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In this paper, a special dielectric property with tailorable negative permittivity is attained in the polyaniline assisted magnetic ultrafine barium ferrite (BaFe12O19) nanorods reinforced epoxy metacomposites. The ultrafine BaFe12O19 nanorods have been prepared through a citrate assisted sol–gel method and a self-propagating combustion combined with high temperature annealing. A thin polyaniline (PANI) layer is introduced on the surface of BaFe12O19 nanorods by in situ polymerization method under the sonication combined with the mechanical stirring in order to improve the dispersion quality and interfacial compatibility between BaFe12O19 and epoxy matrix. The enhanced tensile strength with maximum value of 90.7 MPa for the cured epoxy nanocomposites filled with 10 wt % loading of PANI modified BaFe12O19 (s-BaFe12O19) nanorods was obtained in the tensile tests compared with that of the cured pure epoxy (75.2 MPa) and cured epoxy nanocomposites filled with the same loading of bare BaFe12O19 (u-BaFe12O19) nanorods (74.4 MPa). In the cured s-BaFe12O19/epoxy nanocomposites, the uniform BaFe12O19 distribution has been observed by scanning electron microscope (SEM), which contributes to the improved tensile strength. A negative real permittivity is achieved in both of u-BaFe12O19 and s-BaFe12O19/epoxy nanocomposites at the low frequency range (∼103 Hz). After further increasing frequency, the real permittivity was changed from negative to positive. The permittivity has no u-BaFe12O19 loading dependency for the u-BaFe12O19/epoxy nanocomposites. However, it is decreased with increasing s-BaFe12O19 loadings in the s-BaFe12O19/epoxy nanocomposites, which exhibits the component rather than structure controlled epoxy metacomposites.

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