Synthesis and electrical properties of scheelite Ca1-xSm xMoO4+δ solid electrolyte ceramics

Article abstract of DOI:10.1016/j.materresbull.2010.11.019

Scheelite-type Ca_1-xSm_xMoO_4+δ electrolyte powders were prepared by the sol-gel auto-combustion process. The crystal structure of the samples was determined by employing the techniques of X-ray diffraction (XRD). According to the XRD analysis, the formed continuous series of Ca_1-xSm_xMoO_4+δ (0 ≤ x ≤ 0.3) solid solutions had the structure of tetragonal scheelite, and the lattice parameters increased with increasing x in the Sm-substituted system. Results of sinterability and electrochemical testing revealed that the performances of Sm-doped calcium molybdate were superior to that of pure CaMoO₄. Ca_1-xSm_xMoO_4+δ ceramics show higher sinterability, and the Ca_0.8Sm_0.2MoO_4+δ sample with 98.7% of the theoretical density were obtained after being sintered at 1250 °C for 4 h. The conductivity increased with increasing samarium content, and a total conductivity 9.54 × 10^-3 S cm^-1 at 800 °C could be obtained in Ca_0.8Sm_0.2MoO _4+δ sintered at 1250 °C for 4 h.

Full text of DOI:10.1016/j.materresbull.2010.11.019

Materials Research Bulletin 46 (2011) 185–189
Contents lists available at ScienceDirect
Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Synthesis and electrical properties of scheelite Ca1 Sm MoO solid
4+
d
ꢀx
x
electrolyte ceramics
a,b,
a
a
a
a
Jihai Cheng *, Chenfei Liu , Wenbing Cao , Mingxing Qi , Guoquan Shao
a
Department of Chemistry and Materials Engineering, Hefei University, Hefei 230022, China
b
Key Lab of Powder and Energy Sources Materials, Hefei University, Hefei 230022, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Scheelite-type Ca1 Sm MoO
4+
d
electrolyte powders were prepared by the sol–gel auto-combustion
ꢀx
x
Received 9 February 2010
process. The crystal structure of the samples was determined by employing the techniques of X-ray
Received in revised form 11 November 2010
Accepted 12 November 2010
Available online 21 November 2010
diffraction (XRD). According to the XRD analysis, the formed continuous series of Ca1ꢀxSm MoO4+
x
d
(
0 ꢂ x ꢂ 0.3) solid solutions had the structure of tetragonal scheelite, and the lattice parameters
increased with increasing x in the Sm-substituted system. Results of sinterability and electrochemical
testing revealed that the performances of Sm-doped calcium molybdate were superior to that of pure
Keywords:
CaMoO
4
. Ca1ꢀxSm
x
d d
MoO4+ ceramics show higher sinterability, and the Ca0.8Sm0.2MoO4+ sample with
A. Ceramics
9
8.7% of the theoretical density were obtained after being sintered at 1250 8C for 4 h. The conductivity
B. Chemical synthesis
C. Impedance spectroscopy
D. Electrical properties
ꢀ3
ꢀ1
increased with increasing samarium content, and a total conductivity 9.54 ꢁ 10 S cm at 800 8C could
be obtained in Ca0.8Sm0.2MoO4+ sintered at 1250 8C for 4 h.
d
ß 2010 Elsevier Ltd. All rights reserved.
1
. Introduction
mixed oxides have been of practical interest because of their
attractive luminescence property and possibility of negative
electrode (anode) materials [9,10]. More attention is focused on
a lot of properties such as electrical properties, thermal properties,
luminescence and reflectivity [11–14], which provide a basis for
our understanding of this kind of material. Presently, possible
applications of these materials in solid-state devices as electrodes
and electrolytes have been studied by more and more researchers.
Solid oxide fuel cells (SOFCs) have attracted increasing interest
for their high energy conversion efficiency, fuel flexibility and low
pollution to environment [1–3], and they exhibit relative higher
specific energy among battery systems [4]. Traditional electrolyte
based on yttria-stabilized zirconia (YSZ) is one of the most well
known electrolyte materials for SOFCs. However, the high
operating temperature (above 1000 8C) results in both high costs
and system degradation at the interface between cell components
Various techniques have been developed to synthesize CaMoO
4
fine powders, such as molten salt, microwave irradiation,
coprecipitation, sonochemical method and conventional solid-
state reaction [15–18]. Sol–gel auto-combustion method has been
studied in different ceramic systems recently; it combines the
virtues of the sol–gel process and low temperature combustion
process. This method has the advantages of inexpensive precursors
and resulting nano-sized, homogeneous, highly reactive powders.
Using this method, a single-phase powder can be formed at low
temperature and the properties of the product can be improved.
[
5,6]. Therefore, research and development of electrolyte materials
operating at intermediate temperature have received much
attention previously. Now the electrolyte materials to substitute
the YSZ electrolyte materials have attracted considerable attention
throughout the world.
4
Doped ABO (A = Pb, Sr, Ca; B = Mo, W) scheelite-type materials
have been studied in recent years, these scheelite-type oxides
exhibit a high oxide ion conduction in intermediate temperature
range, e.g., Pb0.9Sm0.1WO4+
shows
a
conductivity of
x
d
In this paper, the Ca1ꢀxSm MoO4+ powders were prepared
d
ꢀ2
ꢀ1
2
ꢁ 10 S cm
at 800 8C [7], which is comparable to that of
using sol–gel auto-combustion process; the phase information and
sinterability were investigated. Furthermore, the electrical prop-
erties were characterized to estimate if it could serve as an
alternative electrolyte for SOFCs.
ꢀ2
ꢀ1
YSZ (3.6 ꢁ 10 S cm
4
at 800 8C). CaMoO is a representative
scheelite compound, and its central Mo metal ion is coordinated by
2
ꢀ
four O ions in tetrahedral symmetry [8]. It can be easily doped
with rare-earth ions, and the doped crystal can be used as an
important function material. Recently, calcium molybdate based
2
. Experimental
Powder samples with the general formula of Ca1ꢀxSmx-
*
Corresponding author. Tel.: +86 551 2158437; fax: +86 551 2158437.
MoO4+d(x = 0–0.3) were synthesized by the sol–gel auto-combus-
E-mail address: cjh@hfuu.edu.cn (J. Cheng).
tion process. To synthesize nanocrystalline Ca1ꢀxSm MoO4+d,
x
0025-5408/$ – see front matter ß 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2010.11.019

1
86
J. Cheng et al. / Materials Research Bulletin 46 (2011) 185–189
[
)
D
F
G
analytical pure Sm(NO
3
)
3
, Ca(NO
3
)
2
, MoO
3
(AR grade, Sinopharm
Chemical Reagent Co. Ltd., China) were used as starting materials.
Sm(NO and Ca(NO were dissolved into deionized water,
MoO was dissolved into ammonia solution, then the two liquids
3
)
3
3 2
)
3
were mixed together under continuous stirring at 80 8C. Citric acid
was then added as both a chelating and a reducing agent (the citric
acid/metal molar ratio was fixed at 1.5), the pH value of the final
solution was adjusted by NH
3
2
ꢃH O. This mixed solution was then
heated under stirring at 80 8C to form a homogeneous sol, the sol
was further heated at 80 8C to get the gel, and then the gel was kept
in an oven at 120 8C for 24 h to get dried gel. The dried gel was
calcined in an oven at 500–700 8C, where the auto-combustion
reaction took place to form the precursors. The powders were
prepared by calcining the precursors at 900 8C for 3 h. The obtained
powders were pressed into pellets under 200 MPa. The pellets
were heated to 1100–1300 8C in air at a heating rate of 1 8C/min
and sintered for 4 h at different temperatures, then cooling down
to room temperature naturally (furnace: KSX
2
-6-16 energy
Fig. 1. X-ray diffraction patterns of Ca1 Sm MoO
4+
d
samples: (a) x = 0, (b) x = 0.1,
ꢀx
x
conservation type fast elevation of temperature electric stove,
Xiangtan Xiangyi instrument Co. Ltd., China).
(c) x = 0.2, and (d) x = 0.3.
[)TD$FIG]
Phase formation of the calcined oxide powders was determined
by X-ray diffraction (XRD: Model D/max-
analysis at room temperature with a Bragg angle range of 10–
08. Relative density of the sintered pellets was measured using the
gB, Rigaku, Japan)
8
Archimedes’ method, and the cross-sectional views of the samples
were observed by scanning electron microscope with 5 kV and
5
mA (SEM: Model JSM-6400, JEOL, Japan). Electrical conductivity
of the electrolyte materials was measured on the sintered pellet
samples. Silver paste was used as electrodes painted on both sides
of each pellet. The measurement of AC impedance was performed
in air with an electrochemical workstation CHI660B (Shanghai
Chen Hua Instrument Co. Ltd., China) in the frequency range 0.1 Hz
to 100 kHz. The measurement curves were conducted in the
temperature range 500–800 8C with an interval of 50 8C. The
conductivity at each temperature was obtained after stabilizing
the specimens for 30 min.
Fig. 2. X-ray diffraction pattern of Ca0.8Sm0.2MoO4+
d
before and after exposing to H
2
atmosphere at 900 8C for 2 h.
3
. Results and discussion
increased with increasing
x in the Sm-substituted system.
x
d
The XRD patterns of the Ca1ꢀxSm MoO4+ powders calcined at
00 8C for 3 h are shown in Fig. 1. All diffraction peaks match well
Furthermore, the volumes of the unit cells increase with increasing
2+
9
x, could be understood by considering the substitution for Ca by
3
+
with JCPDS file No. 08-0144. No additional diffraction peaks were
the trivalent Sm cation having bigger ionic radius. As we know,
3+
2+
2+
3+
found, indicating the substitution of Sm ion to Ca site has taken
the radii of Ca and Sm are 0.099, 0.108 nm, respectively [20,21],
3+
3+
place. The Sm ions dissolved in the crystal lattice of CaMoO
4
when doped with larger sized Sm ions, the tetragonal CaMoO
lattice will expand.
4
completely, and formed a solid solution with single tetragonal
scheelite phase.
In order to verify the stability in a reducing atmosphere, the
sintered samples were exposed to H atmosphere at 900 8C for 2 h
The tetragonal scheelite-type Ca1ꢀxSm
x
MoO4+ solid solution is
2
d
to extend up to x = 0.3 in this system, which is comparable to that
in the previous reports [19]. In this composition range, a shift in the
peak position is not obvious, but, the lattice parameters of the
tetragonal phase changed and the anion lattice distorted with
and phase composition of the resulting specimens was investigat-
ed. Fig. 2 shows the X-ray diffraction patterns of the surface of the
sintered Ca1ꢀxSm
tion pattern of the specimen after H
before untreated sample. That is to say, Scheelite-type Ca1ꢀxSmx-
MoO4+ materials have high chemical stability under reducing
x
MoO4+
before and after exposing. The diffrac-
d
2
treatment was same as that of
3
+
samarium content due to the size differences between Sm and
2+
Ca . Table 1 lists the lattice parameters of Ca1ꢀxSm
x
MoO4+d
d
samples (the lattice parameters were determined by Rietveld
refinement). As can be seen from Table 1, the lattice parameters
atmospheres, and meet the requirement of the electrolyte in
SOFCs.
Table 1
3
ꢀ3
ꢀ3
Sintered density (g cm )
x Value
Lattice constant (nm)
The unit cells’ volume (nm )
Theoretical density (g cm
)
a
c
a
a
x =0
0.5226
0.5345
0.5454
0.5561
1.1439
1.1649
1.1738
1.1959
0.3121
4.259
x =0.1
x =0.2
x =0.3
0.3319
0.3491
0.3694
4.471
4.683
4.893
4.327
4.518
4.713
a
4
Unit cell volume and theoretical density evaluated from the tetragonal scheelite-type structure of CaMoO (JCPDS file No. 08-0144).

[
(
)
T
D
$
F
I
G
]
J. Cheng et al. / Materials Research Bulletin 46 (2011) 185–189
187
[)(TD$FIG]
Fig. 3. Dependence of relative density on sintering temperatures for
Ca1ꢀxSm MoO4+ ceramics.
x
d
Relative density of the sintered pellets was measured using the
Archimedes’ method. Fig. shows the relative density of
Ca1ꢀxSm MoO4+ ceramics sintered at different temperatures.
The density of sintered Sm
3
x
d
x
Ca1ꢀxMoO4+ ceramics increases as
d
temperature is increasing, and a maximum relative density (98.7%)
of the theoretical was obtained at a sintering temperature of
1
250 8C, which satisfied the requirement for SOFCs electrolytes
operating at intermediate temperature. When sintering tempera-
ture is over 1250 8C, the density of sintered ceramics can hardly
improve, so the ideal sintering temperature of Ca1ꢀxSm
ceramics is 1250 8C. On the other hand, the relative density of
Ca1ꢀxSm MoO4+ sintered ceramics decrease on a small scale with
x
MoO4+d
x
d
increasing x, it could be understood by considering the substitution
Fig. 4. Cross-section of Ca0.8Sm0.2MoO4+
d
electrolyte: (a) 1150 8C and (b) 1250 8C.
of the difference of cation sizes.
Fig. 4 shows the cross-sectional views of Ca0.8Sm0.2MoO4+d
electrolyte sintered under 1150 8C and 1250 8C for 4 h. There exists
a small quantity of pores in the sintered pellet at 1150 8C (Fig. 4a).
The sintered electrolyte pellet sintered at 1250 8C (Fig. 4b) is quite
dense and uniform, the cross-sectional view without any obvious
pores and cracks indicating good sintering. Therefore, we selected
the pellets sintered at 1250 8C as the objects of study in this paper.
Fig. 5 shows typical impedance plots for Ca0.8Sm0.2MoO4+d
pellets sintered at 1250 8C for 4 h measured in air at various
temperatures. Impedance spectra at high temperatures (Fig. 5b–d)
[()TD$FIG]
Fig. 5. Impedance spectra of Ca0.8Sm0.2MoO4+
d
tested at: (a) 500 8C, (b) 600 8C, (c) 700 8C, and (d) 800 8C.

1
88
J. Cheng et al. / Materials Research Bulletin 46 (2011) 185–189
Table 2
t (8C)
500
550
600
40.9
1.84 ꢁ 10ꢀ3
650
19.1
3.94 ꢁ 10ꢀ3
700
12.5
6.03 ꢁ 10ꢀ3
750
9.2
8.19 ꢁ 10
800
7.9
9.54 ꢁ 10ꢀ3
R (
V
)
497.6
1.51 ꢁ 10ꢀ4
147.2
5.12 ꢁ 10ꢀ4
ꢀ
1
ꢀ3
s
(S cm
)
show one dominating semicircle, as well as a non-zero axis
intercept on the high frequency side. At lower temperatures
The conductivity at the high temperature region (>650 8C) is
expressed by
(
Fig. 5a), an additional arc at intermediate frequencies becomes
ꢀ
ꢁ
E
1
clearly visible. The resistances associated with these three
processes are referred to as Rb, Ri and Rs, respectively. The
impedance spectra data were fitted with suitable equivalent
circuit of the Zview software to distinguish the bulk resistance and
grain-boundary resistance, and the conductivities were calculat-
ed. The total resistance of electrolyte can be obtained from the
s
T ¼ A1 exp ꢀ
(2)
k
T
with
1
E
1
¼ 2
h
F
þ h
m
(3)
spectra and converted to a conductivity datum
relation:
s, using the
where h
F
is the energy of formation of Frenkel defects and h
m
is the
migration energy of the mobile species. At low temperature
<600 8C), the conductivity being determined by the frozen-in
defects, is given by
(
L
s
¼
(1)
RS
ꢀ
ꢁ
E
2
s
T ¼ A
2
exp ꢀ
(4)
k
T
where L is the sample thickness and S is the electrode area on the
sample surface, and these parameters are compiled in Table 2.
With the increase of the operating temperature, the total
resistance decreases, and the electrical conductivity datum
where
E2 ¼ hm
(5)
increases. The conductivity of Ca0.8Sm0.2MoO4+ sintered at
d
ꢀ
3
ꢀ1
Should the same carrier be mobile in the two temperature
regions, the Frenkel defect formation energy h
from Eqs. (3) and (5) as
1
250 8C for 4 h in air reached 9.54 ꢁ 10 S cm under the testing
F
can be computed
temperature of 800 8C. This value is close to that of other
electrolytes working in intermediate temperature range [20,22],
indicating that samarium doped CaMoO
for intermediate-temperature SOFCs.
4
is a promising electrolyte
h
F
¼ 2ðE
1
ꢀ E
2
Þ
(6)
The activation energies E
1
2
and E , calculated from the two
Representative Arrhenius plots of conductivity measured in air
are shown in Fig. 6. It is remarkable to see that enhanced
conductivities are observed in the samples substituted by
slopes are presented along with Frenkel defect formation energy of
each of the different crystal compositions. It is essentially implied
that the activation enthalpy of the low temperature region is lower
than the one of the high temperature conductivity [19,23]. The
activation energies for Ca0.8Sm0.2MoO4+ sample in the two
temperature regions are 0.55 eV and 0.85 eV, respectively.
ꢀ
5
ꢀ1
samarium (for pure CaMoO
4
, the conductivity is 5.0 ꢁ 10 S cm
ꢀ
1
at 800 8C [23]), it increased gradually with increasing samarium
content within the solubility range. Nevertheless, when the
samarium content exceeded 20%, the conductivity decreased. In
the temperature range of the measurements, the highest
conductivity is observed in the x = 0.2 sample.
d
x
In order to check the charge carriers in Ca1ꢀxSm MoO4+d, the
following oxygen gas concentration cells were constructed using
sample pellets as electrolytes, and the cell EMFs were measured:
The Arrhenius plot is on a logarithmic scale and the activation
x
d
energies for Ca1ꢀxSm MoO4+ samples can be obtained from
ꢀ
O
2
ð0:21 atmÞ; AgCa1ꢀxSm
x
MoO4 dAg;
O
2
ð1 atmÞþ
(7)
þ
Arrhenius plots in Fig. 6. In the case that the conductivities show
small breaks at 600–650 8C, they were calculated in two
temperature regions, above 650 8C and below 600 8C. These
temperatures demarcate the two conductivities contribution by
the two different types of charge carriers [23].
Fig. 7 shows EMFs measured by oxygen gas concentration cells
constructed using samples as the electrolytes, air and 1 atm
oxygen gas as the anode and cathode gas, respectively. Measured
1ꢀx
x
4+
d
EMFs for Ca Sm MoO showed good coincidence with the
[
(
)
T
D
$
F
I
G
]
[()TD$FIG]
Fig. 7. Temperature dependence of the EMF of the cell:
AgjCa1ꢀxSm MoO4+ jAg, O (1 atm). The dashed line denotes the theoretical EMF
obtained by the Nernst equation.
2
O (0.21 atm),
x
d
2
x
d
Fig. 6. Arrhenius plot comparing the total electric conductivity of Ca1ꢀxSm MoO4+ .

[
(
)
T
D
$
F
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G
]
J. Cheng et al. / Materials Research Bulletin 46 (2011) 185–189
189
were studied. Fig. 9 shows the logarithm of electrical conductivity
versus reciprocal temperature of Ca0.8Sm0.2MoO4+ samples
d
sintered for 4 h at different temperatures and then measured at
800 8C in air. As seen from Fig. 9, the electrical conductivity datum
increases with the increase of the sintering temperature, and
reaches an ideal value at 1250 8C. However, when the sintering
temperature exceeded 1250 8C, the conductivity decreased. The
reason may be that the lattice oxygen loss of this system at high
temperatures caused the decrease of electrical conductivity due to
the reduction of charge carrier concentration [25].
4. Conclusions
The sol–gel auto-combustion process was used to synthesize
x
scheelite-type electrolyte materials, Ca1ꢀxSm MoO4+d(x = 0–0.3).
Their phase formation, sintering behaviors and electrochemical
d
Fig. 8. Electrical conductivity of Ca0.8Sm0.2MoO4+ at 800 8C as a function of oxygen
partial pressure.
[()TD$FIG]
properties were investigated. The result shows, when doped
3
+
with larger Sm
expand. Ca1ꢀxSm
than pure CaMoO
of the theoretical density can be obtained after being sintered at
250 8C for 4 h. The total conductivity peaks at x = 0.2 in the
ions, the scheelite lattice of CaMoO
4
will
x
MoO4+ samples show higher sinterability
d
4
x
d
, and the Ca1ꢀxSm MoO4+ sample with 98.7%
1
temperature range of 500–800 8C for Sm-doped CaMoO
4
electrolyte materials.
Acknowledgement
This work was kindly supported by the Natural Science
Foundation of Education, Department of Anhui province under
contract No. KJ2009B045Z.
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