Microwave absorption properties were systematically studied for double-layer carbon black/epoxy resin (cB) and ni 0.6 Zn 0.4 fe 2 o 4 /epoxy resin (F) nanocomposites in the frequency range of 8 to 18 GHz. The ni 0.6 Zn 0.4 fe 2 o 4 nanoparticles were synthesized via high energy ball milling with subsequent sintering while carbon black was commercially purchased. The materials were later incorporated into epoxy resin to fabricate double-layer composite structures with total thicknesses of 2 and 3 mm. The CB1/F1, in which carbon black as matching and ferrite as absorbing layer with each thickness of 1 mm, showed the highest microwave absorption of more than 99.9%, with minimum reflection loss of −33.8 dB but with an absorption bandwidth of only 2.7 GHz. Double layer absorbers with F1/CB1(ferrite as matching and carbon black as absorbing layer with each thickness of 1 mm) structure showed the best microwave absorption performance in which more than 99% microwave energy were absorbed, with promising minimum reflection loss of −24.0 dB, along with a wider bandwidth of 4.8 GHz and yet with a reduced thickness of only 2 mm.In order to address issues induced by high proliferation of electromagnetic interferences in both civil and military applications, efficient microwave absorbers are becoming highly desirable and necessary. For that reason, such material is required to effectively reduce the reflection of electromagnetic (EM) signals over a broad absorption bandwidth. In order to improve the performance of microwave absorption properties, microwave absorbers are designed to meet the specific requirements of simultaneously having strong absorption, wide frequency band, lightweight and small thickness. Improvements can certainly be made to the designs by physical assembling of different types of absorbents 1-5 , chemical decorated absorbents 6,7 as well as by designing multi-layer structures [8][9][10][11] .Microwave absorbers are produced using different kinds of materials including one dimensional (1D) materials such as carbon nanotubes 12-15 , two dimensional (2D) materials such as graphene 16,17 and bulk three dimensional (3D) materials such as ferrites 9,18-21 . The difference in the dimensional structure of the materials would largely affect the microwave absorption performances since different kinds of structures contribute to different www.nature.com/scientificreports www.nature.com/scientificreports/ the F1/CB1 sample showed the best all round performance, in which more than 99% microwave energy was absorbed, with a reflection loss of −24.0 dB and a widest bandwidth of 4.8 GHz at −10 dB, yet it is the thinnest among the three designs, having a total thickness of only 2 mm.

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This paper reports a study of the microwave absorption properties of multiferroic BiFeO 3 (BFO) epoxy resin composites. The effects of various sintering temperatures on the crystalline phase of BFO and its microwave absorption characteristics were critically analyzed. BFO nanoparticles were synthesized by mechanical activation high energy ball milling (HEBM) with post heat treatment over various temperatures ranging from 700 to 800 °C. The XRD results showed by using the HEBM method, BFO phase is formed at a lower sintering temperature of 700 °C compared to conventional solid state reaction due to the enhanced diffusion rates. The phase composition and the grain sizes had significant influence on the permeability, permittivity and reflection loss values of BFO composites measured by a network analyzer in the frequency range from 8 to 18 GHz. It was observed that the purity fraction of BFO phase and the grain sizes increased with the sintering temperature. By increasing the sintering temperature up to 775 °C, the microwave absorption properties were enhanced over a broad working frequency range corresponding to the reflection loss below − 10 dB (i.e. 90% absorption) due to crystalline phase changes. BFO samples sintered at 775 °C demonstrated higher absorption ability with RL min − 40.5 dB over a 1.31 GHz bandwidth, showing that BiFeO 3 has great potential as a microwave absorbing material.

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