| Popis: |
Solid-state ion conduction (SSIC) is a mechanism of electric current which involves the efficient transport of ions through certain crystalline materials. SSIC occurs naturally in the mineral argentite (α-Ag2S) during the growth of wire silver, and can be induced by heating acanthite (β-Ag2S) in a strong thermal gradient. Given that argentite possesses the highest ionic conductivity of any known material, the wire growth process offers a unique opportunity to study the fundamental nature of SSIC. Previous studies noted a relationship between stable Ag isotope fractionation and temperature which warranted further investigation, and thus two sets of growth experiments were devised which enabled the accurate measurement of the thermal gradient during wire formation. Wire silvers were synthesized and subjected to stable Ag isotope analysis, which clarified that the rate of heavy isotope enrichment is an increasing function of the thermal gradient. These observations are potentially relevant for applications in emerging technologies that leverage SSIC, such as atomic switches; tuning the isotopic composition of charge carriers in a device may help optimize certain side effects of ion conduction. In order to better understand the internal ion dynamics of SSIC, a reactive force field (ReaxFF) potential was developed to enable molecular dynamics (MD) simulations of Ag/S-based atomic switches. ReaxFF forcefield optimization is a notoriously difficult task, typically plagued by slow convergence and restricted to training data based on static configurations. A new high-performance optimization algorithm PAGODA was built from the ground up to enable each worker in a parallel genetic algorithm the ability to control arbitrarily parallel instances of the MD engine LAMMPS. This capability makes feasible the use of full MD simulations in the training set, unlocking the door for many new types of training data, including extended crystal structures and multi-phase composites. PAGODA helps to lift restrictions on forcefield optimization and represents a key advance towards workflows that could be used to populate open- source repositories of ReaxFF potentials, analogous to the Basis Set Exchange. With PAGODA, a ReaxFF forcefield was produced which can model SSIC under realistic conditions. The forcefield was validated in simulations of two atomic switch configurations, one of which strongly resembles the initial stages of wire silver formation. |
