Re-oxidation mechanism and kinetics of fine scale Ti-Magnéli phases in fibre form using thermo-gravimetric analysis

This paper describes the manufacture and properties of fine scale (Ø 260 μm) and dense (>96 % theoretical density) fibres consisting of Magnéli (Ti_nO_2n-1) phases for sensing and energy storage applications. In order to understand their operational limits, the re-oxidation of the Magnéli phases in air was examined using thermo-gravimetric analysis at temperatures up to 900 °C under a variety of heating rates. The material was characterised before and after re-oxidation via X-ray diffraction and scanning electron microscopy. The re-oxidation of the Magnéli phases was observed to begin at 650 °C, and the kinetics of the process was studied using the iso-conversional method. The calculated activation energy was consistent with Jander's three-dimensional diffusion model, where oxidation is limited by diffusion of oxygen through a layer of the oxidised product. An activation energy of 0.71 eV was obtained from kinetic analysis of the thermogravimetry data, which is in agreement with previous work on electrical conduction of Magnéli phases using impedance spectroscopy.