用正电子湮没谱学研究多孔铁氧体的缺陷及其对电磁吸波特性的影响

Recently, high-performance radar absorption material especially the ferrite has aroused extensive attention not merely in military field but also in civilian areas. However, the most-used microwave absorption material of ferrite still encounters several shortcomings: the higher surface density, rather narrow effective bandwidth frequency and quite thick coating thickness, which all greatly limit its further application in the new stealth fighter jet. In order to overcome these shortcomings, the material with certain demands should be satisfied: thin thickness, light weight, wide frequency and strong absorption. Moreover, it is such a great difficulty to possess our all effort in adjusting the structure and properties of a single magnetic component to realize the impedance matching and strong absorption. But fortunately, introducing defect in the ferrite probably is an effective approach to achieve the excellent properties. In this project, based on our previous research about the defect structure of some oxides, we will intend to design a kind of MOFs with ZIF structure to gain a porous ferrite frame. Then, the adjustable defects can be employed to improve the impedance matching and enhance the loss. Technology of positron annihilation spectroscopy has been utilized to reveal the defect type and defect concentration. The preparation technology of defect will be developed and the relationship between electromagnetic absorption properties and defect will be discovered. In order to further reveal the physical mechanism of the defect type and defect concentration on the electronic density, electromagnetic parameters and microwave absorption properties, the DFT theoretical calculation has been carried out to prove the internal connection between the defect and microwave performance, which provides theoretical and experimental basis for the use of new lightweight broadband absorption materials with good impedance matching and strong absorption.

研制高性能雷达吸波材料对我国的武器装备和民用领域都有重要的意义。然而面密度大、吸收频带窄、使用涂层厚度大等因素制约铁氧体在新一代隐身战机中的实际应用。为真正实现其“薄、轻、宽、强”等隐身特性，需解决三个关键问题：提高阻抗匹配、增强衰减、降低使用密度。但仅通过调节单一磁性组份的结构和物性而实现阻抗匹配和强吸收，仍面临巨大的挑战，是亟待解决的科学问题。缺陷是调控铁氧体结构和物性的一种有效手段，其内在机制仍待探究。本项目在前期研究氧化物缺陷结构的基础上，拟采用ZIF型MOFs分子设计合成多孔铁氧体骨架，并调控其缺陷改善阻抗匹配及损耗性能；通过正电子湮没谱获得缺陷种类、浓度等信息；研究缺陷结构随制备工艺的演化过程；阐明缺陷与吸波特性之间的构效关系；结合密度泛函计算，从理论上揭示缺陷影响多孔铁氧体电子密度、电磁参数及吸波性能的物理机制，为匹配好、损耗强的新型轻质宽频吸波材料的应用提供理论和实验依据。

新一代飞行器对轻质吸波材料提出了明确的需求，而基于磁性吸收剂的涂层及贴片面密度较大、电磁匹配及吸收频宽均需改善。采用MOFs构筑的新型吸收剂密度低、频带宽，有望成为新一代轻质宽频雷达吸收剂。本项目以MOFs前驱体制备了一系列多孔轻质吸波材料，通过精确设计组成及结构，研究了缺陷结构对电磁波吸收性能的影响规律，获得主要结果如下：(1)采用高温氢气还原法和铝热还原法制备了一系列不同氧缺陷浓度的TiO2介电材料，结合正电子湮没技术分析了缺陷结构，发现还原过程可以将小尺寸的缺陷转换为较大尺寸的缺陷团簇，进而影响周围原子位置及电子的分布，从而增强导电性，DFT理论计算表明，氧缺陷可影响材料本征带隙Eg值及费米能级以下的态密度DOS，改善电子能级的跃迁难易程度并增加电荷载流子浓度进而提高电子导电性；(2)采用铝热还原法制备了不同氧缺陷浓度的SnO2介电材料，DFT计算表明，随氧空位浓度的升高，介电极化先升高后降低，而电导损耗不断升高，损耗和吸波性能先增后减，表明介电极化在决定该氧化物介电行为中的作用与电导损耗相反，甚至起到了决定性作用；(3)发现MOFs金属中心元素及配体种类均会影响衍生物电磁性质。Fe、Co、Ni更有利于获得较高的复磁导率与复介电常数，同时，小分子配体更利于衍生物中金属物种含量的提升；(4)发现双金属及一维结构的构建有利于提升导电性及降低逾渗阈值，影响极化行为。所得材料在10wt%填充度，2mm时，有效吸收频宽可达5.24GHz，基本实现了Ku和X波段的宽频吸收；(5)以ZIF-67为前驱物，经热处理后所得Mo2C/Co@C复合物在35wt%填充度，1.6mm时有效吸收频宽可达6GHz。二维CoAl-LDH表面生长的ZIF-67热处理所得Co@N-C在30wt%填充度时可实现优良吸波性能；(6)开发了不同尺度的复合工艺，实现了组分种类及含量的调节，实现了磁损耗的有效增强及介电损耗的优良匹配。其中，以HKUST-1的MOFs为前驱体，经热处理工艺实现了Co粒子的均匀可控嵌入；以Co3[HCOO]6•DMF的MOFs为前驱体，通过葡萄糖溶液浸渍法，实现了Co/C界面结构的有效调整；以Fe3O4为核，通过原位生长PB壳层获得了一系列磁性复合吸波材料，有效吸收频率可基本覆盖2-18GHz。