Using engineered defects to explore 3D feedback processes in current-driven metal

Abstract

Metals driven by intense electrical current are used in a wide range of applications. However, modeling efforts usually assume metals are homogeneous, when in fact they contain many three-dimensional (3D) perturbations, such as µm-scale resistive inclusions and voids. Focusing on the case of a pit on the surface of a metal rod, magnetohydrodynamic simulations show that the pit causes electrical current density j to redistribute and amplify, thus initiating a feedback loop: j both reacts to and alters the electrical conductivity σ through Joule heating and hydrodynamic expansion, so that j and σ are constantly in flux. Consequently, overheated regions around the pit grow in a direction transverse to j , developing a wide, hot strip – known as a striation in electrothermal instability (ETI) theory – as well as exploding plumes and constantly growing craters on the metal surface. Within the low-density plume, σ increases with rising temperature (opposite the solid/liquid phase), altering the nature of the feedback loop: now overheated regions grow in a direction parallel to j to form elongated plasma filaments. Throughout this process, simulations predict distinctive, time-evolving self-emission patterns, inviting comparison to experiment. However, metal must be ultrapure and ultrasmooth (so self- emission is not dominated by resistive inclusions or machining features), and finally a 10-µm-scale pit must be machined on the surface. Recent experiments satisfy these strict constraints and show promising agreement with simulation, thus providing a glimpse into the remarkably complex 3D dynamics of current-driven metal.