Grain boundaries in complex oxides

Abstract

This research program investigates the properties of grain boundaries in complex multicomponent or nonstoichiometric oxides, including those of interest for electroceramics applications. One area of current research is the relationship between complex lattice defect structures and the ionic space charge at grain boundaries. We focus on TiO2 as a system with lattice defect properties characteristic of many electroceramics, including nonstoichiometry and regimes of high electronic conductivity. Model experiments are performed using a newly developed STEM method to quar, ify the grain boundary accumulation of trivalent (Al3+, Ga3+) and pentavalent (Nb5+) cation dopants. A space charge model is developed which explicitly includes the lattice defect chemistry of TiO2. This work is aimed at making for the first time a detailed, quantitative comparison between space charge theory and grain boundary segregation. The fundamental defect formation energies at grain boundaries and their variation from boundary to boundary are studied. In future work, space charge effects at nanocrystalline grain sizes, and at bimaterial interfaces, emphasizing crystal-glass interfaces germane to the behavior of thin glassy films at grain boundaries, are to be studied. A second area of research concerns the direct measurement of grain boundary thermodynamic properties and size-dependent segregation and transport phenomena in nanocrystaUine oxides. Current efforts are aimed at using high temperature calorimetry to measure the heat of grain coarsening and excess heat capacity of nanocrystalline TiO2, in order to obtain the specific grain boundary enthalpy and entropy and their temperature and composition dependences. With appropriate models for the enthalpy release rate during grain coarsening, in-situ measurement of grain growth kinetic parameters (time exponent, activation energies) is possible. The effect of solute segregation on grain boundary thermodynamic and kinetic properties, and size-dependent segregation phenomena, including the effects of space charge fields on grain growth and superplastic deformation, are to be studied.