Ni2+ guided phase/structure evolution and ultra-wide bandwidth microwave absorption of CoxNi1-x alloy hollow microspheres

A series of bimetallic hexagonal close-packed (HCP) and face-centered cubic (FCC) CoxNi1-x (x = 1, 0.858, 0.813, 0.714, 0.662, 0.137) alloy hollow microspheres (AHMs) with continuously tunable composition and wall thickness were successfully synthesized via one-pot liquid phase reduction. In addition to prolonged reaction time, decreased metal ion concentration, and elevated reaction temperature, Ni2+ addition favors an inside-out Ostwald ripening and Kirkendall diffusion, leading to the formation of CoxNi1-x AHMs with thin wall thickness. Moreover, Ni2+ addition induced the phase transformation from HCP Co to FCC Co and modulated crystal size, internal strain, lattice constant, composition, and wall thickness. Accordingly, Ms decreased linearly as Ni content increased. Alloying of Ni with Co induced significantly enhanced mu", negative e' and ultra-wide bandwidth microwave absorption due to the Plasmon resonance, defect polarization and lattice polarization. The wax-based composites containing 35 wt% Co0.81Ni0.19, 45 wt% Co0.86Ni0.14, or 50 wt% Co0.66Ni0.34 AHMs possessed significantly enhanced absorption capability with maximum RL values of -35.3, -47.3, and -54.6 dB and ultra-wide bandwidths (RL=-10 dB) of 8.16, 9.2, and 10.08 GHz, corresponding to layer thicknesses of 1.9, 1.8, and 2.6 mm, respectively. CoxNi1-x AHMs exhibited stronger absorption, broader bandwidth, and lighter weight than those of other absorbers. Superior microwave absorption performance (MAP) were mainly creditable to the synergy of enhanced permittivity and permeability, high attenuation, and good impedance matching caused by hollow structure, Ni incorporation, and Plasmon resonance. CoxNi1-x AHMs with tunable Ms and excellent MAP might solve electromagnetic pollution and interference problems for further applications in microwave absorption and shielding fields.