Effects of mixing procedure and Bi 2O 3 content on structural and magnetic properties of hexaferrites sintered at low temperature

Abstract

Z-type hexaferrites with composition of Ba 3(Co 0.4Zn 0.6) 2Fe 24O 41+ x wt% Bi 2O 3 (where x=0–2) were prepared by a solid-state reaction method. The effects of different mixing procedures and Bi 2O 3 content on the sintering behavior, phase composition, microstructure and magnetic properties of the samples were investigated. It was confirmed that with the same secondary mixing time ( t 2), increasing the primary mixing time ( t 1) leads to a decrease of the average grain size and an increase of the initial density. The initial density and the distribution of particles and pores determined the sintering density. With t 1=24 h and t 2=6 h, the sintering density reached its maximum. The initial permeability and Q-factor also peaked, which could be attributed to the highest sintering density and compact microstructure. Further studies showed that with increasing Bi 2O 3 content, the major phase changed to the Z-phase; simultaneously, the remnants content of intermediate phase decreased. The Bi 2O 3 additive, as a reaction center, promoted the grain growth at lower temperatures. The decrease of the sintering temperature and the partial agglomeration of Bi 2O 3 on the grain boundaries restrained abnormal grain growth, thus closed pores in grains were not formed. A maximum initial permeability, a higher Q-factor, a cut-off frequency of 900 MHz and a resonance frequency above 1.8 GHz were obtained for the sample with 0.6 wt% Bi 2O 3 sintered at low temperature.