Journal of Power Sources 189 (2009) 888–893
Contents lists available at ScienceDirect
Journal of Power Sources
journal homepage: www.elsevier.com/locate/jpowsour
Influence of CaO on structure and electrical conductivity
of pyrochlore-type Sm
2
Zr
2
O
7
Xiao-Liang Xia, Jia-Hu Ouyang
∗
, Zhan-Guo Liu
Institute for Advanced Ceramics, Department of Materials Science, Harbin Institute of Technology, Harbin 150001, China
article info
Article history:
Received 10 December 2008
Received in revised form
26 December 2008
Accepted 30 December 2008
Available online 14 January 2009
Keywords:
(Sm1-xCax)2Zr2O7-x
Pyrochlore
Electrical conductivity
Impedance spectroscopy
abstract
(Sm
1-x
Ca
x
)
2
Zr
2
O
7-x
(0 ≤ x ≤ 0.100) ceramics were prepared by a solid state reaction process at 1973K
for 10h in air, and were characterized by X-ray diffraction (XRD) and scanning electron microscopy
(SEM). (Sm
1-x
Ca
x
)
2
Zr
2
O
7-x
(0 ≤ x ≤ 0.025) ceramics have a single phase of pyrochlore-type structure;
however (Sm
1-x
Ca
x
)
2
Zr
2
O
7-x
(0.050 ≤ x ≤ 0.100) consist of pyrochlore phase and a small amount of
perovskite-like CaZrO
3
. The electrical conductivity of (Sm
1-x
Ca
x
)
2
Zr
2
O
7-x
ceramics was investigated by
complex impedance spectroscopy over a frequency range of 0.1 Hz to 20 MHz in the temperature range of
573–873 K. The measured electrical conductivity obeys the Arrhenius relation. Both the activation energy
and pre-exponential factor for grain conductivity increase with increasing the CaO content; however,
electrical conductivity of (Sm
1-x
Ca
x
)
2
Zr
2
O
7-x
decreases with increasing the CaO content, which is due to
the increase in structural disordering at 0 ≤ x ≤ 0.025 and the presence of the poorly conducting CaZrO
3
phase at 0.050 ≤ x ≤ 0.100, respectively.
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
In the past few years, pyrochlore compounds with the general
formula A
2
B
2
O
7
, where A is a trivalent rare-earth element and B is a
tetravalent transition metal, are of great technological importance
[1]. These compounds can be used as candidates for catalysts [2,3],
nuclear waste disposal [4,5] and thermal barrier coatings (TBCs)
[6], due to their interesting physical and chemical properties. Espe-
cially, their electrical properties depending on the composition and
the degree of disorder on the cation sites [7] make them promising
hosts for solid electrolytes in high-temperature fuel cells. Oxide-ion
conductors of pyrochlore structure A
2
B
2
O
7
are potential candidates
to substitute materials currently used in the fuel cells [8]. The
advantages of lowering the operation temperature have attracted
great interest worldwide. Enormous amounts of efforts have been
made to improve ionic conductivity of the oxide electrolyte mate-
rials [9,10].
Among the pyrochlores, Gd
2
Zr
2
O
7
has been of keen concern
due to its high oxide-ion conductivity at relatively low tem-
peratures as compared with the commercially used electrolytes
[11]. van Dijk et al. [12] found that the ionic conductivity of
Gd
2
Zr
2
O
7
was the highest among all of the reported pyrochlores.
There have been some efforts to improve the conductivity of
Gd
2
Zr
2
O
7
by substitution at the Gd site. Gd
2
Zr
2
O
7
doped with
5 and 10mol% Sr at the Gd site had a higher total-conductivity
∗
Corresponding author. Tel.: +86 451 86414291; fax: +86 451 86414291.
E-mail address: ouyangjh@hit.edu.cn (J.-H. Ouyang).
than pure Gd
2
Zr
2
O
7
in the temperature range of 773–1023K
[13]. Mandal et al. [14] prepared (Gd
1-x
Nd
x
)
2
Zr
2
O
7
(0 ≤ x ≤ 1.0)
by a solid state reaction route, and found a significant enhance-
ment of ionic conductivity in (Gd
0.5
Nd
0.5
)
2
Zr
2
O
7
at 622–696 K.
Electrical properties were investigated in a La substituted fluorite-
Gd
2
Zr
2
O
7
, and a peak in ionic conductivity was found for the
(Gd
0.8
La
0.2
)
2
Zr
2
O
7
at 1073 K [15]. Díaz-Guillén et al. [16] sub-
stituted La for Gd in pyrochlore-type (Gd
1-x
La
x
)
2
Zr
2
O
7
solid
solutions (0 ≤ x ≤ 1.0) and found that ionic conductivity at tem-
peratures of 773–1023K was almost La-content independent for
x ≤ 0.4. Liu et al. [17] synthesized (Gd
1-x
Sm
x
)
2
Zr
2
O
7
(0 ≤ x ≤ 1.0)
ceramics by the chemical-coprecipitation and calcination method,
and obtained the highest electrical conductivity value at 873 K
for (Gd
0.5
Sm
0.5
)
2
Zr
2
O
7
. Recently, the effect of homovalent A-
site substitution on the electrical conductivity of pyrochlore-type
Gd
2
Zr
2
O
7
was studied, and found that ionic conductivity remained
almost constant or even increased slightly [18].
The electrical conductivity of Sm
2
Zr
2
O
7
pyrochlore phase was
comparable to those of other good oxide-ion conductors in low-
temperature regions [19]. However, to the best of our knowledge,
there is no report to investigate the influence of divalent oxide dop-
ing on electrical conductivity of Sm
2
Zr
2
O
7
ceramic. In the present
work, structure and electrical conductivity of pyrochlore-type
Sm
2
Zr
2
O
7
doped with different contents of CaO were investigated.
2. Experimental procedure
In the present study (Sm
1-x
Ca
x
)
2
Zr
2
O
7-x
(0 ≤ x ≤ 0.100) ceram-
ics were synthesized by a solid state reaction process. The starting
0378-7753/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jpowsour.2008.12.136