Effect of DO<inf>3</inf>Phase on High Frequency Permeability of Fe-Si Powder Cores

Abstract

Fe-Si powder cores have been widely used in energy converting systems and others. In order to use the cores in a several megahertz, silicon content should much higher than that of well-known Fe-6.5% Si alloy. Fe-Si alloys with high silicon content consist of order DO 3 and B2 phases as well as disordered A2 phase. However, the role of DO 3 phase has been controversial in conjunction with good magnetic properties. Cooling rate of water quenched Fe-Si powders during gas-atomization is very high so that the evolution of the ordered phases from non-equilibrium phase changes microstructure and thus magnetic properties of the powder widely during annealing. However, the role of DO 3 phase has been controversial and has not been reported in case of Fe-Si powder up to now. In this study Fe-Si powders with 7 – 11 wt.% Si were gas-atomized into cool water and then sieved to be of a mean diameter of 15 $\mu \mathrm{m}$ . The sieved powders were annealed at $T_{anl}=25-750^{\circ}\mathrm{C}$ for 1 hour in nitrogen atmosphere and then packed into toroid capsules with insulating oil. Structure and magnetic properties were analysed with SEM, VSM, XRD and precision impedance analyser. (200) super lattice peak of B2 phase could be observed in all the as-atomized powders. Detection of (111) peaks of DO 3 phase was possible in the powders annealed at $T_{anl}$ higher than 550, 450 and 25°C in Fe- 7, 9 and 11% Si powders, respectively as shown in Figure. Size of DO 3 phase of Fe-7, 9%Si powder increased with $T_{anl}$ while that of Fe-11 %Si powder was almost unchanged. $H_{c}$ of the Fe-Si powders decreased with increasing $T_{anl}$ . However, there was an increase of $H_{c}$ around evolution temperature of DO 3 phase in Fe - 8, 9% Si powders. Permeability $(\mu^{\prime})$ of the Fe-7, 8, 9% Si powders at 10 MHz decreased with increasing $T_{anl}$ while $\mu$ of Fe-11% Si was almost constant. It was well reported that the electrical resistivity of the Fe-Si alloy changed very much depending on cooling rate in a Si content range of 8 – 20% Si [1]. The reason for the behaviour of $H_{c}$ and $\mu$ with $T_{anl}$ seems to be due to change of electrical resistivity attributed from size increase of DO 3 phase with $T_{anl.}$ More experimental data and detailed discussion will be presented on the conference site.