Thermal expansion and electrical conductivity of CaTi0.9M 0.1O3-δ (M = Fe, Cu, Al)

Article abstract of DOI:10.1007/s10789-008-3015-1

The thermal expansion of CaTi_0.9M_0.1O _3-δ (M = Fe, Cu, Al) samples has been measured, and their lattice parameters have been refined in space group Pnma. The three materials are shown to be mixed conductors, p-type at high

Full text of DOI:10.1007/s10789-008-3015-1

ISSN 0020-1685, Inorganic Materials, 2008, Vol. 44, No. 3, pp. 296–298. © Pleiades Publishing, Ltd., 2008.
Original Russian Text © A.A. Murashkina, A.N. Demina, E.A. Filonova, A.K. Demin, I.S. Korobitsin, 2008, published in Neorganicheskie Materialy, 2008, Vol. 44, No. 3, pp. 350–353.
Thermal Expansion and Electrical Conductivity
of CaTi0.9M0.1O3 – d (M = Fe, Cu, Al)
A. A. Murashkina, A. N. Demina, E. A. Filonova, A. K. Demin, and I. S. Korobitsin
Institute of High-Temperature Electrochemistry, Ural Division, Russian Academy of Sciences,
ul. S. Kovalevskoi 20, Yekaterinburg, 620219 Russia
e-mail: murashkinaaa@mail.ru
Received January 11, 2007; in final form, September 26, 2007
Abstract—The thermal expansion of CaTi0.9M0.1O3 – δ (M = Fe, Cu, Al) samples has been measured, and their
lattice parameters have been refined in space group Pnma. The three materials are shown to be mixed conduc-
tors, p-type at high oxygen partial pressures and n-type at low oxygen pressures. At intermediate oxygen partial
pressures, the conductivity varies little. The acceptor dopants studied are shown to raise the conductivity of cal-
cium titanate in the order Al < Cu < Fe.
DOI: 10.1134/S0020168508030151
INTRODUCTION
found on the IUCr server [10]. We used the Fullprof
program [11].
Perovskite oxides with mixed oxygen-ionic/elec-
tronic conductivity may find application as oxygen-per-
meable membranes in electrochemical converters for
hydrogen production from hydrogen-containing gases,
oxygen generation via air separation, and conversion of
methane to syngas [1]. Doped calcium titanate is poten-
tially attractive as membrane material [2] since it pos-
sesses sufficient stability in reducing atmospheres [3],
high ionic and electronic conductivity [4, 5], and
acceptable oxygen permeability [6].
Electrical conductivity was measured by a four-
probe dc method, using the client.exe software. The
sample, placed in a holder with Pt pressure contacts,
was mounted in a Y2O3-stabilized ZrO2 tube, the walls
of which were fitted with electrodes for an electro-
chemical oxygen pump and oxygen sensor, and the tube
was tightly sealed. The temperature was monitored
with a Pt/Pt–Rh thermocouple. The temperature and
oxygen partial pressure were varied from 298 to
1270 K and from 10–9 to 0.21 × 105 Pa using a Zirkonia-
318 microprocessor controller [7].
Precise knowledge of the structural, electrical, and
thermomechanical properties of materials is crucial for
deciding in what area they can be applied. The purpose
of this work was to study the effect of acceptor doping
on the properties of calcium titanate.
Thermal expansion was measured in air ( pO
=
2
0.21 × 105 Pa) during heating from 298 to 1170 K using
an AF-1 dilatometer. Bar-shaped ceramic samples were
prepared in the same manner as those for conductivity
measurements. The heating/cooling rate was typically
~200 K/h. The linear thermal expansion coefficient at
constant pressure, α, was determined using the well-
known relation
EXPERIMENTAL
Samples for this investigation were prepared by a
standard ceramic processing route, using analytical-
grade ë‡Cé3, Fe2O3, TiO2, Al2O3, and CuO as starting
materials. Appropriate oxide mixtures were reacted at
1470 K for 2 h. Next, the samples were thoroughly
ground and pressed into pellets, which were sintered at
1740 K for 2 h and then quenched to room temperature.
1 ∂L
⎛
⎞
⎠
----- ------
α =
,
(1)
⎝
L0 ∂T
p
as the slope of the ∆L/L(T) curve.
The phase composition of the samples was deter-
mined by x-ray diffraction (XRD) on a DRON-UM1
automated diffractometer (2θ = 20°–80°). The phases
present were identified using JCPDS Powder Diffrac-
tion File data and the fpeak.exe software package [7].
The lattice parameters of individual phases were
refined by the Rietveld profile analysis method [8,
RESULTS AND DISCUSSION
XRD examination showed that the samples with the
nominal compositions CaTi0.9M0.1O3 – δ (M = Fe, Cu,
Al) were single-phase and had an orthorhombically dis-
torted perovskite structure (sp. gr. Pnma). Figure 1
shows the XRD pattern of CaTi0.9Fe0.1O3 – δ. The
9]. The algorithm proposed by Rietveld was imple- refined lattice parameters are listed in Table 1. It can be
mented in many programs, some of which can be seen from these data that the lattice parameters of our
296

THERMAL EXPANSION AND ELECTRICAL CONDUCTIVITY
297
300
200
100
0
–100
10
20
30
40
50
60
70
2θ, deg
Fig. 1. XRD pattern of CaTi Fe
O
.
0.9 0.1 3 – δ
samples are governed not by the size of solute ions the hole concentration and, hence, the conductivity
[12] but by some other factors, e.g., by oxygen sto-
vary as p1O/4
:
2
ichiometry.
σ ∼ p = constp1O/4.
(3)
The conductivity data for CaTi0.9M0.1O3 – δ (M = Fe,
2
Cu, Al) are presented in Fig. 2. The log–log plots of
conductivity versus oxygen partial pressure for calcium
titanate acceptor-doped with Fe, Cu, and Al are charac-
teristic of mixed conductors [13]. At high oxygen par-
tial pressures, the conductivity is nearly proportional to
At intermediate oxygen partial pressures, the plots
in Fig. 2 have a broad plateau, indicating that ionic con-
duction prevails:
ꢀꢀ
ꢀꢀ
'
''
[(Fe, Al)Ti] = 2[VO]; [CuTi] = [VO].
(4)
p1O/4 since oxygen incorporation into lattice sites
2
At low oxygen partial pressures, the conductivity
increases because of the increase in electron concentra-
tion:
increases the carrier (hole) concentration according to
the scheme [14]
1
2
ꢀꢀ
O×O = O2 + VO + 2e'.
(5)
1
2
ꢀꢀ
ꢀ
×
--
--
VO + O2 = OO + 2h .
(2)
ꢀꢀ
ꢀꢀ
Here, the vacancy concentration varies, n = 2[ VO ],
Here, the concentration of oxygen vacancies VO is
and the conductivity is proportional to pO–1/6
:
a constant determined by the doping level. Therefore,
2
Table 1. Lattice parameters of undoped and doped calcium titanate
a, Å
b, Å
c, Å
V, Å3
Rp
Rwp
Rexp
χ2
RBr
Rf
CaTiO3
5.379(5)
5.378(7)
5.382(3)
5.379(9)
7.641(5)
7.630(1)
7.625(4)
7.633(2)
5.441(8) 222.63(3)
5.428(6) 222.75(5)
5.434(4) 223.04(1)
5.435(9) 222.63(3)
22.9
26.3
15.0
42.1
22.9
50.0
3.1
0.4
2.3
0.4
4.1
3.7
1.8
2.4
3.1
2.6
2.5
2.3
CaTi0.9Fe0.1O3 – δ
19.4 25.0
CaTi0.9Al0.1O3 – δ
24.8 33.6
CaTi0.9Cu0.1O3 – δ
26.4 31.1
INORGANIC MATERIALS Vol. 44 No. 3 2008

298
logσ [S/cm]
MURASHKINA et al.
CONCLUSIONS
We refined the lattice parameters of the
CaTi0.9M0.1O3 – δ (M = Fe, Cu, Al) compounds (orthor-
hombic perovskite structure, sp. gr. Pnma).
The electrical conductivity of CaTi0.9M0.1O3 – δ was
measured in wide ranges of oxygen partial pressures
and temperatures. The Fe-, Cu-, andAl-doped materials
(a)
–1.0
–1.2
–1.4
–1.6
–1.8
–2.0
–2.2
ëaTi0.9Fe0.1O3 – δ
ëaTi0.9Cu0.1O3 – δ
ëaTi0.9Al0.1O3 – δ
1170 ä
are mixed conductors, -type at high pO and n-type at
2
low pO . At intermediate oxygen partial pressures, the
2
conductivity varies little. The acceptor dopants studied
are shown to raise the conductivity of calcium titanate
in the order Al < Cu < Fe. The thermal expansion of the
three materials was measured.
–15
–10
–5
0
5
–0.4
–0.6
–0.8
–1.0
–1.2
–1.4
–1.6
–1.8
–2.0
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(b)
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Fig. 2. Log–log plots of conductivity vs. oxygen partial
pressure for CaTi
M
O
.
0.9 0.1 3 – δ
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σ ∼ n = constp–O1/6
.
(6)
2
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order Al < Cu < Fe.
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Fe
290–770
770–1270
290–570
570–800
800–1270
290–1270
14.5
10.6
4.8
0.83
3.3
Al
6.8
10.6
8.5
2.2
2.9
Cu
10.01
0.95
INORGANIC MATERIALS Vol. 44 No. 3 2008