Characterization of Sn-Bi nanoparticles

It is well known that as the size of particles gets small, material properties deviate from those of bulk materials. The bulk phase diagram shows melting and solubility properties of materials, but it does not provide information about the size dependence of those material properties. One method of visualizing the size dependence of the melting temperature and the solubility limits is to construct a temperature-composition phase diagram with a third orthogonal axis of size. A size-dependent phase diagram for the bismuth-tin eutectic system for temperatures greater than the eutectic temperature has previously been mapped out. That study is extended in this study by determining the solid solubilities of the bismuth-tin system as a function of size. From these results, a size dependent phase diagram for temperatures of room temperature and below is mapped out. The two phase diagrams are superimposed to construct a three-dimensional size dependent, T-X-1/r phase diagram showing the portions below room temperature, and above the eutectic temperature.

To grow the particles studied in this experiment, a physical vapor deposition process was employed. Equipment was designed and built so that a bi-metallic, Bi-Sn vapor is evaporated from resistively heated baskets. This vapor is then exposed to a heated substrate for a fixed period of time such that the vapor condenses onto the substrate and grows nanoparticles. The nanoparticles are annealed to ensure that they have an equilibrium structure and composition. The sample is allowed to cool, and is then analyzed by transmission electron microscopy, and energy dispersive x-ray spectroscopy.

It was found that the solid-solubility increases with decreasing particle size, down to a critical size. At this critical size, the two-phase field of the phase diagram pinches off. For sizes smaller than this critical size, it appears that the components are fully soluble in each other.

The data indicate that the ends of the tie-lines marking the compositions of each phase of two-phase particles are detached from the two-phase region boundary. As a result, single-phase data points are found to lie within the apparent two-phase field. This finding is a large deviation from that of bulk materials in which there is a clear and sharp boundary between two-phase and single-phase fields.

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