Design Of Carbon-based Nanocomposite And Its Microwave Absorption Properties

As basic materials of modern aircrafts,weapons and electronic equipments,microwave absorbing material is an important support for the modern stealth technology,which has been widely investigated.In Persian Gulf war,Yugoslav war and Iraq war,microwave absorbing material has greatly improved the survivability of weapons.Moreover,with the rapid development of telecommunication,aerospace technology and wireless electronic devices,electromagnetic(EM)interference and radiation problems have become more and more serious.Electromagnetic pollution has become another major source of pollution following air pollution,water pollution and noise pollution.To effectively solve the problems caused by the pollution to the human environment,researchers have been working on the development of high performance absorbing material.For microwave absorbing material,the primary goal is to design and prepare high performance material owning light weight and strong absorbing in a wide absorption bandwidth.In order to achieve this requirement,the key is to control the electromagnetic parameters of the absorbing material.The composition,crystal structure,morphology,size dimension of the material has significant impact on electromagnetic parameters.In this doctoral dissertation,the morphology,dielectric/magnetic material,crystal defect and thickness of measurement material are studied to control the EM parameters and microwave absorption properties.The main results are as following:(1)We have synthesized Fe3O4 nanocrystals with spherical,triangular and cubic shapes via a convenient one-pot reaction.By changing the type or amount of the surfactant,we could control the morphology of nanocrystals.We further investigated the effects of morphology and size of materials on the electromagnetic parameters and microwave absorption properties.The minimum reflection loss of triangular Fe304 nanoplates reaches-32.1 dB at 11.7 GHz and the bandwidth less than-10 dB is from 10.6 to 13.3 GHz at a thickness of 2.5 mm.(2)We have fabricated the porous Co/CoO micro-rods via a facial and simple method.When the proportion of active substances exceeds the percolation threshold,the electromagnetic parameters of Co/CoO/paraffin complex have an obvious increase.As a result,the porous Co/CoO micro-rods based composite could achieve a broad absorption bandwidth at a thin thickness.The minimum value of RL(-47.96 dB)was obtained at the frequency of 8.08 GHz with the thickness of only 1.76 mm.(3)The flower and rod like Fe3O4/C porous microparticles have been successfully prepared by a water bath reaction.We further investigated the effects of morphology and size of Fe3O4/C porous microparticles on the electromagnetic parameters and microwave absorption properties.Meanwhile,the thickness of the test ring could tune the electromagnetic parameters.The resonance phenomenon in the dielectric spectrum is explained by dimensional resonance.the rod-like Fe3O4/C/paraffin composites have uncommon continuous trinal absorption peaks at a thickness of 2.5 mm that effectively broadens the absorption bandwidth which is from 7.9 to 13.5 GHz.(4)Fe3O4/C composite materials had been successfully synthesized by using one-pot heating procedure.Glucose used as carbon source and reducing agent,and nitrate as the precursor of magnetic nanoparticles,a series of magnetic/carbon nanocomposite have been prepared.Electromagnetic parameters and microwave absorption performance were easily controlled by adjusting the experiment conditions.The excellent microwave absorption properties were obtained by the synergistic effect of carbon and magnetite.The maximum value of reflection loss of Fe3O4/C-1.0 is-46 dB at thickness of 2.9 mm and the absorbing bandwidth from 10.3 to 17.1 GHz.(5)Perovskite oxides(LaFeO3)decorated 2D amorphous carbon nanocomposites have been successfully fabricated on a large scale via a one-pot synthesis.The LaFeO3 nanocomposites with different defect concentrations were controlled by adjusting the mole ratio of reactant.L0.8FOC sample has the best microwave absorption properties in which the maximum absorption is-26.6 dB at 9.8 GHz with a thickness of 2.94 mm and the bandwidth range almost covers all X-band.The polarization caused by the defect can effectively change the relative complex permittivity of the material,so as to realize the control of the microwave absorption properties of the material.This doctoral dissertation provides several strategies for the design of the multi-phase composite microwave absorbing materials by tuning the morphology,dielectric/magnetic material,crystal defect and thickness of measurement material.The electromagnetic parameters varation and the absorbing wave mechanism could help us to reach the target of producing absorbing materials with "thin,light,wide,strong" features.The dissertation could make contributions to the application of carbon-based composites and nanocomposites in the field of microwave absorbing materials.