Atomic Diffusion Along Grain Boundaries

Most solid materials consist of small crystallites, which are also referred to as grains. The areas between the grains, the grain boundaries, often determine the overall behavior of a material. There is, for example, movement of atoms in a solid: atoms jump over energy barriers from one place to another, and the grain boundaries serve as paths along which the atoms move faster than within the grains. So far, this phenomenon is explained by the fact that the energy barriers in grain boundaries are lower than in grains. The movement of atoms along these is faster because it is easier to perform.

Jana Parras, a doctoral candidate at RWTH’s Institute of Physical Chemistry, and her supervisor, Professor Roger De Souza, showed in a study that this explanation does not apply equally to all materials. With the help of a series of detailed simulations, the researchers were able to demonstrate that fast grain boundary diffusion in oxides can result in an effective energy barrier that can be as large as the barrier within the grains. This is due to the fact that oxides consist of charged particles. The grains are charge-neutral, but near grain boundaries, this charge neutrality is often disturbed. Such space-charge zones cause atomic movements to be faster, but not necessarily easier.

Not only does the study result in a deeper understanding of fundamental processes, but also has implications for the application of oxide materials in power engineering, for example as electrolytes in high-temperature fuel cells and other solid electrochemical components. Atomic motion is the key process in the production and pretreatment of oxide materials for specific applications and, due to the large number of degradation processes, plays a central role in affecting the lifetime of a component.

The study received funding from the German Research Foundation DFG within the FieldsMatter Priority Program (SPP1959). The article Grain-boundary diffusion of cations in fluorite-type oxides is faster but not always easier is available online.