Evolution of coating layers during high-temperature annealing and their effects on magnetic behavior of Fe(Si) soft magnetic composites

Abstract

Low-loss Fe(Si) soft magnetic composites (SMCs) with atomic-layer-deposition coated layer were successfully prepared in this work. The continuous, compact and uniform Al2O3 layer is found to form tight bonding with the powder base. Evolution of the coating layers has been investigated under different annealing temperatures and closely linked to the magnetic performance of the composites. Results indicate that 1100 °C was the optimal annealing temperature, at which the Fe(Si) SMCs showed lowest core loss of 1237 mW/cm3 and the highest permeability of 99.7 simultaneously (100 mT/100 kHz). Integrity of the coating layer ensures the maximum grain growth and stress removal of powder particles at the highest possible temperature, so as to reduce the hysteresis loss. Formation of high resistivity oxides and silicates (Al2O3, Fe2SiO4 and 3Al2O3·SiO2) after annealing result in a low inter-particle eddy current loss. Besides, high temperature vacuum annealing is helpful to eliminate impurity atoms and decrease the anomalous eddy current loss. The improvement in effective permeability caused by high-temperature annealing is mainly attributed to the relaxation of residual stresses and the reduction of defects. While excessive temperature leads to the decomposition and destruction of the insulation layer, resulting in a significant increase in hysteresis loss, inter-particle and anomalous eddy current loss.