Nano-alloys Synthesized by Controlled Crystallization from Supercooled Atomic Clusters of Elements
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A.H.W. Ngana) Department of Mechanical Engineering, The University of Hong Kong, Hong Kong, People’s Republic of China (Received 10 June 2003; accepted 10 November 2003)
Materials in nanocrystalline forms are well known to possess unusual and interesting properties when compared to the bulk conditions, and these open up an exciting range of novel applications. The key step involved in the systematic exploitation of nanocrystals for real applications lies in the development of reliable methods to synthesize nanocrystals of arbitrary chemical compositions in a range of crystal sizes. In particular, metallic alloy nanocrystals pose a special challenge. We demonstrate that nano-to-micro-sized crystals of intermetallic nickel–aluminide (Ni3Al) ranging from approximately 3 nm to over 100 nm in size can be synthesized by co-sputtering from elemental Ni and Al onto unheated, incompatible organic substrates, followed by controlled postdeposition heat treatment at mild temperatures. The crystal size of approximately 3 nm here is the smallest ever reported for monolithic ordered Ni3Al.
I. INTRODUCTION
In recent years, nanostructured metallic alloys have been demonstrated to exhibit a range of interesting properties1–3 for novel applications such as, for example, superplastic materials,4 catalysts,5,6 soft magnets,7–9 thermal-electrical sensors,10 gas sensors and hydrogenstorage materials,11 structural components for micronano devices,12 shape memory materials,13 and so forth, and it is highly likely that many more novel properties can be found from metallic alloy nanocrystals (MANCs) if more systematic investigations can be carried out. This calls for the development of generic methods to synthesis MANCs with chemical compositions chosen at will. Traditional means of synthesizing nano-sized powders such as electrochemical or precipitation reactions14 are often material specific, and their applicability to MANCs with arbitrary, yet predetermined chemical compositions is rather limited. The method of solid-state precipitation by aging a supersaturated solid solution can produce nanosized precipitates,1 but such nanocrystals are embedded in a matrix and not in monolithic form, and the applicability is also limited to systems with suitable phase separation and solidus curvature. A technique using pulsedlaser vaporization and condensation has successfully
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J. Mater. Res., Vol. 19, No. 3, Mar 2004
been used to produce nanocrystals of the B2 intermetallic FeAl,15,16 but applicability to a wide range of MANCs has not yet been established. II. EXPERIMENTAL
In this work, we demonstrate the possibility of using magnetron cosputtering on cold, incompatible substrates, followed by controlled annealing in ultrahigh vacuum to synthesize MANCs. The key is to produce a highly supercooled but microscopically homogeneous state of the required chemical composition by co-sputtering, and MANCs can then be grown out from such a far-fromequilibrium state by controlled anne
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