In recent years, there have been a number of reports of enhancements in the ionic conductivity of yttria-stabilised zirconia (YSZ) as the dimensions of the material are reduced, such as when grown as thin films or heterostructures. Improvements in the ionic conductivity are often attributed to an expansive lattice strain caused by a coherent interface between the conducting and insulating layers. Alternatively, regular networks of dislocations are often claimed to be the cause due to the locally strained lattice regularly surrounding a dislocation core. To date, experimental investigations in these systems have led to conflicting reports with conductivity measurements ranging from an enhancement of orders of magnitude  to a modest reduction compared to bulk behaviour . One of the reasons for the disparity in the literature may be due to the large number of differences between nominally similar systems due to variations in fabrication, processing and measurement techniques employed. Another cause of concern when assessing the transport properties of nanoscale ionic conductors is the increased danger of electrical current leakage through either the substrate or experimental setup effecting the measurement, leading to an erroneously interpreted result .
In order to mitigate the issues above we have fabricated highly textured YSZ thin films oriented in the (111) direction onto MgO and sapphire, and the (100) direction onto MgO, LAO and NGO using pulsed laser deposition (PLD). These correspond to a range of lattice mismatches from 4.5% tensile to 18% compressive, and have been grown at a number of thicknesses in order to isolate interfacial effects. Using x-ray diffraction (XRD), secondary ion mass spectroscopy (SIMS), and high-resolution transmission electron microscopy (HR-TEM) the structural and chemical properties of the thin films have been characterised. Electrochemical impedance spectroscopy (EIS) combined with isotope tracer diffusion will be shown to directly and unambiguously measure the oxygen ion transport properties in these films as a function of thickness. This allows compositional, micro- and nano-structural variations observed in the film interfaces to be associated with changes in the conduction properties. We will present evidence to show that a regular dislocation network at YSZ/substrate interfaces does not in fact drastically alter the conduction properties despite being linked to enhanced conduction properties previously .
1. Sillassen, M., et al., Advanced Functional Materials, 2010. 20(13): p. 2071-2076.
2. Gerstl, M., et al., Physical Chemistry Chemical Physics, 2013. 15: p. 1097 - 1107.
3. Kim, H.-R., et al., Physical Chemistry Chemical Physics, 2011. 13(13): p. 6133-6137.