Effect of preparation conditions on the phase composition of ZrO 2-Al2O3-CeO2 powders

Article abstract of DOI:10.1023/A:1021319320332

Data are presented on the phase composition of ZrO₂-Al ₂O₃-CeO₂ powders containing 10, 30, and 70 mol %, prepared via coprecipitation and successive precipitation. The precipitation procedure is shown to have a strong effect on the phase composition of the heat-treated powders and the state (inter- or intracrystalline) of the T-ZrO₂ phase, thereby changing its stability. At an Al ₂O₃ content of 30 mol %, the M-ZrO₂ content is independent of the precipitation procedure.

Full text of DOI:10.1023/A:1021319320332

Inorganic Materials, Vol. 38, No. 12, 2002, pp. 1235–1240. Translated from Neorganicheskie Materialy, Vol. 38, No. 12, 2002, pp. 1455–1461.
Original Russian Text Copyright © 2002 by Podzorova, Il’icheva, Mikhailina, Shevchenko, Shvorneva.
Effect of Preparation Conditions on the Phase Composition
of ZrO2–Al2O3–CeO2 Powders
L. I. Podzorova*, A. A. Il’icheva*, N. A. Mikhailina*,
V. Ya. Shevchenko*, and L. I. Shvorneva**
* Institute of Physicochemical Problems in Ceramic Science, Russian Academy of Sciences,
Ozernaya ul. 48, Moscow, 119361 Russia
** Glass Research Institute, Dushinskaya ul. 7, Moscow, 111024 Russia
e-mail: mnctk@cityline.ru
Received February 13, 2002
Abstract—Data are presented on the phase composition of ZrO2–Al2O3–CeO2 powders containing 10, 30, and
70 mol %, prepared via coprecipitation and successive precipitation. The precipitation procedure is shown to
have a strong effect on the phase composition of the heat-treated powders and the state (inter- or intracrystal-
line) of the T-ZrO2 phase, thereby changing its stability. At an Al2O3 content of 30 mol %, the M-ZrO2 content
is independent of the precipitation procedure.
INTRODUCTION
EXPERIMENTAL
Characterization techniques. The specific surface
of the powders was determined by low-temperature
BET measurements (Autosorb-1 instrument).
The ZrO2–Al2O3 system is of immense importance
in the production of structural ceramics possessing high
strength and fracture toughness owing to the partial sta-
bilization of tetragonal (T) or cubic (C) ZrO2 via doping
with rare-earth or other cations [1, 2].
Particle-size analysis and microstructural examina-
tion were performed using a JSM-35CF scanning elec-
tron microscope (SEM) at an accelerating voltage of
20 kV and magnifications from ×1000 to ×10000.
ZrO2 and Al2O3 do not form solid solutions. The
eutectic temperature in the ZrO2–Al2O3 system is in the
range 2193–1983°C, but the eutectic composition has
not been established with certainty and has been vari-
ously reported to be 45 [3], 30 [4], or 38 mol % ZrO2
[5]. At the same time, according toAdylov et al. [6] sin-
gle-phase T-ZrO2-based materials can be obtained in
the composition range 35–70 mol % ZrO2 under certain
conditions. Such materials must possess a high
strength.
Phase composition was determined by x-ray diffrac-
tion (XRD) analysis on a DRON-3 diffractometer
(CuKα radiation) using JCPDS Powder Diffraction File
data. Quantitative phase analysis was carried out using
XRD intensity data and well-crystallized calibration
1
standards.
The elemental composition of the powders was
determined using a Getan energy-dispersive spectrom-
eter equipped with Si(Li) detectors.
Synthesis of powders. T-ZrO2 was partially stabi-
lized by introducing CeO2. The basic composition con-
tained 88 mol % ZrO2 and 12 mol % CeO2. The Al2O3
content was 10, 30, or 70 mol %.
One advantageous method for preparing ceramic
powders is the sol–gel process, which enables the prep-
aration of materials with controlled structure and parti-
cle size [7–9]. As shown earlier using zirconium and
yttrium hydroxides as an example [9], the precipitation
sequence may have a significant effect on the phase
composition of the reaction products since it influences
the size and elastic strain of the particles.
Hydroxides were precipitated by adding aqueous
1 M solutions of zirconyl chloride, cerium nitrate, and
aluminum nitrate to aqueous 6 M ammonia [7, 10].
In our preparations, we used two procedures:
The purpose of this work was to study the effect of
the precipitation sequence in systems containing Zr, Al,
and Ce cations on the phase composition of the prod-
ucts of subsequent calcination.
(1) First, zirconium and cerium hydroxides (90, 70,
or 30 mol % of the total amount of reagents) were pre-
1
The procedure standardized at the State Institute of Glass.
0020-1685/02/3812-1235$27.00 © 2002 MAIK “Nauka/Interperiodica”

1236
PODZOROVA et al.
(a)
(b)
(c)
(d)
Fig. 1. SEM micrographs of powders prepared by (a, c) successive precipitation and (b, d) coprecipitation, followed by calcination
at 950°C; (a, b) 10, (c, d) 70 mol % Al O .
2
3
cipitated. After 15 min, an appropriate amount of the
aluminum nitrate solution was added.
After drying, the powders were calcined at 950,
1250, 1350, and 1500°C.
(2) The three hydroxides were precipitated in parallel.
RESULTS AND DISCUSSION
Table 1. Specific surface of powders calcined at 950°C
The specific surface of the powders was found to
depend on the precipitation procedure (Table 1).
mol %
Sample no.
Procedure
Coprecipitation
S, m2/g
Al2O3
As apparent from Table 1, the specific surface of the
coprecipitated powders is smaller, presumably because
they contain a large amount of an amorphous phase.
Typical SEM micrographs of the powders are displayed
in Fig. 1.
1
2
3
4
5
6
10
30
70
10
30
70
30
60
100
50
Successive precipitation
The powders with a nearly eutectic composition
(30 mol %Al2O3) are close in specific surface, indepen-
dent of the precipitation procedure.
60
120
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EFFECT OF PREPARATION CONDITIONS ON THE PHASE COMPOSITION
1237
(a)
(b)
(c)
(d)
10
20
30
40
50
60
2θ, deg
Fig. 2. XRD patterns of samples prepared by (a, c) successive precipitation and (b, d) coprecipitation, followed by calcination at
950°C; (a, b) 10, (c, d) 70 mol % Al O .
2
3
INORGANIC MATERIALS Vol. 38 No. 12 2002

1238
PODZOROVA et al.
(a)
α
(b)
30
40
α
50
2θ, deg
Fig. 3. XRD patterns of samples containing 10 mol % Al O , prepared by (a) coprecipitation and (b) successive precipitation, fol-
2
3
lowed by calcination at 1500°C.
The phase composition of the samples was found to α-Al2O3 crystallization occurred in several steps and
depend not only on their elemental composition but
also on the precipitation procedure (Table 2).
that this process was accompanied by partial decompo-
sition of the already present metastable, amorphous
phases CeAlO3 and Ce2O3 · 11Al2O3, which are readily
formed in an oxidizing atmosphere. CeAlO3 is known
to undergo a reversible transformation at 980°C [3],
and Ce2O3 · 11Al2O3 decomposes in air between 1200
and 1400°C.
The powders calcined at 950°C consisted, for the
most part, of a pseudocubic ZrO2-based solid solution,
independent of the precipitation procedure. In the sam-
ple containing 70 mol % Al2O3, a small amount of
γ-Al2O3 was present (Fig. 2).
In the powders prepared via successive precipitation
and containing 10 and 70 mol % Al2O3, the tetragonal
raised from 950 to 1250°C. The likely reasons are that phase converts rapidly into the monoclinic phase as the
Note that the fraction of crystalline phases increased
nonmonotonically as the calcination temperature was
Table 2. Phase composition of heat-treated powders
Weight percent
Phase composi-
tHT, °C
tion
1*
75
2
3
4
5
6
***
C-ZrO2
75
45
88
77
950
76
1250
T-ZrO2
M-ZrO2
α-Al2O3
T-ZrO2
M-ZrO2
α-Al2O3
T-ZrO2
M-ZrO2
α-Al2O3
75
<5
–
80
15
–
80
<5
tr
85
Ͼ5
–
80
15
85
>5
tr
–
1350
85
Ͼ5
–
85
10
Ͼ5
85
<5
Ͼ5
70
15
15
55
20
20
75
15
<5
45
50
<5
80
25
35
20
20
45
30
15
<10
80
1500**
90
<5
<5
Ͼ5
10
* The same samples as in Table 1.
** Grinding in a planetary mill.
*** Pseudocubic ZrO .
2
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EFFECT OF PREPARATION CONDITIONS ON THE PHASE COMPOSITION
1239
(a)
(b)
30
40
50
2θ, deg
Fig. 4. XRD patterns of samples containing 70 mol % Al O , prepared by (a) successive precipitation and (b) coprecipitation, fol-
2
3
lowed by calcination at 1500°C.
(a)
(b)
1
1
2
2
3
3
28 29 30 31 32 33 34 35
28 29 30 31 32 33 34 35 36
2θ, deg
Fig. 5. XRD patterns of powders containing 30 mol % Al O , prepared by (a) successive precipitation and (b) coprecipitation, fol-
2
3
lowed by calcination at (1) 950, (2) 1250, and (3) 1500°C.
temperature is raised to 1500°C, while the coprecipi- cipitation result in intercrystalline and intracrystalline
tated powders with the same compositions contain a
T-ZrO2-based solid solution, as evidenced by the XRD
scans in the range 2θ = 26°–36° displayed in Figs. 3
and 4.
ZrO2, respectively. Thermodynamic calculations indi-
cate that T-ZrO2 stabilization depends on the particle
size and structural perfection of the material. The ther-
mal expansion mismatch between Al2O3 and T-ZrO2
[(7–8) × 10–6 and (9–11) × 10–6 K–1, respectively] gives
Zirconia particles may be present in the host mate-
rial as an intercrystalline or intracrystalline phase [11].
In our experiments, successive precipitation and copre- 0.4%, depending on orientation [6]. In the coprecipi-
rise to thermoelastic strain, which varies from 0.2 to
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PODZOROVA et al.
ACKNOWLEDGMENTS
(a)
Al
Zr
This work was supported by the Russian Foundation
for Basic Research, project no. 00-03-32192.
O
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CONCLUSION
Using different precipitation procedures in the
ZrO2–Al2O3–CeO2 system, one can produce powders
differing not only in particle size but also in crystallin-
ity and, hence, phase composition. Successive precipi-
tation results in intracrystalline T-ZrO2, which contrib-
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INORGANIC MATERIALS Vol. 38 No. 12 2002