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Preparation of YSZ Solid Electrolytes for Solid Oxide Fuel Cells by e-beam Evaporation
Abstract
Zois Sompolos and Panayiotis Yianoulis *
Energy and Environment Laboratory
Physics Department
University of Patras
*yianpan@physics.upatras.gr
The electron-beam evaporation technique has been used to deposit YSZ (ZrO 2 stabilized by 8 wt.% Y 2 O 3 ) thin films
on a variety of porous and non porous substrates. Thin films have been grown on smooth conducting glass, silicon and on
highly porous NiO-YSZ substrates. Films ranging between 1 and 2 μm in thickness have been manufactured. Operating
technical parameters that influence the film properties were studied. 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. Moreover, the film’s morphology has been studied by scanning electron microscopy and atomic force
microscopy. Samples have been also investigated in terms of chemical composition via x-ray photoelectron spectroscopy. It
has been found that the films had good adhesive qualities, however presented cracks when tempered. Furthermore, the
dependence of electron gun power – and subsequently the rate of deposition – played an important role on film morphology.
The latter presented an interesting crystallite structure in the nm range.
Introduction
Oxide ion conductivity was first observed in ZrO 2 [1] in the 1890s and even though since then, a wide range of
materials, some with superior properties, have been introduced, zirconium oxides remain popular, due to their low electronic
conductivity, low cost and ease of production and process. YSZ is ZrO 2 stabilized by 8 wt.% Y 2 O 3 and is part of the fluoritestructure
family, along with CeO 2 –Ln 2 O 3 electrolytes. A large number of techniques are used to fabricate YSZ films, such as
press heating [2], tape casting [3], chemical vapour deposition [4], sputtering and e-beam deposition [5,6]. Vapour deposition
techniques allow strict control on the film’s thickness, porosity, stoichiometry and growth rate during deposition. The
resulting film is very homogenous and adheres well on the substrate. Using that technique we fabricated thin electrolyte films
with a thickness in the 1 - 2 μm range, in order to achieve high oxygen ion conductivity in a Solid Oxide Fuel Cell device.
Our effort is connected with the ongoing search in many laboratories to prepare solid electrolytes with superior performance.
Experimental
The vacuum chamber that is used to manufacture the specimens consists of a glass cylinder cover, 45cm tall and
30cm in diameter that allows optical control during the deposition process. The glass cover is placed upon a metallic base, on
which there are appropriately designed feed-throughs that allow the installation of sensors and other measurement systems.
For the film deposition we used an electron beam gun. That device consists of a W wire that is being subjected to high
voltage (~ 7 kV) and emits electrodes of high kinetic energy. The resulting electron beam is guided by a computer controlled
combined electromagnetic field to a cupper bowl containing
the material that will be deposited (YSZ), as shown in Fig. 1.
The vacuum chamber is connected to a turbomolecular
and a mechanical pump via a strangulation valve. Pirani and
Penning gauges are used to measure the pressure prior and
during the process. The thickness of the film is controlled
during the deposition by the quartz crystal sensor and verified
by a stylus profilometer ex situ.
Other techniques used to characterize the specimens
morphologically and quantitatively are Scanning Electron
Microscopy (SEM), Atomic Force Microscopy (AFM) and X-
ray Photoelectron Spectroscopy (XPS).
Results and Discussion
YSZ films (1 – 2 μm thickness) were deposited on three
different substrates: K-glass (glass covered with a fine
conducting coating based on Sb), optical quartz and NiO-YSZ.
The samples were thoroughly cleaned in ultrasonic bath and
heated before the deposition.
The chamber pressure during deposition was 2×10 -5
mbar and the temperature was that of the environment. The two
ingredients of YSZ have close melting point temperatures
(ZrO 2 : 2700°C and Y 2 O 3 : 2410°C). However, that difference of
approx. 300°C results in a slight variation in the composition of
Figure 1. Vacuum Deposition Device
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