IN ULTRAFINE-GRAINED TITANIUM DIOXIDE

This work has two primary goals. The first is to advance our fundamental understanding of grain boundaries by making direct measurements of grain boundary thermodynamic properties using ultrafine-grained materials. The second is to explore possible deviations from conventional behavior at ultrafine grain sizes, specifically in regards to grain boundary thermodynamic properties and solute segregation to grain boundaries. The first direct measurements of grain boundary thermodynamic properties in an oxide have been made. The excess enthalpy and excess heat capacity of grain boundaries in TiO2 were measured by the calorimetric monitoring of grain growth in samples with an initial mean grain size of 30-70 nm. These samples contain a sufficiently large amount of grain boundary area such that the heat release upon grain growth can be measured accurately by commercial calorimeters. High density, fully oxygenated, phase-pure rutile (TiO2) with a mean grain size <50 nm was prepared by a chemical method and was found to be more appropriate for thes