Synthesis and melting behaviour of Bi, Sn and Sn-Bi nanostructured alloy

Lead-free solders based on Bi-Sn bimetallic nanoclusters with eutectic composition (Bi43Sn57) were synthesized at low temperature by simultaneous reduction reaction from aqueous solution containing bismuth and tin chlorides, using potassium borohydride as a reducing agent. By the same processing route, pure bismuth and tin nanoparticles have also been prepared. Microstructure, morphology and composition of the samples were characterized by X-ray powder diffraction (XRD), transmission (TEM) and scanning electron microscopy (SEM). TEM images of Bi-Sn nanoparticles show average size ranging from 30 to 100 nm. Thermal behaviour of Bi-Sn nanopowders was studied by DSC (differential scanning calorimetry) and a melting temperature (135 C) lower than that of the corresponding microcrystalline sample (139 C) was observed. SEM micrographs of the thermally treated sample up to 400 C show fine spherical grains in the micrometer range with finer powder particles on the surface. XRD powder diffraction analysis indicates the formation of bismuth and tin nanophases with an average particle size of 85 and 126 nm, respectively. The oxidation behaviour of the samples was also investigated. The results obtained have been analyzed in view of theoretical models describing the melting temperature depression of nanoparticles.