Two-phase equilibrium in individual nanoparticles of Bi-Sn
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ZE-DEPENDENT thermodynamic properties that are induced by strong capillarity forces on materials have been observed for many years. Depression of the melting temperature as a function of particle curvature (inverse radius) was first suggested theoretically about 100 years ago,[1] and experimentally shown over 50 years ago.[2] There have been relatively few studies looking into the effects of curvature on alloy systems,[3–6] and nearly all have involved the melting temperature depression. To date, the authors are aware of only three experimental studies of the size dependence of solid solubility and all show an enhancement with increasing surface cur-vature.[6,7,8] In the present article, solid solubility in isolated nanoparticles is studied and a three-dimensional T-X phase diagram is developed with the third axis of reciprocal radius (i.e., particle curvature). Growth conditions, analysis, and a potential theoretical framework for the study of the equilibrium solid solubility of two-phase BiSn nanoparticles are described. II.
EXPERIMENTAL DETAILS
A PHI ultra-high vacuum system was modified to permit the simultaneous deposition of tin and bismuth. High-purity tin and bismuth are individually evaporated through resistive heating of isolated tungsten-wire baskets. The baskets are mounted on a four-pin electrical feedthrough, and isolated from one another by a molybdenum foil between the baskets to prevent cross-contamination. The substrates are standard 3-mm 200 mesh Cu transmission electron C.T. SCHAMP, Graduate Student, and W.A. JESSER, Professor, are with the Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22904. Contact e-mail: [email protected] This article is based on a presentation made in the symposium ‘‘Phase Transformations Within Small-Size Systems: Thermodynamics, Phase Equilibria and Kinetics,’’ which occurred February 14–16, 2005, during the TMS Spring Meeting in San Francisco, CA, under the auspices of the ASMI/MPMD-Phase Transformations, EMPMD/SMD-Chemistry & Physics of Materials, and EMPMD-Nanomaterials Committees. METALLURGICAL AND MATERIALS TRANSACTIONS A
microscopy (TEM) grids supporting amorphous carbon purchased from the Ted Pella Company (Redding, CA). Heating of the substrate block holding these TEM grids are four resistively heated tungsten coils positioned symmetrically around the substrate holder, which is heated to about 90 °C for deposition. Immediately after deposition, the samples were annealed for about 1 hour, which should be adequate to attain equilibrium.[9] A load lock was installed on the ultra-high vacuum chamber to facilitate quick sample loading and unloading. For greater detail, the reader may refer to the Master’s thesis of Schamp.[10] Characterization of the nanoparticles is accomplished with a JEOL* 2010FX field emission analytical electron *JEOL is a trademark of Japan Electron Optics Ltd., Tokyo.
microscope, with an Oxford energy-dispersive spectrometer. The NIST computer program Desktop Spectrum Analyzer was used to analyze th
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