【作者】 伊海波；

【导师】 李发伸；

【作者基本信息】 兰州大学 ， 凝聚态物理， 2012， 博士

【摘要】 不论是电磁屏蔽和抗电磁干扰方面,还是军事雷达隐身方面,都对微波吸收材料有“厚度薄、质量轻、吸波频段宽、吸波能效高”的要求。基于四分之一波长模型,为了得到好的微波吸收性质,对于磁性材料要提高其微波磁导率。基于双各向异性模型,平面型稀土金属间化合物微粉复合材料能够突破Snoek极限的限制,在高的频率下具有高复数磁导率,有望成为新一代的微波磁性材料和微波吸收材料。成分和温度对稀土金属间化合物的磁矩分布有非常重要的影响。一般说来,稀土金属间化合物的磁晶各向异性分为三类：轴型、锥型和平面型磁晶各向异性。根据上述理论,制备出Ce2(Co1-xFex)17(0≤x≤0.8)稀土金属间化合物微粉,研究了元素替代对磁晶各向异性,以及由此对其石蜡复合材料的微波磁性以及微波吸收性质的影响。对其中具有平面型和锥型稀土金属间化合物微粉/石蜡复合材料的微波吸收性质及吸波机理做了深入的分析。同时也研究了氧化物掺杂对磁性微粉/石蜡复合材料的微波吸收特性的影响。研究的主要内容和结果如下：(1)在0≤x≤0.8范围内,随着x值增加,Ce2(Co1-xFex)17的磁晶各向异性有轴型→锥型→平面型的过渡。当x≤0.45时,Ce2(Co1-xFex)17具有轴型磁晶各向异性；当x=0.5和0.55时,Ce2(Co1-xFex)17具有锥型磁晶各向异性；当x≥0.6时,Ce2(Co1-xFex)17具有平面型磁晶各向异性。(2)轴型Ce2(Co1-xFex)17微粉/石蜡复合材料的微波磁性服从Snoek极限限制。在0≤x≤0.45范围内,在同体积浓度下,随着x值增大,轴型Ce2(Co1-xFex)17微粉/石蜡复合材料的起始磁导率升高,自然共振频率降低。(3)平面型Ce2(Co1-xFex)17微粉/石蜡复合材料的微波磁性突破了Sonek极限的限制。在0.6≤x≤0.8范围内,在同体积浓度下,随着x值增大,平面型Ce2(Co1-xFex)17微粉/石蜡复合材料的起始磁导率降低,自然共振频率升高。(4)锥型Ce2(Co1-xFex)17(x=0.5和0.55)微粉/石蜡复合材料微波磁性突破了Sonek极限的限制。(5)在同体积浓度下,锥型和平面型Ce2(Co1-xFex)17(0.5<x<0.8)微粉/石蜡复合材料具有比轴型Ce2(Co1-xFex)17(0<x<0.45)微粉/复合材料高的磁导率。(6)利用微波介电谱和微波磁谱计算得到良好金属短路的单层Ce2(Co1-xFex)17微粉/石蜡复合材料吸波体对应不同涂层厚度的反射损耗曲线。复合材料的反射损耗峰的峰值频率与电磁参数、涂层厚度符合四分之一波长关系。在0.1-18GHz测量频率范围内,随着涂层厚度增加,复合材料的反射损耗峰向低频方向移动；随着涂层增大到一定厚度,复合材料的反射损耗曲线会出现两个甚至两个以上的反射损耗峰。(7)在0.1-18GHz测量频率范围内,随着涂层厚度增大,Ce2(Co1-xFex)17微粉/石蜡复合材料的反射损耗峰峰值先降低后增大。对应到Z=|Zin/Z0|随频率的变化曲线中,在Z=1时反射损耗值最小,具有该对应匹配厚度的吸波涂层材料在该匹配频率下实现完全损耗。在完全损耗情况下的匹配频率处咖>1。(8)改变x值能够有效调节Ce2(Co1-xFex)17微粉/石蜡复合材料的反射损耗特性。同体积浓度下,锥型和平面型Ce2(Co1-xFex)17微粉/石蜡复合材料的完全损耗情况对应的匹配频率相对低于轴型Ce2(Co1-xFex)17微粉/石蜡复合材料。(9)改变磁粉浓度都能够有效调节Ce2(Co1-xFex)17微粉/石蜡复合材料的反射损耗特性。随着磁粉浓度增加,复合材料的复合介电常数和复数磁导率均增加,完全损耗情况下的匹配频率呈降低趋势。(10)通过对反射损耗能的分析,电磁波损耗包括吸波涂层内的损耗以及前后界面反射电磁波在前界面处的干涉损耗,其中以干涉损耗为主要损耗机理。(11)通过直接球磨Ce2(Co0.4Fe0.6)17淬火样品与SiO2粉末的混合物得到SiO2包覆的Ce2(Co0.4Fe0.6)17微粉。制得的SiO2@Ce2(Co0.4Fe0.6)17微粉的XRD数据与Ce2(Co0.4Fe0.6)17微粉完全相同,没有SiO2晶体对应的衍射峰。相比于的Ce2(Co0.4Fe0.6)17微粉,SiO2@Ce2(Co0.4Fe0.6)17微粉具有较低的饱和磁化强度,其石蜡复合材料具有相对较低的复数介电常数和复数磁导率。计算得到的反射损耗峰的峰值频率与电磁参数、涂层厚度符合四分之一波长关系,完全损耗峰对应较高的匹配频率和较低的匹配厚度。在完全损耗情况下的匹配频率处ε/μ>1。氧化物掺杂为针对不同需求微波吸收材料的设计提供了思路。(12)通过直接球磨Ce2(Co0.4Fe0.6)17的办法,制得了Ce2O3@Co0.4Fe0.6纳米晶复合微粉。晶粒尺寸为7.7nm左右的α-Co0.4Fe0.6(?)内米晶分布在Ce2O3中。微粉的颗粒尺寸为0.3-1μm,饱和磁化强度为152.4emu/g,矫顽力为59.5Oe。制备了未取向和取向的Ce2O3@Co0.4Fe0.6微粉石蜡复合材料(35vol%)。经过取向,复合材料的复数介电常数降低,复数磁导率增大,有效改善了阻抗匹配；在涂层厚度为1.5mm-4mm范围内,在各自的峰值频率处,实现了更低的反射损耗。特别在匹配厚度为2.49mm,在匹配频率3.7GHz处实现完全损耗,在该匹配频率处咖>1。更多还原

【Abstract】 ’Thin thickness, light weight, wide band and strong absorption" is required for microwave absorption materials in the electromagnetic compatibility (EMC), anti-electromagnetic interference and military stealth fields. The improvement of the microwave complex permeability is very important for the magnetic microwave absorption materials, according to the quarter-wavelength condition. Based on the double anisotropy model, the rare earth intermetallic compounds with planar magnetocrystalline anisotropy could exceed Snoek’s limit. High permeability and high magnetic resonance frequency can be both obtained. The rare earth intermetallic compounds with planar magnetocrystalline anisotropy show the very significant potential as microwave magnetic materials and microwave absorption materials in the future.The magnetic moment distributing of the rare earth intermetallic compounds is significantly affected by the composition and temperature. Gererally, there are three kinds magnetocrystalline anisotropy for the rare earth intermetallic compounds: unaxial, cone and planar anisotropy.Based on the above theory, the Ce2(Co1-xFex)17(0≤x≤0.8) rare earth intermetallic compounds micropowders were prepared and the magnetocrystalline anisotropy was studied. The intermetallic compounds micropowders/paraffin composites were prepared. The complex permeability and microwave absorption properties were studied. Especially for the planar and cone anisotropy intermetallic compounds micropowders/paraffin composites, the microwave absorption mechanism is deeply analyzed. The affect of the oxide doping on microwave absorption properties for intermetallic compounds micropowders/paraffin composites is also studied. The main results are shown as follows:(1) In the range of0≤x≤0.8, Ce2(Co1-xFex)17intermetallic compounds possess uniaxial magnetocrystalline anisotropy with x≤0.45, possess cone magnetocrystalline anisotropy with x=0.5and0.55, and possess planar magnetocrystalline anisotropy with x≥0.6.(2) The microwave magnetic properties of uniaxial Ce2(Co1-xFex)17micropowders/paraffin composites with0≤x≤0.45obey the Snoek’s limit. The initial permeability increases and the natural resonance frequency decreases with the x value increasing from0to0.45for the uniaxial Ce2(Co1-xFex)17micropowders /paraffin composites with the same volume concentration.(3) The microwave magnetic properties of planar Ce2(Co1-xFex)17micropowders/paraffin composites with0.6≤x≤0.8overcome the Snoek’s limit. The initial permeability decreases and the natural resonance frequency increases with the x value increasing from0.6to0.8for the planar Ce2(Co1-xFex)17micropowders/paraffin composites with the same volume concentration.(4) The microwave magnetic properties of cone Ce2(Co1-xFex)17micro powders/paraffin composites with x=0.5and0.55overcome the Snoek’s limit.(5) The complex permeability of the planar and cone anisotropy intermetallic compounds micropowders/paraffin composites is higher than that of the uniaxial Ce2(Co1-xFex)17micropowders/paraffin composites with the same volume concentration.(6) Simulated microwave reflection result shows that the reflection loss peak frequency, the absorber thickness and the electromagnetic parameters is coincident with the quarter wavelength condition for the Ce2(Co1-xFex)17micropowders/paraffin composites. In the frequency range of0.1-18GHz, with the absorber thickness increasing, the reflection loss peak moves to lower frequency, and two or more reflection loss peaks can be observed.(7) The reflection loss peak value increases first and then decreases with the absorber thickness increasing. The minimum reflection loss peak value is obtained and the perfect loss is achieved as Z=|Zin/Z0|=1. At the matching frequency of the perfect loss conditions, ε/μ>1.(8) The microwave reflection loss characteristics of the Ce2(Co1-xFex)17micropowders/paraffin composites can be modulated by changing the x value. The matching frequencies of the perfect loss conditions for the planar and cone anisotropy intermetallic compounds micropowders/paraffin composites are lower than that for the uniaxial Ce2(Co1-xFex)17micropowders/paraffin composites with the same volume concentration.(9) The microwave reflection loss characteristics of the Ce2(Co1-xFex)17micropowders/paraffin composites can be modulated by changing the volume concentration. With the increase of the volume concentration, both the complex permittivity and permeability increase, and the reflection loss peak of the perfect loss conditions moves to lower frequency.(10) The S11-open, S21-open and S11-short parameters of the Ce2(Co1-xFex)17(x=0.5,0.6) micropowder/paraffin composites of different thickness was measured directly, according to which, the microwave energy loss was calculated. The microwave energy loss includes the electromagnetic loss in the inner of the absorber and the interference loss between the reflection waves from the back interface and the front interface. And the interference loss is the most important contribution.(11) The SiO2@Ce2(Co0.4Fe0.6)17micropowders were prepared by directly ball milling the mixture of the Ce2(Co0.4Fe0.6)17quenched samples and SiO2powders. The XRD patterns are totally same with Ce2(Co0.4Fe0.6)17micropowders, and any diffraction peaks can not be seen in relation to the crystal structure of SiO2. The saturation magnetization of SiO2@Ce2(Co0.4Fe0.6)17micropowders is lower than that of Ce2(Co0.4Fe0.6)17micropowders. The SiO2@Ce2(Co0.4Fe0.6)17micropowders paraffin composites also show lower complex permittivity and lower complex permeability. Simulated microwave reflection result shows that the matching thickness and peak frequency is coincident with the quarter wavelength condition. The matching thickness is thinner and the matching frequency is higher for the perfect matching conditions.(12) The Ce2O3@Co0.4Fe0.6nanocomposites were fabricated by arc melting and followed ball milling. The CoFe nanocrystals are dispersed in the cerium oxide. The complex permittivity, permeability and microwave absorbing properties of Ce2O3@Co0.4Fe0.6powders/paraffin composites with orientation have been studied. The lower permittivity and the higher permeability for the oriented composites have been shown obviously than those for unoriented one. Simulated microwave reflection result shows that the matching thickness and peak frequency is coincident with the quarter wavelength condition. For the oriented one, the minimizing RLm has been reached with the matching thickness of about2.49mm at the corresponding frequency of3.7GHz. At this point, not only are the reflection waves out of phase180°, but also the input impendence of absorber Zin is equal to the impendence of air Z0.更多还原