High-Flux Density and Low-Core Losses Attained for Ferrite/Permalloy Composite Core

Permalloy (Ni₄₇Fe₅₃) microspheres, 8 mum in average diameter, were encapsulated in Ni-Zn ferrite layer by ultrasound enhanced ferrite plating from an aqueous solution at 80degC at pH = 6-9. The ferrite-plated, as well as nonplated (bare) microspheres for comparison were compacted at 5 ton/cm²to toroidal cores, on which complex permeability (mu=mu\´-jmu") and core loss P_cvwere measured. When plated at pH= 9 the composite core exhibited the strongest saturation magnetization, which was about three times stronger than that of conventional bulk ferrite cores. By the ferrite-encapsulation, the core losses decreased by a factor of 10 at 2-5 MHz, because eddy-current loss was suppressed by the insulating Ni-Zn ferrite layer. As compared to the conventional powder cores in which fine particles of Ni₅₀Fe₅₀ or Fe are dispersed in epoxy resins, our composite core (plated at pH = 9) exhibited two or ten times low value of P_cv at 1 MHz, respectively. The value of P_cv in the composite core did not change when temperature was raised from 25 to 100degC, exhibiting thermal stability enough for actual use