Recently there has been a great level of interest in the effect of interfaces in
oxide ionic conductors with a view to eventual application in solid oxide fuel
cells (SOFCs). Enhancements in electrical conductivity of one half to eight orders
of magnitude have been reported in simplified thin film and multi-layered
heterostructure systems. Often this is reported to be enhanced oxygen ion conduction
with little supporting evidence. The aim of this work is to investigate
these reports and develop an understanding of the underlying mechanisms.
Several series of samples were investigated to achieve this goal. The first, designed
to emulate an anomalous result from literature with alternating samarium
doped ceria (SDC) and undoped ceria layers, featured an increasing total number
of layers of equal individual thickness, and thus a constant interfacial density. This
system featured no enhancement in conductivity and exhibited a level of tracer
diffusion comparable with that in bulk SDC. Electron energy loss spectroscopy
(EELS) studies, however, revealed a significant level of Ce III in the undoped layers.
The second and third series used similar materials but tested the hypothesis
proposed in many works, that conductivity enhancement was related to tensile
strain in the conducting material at the heterointerfaces. The second, manipulating
the strain at the interface by varying the dopant (Nd, Sm, Y) in films with
alternating doped and undoped ceria layers and a range of interfacial densities.
This series exhibited minimal change in conductivity with strain or interfacial
density.
The third series replaced the doped ceria with yttria-stabilised zirconia (YSZ)
in order to achieve a higher level of tensile strain. This again featured minimal
change in conductivity. Tracer diffusion and secondary ion mass spectrometry
(SIMS) studies suggested that the undoped ceria layers featured vacancy-rich
regions, close to the interfaces, possibly with compensating Ce III.
The final series of multilayers comprised alternating praseodymium nickel copper
gallate (Pr1.91 Ni0.71 Cu0.24 Ga0.05 O4) and SDC layers which exhibited a high
level of conductivity and evidence of reduced levels of p-type conduction with
decreasing SDC layer thickness, suggesting enhanced ionic conductivity. Oxygen
tracer studies revealed, however, that the dominant charge carrier was not oxygen.
Finally a study of the effect of dislocations in ionic conductors was performed on
deformed single crystal YSZ. Impedance measurements revealed a small enhancement
in conductivity in the orientation parallel to the dislocation cores however
diffusion measurements showed a change that could be negated by the consideration
of the inherent errors.