The role of strain and structure on oxygen ion conduction in nanoscale zirconia and ceria thin films
University of Delaware
Solid oxide fuel cells (SOFCs), an all solid-state energy conversion device, are promising for their high efficiency and materials stability. The solid oxide electrolytes are a key component that must provide high ionic conductivity, which is especially challenging for intermediate temperature SOFCs operating between 500 °C - 700 °C. Doped zirconia and ceria are the most common solid electrolyte materials. Recent reports have suggested that nanoscale ytrria stabilized zirconia (YSZ) thin films may provide better performance in this regard. However, the mechanism behind the increased conductivity of nanoscale thin films is still unclear and the reported experimental results are controversial. In the thesis presented here, the effects of mechanical strain and microstructure on the ionic conductivity have been investigated in ultrathin zirconia- and ceria-based thin films. Reactive RF co-sputtering with metal targets was used to prepare zirconia and ceria based thin films for high purity, modulated composition and thickness. The films were as thin as 10-20 atomic layers thick. X-ray photoelectron spectroscopy, X-ray diffraction and transmission electron microscopy were the main tools to investigate the composition, crystal orientation and microstructure of these sputtered thin films. Microscale interdigitated Pt electrodes were prepared through a lift-off process using photolithography. The electrochemical properties of these sputtered doped zirconia and ceria thin films were investigated using impedance spectroscopy. YSZ thin films deposited on MgO (111) and, especially, MgO (100) showed highly variable crystal orientations, while MgO (110) offered much more stable growth. Regardless of whether the growth was epitaxial or highly disordered polycrystalline, 50 nm thick YSZ thin films on MgO (100), (110), and (111) substrates exhibited similar conductivity with YSZ single crystal. While decreasing the thickness further to 12 nm, the conductivities of YSZ thin films were decreased 3 - 7 times. YSZ thin films deposited on Al 2 O3 obtained a stable epitaxial growth along  (111)YSZ// (0001)Al2 O3 . By tailoring the thickness of YSZ thin film on Al2 O3 from 100 nm to 6 nm, the lattice strain can be increased from nearly 1% to 2%. The corresponding conductivity increased by about 1 order of magnitude and the activation energy decreased from 0.99 eV to 0.79 eV. Ion cleaning of the MgO substrate surface was found to change the YSZ thin films' texture without large change to the conductivity, while ion cleaning of the Al2 O3 substrate surface decreased the crystallinity without changing the texture and reduced the ionic conductivity of YSZ thin films by a factor of 4. Thus, crystallinity not texture was found to determine the ionic conductivity. In addition, a post annealing with a temperature as high as 1000 °C was able to increase the crystallinity of YSZ thin films therefore increasing the conductivity by a factor of 2. Gadolinia doped ceria (GDC) thin films deposited on MgO were randomly oriented along multi axes, suggesting a polycrystalline structure. While, on Al2 O3 , GDC thin films' growth became stable only oriented in (111) orientation, just like YSZ thin films on Al2 O 3 . In the thickness range of 15 nm - 173 nm, the maximum conductivity of GDC thin films was obtained at the thickness of 81 nm. Interestingly, as GDC thin films' thickness increased above 100 nm, the electrical properties changed from a bulk-like conduction to a grain boundary-like conduction.