The Preparation and Properity of Cobalt Nickel Metals and Graphene-based Composites Microwave Absorbers

Author:Wang Ying

Supervisor:han xi jiang

Database:Doctor

Degree Year:2019

Download:63

Pages:151

Size:8882K

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With rapid development and application of radio network technology,numerous electromagnetic(EM)waves enter into living space of human,causing negative issues,such as EM interference and EM radiation pollution.Different from traditional EM shielding materials,microwave absorbers can attenuate EM waves without secondary radiation.To date,microwave absorbers have shortcomings of low absorption intensity and narrow effective absorption bandwidth.Magnetic metals show strong lossy ability to incident EM waves due to larger saturation magnetization and permeability,whereas they also suffer from poor chemical stability and corrosion resistance,high density,complex synthesis process and low output,and easy aggregation.By contrast,conjugated polymers and carbonaceous materials show low density,low cost,good corrosion resistance and easy preparation.Graphene,as one of carbonaceous materials,has unique two-dimensional network structure,good electrical,thermal and mechanical properties,making it a desired substrate for light and efficient microwave absorbers.This dissertation proposes to study the preparation and property of cobalt nickel metals and graphene based composites microwave absorbers,and aims to optimize EM properties and explore EM absorption mechanism by tuning crystal phases,morphologies and atomic ratios of cobalt nickel metals,and by designing and constructing composition and microstructures of graphene based composites.Co assemblies were fabricated with a precursor-direct synthesis route,and mass ratios of hexagonal close packing phase(HCP)/face-centered cubic close packing phase(FCC)in these Co assemblies were 1:0.8,1:1.0,1:1.8,1:3.2,1:13.1 and 1:17.5,respectively,and the corresponding maximum reflection loss(RLmax)at absorber thickness of 1.0 mm were-20.0 dB,-17.0 dB,-32.8 dB,-26.3 dB,-21.7 dB and-19.9dB,where their effective absorption bandwiths over-10.0 dB(EABs<-10 dB)were 2.5GHz,3.6 GHz,4.0 GHz,3.9 GHz,3.3 GHz and 2.6 GHz.Our results indicate that in these Co assemblies HCP phase favors high complex permittivity and FCC phase produces high complex permeability.When mass ratio of HCP/FCC was 1:1.8,Co assembly exhibited superior microwave absorption performance,showing RLmax of-41.0 dB at 9.4 GHz with an absorber thickness of 1.5 mm,and exhibiting effective absorption bandwith is 4.0-17.4 GHz with varying absorber thicknesses of 1.0-5.0 mm.Compared with the solid Co material,porous structure in these Co assemblies plays an important role in reducing complex permittivity,improving impedance matching characteristics and microwave absorption performance.Core-shell CoNi@few layer graphene(FLG)composites were prepared by hydrothermal method and subsequent high-temperature pyrolysis,where atomic ratios of Co/Ni were 1:0.3,1:0.8,and 1:1.8,respectively,and the corresponding mass ratios of CoNi/FLG were 1:0.3,1:0.4 and 1:0.3,and their RLmax were-28.0 dB,-45.5 dB and-15.5 dB with an absorber thickness of 2.0 mm,and the EABs were 5.0 GHz,4.5 GHz and 3.2 GHz,respectively.The results show that dipole polarization at defect sites and interface polarization from core-shell structure contribute to microwave absorption.When Co/Ni atomic ratio was 1:0.8,CoNi@FLG has excellent microwave absorption,strong dielectric loss and relatively weak magnetic loss.Polypyrrole(PPy)nanospheres and graphene oxide(GO)integrated into PPy/GO hybrids byπ-πinteraction and by subsequent reduction of N2H4·H2O,PPy/GO hybrids were transformed into PPy/rGO composites,where mass ratios of PPy/rGO were 1.2:1,0.6:1 and 0.3:1,respectively,and the RLmax were-19.0 dB,-59.2 dB and-10.9 dB,and their absorber thicknesses were 1.5 mm,3.8 mm and 1.0 mm,the corresponding EABs were 2.5 GHz,1.2 GHz and 3.1 GHz,respectively.EM analysis reveals that good microwave absorption of PPy/rGO composites originates from good impedance matching characteristics,multiple polarization loss and conductive loss.A sandwich-like rGO/amorphous carbon microspheres(ACMs)/rGO composite was fabricated via chemical reduction of GO/ACMs/GO,which was prepared through electrostatic interaction between electropositive ACMs modified by polydiallyl dimethyl ammonium chloride and electronegative GO.Compared with ACMs and rGO,rGO/ACMs/rGO composite exhibited enhanced microwave absorption,showing RLmaxax of-21.5 dB(18.0 GHz)and effective bandwidths was 2.6 GHz with an absorber thickness of 1.2 mm.EM parameters and simulation results reveal that well-matched characteristic impedance and proper dielectric loss,as well as multiple reflections and interfacial polarizations derived from the unique microstructure account for the enhanced microwave absorption of rGO/ACMs/rGO.rGO/ACMs/rGO composite exhibited good microwave absorption after being treated at 523 K for 10 d.The EABs of above as-prepared microwave absorbers lie in Ku band(12.0-18.0GHz).Analyzing their microwave absorption mechanisms,it reveals that Co assemblies attenuate EM waves mainly by magnetic loss,while for core-shell CoNi@FLG composite dielectric loss dominates its microwave absorption.In addition,PPy/rGO compsites and rGO/ACMs/rGO composite absorbe EM wave only via dielectric loss.