| 摘要 | 本文采用等离子电弧法，通过改变阳极合金组成和成分、反应气体组成和比例以及控制电弧电流等手段，制备了多种具有不同外壳和内核的纳米胶囊，以及由Al2O3包裹的FeCo纳米胶囊自组装生长的线状、球状、珊瑚状聚集体。利用X射线粉末衍射(XRD)、高分辨透射电镜(HRTEM)、扫描电子显微镜(SEM)、X光电子能谱(XPS)等测试技术，系统研究了各种纳米胶囊的相组成、颗粒形貌、尺寸分布、微观组织结构以及其元素的价态以及聚集体的结构特征。利用荧光分光光度计(FL)、超导量子干涉仪(SQUID)和网络矢量分析仪研究了所制备纳米胶囊的光学性质、磁性质、磁热效应和电磁性能，并初步探讨了纳米胶囊具有优秀电磁吸收性能的物理机制。
通过蒸发Tb70Al30合金，并增加电弧电流和反应H2气压，制备了具有单相TbAl2为内核的TbAl2/Al2O3纳米胶囊。TbAl2/Al2O3纳米胶囊具有壳/核结构，以晶态TbAl2为内核，非晶态Al2O3为外壳。TbAl2/Al2O3纳米胶囊在冻结温度70 K和居里温度100 K 之间表现出超顺磁性。在7 T的磁场变化下，TbAl2/Al2O3纳米胶囊的磁熵变的绝对值随温度降低而快速地增加，在7.5 K时可达到23.83 J Kg-1K-1。
在Ar和H2气氛下，制备了多功能性ZnO包裹的α-Fe固溶体纳米胶囊。该纳米胶囊具有壳/核结构，以晶态α-Fe固溶体为内核，非晶态ZnO为外壳。这种新型α-Fe/ZnO纳米胶囊在室温时呈现铁磁性。随着靶材中Zn浓度的减少，发射光谱中处于388 nm的峰位呈现红移的趋势，这是来自于能带结构的畸变。在对复介电常数和复磁导率进行深入研究之后发现ZnO包裹的α-Fe固溶体纳米胶囊呈现出优异的微波吸收性能，这归功于微结构上的电磁匹配、强烈的自然共振和偶极极化机制的共同作用。
通过改进的等离子电弧法，在Ar,H2和C2H5OH气氛中用碳棒阴极蒸发Fe90Ni10靶材成功制备了以软磁(Fe,Ni)纳米颗粒为内核，以石墨为外壳的(Fe,Ni)/C纳米胶囊。当吸收厚度为2.0 mm时，在整个Ku波段(12.4-18 GHz)上(Fe,Ni)/C纳米胶囊的反射损失RL值都超过了-10 dB；并且在16 GHz处获得了最优RL值 -26.9 dB，RL值超过-20 dB的频宽为13.6-16.6 GHz。另外，当厚度在1.87-2.1 mm范围内改变时，RL值超过-10 dB的频宽没有发生明显的变化。石墨外壳起到了双重作用：其一，使(Fe,Ni)纳米颗粒免于氧化；其二，抑制涡流损耗。另外，石墨外壳和磁性Ni核之间建立了良好的电磁匹配。
通过控制反应电流大小、反应气氛、起弧时间、调整阳极靶材中的组分，我们分别制备了由不同成分Al2O3包覆FeCo纳米胶囊构成的线状聚集体、球状聚集体和珊瑚状聚集体。构成聚集体的纳米胶囊之间的磁性作用和残余电荷作用以及纳米胶囊与铜坩埚之间作用的综合会决定最终聚集体的形貌。阴极和阳极之间的距离、角度、电流大小、弧光大小位置、反应气氛都会影响到聚集体的形貌。三种聚集体在室温时都呈现出铁磁性行为。由于构成聚集体的基本单元都是Al2O3包覆FeCo纳米胶囊，介电性Al2O3为外壳、磁性FeCo为内核，这种特殊的微观结构决定了三种聚集体具有优秀的电磁吸收性能。
用等离子电弧放电法制备得到Ni纳米颗粒作为内核ZnO作为外壳的Ni/ZnO纳米胶囊。用Cole-Cole半圆方法解释了Ni/ZnO纳米胶囊中的双重介电驰豫，用等效的电路模型很好地解释了体系中的双重非线性介电共振。Ni/ZnO纳米胶囊中的磁损耗主要来自于强自然共振。厚度在2.40-2.60 mm范围内，-10分贝的频宽基本没有变化；厚度1.95-2.10 mm范围内，-20分贝的频宽基本没有变化。该体系优秀的电磁吸收性能归功于Ni/ZnO纳米胶囊中特殊的壳/核结构带来的双重非线性介电共振和强自然共振。 |
| 其他摘要 | Different kinds of nanocapsules with different shells and cores and the wire-like, sphere-like, coral-like aggregates self-assembled by Al2O3-coated FeCo nanocapsules were prepared by arc-discharge technique, by changing constitution and composition of the anode, constitution and ratio of the discharging atmosphere and the arc current magnitude. The phase constitution, particles morphologies, size distributions, microstructure and the binding energy of the elements of these different kinds of nanocapsules and the structure characteristics of the aggregates have been studied by means of X-ray diffraction (XRD), High-resolution transmission electron microscopy (HRTEM)， Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) in details. The optical properties, magnetic characteristics and magnetocaloric effect, electromagnetic (EM) properties of the nanocapsules were measured by fluorescence spectrophotometer (FL), Superconducting quantum interface devices (SQUID), network vector analyzer. The physical mechanism of the good EM absorption properties in these nanocapsules was discussed.
A new type of TbAl2/Al2O3 nanocapsules with single-phase intermetallic compound TbAl2 as cores and amorphous Al2O3 as shells has been synthesized by a modified arc-discharging technique. The TbAl2/Al2O3 nanocapsules show superparamagnetic properties between their blocking temperature of 70 K and Curie temperature of 100 K. The absolute value of the change in magnetic entropy increases sharply with decreasing temperature at low-temperature and reaches 23.83 J Kg-1K-1 at 7.5 K in the magnetic fields varying from 0 to 7 T.
Multi-functional ZnO-coated α-Fe solid-solution nanocapsules were fabricated by using a modified arc discharge technique. The nanocapsules have a shell/core structure with crystalline α-Fe solid solution as the core and amorphous ZnO as the shell. The new type of the α-Fe/ZnO nanocapsules exhibit ferromagnetic characteristics at room temperature. The fluorescence properties at room temperature of the nanocapsules were also studied. There is a tendency of redshift for the peak at 388 nm in the FL spectrums as the Zn concentration decreases. The in-depth study of complex permittivity and permeability reveals that the ZnO-coated α-Fe solid-solution nanocapsules exhibit excellent microwave-absorption properties, owing to the consequence of a proper EM match in the microstructure, the strong natural resonance as well as dipolar polarization mechanisms.
The (Fe, Ni)/C nanocapsules with soft magnetic (Fe,Ni) nanoparticles as cores and graphite as shells, have been prepared by a modified arc discharge technique in Ar, H2 and ethanol vapor. For the (Fe, Ni)/C nanocapsules, the RL values exceeding -10 dB are achieved in the whole Ku-band (12.4-18 GHz) for dm = 2.0 mm, and an optimal RL value of -26.9 dB is observed at fm = 16 GHz with the -20 dB bandwidth over the frequency range of 13.6 – 16.6 GHz for dm = 2.0 mm. In addition, bandwidth for the RL values exceeding -10 dB does not change dramatically for the thicknesses of 1.87-2.1 mm. The graphite shell plays the double role that makes the (Fe, Ni) nanoparticles free from oxidation and the separator for suppressing the eddy current loss. In addition, good match is set up between graphite shells and magnetic Ni cores.
Wire-like, sphere-like, coral-like aggregates self-assembled by Al2O3-coated FeCo nanocapsules with different compositions. As a result of magnetic interaction between nanocapsules, remaining charge force and interaction between nanocapsules and copper crucible, the aggregates have different morphology. In addition, the angle and the distance between the anode and cathode, the arc current magnitude, the size and position of the arc and discharging atmosphere can also affect the morphology of the aggregates. Three kinds of aggregates exhibit the ferromagnetic properties. The basic unit of aggregates is Al2O3-coated FeCo nanocapsules, dielectric Al2O3 as shells and magnetic FeCo as cores. Three kinds of aggregates have good EM-absorption properties due to the special microstructure.
The Ni/ZnO nanocapsules with Ni nanoparticles as cores and ZnO as shells were prepared by arc-discharge technique. The Cole-Cole semicircle approach was adopted to explain the dual dielectric relaxations and, the equivalent circuit model was utilized to well explain the dual nonlinear dielectric resonance. The magnetic loss in the Ni/ZnO nanocapsules is mainly caused by the strong natural resonance. RL values exceeding -10 dB are obtained in the whole Ku-band and the whole X-band for 2.50 mm thickness layer. In addition, EM-properties do not change dramatically for the thicknesses of 2.40-2.60 mm for the RL values exceeding -10 dB and for the thickness of 1.95-2.10 mm for the RL values exceeding -20 dB. The excellent EM-absorption-properties are ascribed to the dual nonlinear dielectric resonance and the strong natural resonance from the special core/shell structure in the Ni/ZnO nanocapsules. |
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