Complex thermal evolution of size-stabilized tetragonal zirconia

Article abstract of DOI:10.1016/j.jpcs.2010.03.002

Zirconia nanopowder with doping YO_1.5 contents between 0 and 1 mol% were synthesized by the Pechini method. The crystallite dimensions of the powder, around 10 nm, allows for the size stabilization of the tetragonal polymorph over the thermodynamically stable monoclinic one. As the nanopowders are heated to 1200 °C and subsequently cooled back to room temperature, a complex evolution of the phase composition occurs. Upon heating the tetragonal phase transforms slowly into the monoclinic one and the transition cannot be completed before entering the stability range of the tetragonal phase (above 1150 °C). Upon cooling, on the other hand, the reaction is considerably faster and the complete transformation into the monoclinc phase occurs in a narrow temperature range. Rietveld analysis of the high temperature X-ray patterns revealed as, during heating, the transition is mainly controlled by microstructural parameters and in particular it is triggered by the release of RMS microstrain. Upon cooling, on the other hand, the transition is kinetically controlled by the doping content.

Full text of DOI:10.1016/j.jpcs.2010.03.002

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Journal of Physics and Chemistry of Solids 71 (2010) 1038–1041
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids
journal homepage: www.elsevier.com/locate/jpcs
Complex thermal evolution of size-stabilized tetragonal zirconia
a,n
b
b
b
a
Monica Dapiaggi , Filippo Maglia , Ilenia Tredici , Beatrice Maroni , Giulio Borghini ,
b
Umberto Anselmi Tamburini
a
Universit aꢀ degli Studi di Milano, Dipartimento di Scienze della Terra, via Botticelli 23, I20133 Milano, Italy
Universit aꢀ degli Studi di Pavia, Dipartimento di Chimica Fisica, viale Taramelli 16, I27100 Pavia, Italy
b
a r t i c l e i n f o
a b s t r a c t
Zirconia nanopowder with doping YO1.5 contents between 0 and 1 mol% were synthesized by the
Pechini method. The crystallite dimensions of the powder, around 10 nm, allows for the size
stabilization of the tetragonal polymorph over the thermodynamically stable monoclinic one. As the
nanopowders are heated to 1200 1C and subsequently cooled back to room temperature, a complex
evolution of the phase composition occurs. Upon heating the tetragonal phase transforms slowly into
the monoclinic one and the transition cannot be completed before entering the stability range of the
tetragonal phase (above 1150 1C). Upon cooling, on the other hand, the reaction is considerably faster
and the complete transformation into the monoclinc phase occurs in a narrow temperature range.
Rietveld analysis of the high temperature X-ray patterns revealed as, during heating, the transition is
mainly controlled by microstructural parameters and in particular it is triggered by the release of RMS
microstrain. Upon cooling, on the other hand, the transition is kinetically controlled by the doping
content.
Keywords:
A. nanostrcutures
C. X-ray diffraction
D. phase transitions
&
2010 Elsevier Ltd. All rights reserved.
1
. Introduction
explanation that received the largest consensus, although
the presence of water vapor in crystallite growth, of intrinsic
defects, and of lattice and intergranular strain are also believed to
play a role [9].
The structural and kinetic aspects concerning the stability of the
size stabilized tetragonal zirconia and its transition to the mono-
clinic polymorph has been investigated although in less detail
[10–14]. This transition occurs upon annealing at high temperature
and is believed to be driven by the grain growth of the tetragonal
Zirconium oxide undergoes a transition from its monoclinic to
its tetragonal polymorph at around 1175 1C, and from its tetragonal
to its cubic polymorph at 2370 1C. The low temperature monoclinic
polymorph, the mineral baddeleyite, has very limited practical use,
while the two high-temperature polymorphs are employed in a
variety of technical applications such as, for instance, solid
electrolytes in solid oxide fuel cells and sensors [1]. The stabilization
of the tetragonal and cubic phases is traditionally realized by
2
ZrO . Many of the investigations concerning this point were
doping with lower valence rare earth oxides. Y
frequently used in the zirconia solid solutions of practical relevance;
a minimum amount of Y above 2 and 8 mol% is required for the
stabilization of the tetragonal and cubic polymorphs respectively. It
has been recently shown that tetragonal ZrO can also be size-
2
O
3
is most
performed at room temperature examining samples previously
annealed at higher temperatures. The details of the transition
mechanism and, in particular, its kinetics were consequently
overlooked.
2 3
O
Recent studies of pure zirconia [12–13] and on zirconia doped
2
stabilized at room temperature, without doping, provided that the
grain size is smaller than a critical value of around 10–40 nm
depending on the synthesis condition adopted [2–9].
Since its discovery by Garvie [2–3], size stabilized zirconia has
been the object of numerous studies devoted in particular to the
investigation of the mechanism responsible for the inversion of
phase stability. Among the possible causes, difference in surface
energy between the tetragonal and monoclinic polymorphs is the
2 3
with low amounts of Sc O [14], revealed as the transition
towards the monoclinic phase is kinetically more complex than
previously thought. It is completed, in fact, only during the
cooling stage, with a transition temperature that depends largely
on the thermal history of the sample.
Aim of the present study was to investigate, through in-situ
X-ray diffraction, the structural and microstructural evolution of
2 3 2
Y O –ZrO nanocrystalline solid solutions, with YO1.5 ranging
between 0 and 1 mol%, i.e. below the thermodynamic limit for the
stabilization of the tetragonal phase, in order to elucidate the role
played by the grain size, doping and strain content on the
tetragonal to monoclinic transition.
n
Corresponding author. Tel.: +39 02 50315605; fax: +39 02 50315597.
E-mail address: monica.dapiaggi@unimi.it (M. Dapiaggi).
0022-3697/$ - see front matter & 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jpcs.2010.03.002

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1039
2
. Experimental
residuals. On the basis of DTA–DSC and XRD analyses the starting
material has been in this case annealed at 500 1C for 1 h.
Structural and microstructural characterization of the starting
powders was performed, using the software MAUD [19], using
high quality datasets collected at ESRF (ID31) during the
ZrO
2
nanopowders with a dopant (YO1.5) content ranging
to Zr0.99
between 0 and 1 at% Y (i.e. from pure ZrO
2
0.01 2ꢀd
Y O )
were obtained by a modified Pechini method [15]. An aqueous
solution containing the proper amounts of Zirconyl (IV) nitrate
and yttrium nitrate was mixed with a solution of citric acid in 1:1
experiment HS3654. These high energy patterns, with
˚
a
wavelength of about 0.4 A, allowed indeed a much better
accuracy, in particular for what concerns the microstructural
parameters evaluation, since the reciprocal lattice could be
explored in much higher detail.
(total metal vs citric acid) molar ratio. The solution was then
stirred on a hot plate at 80 1C, increasing its viscosity. After a time
period that depends on the amount of solution employed, the
viscous mass finally turns into a colorless transparent glass.
Further heating provided a white solid, which was subsequently
ground in a mortar and calcined in a furnace, at 500 1C for 1 h.
In-situ high temperature diffraction (an AHT PAP1600 hot
chamber was used) patterns were collected in static air, between
room temperature and 1200 1C. Data were collected during both
heating and cooling stages every 100 1C, the heating rate between
each measuring temperature was of 20 1C per minute.
The results of this analysis are shown in Table 1. All the
samples were free of monoclinic polymorph indicating that the
synthetic procedure adopted was successful in the stabilization of
the high temperature tetragonal polymorph. The cell volume
increases with the doping content coherently with the larger size
3
+
4+
of the Y ion with respect to Zr . The Zr_0 and Zr_05 showed
comparable values of crystallite size (around 12 nm) while Zr_1
showed a slightly smaller value. The grain size is for all the
prepared samples below the previously reported limit for the
The hot chamber is attached to a Philips X’Pert
diffractometer; data collections were performed on a limited 2
range, in order to be able to get the data fast enough to follow the
phase transition easily. The range was from 151 to 501 2 , with a
step size of 0.031 2 and a counting time of 1 s per step.
The wavelength used is Cu K
/21, an antiscatter of 1/21 on the diffracted beam, and a receiving
y–y powder
y
2
complete stabilization of the tetragonal phase in the pure ZrO .
Although there is some scatter in the literature about this limit,
values between 10 and 40 nm are generally reported. No data are
available in the literature for the Zr_05 and Zr_1 compositions as
no previous reports on these compositions are at hand. The RMS
microstrain variation was on the other hand more significative;
the RMS microstrain value for the Zr_05 sample was
approximately twice that of the Zr_1.
Fig. 1 shows the qualitative evolution of the HT-XRD pattern
collected during the heating (upper part) and cooling (lower part)
cycles on the Zr_0 sample. The onset of the formation of the
monoclinic polymorph is clearly visible at temperatures above
600 1C. As the temperature is further increased the amount of the
monoclinic phase increases up to above 50% at 1100 1C; at 1200 1C
the tetragonal polymorph returns to be the only phase
present as at this temperature the tetragonal phase becomes
thermodynamically stable. Similar qualitative trends were
observed also for the other two compositions. Upon cooling, as
the temperature is reduced below 700–800 1C, the monoclinic
fraction reaches values above 95%; finally, at room temperature
y
y
˚
1 (1.5406 A), with an incident slit of
a
1
slit of 0.6 mm, in order to maximize the peak intensity; resolution,
on the other hand, was not of primarily concern in our work due
to considerable line broadening related to the nano-size of our
powder. The sample holder is made of dense alumina, covered
with a thin foil of platinum (Goodfellows, 99.9%), as one of the
peaks of
tetragonal ZrO
a-alumina tend to interfere with the main peak of
2
.
The data were analyzed with the Rietveld method to obtain the
weight percent of the zirconia polymorphs, their cell parameters,
and the profile shape analysis. The sample holder peaks were
fitted with the Le Bail method [16], in order to remove their
influence on the quantitative analysis. The software used was
[
17–18].
As the small number of peaks collected did not allow any
sensible microstructural characterization of the samples during
heating, the crystallite size and the RMS microstrain were
evaluated from the corresponding Lorentzian components of
the peak shape of the modified pseudo-Voigt (function 2 in
GSAS), by keeping the Gaussian component (which approximately
correspond to the instrumental contribution) fixed at all
temperatures. In more details, the crystallite size can be
2
no residual tetragonal ZrO could be observed.
The data collected at each temperature were analyzed using the
Rietveld method, in order to obtain a quantitative evaluation of the
phase composition during heating and cooling. An example of a
Rietveld fit is reported in Fig. 2. Fig. 3 shows the quantitative
analysis at each temperature, in terms of the monoclinic polymorph
wt%, during heating (continuous lines) and cooling (dotted lines),
that allows a direct comparison among the three samples.
The transition is clearly different during the heating and the
cooling stages. Upon cooling the transformation kinetic is
completed within a narrower temperature interval (o100 1C)
and the onset temperature for the tetragonal to monoclinic
transition decreases going from Zr_0 to Zr_1. This indicates, as
estimated from the LX parameter (D¼LX/cos(
y
)) and the RMS
microstrain from the LY parameter (S¼LY tan(
y)). This is clearly
not the ideal way to operate, especially due to the fact that some
strain component varies with the diffraction angle in a way which
is similar to the Caglioti/Gaussian component.
expected,
a larger stability field for the tetragonal phase
increasing the doping content. This behavior, that follows the
well known behavior for ‘‘conventional’’ microcrystalline sample,
was expected as, at the end of the heating cycle, grain coarsening
goes well above the limit for any size stabilizing effect.
3
. Results and discussion
Three different samples were analyzed with dopant (YO1.5
)
content equal to 0% (nominal composition: ZrO
.5% (nominal composition: Zr0.995 , hereafter Zr_05)
and 1% (nominal composition: Zr0.99 , hereafter Zr_1).
2
, hereafter Zr_0),
0
Y
0.005
O
2ꢀ
d
Table 1
Y
0.01
O
2ꢀ
d
Structural and microstructural parameters of the as-prepared powders.
The synthetic route employed in this study is well known to
produce oxides with crystallite and grain sizes in the nanometric
range; the crystallite/grain size of the as-synthesized material is
generally dependent on the annealing treatment. The latter is
necessary to promote the crystallization of the oxide phase,
initially obtained in amorphous form, and to remove the organic
ꢀ
3
Sample
/DSv (nm)
RMS strain (10
)
Cell volume ( A˚ 3
)
Zr0
12.2 (2)
12.7 (1)
9.81 (3)
4.18 (1)
5.97 (4)
2.91 (6)
67.006 (4)
67.037 (4)
67.043 (6)
Zr0.5
Zr1

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M. Dapiaggi et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1038–1041
Hist 1
1
000
Lambda 1.5406 A, L−S cycle 1488
Obsd. and Calc. Profiles
Tetra
2
x10
8
00
6
4
.0
.0
6
00
Mono
4
00
Mono
2.0
2
00
0.0
0
1
000
00
00
25.0
30.0
35.0
2Θ, deg
40.0
45.0
8
6
4
00
Fig. 2. Example of a Rietveld fit of the Zr_05 sample at 800 1C.
2
00
0
3
6
34
30
32
28
2
6
1
20
Zr 0 heating
1
000
Zr 0.5 heating
Zr 1 heating
Zr 0 cooling
Zr 0.5 cooling
Zr 1 cooling
100
8
00
80
60
40
20
0
6
00
4
00
2
00
0
1
000
8
00
6
00
4
00
2
00
3
6
0
34
32
30
28
0
200
400
600
800 1000 1200 1400
2
6
Temperature (°C)
Fig. 1. In-situ high temperature XRD patterns for the Zr_0 sample. Upper part:
Fig. 3. Quantitative analyses of monoclinic polymorph as
a function of
heating cycle, lower part: cooling cycle.
temperature on the heating (solid lines) and on the cooling (dotted lines) stages.
The situation is indeed more complex during the heating cycle.
Not only, in fact, the kinetics is much more shallow, with only about
function of the annealing temperature, for the three compositions.
For all YO1.5 contents the shrinkage of the diffraction peak begins
at an almost identical temperature of around 400 1C and than
proceeds with a slope that is somewhat less pronounced for the
Zr_1 sample than for the other two.
50% of the tetragonal phase that transforms into the monoclinic one
within a 600–700 1C temperature interval, but also the temperature
for the transformation onset does not follow the doping content,
being the Zr_05 sample the first to initiate the transition at ꢁ400 1C
and the Zr_1 the last one at a temperature of ꢁ600 1C.
In the vast majority of the literature, the width of the main
diffraction peak is used for a straight estimation of the crystallite size
using the Scherrer formula. The temperature evolution of the phase
composition of nanocrystalline pure zirconia has been therefore
justified, in most cases, solely on the basis of the grain coarsening.
If applied to our results, where for the first time different low
doping contents are taken into account, such interpretative model is
clearly not sufficient to justify the experimental evidences. The
negligible difference in the initial grain size for the three composi-
tions (Table 1) and the results of Fig. 4 cannot justify the clear
differences in the onset temperature for the phase transition.
To clarify this point an evaluation of the RMS microstrain has
been performed for all the samples at the different annealing
During the heating cycle, the transition is more likely to occur
under microstructural control. Size stabilization is in fact
expected to play the leading role, since the doping contents
employed are well below the minimum limit for the stabilization
of the tetragonal phase.
A qualitative insight on the evolution of the microstructure
during the heating cycle can be gained from Fig. 1 (upper part) by
observing the peak shape evolution of the tetragonal polymorph.
As expected, a shrinkage in the width of the peaks of the
tetragonal phase is observed as the temperature is increased.
Fig. 4 shows the trend of the Lorentzian peak width at 29.91, as a

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2
2
1
1
50
00
50
00
initial highest value of microstrain, starts to release the
microstrain at the lowest temperature which also corresponds
to the beginning of the monoclinic phase formation.
Subsequently, at progressively higher temperature, the same
behavior is followed by the Zr0 and finally the Zr1 sample,
suggesting that microstrain release is the process responsible for
the initiation of the phase transformation at low temperature.
Zr 0
Zr 0.5
Zr 1
4. Conclusions
The thermal behavior of zirconia nanopowders with low yttria
contents was investigated by in-situ high temperature XRD. All
starting materials were characterized by a doping content well
below the thermodynamic limit for the stabilization of the
tetragonal phase. The tetragonal phase was on the other hand
stabilized thanks to the reduced dimension of the grain size.
Room temperature XRD measurements showed as all sample had
a crystallite size around 10 nm while the RMS microstrain content
varied for the different doping contents.
5
0
0
Upon heating the size-stabilized tetragonal phase transforms
partially into the monoclinic one. The onset temperature for this
transformation is strongly dependent on the RMS microstrain
content and occurs at the temperature where the material starts
releasing its stress content. The transition to the monoclinc phase
cannot be completed upon heating before entering, at 1200 1C the
stability range of the tetragonal form. On the other hand, once the
grain size is increased as an effect of annealing, the materials
rapidly transforms into the monoclinic phase cooling down from
0
200
400
600
800
1000
1200
Temperature (°C)
Fig. 4. Thermal evolution of the peak shape and of the monoclinic polymorph
during the heating stage.
0
0
0
0
0
0
0
0
.007
.006
.005
.004
.003
.002
.001
.000
Zr 0
Zr 0.5
Zr 1
1200 1C to room temperature. The onset temperature of the
transition is in this case dependent on the doping content.
Acknowledgements
The authors would like to thank ESRF for providing beam time
(
experiment HS3654), and the staff at ID31, in particular Michela
Brunelli and Andy Fitch for their help during data collection.
Fondazione Cariplo is thanked for funding.
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temperatures (heating cycle). The separation of crystallite size
and of the RMS microstrain contribution to the Lorentzian peaks
was, differently from the case of the high quality synchrotron data
of Table 1, possible in a more approximate way, as we are using a
smaller 2y range for our analyses. For that reason it was chosen to
show the RMS microstrain results only as variations with respect
[
[
[
14] G. Xu, Y.-W. Zhang, C.-S. Liao, C.-H. Yan, Phys. Chem. Chem. Phys. 6 (2004)
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5
[
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[
17] C. Larson, R.B. Von Dreele, General Structure Analysis System (GSAS), Los
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The results are shown in Fig. 5: the relationship between the
RMS microstrain and the onset temperature for the phase
transition (see Fig. 3) is evident. The Zr_05, characterized by the
[
[19] L. Lutterotti, S. Gialanella, Acta Mater. 46 (1998) 101.