【作者】 彭志华；

【导师】 彭景翠；

【作者基本信息】 湖南大学 ， 材料物理与化学， 2010， 博士

【摘要】 碳纳米管(carbon nanotubes, CNTs)具有中空的层状结构和独特的电磁性能,是理想的准一维纳米材料。碳纳米管对电磁波特别是微波的屏蔽与吸收是当今材料科学领域的研究热点之一。人们对碳纳米管材料微波吸收特性的实验研究已取得了较大的进展。近年来,一些性能优异的碳纳米管微波吸收材料相继被合成,在军事隐身和民用微波防辐方面展示出巨大的应用前景。本论文系统探讨了碳纳米管的本征微波吸收机理,重点研究了碳纳米管中氢等离子体的微波吸收机理、碳纳米管／聚合物复合材料的微波介电特性与吸波机理及铁磁性金属填充碳纳米管的微波吸收机理。对碳纳米管的结构特征、制备与纯化、生长机理、理化性质、应用前景、研究领域、研究方法、研究成果及吸波材料的基本理论进行了全面评述,对碳纳米管微波吸收材料的研究进展进行了系统分析,并简要介绍了课题来源和主要研究内容。从碳纳米管电磁参数和微波吸收特性的实验结果出发,分析探讨碳纳米管的微波损耗机理,并得出结论：(1)在微波辐射下,碳纳米管表面形成电偶极子,产生极化电流,电磁波的能量转换为其他形式的能量,同时,这些偶极子和微波场相互作用导致晶格振动以发热的形式引起微波损失；(2)电磁场的交叉极化使螺旋型手性碳纳米管对微波的吸收增加；(3)微波在碳纳米管孔结构中的多重反射和散射将导致电磁波能量的损失；(4)碳纳米管的结构缺陷能增强极化效应,从而增加介电损耗；(5)碳纳米管所具有的小尺寸效应、表面效应(比表面积大)、量子尺寸效应和宏观量子隧道效应等有利于其微波吸收；(6)碳纳米管的电容率较大,但碳纳米管的磁导率相对较小,限制了其作为吸波材料性能的进一步提高,纯的碳纳米管作为微波吸收剂的吸波性能不是很强。对铁催化高压歧化生长的碳纳米管的微波吸收机理进行了深入的探讨。运用双流体理论和唯象模型,导出了碳纳米管中氢等离子体的微波复电容率和微波吸收衰减系数,构建了碳纳米管中氢等离子体的微波吸收模型,从理论上证明了铁催化高压歧化生长的碳纳米管氢等离子体中的带电粒子对微波产生的碰撞吸收是其主要微波吸收机理。计算分析表明：随着碳纳米管氢等离子体中自由电子数密度的增加,最大衰减系数|Attmax|和最大吸收系数αmax增加,吸收峰向高频方向移动；随着电子碰撞有效频率的增加,最大衰减系数|Attmax|和最大吸收系数αmax减小,吸收峰缓慢地向低频方向移动；只要恰当地控制碳纳米管中等离子体的自由电子密度,就能够实现对特定微波段的强吸收。以材料显微结构—性能关联理论为基础,在充分考虑了碳纳米管的几何结构、碳纳米管和聚合物基质各自的复电容率的实部和虚部的基础上,运用等效的电阻-电容(RC)网络模型和对数混合法则,导出了碳纳米管／聚合物(绝缘体)复合材料的复电导率、复电容率和微波吸收系数,构建了其微波吸收模型。同时得出结论：碳纳米管添加到聚合物中,使材料的复电导率、复电容率明显增大,微波吸收能力显著增强。碳纳米管／聚合物复合材料的微波电导率、微波吸收系数与频率之间存在良好的线性关系。系统论述了金属填充碳纳米管的制备方法,深入探讨了铁磁性金属填充碳纳米管的微波吸收机理,理论上证明了铁磁性金属填充碳纳米管对微波的强烈吸收主要是样品中铁磁性金属纳米粒子在微波作用下产生了磁共振的结果。研究发现,随着铁磁性金属填充碳纳米管薄膜厚度的增加,其共振频率vγ,向低频方向移动,随着样品中铁磁性金属纳米粒子含量的增加,vγ向高频方向移动。因此,通过改变样品厚度和碳纳米管中铁磁性纳米粒子含量,可以实现对特定频率微波的强吸收的重要结论。本论文的研究目的是为研制性能优异的碳纳米管微波吸收材料提供理论依据。更多还原

【Abstract】 Carbon nanotubes (CNTs) with both hollow layer structure and unique electromagnetic properties are ideal quasi-one-dimensional nano-materials. At present, the electromagnetic wave (especially microwave) absorbing and shielding properties of CNTs is one of the focus research areas in materials science. Considerable advances have been made experimentally in exploring the microwave absorbing properties of CNTs materials. In recent years, some excellent microwave absorbing materials containing CNTs have been synthesized, having broad application prospects in both military stealth and microwave radiation protection. In this dissertation, the microwave absorbing mechanisms of intrinsic CNTs are studied systematically. We mainly investigate the hydrogen adsorption properties of CNTs, the microwave absorbing mechanisms of hydrogen plasma in CNTs, both the dielectric characteristics and the absorbing mechanisms of CNTs/polymer at microwave frequencies, and the microwave absorbing mechanisms of ferromagnetic metals-filled CNTs etc.The advances in researches for CNTs, such as structural characteristics, synthesis and purification, growth mechanics, physical and chemical properties, applications prospect, research fields, methods and achievements are reviewed entirely. Physical mechanism of electromagnetic wave absorbing materials is described. The research situation about the microwave absorbing materials containing carbon nanotubes are discussed systematically. The source of study and main study content are introduced in brief.Based on the experimental data for both the electromagnetic parameters and the microwave absorbing properties of CNTs, the microwave absorbing mechanisms of CNTs are investigated and revealed systematically. The following conclusions are obtained. (1) Under microwave irradiation, charge dipoles form on the surface of carbon nanotubes. The charge dipoles generate an inductive current in electromagnetic field. The inductive current then causes a loss current and energy dissipation. And the interaction of microwave electric field with charge dipoles causes crystal lattice libration, resulting in the losses of microwave energy through heating. (2)For helical CNTs, the cross-polarization of electromagnetic fields will give rise to the chiral microwave absorption. (3)Microwave is reflected and scattered repeatedly in the pore structures of CNTs, which could also lead to the loss of electromagnetic energy. (4)The structural defects of CNTs can enhance charge polarization, which will cause more dielectric loss. (5)CNTs have a series of novel physical effects, such as small size effect, effect of large specific surface area, quanta-size effect and tunneling effect, contributing to microwave absorption. (6)The complex permittivity of CNTs is large, but their microwave permeability is comparatively small. Therefore, intrinsic CNTs display weak absorption to microwave.The hydrogen adsorption properties of CNTs are first discussed, and then the microwave absorbing mechanisms of HiPco CNTs are investigated. Using double-fluid theory and phenomenological model, the complex permittivity, the attenuation coefficient as well as absorption coefficient of hydrogen plasma in HiPco CNTs at microwave frequencies are deduced. Then a microwave absorption model of hydrogen plasma in HiPco CNTs is developed. It is theoretically proved that microwave energy loss is predominantly caused by the collisional absorption of the charge particle in hydrogen plasma of HiPco CNTs. The numerical results indicate that both the maximum attenuation coefficient and the maximum absorption coefficient increase, and absorption peak shifts towards high frequency with liberal electron density increasing. Both the maximum attenuation coefficient and the maximum absorption coefficient decrease, and absorption peak shifts slowly towards low frequency with electronic effective collision frequency increasing. It is proposed that as long as liberal electron density in plasma in CNTs is modestly controlled, strong microwave absorption, for specified frequency range, can be obtained.Based on both the material microstructure-properties correlation theory and an equivalent resistance-capacitance(RC) network, the complex conductivity, the complex permittivity and electromagnetic wave absorption coefficient of CNTs/polymer composite are theoretically deduced in microwave frequency range using the logarithmic rule. Then a microwave absorption model, which can predict microwave absorbing properties of CNTs/polymer composite, is developed. In this model, the size and shape of CNTs, the real and imaginary parts of the permittivities of CNTs and polymer are considered in detail. It is theoretically proved that the addition of CNTs to the polymer result in explicit increases of the complex conductivity, complex permittivity and microwave absorption coefficient of the sample due to the RC network formation and introduction of conducting paths to the composites. In addition, a good linear relationship between microwave conductivity (or microwave absorption coefficient) of the composite and frequency is found.The preparation methods of metal-filled CNTs are described systematically. Then the microwave absorbing mechanisms of ferromagnetic metals-filled CNTs are investigated. It is theoretically revealed that strong microwave absorption by ferromagnetic metals-filled CNTs is predominantly caused by the magnetic resonance of ferromagnetic metal nanoparticles in the sample. It is found that resonance frequency vy shifts towards low frequency with the increase of ferromagnetic metals-filled CNTs film thickness, and vγshifts towards high frequency with ferromagnetic nanoparticle content percentage increasing in the sample. Thus, strong microwave absorption by ferromagnetic metals-filled CNTs can be realized via the control of the sample thickness or ferromagnetic nanoparticle content percentage among CNTs.The aim of this investigation is to supply valuable theoretical basis for the preparation of excellent microwave absorbing materials containing CNTs.更多还原