| High performance electromagnetic wave absorption(EWA)materials are extremely important in both civil and military fields by reducing electromagnetic pollution and improving the survivability of weapon system.However,EWA materials which not only have the characteristics of "wide,strong,light,thin" but also suitable for working in high temperature and corrosion environments is still an unsolved problem.Polymer derived ceramics(PDCs)with unique nano-multiphase structure,designable chemical composition,tunable electrical/dielectric properties,excellent high temperature thermal and oxidation/corrosion resistance,have become one of the most promissing candidates for novel EWA materials.However,there are very limited publications on the EWA properties of PDCs.Moreover,these reports are mostly focus on dense PDCs bulk materials,no open publications are found on the EMA properties of polymer derived ceramics aerogels(PDCAs).In this paper,PDCAs were innovatively prepared by sol-gel process and freeze-drying technology combined with polymer precursor conversion method.Through the chemical composition and micro structure design,we can control the electrical/dielectric properties and EWA properties.We study the relationship between structure and properties to explore the relative mechanisms.Firstly,different polymer precursors(polysiloxane,polycarbosilane,polysiloxane)were used as raw materials to prepare PDCAs(PDCA-SiCN,PDCA-SiCN,PDCA-SiOC).And PDCA-SiCN as the main research object,the important parameters(synthesis temperature,synthesis time,solvent ratio,pyrolysis temperature)in the preparation process were changed to achieve the composition-structure manipulation of PDCAs and reveal the formation mechanism.Then the SiCN ceramic aerogel composites(PDCA-SiC/SiCN,PDCA-Fe/SiCN,PDCA-Co/SiCN)with multi composition and multilevel structure were designed by microwave-assisted catalysis,reverse microemulsion method,etc.And the formation mechanism of SiCN ceramic aerogel composites was studied.Finally,the effect of composition and structure of PDCAs and composites on the EMA properties was emphatically studied.The relationship among process,composition,structure and properties was established to reveal the EMA mechanism of PDCAs,which provides scientific and theoretical basis for the preparation of high efficiency EMA materials.The main results and contents of this paper are as follows:(1)The design preparation of PDCAs and their composition-structure manipulation:Porous PDCA-SiCNs with a large amount of free carbon and a three-dimensional(3D)networks structure were successfully prepared after gelation(synthesis temperature 150?,synthesis time 20 h,solvent ratio 90 vol%),freeze drying and high temperature pyrolysis(1000 0C),polysiloxane acted as ceramic precursor,divinylbenzene as crosslinking agent and cyclohexane as solvent.PDCA-SiCNs possess characteristics of light weight(0.19 g/cm3),high specific surface areas(134.48 m2/g),hierarchical pore structures(2?120 nm).The formation mechanism is mainly to form a gel network in ultradilute solution by hydrosilylation reaction between polymer precursor(Si-H bond)and divinylbenzene(C=C bond).The freeze-drying technology was applied to reduce capillary force and avoid porous structure collapse.The final production is obtained by the conversion of organic to inorganic after pyrolysis.Meanwhile,the species of precursors,the process parameters of gelation and pyrolysis are designed to control the composition and structure of PDCAs.(2)The EWA properties of PDCAs and relative mechanisms:The effects of composition and structure of PDCAs on the EWA properties of PDCAs were studied.The results showed that the composition and crystallinity of PDCAs,the content and graphitization degree of free carbon,the content of carbon dangling bonds,the porous microstructure and the mass ratio of paraffin wax are the main factors to affect the EWA properties of PDCAs.When the precursor is polysilane,the solvent ratio is 90 vol%,the pyrolysis temperature is 1200?,and the mass ratio of PDCA-SiCN to paraffin is 20:80,the minimum value of reflection loss(RL)is-42.01 dB,the effective absorption bandwidth and corresponding thickness are 6.6 GHz and 3.0 mm,respectively.It can be seen that PDCA-SiCN has a strong absorption capacity and a wide effective absorption band.The mechanism of electromagnetic wave dissipation is mainly dielectric loss,conductive loss and multiple reflections.PDCAs have a unique nanostructure,in which the amorphous matrix can provide dielectric loss,the free carbon dispersed uniformly in the amorphous matrix can provide conductivity loss,and the carbon dangling bonds act as defects to become the dissipative center of electromagnetic wave.Besides,the interfacial polarization between the amorphous matrix and free carbon is also helpful to the EWA properties.Due to the special 3D networks structure,PDCAs can not only cause multiple reflections and scattering,prolong the propagation path of electromagnetic waves and dissipate more electromagnetic energy,but also can adjust electromagnetic parameters and improve the impedance match,which provide necessary conditions for the enhancement of electromagnetic dissipation and benefit to widen the absorption bandwidth.(3)The preparation and EWA properties of SiCN ceramic aerogel composites:In order to further improve the EWA properties,we proposed the design of SiCN composite ceramic aerogels which introduce a variety of components(Fe?Co)and the multilevel structure(zero-dimensional magnetic nanoparticles,one-dimensional SiC nanostructures)based on PDCA-SiCN.The composite of multi compositon is formed by adding ferrocene,cobalt acetate and pyrolyzing to produce magnetic phase,and the composite of multilevel structure is formed by the one-dimensional SiC nanostructure synthesized via microwave catalysis at low temperature.The influence of preparation process on the composition,structure and EWA properties were studied.The results showed that the crystalline phase and crystallinity of SiCN ceramic aerogel composites,the content and graphitization degree of free carbon,the specific surface area,pore structures and the mass ratio of paraffin wax are the main factors to affect the EWA properties of SiCN ceramic aerogel composites.When the thickness is 2.0 mm and the mass ratio of paraffin is 12:88,the minimum RL of PDCA-SiC/SiCN after microwave post-treatment at 600? is-23.89 dB,and the effective absorption bandwidth is 5.2 GHz.When the thickness is 2.5 mm and the mass ratio of paraffin is 20:80,the minimum RL of PDCA-Fe/SiCN with 20 wt%ferrocene is-42.09 dB,and the effective absorption bandwidth is 4.8 GHz.In the-range of 2-40 GHz,when the thickness is 1.6 mm and the mass ratio of praffin is 20:80,the minimum RL of PDCA-Co/SiCN with 0.5 mol/L cobalt acetate concentration is-35.29 dB,and the effective absorption bandwidth is 10.9 GHz.It can be found that SiCN ceramic aerogel composites have strong absorption ability,wide effective absorption band,and the thin thickness.The main reason is that SiCN ceramic aerogels provide suitable dielectric loss(interface polarization,dipole polarization)and conductivity loss,and provide multiple reflections to enhance the electromagnetic dissipation.The composited magnetic phase Fe3Si or Co provides extra resonance loss.The composited one-dimensional SiC whisker/nanofiber acts as a "micro-antenna",which can not only improve the impedance matching,but also enhance the conductivity loss.Furthermore,SiCN ceramic aerogel composites have better EWA properties.(4)The mechanism and model of EWA:According to the special composition and structural characteristics of PDCAs and its composites,a possible model of EWA mechanism is proposed:A kind of composites composed of multi composition and multilevel structure are formed by the free carbon as the main absorbent surrounding the amorphous SiCN matrix with 3D networks structure and adding zero-dimensional magnetic particles and one-dimensional SiC nanostructures.The composition is regulated by the content and order degree of free carbon,the crystallinity of SiCN matrix and introduction of magnetic phase.The design of multilevel structure is realized by formation of zero-dimensional magnetic nanoparticles,one-dimensional SiC nanostructures and PDCAs with 3D networks structure.They provide various ways of electromagnetic wave dissipation to adjust the impedance matching and EWA properties,which obtain a novel EWA material with characteristics of "wide,strong,light,thin". |
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