Technique for Preparing Ultrafine Nanocrystalline Bulk Material of Pure Rare‐Earth Metals

Abstract

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