Atomistics of pre-nucleation layering of liquid metals at the interface with poor nucleants

Sida Ma,Hongbiao Dong,Tao Jing,Adam J Brown,Ruslan L Davidchack,Paul B Howes,Chris Nicklin,Rui Yan,Qijie Zhai

doi:10.1038/s42004-018-0104-1

Abstract

Liquid layering at heterogeneous solid/liquid interfaces is a general phenomenon, which provides structural templates for nucleation of crystalline phases on potent nucleants. However, its efficacy near poor nucleants is incompletely understood. Here we use a combination of X-ray crystal truncation rod analysis and ab initio molecular dynamics to probe the pre-nucleation liquid layering at the sapphire–aluminium solid/liquid interface. At the sapphire side, a ~1.6 aluminium-terminated structure develops, and at the liquid side, two pre-nucleation layers emerge at 950 K. No more pre-nucleation layer forms with decreasing temperature indicating that nucleation of crystalline aluminium through layer-by-layer atomic adsorption of liquid atoms is not favoured. Instead, the appearance of stochastically-formed nuclei near the substrate is supported by our experiments. Nucleation on poor nucleants is dominated by the stochastic nucleation events which are substantially influenced by the pre-nucleation layers that determine the surface structure in contact with the nuclei.

Highlights

Liquid layering at heterogeneous solid/liquid interfaces is a general phenomenon, which provides structural templates for nucleation of crystalline phases on potent nucleants
According to the above thermodynamic analysis, we demonstrate that the interfacial energy reduction Δγ provides the driving force for the pre-nucleation liquid layering; this process is hindered mainly by the interfacial strain energy that arises from the lattice mismatch between the substrate and the partly ordered pre-nucleation layers (PNLs) structure
The poorly ordered in-plane structure of the PNLs at the (0001) sapphire–liquid Al interface is readily understood that Δγ is not sufficiently large to surmount the interfacial strain energy barrier caused by the formation of the PNLs with highly ordered in-plane structure

Summary

Introduction

Liquid layering at heterogeneous solid/liquid interfaces is a general phenomenon, which provides structural templates for nucleation of crystalline phases on potent nucleants. In Classical Nucleation Theory (CNT), nucleation of a crystalline phase is believed to originate from the stochastically formed nuclei[1] (~10–1000 atoms2) driven by energy fluctuations. Such nuclei may manifest as spherical caps when forming on foreign surfaces, which is termed as heterogeneous nucleation. If the liquid layering dominates, heterogeneous nucleation of the crystalline phase proceeds layer-by-layer through a structural templating process; if the energy fluctuations dominate, stochastically formed atomic clusters may become the nuclei and heterogeneous nucleation obey the rules described by the CNT. Heterogeneous nucleation of the crystalline phase on the potent nucleant substrates is most likely to be dominated by the liquid layering, which causes the formation of the PNLs

Methods

Results

Conclusion