Facilesynthesisoflightweight3DhierarchicalNiCo2O4nanoflowers/reducedgrapheneoxidecompositefoamswithexcellentelectromagneticwaveabsorptionperformance

Considering the high filling ratios,high densities,and narrow absorbing bandwidths of the current electromagnetic wave（EMW） absorbers,in this work,we successfully synthesized a 3 D hierarchical NiCo2 O4 nanoflowers/reduced graphene oxide（NiCo2 O4/RGO） composite foam by a simple method under gentle condition.The NiCo2 O4 nanoflowers and unique 3 D foam structure are beneficial to the refraction and scattering of EMW,which endows the prepared 3 D foam with highly efficient EMW absorption performance.When the ratio between NiCo2 O4 and RGO in the foam is 1:1,5% mass fraction of NiCo2 O4/RGO foam in paraffin wax can reach a minimum reflection loss(RLmin) value of-52.2 dB with a thin thickness merely 2.6 mm.Simultaneously,the effective absorption bandwidth（EAB,RL exceeding-10 dB） is7.04 GHz that covers the whole Ku band（10.96-18 GHz）.Moreover,the effects of the thickness of the absorber and the loading ratios of the foam in paraffin wax matrix on the EMW absorption properties are also carefully investigated.The results indicate that the optimum EMW absorption performance of NiCo2 O4/RGO can be tuned in different bands.The EMW absorption mechanism is ascribed to the proper impedance matching and larger dielectric and magnetic loss produced by the synergy of NiCo2 O4 and RGO.Therefore,the NiCo2 O4/RGO hybrid foam is ideal candidate to be used as high-efficient EMW absorbers with low filling ratio,light weight,and broad frequency bandwidths.