【作者】 张淑媛；

【导师】 曹全喜；

【作者基本信息】 西安电子科技大学 ， 材料物理与化学， 2014， 博士

【摘要】 微波隐身材料是能够有效吸收入射电磁波，并将电磁能转化为热能或使电磁波发生散射，从而使目标的回波强度显著减弱的一类电磁功能材料。随着雷达、无线通讯系统、局域网络、个人数字辅助设施及其它的通讯工具的飞速发展，电磁辐射污染及电磁兼容问题日益突出。微波隐身材料成为国防工业与民用技术领域的关键材料。镧系水锰矿由于其优异的电学和磁学特性受到越来越多的关注，本文主要研究La0.7Sr0.3MnO3及改性La0.7Sr0.3MnO3的电磁特性及微波吸收特性。采用理论计算指导实验的研究方式能够节省大量的人力、物力及财力。首先分别采用熔融盐法及化学镀方法制备Fe3N及镀镍空心微球粉体，并购买一定量的羰基铁粉，而后制成电磁参数及反射率测试用吸波剂/环氧树脂复合涂层。以电磁参数为基础，根据均匀传输线理论在Matlab中编程，或依据NRW反射/传输测试方法在HFSS中建立相应的三维结构模型，计算仿真材料的微波反射损耗特性。结果表明，理论计算结果与实验结果基本一致，能够达到性能预测的目的。采用固相反应法在不同合成温度下制备La0.7Sr0.3MnO3吸波粉体，结果表明，当合成温度等于或高于1100℃时可以生成纯净的钙钛矿结构。采用矢量网络分析仪测试La0.7Sr0.3MnO3/环氧树脂复合涂层在8.2~18GHz频段范围内的电磁参数。结果表明，介电常数实部随合成温度的变化由空间电荷极化及界面极化共同决定，而虚部由电导损耗、介电弛豫损耗及界面极化损耗共同决定。磁损耗源于涡流损耗、自然共振及交换共振。根据均匀传输线理论及厚度匹配公式，利用Matlab计算材料在厚度为2mm时的微波吸收特性，结果表明，合成温度为1200℃的样品具有最优的微波吸收特性，位于15.872GHz处的最强吸收峰为-17.650dB，且低于-8dB的带宽为1.770GHz。实测反射率与理论结果基本一致，位于15.120GHz的最强吸收峰为-14.730dB，且低于-8dB的带宽为2.240GHz。采用固相反应法，将原料球磨不同时间，在相同合成温度(1200℃)下制备La0.7Sr0.3MnO3样品。随原料球磨时间的增加，前驱粉体的粒径逐渐减小，且在相同合成温度的情况下，随着前驱样品球磨时间的增加由2h增至12h，所制La0.7Sr0.3MnO3样品的晶粒尺寸逐渐增大。La0.7Sr0.3MnO3样品的介电损耗产生于电导损耗、介电弛豫损耗及界面极化损耗，且多重共振现象归功于过渡金属离子、异质结构、空间电荷极化及界面极化。原料球磨1h所得样品的磁损耗可归功于涡流损耗及自然共振，剩余样品的磁损耗主要归功于涡流损耗、自然共振及交换共振。以电磁参数为基础，分析阻抗匹配特性及损耗系数，利用Matlab计算La0.7Sr0.3MnO3样品的微波反射损耗特性。结果表明，当厚度为2.5mm时，原料球磨时间为3h的La0.7Sr0.3MnO3样品具有最优的微波反射损耗特性，其共振峰位于17.320GHz，最大反射损耗为-29.170dB，且低于-8dB的频宽为4.930GHz，此外，实测反射率与理论计算结果基本一致：16.400GHz处的最大微波反射损耗为-26.043dB，且低于-8dB的吸收带宽为5.560GHz。采用固相反应法，将原料球磨3h并在1200℃烧结，最终合成了纯净钙钛矿结构La0.7Sr0.3Mn1-χTMχO3(TM=Fe、Co或Ni，χ=0，0.05，0.1，0.15，0.2及0.25)粉体。晶粒尺寸随着掺杂浓度的升高逐渐减小，La0.7Sr0.3Mn0.8TM0.2O3(TM=Fe、Co或Ni)具有最小的晶粒尺寸。相对介电常数实部随掺杂浓度的变化主要归功于电子位移极化、离子位移极化、偶极子转向极化、空间电荷极化及界面极化；介电损耗主要包含电导损耗、介电弛豫损耗、界面极化损耗及共振损耗。磁损耗主要归功于涡流损耗、自然共振与交换共振。根据均匀传输线理论计算La0.7Sr0.3Mn1-χTMχO3(TM=Fe、Co或Ni，χ=0，0.05，0.1，0.15，0.2及0.25)样品厚度为2mm时的微波吸收特性。结果表明，过渡金属的引入改善了La0.7Sr0.3MnO3样品的微波吸收特性，当掺杂浓度为15at.%时，Ni-LSM呈现最宽的吸收带宽，低于-8dB的带宽为2.822GHz，且位于11.896GHz处的最强吸收峰为-16.811dB；5at.%Co-LSM、20at.%Co-LSM及25at.%Co-LSM具有宽的低于-8dB的吸收带宽，且当掺杂浓度为25at.%时，12.736GHz处的最强吸收峰为-17.779dB，吸收带宽最大为5.516GHz；20at.%Fe-LSM呈现最优的微波吸收特性，位于10.972GHz处的最大反射损耗为-27.665dB，且低于-8dB的带宽达5.012GHz。通过对比发现20at.%Fe-LSM呈现最优的微波吸收特性，包括最强的微波反射损耗及宽的吸收带宽，采用矢量网络分析仪测试实际样品的微波吸收特性，结果表明，实测反射率与理论计算结果基本一致，位于11.120GHz处的最强吸收峰为-25.986dB，且低于-8dB的带宽为5.760GHz。更多还原

【Abstract】 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.更多还原