Excess Thermodynamic Properties and Size-dependent Segregation Phenomena 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 these measurements han highly defective fine-grained TiO2.x prepared by the inert gas condensation process. Results show a clear temperature and/or grain size dependence of the excess grain boundary enthalpy of TiO2 . The specific grain boundary enthalpy, Hgb, increases from -0.5-1 J/m2 at low temperatures and very fine grain sizes (600-780°C, 30200 nm) to a value of 1.3-1.7 J/m2 averaged over a much larger temperature and size range (600-1300°C, 30 nm-2gm). Size and temperature effects on Hgb are inextricable. Additional evidence of a temperatureor size-dependent Hgb is obtained from measurements of the excess heat capacity of ultrafine-grained samples. The trend in Hgb is not due to experimental artifacts; extraneous contributions from heat-dissipating processes other than grain growth, including the anatase-rutile phase transformation, sintering, and relief of lattice strain, are shown to be negligible.