The Influence of Interfaces on Reactions in Oxide Ceramics

Many technologically important properties of crystalline solids are either determined by or strongly influenced by the presence and behavior of defects. Interfaces between two crystals (grain or phase boundaries) or between a crystal and a gas (surfaces) are perhaps the most technologically important class of defects because of the influence they exert on other defects (point defects, dislocations, pores and second-phase particles). Obtaining a thorough and fundamental description of interfaces including their influence on macroscale properties of solids is a daunting scientific challenge; in many important polycrystalline solids there will be a large number of interfaces with diverse, non-periodic atomic structures. In oxide ceramics, the situation is further complicated by, for example, segregated impurity atoms and amorphous interfacial films. This dissertation will describe a number of experiments involving reactions at interfaces in quite different oxide ceramic systems. By comparing observations in these different systems and with the observations of previous researchers, some general principles applicable to different types of interfaces in oxides will be described. However it will also be seen that in many cases reaction behavior will depend in a complicated way on the details of the interface and the external variables such as temperature. For this research, "reactions" refers primarily to charged point-defect reactions including segregation phenomena and phase transformations in which point-defect movement is a necessary feature. Dislocations will also make several brief appearances. These reactions are of particular practical interest because of their importance in the performance of solid oxide fuel cells and catalysts, both of which are important for alternative-energy technology. The primary experimental tool has been the transmission electron microscope (TEM) which combines high spatial resolution (0.1 nm or better) with the capability to detect many different signals resulting from electron scattering phenomena in solids, most importantly for this research, ionization events and constructive interference of electron waves. Analysis of these scattering phenomena enables local crystallographic, microstructural and compositional information to be obtained. These capabilities (particularly at high spatial resolution) are unique to the TEM and essential for understanding defects in solids; the technique also has several limitations that will be discussed.