Ferromagnetic and excellent microwave absorbing properties of CoNi microspheres and heterogeneous Co/Ni nanocrystallines

CoNi microspheres with different diameters and heterogeneous Co/Ni nanocrystallines were synthesized via changing hydrothermal reaction parameters. The heterogeneous Co/Ni nanocrystallines comprised three kinds of particle morphologies, i.e., nanoflakes, nanospheres and needle-like nanowhiskers. The heterogeneous Co/Ni nanocrystalline sample coating (containing 60 wt% powder) exhibited a maximum reflection loss (RL) of −33 dB at 17.6 GHz and a bandwidth of less than −10 dB covering the 15.04–18.00 GHz range with a coating thickness of 1 mm. The CoNi microsphere sample with diameters in the range of 0.4–2.5 μm exhibited excellent microwave absorption abilities in the C-band (4–8 GHz) and X-band (8–11.5 GHz). However, the sample of chain-like assemblies from CoNi microspheres with a diameter above 2 μm presented poor microwave absorption in the 2–18 GHz range. In contrast, the excellent microwave absorption properties of the heterogeneous Co/Ni nanocrystalline sample in the Ku-band (12–18 GHz) could be attributed to the relatively high permeability (1.63–1.10) and optimal impedance matching between permittivity and permeability.