Development of very thin YSZ films deposited on substrates of varying porosity by e-beam evaporation

Abstract

We present results of our efforts to develop thin films that may prove to be appropriate solid electrolytes for lower operating temperature solid oxide fuel cells. The electron beam evaporation technique has been used to deposit yttrium stabilised zirconia (YSZ: ZrO₂ stabilised with 8 wt-%Y₂O₃) thin films on a variety of porous and non-porous substrates. Thin films have been grown on conducting films on glass, monocrystalline silicon wafers and highly porous NiO/YSZ substrates. Some of the porous substrates have been polished in order to be able to support thinner films. Films ranging from 0·2 to 2 μm in thickness have been manufactured. Submicrometre thin films have successfully been deposited on NiO/YSZ polished substrates. Operating technical parameters that influence the film properties were studied and the influence of substrate structure and deposition rate has been investigated. The film thickness has been measured in situ via a quartz crystal monitor and ex situ by a stylus profilometer. The morphology of the films has been studied by scanning electron microscopy. Samples have also been investigated in terms of chemical composition via X-ray photoelectron spectroscopy. Using the Scotch tape test, it has been found that the films exhibited good adhesive qualities; however, in some occasions, when annealed, cracks appeared. Using a polished – hence smoother – substrate has reduced the occurrence of cracks and other abnormalities. The electron gun power – and subsequently the rate of deposition – played an important role in film morphology. The latter presented an interesting crystallite structure in the nanometre range.