Diffusion and ionic conduction in nanocrystalline ceramics

Abstract

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.

NMR and impedance studies of nanocrystalline and amorphous ion conductors: lithium niobate as a model system

“…5 As the transport properties of these regions can deviate considerably from those of the bulk material, this procedure may have a remarkable impact on the overall ionic diffusivity of the material. 5 Oftentimes grain boundaries in nanocrystalline materials provide fast diffusion pathways for small cations and anions like Li + and F − , respectively, or even larger anions like O 2− , so that an enhancement of the diffusivity is observed. [6][7][8][9][10][11][12][13][14][15][16][17][18] Sometimes, however, the interfacial regions have a blocking effect resulting in a reduction of the ionic conductivity.…”

“…Solid-state nuclear magnetic resonance (NMR) techniques 21,22 in combination with impedance spectroscopy measurements 5,23 were used to probe microscopic as well as macroscopic Li diffusion parameters of LiNbO 3 . In its single crystalline form, lithium niobate is a poor Li conductor with a very small Li diffusion coefficient.…”

“…By the first preparation route the nanocrystallites are formed from smaller building units, similar to other techniques like inert gas condensation 31 , chemical vapor deposition 32 , or pulsed electro-deposition 33 . In contrast to that approach, nanocrystalline materials can also be obtained by reducing the grain size of their coarse grained counterparts, e. g., by sputtering with a radio frequency field or with heavy ions 34 or just by high-energy ball-milling 5,30 .…”

Atomic-scale measurement of ultraslow Li motions in glassyLiAlSi2O6by two-timeL6Wilkening1, Kuhn2, Heitjans3 2008Phys. Rev. B554490View full textAdd to dashboardCite

“…This is due to the often found observation that structurally disordered Li conductors exhibit exceptionally fast ionic diffusivity as compared to their low-defective crystalline counterparts. [9][10][11][12][13][14] Understanding the basics of transport phenomena in such materials in more detail is indispensable in effective battery research. In this context, it will be useful to study Li diffusion over a preferably large dynamic range, i.e., including also extremely slow cation motions.…”

Partial electronic conductivity of nanocrystalline Na2O2

“…This peak value, which is much lower than for mechanically treated oxides [14,15], was almost reached after the sample had been ball milled in Ar atmosphere for 60 min [10]. It is expected to be governed by (i) the defect chemistry, (ii) the increased volume fraction of (locally) disordered interfacial regions [14][15][16][17] as well as (iii) by the non-negligible effect of overlapping space charge zones known to play a pivotal role in nanostructured systems [18].…”