Process optimization and magnetic properties of soft magnetic composite cores based on phosphated and mixed resin coated Fe powders

Fe powders based soft magnetic composites (SMCs) with organic- inorganic multiple-layer coating were fabricated by improved isolation coating followed by pressing and curing. Phosphate and organic resin were selected as the insulting layers. The influences of phosphating process and the composition of organic coating on the magnetic properties of the SMCs and soft magnetic cores have been systematically investigated. The phosphating process was modified for simple and easy operation, suitable for carbonyl iron powders with small particle size. The optimized phosphating method created a complete and continuous phosphate layer on the surface of micron sized particles. Without resin coating, the magnetic cores fabricated with the phosphated powders prepared with 0.5–0.7 wt% phosphoric acid exhibit relatively high permeability and quality factor at high frequency. In addition, a new type of mixed resin with good thermal stability was employed for insolating coating, which provides distinctly higher resistivity and better protective effect than silicone. The optimal content of the mixed resin content is about 3 wt% for fabricating the magnetic cores with low eddy current loss and hysteresis loss by producing a protective layer to isolate magnetic particles and reduce the internal stress. The SMC cores prepared with optimized phosphating process and organic coating exhibit high relative density (93.7%), high electric resistivity (2.23 × 107 Ω·cm), stable permeability (in the frequency range from 5 to 1000 kHz), high quality factor (44.8 at 700 kHz) and relatively low magnetic loss (159.0 kW/m3 at 100 kHz).