Dehydration and crystallization process in sol-gel zirconia thermal and spectroscopic study

Article abstract of DOI:10.1023/B:JTAN.0000032260.31955.6e

Sol-gel zirconia was characterized using high-resolution thermogravimetry (Hi-Res TG) and differential scanning calorimetry (DSC) and compared with X-ray diffraction, Raman scattering, and UV-Vis spectroscopy. ZrO_2-x(OH) _2x·yH₂O annealed below 400°C show typical behavior of amorphous material. As the annealing temperature is increased, the tetragonal and monoclinie phases crystallize. Typical Hi-Res TG curve shows that the samples are continuously dehydrated in a long temperature range, between room temperature and 600°C. The total mass loss relative to the initial mass is of about 29%. The DSC analysis coupled with TG and structural information, indicate that the exothermic processes about 355 and 447°C can be related to the nucleation process of the formation of tetragonal zirconia, with bulk crystallization at 447°C.

Full text of DOI:10.1023/B:JTAN.0000032260.31955.6e

Journal of Thermal Analysis and Calorimetry, Vol. 76 (2004) 755–761
DEHYDRATION AND CRYSTALLIZATION PROCESS
IN SOL–GEL ZIRCONIA
Thermal and spectroscopic study
M. Picquart1*, T. López2, R. Gómez2, E. Torres3, A. Moreno2 and
J. Garcia2
1Universidad Autónoma Metropolitana, Departamento de Física, Apto. Postal 55 534,
09340 México DF, México
2Universidad Autónoma Metropolitana, Departamento de Química, Apto. Postal 55-534,
09340 México DF, México
3Instituto Mexicano del Petróleo, Eje Central Lazaro Cardenas, No. 152, C.P. 07730, México DF,
México
(Received October 28, 2003; in revised form January 20, 2004)
Abstract
Sol–gel zirconia was characterized using high-resolution thermogravimetry (Hi-Res TG) and differ-
ential scanning calorimetry (DSC) and compared with X-ray diffraction, Raman scattering, and
UV-Vis spectroscopy. ZrO2–x(OH)2x×yH2O annealed below 400°C show typical behavior of amor-
phous material. As the annealing temperature is increased, the tetragonal and monoclinic phases
crystallize. Typical Hi-Res TG curve shows that the samples are continuously dehydrated in a long
temperature range, between room temperature and 600°C. The total mass loss relative to the initial
mass is of about 29%. The DSC analysis coupled with TG and structural information, indicate that
the exothermic processes about 355 and 447°C can be related to the nucleation process of the forma-
tion of tetragonal zirconia, with bulk crystallization at 447°C.
Keywords: sol–gel zirconia Raman study, sol–gel zirconia TG-DTA, sol–gel zirconia UV-Vis,
sol–gel zirconia XRD
Introduction
The great variety of applications that display zirconia, has increased the attention to
investigate the factors controlling its structure [1, 2]. Pure ZrO2 present three types of
crystalline structures: tetragonal, monoclinic and cubic, and they are determined by
different factors used in their synthesis. In sol–gel zirconia preparation tetragonal
phase is stable and is transformed into monoclinic through different ways, like in
conventional microcrystalline zirconium oxide [3–7].
*
Author for correspondence: E-mail: mp@xanum.uam.mx
1388–6150/2004/ $ 20.00
Akadémiai Kiadó, Budapest
© 2004 Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht

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PICQUART et al.: SOL–GEL ZIRCONIA
The formation of tetragonal zirconia from amorphous material is well documented
and its characterization is mainly made by XRD studies [8]. However, little evidence is
given concerning thermal process involved in the formation of tetragonal zirconia, and
about its transformation into monoclinic crystalline phase. DSC and TG techniques have
been successfully used to analyze dehydration process in various compounds as oxide
powders [9, 10], minerals [11] and pharmaceutical hydrates [12]. With the aim of
contribute to the identification of zirconia crystalline phases by thermal analysis, in the
present work the evolution of the formation well characterized zirconia obtained from
zirconium alkoxides is reported. XRD, Raman, and UV-Vis spectroscopy supported TG
and DTA results.
Experimental
ZrO2 was prepared by sol–gel method. A solution of 29 cm3 of zirconium n-butoxide
(80% Strem chemicals) and 143 cm3 of ethanol (99.8% Baker) was added drop-wise
during 2 h into 31 cm3 of water until a final molar ratio of H2O/Zr(But)4=20/1 and
EtOH/Zr(But)4=5/1 were obtained. The synthesis process is obtained under constant
stirring and at a reflux temperature of 60°C during 24 h until the gel was formed. The
gel was then dried at 70°C during 12 h in the air. Samples were studied both in the
freshly prepared state (RT) and after annealing in air in a thermally regulated Lindberg
furnace for 4 h at 200, 400, 600, 800 and 900°C and then cooled to room temperature.
The powder XRD spectra were taken at room temperature and after annealing
process. The diffractograms were obtained on a Siemens D-5000 X-ray diffracto-
meter with a CuK radiation source (with a wavelength of 0.15418 nm). The samples
a
were prepared by softly compacting the powder in the specimen holder. An on-line
automatic search system (PDF Data-base) facilitated that observed data match with
JCPDS standards.
Thermal analysis were done in flow of N2 (100 cm3 min–1) on a thermobalance (TA
Instrument, mod. TG-2950). The programmed heating rates were 5, 10, 15,
and 20°C min–1. The mass of the samples used was between 10 and 15 mg. Each
experiment was at least repeated four times. The differential scanning calorimetry (DSC)
curves were obtained in a (TA Instruments DCS-2920). The samples were heated from
RT to 500°C in a N2 atmosphere at a heating rate of 10°C min–1, with isotherms of 5 min
before the start of the process.
Raman spectroscopy measurements were performed at room temperature on a
computerized Spex 1403 double monochromator in combination with the 514.5 nm
line of an Argon laser (Lexel lasers) at a power level of 50 mW at the laser head.
Raman spectra were taken directly from zirconia powder or aggregates in back scat-
tering geometry.
UV-Vis spectra were obtained using the diffuse reflectance technique on a
Varian Cary III equipped with an integrator sphere.
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Results and discussion
X-ray diffraction
The diffractogram of the solids samples ZrO2–x(OH)2x×yH2O annealed below 400°C
show only wide bands (data not shown), which is a typical comportment of
amorphous material. As the annealing temperature increases to 400°C, metastable
tetragonal phase begins to crystallize as it can be seen in Fig. 1a (reflection at 30.3°,
indexed as 011). Small diffraction peaks, indicated by an asterisk (Fig. 1b), are
attributed to the monoclinic phase (reflections at 28.6°, indexed as 111 and 31.8°,
indexed as 111) and can be observed at 600°C. They are more intense at 800°C,
nevertheless, the tetragonal phase is still present at this temperature. A Rietveld
analysis was performed to precise the proportion of each phase. At 400°C, the sample
is in a mixture of amorphous and tetragonal phases. Using the Scherrer equation, it is
possible to estimate the average crystallite size D(nm) in function of the annealing
temperature. The percentages of each phase were calculated in function of the
amount of tetragonal phase (100%, D=10.5 nm) existing in the sample at 400°C. At
600°C there is a mixture of tetragonal (78.9%, D=30.9 nm) and monoclinic (21.1%,
D=5.3 nm) phases and, at 800°C the content of monoclinic phase was increased
(23.3%, D=25.9 nm). It can be observed that the zirconia is nanostructured and the
crystallite size increases as the annealing temperature increases.
Fig. 1 X-rays diffractograms of zirconia at different annealing temperatures:
a – 400°C, b – 600°C and c – 800°C. The monoclinic diffraction peaks are
indicated by an asterisk
Thermal study
Typical TG curves of the ZrO2–x(OH)2x×yH2O are shown in Fig. 2. It can be observed
a continuous dehydration and residual ButOH release process between RT and
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Fig. 2 Typical TG curve during dehydration process of ZrO2–x(OH)2x×yH2O at 10 and 15°C min–1
500°C that can be divided into two steps. The main mass loss (22.5%) is between RT
and 100°C. A second mass loss of 7.5% is observed between 100 and 500°C. No
change in TG was observed after 500°C. The last process (7.5%) corresponds to the
formation of H2O by condensation of surface –OH groups, where can be situated the
sites for the promotion of the nucleus formation of the tetragonal phase.
DSC analysis (Fig. 3), coupled with the previous TG show that the endothermic
peak at 60°C is a consequence of the physical water desorbed as observed previously,
while the exothermic processes at 355 and 447°C are related to the crystallization of the
ZrO2. The presence of the thermal event at 355°C can be related with the initial
formation of nucleus of the metastable tetragonal phase of ZrO2, as observed by X-ray
scattering (Fig. 1), while, at 447°C the bulk crystallization process occurs. The total
enthalpy change for those two processes is 16.2 kJ mol–1, similar to previous works [13].
As was observed in TG analysis no change in DSC curve is observed after 500°C.
Raman scattering
Amorphous, monoclinic, tetragonal and cubic phases have been observed on zirco-
nia. These phases may be identified from one another by means of Raman scattering.
On the basis of experimental results [14, 15] tetragonal zirconia is expected to yield a
spectrum consisting of six bands with frequencies at about 146, 267, 315, 456, 607
and 645 cm–1. Monoclinic zirconia shows fourteen bands around 158, 177, 190, 221,
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Fig. 3 DSC profile of ZrO2–x(OH)2x×yH2O between RT and 500°C at 10°C min–1
306, 333, 347, 381, 475, 500, 537, 558, 615 and 638 cm–1. In the case of the amor-
phous sample a broad band is generally observed at about 550 cm–1.
In Fig. 4 the Raman spectra in the 100–750 cm–1 region of the zirconia at different
annealing temperatures from the fresh sample to 900°C are presented. Between room
temperature and 400°C no peak is observed in agreement with X-ray diffraction results.
It seems that some broad bands are existing suggesting a beginning of crystallization of
the tetragonal phase inside the amorphous material at 400°C. At 600°C the tetragonal
phase is clearly evidenced with bands at 150, 271, 323, 463 and 648.5 cm–1. At 800°C we
have a mixture of monoclinic and tetragonal phases where the bands at 174, 184, 217,
330, 344, 472, 500, 534, 557, 612 and 636 cm–1 are due to the monoclinic phase and the
bands at 141, 262 and 307 cm–1 are due to the tetragonal phase, in agreement with X-ray
diffraction. At 900°C the monoclinic phase is largely predominant in the sample. Some
small bands or shoulders can be observed due to residual tetragonal phase.
UV-Vis
In Fig. 5 the UV-Vis spectra of pure zirconia are presented in function of the anneal-
ing temperature. The band gap is determined from the equation:
a(hn) = A(hn–Eg)m/2
where a(hn) is the absorption coefficient at the hn energy photon, Eg the band gap, m
a coefficient which is 1 for a direct transition and 4 for an indirect transition [16].
From the spectra a straight line is extrapolated to zero absorbance where Eg=hn.
The band gap varies from 5.0 eV for the fresh sample to 3.3 eV for the sample
annealed at 800°C. The thermal treatment favors greatly the semiconducting proper-
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Fig. 4 Raman spectrum of sol–gel ZrO2 at increasing annealing temperatures in the
100–800 cm–1 region
Fig. 5 UV-Vis absorption spectra of ZrO2 sample at different annealing temperatures
ties of the material. This effect is correlated with the formation of defects in the mate-
rials due to oxygen vacancies caused by oxygen surface desorption, phenomenon in
which due to the continuity of the process the energy involved cannot be detected by
thermal analysis.
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Conclusions
The XRD spectrum of the sol–gel ZrO2–x(OH)2x×yH2O obtained in this work when it
is annealed below 400°C show typical behavior of amorphous material. As the an-
nealing temperature is increased above 400°C, metastable tetragonal phase is crystal-
lized. The formation of monoclinic zirconia at 600°C in mixture with tetragonal
phase is identified by Raman spectroscopy. Hi-Res TG curves show that the samples
are continuously dehydrated in a long temperature intervals, between room tempera-
ture and 600°C. The total mass loss mass is of about 30%. The DSC and TG analysis
compared with XRD results indicate that the exothermic process about 355
and 447°C can be associated to the nucleation process of tetragonal phase and the
bulk crystallization of this phase in ZrO2. After 500°C the transformation of
tetragonal zirconia into monoclinic cannot be observed by TG-DSC, showing that
this transformation is continuous and with low energy no detectable by DSC. The
transformation of ZrO2–x(OH)2x×yH2O in tetragonal zirconia and his transformation
into monoclinic during the sol–gel zirconia evolution is proved. No evidence of
ZrO2–x(OH)2x×yH2O direct transformation to monoclinic phase is obtained. As an-
nealing temperature increases, the material changes from isolating to semiconducting
one as it can be observed with the band-gap variation with the annealing temperature.
This evolution of the material is correlated to the existence of the monoclinic phase.
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