Materials Science and Engineering Axxx (2004) xxx–xxx
Carbonate Co-precipitation of Gd2 O3-doped CeO2 solid
solution nano-particles
A.I.Y. Tok∗ , L.H. Luo, F.Y.C. Boey
School of Materials Engineering, Nanyang Technological University, Nanyang Avenue, Singapore
Received 4 March 2004
Abstract
This paper reports on the synthesis of 20 mol% Gd2 O3 -doped CeO2 solid solution (20 GDC) nano-particles via carbonate co-precipitation.
Precursors and calcined particles were characterized using TGA, XRD, BET, FESEM, and TEM. From the diffraction pattern using XRD
with TEM, it was shown that the Gd3+ replaced the Ce4+ lattice in the fluorite structure (FCC) of CeO2 , as opposed to it being a second phase
in the CeO2 structure. The 20 GDC particles were calcined at 700 ◦ C for 2 h, and sintered to >99% density at a very low sintering temperature
of 1150 ◦ C for 4 h.
© 2004 Elsevier B.V. All rights reserved.
Keywords: Gd2 O3 -doped CeO2 ; Carbonate co-precipitation; Nano-particles; Solid oxide fuel cells; Microstructure; Solid solution
1. Introduction
Gadolinium oxide-doped fluorite structured cerium oxide, 20 mol Gd2 O3 –CeO2 (hereafter referred to as 20
GDC for convenience), is a solid solution formed by
replacing the Ce4+ sites of the CeO2 lattice by Gd3+
cations. 20 GDC has been recognized as a low temperature
(500–700 ◦ C operating temperature) electrolyte material
for applications in solid-oxide fuel cells (SOFC), as GDC
has higher ionic conductivity compared to other commonly used materials such as YSZ[(ZrO2 )0.9 (Y2 O3 )0.1 ] and
LSGM(La0.9 Sr0.1 Ga0.8 Mg0.2 O2.85 ) [1]. 20 GDC powders
synthesized via current solid-state reactions require very
high sintering temperatures (1700–1800 ◦ C) [2]. Traditional
ball milling of particles to reduce its size will also introduce
impurities such as silicon into the 20 GDC particles, and this
will severely decrease its ionic conductivity since silicon
form an insulation glassy phase in the grain boundaries [3].
A lower electrolyte sintering temperature is also desired, as
the cathode and anode materials are normally sintered at a
relatively lower temperature of 1100–1300 ◦ C [4]. Therefore, an electrolyte material that can be co-fired together
with the anode/cathode at a lower temperature would be
desired. In addition, nano-particles can impart improved
∗
Corresponding author. Tel.: +65 67904935; fax: +65 67904935.
E-mail address: [email protected] (A.I.Y. Tok).
0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.msea.2004.05.071
mechanical properties to the electrolyte layer, as compared
to those consolidated from micron-sized particles.
Several types of wet-chemical methods have been reported for the synthesis of 20 GDC particles. These include
oxalate co-precipitation [5], sol–gel [6], and hydrothermal
treatment [7]. These wet chemistry-derived powders generally show better reactivity than those obtained via solid-state
methods, but they still require relatively high densification
temperatures of about 1400–1600 ◦ C to reach 99% density.
The limitation here seems to stem from severe agglomeration of the particles, and undesirable morphologies of the
resultant particles. The use of carbonates as the precursor
materials for highly sinterable oxides has shown characteristics of being non-gelatinous, and exhibit significantly weaker
agglomeration after drying [8].
It was also reported that reactive Ce0.8 RE0.2 O1.9 (RE
= La, Nd, Sm, Gd, Dy, Y, Ho, Er, and Yb) powders synthesized via carbonate co-precipitation required an aging temperature of 70 ◦ C and drying in N2 [9], as opposed to using
room temperature aging and air-drying.
2. Experimental procedure
2.1. Powder synthesis
Starting materials used for the 20 GDC synthesis
were cerium nitrate hexa-hydrate and gadolinium nitrate
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A.I.Y. Tok et al. / Materials Science and Engineering Axxx (2004) xxx–xxx
hexa-hydrate [Ce(NO3 )3 ·6H2 O; Gd(NO3 )3 ·6H2 O; 99.99%
purity, Aldrich Chemical Company, Inc.] and ammonium
carbonate [(NH4 )2 CO3 ; 99.99% purity, Alfa Aesar, Johnson Matthey]. The carbonate co-precipitation reaction is as
follows [10]:
0.8Ce(NO3 )3 · 6H2 O+(NH4 )2 CO3 +0.2Gd(NO3 )3 · 6H2 O
→ Ce0.8 Gd0.2 (OH)CO3 · (3–4)H2 O
(1)
carbonate/Ce3+ (+Gd3+ )
molar ratio (R)
The ammonium
significantly affected the precursor properties, and spherical
nano-particles could be produced only in a narrow range of
2.0 < R ≤ 3.0 [10]. In this work, the mole ratio (R) of 2.5
was studied.
The solution of Ce(NO3 )3 ·6H2 O and Gd(NO3 )3 ·6H2 O
mole concentration was prepared to be 0.3 M 150 ml solution
with distilled water. Ammonium carbonate mole concentration was 0.75 M 150 ml solution. One hundred and fifty
milliliters of mixed nitrate solution was dropped very slowly
into 150 ml of a 0.75 M ammonium carbonate solution kept
at room temperature under mild stirring. The resultant suspension was separated into two parts; one part was aged at
70 ◦ C for 1 h and the other was aged at room temperature
for 1 h. The two precursors were then thoroughly washed
with distilled water and rinsed with ethanol respectively. The
precursor aged at 70 ◦ C for 1 h was dried with flowing nitrogen gas for 12 h. The precursor aged at room temperature
was vacuum dried for 12 h. Both precursors were then calcined in flowing oxygen at various temperatures (600, 700
and 800 ◦ C) for 2 h to yield the nano-sized oxide particles.
2.2. Powder characterization
The 20 GDC precursors and particles were characterized using differential thermal analysis (TA Instruments
TGA 2950) and X-ray diffraction (XRD, Model: Rigaku
Dmax2200) operating at 30 kV/40 mA using Cu K␣ radiation in the 2θ range of 20–80◦ at 4◦ 2θ/min.
The crystalline size of the calcined powders were estimated using the X-ray line broadening technique performed
on the (4 2 2) diffraction peak of the ceria lattice using the
Scherrer equation.
Dhkl =
0.89λ
βhkl cos θ
pany (Japan) with four selectable weight functions: 1, sin2θ
× sin2θ, r(θ) × r(θ) and sin2θ sin2θ × r(θ) × r(θ).
This lattice parameter was also compared with results
calculated using slow speed single-peak scanning by the
lattice parameter formula (Eq. (3)).
a
(3)
d=√
2
h + k 2 + l2
In the above equation, the term a, is the CeO2 FCC lattice
parameter, and h, k, l are the crystalline face indexes while
d is the crystalline face space.
Specific surface area of the air-dried precursor and the calcined powders, SBET were measured by Brunaue–Emmett–
Teller analysis (BET, Micromeritics, ASAP 2000), and the
specific surface area was converted to particle size according
to Eq. (4), assuming that the particles were closed spheres
with smooth surface and uniform size.
DBET =
6 × 103
dth SBET
(4)
Here, dth is the theoretical density of the material (g/cm3 )
(solid solution Ce0.8 Gd0.2 O1.9 ) calculated using Eq. (5),
DBET (nm) the average particle size, and SBET the specific
surface area expressed in m2 /g.
4
dth =
[(1 − x)MCe + xMGd + (2 − 0.5x)MO ]
N A a0 3
= 7.2327 g/cm3
(5)
x = 0.2; NA is the Avogadro’s constant and Mi ’s refer to the
atomic weight; a the lattice parameter of CeO2 .
Particle morphology, agglomeration state and crystalline
state of the synthesized precursor and powders were observed via field emission scanning electron microscopy (FESEM, Model: 6340F). The FESEM sample was ultrasonically dispersed into ethanol, and the suspension was spread
on the surface of a copper observation stage. After drying,
the sample was coated with a thin layer of gold for conductivity. The morphology, size and crystalline characterizations
of the calcined powder were observed using transmission
electron microscope (TEM, Model: JEM2010, accelerating
voltage: 200 kV, JEOL, Tokyo, Japan).
2.3. Compaction and sintering
(2)
In this equation, λ is the wavelength of the incident X-rays
(0.15406 nm); θ the diffraction angle; βhkl the corrected
2 = β2 − β2 , where β refer to
half-width given by βhkl
hkl
e
hkl
the measured half-width and βe refer to the half-width of the
equipment, measured using a standard CeO2 powder with
a known crystalline size of larger than 1.0 ␮m. The experimental equipment’s half-width (βe ) is 0.282◦ .
The lattice parameter (a) of the 20 GDC oxide calcined
at 700 ◦ C was determined using a least square refinement
method via a computer software provided by Rigaku Com-
Particles for sintering were dry-pressed into pellets
(∼50 MPa) in a stainless steel die and isostatically pressed
at 200 MPa. The powder compacts were sintered at 1150 ◦ C
for 4 h using a ramp rate of 5 ◦ C/min and naturally cooled
in the furnace.
Two methods were used for density measurements. In
the first, the sintered density of the powder compact (d)
was measured from the green density (d0 ) and the measured
linear shrinkage, L/L0 , using Eq. (6).
d=
d0
(1 − L/L0 )3
(6)
A.I.Y. Tok et al. / Materials Science and Engineering Axxx (2004) xxx–xxx
3
Fig. 2. XRD patterns of the precursors and 20 GDC powders calcined at
different temperatures for 2 h.
Fig. 1. TGA curves of the two precursors in flowing air at a heating rate
of 10 ◦ C/min.
L0 is the initial sample length and L = L0 – L, where L is
the sintered sample length.
The green density (d0 ) of the powder compact was calculated from its weight and geometric dimensions. Relative
sintered density was then obtained by dividing d by the theoretical density of 20 GDC (dth ). A second method was used
to verify the measured density, and this was performed using
gas pycnometry.
3. Results and discussion
3.1. Powder synthesis and characterization
Fig. 1 shows the TGA graph of the synthesized precursors processed using different aging and drying conditions.
Both curves indicate that decomposition mainly occurred via
three distinct stages and was complete at about 700 ◦ C. The
final weight loss (36–40 wt.%) was in good agreement with
that (36–40 wt.%) calculated from the complete decomposition of Ce0.8 Gd0.2 (OH)CO3 ·(3–4)H2 O, with the oxidation
of Ce4+ to Ce3+ . Weight loss below 100 ◦ C could be attributed to absorbed moisture, and weight loss between 100
and 400 ◦ C corresponded to the release of hydrates. The final stage of weight loss signified the de-carbonization of
Ce0.8 Gd0.2 (OH)CO3 . Both TGA curves exhibited similar
nature in the various decomposition stages.
From the TGA results, the calcination temperatures for
the precursors were determined at 600, 700, and 800 ◦ C for
2 h. XRD results (Fig. 2) showed that the as-synthesized precursors were amorphous and all the peaks of the powders
calcined at different temperatures corresponded to the fluorite structure of CeO2 (PDF card number: 34-0394). Increasing intensities were observed as the calcination temperatures
were increased, indicating increasing crystallite growth. No
crystalline phase corresponding to Gd2 O3 could be found
Fig. 3. Single-peak (4 2 2) XRD pattern 20 GDC powders: (a) N2 -dry;
(b) air-dry.
at any calcination temperature, suggesting an intimate mixing of Ce3+ and Gd3+ cations in the precursor. The lattice
parameter of the 20 GDC oxide calcined at 700 ◦ C was determined to be 0.5425 nm using the least square refinement
method. This value agreed well with the theoretical lattice
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Table 1
Crystalline and particle sizes of 20 GDC precursors and powders calcined at various temperatures
Sample
Air-dry
Powder
Powder
Powder
precursor
(600 ◦ C)
(700 ◦ C)
(800 ◦ C)
Surface area (air-dry) (m2 /g)
Calculated DBET (air-dry) (nm)
DXRD (air-dry)/(N2 -dry) (nm)
ϕ (air-dry)
36.8
32.1
27.6
24.7
22.6
25.8
30.0
33.6
17.19/15.60
25.80/25.79
33.01/33.14
1.50
1.16
1.02
parameter of the fluorite structure of CeO2 (theory lattice
parameter, a = 0.5411 nm). As a comparison, the lattice parameter formula in Eq. (3) gave a result of 0.5426 nm. There
was a very slight difference between the calculated and theoretical lattice parameter. This was due to Gd3+ replacing
the Ce4+ lattice in the fluorite structure (FCC) of CeO2 .
The single-peak XRD pattern in Fig. 3 shows that the
powders calcined at the different temperatures were of
nano-crystalline size. Scanning speed used was 0.125◦ /min
and scanning range, 2θ = 87–90◦ .
This crystalline size can be calculated using the
single-peak information together with Eq. (2):
Dhkl =
0.89λ
βhkl cos θ
=√
0.9182
= 17.19 nm
0.89 × 1.54
− 0.2822
× 3.14/180 × cos(88.12/2)
The particle size and the value of specific surface area
of the powders and precursor can also be calculated using
Eq. (4). The calculated results of crystalline size and powder particle size are shown in Table 1. ϕ = DBET /DXRD is
defined as a factor to reflect the agglomeration extent of the
primary crystalline. A ϕ value of 1.0 indicates complete dispersion [10]. The ϕ value decreased gradually with an increase in the calcination temperature. This is interesting to
note, because the ϕ value usually increases with increasing
calcination temperature. However in this case, this decrease
in the ϕ value was due to the crystallite growth rate being
much larger than the agglomeration rate of the particles as
the calcination temperature increased from 600 to 800 ◦ C.
Both the crystalline sizes of air-dry powders and N2 -dry
powders were very similar.
Fig. 4(a) shows the air-dried precursor and Fig. 4(b) shows
the powder calcined at 700 ◦ C for 2 h.
Fig. 5 shows TEM micrographs of the powders synthesized using air-dried precursors calcined at 700 ◦ C for 2 h.
These particles exhibited average particle sizes of 30–50 nm.
In order to characterize the structure of the 20 GDC solid
solution, the diffraction pattern of the particles was obtained
using nano-electron-beam. From the diffraction pattern in
Fig. 6(a), the crystalline face index was determined, and this
is shown in Fig. 6(b). The lattice pattern is quadrangle and
the direction of the crystalline face group is [0 0 1]. All of
these results are in good agreement with the fluorite structure of CeO2 . This reinforces the point that the Gd3+ cations
replaced the Ce4+ cations located on the plane of the Ce4+
lattice. This follows the equation:
CeO2
Gd2 O3 −−→ 2GdCe + VO
+ 3OxO
(7)
In the fluorite unit cell structure, the Ce4+ cations occupies the FCC lattice sites, while the anions (O2− ) are located
at the eight tetrahedral sites. The four remaining octahedral
sites in the FCC lattice remain vacant. The large number
of unoccupied octahedral interstitial sites allows this compound to be used as a nuclear fuel since fission products can
be accommodated in these vacant positions.
Fig. 7 shows a high resolution TEM micrograph of the
nano-particles calcined at 700 ◦ C. Good crystalline state and
crystalline faces can be observed. The distance between the
Fig. 4. FESEM of (a) air-dried precursors (b) calcined at 700 ◦ C for 2 h.
A.I.Y. Tok et al. / Materials Science and Engineering Axxx (2004) xxx–xxx
5
Fig. 5. TEM 20 GDC powder, from air-dried precursor calcined at 700 ◦ C for 2 h.
Fig. 6. (a) Diffraction pattern of the particles using nano-electron-beam. (b) Crystalline face indexes determined from nano-beam diffraction pattern (the
direction of the crystalline face group is [0 0 1]).
crystalline faces was measured to be about 0.539 nm (d).
Since we know that this is a cubic structured material, and
that a is 0.54256 nm (calculated), we can postulate from
Eq. (3) that the crystalline face groups can be (1 0 0), (0 1 0)
or (0 0 1). Since it belongs to the FCC structure, the crystalline face group of {1 0 0} is absent under XRD and diffraction pattern of TEM. However, it can be observed under high
resolution TEM (bright field image).
3.2. Densification behavior of the 20 GDC oxides
Fig. 7. High resolution TEM of 20 GDC powders calcined at 700 ◦ C for
2 h.
Fig. 8 shows the microstructure of 20 GDC nano-particles
sintered at 1150 ◦ C for 4 h from air-dried precursors. Measurements show that the substrate is >99% dense, with very
few residual pores, mainly located along the grain boundaries. Fig. 8(a) shows the fracture surface of the substrate.
Fracture occurred mainly at the grain boundaries. Fig. 8(b)
shows the polished and thermally etched (1100 ◦ C, 0.5 h)
microstructure of the sintered substrate. It can be observed
that the grains are equiaxed and the grain boundaries are
well developed with low porosity. Average grain size was
determined to be 0.78 ␮m using the linear intercept method.
Even though a very low sintering temperature of 1150 ◦ C
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A.I.Y. Tok et al. / Materials Science and Engineering Axxx (2004) xxx–xxx
Fig. 8. (a) Fracture surface of pellet. (b) Polished and thermally etched pellet, both sintered at 1150 ◦ C for 4 h.
was used, this still resulted in a very high sintered density
>99%. This was attributed to the good dispersion obtained
in the carbonate co-precipitated nano-particles.
4. Conclusion
The carbonate co-precipitation method to synthesize 20
GDC nano-particles was successful, and this represents an
easy and cost-effective method. Two aging temperatures and
two dry methods were studied in this experiment. It was concluded that room temperature aging and air-drying allowed
weaker agglomerated nano-particles. This also represents a
simpler processing route. The 20 GDC nano-particles were a
solid solution, where Gd3+ replaced the Ce4+ lattices completely. The nano-particles were about 30–50 nm in size and
had good crystallinity. These nano-particles exhibited excellent sinterability, achieving >99% dense substrates with fine
grains (0.78 ␮m) and low porosity at a low sintering temperature of 1150 ◦ C for 4 h.
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