Microstructure Design and Electrotromagnetic Property Optimization of Polymer Derived SiOC Ceramics

Author:Duan Wen Yan

Supervisor:yin xiao zuo

Database:Doctor

Degree Year:2017

Download:169

Pages:136

Size:11657K

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Polymer derived SiOC ceramics(PDCs-SiOC)possess excellent mechanical properties,high temperature resistance,oxidation resistance,corrosion resistance and the design of their permittivity,which can be a new structural-functional integration material.However,PDCs-SiOC usually exhibit electrical conductivities with values between semiconductors and insulators.The absorption property of PDCs-SiOC was poor because the permittivity of PDCs-SiOC deviated from the optimizing permittivity.Therefore,in order to obtain PDCs-SiOC with excellent absorption properties and broad EAB,it is necessary to design the microstructure and phase composition of PDCs-SiOC through physical/chemical modification.At present,the researches on SiOC ceramics with absorption properties through physical modification approach(adding fillers in PSO)/chemical modification approach(design of the molecular precursor)were not reported.In the paper,the target complex permittivity for the wide band absorption materials are obtained by the theoretical calculation.A/B type and A/B/C type electromagnetic absorption materials are fabricated by optimizing the content,crystallisation degree and microstructure of absorbers through physical modification approach and chemical modification approach.The major contents and results are summarized as follows:(1)Based on the transmission-line theory and metal backpanel model,the target complex permittivity for the wide band absorption materials are obtained by the theoretical calculation.These target values can provide the guidance for designing the microstructure of absorption materials.The main results are as follows:The frequency dispersion effect is needed to obtain the wider EAB when the thickness is below 3mm.When the thickness increases to 4mm and4.5mm,EAB=4.2GHz both can be obtained whenε′=3.8,ε″=2.65-3.15 andε′=3.4,ε″=1.85-2.35.By designing the microstructure of A/B type and A/B/C type electromagnetic absorption materials,reveal the EM absorption mechanism.Based on this,the microstructure of A/B type and A/B/C type PDCs-SiOC are designed.(2)The relation between the content of absorbers and electromagnetic absorption properties of SiC/Si3N4 and SiOC/Si3N4 ceramics are investigated.The main results are as follows:The complex permittivity of SiC/Si3N4 increased with the increase of SiC nanowires(NWs)due to the formation of SiC NWs which offered the mobile charge carriers channels to move and interact with the electromagnetic field over the short range.SiC/Si3N4 ceramics are A/B type electromagnetic absorption materials.When the content of SiC is 10.5wt.%,the RC of composite ceramics(the thickness is 2.3mm)reached to-57dB.SiOC/Si3N4 ceramics were fabeicated by filled with PDCs-SiOC(containing SiC NWs and free carbon)into the pores of Si3N4.SiOC/Si3N4 ceramics show excellent absorption properties when the content of SiOC is low.SiOC/Si3N4 ceramics became electromagnetic shielding materials when the content of SiOC is high.Meanwhile,the formation of SiC NWs can dramatically increase the flexural stress of SiOC/Si3N4 ceramics.(3)A/B/C type CNTs/SiC/SiOC and C/n-SiC/SiOC are prepared by physical modification approach.The main results are as follows:After adding CNTs,unstable factors such as dislocation or grain boundary are present which can decrease the Gibbs free energy,eventually lead to SiC separate from amorphous SiOC ceramics at lower temperature.Fe played the role of catalyst for the formation of SiC NWs.SiC NWs could play the role of bridging two carbon clusters,filling the defects in conductive network.This novel structure is especially benefit for EM absorption.After adding n-SiC,palingenetic SiC nanograins with an average grain diameter smaller than 10 nm and nanosized free carbon were gradually separated from amorphous SiOC phase.SiC and carbon in n-SiC/SiOC ceramic can both lead to electronic dipole polarization.The various interfaces among n-SiC,in-situ formed SiC nanograins,nanosized carbon and amorphous SiOC phase can give rise to interfacial polarization.The electronic dipole polarization and interfacial polarization eventually enhanced the absorption properties.(4)TiC/SiC/SiOC ceramics are derived by tetrabutyl titanate modified PSO.The effect of content of Ti on microstructure evolutions and electromagnetic absorption properties of TiC/SiC/SiOC ceramics are investigated,and electromagnetic absorption properties,and the EM absorption mechanism of which are revealed.The main results are as follows:Firstly,Ti play the role of catalyst and heterogeneous interface,which can decrease the Gibbs free energy,eventually lead to SiC separate from amorphous SiOC ceramics at lower temperature.Secondly,with the increasing content of Ti,TiC can be formed because superfluous Ti can reacte with C.Thirdly,with the increasing content of Ti,the content of TiC increase and the content of SiC correspondingly decrease.The formation of TiC nanocrystalline can promote the more relaxation polarization and dielectric loss in electromagnetic field.The effective absorption bandwidth of TiC/SiC/SiOC ceramics is 4.2GHz,covering the whole X-band.(5)A series of processable HBPSO-VF were synthesized by special chemical modification approach.The effect of chemical modification on microstructure evolutions and electromagnetic absorption properties of C/SiC/SiOC ceramics are investigated.The main results are as follows:This SiOC ceramics show much lower crystallization temperature(decreased from 1450 oC to 1100 oC)and high ceramic yield because of the capability of HBPSO-VF to incorporate metallic iron into the backbone of PSO.When annealed at 1100oC,The minimum reflection coefficient of this nanocrystal-containing ceramic reaches-46 dB,exhibiting a promising prospect as a kind of electromagnetic wave(EMW)absorbing materials.