Reactive Interdiffusion of Components in a Non-Stoichiometric Two‑Layer System of Polycrystalline Titanium and Cobalt Oxides

Abstract

We demonstrated the possibility of using the mathematical form of Darken's theory, applied to the description of the Kirkendall effect in binary systems, to the description of reactive interdiffusion in non-stoichiometric polycrystalline film oxide systems with limited solubility. The aim of the study was the simulation of reactive interdiffusion under vacuum annealing of a thin film system consisting of two non-stoichiometric polycrystalline titanium and cobalt oxides. The nonstoichiometric nature of the system assumes the presence of mobile components, free interstitial cobalt and titanium cations in it. Phase formation occurs as a result of reactive interdiffusion and trapping of mobile components of the system on inter-grain traps. The proposed mechanism describes the formation of complex oxide phases distributed over the depth of the system. A complex empirical research technique was used, involving Rutherford backscattering spectrometry, X-ray phase analysis and modelling methods. The values of the characteristic parameters of the process were determined by numerical analysis of the experimentally obtained distributions of the concentrations of the components within the developed model. During vacuum annealing of a thin film two-layer system of non-stoichiometric TiO2–x–Co1–уO oxides in temperature range T = 773 – 1073 К, the values of the individual diffusion coefficients of cobalt DCo = 5.1·10–8·exp(–1.0 eV/(kT) cm2/s and titanium DTi = 1.38·10–13·exp(–0.31 eV/(kT) cm2/s were determined. It was shown that for T = 1073 K, the phase formation of CoTiO3 with a rhombohedral structure occurs. The extension of the phase formation region of complex cobalt and titanium oxides increases with an increase in the vacuum annealing temperature and at 1073 K it is comparable with the total thickness of the film system. The model allows predicting the distribution of the concentrations of the components over the depth of multilayer nonstoichiometric systems in which reactive interdiffusion is possible.

Keywords

• modelling
• interdiffusion reaction
• kirkendall effect
• interphases
• polycrystalline films
• complex oxides