Studies of Ordering in Small Particles
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U.Herr* **, M.Pollack*, D.L. Olynick*, J.M.Gibson*, R.S. Averback* *University of Illinois at Urbana-Champaign, Urbana, IL61801 "**Institute of Physics, University of Augsburg, 86135 Augsburg, Germany ABSTRACT
Disordered clusters of the intermetallic compounds Ni 3Al and Cu 3Au have been produced using a high pressure sputtering technique. The clusters are either embedded in a film or studied, in-situ in an UHV electron microscope. The evolution of the ordered structure upon annealing is studied. Using a scanning transmission electron microscope, electron diffraction is obtained from individual clusters. Partial ordering is observed in Cu 3Au clusters which have been annealed below the bulk order-disorder transition temperature. Under the experimental conditions, only clusters with sizes of 10-15 nm or larger show ordering. INTRODUCTION
Intermetallic nanoparticles are potentially useful for a variety of structural, catalytic and functional applications. The phase stability and the transformations between phases may be different from the bulk behaviour due to the presence of surfaces and interfaces. The occurrence of nonequilibrium phases in the nanocrystalline state compounds has been observed frequently, e.g. in materials prepared by inert gas condensation [1] or ball milling [2]. A well-known example is the formation of the disordered fcc solid solution in ball milled nanocrystalline Ni 3 AI material [2), which exhibits reordering upon annealing. However, simultaneous grain growth occurs during the annealing treatment. It therefore appears interesting to study the behaviour of individual, well-separated nanoparticles or clusters under comparable conditions. In this study, Ni 3A1 and Cu 3Au have been chosen which both exhibit LI 2 order at low temperatures in the equilibrium pahse diagram [3]. Whereas stoichiometric Ni3 AI remains ordered up to its melting point, L1 2 -ordered Cu 3 Au has a
transition at moderate temperature (663K). Surfaces of Cu 3Au have been subject of various studies using electron and X-ray diffraction [4] techniques which revealed that in systems with first-order bulk transitions a continuous transition occurs at the surface. In the present work, isolated nanoparticles of Ni 3AI and Cu 3Au have been studied under different experimental conditions with the aim to better understand the ordering process in these materials. EXPERIMENT
The clusters used in this study have been prepared by sputtering from stoichiometric targets in Argon atmosphere of about 0.5 - 1 Torr. Based on this technique, two different types of experiments have been conducted. In the first type of experiments, a sputter chamber (base pressure < 10-8 Torr) has been attached directly to a UHV compatible transmission electron microscope (JEOL 200 CX, base pressure in the lower 10-8 Torr range). The clusters were generated by sputtering for about 60 seconds under static Argon atmosphere. Subsequently, the Argon gas was pumped out through the microscope and the clusters were deposited onto a substrate (amorphous Carbon film o
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