Browsing by Author "Stewart, David John"

Now showing 1 - 2 of 2

Restricted
The kinetics and mechanism of formation of compounds of the spinel type
(Te Herenga Waka—Victoria University of Wellington, 1964) Stewart, David John
This thesis describes a study of the kinetics of formation of compounds of the spinel structure from the corresponding oxides, with particular reference to the spinel zinc ferrite (ZnFe2O4). The solid state reaction between zinc oxide and α-ferric oxide to form zinc ferrite has been studied by investigating the effect of particle size, temperature, and initial concentrations of reactants on the rate, A simple method of X-ray diffraction analysis has been developed for following the reaction. From a consideration of the structures of the reactant oxides and the product spinel, a diffusion model for the reaction has been proposed. The experimental data have been compared with several theoretical diffusion equations based on Fick's Law. The rate constants obtained for a range of temperatures have been tabulated and the enthalpy of activation calculated for the reaction of powder samples is 70 Kcals./gm.mole while that for pellets is 120 Kcals./gm.mole. The corresponding entropies of activation are +0.2 to +3.5 cals./deg./mole and -48 to -49 cals./deg./mole respectively, for the temperature ranges considered.
Restricted
The Role of Vapour Phase Transport and the Effect of Reaction Conditions on the Formation of Spinels
(Te Herenga Waka—Victoria University of Wellington, 1967) Stewart, David John
Previous work on the formation of complex oxides (spinels) by solid-state reaction between two simple oxides is reviewed and discussed with reference to possible mechanisms and kinetic models and the effect of reaction conditions on the relative importance of the various reaction processes. The kinetics of reaction between un-compacted ZnO and Fe2O3 powders have been investigated and compared to those of compacted powders. A quantitative analysis method has been developed using Mossbauer spectra. Comparison of the kinetic results obtained by this method with those of an x-ray method have enabled the diffusion rate of the iron ions to be investigated. The importance of vapour phase transport in spinel formation has been established and the volatility of the ZnO has been examined by measuring its rate of evaporation and equilibrium oxygen pressures as a function of temperature. A detailed reaction model is advanced and related to the reaction conditions.