperature that the ESR could withstand is about 400?, which has higher thermal stability than epoxy resin. Compared with the epoxy resin coated SMCs,the ESR prepared samples have better magnetic and mechanical properties. The ISG and DA hybrid resins coated Fe SMCs both exhibit better magnetic properties when compared with the unmodified resin coated samples.ISG hybrid resin exhibits a homogeneous distribution of nano-SiO2 in the polymer matrix,and the sol-gel prepared nano-SiO2 has been chemically bonded with the polymer resin in the ISG. In the DA, the introduced SiO2 nanoparticles have an obvious tendency to agglomerate, which leads to the dual inorganic-organic insulating coating for the coated Fe powders. The ISG prepared SMCs show better magnetic behavior than the DA prepared samples. The optimal annealing temperature for ISG-2 and DA-2 coated Fe SMCs has increased 50?, when compared with that of the unmodified resin prepared samples. The FTIR analysis and the TG measurement revealed that the chemical incorporation of nano-Si02 into the polymer matrix in the ISG-2 improved the chemical stability of the hybrid resin, so the ISG-2 coated Fe SMCs could be annealed at higher temperature. The DA-2 possesses the inorganic coating, which can also withstand the higher annealing temperature.