Synthesis And Property Studies On Rare Earth Substituted W-Type Hexaferrite And PANI/Hexaferrite Composites

W-type hexaferrite is a kind of magnetic material with high saturationmagnetization, which is usually used as high-frequency microwave absorbingmaterial due to its large reflection loss at high frequency. However, the application ofthe hexaferrite microwave absorber was restricted by some disadvantages such asheavy weight, low absorption and narrow absorb bandwidth. In order to enlarge theapplication field of the W-type hexaferrite microwave absorber, it is necessary toadjust and optimize the performance of the hexaferrite. This paper prepared a varietyof rare earth ions substituted W-type hexaferrites, then, take the conductivepolyaniline as shell and the substituted hexaferrite as core, synthesized a series ofpolyaniline-W-type hexaferrite composites. In hope that the strong spin-orbitcoupling effect of the rare earth ions and the excellent dielectric properties of theconductive polyaniline can improve the static magnetic and electromagneticproperties and microwave absorption performance. Based on the internal interactionbetween the microstructure and electromagnetic properties, the general influence ofthe rare earth ions on W-type hexaferrite was discussed in detail. Thecharacterization and performance of the prepared samples were investigated byX-ray diffraction spectrometer, vibrating sample magnetometer, X-ray photoelectronspectroscopy and microwave vector network analyzer etc. The main conclusions areas following:1．The light rare earth ions substituted W-type hexaferriteBa0.9RE0.1Co2Fe16O27(RE=La3+，Nd3+，Sm3+) were synthesized via solid statereaction method. The research results of XRD patterns and microstructureparameters show that the replacement of Ba2+by light rare earth ions reduces the crystal cell parameters. And the crystal cell parameter a, c and crystal cell volumeVcellof the substituted hexaferrite decrease regularly with the decrease of ionicradius of rare earth, the porosity of the substituted hexaferrite increases with thedecrease of ionic radius of rare earth. The static magnetic properties investigationsshow that the light rare earth ions substitution can improve the magnetic propertiesof W-type hexaferrite, and the saturation magnetization Msshows a change regularlywith the increase of rare earth ionic radius: SmBF <NdBF <LaBF. The complexpermittivity and permeability of the light rare earth ions substituted W-typehexaferrite was measured by using the microwave vector network analyzer in thefrequency range of0.5-18GHz. The electromagnetic properties and microwaveabsorption performance were discussed in detail based on the polarizationmechanism and the interaction of the ionic magnetic moment, porosity andsaturation magnetization. Compared with un-substituted hexaferrite, the real (’) andimaginary part (”) of complex permittivity of the substitutes hexaferrite increase,the real part (μ’) of complex permeability of the substituted hexaferrite decreasesslightly and the imaginary part (μ”) increases. The increase order of (’),(”) and (μ”)with La3+，Nd3+，Sm3+ionic radius was: SmBF <NdBF <LaBF. The microwaveabsorption performance of substituted hexaferrite LaBF, NdBF and SmBF wasenhanced due to the improvement of electromagnetic properties. The loss tangenttan increases1.31,1.24and1.12times compared with un-substituted hexaferrite.2．The W-type hexaferrite Ba1-xLaxCo2Fe16O27(x=0.00,0.05,0.01,0.05,0.20)was prepared by sol-gel self-propagating combustion method with different La3+content. The optimum heat treatment condition of pure W-type hexaferrite phase was1200℃/1hby this method, which reduce the sintering temperature and shorten thesintering time effectively. The crystal cell parameters and crystal cell volumedecrease and the porosity increase with the increasing of La3+content. The saturationmagnetization increases at first and then decreases, while the coercivity decreases atfirst and then increases. The X-ray photoelectron spectroscopy was used to measurethe valence states of Fe and co exists in La-substituted and un-substituted hexaferrite.The results show that the valence states of Fe and Co in un-substituted hexaferrite were Fe (III) and Co (II) respectively, while its change to Fe (III)+Fe (II) and Co (II)in La-substituted hexaferrite respectively. It was proved via statistic data that thereplacement of Ba2+by La3+can change a part of Fe3+to Fe2+.3．The polyaniline/hexaferrite composite PANI/Ba0.85La0.15Co2Fe16O27wassynthesized with different polyaniline content. The measurement result of vibratingsample magnetometer show that the saturation magnetization of the compositesdecreases with the increasing of polyaniline content. The complex permittivity of thecomposites increase regularly with the increasing of polyaniline content. Themaximum reflection loss of the composite was-14.95dB at the frequency about9.7GHz, when the PANI:Ba0.85La0.15Co2Fe16O27mass ratio of is7:5, which was morethan double that of pure hexaferrite Ba0.85La0.15Co2Fe16O27(-7.1dB). The discussionon the mechanism shows that the orientation polarization and interface polarizationof the composite were enhanced by the coating of polyaniline effectively, and thedielectric loss part of the composite was improved, consequently, the microwaveabsorbing performance of the composite was enhanced.4．The W-type hexaferrite Ba0.85RE0.15Co2Fe16O27(RE=Gd3+, Tb3+, Ho3+)with different heavy rare earth ions substituted were synthesized via sol-gelself-propagating combustion method. The microstructure parameters and staticmagnetic properties of the synthesized sample was investigated based on thedifference of ionic radius and ionic magnetic moment of rare earth ions. The resultsshows that the crystal cell parameter a, c and crystal cell volume Vcellincrease withthe decreasing of ionic radius of rare earth, and the saturation magnetizationincreases with the decreasing of ionic radius of rare earth. Furthermore, the Gd3+,Tb3+and Ho3+substituted hexaferrites Ba0.85RE0.15Co2Fe16O27were combined withconductive polyaniline under the equal mass ratio to prepared thePANI/Ba0.85RE0.15Co2Fe16O27composite material. The complex permittivity andpermeability of the composite were investigated by microwave vector networkanalyzer. And the results showed that the orientation polarization, interfacepolarization and the impedance matching of the composite were improved by thecoating of polyaniline, which was better for strengthening the dielectric loss part of the composite. Consequently, the microwave absorbing performance of thecomposite was enhanced. The maximum reflection loss ofPANI/Ba0.85Ho0.15Co2Fe16O27composite at about9GHz was-15.1dB, which wasexcellent in the three composite materials.