Preparation And Electromagnetic Properties Of Carbon-Based Microwave Absorption Materials

Electromagnetic（EM）wave absorption materials can effectively reduce the radar signature of weapons and improve their battlefield survivability.They are also widely applied to alleviate the interference between electronic devices and the harm of EM wave to human health.Compared with conventional magnetic absorbers,carbon-based materials are attracting growing research interest for their low density and resistance against oxidation.However,recent studies focus mainly on the synthesis of various micro-nanostructures.To obtain a systematic and quantitative understanding of the electromagnetic properties of carbon-based EM wave absorbers,both calculations and experimentations were conducted in this dissertation.Ideal dielectric curves for carbon-based EM wave absorbers were acquired via high-throughput parameter screening with materials genome methodology,while various absorbers were synthesized.The effect of structural factors,such as graphitization degree,porosity,crystallinity and dimensions,on electromagnetic parameters and EM absorption was studied,so that the dielectric curves of real-world materials can be tuned to approach the ideal ones.Phenomenological models with 3 or 4 parameters about the dielectric curves of carbon-based EM absorbers were established.The high-throughput parameter screening based on them discovered that ideal dielectric curves should have strong frequency dispersive effect and moderate dielectric loss tangent,so that the material have enough attenuation capability without losing impedance matching.These screening results were verified literature results.A random network model was established for conduction-loss materials,and parameter screening showed that moderate conductivity parameter and volume fraction parameter were necessary to achieve ideal absorption performance.This model can fit closely into literature data.To validate the random network model and explore the physical connotations and influencing factors of the two parameters in it,microwave-reduced graphite oxide（MWr GO）was prepared.This material possessed a loose and porous structure,and its conductivity and porosity were further tuned by annealing.This material was dominated by conduction loss,and its dielectric data can be closely fitted by the random network model.The volume fraction parameter can be regulated by the absorber content,while the conductivity parameter,which was intrinsic for certain absorbers,can be tuned with graphitization.The effective absorption bandwidth（EAB）of 6.5 GHz at 2.3 mm and 5.7GHz at 2.0 mm was achieved.The closer the two parameters fitted from real-world materials were to the ideal values from screening,the better the absorption performance was.The outstanding EM wave absorption performance was ascribed to the moderate graphitization level,thin layered structure,high porosity,and good dispersibility.These findings showed that parameter screening based on the random network model was instructive for experimentation.To further expound the effect of pore volume on dielectric properties,hierarchically porous carbon materials were synthesized with Pechini method,of which the specific pore volume was regulated with the precursor ratio.This material was also conduction-dominated and matched well with the random network model.By implementing the materials genome paradigm,the experiments were optimized guided by parameter screening,and an EAB of 6.2 GHz at 2.2 mm was achieved.The mechanism how porosity affected EM wave absorption was unveiled:enlarging the specific pore volume first promoted the formation of resistor-capacitor random network,and then led to a higher volume fraction parameter and lower conductivity parameter.These findings provided new tools to tune the absorption performance of conduction-loss absorbers.To investigate the synergistic effect of multiple microwave attenuation mechanisms,a hierarchically porous Fe₃O₄/C absorber was prepared with biomass as the carbon source and template,and the hollow fibrous structure of cotton was retained.With increasing pyrolysis temperature,both carbon and Fe₃O₄ underwent crystallization and subsequent grain growth,which was accompanied with the shrinkage and collapse of mesopores.The conduction and dielectric polarization loss capability was also enhanced.Fe₃O₄ majorly contributed to the dielectric rather than magnetic loss by providing conduction,dielectric polarization,and interfacial polarization,as well as catalyzing the graphitization of carbon.The sample pyrolyzed at 600℃gave the best EAB of 4.50 GHz at 2.0 mm,proving that the 3-parameter and 4-parameter models can qualitatively guide the development of microwave absorbers with complex attenuation mechanisms.To study the effect of size and composition factors of nanocomposites on their dielectric performances,the diameter of nitrogen-doped carbon nanotubes（NCNTs）was tailored with the“Trojan catalyst”strategy.It was discovered that smaller diameter facilitated the formation of conductive network,and brought about larger surface area.As a result,conduction loss and interfacial polarization were enhanced.The Ni O-Mg O solid solution not only acted as the precursor and support of the catalyst,but also adjusted the impedance.It has been shown again that the closer the dielectric curves were to the screening results,the better absorption performance was obtained.The highest EAB was4.4 GHz at 1.6 mm.