Charged grain boundaries and the microstructural evolution of ionic ceramics

The macroscopic properties of polycrystalline ionic ceramics are determined by the doping levels, point defects, and their interaction with the microstructure, as they are specified via processing and the target application. Here, the starting powders react, densify, and coarsen into microstructurally tailored grain topologies that are aimed to enhance (but sometimes limit) the performance of the device that are part of. The extent of these interactions varies with grain size, crystallographic orientation, and misorientation distribution, as well as applied fields, such as stress or electric fields. In order to understand the grain boundary characteristics, including their electrochemical properties and the driving forces that control grain coarsening, a thermodynamically consistent diffuse interface theory has been developed. The theory naturally incorporates the effects of grain boundary drag as imposed by the interfacially accumulated charged defects on the grain growth of polycrystalline ceramics. Applications to materials such as YSZ, GCO, and STO (and comparisons against experimental results) are presented

Creep Modelling of P91 Steel for High Temperature Power Plant Applications

AbstractThere has been considerable interest in the development of continuum damage (CDM) mechanism based model for creep life predictions of 9CrMoNbV steel. It is reported that the steel has high dislocation density in normalized and tempered condition whereas with creep exposure it goes down significantly. The paper examines one of the recent models and attempts to incorporate this as an additional damage parameter. This has resulted in much better prediction of creep stain time plots for this steel

Electric-field-assisted processing of ceramics: Nonthermal effects and related mechanisms

Field-assisted processing methods, such as spark plasma sintering and flash sintering, have considerably expanded the toolbox of ceramic engineering. Depending on the conditions, substantial electric currents may flow through the material resulting in fast heating rates due to Joule heating. Here, we focus on nonthermal effects induced by electric fields during processing of fluorite- and perovskite-based ceramics. The fundamentals of how a field can directly modify defect formation and migration in crystals are discussed. In addition, the interplay of ion transport and electrical conductivity is considered, this interplay being crucial to understanding nonthermal effects caused by electric fields (as in memristive switching). Electrochemical reactions leading to new phases or reduction are also described, as are densification rates and sintering parameters that are significantly affected even though the sample temperature is held constant. Finally, as grain-boundary properties and segregation are changed by ion transport, we describe how both retardation and acceleration of grain growth can be achieved including graded microstructures