SURFACE MELTING ENHANCED BY CURVATURE EFFECTS

Recent unambiguous experiments (melting of ultrafine particles, premel ting at the surface of a bulk crystal, superheating, etc.) offer clear evidence of the key role of the surface in determining the melting of a material. In this work we concentrate our attention on spherical an d non-spherical nanometric lead inclusions. We report experimental res ults on Pb/SiO and Pb/Al2O3 systems obtained at different temperatures by two techniques: high-sensitivity optical reflectance and dark-fiel d electron microscopy. The main result is the existence, below the mel ting temperature and at the surface of the inclusion, of a liquid laye r whose thickness is much larger than that observed on the bulk (zero curvature). This thickness, which depends on local curvature, increase s continuously with temperature until a uniform curvature of the solid core is attained; then the core melts suddenly. A phenomenological mo del, based on the minimization of the free energy, is proposed and rep orted in detail. It represents a significant improvement compared to p revious theoretical approaches related to well-known thermodynamic siz e-effect models, particularly insofar as the agreement with the experi mental results is concerned.