Combined Cryo and Room-Temperature Ball Milling to Produce Ultrafine Halide Crystallites
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NANOCRYSTALLINE materials constitute an important class of materials. The sole characteristic of nanomaterials is the size effect i.e., the properties of these materials depend on the crystallite size or grain size. The preparation and characterization of these novel materials is very much needed to probe the effect of size on the properties. These materials can be synthesized by using either the bottom-up or the top-down approach.[1] The bottom-up approach refers to the build up of the nanomaterials from the bottom; atom-by-atom, layerby-layer or cluster-by-cluster. In the top-down approach the micron- or submicron-sized microstructure changes to nanostructures for the bulk material. Attrition or mechanical milling is a typical top-down method in preparing nanoparticles. The communition of powders to a small size by using high-energy ball-milling is termed mechanical milling.[2–5] The process is carried out in a high-energy ball mill, and it involves repeated deformation, cold-welding, fracturing, and dynamic recrystallization to achieve an extremely fine grain size.[2] The force of the impact exerted by the balls plastically deforms the power leading to work hardening and fracture. The main attribute of this process is the refinement of crystallite size to nanometric range. The biggest disadvantage with the top-down approach is the generation of structural defects in the material. Substantial amount of research work has been AKASH VERMA, Student, and KRISHANU BISWAS, Assistant Professor, are with the Department of Materials Science and Engineering, Indian Institute of Technology, Kanpur, U.P. 208016, India. Contact e-mail: [email protected] CHANDRA SEKHAR TIWARY, Student, AMIT KUMAR MONDAL, Project Associate, and KAMANIO CHATTOPADHYAY, Professor, are with the Department of Materials Engineering, Indian Institute of Science, Bangalore, Karnataka 560012, India. Manuscript submitted April 4, 2010. Article published online November 4, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
done in preparation and in characterization of metallic, oxide, nitride, and carbide nanoparticles.[1] To the best of the authors’ knowledge, there is no research work available dealing with preparation and characterization of ultrafine halide nanoparticles. The halides are very reactive and thus these nanoparticles cannot be prepared using bottom-up methods. One needs to use the topdown approach to synthesize these nanoparticles. The present article deals with the preparation and detailed characterization of sodium chloride nanoparticles by combined low temperature (cryo) and roomtemperature (RT) vacuum milling.[3–5] Cryomilling is a variation of mechanical milling in which metallic or ceramic powders are milled in cryogenic slurry (liquid N2) or at cryogenic temperatures such that nanostructures are obtained. It is well known that the cooling of powders is an effective way to accelerate the fracturing process.[3–5] In addition, extremely low temperatures in cryomilling suppress recovery and recrystallization processes, causing rapid
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