Kinetics and mechanisms of solid-state phase transitions and reactions

Abstract

The phase transition of C,a- to ^p-ferrosilicon has been studied, both with and without a doping of 1% aluminium, at high temperature by time-resolved simultaneous X-ray powder diffraction and iron K-edge EXAFS at four temperatures, and also by high temperature time-resolved Mossbauer spectroscopy at two temperatures. The extent of the phase transition and its evolution with time has been determined using a novel method. The results have been analysed in terms of theories of solid state kinetics. Some tentative conclusions for the mechanism of the phase transition have been drawn. Approximate values for the activation energy of the phase transitions, with and without 1% aluminium doping, have been derived. The structures of the phases, with and without 1% aluminium doping, have been examined by techniques including X-ray powder diffraction, Fe K-edge EXAFS and Si K-edge EXAFS and Mossbauer spectroscopy, in order to gain insight on how the presence of the dopant might affect the phase transition. The formation of mullite from precursors formed by the hydrothermal processing of aluminium- and silicon-acetates, both undoped and with the inclusion of iron and vanadium dopants, has been investigated using flat-plate room temperature X-ray powder diffraction of samples heated at a variety of temperatures and for different durations of time. The structures of the precursors and the mullite formed have been examined by techniques including X-ray powder diffraction, Fe K-edge EXAFS, Mossbauer spectroscopy and MAS NMR. The formation of 5% iron-doped mullite from a precursor formed by the hydrothermal processing of aluminium-, silicon- and iron acetates has also been monitored by time-resolved X-ray powder diffraction.