Stabilization of Undercooled State via Passivating Layers

Abstract

<p>Advances in hybrid, high‐density and flexible/wearable electronics demand low temperature metal processing. Undercooled metals have emerged as a solution to low temperature soldering and printing of conductive traces. The process of undercooling, however, relies on frustration of liquid‐solid transition mainly through increase in activation energy by: i) elimination heterogeneous nucleants, or ii) frustrating homogeneous nucleation. We inferred that passivating oxide layers present an active platform that can 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 hence slows 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. Herein, the role of surface oxide structure in enhancing and maintaining undercooling is demonstrated. We demonstrate that; i) the integrity of the passivation oxide is critical in stabilizing undercooled particle, a key tenet in developing heat‐free solders, ii) that inductive effects play a critical role in undercooling, and iii) that magnitude of the effect of the passivating oxide can be larger that of size in undercooling.</p>