Yttrium-doped barium zirconate powders synthesized by the gel-casting method

Article abstract of DOI:10.1111/j.1551-2916.2010.03609.x

BaZr_0.9Y_0.1O_2.95 powders were synthesized using both gel-casting process and solid-state reaction. The gel-casting process with citric acid as the complexing agent could result in nanoparticles with a single perovskite phase. Lower calcining temperature and shorter holding time were required to synthesize the BaZr_0.9Y_0.1O _2.95 powders than the traditional solid-state reaction method. The work demonstrates that the gel-casting process is a simple, fast, and convenient method for preparing a high-temperature proton conductor BaZr _0.9Y_0.1O_2.95 powder.

Full text of DOI:10.1111/j.1551-2916.2010.03609.x

J. Am. Ceram. Soc., 93 [6] 1572–1575 (2010)
DOI: 10.1111/j.1551-2916.2010.03609.x
r 2010 The American Ceramic Society
ournal
J
Yttrium-Doped Barium Zirconate Powders Synthesized
by the Gel-Casting Method
Dongyun Gao and Ruisong Guow
Key Laboratory of Advanced Ceramics and Machining Technology of Ministry of Education,
Tianjin University, Tianjin 300072, China
BaZr0.9Y0.1O2.95 powders were synthesized using both gel-
casting process and solid-state reaction. The gel-casting process
with citric acid as the complexing agent could result in nano-
particles with a single perovskite phase. Lower calcining tem-
perature and shorter holding time were required to synthesize
the BaZr0.9Y0.1O2.95 powders than the traditional solid-state
reaction method. The work demonstrates that the gel-casting
process is a simple, fast, and convenient method for preparing a
high-temperature proton conductor BaZr0.9Y0.1O2.95 powder.
polyacrylamide. Without a separate drying step, direct pyrolysis
yields very fine and highly dispersed powders of mixed oxides
and solid solutions.
In this study, both powder gel-casting process and solid-state
reaction were applied to synthesize the high-temperature proton
conductor BaZr0.9Y0.1O2.95. A comparison of the two tech-
niques is presented.
1
2
II. Experimental Procedure
(
1) Materials
I. Introduction
Ba(NO
3 2
) , Y(NO
3
)
3
ꢀ 6H
2
O, Zr(NO
3
)
2
ꢀ 5H
2
O, CA, acrylamide,
0
0
3
HE Y-doped BaZrO (BZY) ceramic is one of the most
N,N -methylenebisacrylamide, and a,a -azoisobutyronitrile were
purchased from Kewei Company (Tianjin, China). All the ma-
terials had a purity of 99.9%.
T
protonic conductivity and excellent chemical and mechanical
promising proton-conducting materials for solid oxide fuel
cell applications. It has desirable properties such as high
1
stability. However, the refractory nature of BaZrO
3
-based ce-
(
2) Sample Preparation
ramics results in small grain sizes and high total grain-boundary
area. Consequently, this produces an overall low conductivity.
3
Many efforts are made to densify the BaZrO -based ceramics.
The most common method is to elevate the sintering tempera-
ture and prolong the holding time. BaZr0.9Y0.1O2.95 samples of
7% theoretical density can be obtained at 17151C and a hold-
ing time of 30 h. Different kinds of sintering aids were tried to
lower the sintering temperature of BaZrO -based ceramics.
Among them, ZnO is the most effective one, which can enhance
both densification and uniform grain growth. This results in
high densities (493%) at reduced sintering temperature
13001C).
Wet chemical route, such as sol–gel, coprecipitation, com-
bustion synthesis, are also used to synthesize doped-BaZrO
These methods can produce a homoge-
neous precursor powder at a lower temperature than the tradi-
tional solid-state reaction.
Gel-casting is widely known as an attractive ceramic-forming
process for making high-quality, complex-shaped ceramic
parts. This method can also be used to synthesize multicom-
ponent oxide powder of ceramics and glasses. French scientist
Andre Douy first described this process in 1989 and applied the
method to synthesize several kinds of powders such as
2
(A) Gel-Casting Method: Ba(NO
3
)
, Y(NO )
2 3 3
2
ꢀ 6H O,
and Zr(NO
)
ꢀ 5H
O were weighted in a stoichiometric ratio.
3
2
2
These compounds were then dissolved in deionized water sep-
arately. The solutions were heated to approximately 601C to
accelerate the dissolving. Then, the three kinds of solution were
mixed together and CA was added to the solution in a ratio of
CA to nitrate of a 2:1. Ammonia was added to adjust the pH
value to 7–8. A premix solution was prepared by dissolving 8 g
of acrylamide and 0.8 g of N,N -methylenebisacrylamide in 100
mL of distilled water. This premix solution was added to the
above nitrate solution under stirring at 2:5 premix solution to
9
3
3
0
4
,5
(
0
nitrate ratio. A few milligrams of the initiator, a,a -azoisobuty-
ronitrile, were added to initiate the polymerization. The solution
with all the ingredients was heated in a dryer to 801C. About 30
min later, the solution became a translucent gel. The gel was
dried at 1201C for 24 h and then calcined at 8001C for 3 h to
eliminate the organic ingredients. The amorphous powder ob-
tained was calcined at 12001C for 4 h to synthesize BaZr0.9
3
6
–9
precursor powder.
1
0
Y
0.1
O
2.95 powder. The flow chart of this process is shown in
Fig. 1.
BaZr0.9Y O2.95 powders were also prepared using the same
0.1
procedure illustrated in Fig. 1 without the steps inside the dash-
dotted frame. No CA was used to see if cations can still be fixed
in situ homogenously in the polyacrylamide network formed by
acrylamide, methylenebisacrylamide, and azoisobutyronitrile.
(B) Solid-State Reaction Method: The procedure for the
11,12
CaAl O , La0.8Sr0.2MnO , BaZrO , and CaAl Si O .
1
2
9
3
3
2
2
8
The powder gel-casting process combines the advantages of
the strong chelation ability of citric acid (CA) in aqueous solu-
tion with the steric entrapment effect of an organic polymer.
Stable solutions of citrates are easily and rapidly gelled by in situ
formation of an auxiliary three-dimensional organic network by
preparation of BaZr0.9
solid-state reaction is mixing BaCO
ichiometric ratio and milling the powders in deionized water for
2 h. After drying, the powders were calcined at 14001C for 10 h
in air atmosphere to synthesize the BaZr0.9 2.95 phase.
Y
0.1
O2.95 powders by the traditional
, ZrO , and Y in a sto-
O
2 3
3
2
1
T. Gur—contributing editor
0.1
Y O
In this paper, three kinds of BZY powders are prepared: gel-
cast BZY powder with CA, gel-cast powder without CA, and
BZY powder synthesized by the traditional solid-state reaction.
They are referred as gel-cast BZY–CA, gel-cast BZY, and SSR
BZY, respectively.
Manuscript No. 26289. Received June 2, 2009; approved December 16, 2009
This work was financially supported by the National Natural Science Foundation of
China under grant No. 50872090.
wAuthor to whom correspondence should be addressed. e-mail: rsguo@tju.edu.cn
1
572

June 2010
Rapid Communications of the American Ceramic Society
1573
CH COONH
4
3 C(OH)COOH + 2Y
Ba(NO )
Zr(NO ) ·5H O
Y(NO ) ·6H O
2
3
+
+
In deionized water In deionized water In deionized water
Y [C H O (NH ) ] + 6H
2
6
6
7
3 2 3
(4)
CH COONH
2
4
Mixing and stirring at 60 80 °C
Citric acid
Metal-citric acid complex
NH ·H O
CH COONH
2
C(OH)COOH + Zr
CH
2
COONH
3
NH
3
CH
2
COO
4
4
+
+
2
C(OH)COOH Zr HOOCC(OH) + 4H
CH COONH NH CH COO
Initiator
Acrylamide monomer
(
α,α’-azoisobutyronitrile) Heating at 80 90 °C N,N’-methylenbisacrylamide
CH COONH
(5)
2
4
2
3
3
2
Gel
0
With the addition of an initiator (a,a -azoisobutyronitrile) to
the solution, the chain-building monomers (acrylamide) form
Drying
0
long chains with the cross-linking monomers (N,N -methyl-
Calcination at 800 °C to eliminate the organic
Calcination at 1200 °C to synthesize perovskite phase
enebisacrylamide). A very high-molecular-weight polymer,
which fills the space and traps the solvent molecules into its
branches, is formed. The macroscopic result is a polymer–
solvent gel. The gelation time is about 1 h under 801–901C.
The gel system is pure organic and no foreign ions are intro-
duced to contaminate the solution.
BaZr
Y
O
powder
Fig. 1. Flow chart of BaZr0.9
gel-casting method.
0.1
Y O
2.95 powders synthesized by the
In the gel-casting process, an increased homogeneity of the
product is expected because the mixing of the reagents occurs at
a molecular level in the solution.
1
2
Douy had found that the stability of the complexes is much
higher if the pH is raised to neutrality (pH 6–8). Many com-
plexes between CA and metal ions (La, Pb, Bi, y) are insoluble
in acidic media but by adding a solution of ammonia as a base, a
(
3) Sample Characterization
Thermogravimetry (TG) and differential scanning calorimetry
DSC) of the dried gel were performed using a NETZSCH STA
49C (Netzsch-Geratebau GmbH, Selb, Germany) simulta-
(
4
¨
12
stable clear solution is obtained rapidly. This complies with
our experiments in the synthesis of BaZr0.9Yb0.1O2.95 powders.
neous TG–DSC analyzer. Samples were heated at 101C/min
from ambient temperature to 13001C. Powder X-ray diffraction
31
The complex of CA and Yb precipitated at a low pH value
acidic), but the precipitation gradually disappeared by adding
(
XRD) data were obtained using a Rigaku D/max 2500 V/PC
(
automatic powder diffractometer (Rigaku Corporation, Tokyo,
Japan) with CuKa radiation. Intensities were obtained in the 2y
range between 201 and 901 with a step size of 0.021 and a mea-
suring time of 3 s at each step. XRD data were analyzed using
Jade 5.0 software. The grain size and morphology of gel-cast
powders and SSR BZY powders were characterized using JEM-
drops of NH ꢀ H O solution adjust the pH to neutrality. Al-
3
2
though precipitation did not emerge in the acidic solution of
1400 transmission electronic microscopy (TEM) (JEOL Ltd.,
Tokyo, Japan). The specific area was measured at 77 K by
NOVA 2000 Series Specific Area Analyzer of Quantachrome
Instruments (Quantachrome Instruments, Boynton Beach, FL),
with N as the adsorbate gas. Samples were degassed at 2001C
2
for 4 h. The specific surface areas were calculated using the BET
multipoint method with five data points. A crystallite size of the
12001C-calcined gel-cast BZY–CA powders was determined by
the X-ray line broadening and calculated using the Scherrer
equation:
0
:9l
D ¼
(1)
b cos y
where D is crystallite size in nanometers, l the radiation wave
length (0.15405 nm for CuKa), b the corrected full width at half
maximum, and y is the diffraction angle.
III. Results and Discussion
(
1) Gelation Process
The solution of Ba(NO
O forms a complex with CA as the chelating agent in the
3 2
) , Y(NO
)
3 3
ꢀ 6H
2
O, and Zr(NO
)
3 2
ꢀ
5H
2
first step of the gel-casting process. The chemical reactions are as
follows:
CH
2
COOH
CH COONH
2
4
C(OH)COOH + 2H
+
+
C(OH)COOH + 2NH
4
CH
2
COOH
CH COONH
(2)
(3)
2
4
CH COONH
CH COONH
2 3
C(OH)COOH Ba + 2H
2
4
2
+
+
C(OH)COOH + Ba
Fig. 2. Thermogravimetry (TG)–differential scanning calorimetry
(DSC) curves of BZY gel (a) complexed with citric acid, (b) without
citric acid.
CH COONH
CH COONH
2
4
2
3

1
574
Rapid Communications of the American Ceramic Society
Vol. 93, No. 6
gel-cast BZY–CA, because of the less amount of fuel, thus ren-
dering the weight loss of gel-cast BZY to be less than gel-cast
BZY–CA.
Based on the DSC–TG curves, the temperature of eliminating
the organics is fixed at 8001C with 3 h of holding time, and that
of synthesizing the perovskite phase is set at 12001C with 4 h of
holding time.
(
3) Powder Process and Calcination
The elimination of the organics in the gel at 8001C is a thermally
induced redox reaction between the oxidizer and the fuel. Based
on the principles of the propellant chemistry, the metallic ni-
trates are used as oxidizers as well as metal element sources. CA,
0
0
acrylamide, N,N -methylenebisacrylamide, and a,a -azoisobuty-
ronitrile act as fuels apart from their chelating and gelating ef-
fects in the solution. All the fuels are the sources of C and H,
which form CO and H O and liberate heat when burned. Based
2
2
Fig. 3. X-ray diffraction patterns of BZY powders synthesized by the
gel-cast and SSR method: (a) gel-cast BZY after elimination of organics
at 8001C, (b) gel-cast BZY–CA after elimination of organics at 8001C,
on the amount of each reactant used, the chemical reaction is
proposed as follows:
(
1
c) SSR BZY calcined at 14001C for 10 h, (d) gel-cast BZY calcined at
2001C for 4 h, and (e) gel-cast BZY–CA calcined at 12001C for 4 h.
BaðNO Þ þ 0:9ZrðNO Þ ꢀ 5H2O þ 0:1YðNO Þ ꢀ 6H2O
3
2
3
2
3 3
þ 2C6H8O7 ꢀ H2O þ 4NH4OH þ 2:4H2C ¼ CHCONH2
þ 0:1H2C ¼ ðCHCONHÞ CH2 þ 17:1O2 ! BaZr0:9Y0:1O3ꢁd
2
þ 19:8CO2 þ 31:6H2O þ 5:35N2
ð6Þ
BaZr0.9Y0.1O2.95, ammonia was still added in order to maintain
the consistency with our serial experiments of different dopants.
The actual reaction may not be exactly as proposed because
may be NO, NO , or other forms. In addition, the amount of
NH OH used to adjust the pH may fluctuate slightly every time.
N
2
2
(
2) TG–DSC Analyses
4
0
The TG–DSC thermograms of BZY gel complexed with CA
and without CA are shown in Figs. 2(a) and (b), respectively.
In Fig. 2(a), the sharp exothermic peak of gel-cast BZY–CA
at 248.11C is due to the reaction between the nitrates and CA,
acrylamide, N,N -methylenebisacrylamide, and a,a -azoisobuty-
ronitrile. This reaction releases abundant CO and H O, which
corresponds to the sharp weight loss in the TG curve. The exo-
thermic peak at 476.71C is caused by the oxidization of the res-
idue carbon from the combustion of the foregoing reaction. Less
heat is released after 6001C, unlike the gel-cast BZY, the heat
releasing of which lasts to the end of the testing temperature of
1
phase transformation of monoclinic to tetrahedral ZrO . Almost
no weight change of the sample is observed after 8001C. The
total weight loss of 95.27% is attributed to the elimination of all
the water and organics in the gel.
a,a -Azoisobutyronitrile was not considered in the equation due
to its small quantity.
After this redox reaction, the organics were eliminated from
the gel, and several phases were detected in the powder, as in-
dicated in Figs. 3(a) and (b).
0
0
2
2
The gel-cast BZY–CA powders were mainly a mixture of Ba
ZrO and BaCO , and the gel-cast BZY powders were a mixture
of BaZrO , BaCO , and ZrO . BaCO is formed by Ba react-
3
3
21
3
3
2
3
2
ing with the CO gas produced in the redox reaction.
After the elimination of organics, the powders were calcined
at 12001C for 4 h to yield a well-crystallized BZY powder. Fig-
ures 3(c)–(e) show the XRD patterns of the powders synthesized
by the gel-casting method at 12001C/4 h and SSR at 14001C/10
h. It can be seen from the figure that gel-cast BZY–CA powder
forms a single perovskite phase, while two other phases, ZrO2
3001C. The small exothermal peak at 1163.61C is caused by the
2
0.14
and Zr0.86Y O1.93, are detectable in the gel-cast BZY powder.
In Fig. 2(b), a sharp exothermic peak appears at 254.11C. The
exothermic peaks of 345.8, 424.3, and 592.31C are attributed to
the burning off of acrylamide, N,N -methylenebisacrylamide,
This indicates that without CA as a chelating agent, the distri-
bution of cations is inhomogeneous in the gel network formed
0
0
0
by acrylamide, N,N -methylenebisacrylamide, and a,a -azoi-
sobutyronitrile. This is consistent with the phenomena in the
experiment: there is always some water separating out several
minutes after the initial gelation of the gel-cast BZY solution,
and the pH value of the water is 1–2. This implies that some
nitrates are not steadily fixed in the gel by the organic network,
even though most of them are immobile in the gel. A single
perovskite phase is formed in the SSR BZY powder too, al-
though this is achieved with a much higher calcining tempera-
ture and a longer holding time than the gel-casting method.
0
and a,a -azoisobutyronitrile. The weak endothermic peak of
8
151C is probably caused by the g2b phase transition of
BaCO . The weight change after 9001C is negligible, but the
3
exothermal effect lasted until the end point of the test at 13001C.
This is perhaps induced by the amorphous phase transforming
to a crystal phase. The overall weight loss is 81.28% in the test-
ing temperature range, less than that of the gel-cast BZY–CA,
which is 95.27%. Apart from the lack of a chelating agent, the
combustion process in gel-cast BZY is not as intense as in the
Fig. 4. TEM photographs of gel-cast and SSR powders after calcination at (a) 8001C, (b) 12001C, and (c) SSR BZY powders calcined at 14001C.

June 2010
Rapid Communications of the American Ceramic Society
1575
The XRD patterns in Fig. 3 suggest that the gel-casting
ular level, the powders have a high degree of chemical homoge-
neity. For gel-cast powders, 12001C/4 h is enough for the
synthesis of a single perovskite phase, while 14001C/10 h is suit-
able for the SSR powders.
method can reduce the synthetic temperature. It is clear that
lower calcining temperature is one of the advantages of the
powder’s liquid solution preparation technology over solid-state
reaction.
In all the three samples, the peaks shift to the lower 2y, which
could be explained by the ion size difference between
31
VIII
41 VIII
13
5 0.098 nm). The cell
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(R
5 0.1159 nm) and Zr (R
1
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(
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2
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0.41815 nm).
Figure 4 shows the TEM photographs of gel-cast powders
3
phase
(
3
complexed with CA after calcination at 8001 and 12001C. The
TEM photograph of SSR BZY was presented in the same figure
for comparison. The powders after the elimination of organics
at 8001C are foam-like and fluffy flakes formed by small crys-
tallites. The 12001C-calcined powders are still puffy, with a grain
size of B50 nm, calculated from the (110) XRD diffraction
peak. The crystallite size of SSR BZY calcined at 14001C was
4
5
1
6
4
100 nm, determined with the same method. Not only bigger
7
particle size but also there is agglomeration in the SSR BZY
powders. The specific area of gel-cast BZY–CA calcined at
0.2
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0.2
2
8
12001C was 7.53 m /g. The grain size of BaZrO
3
powders syn-
6
thesized by Sin et al. using the same gel-casting method was
B35 nm. CA combustion process could obtain BaZrO powders
of B30 nm. Glycine nitrate combustion synthesis process could
synthesize BaZr0.9 2.95 powders with a specific surface area
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3
9
H. P. Kumar, C. Vijayakumar, C. N. George, S. Solomon, R. Jose, J. K.
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0.1
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2
4
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11
12
IV. Conclusions
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The present work demonstrates that the gel-casting process is a
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13
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0.1
ture proton conductor BaZr0.9Y O2.95 powder. As the mixing
6
&
of the constituents in the gel formation is achieved at a molec-