Aluminum substituted low loss Z-type hexaferrites for antenna applications

Z-type hexaferrites in series (Ba0.5Sr0.5)3Co2Fe22−xAlxO38+1.0 wt% H3BO3 (where x=0.0, 0.4, 0.8, 1.2, 1.6, 2.0, and 2.4) were prepared by a conventional ceramic technique. Aluminum substitution iron has been used to enhance sintering density and direct current (DC) resistivity, and decrease magneto-dielectric losses of the ferrites. Meanwhile H3BO3 addition has been used to lower sintering temperature, reduce permeability and enhance resonance frequency. A second phase of Al4B2O9 was found in the XRD patterns (where x=2.0, 2.4). The replacement of Fe3+ ions by Al3+ ions promoted sintering and increased density of the ferrites, which resulted in the increase of real permittivity. Comparatively, a ferrite sample with x=1.6 presented the best performances in terms of densification, microstructure, magneto-dielectric property, and loss tangent. Since loss tangent was kept low with the frequency ranging from 10 MHz to 1.2 GHz, such magneto-dielectric material could be used in this frequency range for antenna miniaturization.