Review of the Current Experimental Data and their Agreement with Theory

Review of the Current Experimental Data and their Agreement with Theory 

Recent transport measurements have been carried out on nanocrystalline thin films - either as single layers on an inert substrate or as multilayers - and in these cases the interfaces were more well-defined than in compacted samples. The examination of interfaces using TEM is also simpler to interpret, as the samples are generally more uniform. However, there is normally a lattice mismatch between the film and the substrate, or between alternating layers, such that the degree of mismatch may be large and lead to disorder and strain in the interface. The nature of the interface is therefore very dependent on the lattice parameters of the layers and the preparation conditions. These points must be borne in mind when discussing the transport results. 

Although the emphasis here will, by necessity, be placed on more recent data, several key reviews of transport in nanocrystalline ionic materials have been presented, the details of which will be outlined first. An international workshop on interfacially controlled functional materials was conducted in 2000, the proceedings of which were published in the journal Solid State Ionics (Volume 131), focusing on the topic of atomic transport. In this issue, Maier [29] considered point defect thermodynamics and particle size, and Tuller [239] critically reviewed the available transport data for three oxides, namely cubic zirconia, ceria, and titania. Subsequently, in 2003, Heitjans and Indris [210] reviewed the diffusion and ionic conductivity data in nanoionics, and included some useful tabulations of data. A review of nanocrystalline ceria and zirconia electrolytes was recently published [240], as have extensive reviews of the mechanical behavior (hardness and plasticity) of both metals and ceramics [13, 234]. 

The approach taken here is to consider data according to the type of material and, 

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