Synthesis and characterization of alumina- and zirconia-based powders obtained by the ultrasonic spray pyrolysis

Article abstract of DOI:10.1007/s10973-006-8075-3

The ultrasonic spray pyrolysis (USP) technique was used for synthesis of alumina- and zirconia-based powders. The starting agents were aqueous solutions, atomized by the ultrasonic spray generator and pyrolized in the furnace under the open-air conditions. The powders prepared by USP were in the form of solid and hollow aggregates (spheres) consisted of nanosize amorphous grains as determined by the microscopy and the X-ray diffraction techniques. The alumina-based powders were consolidated by the pulse plasma sintering resulting in single-phase materials. Different behavior of solid and hollow particles during the isostatic sintering is found; a higher degree of deformation of spheres is observed in the second case. Springer-Verlag 2007.

Full text of DOI:10.1007/s10973-006-8075-3

Journal of Thermal Analysis and Calorimetry, Vol. 88 (2007) 1, 65–69
SYNTHESIS AND CHARACTERIZATION OF ALUMINA- AND
ZIRCONIA-BASED POWDERS OBTAINED BY THE ULTRASONIC
SPRAY PYROLYSIS
W. Kucza1*, J. ObÓ·kowski1, R. Gajerski1, S. ˜abuí1, M. Danielewski1, A. MaÓecki1,
J. Morgiel2 and A. Michalski3
1Faculty of Materials Science and Ceramics, AGH University of Science and Technology, Al. Mickiewicza 30
Cracow 30-059, Poland
2Polish Academy of Sciences, Institute of Metallurgy and Materials Sciences, Reymonta 25, Cracow 30-059, Poland
3Warsaw University of Technology, Faculty of Materials Science and Engineering, WoÓoska 141, Warsaw 02-507, Poland
The ultrasonic spray pyrolysis (USP) technique was used for synthesis of alumina- and zirconia-based powders. The starting agents
were aqueous solutions, atomized by the ultrasonic spray generator and pyrolized in the furnace under the open-air conditions.
The powders prepared by USP were in the form of solid and hollow aggregates (spheres) consisted of nanosize amorphous grains as
determined by the microscopy and the X-ray diffraction techniques. The alumina-based powders were consolidated by the pulse
plasma sintering resulting in single-phase materials. Different behavior of solid and hollow particles during the isostatic sintering is
found; a higher degree of deformation of spheres is observed in the second case.
Keywords: alumina, plasma sintering, SEM, spray pyrolysis, TEM, zirconia
Introduction
and fine-grained materials [8]. Therefore additives
are used for preventing the grain growth during a ther-
mal treatment. The oxide Cr2O3 is isostructural to
Al2O3, forms solid solutions with alumina above
1400°C [8, 9] what eventually results in better me-
chanical and thermal performances. The same strat-
egy is adopted in the case of zirconia, where additives
of Y2O3 and CeO2 stabilize the structure and improve
thermal shock resistance [10].
The first target of the present study was to com-
bine advantages offered by ceramic materials and by
the spray pyrolysis technique. As a result, synthesis
and characterization of alumina-based powders (pure
Al2O3 and Al2O3–2%Cr2O3) and zirconia-based pow-
ders (ZrO2–10%Y2O3 and ZrO2–15%CeO2) are pre-
sented. The second one was manufacturing of materi-
als by the pulse plasma sintering of the obtained pow-
ders and their preliminary characterization. The influ-
ence of sintering conditions on morphology of these
materials is presented.
Ceramic powders found many applications in modern
industry as catalysts, adsorbents, abrasives, bio-
materials, solid electrolytes or ionic barriers, thermal
barriers, etc. In this aspect alumina- and zirco-
nia-based materials, in the form of dense or porous
materials, are considered to be very important be-
cause of their strength, thermal stability, high chemi-
cal resistance [1] and catalytic properties [2]. Spray
pyrolysis-based techniques become more and more
popular route for ceramic powder fabrication as a re-
sult of its simplicity, high homogeneity of obtained
materials, versatility, low cost and applicability at the
open atmosphere condition [3–7]. Moreover, such
techniques offer obtaining of metastable phases [4]
and/or nanomaterials [4, 6] in one-step process. Spray
pyrolysis involves liquid precursor atomization, its
transport by a carrier gas and a reaction in a furnace.
In order to reduce powder loss a system of electro-
static filters are used. Key parameters influencing
morphology of powders obtained by spray pyrolysis
(porosity, grain size, particles shapes) are: the precur-
sors [3, 6, 7], operating temperature [6], carrier
gas [3] (neutral gas or its mixture with hydrogen), its
flow rate, etc.
Experimental
Powders preparation
Alumina- and zirconia-based powders were synthe-
sized by the ultrasonic spray pyrolysis method. Aque-
It is well known that mechanical properties of
alumina can be increased by producing homogenous
*
Author for correspondence: witek@agh.edu.pl
1388–6150/$20.00
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
© 2007 Akadémiai Kiadó, Budapest

KUCZA et al.
Table 1 Solution concentrations and pyrolysis conditions
Nominal composition/mol%
Temperature/°C
800
Air flow/m3 h–1
0.20
Solution concentration/M
0.08
Al(NO3)3⋅9H2O
Al2O3
0.08
Al(NO3)3⋅9H2O+Cr(NO3)3⋅9H2O (2%)
Al2O3–2%Cr2O3
ZrO2–10%Y2O3
ZrO2–15%CeO2
800
860
900
0.30
0.45
0.15
0.08
ZrO(NO3)3⋅nH2O+Y2O3 (5%)
0.08
ZrO(NO3)3⋅H2O+NH4[Ce(NO3)5]⋅4H2O (15%)
ous precursors were obtained by dissolution of the in-
organic salts Al(NO3)3⋅9H2O, Cr(NO3)3⋅9H2O,
NH4[Ce(NO3)5]⋅4H2O and the oxide Y2O3
(a.p., POCH, Poland) in appropriate ratios in distilled
water. The obtained solutions were atomized by the
ultrasonic nebulizer (frequency: 2.6 MHz) and then
transported by a carrier gas into the furnace where py-
rolysis occurred. In order to reduce costs, the open at-
mosphere condition was selected in the present study.
For collecting of the obtained ceramic powders an
electrostatic filter at the outlet of the furnace was
used. More detailed description of the applied experi-
mental set is given in [11]. Several experiments were
carried out prior to this study in order to optimize the
process parameters such as solution concentrations,
furnace temperature and the carrier gas flow. The fi-
nal compositions of aqueous solutions and synthesis
conditions are collected in Table 1.
croscopy (SEM, Hitachi S3500N) and Transmission
Electron Microscopy (TEM, Philips CM20, 200 kV).
Results and discussion
Powders
The powders obtained by spray pyrolysis of the nomi-
nal compositions of Al2O3 and Al2O3–2%Cr2O3 were
thermally analyzed. These results as well as those of
evolving gases analyses are presented in Figs 1a
and b, respectively. For both compositions mass
losses are observed pointing at significant non-
stoichiometry of the as-received powders, however in
the case of Al2O3–2%Cr2O3 the mass loss is twice
lower. Quadrupole mass spectrometry measurements
show that nonstoichiometry of Al2O3 is related to de-
composition of residual nitrate used in synthesis
(200–500°C) and oxygen release (800–1000°C),
whereas for Al2O3–2%Cr2O3 to oxygen release only
(800–1000°C). In both cases mass losses are dimin-
ished for temperatures higher than 1000°C, therefore
this value was taken as a down limit for the pulse
plasma sintering process.
Sintering
The powders received by spray pyrolysis were con-
solidated using the pulse plasma sintering.
The sintering was carried out under a load of
600 kg cm–2 and the air pressure of 10–2 Pa. The whole
process was divided into three steps: (1) in order to
eliminate adsorbed gases samples were preheated at
150°C for 100–150 s, (2) pulse sintering was carried
out at the temperature range 1000–1200°C for
100–300 s, and (3) cooling to a temperature 100°C,
for about 250 s.
XRD spectra of as-received alumina-based pow-
ders are shown in Fig. 2. The obtained patterns are
characteristic for amorphous materials (background
reflections are present only). Scanning electron mi-
crographs of the ceramic powders are presented in
Figs 3–5; for the nominal compositions:
Al2O3–2%Cr2O3,
ZrO2–10%Y2O3
and
ZrO2–15%CeO2, respectively. The obtained powders
consist of spherical particles with a diameter between
20 nm–10 microns, depending on precursor and
sintering conditions. The spherical geometry is re-
lated to the drying of droplets produced by the ultra-
sonic atomizer. These particles are solid or hollow
spheres built from nanocrystallites, as can be seen in
Fig. 6, where TEM micrographs of zirconia based
powder are presented.
Materials characterization
Thermal studies of the obtained powders (TG and
DTA) were carried out using TG-DTA thermobalance
(2960 SDT, TA Instruments). Evolving gas analysis
(EGA) was made by means of the quadrupole mass
spectrometer (QMD 300 ThermoStar, Balzers). Phase
characterization was carried out using powder X-ray
diffractometer (Co lamp). Powders and sinters
morphologies were analyzed by Scanning Electron Mi-
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ALUMINA- AND ZIRCONIA-BASED POWDERS
Fig. 2 XRD patterns of as-received powders of nominal com-
positions: a – Al2O3 and b – Al2O3–2%Cr2O3
Fig. 3 SEM micrograph of the alumina-based powder of the
nominal composition Al2O3–2%Cr2O3
Fig. 1 DTA/TG and QMS measurements for powders of nom-
inal compositions: a – Al2O3 and b – Al2O3–2%Cr2O3
to note a thorough evolution of XRD patterns after rel-
atively short time of the thermal treatment (sintering
lasted up to 300 s). SEM micrographs of fractures of
alumina samples sintered at 1100 and 1200°C are pre-
sented in Figs 8a and b, respectively. The higher de-
gree of consolidation was found for the higher temper-
ature, the density measurements carried out by the
Archimedes’ method showed densities: 2.70 and
3.10 g cm–3, respectively. Similar studies were carried
out for the powder of the nominal composition
Sinters
In this paper consolidation of alumina-based powders
by the pulse plasma sintering is presented. XRD pat-
terns of sintered materials are shown in Figs 7a and b,
for compositions Al2O3 and Al2O3–2%Cr2O3, respec-
tively. The main phase in both cases was the hexagonal
alumina (JCPDS-ICDD file no. 046-1212). It is worth
J. Therm. Anal. Cal., 88, 2007
67

KUCZA et al.
Fig. 4 SEM micrograph of the zirconia-based powder of the
nominal composition ZrO2–10%Y2O3
Fig. 7 XRD patterns of alumina based materials obtained by
pulse plasma sintering of powders: a – Al2O3 and
b – Al2O3–2%Cr2O3
Fig. 5 SEM micrograph of the zirconia-based powder of the
nominal composition ZrO2–15%CeO2
Fig. 8 SEM micrographs of fractures of alumina powder
(nominal composition: Al2O3) sintered for 300 s at
a – 1100°C and b – 1200°C
Fig. 6 TEM micrographs of the zirconia-based powder of the
nominal composition ZrO2–15%CeO2
that the pulse plasma sintering prevents particle growth
for the times applied in this study.
Al2O3–2%Cr2O3. In this case the sintering temperature
remained constant (1100°C) whereas times were var-
ied: (a) 100 and (b) 300 s. The morphology of these
samples are presented in Figs 9a and b. The determined
density for the sample sintered at 1100°C was even
higher that that for the pure Al2O3 sintered at 1200°C
that is related to chromia additive. It is worth to note
Additionally TEM micrographs of Al2O3 mate-
rial sintered at the lowest temperature of 1000°C for
300 s are shown in Figs 10a and b. The samples were
prepared using FIB technique prior to microscope ob-
servations. The growth of necks between neighboring
spheres (Fig. 10a) and their deformation (Fig. 10b) as
results of the isostatic sintering are observed. The de-
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J. Therm. Anal. Cal., 88, 2007

ALUMINA- AND ZIRCONIA-BASED POWDERS
formances of these particles under the thermal shocks
and erosion.
Conclusions
Alumina- and zirconia-based powders were synthe-
sized by spray pyrolysis. Alumina-based powders
were sintered by means of the pulse plasma sintering.
After the sintering, XRD patters show evolution from
the amorphous type to the well-crystallized one.
Morphologies of powders and sinters were studied by
SEM and TEM techniques; powders were in the form
of full or hollow spherical particles consisted of
nano-grains. These particles were distorted during the
sintering with degree of deformation depending on
whether particles were hollow or not. Sintering stud-
ies show that alumina powders obtained by spray py-
rolysis are good starting precursors for obtaining both
porous and dense materials by varying the processing
conditions, e.g. for catalysis or biomaterial applica-
tions, respectively.
Fig. 9 SEM micrographs of fractures of chromia doped alu-
mina of (Al2O3–2%Cr2O3) powders sintered at 1100°C
for a – 100 and b – 300 s
Acknowledgements
The authors are grateful to the Polish State Committee for Scien-
tific Research for financial support via grant no. 3 T08C 041 28.
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DOI: 10.1007/s10973-006-8075-3
J. Therm. Anal. Cal., 88, 2007
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