Overview and Areas for Future Development

In the foregoing sections we have presented the available information on the structure and transport in ion-conducting nanocrystals, and shall now attempt to summarize and assess the present state of knowledge. 

The only way to rationalize the transport data for nanocrystalline ionic materials is to consider the systems separately, from the viewpoint of the level of defects in the crystals and the nature ofthe samples. In the case of lowly defective systems, such as the alkaline earth fluorides, there is good evidence for a conductivity enhancement, and the data can be explained in terms of models based on the space-charge layer. A key experiment here was the observation of enhanced conductivity in very thin alternating Cafb/Bafb layers when measured perpendicular to the layers [298]. This was explained as being due to the space-charge layers overlapping and saturating the layers. However, this observation is difficult to explain in terms of a model based on surface mismatch. 

Although extensive data are available on ceria and zirconia systems, the results 

Angstroms, which in turn makes the space-charge layer effects negligible. Thus, the 

The application of nanocrystalline metal oxides in sensor devices is now well-established, and should produce benefits in terms of improved sensitivity and speed of response. On a similar note, nanomaterials have become increasingly important in battery technology, particularly in the development of lithium solid-state batteries [106, 303, 304]. Nanocrystalline oxides offer many advantages in SOFCs, primarily by increasing the surface area of the materials and hence the catalytic activity [305, 306], and this is especially important for lowering the cell s operating temperature. Overall, however, it remains clear that further research into the 

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