【作者】 曹静；

【导师】 付乌有；

【作者基本信息】 吉林大学 ， 凝聚态物理， 2010， 硕士

【摘要】 随着现代社会与科学技术的飞速发展,由于大范围使用移动电话、局域网以及雷达设备等等,造成了越来越严重的电磁干扰,给人们的生活带来了诸多不便。因此,微波吸收材料不仅应用在军事领域中而且在人们的生产与生活的各个方面都有很重要的作用。鉴于上述原因,我们急需要研究发展经济节约、吸收带宽、吸波效果好的微波吸收材料。本文采用化学液相法制备出放射状结构ZnO/CoFe₂O₄纳米复合材料,ZnO/Fe₃O₄纳米复合材料。对其形貌、成分、磁性质、微波吸收特性等进行了系统的研究。首先,在水浴静止条件下低温制备出放射状ZnO纳米管束,随后,利用共沉淀法,90oC条件下在ZnO表面包覆CoFe₂O₄纳米颗粒。对其成分、形貌分析可知, ZnO/CoFe₂O₄纳米复合材料具有完整的ZnO、CoFe₂O₄晶形,并且在包覆过程中ZnO的放射管状形貌没有被破坏。放射管状ZnO/CoFe₂O₄纳米复合材料对于微波的吸收性能无论是吸收强度还是吸收频率宽度都显著优于单一ZnO纳米管束或单一CoFe₂O₄纳米颗粒。当磁性与介电性质相匹配时微波吸收性能会明显增强, ZnO的质量百分含量为60%,频率在9.1GHz（4.1-13.2GHz）的宽范围内其微波吸收值均超过10dB,在8.5GHz处反射损失达到28.3dB,复合材料的微波吸收性能达到最优。从微波吸收频率宽度看,放射管状ZnO/CoFe₂O₄纳米复合材料的微波吸收频率宽度比单纯的放射管状ZnO纳米管束的情况宽接近三倍。ZnO/CoFe₂O₄纳米复合材料的优异的微波吸收性能得益于ZnO的放射管状形貌,以及CoFe₂O₄纳米颗粒之间、CoFe₂O₄纳米颗粒与ZnO之间形成的多重界面。当放射管状ZnO纳米管束分散在酚醛树脂胶接剂中并固化成型后会在其中形成三维的传导路径,微波能量会在这些传导网格中形成耗散电流,最终将能量消耗掉。其次,电磁波辐射到测试板上,一部分微波的能量被吸收,剩余的微波就会被其中存在的大量的界面漫反射。在这个漫反射的过程中,微波进入了另一次能量形成耗散电流被消耗掉的过程。以亚硫酸钠为还原剂,直接得到了Fe₃O₄纳米颗粒,并成功包覆在放射状ZnO纳米管表面,ZnO与Fe₃O₄纳米颗粒层之间结合紧密,并且复合材料中不含有其它杂质。其微波吸收性能相比于单一的放射状ZnO或单一Fe₃O₄纳米颗粒具有明显的优势。更多还原

【Abstract】 The preparation of nanocomposite particles is a great challenge in the fields of synthetic chemistry and materials science, because nanocomposite particles have unique structural, mechanical, electronic, magnetic and optical proterties. With the development of technology and science, the request and design of materials is becoming higher and higher. Single non-conductivity materials or magnetic materials can not meet the growth of application requirements. In the present work, we report for the first time the actinomorphic tubular ZnO/CoFe₂O₄ nanocomposites and actinomorphic tubular ZnO/Fe₃O₄ by a simple chemical solution method. Characterization was accomplished using various techniques, such as powder X-ray diffraction, scanning electron microscopy, X-ray energydispersive spectroscopy, and so on.ZnO nanotubes bundles were synthesized by a single solution method at a mild temperature of 90 oC. From the X-ray powder diffraction patterns of the as-prepared pure ZnO, there is no diffraction peaks from other crystalline forms are detected, which indicates a high purity and crystallinity of these ZnO samples. The FESEM and TEM images indicate the detailed morphology of pure ZnO, the wall thickness of the ZnO nanotubes is about 60 nm, inner diameters of the tubes are about 350 nm, and each tube outer wall has the obvious six edge angles, which because of ZnO have a hexagonal structure.CoFe₂O₄ nanoparticles have been prepared at 90oC through coprecipitated method. We synthesize ZnO/CoFe₂O₄ nanocomposites with the same method. The diffraction peaks corresponding to both ZnO and CoFe₂O₄ can be seen clearly. Besides, there are no other diffraction peaks except ZnO and CoFe₂O₄. Hence, it is concluded that the as-synthesized core/shell structured composites are composed of crystalline ZnO and CoFe₂O₄. In comparison with FESEM images of pure ZnO, the average length of the ZnO nanotubes has not obviously changed, while the six edge angles of the outer wall have disappeared, and some small particles can clearly be found on the surface of the ZnO nanotube. It indicates that ZnO nanotubes have been coated with CoFe₂O₄ nanoparticles, and the size of CoFe₂O₄ nanoparticles are below 40 nm. From FESEM images of ZnO/CoFe₂O₄ composites with different relative content of ZnO. It can be seen that with the decreasing of relative content of ZnO, the coating thickness increases significantly. However, in compari- son with Figure 2a, the tubular structures are not obviously changed. The EDX pattern of the ZnO nanotubes shows only the presence of O and Zn elements. Co and Fe elements are found to be present after the nanotube is coated with CoFe₂O₄ nanoparticles, which provides powerful evidence for the successful coating of CoFe₂O₄ on the surface of ZnO nanotubes. This is consistent with the broad peaks of CoFe₂O₄ in the XRD spectra. It can be seen that the coating of CoFe₂O₄ nanoparticles is continuous and uniform. From the TEM image of typical area of actinomorphic tubular ZnO/CoFe₂O₄ nanocomposites. It can be seen that CoFe₂O₄ is coating the surface of ZnO （average 100 nm thick） as a thin layer. It also illustrates that CoFe₂O₄ nanoparticles coat ZnO nanotubes compactly, and there is no obvious crack in the core-shell of the ZnO/CoFe₂O₄ structure. This is in agreement with the result which is concluded from SEM. The magnetic properties of the assynthesized CoFe₂O₄ and the representative actinomorphic tubular ZnO/CoFe₂O₄ nanocomposites were measured at room temperature. A similar behavior for the actinomorphic tubular ZnO/CoFe₂O₄ nanocomposites was observed. In contrast, Ms of the ZnO coated with CoFe₂O₄ decreased to 27 emu/g, mainly due to the volume of the nonmagnetic of the total sample volume. From the the RAM reflectivity far field RCS method spectrum of five microwave test plates. It can be seen that the reflection loss of pure ZnO nanotubes and CoFe₂O₄ nanoparticles are rather low for all frequencies between 2～18 GHz and the peak values are 8.3 and 10.5 dB, respectively. For actinomorphic tubular ZnO/CoFe₂O₄ nanocomposites, the microwave absorption is evidently improved （much better than that of both ZnO nanotubes and CoFe₂O₄ particles）. When the conthet of ZnO is 60%, the maximum reflection loss is 28.3 dB. The maximum reflection loss increases from 10.5 dB to about 28.3 dB for the weight ratio of CoFe₂O₄ = 40%. When the weight ratio of ZnO is 60%, the composites have good compatible dielectric and magnetic properties, and hence the microwave absorbing properties show the maximum value. However, when the weight ratio of ZnO is 40%, the maximum reflection loss decreases to 17.2 dB, which may be due to deteriorating the dielectric property when the weight ratio of CoFe₂O₄ exceeds a critical value. Hence, the CoFe₂O₄ particle-functionalized ZnO nanotubes exhibit a better microwave absorption. The improvement of microwave absorption obviously originates from the combination of ZnO nanotubes and CoFe₂O₄ nanoparticles. Therefore, the difference onreflection loss maximum as a function of the samples is associated with the magnetocrystalline anisotropy and structure anisotropy of as-synthesized ZnO nanotubes/CoFe₂O₄ nanocomposites.The synthesis of actinomorphic tubular ZnO/Fe₃O₄ nanocomposites was by a simple chemical solution method. For Fe₃O₄ nanoparticles, the typical characteristics of superparamagnetic behavior were observed, namely almost immeasurable coercivity and remanence. Saturated magnetization （Ms） were estimated to be Ms = 71.05 emu/g. In contrast, Ms of the ZnO coated with Fe₃O₄ decreased to 30.37 emu/g, mainly attributing to the volume of the non-magnetic to the total sample volume. Furthermore, the coercivity and remanence for the ZnO/Fe₃O₄ nanocomposites were also observed to tend towards zero, which is consistent with superparamagnetic behavior at the nanoscale dimensions of inorganic magnetic particles. Compared with that of pure ZnO nanotubes and Fe₃O₄ nanoparticles, enhanced electromagnetic wave absorption of actinomorphic tubular ZnO/Fe₃O₄ nanocomposites at 2¹8 GHz was observed and the possible mechanism was discussed. First, the actinomorphic tubular ZnO cluster comprises many crystalline tubes which are outwardly extended. The three-dimensional conductive paths will be formed when the actinomorphic tubular ZnO clusters distribute in the matrix. As electromagnetic wave impenetrate the cellular coating, the energy is induced into dissipative current by the conductive networks. Second, there exist multiple interfaces between Fe₃O₄ nanoparticles and Fe₃O₄ nanoparticles as well as Fe₃O₄ nanoparticles and ZnO nanotubes. The electromagnetic wave radiating on absorbents is partially absorbed, while the surplus electromagnetic wave will present diffuse reflections due to the multiple interfaces. During the process of diffuse reflections, the electromagnetic wave enters another process which the energy is induced into dissipative current.更多还原