Transformation of Yttrium-Doped Hydrated Zirconium into Tetragonal and Cubic Nanocrystalline Zirconia

Article abstract of DOI:10.1006/jssc.1998.8056

Nanostructured yttrium-stabilized zirconia powders, with yttria concentrations between 0.0 and 10.0 mol%, were prepared via the hydrolysis of an aqueous solution of zirconyl and yttrium chloride, and ammonium hydroxide. Powder phases were characterized by using X-ray powder diffraction; their crystalline structures were refined with the Rietveld technique. When samples were annealed below 200°C, their diffraction patterns corresponded to an amorphous atom distribution and were independent of yttria concentration. The doped amorphous phases crystallized, at 400°C, into tetragonal or cubic nanocrystalline zirconia, which were stabilized by yttrium. These results suggest that yttrium atoms served as a substitute for zirconium atoms not only in the crystalline phases but also in the amorphous phases, which are determined by the fast condensation of zirconyl clusters. Nondoped samples contained a mixture of monoclinic and tetragonal nanocrystalline zirconia; those with 2.5 to 5.0 mol% yttria contained only the tetragonal zirconia nanophase, and those with 7.5 to 10.0 mol% had only the nanocrystalline cubic phase. The average crystallite size of the nanophases diminished when Y₂O₃concentration was increased.

Full text of DOI:10.1006/jssc.1998.8056

Journal of Solid State Chemistry 142, 409—418 (1999)
Article ID jssc.1998.8056, available online at http://www.idealibrary.com on
Transformation of Yttrium-Doped Hydrated Zirconium
into Tetragonal and Cubic Nanocrystalline Zirconia
X. Bokhimi,*ꢀꢁ A. Morales,* A. Garcıa-Ruiz,- T. D. Xiao,‡ H. Chen,m and P. R. Struttm
*Institute of Physics, The National University of Mexico (UNAM), A.P. 20-364, 01000 Mexico, Mexico; -UPIICSA, COFAA, The National Polytechnic Institute,
Te& No. 950 Esq. Resina, 08400 Mexico D.F., Mexico; ‡Inframat Corporation, 20 Washington Avenue, Suite 106, North Haven, Connecticut 06473;
mPrecision Manufacturing Center, PMC, University of Connecticut, Storrs, Connecticut 06269
Received October 27, 1997; in revised form October 9, 1998; accepted October 10, 1998
using solid-state reaction, with yttria and zirconia precur-
Nanostructured yttrium-stabilized zirconia powders, with yt- sors, requires large atom diffusion, which involves high
tria concentrations between 0.0 and 10.0 mol%, were prepared
via the hydrolysis of an aqueous solution of zirconyl and yttrium
chloride, and ammonium hydroxide. Powder phases were charac-
terized by using X-ray powder diffraction; their crystalline struc-
tures were refined with the Rietveld technique. When samples
were annealed below 200°C, their diffraction patterns correspon-
ded to an amorphous atom distribution and were independent of
yttria concentration. The doped amorphous phases crystallized,
at 400°C, into tetragonal or cubic nanocrystalline zirconia, which
were stabilized by yttrium. These results suggest that yttrium
atoms served as a substitute for zirconium atoms not only in the
temperatures. When zirconia is synthesized using soft chem-
istry based on hydrolysis, the zirconyl cluster is always
involved (1); therefore, when yttrium-doped zirconia is pre-
pared by these methods, zirconyl cluster must be affected by
yttrium atoms.
Hoping to obtain some information about the effect of
yttrium on zirconyl cluster, we studied in detail the crystal-
lography of nanocrystalline yttrium-doped zirconia
(Y O /ZrO ), prepared via an advanced chemical route.
ꢄ ꢆ
ꢄ
The evolution of the amorphous and crystalline phases and
crystalline phases but also in the amorphous phases, which are their average crystallite size was analyzed as a function of
determined by the fast condensation of zirconyl clusters. Non-
doped samples contained a mixture of monoclinic and tetragonal
nanocrystalline zirconia; those with 2.5 to 5.0 mol% yttria con-
tained only the tetragonal zirconia nanophase, and those with 7.5
to 10.0 mol% had only the nanocrystalline cubic phase. The
average crystallite size of the nanophases diminished when Y2O3
concentration was increased. ( 1999 Academic Press
Key Words: zirconyl cluster; nanocrystalline zirconia; yttrium-
doped zirconia; tetragonal zirconia; cubic zirconia; hydrated
zirconium; X-ray diffraction; Rietveld technique.
yttria concentration and temperature.
Due to its superior properties, considerable attention has
been focused on the development of materials based on
zirconia. For example, they are chemically stable, have high
hardness and mechanics strength (5—7), exhibit refractori-
ness (8), and are good ionic conductors (9, 10).
Zirconia is widely used in thermal barrier coatings of
advanced engines, where extremely high temperatures are
required, as well as to prepare milling balls, refractors,
oxygen sensors, fuel cells barriers, and electronic ceramics.
Pure microcrystalline zirconia can exist in three different
crystalline phases. At low temperatures, the stable phase is
monoclinic; it is transformed into a tetragonal phase when it
is heated at 1170°C. This tetragonal structure is transformed
into a cubic one when samples are heated at 2370°C. The
above transformations are accompanied by a unit cell vol-
ume change that, in some cases, is used to improve the
mechanical properties of this material (11).
1. INTRODUCTION
The amorphous structure of hydrated zirconium (a sys-
tem with aquo, hydro, and oxo bindings) and its crystalliza-
tion into nanophases of pure zirconia depend on the
behavior of zirconyl cluster (Fig. 1), (Zr (OH) (OH ) )ꢃ>
ꢂ
ꢃ
ꢄ ꢁꢅ
(1—3). The amorphous phase occurs because these clusters
condense rapidly, avoiding crystallization, and has a local
order similar to that of tetragonal zirconia.
The high-temperature crystalline phases can be stabilized
at low temperatures. This occurs by doping zirconia with
low-valent cations, such as Mgꢄ>, Caꢄ>, Yꢆ>, and rare
earth cations, which incorporate into zirconia crystalline
structure, as substitutes for zirconium cations.
Tetragonal and cubic zirconia can be stabilized by doping
them with large cations like yttrium (4). Their synthesis
Many methods for the preparation of conventional
ꢁTo whom correspondence should be addressed.
micrometer and submicrometer-sized ZrO have been
ꢄ
409
0022-4596/99 $30.00
Copyright ( 1999 by Academic Press
All rights of reproduction in any form reserved.

410
BOKHIMI ET AL.
extensively developed (5—8, 12). These include coprecipita-
tion, microemulsion, and sol-gel synthesis.
Recently, attention has been focused on reducing zirconia
crystallite size from the micrometer to the nanometer di-
mensions; for example, spherical zirconia powders com-
posed of crystallites with an average size of 20 nm were
prepared by nebulized spray pyrolysis (13).
Nanostructured zirconia offers several advantages, espe-
cially at high temperatures (4, 14—16): low thermal conduct-
ivity, reduced sintering temperature, improved mechanical
properties, and superplasticity. Currently, a number of pro-
duction techniques are available for the synthesis of nano-
structured oxide and nonoxide materials (17, 18). These
include laser ablation, microwave plasma synthesis, spray
conversion, flame pyrolysis, inert gas condensation, and
sol-gel synthesis. These synthesis techniques are also being
employed to prepare ultrafine and nanostructured
Y O /ZrO (4, 19—33).
FIG. 1. Tetrameric structural unit associated with the zirconyl cluster.
ꢄ ꢆ
ꢄ
FIG. 2. X-ray diffraction patterns for the different yttria concentrations; samples were annealed in air at 200°C. Tick marks correspond to tetragonal
zirconia.

YTTRIUM-DOPED HYDRATED ZIRCONIUM
411
FIG. 3. X-ray diffraction patterns of nondoped samples annealed at different temperatures. Upper tick marks correspond to monoclinic zirconia;
lower tick marks correspond to tetragonal zirconia.
In the present work, yttrium-doped zirconia was obtained ultrasonicated, dried, and then annealed at temperatures
via an advanced processing route that combines a precipita- between 100 and 800°C to obtain nanostructured
tion hydrolysis with an atomized spraying and ultrasonic Y O /ZrO powders. Yttria (Y O ) concentrations were
ꢄ ꢆ
ꢄ
ꢄ ꢆ
processing. Phase atomic arrangements (amorphous and 0.0, 2.5, 5.0, 7.0, and 10.0 mol%.
crystalline) were analyzed in detail with X-ray powder dif-
fraction, by refining the crystalline structures with the Riet-
veld technique.
Characterization
X-ray diffraction measurements were performed at room
temperature with CuKa radiation. Sample powders were
packed in a glass holder. Intensity was measured by step
scanning in the 2h range between 20 and 110°, with a 2h step
2. EXPERIMENTAL
Sample Preparation
Y O /ZrO powders were synthesized by hydrolysis. An of 0.02° for 2 s per point. Crystalline structures were refined
ꢄ ꢆ
aqueous solution of zirconyl chloride (ZrOCl . 8H O), yt- with the Rietveld technique by using DBWS-9411 (34) and
ꢄ
ꢄ
ꢄ
trium chloride, and ammonium hydroxide (NH OH) was WYRIET (35) programs; peak profiles were modeled with
ꢂ
ultrasonically sprayed into the reaction vessel containing a pseudo-Voigt (36) function that depends on average crys-
a diluted solution of NH OH in deionized water, while the tallite size (37). Standard deviations, which show the last
ꢂ
vessel was vigorously stirred. The reaction product was figure variations of a number, are given in parentheses.

412
BOKHIMI ET AL.
FIG. 4. X-ray diffraction patterns of the samples with 2.5 mol% yttria annealed at different temperatures. Tick marks and indices correspond to
tetragonal zirconia.
When these deviations correspond to Rietveld refined para- position of the main diffraction peak of the amorphous
meters, their values are estimates not of the probable error structure and the ticks corresponding to tetragonal zirconia
in the analysis as a whole, but only of minimum possible (Fig. 2), the amorphous structure had the tetragonal zirco-
probable errors based on their normal distribution (38).
nia local order.
The local order of the nondoped amorphous hydrated
zirconium is determined by the zirconyl cluster
(Zr (OH) (OH ) )ꢃ> (33, 39, 1). This cluster is a tetramer
3. RESULTS AND DISCUSSION
ꢂ
ꢃ
ꢄ ꢁꢅ
When the samples were annealed below 200°C, they were (Fig. 1) forming a lightly distorted square plane with four
amorphous (Fig. 2) and crystallized when heated at 400°C zirconium atoms at corners (40), similar to the faces of cubic
(Figs. 3 through 7).
zirconia (2, 41). A comparison of Clearfield’s (40) and Bok-
The X-ray diffraction patterns of the amorphous phases himi et al.’s (42) results on zirconia suggests that zirconyl
were equal and independent of yttrium concentration (Fig. symmetry is similar to the one observed in tetragonal zirco-
2). Despite the high yttrium concentrations, they had the nia, which is the phase formed when the amorphous phase
same distribution observed in nondoped hydrated zirco- crystallizes (42).
nium (3). This means that in some way, yttrium was dis-
Since yttrium is dissolved in the amorphous phase, and
solved into the hydrated-zirconium amorphous structure, this phase is based on zirconyl cluster, yttrium atoms could
forming an amorphous solid solution. As indicated by the serve as a substitute for zirconium atoms in this cluster,

YTTRIUM-DOPED HYDRATED ZIRCONIUM
413
FIG. 5. X-ray diffraction patterns of the samples with 5.0 mol% yttria annealed at different temperatures. Tick marks correspond to tetragonal
zirconia.
producing the yttrium-doped zirconyl ion (Zr
Y
To quantify the evolution of the crystalline phases with
ꢂ(ꢁ\V) ꢂV
(OH) (OH ) )(ꢃ\ꢂV)>. As in pure hydrated zirconium (1, yttrium concentration and temperature, their crystalline
ꢃ
ꢄ ꢁꢅ
43), the yttrium-doped amorphous structure would occur by structures were refined with the Rietveld technique. Cubic
the fast condensation of these ions and zirconyl clusters via zirconia was refined with a cubic unit cell with space group
olation between tetramers. But, because yttrium valence is Fm3m; tetragonal zirconia, with a tetragonal unit cell with
smaller than zirconium valence, some of the double hy- space group P4 /nmc; and monoclinic zirconia, with a
ꢄ
droxyl bridges (Fig. 1) would have only one hydroxyl. The monoclinic unit cell with space group P2 /c. Figure 8 shows
ꢁ
number of single-hydroxyl bridges will depend on yttrium a typical Rietveld refinement plot.
concentration.
Only the nondoped samples had monoclinic zirconia
When samples were annealed at 400°C, the amorphous (Fig. 3). Its concentration, average crystallite size, and lattice
structure crystallized to nanocrystalline monoclinic, tetra- parameters increased with temperature (Tables 1 and 2). In
gonal or cubic zirconia, depending on yttria concentration. this system, stabilization of the tetragonal phase was pro-
In the samples without yttrium, the crystallized phases were duced by hydroxyl groups contained in its crystalline struc-
a mixture of monoclinic and tetragonal zirconia (Fig. 3). For ture, as in sol-gel zirconia samples (42). The annealing
2.5 and 5.0 mol% Y O only the tetragonal phase was above 400°C dehydroxylized the phase, destabilizing the
ꢄ ꢆ
observed (Figs. 4 and 5), but for 7.0 and 10.0 mol% Y O
tetragonal structure that eventually was transformed into
monoclinic zirconia.
ꢄ ꢆ
the phase was cubic zirconia (Figs. 6 and 7).

414
BOKHIMI ET AL.
FIG. 6. X-ray diffraction patterns of the samples with 7.0 mol% yttria after they were annealed at different temperatures. Tick marks and indices
correspond to cubic zirconia.
In yttrium-doped samples, the monoclinic phase was ab- be associated to either a tetragonal or a cubic phase. They
sent, even when they were annealed at high temperatures were modeled with a cubic and a tetragonal structure; the
(Table 1). The absence of either monoclinic zirconia or best fit corresponded to the tetragonal structure. The
segregated yttrium compounds indicated that yttrium was sample annealed at 400°C had a bimodal size distribution
well dispersed in the samples. This result shows the (Fig. 4 and Tables 1 and 3) that disappeared after annealing
goodness of the present synthesis method for preparing the sample at 600°C (Table 3).
single-phase samples of nanocrystalline yttrium-doped
zirconia.
The observable distinctive differences between cubic and
tetragonal phases were the presence of the reflection (012) in
As in microcrystalline samples, in the nanostructured the tetragonal phase (Fig. 4), and the splitting of the (002),
zirconia reported in the present work yttrium stabilized its (113), and (004) reflections of cubic phase (Fig. 6). Each of
tetragonal and cubic phases (Table 1), but synthesis temper- these three reflections split into two peaks of the tetragonal
ature was very low. In contrast to nondoped samples, the phase: (002), (110); (013), (121); and (004) (220) for (002), (113),
tetragonal phase was stable even when the sample was and (004), respectively (Fig. 4). In the present case, the
annealed at 800°C (Table 1).
differences were more evident when the peak width
Samples with 2.5 mol% Y O had only nanocrystalline decreased. For broad diffraction peaks, if one would assume
ꢄ ꢆ
tetragonal zirconia (Fig. 4). Diffraction patterns of the sam- a cubic phase, the split appeared as an apparent asymmetry
ples annealed at 400 and 600°C had broad peaks that could of (002), (113), and (004) reflections (Fig. 6).

YTTRIUM-DOPED HYDRATED ZIRCONIUM
415
FIG. 7. X-ray diffraction patterns of the samples with 10.0 mol% yttria annealed at different temperatures. Tick marks correspond to cubic zirconia.
When Y O concentration was 5.0 mol%, the structure one observed in the samples with 5.0 mol% Y O (Tables 3
ꢄ ꢆ
ꢄ ꢆ
was tetragonal, but the lattice parameters tended to corres- and 4).
The cubic structure was also the only one observed in the
pond to a cubic phase; c/(a(2) ratio approached 1.0 (Table
3). The unit cell expanded slightly. In the diffraction pat- samples with 10.0 mol% Y O (Fig. 7). The average crystal-
terns, the (002) and (110) reflections were not visually distin- lite size, however, was smaller than in the samples with
ꢄ ꢆ
guished from each other, even when samples were annealed 7.0 mol% yttria. It was 5.7 (2) nm when the sample was
at 800°C; the refinement, however, distinguished them. For annealed at 400°C and increased to only 16.8 (3) nm after
this yttria concentration, at a fixed annealing temperature, annealing the sample at 800°C. The increase on yttria con-
the average crystallite size was smaller than the one ob- centration also expanded the unit cell (Table 4).
served for 2.5 mol% yttria (Table 3).
For 7.0 mol% yttria, at all annealing temperatures, the
sample had only nanocrystalline cubic zirconia (Fig. 6
and Table 1). The (002), (113), and (004) reflections
4. CONCLUSIONS
Via the synthesis method described in the present work,
did not show the asymmetry or splitting characteristic of the for yttria concentrations between 2.5 and 10 mol%, it was
tetragonal structure. Lattice parameters slightly decreased possible to produce single-phase tetragonal and cubic yt-
with temperature (Table 4). At any annealing temper- tria-stabilized zirconia at temperatures as low as 400°C; the
ature, the average crystallite size was smaller than phases were nanocrystalline.

416
BOKHIMI ET AL.
FIG. 8. Rietveld refinement plot of the non-doped sample after it was annealed at 600°C (R "0.080). Upper tick marks correspond to monoclinic
ꢀꢁ
zirconia (R "0.014); lower tick marks correspond to tetragonal zirconia (R "0.012).
$
$
TABLE 1
Yttrium-doped samples annealed at 200°C were amorph-
Zirconia: Phase Composition as a Function of Yttria Content and
Temperature
ous. Their X-ray diffraction patterns were independent of
yttrium concentration and identical to the one observed in
the nondoped samples. This result and the fact that the
phases formed during crystallization contained yttrium in
their crystalline structure suggest that yttrium atoms were
incorporated in zirconyl molecular structure. The most like-
ly structure sites for yttrium atoms in zirconyl cluster are
those occupied by zirconium; this would transform this
Yttria
concentration
(mol%)
¹
(°C)
Cubic
(wt%)
Tetragonal
(wt%)
Monoclinic
(wt%)
0.0
0.0
0.0
400
600
800
0.0
0.0
0.0
18.0 (1)
2.6 (1)
2.2 (1)
82.0 (1)
97.4 (1)
97.8 (1)
cluster into the yttrium-doped zirconyl ion, (Zr
Y
2.5
400
0.0
54.6 (5)
45.4 (4)
100.0
0.0
ꢂ(ꢁ\V) ꢂV
(OH) (OH ) )(ꢃ\ꢂV)>, which would be the basic ion of
ꢃ
ꢄ ꢁꢅ
2.5
2.5
600
800
0.0
0.0
0.0
0.0
100.0
TABLE 2
5.0
400
0.0
76 (1)
24 (1)
100.0
0.0
Monoclinic Zirconia: Lattice Parameters as a Function of
Yttria Content and Temperature
5.0
5.0
600
800
0.0
0.0
0.0
0.0
100.0
Yttria
concentration
(mol %)
Average
crystallite
(°C) size (nm)
7.0
7.0
7.0
400
600
800
100.0
100.0
100.0
0.0
0.0
0.0
0.0
0.0
0.0
¹
a
b
c
(nm)
(nm)
(nm)
b (°)
0.0
0.0
0.0
400
600
800
12.3 (6) 0.5144 (2) 0.5183 (2) 0.5304 (2) 98.64 (2)
21.4 (6) 0.51448 (4) 0.52033 (4) 0.53131 (4) 99.20 (1)
30.4 (8) 0.51439 (2) 0.52030 (3) 0.53146 (3) 99.20 (1)
10.0
10.0
10.0
400
600
800
100.0
100.0
100.0
0.0
0.0
0.0
0.0
0.0
0.0

YTTRIUM-DOPED HYDRATED ZIRCONIUM
417
TABLE 3
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7.0
7.0
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400
600
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8.7 (5)
14.6 (3)
22.4 (4)
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