Influences of Bi2O3 additive on the microstructure, permeability, and power loss characteristics of Ni-Zn ferrites

Nickel-zinc ferrite materials containing different Bi₂O₃ concentrations have been prepared by the conventional ceramic technique. Micrographs have clearly revealed that the Bi₂O₃ additive promoted grain growth. When the Bi₂O₃ content reached 0.15 wt%, a dual microstructure with both small grains (<5 μm) and some extremely large grains (>50 μm) appeared. With higher Bi₂O₃ content, the samples exhibited a very large average grain size of more than 30 μm. The initial permeability gradually decreased with increasing Bi₂O₃ content. When the Bi₂O₃ content exceeded 0.15 wt%, the permeability gradually decreased with frequency due to the low-frequency resonance induced by the large grain size. Neither the sintering density nor the saturation magnetization was obviously influenced by the Bi₂O₃ content or microstructure of the samples. However, power loss (Pcv) characteristics were evidently influenced. At low flux density, the sample with 0.10 wt% Bi₂O₃, which was characterized by an average grain size of 3-4 μm and few closed pores, displayed the lowest Pcv, irrespective of frequency. When the flux density was equal to or greater than the critical value of 40 mT, the sample with 0.20 wt% Bi₂O₃, which had the largest average grain size, displayed the lowest Pcv.