Experimental studies of crystal nucleation: metals and colloids

Abstract

Crystallization is one of the most important phase transformations of first order. In the case of metals and alloys, the liquid phase is the parent phase of materials production. The conditions of the crystallization process control the as-solidified material in its chemical and physical properties. Nucleation initiates the crystallization of a liquid. It selects the crystallographic phase, stable or meta-stable. Its detailed knowledge is therefore mandatory for the design of materials. We present techniques of containerless processing for nucleation studies of metals and alloys. We demonstrate the power of these methods for crystal nucleation of stable solids but in particular also for investigations of crystal nucleation of metastable solids at extreme undercooling. This concerns the issue of heterogeneous versus homogeneous nucleation and non-equilibrium conditions. The results are analyzed within classical nucleation theory, where the activation energy of homogeneous nucleation depends on the interfacial energy and the difference of Gibbs free energies of solid and liquid. The interfacial energy acts as barrier for the nucleation process. Its experimental determination is difficult in the case of metals. In the second part of this work we therefore explore the use of colloidal suspensions as models for the crystallization process. Their nucleation process is observed in situ by optical techniques and ultra-small angle X-ray diffraction using high intensity synchrotron radiation. It allows an unambiguous discrimination of homogeneous and heterogeneous nucleation as well as the determination of the interfacial free energy of the solid-liquid interface. Our results are used to construct Turnbull plots of colloids, which are discussed in relation to Turnbull plots of metals and support the hypothesis that colloids are useful model systems to investigate crystal nucleation.

Invited review submitted for a special issue of J. Chem. Phys. "Overview Articles - Special Topic: Nucleation: New Concepts and Discoveries on the physics of nucleation". Submitted 2016 May 10. 61 pages 189 References 25 Figures. Abstract above is slightly shoretened to match ArXive rules v2 - added reprint permissions to all figures

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• Kinetic phase diagram for nucleation and growth of competing crystal polymorphs in charged colloidal suspensions
• d-SEAMS: Deferred Structural Elucidation Analysis for Molecular Simulations
• Interfacial Line Energy of a Topological Phase
• The Droplet Formation-Dissolution Transition in Different Ensembles: Finite-Size Scaling from Two Perspectives
• From particle condensation to polymer aggregation
• Peaked bulk crystal nucleation in charged sphere melts from salt concentration dependent crystallization experiments at very low metastability