Do we have a probe for the initial stages of solid state reactions?

Abstract

The aim of this work is to show that ReflEXAFS can be used as a tool for investigating the initial stages of solid state reactions, when local chemical equilibrium is not yet attained at the interfaces. A very classical solid state reaction, i.e. the formation of Ni–Al spinel starting from the parent oxides (NiO + Al₂O₃ → NiAl₂O₄) has been investigated by monitoring with Ni–K edge ReflEXAFS the time evolution of the local chemical environment of Ni in a reacting NiO thin layer. At temperatures above 1000 °C, a progressive evolution from an NiO environment to a spinel environment has been obtained with increasing firing times. At lower temperatures (≅ 930 °C) the NiO film does not react but undergoes a structural rearrangement, which is seemingly related to the process by which local chemical equilibrium is attained at the interface.