Melting at the Limit of Superheating

Theories on superheating-melting mostly involve vibrational and mechanical instabilities, catastrophes of entropy, volume and rigidity, and nucleation-based kinetic models. The maximum achievable superheating is dictated by nucleation process of melt in crystals, which in turn depends on material properties and heating rates. We have established the systematics for maximum superheating by incorporating a dimensionless nucleation barrier parameter and heating rate, with which systematic molecular dynamics simulations and dynamic experiments are consistent. Detailed microscopic investigation with large-scale molecular dynamics simulations of the superheating-melting process, and structure-resolved ultrafast dynamic experiments are necessary to establish the connection between the kinetic limit of superheating and vibrational and mechanical instabilities, and catastrophe theories. INTRODUCTION Melting and freezing as first-order phase changes and their related kinetics, are of ubiquitous theoretical and experimental interest in condensed matter physics, materials science and engineering, geophysics and planetary sciences.[1] Metastable superheating and undercooling are inherent in melting and freezing processes. Determining the degree to which a solid can be superheated and a liquid undercooled, is a fundamental and challenging issue. Experimental investigation of the maximum superheating is particularly difficult due to the existence of heterogeneous nucleation sites (e.g. free surfaces and defects), and the difficulty in achieving high heating rates while making sensible measurements. Theoretical efforts in understanding superheating-melting have been seriously undermined by the paucity in superheating data. Molecular dynamics (MD) simulations have been utilized to probe melting and freezing processes at atomic level, and serve an important complementary approach to theoretical and experimental techniques. Previous superheating-melting theories[1] including Lindemann and Born’s criteria, orderdisorder transition, catastrophes of entropy, volume and rigidity, and nucleation-based kinetic models are briefly reviewed. We present certain details on the recently developed systematics for the maximum superheating and undercooling.[1] Experimental, theoretical and simulation directions for future investigations of superheatingmelting process are presented. SUPERHEATING-MELTING THEORIES Solids differ distinctly from liquids in both their long-range order and ability to resist shearing. The definitions and criteria for melting mostly involve vibrational and mechanical instabilities and order-disorder transitions.[1] Lindemann’s vibrational criterion[1] states that melting occurs at the onset of an instability when the atomic displacements (e.g. the root-meansquared displacements) during thermal vibrations exceed a certain threshold. Born’s mechanical criterion[1] states that the stability against shearing stress vanishes (e.g. for cubic lattice c44 = 0 where c44 is the elastic constant in Downloaded 06 Mar 2006 to 131.215.240.9. Redistribution subject to AIP license or copyright, see http://proceedings.aip.org/proceedings/cpcr.jsp Voigt’s notation) and such shearing instability is essentially melting. Melting is also interpreted as structure transition from order to disorder as proposed by Lennard-Jones and Devonshire.[1] Such a order-disorder transition is arguably attributed by Cahn[1] to the spontaneous production of intrinsic lattice defects.