| The development of modern society is bringing increasing problems of electromag-netic radiation and pollution. Microwave absorbing materials (MAM) dissipating inci-dent microwave energy, are widely used in electromagnetic interference, electromag-netic compatibility, information security and human body protection applications. Highabsorption, broad absorption bandwidth and lightweight are the requirement of high per-formance MAM. Most magnetic absorbing materials have the disadvantage of relativelyhigh density and high fller loading, which would restrict their usage. Carbon fllers haveexcellent electrical conductivity and dielectric properties, low density, as well as corro-sion resistance, are getting more and more attention in the research of MAM. Because ofitsadvantagesofsoftness, cohesivenessandeasytotailorintocertainshape, rubberbasedMAM can be used as the absorbing layer of devices with complex structures or inner sur-facesofcavities. Inthisthesis,hydrogenatedacrylonitrile–butadienerubber(HNBR)wasblended with multi-walled carbon nanotubes (MWCNT) and rare earth (RE) compoundsto prepare high performance MAM. Dependence of the type of carbon fllers, rare earthcompounds and their combinations on the microwave absorbing properties was studied.The effect of phase morphology in polymer matrix and fller dispersion on enhancing themicrowave absorbing properties was also investigated.Conductivecarbonblack(CCB),carbonfber(CF)andMWCNTwereblendedwithHNBR to prepare MAM, and their electromagnetic characteristics and microwave ab-sorbing properties were studied. HNBR/MWCNT composites had higher electrical con-ductivity, real part and imaginary part of complex permittivity than HNBR/CCB andHNBR/CF composites at a given fller loading. The relationship between permittivity of the composites and their fller loading can be described using the Maxwell–Garnettmodel. HNBR/MWCNTcompositeshadthebestmicrowaveabsorbingpropertiesamongthethreekindsofcomposites,HNBR/MWCNT(100/10)compositehadthelowestrefec-tion loss of49:3dB, while HNBR/CCB (100/15) and HNBR/CF (100/30) compositeshad the refection losses of13:1dB and7:1dB, respectively.MWCNT and CF were added into HNBR/carbonyl iron powder (CIP) compositesto prepare MAM. MWCNT and CF can signifcantly increase the complex permittivityand dielectric loss of HNBR/CIP composites. The HNBR/CIP/carbon fller compositeshad lower refection loss, lower matching frequency and broader absorption bandwidththan the HNBR/CIP composites. HNBR/CIP/reduced graphene oxide (rGO) compositeswere prepared by in situ reducing graphene oxide using hydrazine, and rGO was exfoli-ated in HNBR matrix. HNBR/CIP/rGO composite had higher imaginary part of complexpermittivity and dielectric loss than that of HNBR/CIP composite, and the microwaveabsorbing properties of HNBR/CIP composite could be improved.Rare earth modifed HNBR composites were prepared by in situ reactions betweenlanthanum oxide (La2O3) and acrylic acid (AA). The in situ reactions were traced usinginfraredspectra, X-raydiffractionanalysisandscanningelectronmicroscopy. La2O3wasconverted to lanthanum acrylate (La(AA)3) after the in situ reactions, and became nano-size particles with narrower size distribution after vulcanization. The La(AA)3couldreinforce HNBR remarkably and increase the glass transition temperature of HNBR aswell. Different kinds of rare earth acrylates modifed HNBR/MWCNT composites wereprepared,andtheirdielectric,microwaveabsorbingandmechanicalpropertieswerestud-ied. The acrylates of light rare earth elements (Nd, Sm, Eu) formed large aggregates inHNBR matrix, while the acrylates of heavy rare earth elements (Gd, Tb, Dy, Ho, Er,Tm, Yb, Lu) were dispersed in HNBR matrix as fne particles. The acrylates of heavyrare earth elements could increase the complex permittivity, dielectric loss and dielectricrelaxation time, improve the microwave absorbing properties of HNBR/MWCNT com-posites, while the acrylates of light rare earth elements had little effect on the microwaveabsorbing properties of HNBR/MWCNT composite.HNBR/lithium acrylate (LiAA) composites were prepare using in situ reactions be-tween lithium hydroxide and AA. The in situ prepared LiAA was uniformly dispersedin HNBR matrix, and signifcantly increased the tensile strength and modulus of HNBR.The electrical conductivity of HNBR/LiAA increased by fve orders of magnitude af- ter water absorption, and increased even further by adding lanthanum chloride (LaCl3)or cerium chloride (CeCl3). The HNBR/LiAA/RECl3composites after water absorptionhad higher complex permittivity and dielectric loss, longer dielectric relaxation time, aswell as improved microwave absorbing properties than the composites without water ab-sorption.Immiscible HNBR and ethylene–propylene rubber (EPR) blends were used as thematrix for MAM, and MWCNT and CIP were selectively dispersed in HNBR and EPR,respectively. Composites with different fller distribution could be obtained by changingHNBR/EPR ratio. The infuence of phase morphology on the electromagnetic charac-teristics, microwave absorbing and mechanical properties of the composites was investi-gated. WhileHNBRformedcontinuousphase,theHNBR/EPR/MWCNT/CIPcompositehad the highest imaginary part of complex permittivity, dielectric loss, and the best mi-crowave absorbing properties. It had the refection loss of56:8dB, lower than thatof HNBR/MWCNT/CIP composite with HNBR as the only matrix. While EPR formedcontinuousphase, theHNBR/EPR/MWCNT/CIPcompositehadwidestabsorptionband-width of5.5GHz.Polyvinyl chloride (PVC) and crosslinked acrylonitrile–butadiene rubber powder(P-NBR) were used as MAM matrix to prepare PVC/MWCNT composites with differentMWCNTloadinganddispersionstate. TheeffectofMWCNTdispersionontheelectricalconductivity, dielectric, microwave absorbing properties of MWCNT composites was in-vestigated. WhentheMWCNTloadingwashigherthan4phr,thePVC/P-NBR/MWCNTcomposites with nonuniformly dispersed MWCNT, had higher electrical conductivity,imaginary part of complex permittivity, dielectric loss, and better microwave absorbingproperties than PVC/MWCNT composites. |
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