Synthesis And Microwave Absorbing Properties Of Bio-dirived Carbon Nanocomposites

Carbonaceous materials are considered to be one of the most promising microwave absorbing materials for their low density,adjustability and strong dielectric loss.Biomass,as a waste or by-product of industry,agriculture,animal husbandry and manufacturing,is an ideal natural carbon source with advantages of low cost and high availability.Nevertheless,the morphology and structure of the bio-derived carbon is usually similar to those of its original precursor due to the toughness of biomass,which limits the interaction between microwave absorbers and electromagnetic waves as well as the loss mechanism of absorbers.Therefore,the development of novel and highly efficient bio-derived carbonaceous microwave absorbing materials is of great significance for their large-scale production and practical application,and promoting the diversified development of bio-derived carbon materials.Due to their limited conductivity,graphene cages have hardly been investigated in the field of microwave absorbing material.Given this issue,a novel three-dimensional interconnected graphene cage was synthesized using bio-precursor as carbon source in the first work.Firstly,the oxygen-rich and defect-ridden porous carbon was obtained by pre-oxidation and activation treatment,and then further oxidized by concentrated nitric acid.Then,the Ni nanoparticles were uniformly loaded in the porous carbon by electrostatic interaction and physical adsorption.Consequently,the carbon gases released from the precursor were deposited and precipitated on the Ni nanoparticles and grew into three-dimensional interconnected graphene nanocages under high-temperature calcination.Through HRTEM,TG,Raman spectra,XPS and so on characterizations,it was found that the amount of catalyst Ni mainly affects the morphology,the proportion of KOH mainly affects the microstructure,and the temperature mainly affects the degree of graphitization of 3D interconnected graphene nanocages.When Ni²⁺-C500 mass ratio is 1,KOH-C500mss ratio is 1,and the calcination temperature is 750℃,the conductive loss and thus the attenuation capability of 3D interconnected graphene nanocages is enhanced.The results show that the minimum reflection loss of 3DIGC-750 achieves-51.08 d B,and the effective absorption bandwidth covers the whole Ku and X bands with the thicknesses of 1.45-2.35mm.Additionally,in order to improve the impedance matching of CNTs and enhance their electromagnetic properties,in the second work,a three-dimensional carbon hybrid nanocomposite of nitrogen-doped carbon/CNTs has been synthesized by using bio-derived porous carbon as support and dicyanodiamine as external carbon source via a simple and mild one pot method with Ni nanoparticles as catalyst to connect porous carbon with CNTs,in which the adjustment of morphology and graphitization degree of the as-prepared samples were realized by modifying the amount of dicyanodiamine and the temperature of pyrolysis,respectively.Combined with TEM,HRTEM,Raman spectra,XPS and other characterization methods,the results show that the length-diameter ratio of CNTs is affected by the dosage of external carbon source.When the dicyandiamine-C500 ratio is 4,the samples show the best impedance matching;When the temperature is 750℃,the balance between polarization loss,conductive loss and impedance matching was achieved.The results show that the minimum reflection loss of N-AC/CNTs-750 is as high as-61.76 d B and the effective absorption bandwidth achieves 4.46 GHz at a thickness of only 1.60 mm.