Diffusion and ionic conduction in nanocrystalline ceramics
We review case studies of diffusion in nanocrystalline ceramics, i.e. polycrystalline non-metallic materials with average grain sizes typically in the range from 5 to 50 nm. The experimental methods applied are, on the one hand, tracer diffusion or conductivity methods which are sensitive to macroscopic transport, and, on the other hand, NMR techniques which, complementarily to the previous ones, give access to microscopic diffusion parameters like atomic hopping rates and jump barrier heights. In all cases the diffusion properties of the samples, whether single-phase systems or composites, are dominated by their grain boundaries and interfacial regions, respectively. In principle, all experimental techniques allow one to discriminate between contributions to the diffusion from the crystalline grains and those from the interfacial regions. Corresponding examples are presented for SIMS and impedance measurements on oxygen conductors. NMR studies for various nanocrystalline lithium ion conductors reveal that two lithium species with different diffusivities are present. Comparison with the coarse grained counterparts shows that the slower ions are located inside the crystallites and the faster ones in the structurally disordered interfacial regions. Investigations on composite materials exhibit phenomena which can be explained by the percolation of fast diffusion pathways being formed by the interfaces between the two components.