ZnFe2O4@SiO2@Polypyrrole nanocomposites with efficient electromagnetic wave absorption properties in the K and Ka band regions

A series of ZnFe2O4@SiO2@PPy nanocomposites with different SiO2 contents were successfully fabricated using a combination of sol-gel and in-situ polymerization processes. Spherical ZnFe2O4 particles (mean diameter similar to 300 nm) were first synthesized, then coated successively with conformal layers of SiO2 and polypyrrole (PPy). The electromagnetic wave (EMW) absorption properties of the resulting ZnFe2O4@(SiO2)(x)@PPy nanocomposites (where x = the volume of TEOS used in the synthesis) were subsequently investigated in the K band (18-26.5 GHz) and K-a band (26.5-40 GHz). Results show that the EMW absorption properties of the nanocomposites can be precisely tuned by controlling the thickness of the SiO2. Compared with ZnFe2O4@PPy, the ZnFe2O4@SiO2)(x)@PPy composites exhibited enhanced reflection losses and broader effective absorption bandwidth (EAB, reflection loss less than -10 dB). The ZnFe2O4@(SiO2)(1.0)@PPy nanocomposite offered the best EMW absorption performance, with a minimum reflection loss (RLmin) of -29.72 dB at 24.96 GHz (EAB of 7.0 GHz, 19.5-26.5 GHz) at 1.5 mm thickness and 36.75 dB at 38.38 GHz (EAB of 9.56 GHz, 30.44-40 GHz) at 1.0 mm thickness. The main microwave absorption mechanisms used by the ZnFe2O4@SiO2@PPy composites were magnetic losses (ZnFe2O4 nanoparticles), dielectric losses (ZnFe2O4, SiO2 and PPy) and interfacial relaxation losses (at ZnFe2O4-SiO2-PPy interfaces). Results guide the development of improved microwave absorbers in the K and K-a bands.