【作者】 曹晓非；

【导师】 孙康宁；

【作者基本信息】 山东大学 ， 材料学， 2009， 博士

【摘要】 近年来,由于电子电气设备在工业、民用及军事领域中的应用迅速增长,使得由此产生的电磁辐射成为一种环境污染,而0.5-3 GHz低频段内的电磁辐射已被证明会对人体组织器官产生较严重的危害作用。在现有的电磁辐射防护措施中,辐射吸收是一种实用而且有效的方法。因此,对于民用低频吸波材料进行研究和制备具有非常重要的意义。与其他材料相比,尖晶石型铁氧体及六方铁氧体在吸波层厚度、有效吸收频宽等方面具有优势。但由溶胶-凝胶法制备的高质量纳米铁氧体的微波吸收能力大多集中在高频区。有研究表明,采用固相反应法制备的锂锌铁氧体在低频区具有较好的吸波性能,但是固相反应法本身的缺点限制了这种材料的进一步应用。因此,本文利用溶胶凝胶法结合后期煅烧制备了在低频区具有良好吸波性能的微米级锂锌铁氧体。本文首先利用溶胶-凝胶法结合后期煅烧成功制备了微米级纯锂锌铁氧体,并利用X射线衍射、扫描电镜及网络矢量分析仪等手段对其微观结构和吸波性能进行了研究。借助反应动力学和铁磁共振理论,研究了热处理工艺参数对其低频吸波性能的影响,发现最终煅烧温度决定颗粒粒径,不同的升温速率影响后期煅烧过程中锂和锌的实际汽化损失量,而纯锂锌铁氧体的实际颗粒大小和物相组成会对其由铁磁共振引起的低频微波吸收产生影响。以200℃/h升至500℃下煅烧及以240℃/h升至1200℃下煅烧所得的纯锂锌铁氧体均在0.5-3 GHz的频段内具有吸波性能,而后者的吸波效果更好,其在2.8 GHz及10 mm的吸波层厚度下具有最高的反射衰减,可达-25.89 dB。该制备工艺尚未见报道。但这种纯锂锌铁氧体主要在2.5-3 GHz内具有微波吸收能力,其有效吸波频带仍需要向低频区移动。为增强其在0.5-3 GHz内的吸波能力以及拓宽其吸波频宽,本文利用相同工艺参数的溶胶凝胶法结合后期煅烧,对锂锌铁氧体分别进行了镁、铜、镧及铈的不等量掺杂。对掺杂后所得锂锌铁氧体进行微观结构及吸波性能的检测并与纯锂锌铁氧体进行对比。结果发现,掺杂镁、铜、镧及铈会改变锂锌铁氧体晶格中金属离子的分布状态,而掺杂镧和铈还会引起锂锌铁氧体磁晶各向异性的变化。这些变化导致相应的等效磁场发生改变,进而对铁磁共振引起的低频吸波性能产生影响。另外,镁、铜、镧、铈在锂锌铁氧体中均具有最大掺杂量。若实际掺杂量超过这一限定值,所得铁氧体材料中会分别形成MgFe₂O₄,CuFe₂O₄,LaFeO₃和CeO₂。一方面,MgFe₂O₄,CuFe₂O₄和LaFeO₃的产生会引起掺杂锂锌铁氧体生成量的减少。另一方面,MgFe₂O₄,CuFe₂O₄,LaFeO₃的低频吸波能力相对较弱。会使得实际铁氧体材料的低频吸波能力进一步减弱。但当铈的掺杂量超过限定值时,CeO₂的形成并不影响实际所得铁氧体中锂锌铁氧体的数量。值得注意的是,当铈掺杂到一定量时,实际铁氧体中CeO₂的富集以及其与锂锌铁氧体的复合比例将使材料的介电损耗得到增强,进而大幅提高其低频吸波能力。所得铁氧体材料在1.68 GHz及10 mm的吸波层厚度下的反射阻断衰减最高值可达-28.06 dB,并且其有效吸波频带分布在1.25-2.25频段内,带宽可达1 GHz。因此,这种实际制备的铁氧体复合材料在0.5-3 GHz频段内是一种很有实用化前景的吸波材料。根据全貌分析法及复合材料有效电磁参数的计算公式,本文利用MATLAB编写了可对不同频率点处复合吸波材料的反射衰减、吸波层厚度及组分配比间的关系进行表征的计算模型。应用此模型得到一系列有效的锂锌铁氧体/乙炔碳黑复合吸波剂的相关数据。从中取三种组分利用球磨法制备了不同的锂锌铁氧体/乙炔碳黑复合材料,并对其吸波性能进行检测。结果表明,根据上述预分析模型设计并实际制备的复合材料,其低频吸波性能确实优于纯锂锌铁氧体。这表明该预分析模型和预设计方法对于低频吸波材料的制备有所助益。细颗粒吸波剂在聚合物中的快速均匀分散对于涂敷型吸波材料的吸波性能、机械性能和实用化都有着非常重要的影响。为推进吸波涂料的实用化,本文研制了往复射流分散法及相应装置,并对不饱和聚酯树脂中的微米级锂锌铁氧体及纳米碳管进行了分散实验。对分散后前驱体混合物的粘度及由其固化所得复合材料的断口表面的相关检测发现,相对于超声分散法的处理结果,经往复射流分散处理后的锂锌铁氧体及纳米碳管在聚合物基体中均呈现更为均匀的分布状态,且分散处理时间更短。该技术不仅可快速对微米级吸波剂实施均匀分散,而且对纳米吸波剂颗粒也有很好的分散效果。因此,往复射流分散对于聚合物中的细颗粒吸波剂将是一种有效的新型分散技术。更多还原

【Abstract】 In recent years,electromagnetic radiation has become a specific type of environmental pollution,due to the rapid growth in utilization of electrical and electronic devices in industrial,commercial and military applications.And electromagnetic radiation in frequency range of 0.5-3 GHz has been proved to be harmful to human tissues and organs severely.Radiation absorption is quite practical and effective among the present measures of radiation protection. Therefore,it would have great significance to study and prepare some effective low-frequency microwave absorbing materials in civil utilization.As far as thickness and working frequency bandwidth are concerned,spinel ferrites and hexaferrites have obvious advantages.However,nano-size ferrites with high quality,which are prepared by sol-gel method,usually have microwave absorption in high frequency region.Some researches have indicated that micron-size pure lithium zinc ferrite（LiZn ferrite） prepared by solid-state reaction method have good low-frequency microwave absorption.But further application of this material is restricted by some inherent disadvantages.So in present work, sol-gel process was combined with subsequent calcination in order to prepare micron-size LiZn ferrite,which could exhibit good microwave absorbing properties in the frequency range of 0.5-3 GHz.Firstly,micron-size pure LiZn ferrites were successfully prepared by a combination of sol-gel process with subsequent calcination,and the microstructure and microwave absorbing properties of them were characterized by X-ray diffraction（XRD）,scanning electron microscopy（SEM） and network vector analyzer.The effects of calcination processing parameters on low-frequency microwave absorption were also studied according to kinetics of reaction and ferromagnetic resonance theory.The particle diameter is determined by final calcination temperature,and the heat-treatment speed rate has effect on vaporization loss of lithium and zinc during the calcination.The differences of actual particle size and phase composition of pure LiZn ferrites would have a further influence on their low-frequency microwave absorption generated from ferromagnetic resonance.Pure LiZn ferrite calcined at 500℃with a heating rate of 200℃/h and the one calcined at 1200℃with a heating rate of 240℃/h both exhibite microwave absorption in the frequency range of 0.5-3 GHz,and the latter has better wave absorbing properties.A peak value of reflection loss of-25.89 dB can be obtained for absorber thickness of 10 mm at 2.8 GHz,and this preparation method for LiZn ferrite microwave absorbers has not been reported at present. However,this pure LiZn ferrite mainly has microwave absorption in the frequency range of 2.5-3 GHz,and this working frequency band should be shifted towards lower frequency region.In order to improve microwave absorption and widen working frequency bandwidth in the frequency range of 0.5-3 GHz,LiZn ferrites doped with different amount of magnesium,copper,lanthanum and cerium were prepared by a combination of sol-gel process with subsequent calcination,respectively.The microstructure and low-frequency microwave absorption of these as-doped LiZn ferrites were characterized and compared with pure LiZn ferrite prepared by the same process.The results indicate that doping with magnesium,copper,lanthanum and cerium can modify the distribution of metallic irons within the crystal lattice of LiZn ferrite.And magnetocrystalline anisotropy of LiZn ferrite can be modified after being doped with lanthanum and cerium.All these modifications lead to changes of corresponding equivalent magnetic field,and then have a further effect on low-frequency microwave absorption generated from ferromagnetic resonance. In addition,there is a maximum doping amount for magnesium-doped, copper-doped,lanthanum-doped and cerium-doped LiZn ferrites.If the actual doping amount exceeds the limited amount,MgFe₂O₄,CuFe₂O₄,LaFeO₃ and CeO₂ will be formed in as-prepared ferrites.On one hand,the formation of MgFe₂O₄, CuFe₂O₄ and LaFeO₃ decrease the amount of as-doped LiZn ferrite.On the other hand,MgFe₂O₄,CuFe₂O₄ and LaFeO₃ have relatively poor microwave absorption in low frequency region.As a result,the low-frequency microwave absorption of actual ferrite becomes much worse.However,the formation of CeO₂ has no effect on the amount of LiZn ferrite in actual ferrite when the actual doping amount of cerium exceeds the limited value. It needs to be mentioned that when the doping amount of cerium reaches a specific value,the enrichment of CeO₂ and suitable component ratio between CeO₂ and LiZn ferrite can strengthen the dielectric attenuation for microwave,and improve the low-frequency microwave absorption of actual ferrite material greatly.A peak value of reflection loss of-28.06 dB can be obtained for absorber thickness of 10 mm at 1.68 GHz,and the distribution of working frequency band is in the frequency range of 1.25-2.25 GHz.Therefore,this as-prepared ferrite composite material is a practical and promising microwave absorber in the frequency range of 0.5-3 GHz.According to panorama analysis method and the formula of effective electromagnetic parameters of composite materials,MATLAB was used to compile a model for characterizing the correlations between the reflection loss, absorber thickness and component ratio of composite microwave absorbers at different frequency points.By utilizing this model,a plenty of data about predictive LiZn ferrite/ acetylene carbon black（ACB） composite microwave absorbers,such as matching conditions and component ratios,had been achieved. According to these data,three LiZn ferrite/ACB composite materials with different component ratios were then prepared by ball milling method,and their microwave absorbing properties were measured.The results indicate that these as-prepared composite materials,which are designed by this predictive analysis model mentioned above,do have better low-frequency microwave absorption than pure LiZn ferrite.And this indicates that the predictive analysis model and pre-design method can be useful for the preparation of low-frequency microwave absorbers.In microwave absorbing coating,rapid and uniform dispersion of fine microwave absorbers in polymer has a significant influence on its actual mechanical properties,microwave absorption and practical applications.In this paper,reciprocating fluid injection process（RFIP） and the dispersing device for RFIP were developed in order to promote further practical applications for microwave absorbing coating.And they were used in dispersion experiment for micro-size LiZn ferrite and carbon nanotubes（CNTs） in unsaturated polyester resin（UPR）.Viscosity of dispersed precursor mixture and the fracture surface of final composite cured from precursor mixture were characterized.And it can be discovered from the results that in polymer matrix,LiZn ferrite and CNTs dispersed by RFIP could both exhibit more homogenous distribution within less processing time than the ones dispersed by ultrasonication.This technology is not only applicable for rapid and homogeneous dispersion of micro-size microwave absorbers,but also has an excellent dispersive action on nano-size microwave absorbing agents.Therefore,RFIP will be a novel and effective process for dispersing fine microwave absorbing agents in polymer.更多还原