Prof. Dr. Wolfgang Rheinheimer
Institute head

Institute for Manufacturing Technologies of
Ceramic Components and Composites (IMTCCC)

University of Stuttgart

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PDFs of my papers on Researchgate

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

Wolfgang's research group on interfaces focusses on the fundamentals of interfaces in functional ceramics, specifically, the impact on processing and electric properties. The focus is on interfaces in ionic conductors for Li, H and O conducting electrochemical systems.

 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. Particularly, the performance of CeO2, LixLayTiO3 and BaZrO3 suffers from poor grain boundary conductivity due to space charge.

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.


Grain growth transitions in functional ceramics

In general, microstructure evolution is believed to be a thermally activated process due to its dependence on mass transport by diffusion. However, in functional ceramics as the perovskite SrTiO3 and related materials, non-Arrhenius behavior occurs during microstructure evolution: at higher temperatures, finer microstructures can occur. This unexpected behavior is associated with bimodal microstructures and segregation. Its complete understanding allows tailoring microstructures according to a given need. Controlling the grain growth rate allows well-controlled unimodal fine-grained or coarse microstructures. Even quasi-single crystalline microstructures can be obtained with grains of a size of 100ds of µm, if segregation and space charge are carefully engineered. This high degree of microstructure control offers immense potential to tailor properties: both ionic and electronic conductivities of grain boundaries are of central importance e.g. for proton conductors (BaZrO3), Li conductors (LixLayTiO3) and oxygen conductors (CeO2) and many other applications.

W. Rheinheimer & M. J. Hoffmann: ‘Grain growth in perovskites: What is the impact of boundary transitions?’, Current Opinion in Solid State and Materials Science, 2016
W. Rheinheimer & M. J. Hoffmann: ‘Non-Arrhenius behavior of grain growth in strontium titanate: New evidence for a structural transition of grain boundaries’, Scripta Materialia, 2015
W. Rheinheimer, M. Bäurer & M. J. Hoffmann: ‘A reversible wetting transition in strontium titanate and its influence on grain growth and the grain boundary mobility’, Acta Materialia, 2015


Simulation of bimodal grain growth

To obtain full microstructural control, a careful analysis of bimodal microstructure evolution is needed. This can only be achieved by establishing a digital twin, e.g. using a phase field model for bimodal microstructure evolution. The obtained numbers allow investigating nucleation behavior of bimodal microstructures.

W. Rheinheimer, E. Schoof, M. Selzer, B. Nestler & M. J. Hoffmann: „Non-Arrhenius grain growth in strontium titanate: Quantification of bimodal grain growth“, Acta Materialia, 2019

Anisotropy and its impact on microstructure evolution

Due to the crystalline nature of ceramics, lattices have anisotropic properties. As a consequence, all grain boundary properties are anisotropic as well, e.g. the grain boundary energy, and mobility, but also electric properties and segregation. To evaluate the impact of anisotropy on microstructures and properties, careful model experiments are needed. For example, the anisotropy of the grain boundary energy can be approached by observing the shape of pores or grains in microstructures. Statistical approaches reveal the texture of the grain boundary plane orientation (Grain Boundary Plane distribution) and allow the identification of important grain boundary configurations.

W. Rheinheimer, C. A. Handwerker & J. E. Blendell: ‘
Equilibrium and kinetic shapes of grains in polycrystals’, Acta Materialia, 2020
 W. Rheinheimer, D. Lowing & J.E. Blendell: ‘Grain growth in NiO-MgO and its dependence on faceting and the equilibrium crystal shape’, Scripta Materialia, 2020
 W. Rheinheimer, Fabian J. Altermann & M. J. Hoffmann: ‘The equilibrium crystal shape of strontium titanate: Impact of donor doping’, Scripta Materialia, 2017
 W. Rheinheimer, M. Bäurer, H. Chien, G. S. Rohrer, C. A. Handwerker, J. E. Blendell & M. J. Hoffmann: ‘The equilibrium crystal shape of strontium titanate and its relationship to the grain boundary plane distribution’, Acta Materialia, 2015

 W. Rheinheimer, M. Bäurer, C. A. Handwerker, J. E. Blendell & M. J. Hoffmann: ‘Growth of single crystalline seeds into polycrystalline strontium titanate: Anisotropy of the mobility, intrinsic drag effects and kinetic shape of grain boundaries’, Acta Materialia, 2015

Space charge, grain boundary segregation and solute drag in functional ceramics

Space charge at a grain boundary forms for thermodynamic reasons: The grain boundary is a 2D lattice defect and results in a fraction of broken bonds and, as a result, lattice stresses. In response, the grain boundary restructures by segregating point defects to the grain boundary core. As these defects bring a charge to the grain boundary plane. This charge is shielded by an accumulation of point defects with inverse polarity next to the grain boundary core. Space charge and segregation are common in functional ceramics and sometimes decrease the performance by orders of magnitude due to the resulting Schottky barriers. Less well-known is the dependence of microstructure evolution on space charge: segregated defects can dominate densification and grain boundary migration. The underlying physics are known since the 60s from metals (‘solute drag’). Accordingly, tailoring microstructure evolution in functional ceramics for a given application needs a fundamental understanding of space charge and segregation. This fundamental understanding is supported by well-established models from the metals community that need to be extended to account for the additional complexity of ionic polycrystals.

W. Rheinheimer & M. J. Hoffmann: ‘Grain growth in perovskites: What is the impact of boundary transitions?’, Current Opinion in Solid State and Materials Science, 2016
 K.S.N. Vikrant, W. Rheinheimer, H. Sternlicht, M. Bäurer & R. E. García: ‘Electrochemically-driven abnormal grain growth in ionic ceramics’, Acta Materialia, 2020
 K. S. N Vikrant, W. Rheinheimer, & R. E. García: ‘Electrochemical drag effect on grain boundary motion in ionic ceramics’, npj Computational Materials, 2020
 W. Rheinheimer, X. L. Phuah, H. Wang, F. Lemke, M. J. Hoffmann & H. Wang: 'The role of point defects and defect gradients in flash sintering of perovskite oxides’, Acta Materialia, 2019
 W. Rheinheimer, J. P. Parras, J.-H. Preusker, R. A. De Souza & M. J. Hoffmann: ‘Grain growth in strontium titanate in electric fields: The impact of space charge on the grain boundary mobility’, Journal of the American Ceramic Society, 2019

Grain boundary adsorption and its interplay with functional properties

The occurrence of adsorption and grain boundary phases in ceramic materials is known since the 90s, when the engineering ceramic Si3N4 was the investigated in detail. This concept bases on the thermodynamic stabilization of adsorption layers by a reduction of the grain boundary energy and is known under the terms critical wetting, intergranular glassy film (IGF) or complexion.

Recently, the phenomenon of grain boundary phases was revisited in the context of microstructure evolution and functional properties. For example, in LMO-LLTO half cells for solid state batteries, nm-thick layers of amorphous grain boundary phases result in enormous interfacial resistance. Such an interface is unusable for solid-state batteries.

P. Xu, W. Rheinheimer, S.N. Shuvo, O. Levit, Y. Ein-Eli & L. Stanciu: ‘Origin of high interfacial resistances in solid-state batteries: interdiffusion and amorphous film formation in LLTO/LMO half cells’, ChemElectroChem, 2019

 W. Rheinheimer & M. J. Hoffmann: „Grain growth in perovskites: What is the impact of boundary transitions?“, Current Opinion in Solid State and Materials Science, 2016
 W. Rheinheimer, M. Bäurer & M. J. Hoffmann: ‘A reversible wetting transition in strontium titanate and its influence on grain growth and the grain boundary mobility’, Acta Materialia, 2015

Grain boundary structure and microstructure evolution

On atomistic scale, the motion of grain boundaries is believed to base on the movement of steps and grain boundary dislocations (‘disconnections’). This mechanism is overlaid by other grain boundary effects as e.g. space charge and solute drag.

H. Sternlicht, W. Rheinheimer, A. Mehlmann, A. Rothschild, M. J. Hoffmann & W. D. Kaplan: „The mechanism of grain growth at general grain boundaries in SrTiO3“, Scripta Materialia, 2020
H. Sternlicht, W. Rheinheimer, R. E. Dunin-Borkowski, M. J. Hoffmann & W. D. Kaplan: ‘Characterization of grain boundary disconnections in SrTiO3 part I: The dislocation component of grain boundary disconnections’, Journal of Materials Science, 2019
H. Sternlicht, W. Rheinheimer, J. Kim, E. Liberti, A. I. Kirkland, M. J. Hoffmann & W. D. Kaplan: ‘Characterization of grain boundary disconnections in SrTiO3 part II: The influence of superimposed disconnections on image analysis’, Journal of Materials Science, 2019