Chemical Reactions within Fe/AlN Layered Nanocomposites: A Simulation Study based on Crystal-Chemical Atomic Dynamics

Abstract

Layered nanocomposites made of metallic iron and aluminum nitride are subject to unexpected chemical reactions, resulting in a spontaneous formation of iron nitrides and a partial reduction to metallic aluminum. Since bulk thermochemical data are unable to rationalize the above finding, atomistic computer simulations based on the crystal-chemical atomic dynamics (CCAD) approach have been performed in the search for an explanation. The computational setup mimics a total number of about 1000 atoms moving over a time frame of 74 ps. When AlN molecules are sputtered on the iron surface under the experimental radio frequency (rf) conditions, the molecules are found to be chemically unstable upon hitting the surface, immediately breaking apart into individual atoms. Atomic nitrogen enters the Fe crystal to acquire quasi-octahedral coordination, leaving Al atoms behind on the surface. The reaction results in a stronger bonding of the nitride ion in the crystal compared to the covalently bonded nitrogen atom in the molecule. As a consequence, a small amount of Fe lattice expansion (2.5%) as well as a partial buildup of an iron/aluminum alloy is observed in the reaction zone near the surface of the bulk material.