Fabrication and microwave absorption properties of multi-hexaferrite–epoxy composites

Abstract

Multi-hexaferrites (Sr3-xMxCo2Fe24Oa; M = none, La, Ca, Ce, Y; x = 0.3) with more than two types of hexaferrite phases were synthesized by solid-state reactions and doping of various cations in the Z-type Sr3Co2 hexaferrite. Multi-hexaferrites consisting of Z + U + W, W + M, X + W, W + M, and X + W + Z phases were formed in the samples with M = none, La, Ca, Ce, and Y, respectively, upon first and second calcinations at 1160 and 1210 °C. The microstructures were observed by scanning electron microscopy. M–H curves of the sintered bodies were measured using a B–H loop tracer. Complex permittivity and permeability measurements (100 MHz ≤ f ≤ 18 GHz) were performed on hexaferrite–epoxy composite samples. The microwave absorption properties were evaluated by reflection loss (RL) calculations based on the complex permittivity and permeability spectra. For the undoped sample, the minimum RL (-47.6 dB) corresponding to the maximum microwave absorption was obtained at f =4.5 GHz and thickness d =4.6 mm. The frequency corresponding to the RL peak was increased up to 17.3 GHz, while the optimal thickness gradually decreased upon the doping with Ca, Y, Ce, and La cations. The peak shift to high frequency is attributed to the increase of magnetic hardness of the mult-hexaferrites. Broad-band EM absorption properties are also achieved owing to the overlaps of the hard–soft magnetic properties of the multi-hexaferrites.