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

Abstract

Two kinds of Pb± Al granular sample (with nanometre-sized Pb particles embedded in an Al matrix) were prepared by using melt-spinning and ballmilling techniques. The Pb particles were synthesized with similar particle sizes, about 5± 30nm 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± 40K, whereas the irregularly shaped Pb particles in the ball-milled sample melted about 13K 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.

Nanoscale lead and noble gas inclusions in aluminum: Structures and properties

“…They were later verified by Goswami and Chattopadhyay (1997) and Johnson and his collaborators (Grå baek et al, 1992;Johnson et al, 1990; for further details see also Andersen and Johnson, 1995, and references therein). The results have also been confirmed by Sheng and his collaborators in Shenyang (Sheng et al, 1996(Sheng et al, , 1998 who also noted that lead inclusions in ball milled aluminum-lead alloys often had random orientations and, subsequently, loss of distinct facets. This group also invented a process for making alloys with high lead concentrations by the use of consecutive rolling and folding of multilayer lead aluminum sandwiches that eventually would introduce break-up of the layers into a nanostructured material .…”

“…Melting was found to be curvature driven with a melting point depression that was largest for the inclusions with the smallest radius of curvature (Kofman et al, 1994;Lereah et al, 2001). The discrepancy between observations of superheating effects and premelting effects was solved in 1996 when Sheng and collaborators (Sheng et al, 1996) presented evidence for the assumption that superheating of the lead inclusions in aluminum was associated with the low energy of the interface facets. Using in-situ TEM analysis, they showed that faceted and aligned lead inclusions in rapidly solidified alloys melted significantly above the bulk melting point of lead, while similar inclusions in ball milled alloys without preferred orientation and without low-energy facets melted below the bulk melting point, e.g., under conditions where ␥ sm Ͼ ␥ lm .…”

“…The size effect also leads to significant deviations in melting point for nanoscale inclusions as their size is decreased. Free nanoscale crystals are nearly always observed to display a size-dependent decrease in the melting point (Couchman and Jesser, 1977;Kofman et al, 1994;Peters et al, 1998), while for inclusions both a decrease and an increase of the melting point have been observed (Sheng et al, 1996), depending on whether the inclusion shape is defined by low-energy facets or not. Faceting of the inclusions is related to the anisotropy of the interface energy that can be inter-preted as a ratio of separation of distances observed for specific interface planes of an embedded crystal when elastic and other contributions to the energy that would affect particle shape are negligible.…”

Size dependence of nanostructures: Impact of bond order deficiency

“…An MD calculation suggested that a 115 K overheating occurs to Pb(111) films confined in an Al(111) matrix [262]. The T m suppression for a free surface is attributed to the reduced degree of confinement, and hence the increased entropy of the molecules at the surface compared with atoms in the bulk, whereas the T m elevation or depression of the embedded nanosolids depends on the coherency between the nanosolids and the embedding matrix [263,264].…”

Effect of Sputtering Area Ratio of Gold/Alumina Target on Microstructure and Optical Absorption Properties of Au Nanoparticles dispersed in Amorphous Alumina Dielectric films

“…However, for the embedded NPs the melting point is not only related to NPs' size, structure and shape; but also affected by the embedding matrix. For some matrices, melting of the embedded NPs occurs in lower temperature than its bulk state, while it is possible for the same NPs to have superheating above the melting point in some other matrices [31][32][33][34][35]. Qi et al [36] have developed a new model to accounting for the size and shape dependent superheating of nanoparticles embedded in a matrix where the particle shape is considered by introducing a shape factor.…”