Electrical properties of YSZ/Al
2
O
3
composite and YSZ/Al
2
O
3
interface studied by
impedance spectroscopy and finite element modelling
Fan Yang, Xiaofeng Zhao, Ping Xiao ⁎
Materials Science Centre, School of Materials, University of Manchester, M1 7HS, UK
abstract article info
Article history:
Received 24 November 2009
Received in revised form 17 March 2010
Accepted 30 April 2010
Keywords:
YSZ/Al
2
O
3
composite
Impedance
Interface
Finite element modelling
Electrical properties of the YSZ/Al
2
O
3
composite were measured with use of impedance spectroscopy.
Electrical conductivity of the composite decreases with increase in the Al
2
O
3
volume fraction and shows
typical percolation behaviour. Decrease in the conductivity around five orders of magnitude occurred when
the volume fraction of Al
2
O
3
in the composite exceeds a critical level of 86.1%. The electrical conductivity of
the YSZ/Al
2
O
3
interface was examined based on the relaxation frequency analysis and finite element
modelling. It was concluded that the YSZ/Al
2
O
3
interface conductivity is higher than that of the YSZ grain
boundary but similar to, or lower than that of the YSZ grain.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
Yttria stabilized zirconia (YSZ) and Al
2
O
3
are important ceramic
materials and have wide range of applications in industry. Their YSZ/Al
2
O
3
composite, which should combine the properties of individual compo-
nent, should increase the range of its application. For example, the
YSZ/Al
2
O
3
composite has been proposed as electrolyte for planar Solid
Oxide Fuel Cell (SOFC) because of its enhanced mechanical and thermal
properties compared with conventional YSZ [1]. On the other hand, owing
to the immiscibility of YSZ and Al
2
O
3
(the solubility of Al
2
O
3
in 8 mol.% YSZ
is around 1 wt.% when sintered at 1500 °C [2]), YSZ/Al
2
O
3
composite is
also of scientific interest in understanding the fundamental behaviours of
a diphase system [3]. It is also a good system for studying the physical
properties of the ceramic/ceramic interface.
Electrical properties of the YSZ/Al
2
O
3
composite are not new topics
and have been investigated by a number of researchers [1–8]. However,
most of the studies have been focused on the effect of adding a small
amount of Al
2
O
3
(less than 5 wt.%) to YSZ [4–8]. A review on the effect of
the Al
2
O
3
addition on the electrical properties of YSZ grain and grain
boundary can be found in Ref. [9,10]. It is summarized that the Al
2
O
3
addition below the solubility limit decreases the grain boundary
conductivity by increasing the Schottky barrier height, while the Al
2
O
3
addition above the solubility limit increases the grain boundary
conductivity by increasing the grain-to-grain contact area by scavenging
SiO
2
from the boundaries between grains [9,10]. For a much wider range
of composition, most of work has reported the electrical properties of
YSZ/Al
2
O
3
composite with the Al
2
O
3
fraction less than 30 wt.% (around
40 v.%). For example, Feighery and Irvine [2] doped up 24 wt.% Al
2
O
3
into 8 mol% YSZ and found that 10 wt.% Al
2
O
3
can be added to 8 mol.%
YSZ without any significant decreases in ionic conducting properties.
Further additions of Al
2
O
3
cause a rapid decrease in conductivity due to
the large volume fraction of insulating Al
2
O
3
phase. Mori et al. [1]
studied the composite with Al
2
O
3
content up to 30 wt.%, and found the
conductivity only decreased less than one order of magnitude.
Although it is predictable that the conductivity of the YSZ/Al
2
O
3
decreases with further increase in the Al
2
O
3
volume fraction, it is still of
interest and importance to study the electrical properties of the
composite covering the whole composition range because it can provide
valuable information on the diphase system. However, the only work
covered the whole composition range of YSZ/Al
2
O
3
composite, limited
to the authors’ knowledge, is reported by Peko et al. [3]. They found a
strong influence from the microstructural features on the final electrical
response of the composite.
On the other hand, among numerous studies related to the electrical
properties of the YSZ/Al
2
O
3
composite, the effect of the YSZ/Al
2
O
3
interface has been rarely discussed. Kumar et al. [11,12] examined the
normalized conductivity (conductivity of a composite/volume fraction of
active phase) and proposed that the transport of oxygen ions was
enhanced at the YSZ/Al
2
O
3
interface because of the creation of space
charge regions in the vicinity of the YSZ-Al
2
O
3
phase boundary. However,
due to the blocking effect of Al
2
O
3
, the enhancement of conductivity is not
obvious. Guo [5] studied the grain boundary resistance in yttria and
alumina codoped-zriconia and discussed the effect of the ZrO
2
/Al
2
O
3
interface. It was proposed the resistance of the Al
2
O
3
/ZrO
2
interface is
reduced because of the Al
2
O
3
/ZrO
2
interface potential is lower than the
grain boundary interface potential of ZrO
2
. However, the resistance-
reducing effect was not obvious due to the pores and the amorphous
Solid State Ionics 181 (2010) 783–789
⁎ Corresponding author.
E-mail address: ping.xiao@manchester.ac.uk (P. Xiao).
0167-2738/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ssi.2010.04.036
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