Effects of different sintering temperature and Nb2O5 content on structural and magnetic properties of Z-type hexaferrites

Nb-doped Z-type hexaferrites (Ba3(Co0.4Zn0.6)2Fe24O41) with composition of Ba3(Co0.4Zn0.6)2Fe24O41+x Nb2O5 (where x=0.0, 0.1, 0.2, 0.4, 0.6, 0.8, 1.2, 1.6 and 2.0 wt%) were prepared by a solid-state reaction method. The effects of different sintering temperature (Ts) and Nb2O5 content on the sintering behaviors, phase composing, microstructure, and magnetic properties of the samples were investigated. The results from X-ray diffraction and scanning electron microscopy show that as the amount of Nb2O5 additive increases, the major phase changes to Z-phase, Simultaneously, M-phase and a small amount of niobate phase appear. The Nb2O5 additive promotes the grain growth as reaction center at lower sintering temperature (1220 °C), but at higher temperature (1260 °C), niobate phase separated out in grain boundaries as secondary phase will restrain abnormal grain growth, so closed pores in grains are not formed. The Nb2O5 additive can enhance densification, improve initial permeability of hexaferrites by increasing the grain growth of hexaferrite and the displacement of ions in the sintering process due to the aberration and activation of crystal lattice, which is accompanied by the solubility of Nb5+ in the hexaferrites. A relative density of 96%, maximum initial permeability (32–33), minimum coercivity (454–455 A/m) and resonance frequency above 400 MHz were obtained for the sample with 0.8 wt% Nb2O5 sintered at 1260 °C for 6 h.