Superheating and melting-point depression of Pb nanoparticles embedded in Al matrices

Two kinds of Pb-Al granular sample (with nanometre-sized Pb particles embedded in an Al matrix) were prepared by using melt-spinning and ball-milling techniques. The Pb particles were synthesized with similar particle sizes, about 5-30 nm in diameter, but were observed to have different Pb-Al interfaces for samples prepared using different approaches. In the melt-spun Pb-Al samples, the Pb nanoparticles were seen to have an epitaxial relationship with the Al matrix, while the Pb particles in the ball-milled sample were seen to be randomly oriented with respect to the Al matrix. The melting behaviour of the Pb particles was investigated by differential scanning calorimetry. It was found that the faceted Pb nanoparticles in the melt-spun sample could be superheated by about 11-40 K, whereas the irregularly shaped Pb particles in the ball-milled sample melted about 13 K below its equilibrium point. We suggest that different interface structures, which cause variations in energy differences between solid-Pb-solid-Al and liquid-Pb-solid-Al interfaces, are responsible for the different melting behaviour of Pb nanoparticles.