Interfaces in functional ceramics: How to tweak a material’s properties?

• Applicant: Professor Dr.-Ing. Wolfgang Rheinheimer
• Subject Area: Synthesis and Properties of Functional Materials
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 435025113

Interfaces play a key role in materials processing and microstructure evolution of ceramic materials. Beyond that, many material’s properties are dominated by interfacial properties. This is most notable for mechanical properties of ceramics where the overall fracture behavior depends on the grain boundary structure. For the classical engineering ceramics silicon nitride and alumina, grain boundary fracture is well-known to depend on the formation of amorphous layers depending on the dopant concentrations. In alumina and many other systems, the thermodynamics of these layers were investigated in detail (‘complexions’). It was shown that the complexions follow phase-like behavior enabling the investigation of grain boundary phase and TTT diagrams. But also conducting materials critically depend on the conductivity of internal interfaces. In most ceramic systems, such interfaces involve a charged core with an adjacent space charge. As soon as charge carriers are transported through the material, this space charge acts as Schottky barrier resulting in very low grain boundary conductivity. This poses a significant problem for applications in the field of ionic conductors as e.g. electrolytes in SOFC or solid-state electrolytes for Li batteries.For a few model systems (alumina, zirconia, titania and strontium titanate), our knowledge on the relationship between fundamental thermodynamics (i.e. defects and their chemistry) and some grain boundary properties is relatively well established. Beyond these systems, little is known on interfaces and their properties. This proposed project aims on both completing our knowledge on model systems and extending our current modelling such that space charge is added to the complexions framework. But more importantly, this framework and knowledge will be extended to more complicated material systems that have a higher relevance for electronic applications.In this regard, not only equilibrium situations are of interest, but also the kinetics of grain boundary phase transitions. If grain boundary phase and TTT diagrams can be established, roadmaps for tailored processing, microstructure evolution and optimized grain boundary properties become available.

• DFG Programme: Independent Junior Research Groups
• International Connection: Austria, USA
• Cooperation Partners: Professor Dr. Jürgen Fleig; Professor R. Edwin García, Ph.D.