Preparation and Electromagnetic Properties of Graphene/Ferrite/Polyimide Ternary Composites

Author:Liu Pei Jiang

Supervisor:yao zheng jun


Degree Year:2018





With the development of advanced radar detection technology,the abilities of the aircraft detection system to search and track targets are greatly improved.The threat of traditional weapon system becomes more serious,and stealth technology is an effective means to improve the survival,protection and deep strike of weapon system.In daily life,electromagnetic(EM)pollution not only disturbs the operation of devices,but also effect human nervous system,cardiovascular system,immune system and endocrine system.As the earliest absorbing materials,although coating absorbing materials can prevent EM interference to a certain extent,they suffer from large density,narrow bandwidth,heavy weight and easy abrasion,which limit their practical application.In this manuscript,graphene sheets(GN)exhibiting dielectric loss was combined with Co0.2Ni0.4Zn0.4Fe2O4 ferrites(CNZF)exhibiting magnetic loss to produce graphene/ferrite(GN/CNZF)absorbers.Afterwards,the absorbers with high efficience absorbing capability were employed to reinforce the polyimide(PI)with great wave transparency and excellent mechanical property to obtain novel absorbing bearing ternary composites.This paper focused on several key issues,such as synthesis of monomers and optimization of EM properties,preparation of absorbers and optimization of microwave absorption performance,and matching property between mechanical properties and absorbing properties.In order to obtain suitable graphene and ferrite absorbing monomers,the preparation and EM property have been studied.The influences of different methods and reductive degrees on EM properties of GN were investigated,and the influence of different methods on morphology and EM properties of ferrite was also discussed.The GN obtained by hydrothermal method had a lower reductive degree than that obtained by high temperature reduction method.With the temperature increasing,the reductive degree increased.The GN exhibited dielectric loss due to functional groups and defects.Ferrites produced by sol-gel method and hydrothermal method had a dimensions of100500nm and 1535nm,respectively,and mainly presented magnetic loss,which mostly comes from eddy current loss and natural resonance loss.However,due to the effect of Snoek’s limit,the permeabilities decreased with the increase of frequency.On the basis of suitable absorbing monomers,GN/CNZF absorbers were synthesized by mechanical blending method and hydrothermal method,and the EM properties and absorbing methanisms were investigated.In the absorber prepared by mechanical blending method,ferrites had great agglomeration,while the ferrite nanoparticles can uniformly disperse on the surface of graphene in the absorber prepared by hydrothermal method.The EM properties indicated that the dielectric loss of absorber prepared by mechanical blending method was above 0.4,which was higher than that prepared by hydrothermal method.The magnetic loss decreased with the increase of frequency.However,for absorber prepared by hydrothermal method,a broad resonance peak appearred at lower frequency.Such phenomenon may be due to the increase of magnetic anisotropic field,which is caused by the lattice defects,inclusions and internal stresses in nanoparticles.For absorbers prepared by mechanical blending method,GN/CNZF-4 exhibited a remarkable absorption performance,with a maximum reflectivity of–44.8 dB at a thickness of 1.6 mm and a band range below–10 dB of 2.7GHz.GN/CNZF-8 prepared by hydrothermal method showed the maximum reflection loss(RL)of–56.1 dB at a thickness of 4.6 mm,and the effective bandwidth was 3.2 GHz.The enhanced absorbing performance is ascribed to the synergistic effect of dielectric loss and magnetic loss.Absorbers with excellent microwave absorbing capability were used to reinforce PI resin,and the effects of the processing technologies and contents of graphene and absorber on mechanical properties were studied systematically.The optimal processing technology for PI resin pointed out that:molding temperature for the first stage is 260270℃,molding temperature for the second stage is 320340℃,and the molding pressure is 1520 MPa.The absorbers caused the decrease of the thermal stability,while two main weight loss at 250500℃and 500650℃appeared in TGA curves,originating from the thermal decomposition of graphene and polyimide chain,respectively.For the ternary composite,the addition of absorbers deteriorated the mechanical properties of PI.When the absorber content is35wt%,the tensile strength,bending strength and impact toughness decreased 19.4%,28.6%and37.2%respectively.The major reason for these deterioration is that the addition of a large number of inorganic particles reduces the dispersion,and further decrease the interaction with PI resin.However,thanks to the excellent mechanical properties of matrix resin,the composite material can meet the requirements of practical application for material strength.The study of microwave absorption property of the ternary composites showed that with the practical thickness of 4.0 mm,GN/CNZF/PI-5 possessed the maximum RL value of-33.1dB,a band range below–5 dB reached 6.3 GHz,and below–10 dB reached 3.2 GHz.When the matching thickness is 4.7 mm,the maximum RL value is-54.0dB,and below–10 dB reached 4.1 GHz.The absorbing property of the ternary composite mainly depend on the effect of resistance loss,dielectric loss and magnetic loss.In addition,the impedance matching rate of the composite gradually decreased with the increase of the absorber contents,and attenuation coefficientαshowed the opposite trend.With the increase of the absorber content,attenuation coefficientαwill increase,and this increase is obvious at high frequency range.