Team Interfaces in Functional Ceramics

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 (Li_xLa_yTiO₃) and oxygen conductors (CeO₂) and many other applications.

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.

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.

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.

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.

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.