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电磁波吸收复合物微波等效参数研究
【作者】 刘涛；
【导师】 邓龙江；
【作者基本信息】 电子科技大学 ， 电子信息材料与元器件， 2010， 硕士
【摘要】 随着雷达探测技术的发展,传统的吸波材料已无法满足现实的需求,铁基纳米晶软磁材料具有的高饱和磁化强度和低矫顽力以及特有的纳米效应,使其成为新一代的高频磁性材料。根据磁极限关系式,颗粒片状化有利于提高微波磁导率,扩展吸收频带。基于上述考虑,本课题采用湿法球磨工艺制备了片状FeSiAl纳米晶合金,深入研究制备工艺(球磨机转速、球料比、球磨时间等)对纳米晶合金的形貌及静态和动态磁性能的影响,从而确定最佳的工艺条件。雷达吸收材料通常是由吸收剂与基体(黏合剂)混合形成的复合物。吸波材料性能的优劣与复合物的等效磁导率与介电常数密切相关。计算磁性吸波复合物等效电磁参数一直是学术研究的重大基础问题之一,其核心内容包括混合方程和共振机理的研究和表达。解决了这两个难点,我们就可以准确快速的设计出具有预期电磁性能的材料。前人研究发现在基体中一致排列的片状铁磁颗粒有利于吸波性能的提高。为了进行深入的研究,作者利用商业CST微波工作室软件进行建模仿真,研究一致排列的片状铁磁颗粒在基体中的取向及空间分布对等效磁导率的影响,并将这两个因素引入到Maxwell-Garnett混合公式中,做适当的改进。计算得到不同模型的等效磁导率的理论值与仿真结果比较一致。此外,当片状颗粒在基体中的取向不唯一时,作者采用了一种半经验性的理论公式成功预测了复合材料的有效磁导率。铁基纳米晶颗粒在吉赫兹频段的主要磁共振机理是自然共振和交换共振。因此,Landau-Lifshitz-Gilbert方程可以用于计算本征磁导率。本文重点研究了不同形貌(球形,片状,纤维状)的铁基纳米晶颗粒在基体中随机取向及一致取向条件下的共振表达式。考虑到渗流效应以及颗粒间偶极偶极相互作用对等效电磁参数的影响,对Maxwell-Garnett方程作进一步的改进。计算得到的两种取向条件下片状铁基纳米晶复合物的有效磁导率与实验值非常吻合。这意味着直接从磁性材料静态参数出发来计算不同取向复合物等效微波参数成为可能,为磁性吸波材料性能设计奠定了理论基础。
【Abstract】 With the development of radar technology, the traditional absorbing materials have been unable to meet the need at present, Iron-based nanocrystalline soft magnetic materials have become promising high-frequency magnetic materials owning to high saturation magnetization, low coercivity and obvious nano-effects. According to magnetic bounds relations, high microwave permeability and extended absorption band can be obtained for flake magnetic particles. Based on the above considerations, wet milling process is used for preparation of FeSiAl nanocrystalline flake particles. We delve into the effect of preparation technology (Speed of ball mill, Ration between balls and samples, Milling time) on the morphology, static and dynamic magnetic properties of nanocrystalline alloys and thus obtain the optimum conditions.Radar absorbing materials (RAM) are usually composites in which the absorber and matrix (adhesive) are mixed at appropriate proportion in practical application. The performance of RAM is closely related to effective permeability and permittivity of composites. Therefore, the calculations of effective electromagnetic parameters of magnetic absorbing composites have been one of the most significant basic researches, whose core contents include the investigation on resonance mechanism and expression on mixing formula. We can quickly and accurately design absorbing materials with expected electromagnetic properties on the basis of figuring out these issues. Previous studies found that aligned flake ferromagnetic particles in the matrix is advantageous to the improvement of absorbing properties. For in-depth study, commercial software CST ? microwave studio is used to investigate spatial orientation and position distribution effect on the effective permeability of composites consisting of aligned ferromagnetic flakes and Maxwell-Garnett mixing formula is improved considering these two factors. The effective permeabilities of simulation models calculated by virtue of corrected Maxwell-Garnett mixing formula are nearly consistent with simulation results. Furthermore, semi-empirical theoretical formula can successfully predict the effective permeability of composites in which the flakes are not aligned.Main resonance mechanisms of ferromagnetic nanocrystalline particles in GHz band are natural resonance and exchange resonance. Therefore, Landau-Lifshitz-Gilbert equation can be used to obtain the expression of intrinsic permeability. In this paper, we put emphasis on the investigation about the resonance expressions of ferromagnetic nanocrystalline particles with different shapes (sphere, flake, fiber), whose orientations are aligned and random in the matrix. Maxwell-Garnett mixing rule is further improved considering percolation threshold and dipole-dipole interaction between particles effect on the effective electromagnetic parameters. The effective permeabilities of flake composites for two kinds of orientation cases have been calculated and an excellent agreement between theoretical and experiment results is obtained. This means that it is possible for us to calculate effective electromagnetic parameters for different orientation cases from static magnetic parameters, which laid a theoretical foundation for the performance design of magnetic absorbing materials.