Abstract
A few experiments have detected icosahedral superclusters in undercooled liquids. These superclusters survive above the crystal melting temperature Tm because all their surface atoms have the same fusion heat as their core atoms, and are melted by liquid homogeneous and heterogeneous nucleation in their core, depending on superheating time and temperature. They act as heterogeneous growth nuclei of crystallized phase at a temperature Tc of the undercooled melt. They contribute to the critical barrier reduction, which becomes smaller than that of crystals containing the same atom number n. After strong superheating, the undercooling rate is still limited because the nucleation of 13-atom superclusters always reduces this barrier, and increases Tc above a homogeneous nucleation temperature equal to Tm/3 in liquid elements. After weak superheating, the most stable superclusters containing n = 13, 55, 147, 309 and 561 atoms survive or melt and determine Tc during undercooling, depending on n and sample volume. The experimental nucleation temperatures Tc of 32 liquid elements and the supercluster melting temperatures are predicted with sample volumes varying by 18 orders of magnitude. The classical Gibbs free energy change is used, adding an enthalpy saving related to the Laplace pressure change associated with supercluster formation, which is quantified for n = 13 and 55.
Highlights
An undercooled liquid develops special clusters that minimize the energy locally which are incompatible with space filling [1,2,3]
The formation of icosahedral nanoclusters has often been studied by molecular dynamics simulations into or out of liquids [5,6,7,8]
The classical Gibbs free energy change for a growth nucleus formation in a melt is given in Equation (1): ΔG1ls = θ
Summary
An undercooled liquid develops special clusters that minimize the energy locally which are incompatible with space filling [1,2,3]. An enthalpy saving εv per volume unit of critical radius clusters equal to εls × ΔHm/Vm was introduced in the Gibbs free energy change ΔG2ls which gives rise to spherical clusters that transform the critical energy barrier into a less effective energy barrier, thereby inducing crystal growth around them at a temperature Tc much higher than the theoretical homogeneous nucleation temperature equal to Tm/3. Superclusters containing 13 and 55 atoms have an energy saving coefficient εnm0 which is quantified This coefficient εnm0 associated with an n-atom supercluster strongly depends on n up to the critical number nc of atoms, giving rise to crystal spontaneous growth when εnm0 is equal to 0.217 in liquid elements [15]. Reduced s-state density of superclusters depending on their radius and electronic specific heat of Cu, Ag and Au n-atom superclusters are studied, imposing a relative variation of Fermi energies during their formation in noble metal liquid state equal to −2/3 of the relative volume change. Analysis of the influence of Cu superheating time on the undercooling rate
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