Impact of the presence of grain boundaries on the in-plane ionic conductivity of thin film gadolinium-doped cerium oxide

The ionic in-plane conductivity of thin film Gd-doped CeO2 (GDC) at temperatures between 500°C and 750°C is of interest because GDC has the potential to be a highly conductive electrolyte in a thin-film solid oxide fuel cell (SOFC). There have been many research groups that have deposited GDC thin films (mostly polycrystalline material) that vary widely in reported conductivity values. The assertion in the literature is that a sub-micron grain size in polycrystalline GDC thin films is necessary for observing a high ionic conductivity in the 500-700C temperature range. An experimental investigation that directly tests this assertion by comparing the in-plane conductivity of GDC films that are largely identical except for the presence of grain boundaries is lacking in the literature.In this work, the assertion that sub-micon grain boundaries enhance the in-plane ionic conduction observed in GDC thin films is examined. To rigorously test this assertion it must be possible to grow GDC films on highly insulating substrates that are identical except for the presence or lack of grain boundaries. While it is straight forward to grow polycrystalline GDC films on highly insulating Al2O3 substrates by RF magnetron sputtering, methods for depositing single-crystal thin-film GDC on Al2O3 substrates (no grain boundaries) have not been reported and are therefore developed. The in-plane conductivity of single-crystal and polycrystalline GDC films are measured in the temperature range between 400-700C. The conductivity of the films are analyzed to extract the impact on GDC ionic conduction of the presence of boundaries separating sub-micron grains. The conclusion drawn from this study is that the presence of these does not enhance, but rather degrades the in-plane ionic conductivity of GDC thin films.