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Role of surface structure/composition in altering solidification behavior is demonstrated through undercooling of core-shell metal particles. Autonomous surface speciation in the passivating oxide plays a critical role in the relaxation of a molten metal due to divergence in the concentration and composition of oxidizing species across the thickness of the oxide layer. In an unreactive environment, surface speciation is dictated by flux, cohesive energy density, and surface energy minimization. Under oxidizing conditions (e.g., ambient), however, reduction potential, curvature, and surface plasticity dictate the spatial order and concentration(s) across thin passivating oxide layers. It is therefore important to redefine solubility beyond the limitations of Hume-Rothery rules by substituting electronegativity for redox potential and cohesive energy density. Increasing number of components in an alloy does not necessarily lead to increased undercooling, but a maximum is observed around two to three components. These results lead to an empirical observation that ΔG LS (Gibbs' free energy for liquid-solid transition) around the freezing point can be understood from enthalpy and surface tension balance, with the degree of undercooling being a proportionality term for the enthalpic component. This work extends our understanding of classical nucleation theory by illustrating the importance of asymmetry in surface work in phase change, largely due to changes in structure of nanoscale passivating oxides. This phenomena is critical in development of low-temperature solders. [ABSTRACT FROM AUTHOR]

Undercooling metals relies on frustration of liquid–solid transition mainly by an increase in activation energy. Passivating oxide layers are a way to isolate the core from heterogenous nucleants (physical barrier) while also raising the activation energy (thermodynamic/kinetic barrier) needed for solidification. The latter is due to composition gradients (speciation) that establishes a sharp chemical potential gradient across the thin (0.7–5 nm) oxide shell, slowing homogeneous nucleation. When this speciation is properly tuned, the oxide layer presents a previously unaccounted for interfacial tension in the overall energy landscape of the relaxing material. We demonstrate that 1) the integrity of the passivation oxide is critical in stabilizing undercooled particle, a key tenet in developing heat‐free solders, 2) inductive effects play a critical role in undercooling, and 3) the magnitude of the influence of the passivating oxide can be larger than size effects in undercooling. [ABSTRACT FROM AUTHOR]

Cover Picture: Stabilization of Undercooled Metals via Passivating Oxide Layers (Angew. Engineering composition across a thin surface oxide layer of a metal particle, and stabilizing it with appropriate ligands, leads to a sharp surface stress that frustrates solidification. Engineering composition across a thin surface oxide layer of a metal particle, and stabilizing it with appropriate ligands, leads to a sharp surface stress that frustrates solidification. [Extracted from the article]

Energy landscape, inductive effect, metastability, phase transformation, stable undercooling Es bildet sich ein unterkühltes flüssiges Metallteilchen, was ein neues Konzept für das gezielte Engineering der Energielandschaft eines Materials unter Verwendung einer dünnen Oberflächenschicht aufzeigt, wie Martin Thuo et al. in ihrem Forschungsartikel auf S. 5993 beschreiben. Keywords: energy landscape; inductive effect; metastability; phase transformation; stable undercooling EN energy landscape inductive effect metastability phase transformation stable undercooling 5633 5633 1 03/03/21 20210308 NES 210308 B Kühle Frustration b . [Extracted from the article]