Advances in Ma terials Physics and Che mist ry, 2012, 2, 5-8
doi:10.4236/ampc.2012.24B002 Published Online December 2012 (htt p://www.SciRP.org/journal/ampc)
Preparation, Characterization and Thermal Expansion of Pr
Co-Dopant in Samarium Doped Ceri a
V. Venkatesh, V. Prasha nth Kumar, R. Say anna, C. Vishnuvardhan Reddy
Dept.of Physics, Osmania Un ivers it y, H yderabad, INDIA
Received 2012
ABSTRACT
The compositions Ce0.8-xSm0.2PrxO2-δ (X=0, 0.02, 0.04, 0.06) were prepared through the sol–gel route. The effect of Pr addition on
the crystal structure, densification and thermal expansion of Ce0.8Sm0.2O2-δ was studied. The phase identification and morphology
was studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). X-ray diffraction analysis showed that all the
samples exhibit a fluorite structure. The lattice parameters were determined by X-ray powder diffraction. Lattice parameters and
volume of the unit cell increases with Pr doping. Density of the all samples is more than 90% of theoretical density. The thermal
expansion was measured using dilatometric technique in the temperature range 30–1000°C. It was observed that the thermal expan-
sion increased linearly with increas ing te mperature for all the s amples.
Keywords: Solid Oxide Fuel Cells; Sol-Gel; X-Ray Diffraction; Scaning Electron Micro s copy; Thermal Expansion
1. Introduction
Solid oxide fuel cells (SOFCs) convert ch emical energy directly
into electrical energy with high efficiency, environmental
friendliness, and great flexibility in the choice of fuel [1-3].
Electrolytes used for SOFCs are usually the main components
determining the performance of the fuel cell. Yttria stabilized
zirconia (YSZ) is well established electrolyte, which can be
used in commercial SOFCs. A typical high-temperature SOFC
uses 8 mol% YSZ as ele ctrolyte, usually operated at temperat ures
as high as 800-1000oC to obtain the required level of ionic
conductivity. However, such high operating temperat ures resul t
in expen sive fabricatio n costs an d accelerate th e degradatio n of
fuel cell systems. Therefore, strong motivation to search for new,
improved oxide-ion electrolytes at intermediate temperatures
(400-700oC) persists. Lowering the operating temperatu re to an
intermediate temperature (400-700°C) significantly enhances
the long-term performance stability, lessens sealing problem,
widens the material selection, and allows the use of low-cost
metallic interconnects, thereby accelerating the co mmer ciali-
zation of SOFC technology[4]. Doped ceria has been acknow-
ledged as a poten tial el ectrol yte material for IT-SOFCs b ecause
of their high ionic conductivity and good compatibility with
electrodes[5-6].
The ionic conductivities of ceria based electrolytes doped
with various dopants(e.g., Sm3+, Gd3+, Y3+, Ca2+, Sr2+) at differ-
ent dopant concentrations have been extensively investigated
[7-13]. Sm3+ is considered one of the best dopants for ceria-
based solid electrolytes currently available [14-16]. The co-
doping technique has been effective method for improving the
condu ctivity and lead s to thermal expansio n match between the
electrodes and electrolyte in ceria based electrolytes for the
IT-SOFCs [7] .
In the present paper we report the sol-gel synthesis of
Ce0.8-xSm0.2PrxO2-δ for (0≤x≤ 0.06). The powder characteristics
such as cr ystallite s ize, sur fac e area and ther mal expansio n have
been determined as a function of x.
2. Experimental
The sample with the general formula Ce0.8-xSm0.2PrxO2-δ (x=0 ,
0.02, 0.04, 0.06) were synthesized by sol-gel method. Ceric
ammonium nitrate ((NH4)2Ce(NO3)6) (Merck, 99%), samarium
oxide (Sm2O3) (Himedia-99.9%) and Praseodymium oxide
(Pr6O11) (Himedia-99.9%) were used as starting materials.
Stoichiometric amounts of samarium oxide and praseodymium
oxide powders were mixed with nitric acid and placed on a
heater at 50°C to convert into nitrates. Stoichiometric amount
of Ceric ammonium nitrate was dissolved in distilled water.
Nitrate solutions were added to Ceric ammonium nitrate
solution and stirred properly. Citric acid was added to the
whole solution in 1׃1 molar metal ratio to bind the metal ions.
The pH of solution was adjusted to ≈7 by adding ammonia.
After evaporating the water, ethylene glycol was added and
heated at about 90°C until a gel-type solution was formed. The
gel was dried at 150°C and then decomposed at 250 °C in air
for 2 h to decompose nitrates and all organic materials. The
resultant ash was ground to get a fine homogeneous powder
after that the powder was calcined at 600°C for 2 hours. The
oxidation of Ce3+ to Ce4+ occurred during this stage [5].
Intermediate calcination and grounding of the synthesized
powders were finally pressed in to pellets dimension 10mm in
diameter 2 mm in t hi ckness, and then pel lets were sin ter ed i n ai r
at 1300°C for 4 hours. The densities of sintered samples meas-
ured in xylene by Archimedes principle. The sintered samples
have above 90% of the theoretical density.
Phase identification and crystallographic information of the
samples were obtained from the X-ray data by using P ANalyti-
cal X ’Pert Pro X-ray diffractometer (XRD) with Cu Kα radia-
tion (λ=1.54056 Å operated at 40kV and 30mA) at room tem-

V. VENKATESH ET AL.
6
perature in the range 20°≤2θ ≤80°. Lattice parameter s were
calculat ed b y fitti ng th e ob served r eflectio ns with a l east-square
refinement UNIT CELL program. The surface morphology of
the samples was observed using the scanning electron micro-
scopy (SEM) (ZIESS EVO-18).
The Thermal expansion measurements were carried out with
a Netzsch push-rod dilatometer (DIL: Model Netzsch DIL 402
PC, Germany). Thermal expansion coefficient of the sintered
sample pellets were measured using a constant heating rate of
3°C/min in the temperature range 30-1000°C. The rectangular
samples of dimension 25mm × 6mm × 6mm were used these
measu remen ts. Th e rectan gular p ell ets wer e pres sed in a hydraulic
press and sintered at 1300°C. Standard aluminum (Al2O3) ref-
erence sa mpl e was used fo r the temperat ure calibration.
3. Results and Discussion
The X-ray diffraction patterns of the prepared system Ce0.8-x
Sm0.2PrxO2-δ are shown in Figure 1. Praseodymium as co- do-
pant into samarium doped ceria (SDC) ceramics sintered at
1300°C shows cubic fluorite structure with space group Fm3m
(JCPDS powder dif frac ti on File n o. 7 5-0158).
The crystallite size (DXRD) is calculated according to the
Scherrer equat ion[17] .
( )
XRD
D 0.9 cos
λβ θ
=
(1)
where, λ is the wavelength of the radiation, θ is the diffraction
angle and β is the full width in radians at half maximum inten-
sity of powder pattern at 2θ. The crystallite sizes of th e sample
powders calculated by the Scherrer formula are in between
46nm and 59nm.
In praseodymium, the ionic state changes from Pr3+ to Pr4+
under oxidizing process [18]. The introduction of Pr4+ in to Ce4+
can cause a small shift in the ceria peaks. This shift is indicative
of a change in the lattice parameter. The lattice parameter is
increased with an increase Pr content due to the difference in
ionic radii of Ce4+ (0.96Å) and Pr4+(1.14Å ) in an oxide solid
solution [19]. As praseodymium content increases, the lattice
parameter i ncreases, this i s indicat es that Pr has been disso lved
into Ce site in Ce0.8-xSm0.2PrxO2-δ and in si ng l e phase structure
20 30 40 50 60 70 80
(420)
(331)
(400)
(222)
(311)
(220)
(200)
(111)
x=0.06
x=0.04
x=0.02
x=0
Intensity (arb.u))
2 Theta(de g)
Figure 1. XRD pattern of Ce0.8 -xSm0.2PrxO2− δ (0≤x ≤0.06).
is formed. The lattice parameters, the unit cell volume and the
type of structure of the system are presented in the Table.
x Structure A (Å) Volume (Å3)
0 Cubic 5.432 160.28
0.02 C ubic 5.436 160.71
0.04 C ubic 5.437 160.77
0.06 C ubic 5.438 160.82
Figure 2 show the SEM photographs are clears that less
porous is res idual , in acco rd an ce with the r elati ve den sit y of the
sintered pellets.
Relatively dense samples, with density greater than 90% of
the theo retical values are requ ired for the measurement o f TEC
[20].
TH
D 0.6023zM V=××
(2)
X = 0.0
X = 0.022
X = 0.06
Figure 2. SEM Photographs of Ce0.8Sm0.2O2-δ, Ce0.78 Sm0.2Pr0.02 O2-δ
and Ce0.74Sm0.2Pr0.06O2-δ.

V. VENKATESH ET AL.
7
Theoretical density can be calculated using by equation (2)
where V (in Å3) is the volume of the unit cell as determined by
XRD, M (in atomic mass uni ts) is th e mass of one formul a unit,
z is the number of such formula units in one unit cell of the
crystal. The possible departures from stoichiometry have not
been taken into accou nt in calculating M. The bulk density (d),
the theoretical density (dth), and d/dth (%) are summarized in
Table 1. All samples have densities above 90% of the theoretical
value.
The thermal expansion coefficients of electrolyte and elec-
trodes should match, to avoid micro-cracks in between th em for
the operati0n of SOFC device at high temperatures. The ther-
mal expansion data of Ce0.8-xSm0.2PrxO2-δ (x=0, 0.02, 0.04, 0.06)
obtained in the temperature range 30- 1000°C in air is shown in
Figure 3. The thermal expansion depends on the electrostatic
forces within the lattice, which depends on the concentration of
positive and negative charges and their distances within the
lattice. The thermal expansion increases due to the decrease in
attractive forces. Ther mal e xpan sio n of a latt ice i s char acterized
by a steady thermal expansion coefficient (α), for a certain
structure and fixed oxygen t o metal st oichio metry. The slope of
thermal expansion curves for all the composition s are increased
with temperature.
The thermal expansion coefficients (TEC) are calculated
from the thermal expansion curves and values are listed in Ta-
ble 2. The results of the present study are in contrast with the
reported values. [10, 12-13, 21 -24]. The disparity in the results
may be due to preparation condition, non-stoichiometry of
oxygen and oxidizing process of Pr.
Table 1. Density measurements of Ce0.8-xSm0.2PrxO2-δ (0≤x ≤0.06).
x Bulk Density (d) Theoretical Density (dth) d/dth%
0 6.769 7.216 93
0.02 6.934 7.197 96
0.04 6.725 7.195 93
0.06 6.818 7.192 94
200 400 600 8001000
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
(dL/L)%
T emp erature (0C )
x=0
x=0.02
x=0.04
x=0.06
Figure 3. Temperature variation of thermal expansion of Ce0.8-x
Sm0.2PrxO2 (0≤x≤0.06).
Table 3. Thermal expansion co-efficient of Ce0.8-xSm0.2PrxO2-δ (0≤x
≤0.06).
x TEC (10-6/oC)
30- 600oC 30- 800oC 30- 1000oC
0 11.51 12.25 12.32
0.02 11.17 12.36 12.66
0.04 12.03 13.31 13.78
0.06 11.52 13.18 13.02
4. Conclusions
Praseodymium and samarium co-doped ceria samples Ce0.8−x
Sm0.2PrxO2−δ (x=0.00, 0.02, 0.04 and 0.06) are successfully
synthesized through the sol–gel method. Dense ceramics are
obtained by sintering the pellets at 1300°C for 4 hours. The
relative densities are over 90% of the theoretical density and
these results are consistent with the SEM studies. The lattice
parameter increased with increasing Pr content. Thermal ex-
pansion increased linearly with increasing temperature for all
the sa mples. Th e val ues o f t her mal expan sio n coeffici ent s of all
the compositions are in the range of 12.25×10−6/°C to 13.31 ×
10−6/°C. The present co-doped ceria materials can be used as
possible electrolyte materi al for I T-SOFC applications.
5. Acknowledgements
One of the author, V.Venkatesh, thanks to the University Grant
Commission (UGC), New Delhi, India for the financial assis-
tance u nder th e scheme of Resear ch Fello wships in Science for
Meritorious Students (RFSMS).
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