| Microwave absorbing materials have been used widely in civil and military fields,and they have become the researching hotspot in anti electromagnetic radiation and military stealth equipments in many countries.In most of absorbing materials,the magnetic metal alloys and ferrites are the main components because of its good absorption properties and low cost.However,the defect of high density is restricted their usefulness in applications where lightweight materials are required such as advanced microwave absorbents.In addition,conductive polymer with low density,diverse structures,excellent environmental stability and the specific conductivity has been also studied widely,but a single component of conductive polymer is not ideal because of its poor conductivity and narrow absorption bands.So,absorbing materials with light quality,excellent absorption performance and wide frequency band have become hot topics among researchers.Fly ash cenospheres(FACs)are primarily a by-product from power generation plants and have posed disposal and environmental problems for human beings.Nevertheless,FACs possess favorable characteristics with hollow spherical morphology,lightweight mass and high chemical stability,and hence are marked as substrates on which to fabricate lightweight and functional composites.In this thesis,the FACs were employed as substrates,the composite absorbers of FACs/Ni-Co(Fe)-P and FACs/barium ferrite with core-shell hollow structure were obtained by a modified electroless plating and a sol-gel self-propagating combustion technology,respectively,and the lightweight absorbers of FACs/barium ferrite/PAN1 with core-shell-shell hollow structure were also acquired by sol-gel self-propagating combustion method combining in-situ doping polymerization deposition technique.The morphologies,constituents,crystal structure,magnetic and microwave absorption properties of the as-prepared samples were characterized comprehensively.In addition,the mechanism and excellent properties of the microwave absorbing materials were also discussed.First of all,we employed a low-cost three-step pretreatment procedure on FACs,including an acid pickling step with nitric acid(10 mL/L),a coupling process with y-aminopropyltriethoxy silane(KH550,APS)and an activating step using silver nitrate as the activator.The results suggested that a pretreatment of FACs by APS was introduced prior to electroless plating,which improved the continuity and uniformity of the alloy films remarkably.Furthermore,when the FACs modified by APS were immersed into activation solution which consists of silver nitrate,ammonia and tartrate salt,the Ag catalyst particles were deposited on the surface of FACs directly and as catalytic activity centers for subsequent plating reaction.The magnetic composite material with core-shell structure of FACs/Ni-Co(Fe)-P were obtained by depositing Ni-Co(Fe)-P alloy films on FACs through modified electroless plating.The effects of preparation process,compostion change on the crystal structure,magnetic and microwave absorption properties of these composite materials have been studied.On the basis of the plating bath experiments,mole ratios of Ni2+/Co2+ of 1,NaH2PO2·H2O concentration of 0.20 mol/L,pH of 9.0 and temperature of 85? seem to be optimal for preparing electroless Ni-Co-P alloy deposits,and the density of FACs/Ni-Co-P was about 1.44 g/cm3,while the FACs/Ni-Fe-P was 1.38 g/cm3.The variety of alloy constituents Ni,Co(Fe),P and heat treatment temperature of FACs/Ni-Co(Fe)-P composite had obvious effects on coating structure,magnetic and absorbing performances.The as-plated Ni-Co(Fe)-P alloy coating was amorphous nature when the n(Ni2+):n(Co2+ or Fe2+)?1,while the crystal behavior of the composite can be improved by increasing sintering temperature.The value of Ms and Mr of the FACs/Ni-Co-P composite decreased with the increase of mole ration of Ni2+/Co2+,respectively,but Hc was increased.To FACs/Ni-Fe-P composite materials,the value of Ms,Mr and He decreased with the increase of mole ration of Ni2+ and Fe2+.The magnetic properties of these composite got better as calcining temperature increased.Moreover,the absorption properties of FACs/Ni-Co-P composite was enhanced with the increase of Ni in materials,the minimum reflection loss reached-20.64 dB,and the absorption bandwidth was 3.06 GHz at-10 dB when the thickness of absorption layer was 1.5 mm.While the minimum reflection loss of FACs/Ni-Fe-P composite materials reached-31.28 dB,and the absorption bandwidth was 1.9 GHz at-20 dB when the matching thickness of absorption layer was 1.5 mm.Also,the microwave absorption properties were promoted remarkably by heat treatment.The magnetic composite materials with core-shell structure of barium ferrite coated on FACs were prepared by sol-gel self-propagating combustion method.The crystal structure,magnetic and microwave absorption properties of composite materials as a function of mass rations of barium ferrite and FACs have been studied.Continuous and uniform coatings of barium ferrite were found on the surfaces of the FACs,and the thickness was between 5 nm and 15 nm.The size of barium ferrite particles was between 30 nm and 60 nm and belonged to single domain.The density of composite material was 3.11 g/cm3 when the mass rations of barium ferrite and FACs of 4:6.The XRD results showed that the composites were composed of barium ferrite,hematite,minor mullite and quartz.The magnetic properties of composite material enhanced with increasing the mass rations of barium ferrite and FACs,and the maximum values of Ms,Mr and He reached 43.6 emu/g,30.5 emu/g and 5239 Oe,respectively.Furthermore,the microwave absorption properties of barium FACs/ferrite composites were increased with increasing the mass rations of barium.ferrite and FACs.The minimum reflection loss decreased from-24.6 dB to-29.2 dB with the decrease of the mass ratio of barium ferrite and FACs,while the absorption bandwidth at-10 dB increased from 2.7 GHz to 4.5 GHz.The composite material with excellent absorption properties was attributed to the electromagnetic complementarity between barium ferrite and FACs,the special core-shell structure of composites and hollow structure of FACs,interfacial effects,and so forth.In order to improve the microwave absorption properties of composite powde,the composite material with core-shell-shell structure of FACs/barium ferrite/polyaniline were synthetized by sol-gel self-propagating combustion method combining in-situ doping polymerization deposition technique.The effects of different mass PANI on the crystal structure,magnetic and microwave absorption properties of composite materials have been investigated.The results showed that the diffraction peak intensity of PANI enhanced with the increase of PANI content,but the peak intensity of barium ferrite was decreased.The magnetic properties of composite materials decreased slowly with increasing the PANI content,and the maximum values of Ms,Mr and He reached 16.0 emu/g,10.5 emu/g and 4726 Oe,respectively.Furthermore,the absorption properties of composite materials were raised with the increase of PANI content,meanwhile,the matching thickness decreased.The samples possess the favorable absorption properties when the PANI content was 5%and 10%,the minimum reflection loss reached-37.55 dB and-42.94 dB,and the absorption bandwidth was 4.6 GHz and 5.8 GHz at-15 dB,respectively,the matching thickness of two samples were 1.2 mm.The enhancement microwave absorption property was related to the increasing magnetic and dielectric loss because of PANI,having better impedance match among different layers,prolonging the communication process between multilayer and hollow structure,coupling effects,and so on. |
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