Controllable Synthesis of Cu x Fe 3− x O 4 @Cu Core–Shell Hollow Spherical Chains for Broadband, Lightweight Microwave Absorption

Cu x Fe 3− x O 4 @Cu (0< x <1.30) core–shell hollow spherical chains (HSCs) with tunable composition were prepared through magnetic‐field‐induced solvothermal and liquid‐phase reduction approach. The influence of [Cu 2+ ] on composition, phase, morphology, and microstructure of the products was confirmed by energy‐dispersive X‐ray spectrometry, X‐ray diffraction, X‐ray photoelectron spectrometry, scanning electron microscopy, and transmission electron microscopy analyses. Cu 0 content, Cu 2+ substitution, and crystal size in Cu x Fe 3− x O 4 @Cu HSCs can be adjusted by changing [Cu 2+ ]. Increasing [Cu 2+ ] decreases the mean crystal size and increases the Cu 0 content and Cu 2+ substitution. The saturation magnetization ( M s ) of Cu x Fe 3− x O 4 @Cu HSCs linearly decreases, and the coercivity ( H c ) varies in the Boltzmann mode. The microwave‐absorbing properties of Cu x Fe 3− x O 4 @Cu HSCs decline first and then increase, and the sample with x= 1.300 exhibits the optimal properties. Cu 1.300 Fe 1.700 O 4 @Cu HSCs with 44 wt % mass fraction obtain the minimum resonance loss ( R L ) of −45.1 dB at 3.44 GHz and the absorption bandwidth ( R L ≤−20 dB) of 9.9 GHz. Cu 1.300 Fe 1.700 O 4 @Cu HSCs exhibits stronger absorption, lighter weight, and a broader absorption band than the other samples. The improved absorption performance may be ascribed to the hollow/porous core–shell structures, Cu 2+ substitution, and Cu 0 shell with high conductivity, leading to enhanced permittivity, matching, and resonance absorption.