Enhanced microwave electromagnetic properties of core/shell/shell-structured Ni/SiO₂/polyaniline hexagonal nanoflake composites with preferred magnetization and polarization orientations

Core/shell/shell-structured Ni/SiO₂/polyaniline hexagonal nanoflakes possessing in-plane [111] easy magnetization (M) and out-of-plane interfacial polarization (P) are synthesized by a three-step liquid chemical method, and their physicochemical properties and growth mechanism are investigated. Three characteristic types of paraffin-bonded ring-shaped nanoflake composites having random (R), vertical–horizontal (V–H), and horizontal–vertical (H–V) orientations of the orthogonal M and P to their two major surfaces are prepared by randomly, vertically, and horizontally aligning the nanoflakes in the paraffin matrix under a magnetic alignment and thermal curing process. The composites are evaluated experimentally and theoretically in the L–Ku (1–18 GHz) bands of microwaves in order to investigate the orientation effect of the orthogonal M and P on their microwave electromagnetic properties. The in-plane M in the H–V-oriented composite and the out-of-plane P in the V–H-oriented composite, which are parallel to the effective magnetic and electric field vectors of incident microwaves, result in a significant enhancement in permeability with multiple magnetic natural resonances and an obvious improvement in permittivity in comparison with other composites, respectively. The observations agree with the theoretical predictions based on the Landau–Lifshitz–Gilbert equation and Bruggeman's effective medium theory for permeability and the Debye's polarization theory for permittivity. As a result, the H–V-oriented composite achieves the best microwave electromagnetic impedance matching and absorption with a broad absorbing bandwidth of 4 GHz, a wide thickness range of 7–10 mm, and a minimal reflection loss of −41.5 dB in the Ku (12–18 GHz) band.