Study of an iron phosphate soft magnetic composite and its stability at high temperatures

Most studies on soft magnetic composites (SMC) have focused on traditional compaction. However, this consolidation method leads to low densities and therefore, poor magnetic properties. For this reason, this work focuses on the study of an iron phosphate (Fe3(PO4)2) coating to develop SMCs consolidated through a novel field-assisted sintering technology (FAST) and compares it with traditional compaction. A direct relationship is demonstrated between the thickness of the coating obtained and the amount of reagent added to the synthesis as well as the inverse relationship between the thickness of the coating and the powder/acetone ratio. By contrast, thermal analysis shows that the desired phase Fe3(PO4)2 is stable up to 900 °C. Between 900 and 960 °C, iron phosphate decomposes into Fe–P and SiO2. At higher temperatures, the decomposition process is completed, and the Fe–P phase reacts with Fe–3Si particles, resulting in an FeSiP matrix with a non-continuous layer of surrounding SiO2. The highest compact density and static magnetic properties are obtained for the SMC prepared with 2 wt% H3PO4 and a powder/acetone ratio of 2.5 g/mL consolidated by FAST, providing a density of 6.59 g/cm3, a magnetic saturation of 1.752 T and a coercivity of 313.4 A/m. In addition, this SMC results in the highest permeability in the entire audio-frequency range (40 up to 100 kHz) as well as high electrical resistivity (3.18·105 μΩ cm) and the lowest power losses up to 10–20 kHz (31.7 mW/cm3 at B = 50 mT and f = 10 kHz). However, cold-press consolidation leads to higher operating frequencies, reaching a permeability of 24 up to 1 MHz, and the lowest power losses for frequencies greater than 10–20 kHz (414 mW/cm3 at 100 kHz and B = 50 mT) due to the reduction of eddy currents obtained by the higher electrical resistivity and lower number of coating defects.