Technique for Preparing Ultrafine Nanocrystalline Bulk Material of Pure Rare‐Earth Metals
Technique for Preparing Ultrafine Nanocrystalline Bulk Material of Pure Rare‐Earth Metals
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DOI：
10.1002/adma.200502619
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发表时间：
2006-05
影响因子：
29.4
通讯作者：
Xiaoyan Song;Jiuxing Zhang;M. Yue;E. Li;H. Zeng;Nian-Hu Lu;Meiling Zhou;T. Zuo
中科院分区：
文献类型：
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作者：
Xiaoyan Song;Jiuxing Zhang;M. Yue;E. Li;H. Zeng;Nian-Hu Lu;Meiling Zhou;T. Zuo
未成对的4f和5f电子的独特构型及其丰富的能级结构使稀土金属具有许多特殊的物理化学性质，如高电导率、大磁矩和非常高的络合反应活性。基于这些性能，稀土金属和化合物已被广泛应用于永磁体、汽车催化剂、超导体等。对高纯稀土氧化物和稀土金属的需求预计将增加，特别是在耐腐蚀性、储热和分散方面，以及在环境友好型应用中，如涂料和塑料颜料、水泥制造中降低焙烧温度和帮助节约能源的应用，以及由于寻找氯氟烃(CFC)替代品而产生的制冷部件。对于纳米级稀土金属，由于总表面积或晶界面积的显著增加，在晶体结构、界面、热力学和相变等方面呈现出一些新的特征。因此，可以预期显著改善光学、电学、磁学和催化性能。然而，由于具有极高的化学反应活性，因此需要相当严格的设备来保持产品的高纯度，因此纳米结构纯稀土金属的制备和表征仍然是纳米科学和纳米技术中的一大挑战。因此，纳米稀土金属的许多重要特性，如物理、化学、热和机械特性，到目前为止还很少被报道。然而，与这些特性相对应的研究无论是对于纳米科学和纳米技术的发展，还是对于扩大稀土金属的应用都具有重要的意义。鉴于此，我们在本工作中展示了如何制备一些典型稀土金属成员的纳米结构块体材料，为表征纳米级稀土金属的物理和化学性质奠定了基础。在过去的二十年里，人们发展了许多合成纳米块体材料的技术，如惰性气体冷凝固结、电沉积、强塑性变形、非晶态固体的晶化、表面机械磨损和粉末冶金。然而，在100 nm以下的大范围内很难制备出晶粒度可控的纳米晶材料。此外，在粉末冶金固结纳米颗粒的过程中，合成的块体中的颗粒尺寸通常大于初始颗粒尺寸。特别是，在传统的粉末冶金工艺中，纳米颗粒的快速粗化经常发生，导致形成亚微米甚至微米级的颗粒。利用一种新型的“无氧”(氧浓度为0.5ppm)原位合成技术，将惰性气体冷凝与放电等离子烧结(SPS)在一个完全封闭的系统中相结合，制备了具有超细(<；20 nm)纳米颗粒的纯稀土金属(Nd、Sm、Gd和Tb)纳米晶块体材料。考虑到文献中提出的SPS固结的特殊机理，我们设计了一种按顺序排列的制备方案，即纳米颗粒的非晶化、纳米颗粒内短程有序团簇的成核和生长以及完全纳米化，如图1所示。通过这种方法，我们实现了纯稀土金属块体纳米晶材料的制备。这项技术最显著的优点是，所得到的纳米晶块体的颗粒尺寸明显小于初始纳米颗粒尺寸，据我们所知，这是第一次证明，我们改进的粉末冶金技术可以改变传统上公认的初始粉末颗粒尺寸与烧结体颗粒尺寸之间的关系。借助于这一技术，具有可控晶粒度的纳米晶块体材料CO M M U N IC A TI O N S
The unique configuration of unpaired 4f and 5f electrons and the rich structures of their energy levels enable rare-earth metals to possess many particular physical and chemical properties, such as high electrical conductivity, large magnetic moment, and very high complexation reactivity. Based on these properties, the rare-earth metals and compounds have been applied extensively in permanent magnets, autocatalysts, superconductors, etc. Demand for high-purity rareearth oxides and rare-earth metals is expected to increase particularly for use in corrosion resistance, heat storage and dispersal, and also in environmentally friendly applications such as in pigments for paint and plastics, in cement manufacture to reduce the temperature of calcination and help save energy, and in refrigeration components arising from the search for chlorofluorocarbon (CFC) replacements. For the nanoscale rare-earth metals, because of the significantly increased total surface area or the grain boundary area, some new features show in the crystal structures, interface, thermodynamics, and phase transitions. Consequently, remarkably improved optical, electronic, magnetic, and catalysis properties can be expected. However, because of the extremely high chemical reactivity and hence the considerably rigorous equipment requirements to preserve a high purity of the product, the preparation and characterization of nanostructured pure rare-earth metals are still big challenges in nanoscience and nanotechnology. Thus, many important features of nanoscale rare-earth metals, such as the physical, chemical, thermal, and mechanical characteristics have rarely been reported so far. The research corresponding to these characteristics is of great importance, however, both for the development of nanoscience and nanotechnology and for extending the applications of the rareearth metals. In this consideration, we demonstrate in the present work how to prepare nanostructured bulk materials of some typical members of the rare-earth metals, laying the foundation for characterizing the physical and chemical properties of nanoscale rare-earth metals. During the past two decades, a number of techniques have been developed to synthesize nanocrystalline bulk materials, such as inert gas condensation and consolidation, electrodeposition, severe plastic deformation, crystallization of amorphous solids, surface mechanical attrition, and powder metallurgy. However, it is hard to produce nanocrystalline materials with controllable grain sizes in a wide range below 100 nm. Furthermore, in powder metallurgy for the consolidation of nanoparticles, the grain size in the synthesized bulk is generally larger than the initial particle size. Particularly, in conventional powder metallurgy processes, a rapid coarsening of nanoparticles occurs very often, leading to the formation of grains in the submicrometer or even micrometer range. Using a new “oxygen-free” (oxygen concentration < 0.5 ppm) in-situ synthesis, where inert gas condensation was combined with spark plasma sintering (SPS) in an entirely closed system, we prepared nanocrystalline bulk material of pure rare-earth metals (Nd, Sm, Gd, and Tb) with ultrafine (< 20 nm) nanograins. Taking into account the special mechanisms of SPS consolidation, which were proposed in the literature and were recently developed in our previous work, we designed a preparation scheme with sequentially arranged processes of: amorphization of nanoparticles, nucleation and growth of the short-range ordered “clusters” inside the nanoparticle, and the complete nanocrystallization, as shown in the diagram in Figure 1. By this approach we have realized the preparation of bulk nanocrystalline materials of pure rare-earth metals. The most significant advantage of this technique is that the grain size of the resultant nanocrystalline bulk is distinctly smaller than the initial nanoparticle size, which is the first demonstration to the best of our knowledge that the traditionally accepted relationship between the size of the initial powder particles and the grain size of the sintered bulk can be changed by our modified powder metallurgy technology. In virtue of this technique, nanocrystalline bulk materials with controllable grain C O M M U N IC A TI O N S