| 摘要 | 本文采用等离子电弧法，通过改变阳极合金的组成和成分、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GdAl2纳米胶囊自组装生长的珊瑚状三维宏观聚集体。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)电子能量损失谱(EELS)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、微观组织结构及其元素的价态以及聚集体的结构特征。利用超导量子干涉仪（SQUID）研究了所制备的纳米胶囊的磁性和磁热效应并初步探讨了纳米胶囊中在低温下具有高磁熵变的物理机制。
在Ar和H2混合气氛中，制备了具有壳/核结构的Co(Cr)和Cr(Co)固溶体纳米胶囊。Co(Cr)纳米胶囊具有不规则的球形，而Cr(Co)纳米胶囊的形貌为不规则多边形。Cr(Co)纳米胶囊的磁化强度，随其中Co含量的增加而增加。其磁化强度来自于bcc Cr晶格中未被补偿的Cr原子的磁矩和Co原子磁矩以及小的Co颗粒的磁矩。Co(Cr)和Cr(Co)纳米胶囊在冻结温度以下显示的磁滞特征起源于低温下各向异性的能垒对颗粒磁矩的转动的阻碍占主导作用。
通过蒸发Gd80Al20合金，并增加电弧电流和反应H2气压，制备了具有单相GdAl2为内核的GdAl2纳米胶囊。同时，也制备了由GdAl2纳米胶囊自组装生长的珊瑚状三维宏观聚集体。所制备的宏观聚集体由纳米胶囊、絮状结构、小团簇、大团簇和大的分枝在不同的尺度上逐级构成，说明了聚集体的结构具有自相似特征。GdAl2纳米胶囊具有壳/核结构，以晶态GdAl2为内核，非晶态Al2O3为外壳。在5 T的磁场变化下，GdAl2纳米胶囊的磁熵变绝对值随温度的降低而快速增加，在7.5 K时可达到14.5 J/kg K。
进一步，在Ar和H2气氛下，蒸发GdxAl100-x (x=50, 60, 70, 80和90)合金，我们成功地合成了金属间化合物GdAl2/Al2O3纳米胶囊。不同纳米胶囊内核的相组成受控于进入腔体中的Gd和Al原子比例，而进入腔体中Gd和Al原子数量又决定于初始阳极合金中各金属单质的初始含量和其沸点的高低。GdAl2/Al2O3纳米胶囊的形貌为不规则的球形，且具有典型的壳核结构，外壳为非晶Al2O3，内核为晶态GdAl2。在5到300K这个温度区间，GdAl2/Al2O3纳米胶囊分别处于冻结态、超顺磁态和顺磁态。GdAl2/Al2O3纳米胶囊的居里温度低于块体GdAl2的居里温度，且随GdAl2 晶格中替代Al原子的增加而降低。从180 K到5 K，GdAl2/Al2O3纳米胶囊的磁熵变随温度降低而连续地增加。当纳米胶囊进入冻结态时，磁熵变会迅速增加，在7.5 K和7 T的磁场变化下，由GdxAl100-x (x=70, 80,90)制备的GdAl2/Al2O3纳米胶囊的磁熵变可达 19.5、19.1和32.5 Jkg-1K-1。在冻结态时，GdAl2/Al2O3纳米胶囊出现大的磁熵变的物理机制是：高磁化强度及小的磁晶各向异性能垒导致的大 的出现。同时，发现处于冻结态的GdAl2/Al2O3纳米胶囊，其磁熵变-S与1/T呈较好的线性关系，且各向异性能垒的大小影响着直线斜率的大小。
通过等离子电弧法，在Ar和CH4气氛中蒸发Fe80B20 合金，成功制备了以 -Fe 和 Fe3B为内核，以石墨为外壳的FeB(C)纳米胶囊。石墨外壳显示了强烈的抗酸性腐蚀性。FeB(C)纳米胶囊的冻结温度为300K，其室温下的饱和磁化强度为80 Am2/kg，要远大于Fe (B)的57 Am2/kg，这归因于所制备样品中非磁性相的减少、软磁相Fe3B的存在以及非磁外壳的形成和几个内核共有一个晶态的C外壳的新型纳米胶囊的出现。 |
| 其他摘要 | Different kinds of nanocapsules with different shells and cores and the macro-aggregates self-assembled by GdAl2 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 the different kinds of nanocapsules and the structure characteristics of the macro-aggregates have been studied by means of X-ray diffraction (XRD), High-resolution transmission electron microscopy (HRTEM), Scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), Electron energy loss spectroscopy spectrum (EELS) in details. The magnetic characteristics of the nanocapsules and magnetocaloric effect of GdAl2 nanocapsules were measured by Superconducting quantum interface devices (SQUID). The physical mechanism of the high change of magnetic entropy appeared in blocking state GdAl2 nanocapsules was discussed.
Co(Cr) and Cr(Co) solid-solution nanocapsules have been fabricated by arc-discharge in atmosphere of argon and hydrogen. Cr(Co) and Co(Cr) solid solution nanocapsules show shell/core structure with irregular tetragonal and spherical shape characteristics. The magnetization of Cr(Co) increase with increasing Co content in the nanocapsules. The magnetization of Cr(Co) nanocapsules is contributed by the uncompensated moments in bcc chromium lattice, the moments of Co atoms in the bcc lattice and small cobalt particles.
A new type of GdAl2 nanocapsules with single-phase intermetallic compound GdAl2 as cores and amorphous Al2O3 as shells has been synthesized by the arc-discharge technique with modified strategies. Meanwhile, novel three-dimension coral-like hierarchical branching macro-aggregates have been self-assembled by disordered nanocapsules synthesized simultaneously in the arc-discharge process. The aggregates are constructed by nanocapsules, flocculent structure, small clusters, large clusters and big braches in different dimension, which indicates the self-similar characteristics of the aggregates. GdAl2 nanocapsule has shell/core structure with crystal GdAl2 as cores and amorphous Al2O3 as shell. The magnetocaloric effect of the GdAl2 nanocapsules were measured between 5 and 165 K and the absolute value of change of magnetic entropy of GdAl2 nanocapsules reaches 14.5 J kg-1 K-1 at 7.5 K in the magnetic fields varying from 0 to 5 T.
Furthermore, GdAl2/Al2O3 nanocapsules with crystalline cores of GdAl2 compound and shells of amorphous Al2O3 were prepared by using the modified arc-discharge technique, evaporating GdxAl100-x (x = 50, 60, 70, 80 and 90) alloys. The phase constitution of the as-prepared nanocapsules was determined by the ratio of Gd and Al atoms entering into the chamber. The morphology of the GdAl2/Al2O3 nanocapsules is irregular spherical shape. The GdAl2/Al2O3 nanocapsules behave as blocking, superparamagnetic and paramagnetic states in three temperature ranges in between 5 and 300 K. The Curie temperature of the GdAl2/Al2O3 nanocapsules is lower than that of bulk GdAl2 and it decreases with the increase of substitution Al atoms in GdAl2 lattice. According to isothermal magnetization measurements from 5 to 180 K, magnetic entropy change of the GdAl2/Al2O3 nanocapsules continuously increases with decreasing temperature T, which rapidly enhances when they enter into the blocking state. The largest entropy change at 7.5 K reached 19.1, 19.4, 32.5 J kg-1 K-1 at the variation of a magnetic field of 7 T for three GdAl2/Al2O3 nanocapsules synthesized by arc-discharging Gd70Al30, Gd80Al20 and Gd90Al10 alloys respectively. The appearance of a large entropy change in the blocking state was ascribed to a lower anisotropy energy barrier and large moments density of these nanocapsules. The linear relation between the entropy change and the reciprocal of the temperature (1/T) in the blocking state was found, which was discussed in terms of superparamagnetism and magnetocaloric theory.
FeB(C) nanocapsules were prepared by arc-discharging Fe80B20 alloy in Ar and CH4. From X-ray diffraction and transmission electron microscopy analysis, the FeB(C) nanocapsules show core/shell characteristic with -Fe and Fe3B as cores and graphite as shells. The formation mechanism of the FeB(C) nanocapsules is discussed. The graphite shells display strong anti-acid effect. The saturation magnetization at room temperature of the FeB(C) nanocapsules is much higher than that of Fe(B) nanocapsules. The blocking temperature of FeB(C) nanocapsules is above 300 K. |
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