Electromagnetic And Microwave Absorption Properties Of La_0.7Sr_0.3MnO₃and Modified La_0.7Sr_0.3MnO₃

Microwave absorption materials are kinds of functional materials, which canabsorb or dissipate electromagnetic waves by converting electromagnetic energy intoheat or destructive interference, in this way, the intensity of target echo will beweakened. With the burgeoning development of radar, wireless communication system,local network, personal digital assistant and other communication devices,electromagnetic pollution and electromagnetic interference are becoming more andmore serious. Microwave absorption materials have been the key point in the militaryand civilian technology. Lanthanum manganese oxides have sparked a surge of interestfor their superior electronic and magnetic performance. This thesis mainly focuses onthe electromagnetic and microwave absorption performance of La0.7Sr0.3MnO3andmodified La0.7Sr0.3MnO3.With the combination of computational science and experiment, a lot of humanresources, materials and financial resources can be saved. Fe3N and Ni coated hollowspheres were synthesized by the molten-salt method and chemical plating method,respectively. Appropriate carbonyl iron powders were purchased. The absorber/epoxycomposites were prepared for the measurement of electromagnetic parameters andreflectivity. It is found that eddy current loss, domain wall resonance and naturalresonance contributed to magnetic loss. Based on the database of electromagneticparameters, microwave absorption performance has been calculated according touniform transmission line theory and NRW reflection/transmission method by Matlaband HFSS, respectively. It is obtained that the experiment data were in good agreementwith the theoretical results, which can predict microwave absorption properties ofmaterials.La0.7Sr0.3MnO3powders have been synthesized by solid state reaction method. It isfound that the pure perovskite structure could be formed when the calcinationstemperature was equal to or higher than1100℃. Electromagnetic parameters in therange of8.2~18GHz were measured. It is found that the variation of with sinteringtemperature was determined by space charge polarization and interfacial polarization, was induced by conductance loss, delectric relaxation loss and interfacial polarizationloss. Magnetic loss of La0.7Sr0.3MnO3was attributed to eddy current loss, naturalresonance and exchange resonance loss. Microwave absorption performance ofLa0.7Sr0.3MnO3with a thickness of2mm was calculated using Matlab. It is found thatLa0.7Sr0.3MnO3calcined at1200℃exhibited the optimal microwave absorption properties, the maximum reflection loss was-17.650dB at15.872GHz, and thebandwidth below-8dB was1.770GHz. Then the corresponding composite used for themeasurement of reflection loss was fabricated. It is found that the experiment data werein good consistent with the theoretical results. The maximum reflection loss was-14.730dB located at15.120GHz, and the bandwidth below-8dB was2.240GHz.La0.7Sr0.3MnO3powders were prepared by solid state reaction method aftercalcinated at1200℃for4h. The precursor samples were obtained by milling rawmaterials for different time (0h,1h,2h,3h,6h,9h, and12h). La0.7Sr0.3MnO3sampleshave been labeled as S0, S1, S2, S3, S6, S9and S12. It is found that the particle size ofraw materials became small with the increase of grinding time, and the crystalline sizeof the samples were enlarged with the grinding time increase from2h to12h. Dielectricloss originated from conductance loss, dielectric relaxation loss and interfacialpolarization loss, the multiple resonance phenomena could be attributed to transitionmetal ions, heterostructure, space charge polarization and interfacial polarization. It isobtained that magnetic loss of S2, S3, S6, S9and S12originated from eddy current loss,natural resonance and exchange resonance, magnetic loss of S1was attributed to eddycurrent loss and natural resonance. After analysis of impedance matching performanceand attenuation constant, microwave absorption performance of La0.7Sr0.3MnO3wascalculated using Matlab. It is found that S3exhibited the optimal microwave absorptionproperties with a thickness of2.5mm. The maximum reflection loss was-29.170dB at17.320GHz, and the-8dB absorbing bandwidth was4.930GHz. Meanwhile, thecorresponding composite used for the measurement of reflection loss was fabricated. Itis found that the experiment data were in good coincident to the theoretical results. Themaximum reflection loss was-26.043dB at16.400GHz, and the-8dB absorbingbandwidth was5.560GHz.The pure perovskite structure La0.7Sr0.3Mn1-χTMχO3(TM=Fe, Co or Ni, χ=0,0.05,0.1,0.15,0.2,0.25) powders were successfully synthesized after calcinated at1200℃by solid stated reaction method, with raw materials grinding for3h. With the increase ofdoping concentration, crystalline size decreased, and La0.7Sr0.3Mn0.8TM0.2O3(TM=Fe,Co or Ni) had the smallest crystalline size. The changes of with doping concentrationcan be attributed to ionic polarization, electronic displacement polarization, electricdipole polarization, space charge polarization and interfacial polarization. Conductanceloss, dielectric relaxation loss, interfacial polarization loss and resonance losscontributed to dielectric loss. Magnetic loss was attributed to eddy current loss, naturalresonance and exchange resonance. The dielectric loss, magnetic loss, impedance matching and attenuation performance of La0.7Sr0.3Mn1-χTMχO3(TM=Fe, Co or Ni, χ=0,0.05,0.1,0.15,0.2,0.25) were analyzed. Microwave absorption properties ofLa0.7Sr0.3Mn1-χTMχO3(TM=Fe, Co or Ni, χ=0,0.05,0.1,0.15,0.2,0.25) werecalculated using Matlab. It is found that the incorporation of transition metal greatlyimprove the microwave absorption performance of La0.7Sr0.3MnO3. The maximumreflection loss of15at.%Ni doped LSM was-16.8113dB at11.896GHz, and the-8dBabsorbing bandwidth was2.822GHz.5at.%,20at.%and25at.%Co-doped LSMexhibited good microwave absorption properties. The maximum reflection loss of25at.%Co-doped LSM was-17.779dB at12.736GHz, and the-8dB absorbingbandwidth was5.516GHz.20at.%Fe-LSM exhibited the optimal microwave absorptionperformance, the maximum reflection loss was-27.665dB at10.972GHz, and the-8dBabsorbing bandwidth was5.012GHz. Meanwhile, the20at.%Fe-LSM/epoxy composite,used for the measurement of reflection loss, was fabricated. The experiment data werein good agreement with the theoretical results. The maximum reflection loss was-25.986dB at11.120GHz, and the-8dB absorbing bandwidth was5.760GHz.